literature review structure types

Literature Review: Types of literature reviews

• Traditional or narrative literature reviews
• Scoping Reviews
• Systematic literature reviews
• Annotated bibliography
• Keeping up to date with literature
• Finding a thesis
• Evaluating sources and critical appraisal of literature
• Managing and analysing your literature
• Further reading and resources

Types of literature reviews

The type of literature review you write will depend on your discipline and whether you are a researcher writing your PhD, publishing a study in a journal or completing an assessment task in your undergraduate study.

A literature review for a subject in an undergraduate degree will not be as comprehensive as the literature review required for a PhD thesis.

An undergraduate literature review may be in the form of an annotated bibliography or a narrative review of a small selection of literature, for example ten relevant articles. If you are asked to write a literature review, and you are an undergraduate student, be guided by your subject coordinator or lecturer.

The common types of literature reviews will be explained in the pages of this section.

• Narrative or traditional literature reviews
• Critically Appraised Topic (CAT)
• Scoping reviews
• Annotated bibliographies

These are not the only types of reviews of literature that can be conducted. Often the term "review" and "literature" can be confusing and used in the wrong context. Grant and Booth (2009) attempt to clear up this confusion by discussing 14 review types and the associated methodology, and advantages and disadvantages associated with each review.

Grant, M. J. and Booth, A. (2009), A typology of reviews: an analysis of 14 review types and associated methodologies . Health Information & Libraries Journal, 26 , 91–108. doi:10.1111/j.1471-1842.2009.00848.x

What's the difference between reviews?

Researchers, academics, and librarians all use various terms to describe different types of literature reviews, and there is often inconsistency in the ways the types are discussed. Here are a couple of simple explanations.

• The image below describes common review types in terms of speed, detail, risk of bias, and comprehensiveness:

"Schematic of the main differences between the types of literature review" by Brennan, M. L., Arlt, S. P., Belshaw, Z., Buckley, L., Corah, L., Doit, H., Fajt, V. R., Grindlay, D., Moberly, H. K., Morrow, L. D., Stavisky, J., & White, C. (2020). Critically Appraised Topics (CATs) in veterinary medicine: Applying evidence in clinical practice. Frontiers in Veterinary Science, 7 , 314. https://doi.org/10.3389/fvets.2020.00314 is licensed under CC BY 3.0

• The table below lists four of the most common types of review , as adapted from a widely used typology of fourteen types of reviews (Grant & Booth, 2009).  

Grant, M.J. & Booth, A. (2009).  A typology of reviews: An analysis of 14 review types and associated methodologies. Health Information & Libraries Journal, 26 (2), 91-108. https://doi.org/10.1111/j.1471-1842.2009.00848.x

See also the Library's  Literature Review guide.

Critical Appraised Topic (CAT)

For information on conducting a Critically Appraised Topic or CAT

Callander, J., Anstey, A. V., Ingram, J. R., Limpens, J., Flohr, C., & Spuls, P. I. (2017).  How to write a Critically Appraised Topic: evidence to underpin routine clinical practice.  British Journal of Dermatology (1951), 177(4), 1007-1013. https://doi.org/10.1111/bjd.15873 

Books on Literature Reviews

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Systematic Reviews

• Types of Literature Reviews

What Makes a Systematic Review Different from Other Types of Reviews?

• Planning Your Systematic Review
• Database Searching
• Creating the Search
• Search Filters and Hedges
• Grey Literature
• Managing and Appraising Results
• Further Resources

Reproduced from Grant, M. J. and Booth, A. (2009), A typology of reviews: an analysis of 14 review types and associated methodologies. Health Information & Libraries Journal, 26: 91–108. doi:10.1111/j.1471-1842.2009.00848.x

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Types of Literature Review

There are many types of literature review. The choice of a specific type depends on your research approach and design. The following types of literature review are the most popular in business studies:

Narrative literature review , also referred to as traditional literature review, critiques literature and summarizes the body of a literature. Narrative review also draws conclusions about the topic and identifies gaps or inconsistencies in a body of knowledge. You need to have a sufficiently focused research question to conduct a narrative literature review

Systematic literature review requires more rigorous and well-defined approach compared to most other types of literature review. Systematic literature review is comprehensive and details the timeframe within which the literature was selected. Systematic literature review can be divided into two categories: meta-analysis and meta-synthesis.

When you conduct meta-analysis you take findings from several studies on the same subject and analyze these using standardized statistical procedures. In meta-analysis patterns and relationships are detected and conclusions are drawn. Meta-analysis is associated with deductive research approach.

Meta-synthesis, on the other hand, is based on non-statistical techniques. This technique integrates, evaluates and interprets findings of multiple qualitative research studies. Meta-synthesis literature review is conducted usually when following inductive research approach.

Scoping literature review , as implied by its name is used to identify the scope or coverage of a body of literature on a given topic. It has been noted that “scoping reviews are useful for examining emerging evidence when it is still unclear what other, more specific questions can be posed and valuably addressed by a more precise systematic review.” [1] The main difference between systematic and scoping types of literature review is that, systematic literature review is conducted to find answer to more specific research questions, whereas scoping literature review is conducted to explore more general research question.

Argumentative literature review , as the name implies, examines literature selectively in order to support or refute an argument, deeply imbedded assumption, or philosophical problem already established in the literature. It should be noted that a potential for bias is a major shortcoming associated with argumentative literature review.

Integrative literature review reviews , critiques, and synthesizes secondary data about research topic in an integrated way such that new frameworks and perspectives on the topic are generated. If your research does not involve primary data collection and data analysis, then using integrative literature review will be your only option.

Theoretical literature review focuses on a pool of theory that has accumulated in regard to an issue, concept, theory, phenomena. Theoretical literature reviews play an instrumental role in establishing what theories already exist, the relationships between them, to what degree existing theories have been investigated, and to develop new hypotheses to be tested.

At the earlier parts of the literature review chapter, you need to specify the type of your literature review your chose and justify your choice. Your choice of a specific type of literature review should be based upon your research area, research problem and research methods.  Also, you can briefly discuss other most popular types of literature review mentioned above, to illustrate your awareness of them.

[1] Munn, A. et. al. (2018) “Systematic review or scoping review? Guidance for authors when choosing between a systematic or scoping review approach” BMC Medical Research Methodology

  John Dudovskiy

How To Structure Your Literature Review

3 options to help structure your chapter.

By: Amy Rommelspacher (PhD) | Reviewer: Dr Eunice Rautenbach | November 2020 (Updated May 2023)

Writing the literature review chapter can seem pretty daunting when you’re piecing together your dissertation or thesis. As  we’ve discussed before , a good literature review needs to achieve a few very important objectives – it should:

• Demonstrate your knowledge of the research topic
• Identify the gaps in the literature and show how your research links to these
• Provide the foundation for your conceptual framework (if you have one)
• Inform your own  methodology and research design

To achieve this, your literature review needs a well-thought-out structure . Get the structure of your literature review chapter wrong and you’ll struggle to achieve these objectives. Don’t worry though – in this post, we’ll look at how to structure your literature review for maximum impact (and marks!).

But wait – is this the right time?

Deciding on the structure of your literature review should come towards the end of the literature review process – after you have collected and digested the literature, but before you start writing the chapter. 

In other words, you need to first develop a rich understanding of the literature before you even attempt to map out a structure. There’s no use trying to develop a structure before you’ve fully wrapped your head around the existing research.

Equally importantly, you need to have a structure in place before you start writing , or your literature review will most likely end up a rambling, disjointed mess. 

Importantly, don’t feel that once you’ve defined a structure you can’t iterate on it. It’s perfectly natural to adjust as you engage in the writing process. As we’ve discussed before , writing is a way of developing your thinking, so it’s quite common for your thinking to change – and therefore, for your chapter structure to change – as you write. 

Need a helping hand?

Like any other chapter in your thesis or dissertation, your literature review needs to have a clear, logical structure. At a minimum, it should have three essential components – an  introduction , a  body   and a  conclusion . 

Let’s take a closer look at each of these.

1: The Introduction Section

Just like any good introduction, the introduction section of your literature review should introduce the purpose and layout (organisation) of the chapter. In other words, your introduction needs to give the reader a taste of what’s to come, and how you’re going to lay that out. Essentially, you should provide the reader with a high-level roadmap of your chapter to give them a taste of the journey that lies ahead.

Here’s an example of the layout visualised in a literature review introduction:

Your introduction should also outline your topic (including any tricky terminology or jargon) and provide an explanation of the scope of your literature review – in other words, what you  will   and  won’t   be covering (the delimitations ). This helps ringfence your review and achieve a clear focus . The clearer and narrower your focus, the deeper you can dive into the topic (which is typically where the magic lies). 

Depending on the nature of your project, you could also present your stance or point of view at this stage. In other words, after grappling with the literature you’ll have an opinion about what the trends and concerns are in the field as well as what’s lacking. The introduction section can then present these ideas so that it is clear to examiners that you’re aware of how your research connects with existing knowledge .

2: The Body Section

The body of your literature review is the centre of your work. This is where you’ll present, analyse, evaluate and synthesise the existing research. In other words, this is where you’re going to earn (or lose) the most marks. Therefore, it’s important to carefully think about how you will organise your discussion to present it in a clear way. 

The body of your literature review should do just as the description of this chapter suggests. It should “review” the literature – in other words, identify, analyse, and synthesise it. So, when thinking about structuring your literature review, you need to think about which structural approach will provide the best “review” for your specific type of research and objectives (we’ll get to this shortly).

There are (broadly speaking)  three options  for organising your literature review.

Option 1: Chronological (according to date)

Organising the literature chronologically is one of the simplest ways to structure your literature review. You start with what was published first and work your way through the literature until you reach the work published most recently. Pretty straightforward.

The benefit of this option is that it makes it easy to discuss the developments and debates in the field as they emerged over time. Organising your literature chronologically also allows you to highlight how specific articles or pieces of work might have changed the course of the field – in other words, which research has had the most impact . Therefore, this approach is very useful when your research is aimed at understanding how the topic has unfolded over time and is often used by scholars in the field of history. That said, this approach can be utilised by anyone that wants to explore change over time .

For example , if a student of politics is investigating how the understanding of democracy has evolved over time, they could use the chronological approach to provide a narrative that demonstrates how this understanding has changed through the ages.

Here are some questions you can ask yourself to help you structure your literature review chronologically.

• What is the earliest literature published relating to this topic?
• How has the field changed over time? Why?
• What are the most recent discoveries/theories?

In some ways, chronology plays a part whichever way you decide to structure your literature review, because you will always, to a certain extent, be analysing how the literature has developed. However, with the chronological approach, the emphasis is very firmly on how the discussion has evolved over time , as opposed to how all the literature links together (which we’ll discuss next ).

Option 2: Thematic (grouped by theme)

The thematic approach to structuring a literature review means organising your literature by theme or category – for example, by independent variables (i.e. factors that have an impact on a specific outcome).

As you’ve been collecting and synthesising literature , you’ll likely have started seeing some themes or patterns emerging. You can then use these themes or patterns as a structure for your body discussion. The thematic approach is the most common approach and is useful for structuring literature reviews in most fields.

For example, if you were researching which factors contributed towards people trusting an organisation, you might find themes such as consumers’ perceptions of an organisation’s competence, benevolence and integrity. Structuring your literature review thematically would mean structuring your literature review’s body section to discuss each of these themes, one section at a time.

Here are some questions to ask yourself when structuring your literature review by themes:

• Are there any patterns that have come to light in the literature?
• What are the central themes and categories used by the researchers?
• Do I have enough evidence of these themes?

PS – you can see an example of a thematically structured literature review in our literature review sample walkthrough video here.

Option 3: Methodological

The methodological option is a way of structuring your literature review by the research methodologies used . In other words, organising your discussion based on the angle from which each piece of research was approached – for example, qualitative , quantitative or mixed  methodologies.

Structuring your literature review by methodology can be useful if you are drawing research from a variety of disciplines and are critiquing different methodologies. The point of this approach is to question  how  existing research has been conducted, as opposed to  what  the conclusions and/or findings the research were.

For example, a sociologist might centre their research around critiquing specific fieldwork practices. Their literature review will then be a summary of the fieldwork methodologies used by different studies.

Here are some questions you can ask yourself when structuring your literature review according to methodology:

• Which methodologies have been utilised in this field?
• Which methodology is the most popular (and why)?
• What are the strengths and weaknesses of the various methodologies?
• How can the existing methodologies inform my own methodology?

3: The Conclusion Section

Once you’ve completed the body section of your literature review using one of the structural approaches we discussed above, you’ll need to “wrap up” your literature review and pull all the pieces together to set the direction for the rest of your dissertation or thesis.

The conclusion is where you’ll present the key findings of your literature review. In this section, you should emphasise the research that is especially important to your research questions and highlight the gaps that exist in the literature. Based on this, you need to make it clear what you will add to the literature – in other words, justify your own research by showing how it will help fill one or more of the gaps you just identified.

Last but not least, if it’s your intention to develop a conceptual framework for your dissertation or thesis, the conclusion section is a good place to present this.

Example: Thematically Structured Review

In the video below, we unpack a literature review chapter so that you can see an example of a thematically structure review in practice.

Let’s Recap

In this article, we’ve  discussed how to structure your literature review for maximum impact. Here’s a quick recap of what  you need to keep in mind when deciding on your literature review structure:

• Just like other chapters, your literature review needs a clear introduction , body and conclusion .
• The introduction section should provide an overview of what you will discuss in your literature review.
• The body section of your literature review can be organised by chronology , theme or methodology . The right structural approach depends on what you’re trying to achieve with your research.
• The conclusion section should draw together the key findings of your literature review and link them to your research questions.

If you’re ready to get started, be sure to download our free literature review template to fast-track your chapter outline.

Psst… there’s more!

This post is an extract from our bestselling short course, Literature Review Bootcamp . If you want to work smart, you don't want to miss this .

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27 Comments

Great work. This is exactly what I was looking for and helps a lot together with your previous post on literature review. One last thing is missing: a link to a great literature chapter of an journal article (maybe with comments of the different sections in this review chapter). Do you know any great literature review chapters?

I agree with you Marin… A great piece

I agree with Marin. This would be quite helpful if you annotate a nicely structured literature from previously published research articles.

Awesome article for my research.

I thank you immensely for this wonderful guide

It is indeed thought and supportive work for the futurist researcher and students

Very educative and good time to get guide. Thank you

Great work, very insightful. Thank you.

Thanks for this wonderful presentation. My question is that do I put all the variables into a single conceptual framework or each hypothesis will have it own conceptual framework?

Thank you very much, very helpful

This is very educative and precise . Thank you very much for dropping this kind of write up .

Pheeww, so damn helpful, thank you for this informative piece.

I’m doing a research project topic ; stool analysis for parasitic worm (enteric) worm, how do I structure it, thanks.

comprehensive explanation. Help us by pasting the URL of some good “literature review” for better understanding.

great piece. thanks for the awesome explanation. it is really worth sharing. I have a little question, if anyone can help me out, which of the options in the body of literature can be best fit if you are writing an architectural thesis that deals with design?

I am doing a research on nanofluids how can l structure it?

Beautifully clear.nThank you!

Lucid! Thankyou!

Brilliant work, well understood, many thanks

I like how this was so clear with simple language 😊😊 thank you so much 😊 for these information 😊

Insightful. I was struggling to come up with a sensible literature review but this has been really helpful. Thank you!

You have given thought-provoking information about the review of the literature.

Thank you. It has made my own research better and to impart your work to students I teach

I learnt a lot from this teaching. It’s a great piece.

I am doing research on EFL teacher motivation for his/her job. How Can I structure it? Is there any detailed template, additional to this?

You are so cool! I do not think I’ve read through something like this before. So nice to find somebody with some genuine thoughts on this issue. Seriously.. thank you for starting this up. This site is one thing that is required on the internet, someone with a little originality!

I’m asked to do conceptual, theoretical and empirical literature, and i just don’t know how to structure it

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• What is a Literature Review? | Guide, Template, & Examples

What is a Literature Review? | Guide, Template, & Examples

Published on 22 February 2022 by Shona McCombes . Revised on 7 June 2022.

What is a literature review? A literature review is a survey of scholarly sources on a specific topic. It provides an overview of current knowledge, allowing you to identify relevant theories, methods, and gaps in the existing research.

There are five key steps to writing a literature review:

• Search for relevant literature
• Evaluate sources
• Identify themes, debates and gaps
• Outline the structure
• Write your literature review

A good literature review doesn’t just summarise sources – it analyses, synthesises, and critically evaluates to give a clear picture of the state of knowledge on the subject.

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Table of contents

Why write a literature review, examples of literature reviews, step 1: search for relevant literature, step 2: evaluate and select sources, step 3: identify themes, debates and gaps, step 4: outline your literature review’s structure, step 5: write your literature review, frequently asked questions about literature reviews, introduction.

• Quick Run-through
• Step 1 & 2

When you write a dissertation or thesis, you will have to conduct a literature review to situate your research within existing knowledge. The literature review gives you a chance to:

• Demonstrate your familiarity with the topic and scholarly context
• Develop a theoretical framework and methodology for your research
• Position yourself in relation to other researchers and theorists
• Show how your dissertation addresses a gap or contributes to a debate

You might also have to write a literature review as a stand-alone assignment. In this case, the purpose is to evaluate the current state of research and demonstrate your knowledge of scholarly debates around a topic.

The content will look slightly different in each case, but the process of conducting a literature review follows the same steps. We’ve written a step-by-step guide that you can follow below.

Prevent plagiarism, run a free check.

Writing literature reviews can be quite challenging! A good starting point could be to look at some examples, depending on what kind of literature review you’d like to write.

• Example literature review #1: “Why Do People Migrate? A Review of the Theoretical Literature” ( Theoretical literature review about the development of economic migration theory from the 1950s to today.)
• Example literature review #2: “Literature review as a research methodology: An overview and guidelines” ( Methodological literature review about interdisciplinary knowledge acquisition and production.)
• Example literature review #3: “The Use of Technology in English Language Learning: A Literature Review” ( Thematic literature review about the effects of technology on language acquisition.)
• Example literature review #4: “Learners’ Listening Comprehension Difficulties in English Language Learning: A Literature Review” ( Chronological literature review about how the concept of listening skills has changed over time.)

You can also check out our templates with literature review examples and sample outlines at the links below.

Download Word doc Download Google doc

Before you begin searching for literature, you need a clearly defined topic .

If you are writing the literature review section of a dissertation or research paper, you will search for literature related to your research objectives and questions .

If you are writing a literature review as a stand-alone assignment, you will have to choose a focus and develop a central question to direct your search. Unlike a dissertation research question, this question has to be answerable without collecting original data. You should be able to answer it based only on a review of existing publications.

Make a list of keywords

Start by creating a list of keywords related to your research topic. Include each of the key concepts or variables you’re interested in, and list any synonyms and related terms. You can add to this list if you discover new keywords in the process of your literature search.

• Social media, Facebook, Instagram, Twitter, Snapchat, TikTok
• Body image, self-perception, self-esteem, mental health
• Generation Z, teenagers, adolescents, youth

Search for relevant sources

Use your keywords to begin searching for sources. Some databases to search for journals and articles include:

• Your university’s library catalogue
• Google Scholar
• Project Muse (humanities and social sciences)
• Medline (life sciences and biomedicine)
• EconLit (economics)
• Inspec (physics, engineering and computer science)

You can use boolean operators to help narrow down your search:

Read the abstract to find out whether an article is relevant to your question. When you find a useful book or article, you can check the bibliography to find other relevant sources.

To identify the most important publications on your topic, take note of recurring citations. If the same authors, books or articles keep appearing in your reading, make sure to seek them out.

You probably won’t be able to read absolutely everything that has been written on the topic – you’ll have to evaluate which sources are most relevant to your questions.

For each publication, ask yourself:

• What question or problem is the author addressing?
• What are the key concepts and how are they defined?
• What are the key theories, models and methods? Does the research use established frameworks or take an innovative approach?
• What are the results and conclusions of the study?
• How does the publication relate to other literature in the field? Does it confirm, add to, or challenge established knowledge?
• How does the publication contribute to your understanding of the topic? What are its key insights and arguments?
• What are the strengths and weaknesses of the research?

Make sure the sources you use are credible, and make sure you read any landmark studies and major theories in your field of research.

You can find out how many times an article has been cited on Google Scholar – a high citation count means the article has been influential in the field, and should certainly be included in your literature review.

The scope of your review will depend on your topic and discipline: in the sciences you usually only review recent literature, but in the humanities you might take a long historical perspective (for example, to trace how a concept has changed in meaning over time).

Remember that you can use our template to summarise and evaluate sources you’re thinking about using!

Take notes and cite your sources

As you read, you should also begin the writing process. Take notes that you can later incorporate into the text of your literature review.

It’s important to keep track of your sources with references to avoid plagiarism . It can be helpful to make an annotated bibliography, where you compile full reference information and write a paragraph of summary and analysis for each source. This helps you remember what you read and saves time later in the process.

You can use our free APA Reference Generator for quick, correct, consistent citations.

To begin organising your literature review’s argument and structure, you need to understand the connections and relationships between the sources you’ve read. Based on your reading and notes, you can look for:

• Trends and patterns (in theory, method or results): do certain approaches become more or less popular over time?
• Themes: what questions or concepts recur across the literature?
• Debates, conflicts and contradictions: where do sources disagree?
• Pivotal publications: are there any influential theories or studies that changed the direction of the field?
• Gaps: what is missing from the literature? Are there weaknesses that need to be addressed?

This step will help you work out the structure of your literature review and (if applicable) show how your own research will contribute to existing knowledge.

• Most research has focused on young women.
• There is an increasing interest in the visual aspects of social media.
• But there is still a lack of robust research on highly-visual platforms like Instagram and Snapchat – this is a gap that you could address in your own research.

There are various approaches to organising the body of a literature review. You should have a rough idea of your strategy before you start writing.

Depending on the length of your literature review, you can combine several of these strategies (for example, your overall structure might be thematic, but each theme is discussed chronologically).

Chronological

The simplest approach is to trace the development of the topic over time. However, if you choose this strategy, be careful to avoid simply listing and summarising sources in order.

Try to analyse patterns, turning points and key debates that have shaped the direction of the field. Give your interpretation of how and why certain developments occurred.

If you have found some recurring central themes, you can organise your literature review into subsections that address different aspects of the topic.

For example, if you are reviewing literature about inequalities in migrant health outcomes, key themes might include healthcare policy, language barriers, cultural attitudes, legal status, and economic access.

Methodological

If you draw your sources from different disciplines or fields that use a variety of research methods , you might want to compare the results and conclusions that emerge from different approaches. For example:

• Look at what results have emerged in qualitative versus quantitative research
• Discuss how the topic has been approached by empirical versus theoretical scholarship
• Divide the literature into sociological, historical, and cultural sources

Theoretical

A literature review is often the foundation for a theoretical framework . You can use it to discuss various theories, models, and definitions of key concepts.

You might argue for the relevance of a specific theoretical approach, or combine various theoretical concepts to create a framework for your research.

Like any other academic text, your literature review should have an introduction , a main body, and a conclusion . What you include in each depends on the objective of your literature review.

The introduction should clearly establish the focus and purpose of the literature review.

If you are writing the literature review as part of your dissertation or thesis, reiterate your central problem or research question and give a brief summary of the scholarly context. You can emphasise the timeliness of the topic (“many recent studies have focused on the problem of x”) or highlight a gap in the literature (“while there has been much research on x, few researchers have taken y into consideration”).

Depending on the length of your literature review, you might want to divide the body into subsections. You can use a subheading for each theme, time period, or methodological approach.

As you write, make sure to follow these tips:

• Summarise and synthesise: give an overview of the main points of each source and combine them into a coherent whole.
• Analyse and interpret: don’t just paraphrase other researchers – add your own interpretations, discussing the significance of findings in relation to the literature as a whole.
• Critically evaluate: mention the strengths and weaknesses of your sources.
• Write in well-structured paragraphs: use transitions and topic sentences to draw connections, comparisons and contrasts.

In the conclusion, you should summarise the key findings you have taken from the literature and emphasise their significance.

If the literature review is part of your dissertation or thesis, reiterate how your research addresses gaps and contributes new knowledge, or discuss how you have drawn on existing theories and methods to build a framework for your research. This can lead directly into your methodology section.

A literature review is a survey of scholarly sources (such as books, journal articles, and theses) related to a specific topic or research question .

It is often written as part of a dissertation , thesis, research paper , or proposal .

There are several reasons to conduct a literature review at the beginning of a research project:

• To familiarise yourself with the current state of knowledge on your topic
• To ensure that you’re not just repeating what others have already done
• To identify gaps in knowledge and unresolved problems that your research can address
• To develop your theoretical framework and methodology
• To provide an overview of the key findings and debates on the topic

Writing the literature review shows your reader how your work relates to existing research and what new insights it will contribute.

The literature review usually comes near the beginning of your  dissertation . After the introduction , it grounds your research in a scholarly field and leads directly to your theoretical framework or methodology .

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• UConn Library
• Literature Review: The What, Why and How-to Guide
• Introduction

Literature Review: The What, Why and How-to Guide — Introduction

• Getting Started
• How to Pick a Topic
• Strategies to Find Sources
• Evaluating Sources & Lit. Reviews
• Tips for Writing Literature Reviews
• Writing Literature Review: Useful Sites
• Citation Resources
• Other Academic Writings

What are Literature Reviews?

So, what is a literature review? "A literature review is an account of what has been published on a topic by accredited scholars and researchers. In writing the literature review, your purpose is to convey to your reader what knowledge and ideas have been established on a topic, and what their strengths and weaknesses are. As a piece of writing, the literature review must be defined by a guiding concept (e.g., your research objective, the problem or issue you are discussing, or your argumentative thesis). It is not just a descriptive list of the material available, or a set of summaries." Taylor, D.  The literature review: A few tips on conducting it . University of Toronto Health Sciences Writing Centre.

Goals of Literature Reviews

What are the goals of creating a Literature Review?  A literature could be written to accomplish different aims:

• To develop a theory or evaluate an existing theory
• To summarize the historical or existing state of a research topic
• Identify a problem in a field of research 

Baumeister, R. F., & Leary, M. R. (1997). Writing narrative literature reviews .  Review of General Psychology , 1 (3), 311-320.

What kinds of sources require a Literature Review?

• A research paper assigned in a course
• A thesis or dissertation
• A grant proposal
• An article intended for publication in a journal

All these instances require you to collect what has been written about your research topic so that you can demonstrate how your own research sheds new light on the topic.

Types of Literature Reviews

What kinds of literature reviews are written?

Narrative review: The purpose of this type of review is to describe the current state of the research on a specific topic/research and to offer a critical analysis of the literature reviewed. Studies are grouped by research/theoretical categories, and themes and trends, strengths and weakness, and gaps are identified. The review ends with a conclusion section which summarizes the findings regarding the state of the research of the specific study, the gaps identify and if applicable, explains how the author's research will address gaps identify in the review and expand the knowledge on the topic reviewed.

• Example : Predictors and Outcomes of U.S. Quality Maternity Leave: A Review and Conceptual Framework:  10.1177/08948453211037398  

Systematic review : "The authors of a systematic review use a specific procedure to search the research literature, select the studies to include in their review, and critically evaluate the studies they find." (p. 139). Nelson, L. K. (2013). Research in Communication Sciences and Disorders . Plural Publishing.

• Example : The effect of leave policies on increasing fertility: a systematic review:  10.1057/s41599-022-01270-w

Meta-analysis : "Meta-analysis is a method of reviewing research findings in a quantitative fashion by transforming the data from individual studies into what is called an effect size and then pooling and analyzing this information. The basic goal in meta-analysis is to explain why different outcomes have occurred in different studies." (p. 197). Roberts, M. C., & Ilardi, S. S. (2003). Handbook of Research Methods in Clinical Psychology . Blackwell Publishing.

• Example : Employment Instability and Fertility in Europe: A Meta-Analysis:  10.1215/00703370-9164737

Meta-synthesis : "Qualitative meta-synthesis is a type of qualitative study that uses as data the findings from other qualitative studies linked by the same or related topic." (p.312). Zimmer, L. (2006). Qualitative meta-synthesis: A question of dialoguing with texts .  Journal of Advanced Nursing , 53 (3), 311-318.

• Example : Women’s perspectives on career successes and barriers: A qualitative meta-synthesis:  10.1177/05390184221113735

Literature Reviews in the Health Sciences

• UConn Health subject guide on systematic reviews Explanation of the different review types used in health sciences literature as well as tools to help you find the right review type
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Organizing Your Social Sciences Research Paper

• 5. The Literature Review
• Purpose of Guide
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• Independent and Dependent Variables
• Glossary of Research Terms
• Reading Research Effectively
• Narrowing a Topic Idea
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A literature review surveys prior research published in books, scholarly articles, and any other sources relevant to a particular issue, area of research, or theory, and by so doing, provides a description, summary, and critical evaluation of these works in relation to the research problem being investigated. Literature reviews are designed to provide an overview of sources you have used in researching a particular topic and to demonstrate to your readers how your research fits within existing scholarship about the topic.

Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper . Fourth edition. Thousand Oaks, CA: SAGE, 2014.

Importance of a Good Literature Review

A literature review may consist of simply a summary of key sources, but in the social sciences, a literature review usually has an organizational pattern and combines both summary and synthesis, often within specific conceptual categories . A summary is a recap of the important information of the source, but a synthesis is a re-organization, or a reshuffling, of that information in a way that informs how you are planning to investigate a research problem. The analytical features of a literature review might:

• Give a new interpretation of old material or combine new with old interpretations,
• Trace the intellectual progression of the field, including major debates,
• Depending on the situation, evaluate the sources and advise the reader on the most pertinent or relevant research, or
• Usually in the conclusion of a literature review, identify where gaps exist in how a problem has been researched to date.

Given this, the purpose of a literature review is to:

• Place each work in the context of its contribution to understanding the research problem being studied.
• Describe the relationship of each work to the others under consideration.
• Identify new ways to interpret prior research.
• Reveal any gaps that exist in the literature.
• Resolve conflicts amongst seemingly contradictory previous studies.
• Identify areas of prior scholarship to prevent duplication of effort.
• Point the way in fulfilling a need for additional research.
• Locate your own research within the context of existing literature [very important].

Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper. 2nd ed. Thousand Oaks, CA: Sage, 2005; Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1998; Jesson, Jill. Doing Your Literature Review: Traditional and Systematic Techniques . Los Angeles, CA: SAGE, 2011; Knopf, Jeffrey W. "Doing a Literature Review." PS: Political Science and Politics 39 (January 2006): 127-132; Ridley, Diana. The Literature Review: A Step-by-Step Guide for Students . 2nd ed. Los Angeles, CA: SAGE, 2012.

Types of Literature Reviews

It is important to think of knowledge in a given field as consisting of three layers. First, there are the primary studies that researchers conduct and publish. Second are the reviews of those studies that summarize and offer new interpretations built from and often extending beyond the primary studies. Third, there are the perceptions, conclusions, opinion, and interpretations that are shared informally among scholars that become part of the body of epistemological traditions within the field.

In composing a literature review, it is important to note that it is often this third layer of knowledge that is cited as "true" even though it often has only a loose relationship to the primary studies and secondary literature reviews. Given this, while literature reviews are designed to provide an overview and synthesis of pertinent sources you have explored, there are a number of approaches you could adopt depending upon the type of analysis underpinning your study.

Argumentative Review This form examines literature selectively in order to support or refute an argument, deeply embedded assumption, or philosophical problem already established in the literature. The purpose is to develop a body of literature that establishes a contrarian viewpoint. Given the value-laden nature of some social science research [e.g., educational reform; immigration control], argumentative approaches to analyzing the literature can be a legitimate and important form of discourse. However, note that they can also introduce problems of bias when they are used to make summary claims of the sort found in systematic reviews [see below].

Integrative Review Considered a form of research that reviews, critiques, and synthesizes representative literature on a topic in an integrated way such that new frameworks and perspectives on the topic are generated. The body of literature includes all studies that address related or identical hypotheses or research problems. A well-done integrative review meets the same standards as primary research in regard to clarity, rigor, and replication. This is the most common form of review in the social sciences.

Historical Review Few things rest in isolation from historical precedent. Historical literature reviews focus on examining research throughout a period of time, often starting with the first time an issue, concept, theory, phenomena emerged in the literature, then tracing its evolution within the scholarship of a discipline. The purpose is to place research in a historical context to show familiarity with state-of-the-art developments and to identify the likely directions for future research.

Methodological Review A review does not always focus on what someone said [findings], but how they came about saying what they say [method of analysis]. Reviewing methods of analysis provides a framework of understanding at different levels [i.e. those of theory, substantive fields, research approaches, and data collection and analysis techniques], how researchers draw upon a wide variety of knowledge ranging from the conceptual level to practical documents for use in fieldwork in the areas of ontological and epistemological consideration, quantitative and qualitative integration, sampling, interviewing, data collection, and data analysis. This approach helps highlight ethical issues which you should be aware of and consider as you go through your own study.

Systematic Review This form consists of an overview of existing evidence pertinent to a clearly formulated research question, which uses pre-specified and standardized methods to identify and critically appraise relevant research, and to collect, report, and analyze data from the studies that are included in the review. The goal is to deliberately document, critically evaluate, and summarize scientifically all of the research about a clearly defined research problem . Typically it focuses on a very specific empirical question, often posed in a cause-and-effect form, such as "To what extent does A contribute to B?" This type of literature review is primarily applied to examining prior research studies in clinical medicine and allied health fields, but it is increasingly being used in the social sciences.

Theoretical Review The purpose of this form is to examine the corpus of theory that has accumulated in regard to an issue, concept, theory, phenomena. The theoretical literature review helps to establish what theories already exist, the relationships between them, to what degree the existing theories have been investigated, and to develop new hypotheses to be tested. Often this form is used to help establish a lack of appropriate theories or reveal that current theories are inadequate for explaining new or emerging research problems. The unit of analysis can focus on a theoretical concept or a whole theory or framework.

NOTE: Most often the literature review will incorporate some combination of types. For example, a review that examines literature supporting or refuting an argument, assumption, or philosophical problem related to the research problem will also need to include writing supported by sources that establish the history of these arguments in the literature.

Baumeister, Roy F. and Mark R. Leary. "Writing Narrative Literature Reviews."  Review of General Psychology 1 (September 1997): 311-320; Mark R. Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper . 2nd ed. Thousand Oaks, CA: Sage, 2005; Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1998; Kennedy, Mary M. "Defining a Literature." Educational Researcher 36 (April 2007): 139-147; Petticrew, Mark and Helen Roberts. Systematic Reviews in the Social Sciences: A Practical Guide . Malden, MA: Blackwell Publishers, 2006; Torracro, Richard. "Writing Integrative Literature Reviews: Guidelines and Examples." Human Resource Development Review 4 (September 2005): 356-367; Rocco, Tonette S. and Maria S. Plakhotnik. "Literature Reviews, Conceptual Frameworks, and Theoretical Frameworks: Terms, Functions, and Distinctions." Human Ressource Development Review 8 (March 2008): 120-130; Sutton, Anthea. Systematic Approaches to a Successful Literature Review . Los Angeles, CA: Sage Publications, 2016.

Structure and Writing Style

I.  Thinking About Your Literature Review

The structure of a literature review should include the following in support of understanding the research problem :

• An overview of the subject, issue, or theory under consideration, along with the objectives of the literature review,
• Division of works under review into themes or categories [e.g. works that support a particular position, those against, and those offering alternative approaches entirely],
• An explanation of how each work is similar to and how it varies from the others,
• Conclusions as to which pieces are best considered in their argument, are most convincing of their opinions, and make the greatest contribution to the understanding and development of their area of research.

The critical evaluation of each work should consider :

• Provenance -- what are the author's credentials? Are the author's arguments supported by evidence [e.g. primary historical material, case studies, narratives, statistics, recent scientific findings]?
• Methodology -- were the techniques used to identify, gather, and analyze the data appropriate to addressing the research problem? Was the sample size appropriate? Were the results effectively interpreted and reported?
• Objectivity -- is the author's perspective even-handed or prejudicial? Is contrary data considered or is certain pertinent information ignored to prove the author's point?
• Persuasiveness -- which of the author's theses are most convincing or least convincing?
• Validity -- are the author's arguments and conclusions convincing? Does the work ultimately contribute in any significant way to an understanding of the subject?

II.  Development of the Literature Review

Four Basic Stages of Writing 1.  Problem formulation -- which topic or field is being examined and what are its component issues? 2.  Literature search -- finding materials relevant to the subject being explored. 3.  Data evaluation -- determining which literature makes a significant contribution to the understanding of the topic. 4.  Analysis and interpretation -- discussing the findings and conclusions of pertinent literature.

Consider the following issues before writing the literature review: Clarify If your assignment is not specific about what form your literature review should take, seek clarification from your professor by asking these questions: 1.  Roughly how many sources would be appropriate to include? 2.  What types of sources should I review (books, journal articles, websites; scholarly versus popular sources)? 3.  Should I summarize, synthesize, or critique sources by discussing a common theme or issue? 4.  Should I evaluate the sources in any way beyond evaluating how they relate to understanding the research problem? 5.  Should I provide subheadings and other background information, such as definitions and/or a history? Find Models Use the exercise of reviewing the literature to examine how authors in your discipline or area of interest have composed their literature review sections. Read them to get a sense of the types of themes you might want to look for in your own research or to identify ways to organize your final review. The bibliography or reference section of sources you've already read, such as required readings in the course syllabus, are also excellent entry points into your own research. Narrow the Topic The narrower your topic, the easier it will be to limit the number of sources you need to read in order to obtain a good survey of relevant resources. Your professor will probably not expect you to read everything that's available about the topic, but you'll make the act of reviewing easier if you first limit scope of the research problem. A good strategy is to begin by searching the USC Libraries Catalog for recent books about the topic and review the table of contents for chapters that focuses on specific issues. You can also review the indexes of books to find references to specific issues that can serve as the focus of your research. For example, a book surveying the history of the Israeli-Palestinian conflict may include a chapter on the role Egypt has played in mediating the conflict, or look in the index for the pages where Egypt is mentioned in the text. Consider Whether Your Sources are Current Some disciplines require that you use information that is as current as possible. This is particularly true in disciplines in medicine and the sciences where research conducted becomes obsolete very quickly as new discoveries are made. However, when writing a review in the social sciences, a survey of the history of the literature may be required. In other words, a complete understanding the research problem requires you to deliberately examine how knowledge and perspectives have changed over time. Sort through other current bibliographies or literature reviews in the field to get a sense of what your discipline expects. You can also use this method to explore what is considered by scholars to be a "hot topic" and what is not.

III.  Ways to Organize Your Literature Review

Chronology of Events If your review follows the chronological method, you could write about the materials according to when they were published. This approach should only be followed if a clear path of research building on previous research can be identified and that these trends follow a clear chronological order of development. For example, a literature review that focuses on continuing research about the emergence of German economic power after the fall of the Soviet Union. By Publication Order your sources by publication chronology, then, only if the order demonstrates a more important trend. For instance, you could order a review of literature on environmental studies of brown fields if the progression revealed, for example, a change in the soil collection practices of the researchers who wrote and/or conducted the studies. Thematic [“conceptual categories”] A thematic literature review is the most common approach to summarizing prior research in the social and behavioral sciences. Thematic reviews are organized around a topic or issue, rather than the progression of time, although the progression of time may still be incorporated into a thematic review. For example, a review of the Internet’s impact on American presidential politics could focus on the development of online political satire. While the study focuses on one topic, the Internet’s impact on American presidential politics, it would still be organized chronologically reflecting technological developments in media. The difference in this example between a "chronological" and a "thematic" approach is what is emphasized the most: themes related to the role of the Internet in presidential politics. Note that more authentic thematic reviews tend to break away from chronological order. A review organized in this manner would shift between time periods within each section according to the point being made. Methodological A methodological approach focuses on the methods utilized by the researcher. For the Internet in American presidential politics project, one methodological approach would be to look at cultural differences between the portrayal of American presidents on American, British, and French websites. Or the review might focus on the fundraising impact of the Internet on a particular political party. A methodological scope will influence either the types of documents in the review or the way in which these documents are discussed.

Other Sections of Your Literature Review Once you've decided on the organizational method for your literature review, the sections you need to include in the paper should be easy to figure out because they arise from your organizational strategy. In other words, a chronological review would have subsections for each vital time period; a thematic review would have subtopics based upon factors that relate to the theme or issue. However, sometimes you may need to add additional sections that are necessary for your study, but do not fit in the organizational strategy of the body. What other sections you include in the body is up to you. However, only include what is necessary for the reader to locate your study within the larger scholarship about the research problem.

Here are examples of other sections, usually in the form of a single paragraph, you may need to include depending on the type of review you write:

• Current Situation : Information necessary to understand the current topic or focus of the literature review.
• Sources Used : Describes the methods and resources [e.g., databases] you used to identify the literature you reviewed.
• History : The chronological progression of the field, the research literature, or an idea that is necessary to understand the literature review, if the body of the literature review is not already a chronology.
• Selection Methods : Criteria you used to select (and perhaps exclude) sources in your literature review. For instance, you might explain that your review includes only peer-reviewed [i.e., scholarly] sources.
• Standards : Description of the way in which you present your information.
• Questions for Further Research : What questions about the field has the review sparked? How will you further your research as a result of the review?

IV.  Writing Your Literature Review

Once you've settled on how to organize your literature review, you're ready to write each section. When writing your review, keep in mind these issues.

Use Evidence A literature review section is, in this sense, just like any other academic research paper. Your interpretation of the available sources must be backed up with evidence [citations] that demonstrates that what you are saying is valid. Be Selective Select only the most important points in each source to highlight in the review. The type of information you choose to mention should relate directly to the research problem, whether it is thematic, methodological, or chronological. Related items that provide additional information, but that are not key to understanding the research problem, can be included in a list of further readings . Use Quotes Sparingly Some short quotes are appropriate if you want to emphasize a point, or if what an author stated cannot be easily paraphrased. Sometimes you may need to quote certain terminology that was coined by the author, is not common knowledge, or taken directly from the study. Do not use extensive quotes as a substitute for using your own words in reviewing the literature. Summarize and Synthesize Remember to summarize and synthesize your sources within each thematic paragraph as well as throughout the review. Recapitulate important features of a research study, but then synthesize it by rephrasing the study's significance and relating it to your own work and the work of others. Keep Your Own Voice While the literature review presents others' ideas, your voice [the writer's] should remain front and center. For example, weave references to other sources into what you are writing but maintain your own voice by starting and ending the paragraph with your own ideas and wording. Use Caution When Paraphrasing When paraphrasing a source that is not your own, be sure to represent the author's information or opinions accurately and in your own words. Even when paraphrasing an author’s work, you still must provide a citation to that work.

V.  Common Mistakes to Avoid

These are the most common mistakes made in reviewing social science research literature.

• Sources in your literature review do not clearly relate to the research problem;
• You do not take sufficient time to define and identify the most relevant sources to use in the literature review related to the research problem;
• Relies exclusively on secondary analytical sources rather than including relevant primary research studies or data;
• Uncritically accepts another researcher's findings and interpretations as valid, rather than examining critically all aspects of the research design and analysis;
• Does not describe the search procedures that were used in identifying the literature to review;
• Reports isolated statistical results rather than synthesizing them in chi-squared or meta-analytic methods; and,
• Only includes research that validates assumptions and does not consider contrary findings and alternative interpretations found in the literature.

Cook, Kathleen E. and Elise Murowchick. “Do Literature Review Skills Transfer from One Course to Another?” Psychology Learning and Teaching 13 (March 2014): 3-11; Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper . 2nd ed. Thousand Oaks, CA: Sage, 2005; Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1998; Jesson, Jill. Doing Your Literature Review: Traditional and Systematic Techniques . London: SAGE, 2011; Literature Review Handout. Online Writing Center. Liberty University; Literature Reviews. The Writing Center. University of North Carolina; Onwuegbuzie, Anthony J. and Rebecca Frels. Seven Steps to a Comprehensive Literature Review: A Multimodal and Cultural Approach . Los Angeles, CA: SAGE, 2016; Ridley, Diana. The Literature Review: A Step-by-Step Guide for Students . 2nd ed. Los Angeles, CA: SAGE, 2012; Randolph, Justus J. “A Guide to Writing the Dissertation Literature Review." Practical Assessment, Research, and Evaluation. vol. 14, June 2009; Sutton, Anthea. Systematic Approaches to a Successful Literature Review . Los Angeles, CA: Sage Publications, 2016; Taylor, Dena. The Literature Review: A Few Tips On Conducting It. University College Writing Centre. University of Toronto; Writing a Literature Review. Academic Skills Centre. University of Canberra.

Writing Tip

Break Out of Your Disciplinary Box!

Thinking interdisciplinarily about a research problem can be a rewarding exercise in applying new ideas, theories, or concepts to an old problem. For example, what might cultural anthropologists say about the continuing conflict in the Middle East? In what ways might geographers view the need for better distribution of social service agencies in large cities than how social workers might study the issue? You don’t want to substitute a thorough review of core research literature in your discipline for studies conducted in other fields of study. However, particularly in the social sciences, thinking about research problems from multiple vectors is a key strategy for finding new solutions to a problem or gaining a new perspective. Consult with a librarian about identifying research databases in other disciplines; almost every field of study has at least one comprehensive database devoted to indexing its research literature.

Frodeman, Robert. The Oxford Handbook of Interdisciplinarity . New York: Oxford University Press, 2010.

Another Writing Tip

Don't Just Review for Content!

While conducting a review of the literature, maximize the time you devote to writing this part of your paper by thinking broadly about what you should be looking for and evaluating. Review not just what scholars are saying, but how are they saying it. Some questions to ask:

• How are they organizing their ideas?
• What methods have they used to study the problem?
• What theories have been used to explain, predict, or understand their research problem?
• What sources have they cited to support their conclusions?
• How have they used non-textual elements [e.g., charts, graphs, figures, etc.] to illustrate key points?

When you begin to write your literature review section, you'll be glad you dug deeper into how the research was designed and constructed because it establishes a means for developing more substantial analysis and interpretation of the research problem.

Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1 998.

Yet Another Writing Tip

When Do I Know I Can Stop Looking and Move On?

Here are several strategies you can utilize to assess whether you've thoroughly reviewed the literature:

• Look for repeating patterns in the research findings . If the same thing is being said, just by different people, then this likely demonstrates that the research problem has hit a conceptual dead end. At this point consider: Does your study extend current research?  Does it forge a new path? Or, does is merely add more of the same thing being said?
• Look at sources the authors cite to in their work . If you begin to see the same researchers cited again and again, then this is often an indication that no new ideas have been generated to address the research problem.
• Search Google Scholar to identify who has subsequently cited leading scholars already identified in your literature review [see next sub-tab]. This is called citation tracking and there are a number of sources that can help you identify who has cited whom, particularly scholars from outside of your discipline. Here again, if the same authors are being cited again and again, this may indicate no new literature has been written on the topic.

Onwuegbuzie, Anthony J. and Rebecca Frels. Seven Steps to a Comprehensive Literature Review: A Multimodal and Cultural Approach . Los Angeles, CA: Sage, 2016; Sutton, Anthea. Systematic Approaches to a Successful Literature Review . Los Angeles, CA: Sage Publications, 2016.

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Organizing and Creating Information

• Citation and Attribution

What Is a Literature Review?

Review the literature, write the literature review, further reading, learning objectives, attribution.

This guide is designed to:

• Identify the sections and purpose of a literature review in academic writing
• Review practical strategies and organizational methods for preparing a literature review

A literature review is a summary and synthesis of scholarly research on a specific topic. It should answer questions such as:

• What research has been done on the topic?
• Who are the key researchers and experts in the field?
• What are the common theories and methodologies?
• Are there challenges, controversies, and contradictions?
• Are there gaps in the research that your approach addresses?

The process of reviewing existing research allows you to fine-tune your research question and contextualize your own work. Preparing a literature review is a cyclical process. You may find that the research question you begin with evolves as you learn more about the topic.

Once you have defined your research question , focus on learning what other scholars have written on the topic.

In order to  do a thorough search of the literature  on the topic, define the basic criteria:

• Databases and journals: Look at the  subject guide  related to your topic for recommended databases. Review the  tutorial on finding articles  for tips. 
• Books: Search BruKnow, the Library's catalog. Steps to searching ebooks are covered in the  Finding Ebooks tutorial .
• What time period should it cover? Is currency important?
• Do I know of primary and secondary sources that I can use as a way to find other information?
• What should I be aware of when looking at popular, trade, and scholarly resources ? 

One strategy is to review bibliographies for sources that relate to your interest. For more on this technique, look at the tutorial on finding articles when you have a citation .

Tip: Use a Synthesis Matrix

As you read sources, themes will emerge that will help you to organize the review. You can use a simple Synthesis Matrix to track your notes as you read. From this work, a concept map emerges that provides an overview of the literature and ways in which it connects. Working with Zotero to capture the citations, you build the structure for writing your literature review.

How do I know when I am done?

A key indicator for knowing when you are done is running into the same articles and materials. With no new information being uncovered, you are likely exhausting your current search and should modify search terms or search different catalogs or databases. It is also possible that you have reached a point when you can start writing the literature review.

Tip: Manage Your Citations

These citation management tools also create citations, footnotes, and bibliographies with just a few clicks:

Zotero Tutorial

Endnote Tutorial

Your literature review should be focused on the topic defined in your research question. It should be written in a logical, structured way and maintain an objective perspective and use a formal voice.

Review the Summary Table you created for themes and connecting ideas. Use the following guidelines to prepare an outline of the main points you want to make. 

• Synthesize previous research on the topic.
• Aim to include both summary and synthesis.
• Include literature that supports your research question as well as that which offers a different perspective.
• Avoid relying on one author or publication too heavily.
• Select an organizational structure, such as chronological, methodological, and thematic.

The three elements of a literature review are introduction, body, and conclusion.

Introduction

• Define the topic of the literature review, including any terminology.
• Introduce the central theme and organization of the literature review.
• Summarize the state of research on the topic.
• Frame the literature review with your research question.
• Focus on ways to have the body of literature tell its own story. Do not add your own interpretations at this point.
• Look for patterns and find ways to tie the pieces together.
• Summarize instead of quote.
• Weave the points together rather than list summaries of each source.
• Include the most important sources, not everything you have read.
• Summarize the review of the literature.
• Identify areas of further research on the topic.
• Connect the review with your research.
• DeCarlo, M. (2018). 4.1 What is a literature review? In Scientific Inquiry in Social Work. Open Social Work Education. https://scientificinquiryinsocialwork.pressbooks.com/chapter/4-1-what-is-a-literature-review/
• Literature Reviews (n.d.) https://writingcenter.unc.edu/tips-and-tools/literature-reviews/ Accessed Nov. 10, 2021

This guide was designed to: 

• Identify the sections and purpose of a literature review in academic writing 
• Review practical strategies and organizational methods for preparing a literature review​

Content on this page adapted from: 

Frederiksen, L. and Phelps, S. (2017).   Literature Reviews for Education and Nursing Graduate Students.  Licensed CC BY 4.0

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Lau F, Kuziemsky C, editors. Handbook of eHealth Evaluation: An Evidence-based Approach [Internet]. Victoria (BC): University of Victoria; 2017 Feb 27.

Handbook of eHealth Evaluation: An Evidence-based Approach [Internet].

Chapter 9 methods for literature reviews.

Guy Paré and Spyros Kitsiou .

9.1. Introduction

Literature reviews play a critical role in scholarship because science remains, first and foremost, a cumulative endeavour ( vom Brocke et al., 2009 ). As in any academic discipline, rigorous knowledge syntheses are becoming indispensable in keeping up with an exponentially growing eHealth literature, assisting practitioners, academics, and graduate students in finding, evaluating, and synthesizing the contents of many empirical and conceptual papers. Among other methods, literature reviews are essential for: (a) identifying what has been written on a subject or topic; (b) determining the extent to which a specific research area reveals any interpretable trends or patterns; (c) aggregating empirical findings related to a narrow research question to support evidence-based practice; (d) generating new frameworks and theories; and (e) identifying topics or questions requiring more investigation ( Paré, Trudel, Jaana, & Kitsiou, 2015 ).

Literature reviews can take two major forms. The most prevalent one is the “literature review” or “background” section within a journal paper or a chapter in a graduate thesis. This section synthesizes the extant literature and usually identifies the gaps in knowledge that the empirical study addresses ( Sylvester, Tate, & Johnstone, 2013 ). It may also provide a theoretical foundation for the proposed study, substantiate the presence of the research problem, justify the research as one that contributes something new to the cumulated knowledge, or validate the methods and approaches for the proposed study ( Hart, 1998 ; Levy & Ellis, 2006 ).

The second form of literature review, which is the focus of this chapter, constitutes an original and valuable work of research in and of itself ( Paré et al., 2015 ). Rather than providing a base for a researcher’s own work, it creates a solid starting point for all members of the community interested in a particular area or topic ( Mulrow, 1987 ). The so-called “review article” is a journal-length paper which has an overarching purpose to synthesize the literature in a field, without collecting or analyzing any primary data ( Green, Johnson, & Adams, 2006 ).

When appropriately conducted, review articles represent powerful information sources for practitioners looking for state-of-the art evidence to guide their decision-making and work practices ( Paré et al., 2015 ). Further, high-quality reviews become frequently cited pieces of work which researchers seek out as a first clear outline of the literature when undertaking empirical studies ( Cooper, 1988 ; Rowe, 2014 ). Scholars who track and gauge the impact of articles have found that review papers are cited and downloaded more often than any other type of published article ( Cronin, Ryan, & Coughlan, 2008 ; Montori, Wilczynski, Morgan, Haynes, & Hedges, 2003 ; Patsopoulos, Analatos, & Ioannidis, 2005 ). The reason for their popularity may be the fact that reading the review enables one to have an overview, if not a detailed knowledge of the area in question, as well as references to the most useful primary sources ( Cronin et al., 2008 ). Although they are not easy to conduct, the commitment to complete a review article provides a tremendous service to one’s academic community ( Paré et al., 2015 ; Petticrew & Roberts, 2006 ). Most, if not all, peer-reviewed journals in the fields of medical informatics publish review articles of some type.

The main objectives of this chapter are fourfold: (a) to provide an overview of the major steps and activities involved in conducting a stand-alone literature review; (b) to describe and contrast the different types of review articles that can contribute to the eHealth knowledge base; (c) to illustrate each review type with one or two examples from the eHealth literature; and (d) to provide a series of recommendations for prospective authors of review articles in this domain.

9.2. Overview of the Literature Review Process and Steps

As explained in Templier and Paré (2015) , there are six generic steps involved in conducting a review article:

• formulating the research question(s) and objective(s),
• searching the extant literature,
• screening for inclusion,
• assessing the quality of primary studies,
• extracting data, and
• analyzing data.

Although these steps are presented here in sequential order, one must keep in mind that the review process can be iterative and that many activities can be initiated during the planning stage and later refined during subsequent phases ( Finfgeld-Connett & Johnson, 2013 ; Kitchenham & Charters, 2007 ).

Formulating the research question(s) and objective(s): As a first step, members of the review team must appropriately justify the need for the review itself ( Petticrew & Roberts, 2006 ), identify the review’s main objective(s) ( Okoli & Schabram, 2010 ), and define the concepts or variables at the heart of their synthesis ( Cooper & Hedges, 2009 ; Webster & Watson, 2002 ). Importantly, they also need to articulate the research question(s) they propose to investigate ( Kitchenham & Charters, 2007 ). In this regard, we concur with Jesson, Matheson, and Lacey (2011) that clearly articulated research questions are key ingredients that guide the entire review methodology; they underscore the type of information that is needed, inform the search for and selection of relevant literature, and guide or orient the subsequent analysis. Searching the extant literature: The next step consists of searching the literature and making decisions about the suitability of material to be considered in the review ( Cooper, 1988 ). There exist three main coverage strategies. First, exhaustive coverage means an effort is made to be as comprehensive as possible in order to ensure that all relevant studies, published and unpublished, are included in the review and, thus, conclusions are based on this all-inclusive knowledge base. The second type of coverage consists of presenting materials that are representative of most other works in a given field or area. Often authors who adopt this strategy will search for relevant articles in a small number of top-tier journals in a field ( Paré et al., 2015 ). In the third strategy, the review team concentrates on prior works that have been central or pivotal to a particular topic. This may include empirical studies or conceptual papers that initiated a line of investigation, changed how problems or questions were framed, introduced new methods or concepts, or engendered important debate ( Cooper, 1988 ). Screening for inclusion: The following step consists of evaluating the applicability of the material identified in the preceding step ( Levy & Ellis, 2006 ; vom Brocke et al., 2009 ). Once a group of potential studies has been identified, members of the review team must screen them to determine their relevance ( Petticrew & Roberts, 2006 ). A set of predetermined rules provides a basis for including or excluding certain studies. This exercise requires a significant investment on the part of researchers, who must ensure enhanced objectivity and avoid biases or mistakes. As discussed later in this chapter, for certain types of reviews there must be at least two independent reviewers involved in the screening process and a procedure to resolve disagreements must also be in place ( Liberati et al., 2009 ; Shea et al., 2009 ). Assessing the quality of primary studies: In addition to screening material for inclusion, members of the review team may need to assess the scientific quality of the selected studies, that is, appraise the rigour of the research design and methods. Such formal assessment, which is usually conducted independently by at least two coders, helps members of the review team refine which studies to include in the final sample, determine whether or not the differences in quality may affect their conclusions, or guide how they analyze the data and interpret the findings ( Petticrew & Roberts, 2006 ). Ascribing quality scores to each primary study or considering through domain-based evaluations which study components have or have not been designed and executed appropriately makes it possible to reflect on the extent to which the selected study addresses possible biases and maximizes validity ( Shea et al., 2009 ). Extracting data: The following step involves gathering or extracting applicable information from each primary study included in the sample and deciding what is relevant to the problem of interest ( Cooper & Hedges, 2009 ). Indeed, the type of data that should be recorded mainly depends on the initial research questions ( Okoli & Schabram, 2010 ). However, important information may also be gathered about how, when, where and by whom the primary study was conducted, the research design and methods, or qualitative/quantitative results ( Cooper & Hedges, 2009 ). Analyzing and synthesizing data : As a final step, members of the review team must collate, summarize, aggregate, organize, and compare the evidence extracted from the included studies. The extracted data must be presented in a meaningful way that suggests a new contribution to the extant literature ( Jesson et al., 2011 ). Webster and Watson (2002) warn researchers that literature reviews should be much more than lists of papers and should provide a coherent lens to make sense of extant knowledge on a given topic. There exist several methods and techniques for synthesizing quantitative (e.g., frequency analysis, meta-analysis) and qualitative (e.g., grounded theory, narrative analysis, meta-ethnography) evidence ( Dixon-Woods, Agarwal, Jones, Young, & Sutton, 2005 ; Thomas & Harden, 2008 ).

9.3. Types of Review Articles and Brief Illustrations

EHealth researchers have at their disposal a number of approaches and methods for making sense out of existing literature, all with the purpose of casting current research findings into historical contexts or explaining contradictions that might exist among a set of primary research studies conducted on a particular topic. Our classification scheme is largely inspired from Paré and colleagues’ (2015) typology. Below we present and illustrate those review types that we feel are central to the growth and development of the eHealth domain.

9.3.1. Narrative Reviews

The narrative review is the “traditional” way of reviewing the extant literature and is skewed towards a qualitative interpretation of prior knowledge ( Sylvester et al., 2013 ). Put simply, a narrative review attempts to summarize or synthesize what has been written on a particular topic but does not seek generalization or cumulative knowledge from what is reviewed ( Davies, 2000 ; Green et al., 2006 ). Instead, the review team often undertakes the task of accumulating and synthesizing the literature to demonstrate the value of a particular point of view ( Baumeister & Leary, 1997 ). As such, reviewers may selectively ignore or limit the attention paid to certain studies in order to make a point. In this rather unsystematic approach, the selection of information from primary articles is subjective, lacks explicit criteria for inclusion and can lead to biased interpretations or inferences ( Green et al., 2006 ). There are several narrative reviews in the particular eHealth domain, as in all fields, which follow such an unstructured approach ( Silva et al., 2015 ; Paul et al., 2015 ).

Despite these criticisms, this type of review can be very useful in gathering together a volume of literature in a specific subject area and synthesizing it. As mentioned above, its primary purpose is to provide the reader with a comprehensive background for understanding current knowledge and highlighting the significance of new research ( Cronin et al., 2008 ). Faculty like to use narrative reviews in the classroom because they are often more up to date than textbooks, provide a single source for students to reference, and expose students to peer-reviewed literature ( Green et al., 2006 ). For researchers, narrative reviews can inspire research ideas by identifying gaps or inconsistencies in a body of knowledge, thus helping researchers to determine research questions or formulate hypotheses. Importantly, narrative reviews can also be used as educational articles to bring practitioners up to date with certain topics of issues ( Green et al., 2006 ).

Recently, there have been several efforts to introduce more rigour in narrative reviews that will elucidate common pitfalls and bring changes into their publication standards. Information systems researchers, among others, have contributed to advancing knowledge on how to structure a “traditional” review. For instance, Levy and Ellis (2006) proposed a generic framework for conducting such reviews. Their model follows the systematic data processing approach comprised of three steps, namely: (a) literature search and screening; (b) data extraction and analysis; and (c) writing the literature review. They provide detailed and very helpful instructions on how to conduct each step of the review process. As another methodological contribution, vom Brocke et al. (2009) offered a series of guidelines for conducting literature reviews, with a particular focus on how to search and extract the relevant body of knowledge. Last, Bandara, Miskon, and Fielt (2011) proposed a structured, predefined and tool-supported method to identify primary studies within a feasible scope, extract relevant content from identified articles, synthesize and analyze the findings, and effectively write and present the results of the literature review. We highly recommend that prospective authors of narrative reviews consult these useful sources before embarking on their work.

Darlow and Wen (2015) provide a good example of a highly structured narrative review in the eHealth field. These authors synthesized published articles that describe the development process of mobile health ( m-health ) interventions for patients’ cancer care self-management. As in most narrative reviews, the scope of the research questions being investigated is broad: (a) how development of these systems are carried out; (b) which methods are used to investigate these systems; and (c) what conclusions can be drawn as a result of the development of these systems. To provide clear answers to these questions, a literature search was conducted on six electronic databases and Google Scholar . The search was performed using several terms and free text words, combining them in an appropriate manner. Four inclusion and three exclusion criteria were utilized during the screening process. Both authors independently reviewed each of the identified articles to determine eligibility and extract study information. A flow diagram shows the number of studies identified, screened, and included or excluded at each stage of study selection. In terms of contributions, this review provides a series of practical recommendations for m-health intervention development.

9.3.2. Descriptive or Mapping Reviews

The primary goal of a descriptive review is to determine the extent to which a body of knowledge in a particular research topic reveals any interpretable pattern or trend with respect to pre-existing propositions, theories, methodologies or findings ( King & He, 2005 ; Paré et al., 2015 ). In contrast with narrative reviews, descriptive reviews follow a systematic and transparent procedure, including searching, screening and classifying studies ( Petersen, Vakkalanka, & Kuzniarz, 2015 ). Indeed, structured search methods are used to form a representative sample of a larger group of published works ( Paré et al., 2015 ). Further, authors of descriptive reviews extract from each study certain characteristics of interest, such as publication year, research methods, data collection techniques, and direction or strength of research outcomes (e.g., positive, negative, or non-significant) in the form of frequency analysis to produce quantitative results ( Sylvester et al., 2013 ). In essence, each study included in a descriptive review is treated as the unit of analysis and the published literature as a whole provides a database from which the authors attempt to identify any interpretable trends or draw overall conclusions about the merits of existing conceptualizations, propositions, methods or findings ( Paré et al., 2015 ). In doing so, a descriptive review may claim that its findings represent the state of the art in a particular domain ( King & He, 2005 ).

In the fields of health sciences and medical informatics, reviews that focus on examining the range, nature and evolution of a topic area are described by Anderson, Allen, Peckham, and Goodwin (2008) as mapping reviews . Like descriptive reviews, the research questions are generic and usually relate to publication patterns and trends. There is no preconceived plan to systematically review all of the literature although this can be done. Instead, researchers often present studies that are representative of most works published in a particular area and they consider a specific time frame to be mapped.

An example of this approach in the eHealth domain is offered by DeShazo, Lavallie, and Wolf (2009). The purpose of this descriptive or mapping review was to characterize publication trends in the medical informatics literature over a 20-year period (1987 to 2006). To achieve this ambitious objective, the authors performed a bibliometric analysis of medical informatics citations indexed in medline using publication trends, journal frequencies, impact factors, Medical Subject Headings (MeSH) term frequencies, and characteristics of citations. Findings revealed that there were over 77,000 medical informatics articles published during the covered period in numerous journals and that the average annual growth rate was 12%. The MeSH term analysis also suggested a strong interdisciplinary trend. Finally, average impact scores increased over time with two notable growth periods. Overall, patterns in research outputs that seem to characterize the historic trends and current components of the field of medical informatics suggest it may be a maturing discipline (DeShazo et al., 2009).

9.3.3. Scoping Reviews

Scoping reviews attempt to provide an initial indication of the potential size and nature of the extant literature on an emergent topic (Arksey & O’Malley, 2005; Daudt, van Mossel, & Scott, 2013 ; Levac, Colquhoun, & O’Brien, 2010). A scoping review may be conducted to examine the extent, range and nature of research activities in a particular area, determine the value of undertaking a full systematic review (discussed next), or identify research gaps in the extant literature ( Paré et al., 2015 ). In line with their main objective, scoping reviews usually conclude with the presentation of a detailed research agenda for future works along with potential implications for both practice and research.

Unlike narrative and descriptive reviews, the whole point of scoping the field is to be as comprehensive as possible, including grey literature (Arksey & O’Malley, 2005). Inclusion and exclusion criteria must be established to help researchers eliminate studies that are not aligned with the research questions. It is also recommended that at least two independent coders review abstracts yielded from the search strategy and then the full articles for study selection ( Daudt et al., 2013 ). The synthesized evidence from content or thematic analysis is relatively easy to present in tabular form (Arksey & O’Malley, 2005; Thomas & Harden, 2008 ).

One of the most highly cited scoping reviews in the eHealth domain was published by Archer, Fevrier-Thomas, Lokker, McKibbon, and Straus (2011) . These authors reviewed the existing literature on personal health record ( phr ) systems including design, functionality, implementation, applications, outcomes, and benefits. Seven databases were searched from 1985 to March 2010. Several search terms relating to phr s were used during this process. Two authors independently screened titles and abstracts to determine inclusion status. A second screen of full-text articles, again by two independent members of the research team, ensured that the studies described phr s. All in all, 130 articles met the criteria and their data were extracted manually into a database. The authors concluded that although there is a large amount of survey, observational, cohort/panel, and anecdotal evidence of phr benefits and satisfaction for patients, more research is needed to evaluate the results of phr implementations. Their in-depth analysis of the literature signalled that there is little solid evidence from randomized controlled trials or other studies through the use of phr s. Hence, they suggested that more research is needed that addresses the current lack of understanding of optimal functionality and usability of these systems, and how they can play a beneficial role in supporting patient self-management ( Archer et al., 2011 ).

9.3.4. Forms of Aggregative Reviews

Healthcare providers, practitioners, and policy-makers are nowadays overwhelmed with large volumes of information, including research-based evidence from numerous clinical trials and evaluation studies, assessing the effectiveness of health information technologies and interventions ( Ammenwerth & de Keizer, 2004 ; Deshazo et al., 2009 ). It is unrealistic to expect that all these disparate actors will have the time, skills, and necessary resources to identify the available evidence in the area of their expertise and consider it when making decisions. Systematic reviews that involve the rigorous application of scientific strategies aimed at limiting subjectivity and bias (i.e., systematic and random errors) can respond to this challenge.

Systematic reviews attempt to aggregate, appraise, and synthesize in a single source all empirical evidence that meet a set of previously specified eligibility criteria in order to answer a clearly formulated and often narrow research question on a particular topic of interest to support evidence-based practice ( Liberati et al., 2009 ). They adhere closely to explicit scientific principles ( Liberati et al., 2009 ) and rigorous methodological guidelines (Higgins & Green, 2008) aimed at reducing random and systematic errors that can lead to deviations from the truth in results or inferences. The use of explicit methods allows systematic reviews to aggregate a large body of research evidence, assess whether effects or relationships are in the same direction and of the same general magnitude, explain possible inconsistencies between study results, and determine the strength of the overall evidence for every outcome of interest based on the quality of included studies and the general consistency among them ( Cook, Mulrow, & Haynes, 1997 ). The main procedures of a systematic review involve:

• Formulating a review question and developing a search strategy based on explicit inclusion criteria for the identification of eligible studies (usually described in the context of a detailed review protocol).
• Searching for eligible studies using multiple databases and information sources, including grey literature sources, without any language restrictions.
• Selecting studies, extracting data, and assessing risk of bias in a duplicate manner using two independent reviewers to avoid random or systematic errors in the process.
• Analyzing data using quantitative or qualitative methods.
• Presenting results in summary of findings tables.
• Interpreting results and drawing conclusions.

Many systematic reviews, but not all, use statistical methods to combine the results of independent studies into a single quantitative estimate or summary effect size. Known as meta-analyses , these reviews use specific data extraction and statistical techniques (e.g., network, frequentist, or Bayesian meta-analyses) to calculate from each study by outcome of interest an effect size along with a confidence interval that reflects the degree of uncertainty behind the point estimate of effect ( Borenstein, Hedges, Higgins, & Rothstein, 2009 ; Deeks, Higgins, & Altman, 2008 ). Subsequently, they use fixed or random-effects analysis models to combine the results of the included studies, assess statistical heterogeneity, and calculate a weighted average of the effect estimates from the different studies, taking into account their sample sizes. The summary effect size is a value that reflects the average magnitude of the intervention effect for a particular outcome of interest or, more generally, the strength of a relationship between two variables across all studies included in the systematic review. By statistically combining data from multiple studies, meta-analyses can create more precise and reliable estimates of intervention effects than those derived from individual studies alone, when these are examined independently as discrete sources of information.

The review by Gurol-Urganci, de Jongh, Vodopivec-Jamsek, Atun, and Car (2013) on the effects of mobile phone messaging reminders for attendance at healthcare appointments is an illustrative example of a high-quality systematic review with meta-analysis. Missed appointments are a major cause of inefficiency in healthcare delivery with substantial monetary costs to health systems. These authors sought to assess whether mobile phone-based appointment reminders delivered through Short Message Service ( sms ) or Multimedia Messaging Service ( mms ) are effective in improving rates of patient attendance and reducing overall costs. To this end, they conducted a comprehensive search on multiple databases using highly sensitive search strategies without language or publication-type restrictions to identify all rct s that are eligible for inclusion. In order to minimize the risk of omitting eligible studies not captured by the original search, they supplemented all electronic searches with manual screening of trial registers and references contained in the included studies. Study selection, data extraction, and risk of bias assessments were performed inde­­pen­dently by two coders using standardized methods to ensure consistency and to eliminate potential errors. Findings from eight rct s involving 6,615 participants were pooled into meta-analyses to calculate the magnitude of effects that mobile text message reminders have on the rate of attendance at healthcare appointments compared to no reminders and phone call reminders.

Meta-analyses are regarded as powerful tools for deriving meaningful conclusions. However, there are situations in which it is neither reasonable nor appropriate to pool studies together using meta-analytic methods simply because there is extensive clinical heterogeneity between the included studies or variation in measurement tools, comparisons, or outcomes of interest. In these cases, systematic reviews can use qualitative synthesis methods such as vote counting, content analysis, classification schemes and tabulations, as an alternative approach to narratively synthesize the results of the independent studies included in the review. This form of review is known as qualitative systematic review.

A rigorous example of one such review in the eHealth domain is presented by Mickan, Atherton, Roberts, Heneghan, and Tilson (2014) on the use of handheld computers by healthcare professionals and their impact on access to information and clinical decision-making. In line with the methodological guide­lines for systematic reviews, these authors: (a) developed and registered with prospero ( www.crd.york.ac.uk/ prospero / ) an a priori review protocol; (b) conducted comprehensive searches for eligible studies using multiple databases and other supplementary strategies (e.g., forward searches); and (c) subsequently carried out study selection, data extraction, and risk of bias assessments in a duplicate manner to eliminate potential errors in the review process. Heterogeneity between the included studies in terms of reported outcomes and measures precluded the use of meta-analytic methods. To this end, the authors resorted to using narrative analysis and synthesis to describe the effectiveness of handheld computers on accessing information for clinical knowledge, adherence to safety and clinical quality guidelines, and diagnostic decision-making.

In recent years, the number of systematic reviews in the field of health informatics has increased considerably. Systematic reviews with discordant findings can cause great confusion and make it difficult for decision-makers to interpret the review-level evidence ( Moher, 2013 ). Therefore, there is a growing need for appraisal and synthesis of prior systematic reviews to ensure that decision-making is constantly informed by the best available accumulated evidence. Umbrella reviews , also known as overviews of systematic reviews, are tertiary types of evidence synthesis that aim to accomplish this; that is, they aim to compare and contrast findings from multiple systematic reviews and meta-analyses ( Becker & Oxman, 2008 ). Umbrella reviews generally adhere to the same principles and rigorous methodological guidelines used in systematic reviews. However, the unit of analysis in umbrella reviews is the systematic review rather than the primary study ( Becker & Oxman, 2008 ). Unlike systematic reviews that have a narrow focus of inquiry, umbrella reviews focus on broader research topics for which there are several potential interventions ( Smith, Devane, Begley, & Clarke, 2011 ). A recent umbrella review on the effects of home telemonitoring interventions for patients with heart failure critically appraised, compared, and synthesized evidence from 15 systematic reviews to investigate which types of home telemonitoring technologies and forms of interventions are more effective in reducing mortality and hospital admissions ( Kitsiou, Paré, & Jaana, 2015 ).

9.3.5. Realist Reviews

Realist reviews are theory-driven interpretative reviews developed to inform, enhance, or supplement conventional systematic reviews by making sense of heterogeneous evidence about complex interventions applied in diverse contexts in a way that informs policy decision-making ( Greenhalgh, Wong, Westhorp, & Pawson, 2011 ). They originated from criticisms of positivist systematic reviews which centre on their “simplistic” underlying assumptions ( Oates, 2011 ). As explained above, systematic reviews seek to identify causation. Such logic is appropriate for fields like medicine and education where findings of randomized controlled trials can be aggregated to see whether a new treatment or intervention does improve outcomes. However, many argue that it is not possible to establish such direct causal links between interventions and outcomes in fields such as social policy, management, and information systems where for any intervention there is unlikely to be a regular or consistent outcome ( Oates, 2011 ; Pawson, 2006 ; Rousseau, Manning, & Denyer, 2008 ).

To circumvent these limitations, Pawson, Greenhalgh, Harvey, and Walshe (2005) have proposed a new approach for synthesizing knowledge that seeks to unpack the mechanism of how “complex interventions” work in particular contexts. The basic research question — what works? — which is usually associated with systematic reviews changes to: what is it about this intervention that works, for whom, in what circumstances, in what respects and why? Realist reviews have no particular preference for either quantitative or qualitative evidence. As a theory-building approach, a realist review usually starts by articulating likely underlying mechanisms and then scrutinizes available evidence to find out whether and where these mechanisms are applicable ( Shepperd et al., 2009 ). Primary studies found in the extant literature are viewed as case studies which can test and modify the initial theories ( Rousseau et al., 2008 ).

The main objective pursued in the realist review conducted by Otte-Trojel, de Bont, Rundall, and van de Klundert (2014) was to examine how patient portals contribute to health service delivery and patient outcomes. The specific goals were to investigate how outcomes are produced and, most importantly, how variations in outcomes can be explained. The research team started with an exploratory review of background documents and research studies to identify ways in which patient portals may contribute to health service delivery and patient outcomes. The authors identified six main ways which represent “educated guesses” to be tested against the data in the evaluation studies. These studies were identified through a formal and systematic search in four databases between 2003 and 2013. Two members of the research team selected the articles using a pre-established list of inclusion and exclusion criteria and following a two-step procedure. The authors then extracted data from the selected articles and created several tables, one for each outcome category. They organized information to bring forward those mechanisms where patient portals contribute to outcomes and the variation in outcomes across different contexts.

9.3.6. Critical Reviews

Lastly, critical reviews aim to provide a critical evaluation and interpretive analysis of existing literature on a particular topic of interest to reveal strengths, weaknesses, contradictions, controversies, inconsistencies, and/or other important issues with respect to theories, hypotheses, research methods or results ( Baumeister & Leary, 1997 ; Kirkevold, 1997 ). Unlike other review types, critical reviews attempt to take a reflective account of the research that has been done in a particular area of interest, and assess its credibility by using appraisal instruments or critical interpretive methods. In this way, critical reviews attempt to constructively inform other scholars about the weaknesses of prior research and strengthen knowledge development by giving focus and direction to studies for further improvement ( Kirkevold, 1997 ).

Kitsiou, Paré, and Jaana (2013) provide an example of a critical review that assessed the methodological quality of prior systematic reviews of home telemonitoring studies for chronic patients. The authors conducted a comprehensive search on multiple databases to identify eligible reviews and subsequently used a validated instrument to conduct an in-depth quality appraisal. Results indicate that the majority of systematic reviews in this particular area suffer from important methodological flaws and biases that impair their internal validity and limit their usefulness for clinical and decision-making purposes. To this end, they provide a number of recommendations to strengthen knowledge development towards improving the design and execution of future reviews on home telemonitoring.

9.4. Summary

Table 9.1 outlines the main types of literature reviews that were described in the previous sub-sections and summarizes the main characteristics that distinguish one review type from another. It also includes key references to methodological guidelines and useful sources that can be used by eHealth scholars and researchers for planning and developing reviews.

Typology of Literature Reviews (adapted from Paré et al., 2015).

As shown in Table 9.1 , each review type addresses different kinds of research questions or objectives, which subsequently define and dictate the methods and approaches that need to be used to achieve the overarching goal(s) of the review. For example, in the case of narrative reviews, there is greater flexibility in searching and synthesizing articles ( Green et al., 2006 ). Researchers are often relatively free to use a diversity of approaches to search, identify, and select relevant scientific articles, describe their operational characteristics, present how the individual studies fit together, and formulate conclusions. On the other hand, systematic reviews are characterized by their high level of systematicity, rigour, and use of explicit methods, based on an “a priori” review plan that aims to minimize bias in the analysis and synthesis process (Higgins & Green, 2008). Some reviews are exploratory in nature (e.g., scoping/mapping reviews), whereas others may be conducted to discover patterns (e.g., descriptive reviews) or involve a synthesis approach that may include the critical analysis of prior research ( Paré et al., 2015 ). Hence, in order to select the most appropriate type of review, it is critical to know before embarking on a review project, why the research synthesis is conducted and what type of methods are best aligned with the pursued goals.

9.5. Concluding Remarks

In light of the increased use of evidence-based practice and research generating stronger evidence ( Grady et al., 2011 ; Lyden et al., 2013 ), review articles have become essential tools for summarizing, synthesizing, integrating or critically appraising prior knowledge in the eHealth field. As mentioned earlier, when rigorously conducted review articles represent powerful information sources for eHealth scholars and practitioners looking for state-of-the-art evidence. The typology of literature reviews we used herein will allow eHealth researchers, graduate students and practitioners to gain a better understanding of the similarities and differences between review types.

We must stress that this classification scheme does not privilege any specific type of review as being of higher quality than another ( Paré et al., 2015 ). As explained above, each type of review has its own strengths and limitations. Having said that, we realize that the methodological rigour of any review — be it qualitative, quantitative or mixed — is a critical aspect that should be considered seriously by prospective authors. In the present context, the notion of rigour refers to the reliability and validity of the review process described in section 9.2. For one thing, reliability is related to the reproducibility of the review process and steps, which is facilitated by a comprehensive documentation of the literature search process, extraction, coding and analysis performed in the review. Whether the search is comprehensive or not, whether it involves a methodical approach for data extraction and synthesis or not, it is important that the review documents in an explicit and transparent manner the steps and approach that were used in the process of its development. Next, validity characterizes the degree to which the review process was conducted appropriately. It goes beyond documentation and reflects decisions related to the selection of the sources, the search terms used, the period of time covered, the articles selected in the search, and the application of backward and forward searches ( vom Brocke et al., 2009 ). In short, the rigour of any review article is reflected by the explicitness of its methods (i.e., transparency) and the soundness of the approach used. We refer those interested in the concepts of rigour and quality to the work of Templier and Paré (2015) which offers a detailed set of methodological guidelines for conducting and evaluating various types of review articles.

To conclude, our main objective in this chapter was to demystify the various types of literature reviews that are central to the continuous development of the eHealth field. It is our hope that our descriptive account will serve as a valuable source for those conducting, evaluating or using reviews in this important and growing domain.

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• Cite this Page Paré G, Kitsiou S. Chapter 9 Methods for Literature Reviews. In: Lau F, Kuziemsky C, editors. Handbook of eHealth Evaluation: An Evidence-based Approach [Internet]. Victoria (BC): University of Victoria; 2017 Feb 27.
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• Introduction
• Overview of the Literature Review Process and Steps
• Types of Review Articles and Brief Illustrations
• Concluding Remarks

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Literature Reviews

What this handout is about.

This handout will explain what literature reviews are and offer insights into the form and construction of literature reviews in the humanities, social sciences, and sciences.

Introduction

OK. You’ve got to write a literature review. You dust off a novel and a book of poetry, settle down in your chair, and get ready to issue a “thumbs up” or “thumbs down” as you leaf through the pages. “Literature review” done. Right?

Wrong! The “literature” of a literature review refers to any collection of materials on a topic, not necessarily the great literary texts of the world. “Literature” could be anything from a set of government pamphlets on British colonial methods in Africa to scholarly articles on the treatment of a torn ACL. And a review does not necessarily mean that your reader wants you to give your personal opinion on whether or not you liked these sources.

What is a literature review, then?

A literature review discusses published information in a particular subject area, and sometimes information in a particular subject area within a certain time period.

A literature review can be just a simple summary of the sources, but it usually has an organizational pattern and combines both summary and synthesis. A summary is a recap of the important information of the source, but a synthesis is a re-organization, or a reshuffling, of that information. It might give a new interpretation of old material or combine new with old interpretations. Or it might trace the intellectual progression of the field, including major debates. And depending on the situation, the literature review may evaluate the sources and advise the reader on the most pertinent or relevant.

But how is a literature review different from an academic research paper?

The main focus of an academic research paper is to develop a new argument, and a research paper is likely to contain a literature review as one of its parts. In a research paper, you use the literature as a foundation and as support for a new insight that you contribute. The focus of a literature review, however, is to summarize and synthesize the arguments and ideas of others without adding new contributions.

Why do we write literature reviews?

Literature reviews provide you with a handy guide to a particular topic. If you have limited time to conduct research, literature reviews can give you an overview or act as a stepping stone. For professionals, they are useful reports that keep them up to date with what is current in the field. For scholars, the depth and breadth of the literature review emphasizes the credibility of the writer in his or her field. Literature reviews also provide a solid background for a research paper’s investigation. Comprehensive knowledge of the literature of the field is essential to most research papers.

Who writes these things, anyway?

Literature reviews are written occasionally in the humanities, but mostly in the sciences and social sciences; in experiment and lab reports, they constitute a section of the paper. Sometimes a literature review is written as a paper in itself.

Let’s get to it! What should I do before writing the literature review?

If your assignment is not very specific, seek clarification from your instructor:

• Roughly how many sources should you include?
• What types of sources (books, journal articles, websites)?
• Should you summarize, synthesize, or critique your sources by discussing a common theme or issue?
• Should you evaluate your sources?
• Should you provide subheadings and other background information, such as definitions and/or a history?

Find models

Look for other literature reviews in your area of interest or in the discipline and read them to get a sense of the types of themes you might want to look for in your own research or ways to organize your final review. You can simply put the word “review” in your search engine along with your other topic terms to find articles of this type on the Internet or in an electronic database. The bibliography or reference section of sources you’ve already read are also excellent entry points into your own research.

Narrow your topic

There are hundreds or even thousands of articles and books on most areas of study. The narrower your topic, the easier it will be to limit the number of sources you need to read in order to get a good survey of the material. Your instructor will probably not expect you to read everything that’s out there on the topic, but you’ll make your job easier if you first limit your scope.

Keep in mind that UNC Libraries have research guides and to databases relevant to many fields of study. You can reach out to the subject librarian for a consultation: https://library.unc.edu/support/consultations/ .

And don’t forget to tap into your professor’s (or other professors’) knowledge in the field. Ask your professor questions such as: “If you had to read only one book from the 90’s on topic X, what would it be?” Questions such as this help you to find and determine quickly the most seminal pieces in the field.

Consider whether your sources are current

Some disciplines require that you use information that is as current as possible. In the sciences, for instance, treatments for medical problems are constantly changing according to the latest studies. Information even two years old could be obsolete. However, if you are writing a review in the humanities, history, or social sciences, a survey of the history of the literature may be what is needed, because what is important is how perspectives have changed through the years or within a certain time period. Try sorting through some other current bibliographies or literature reviews in the field to get a sense of what your discipline expects. You can also use this method to consider what is currently of interest to scholars in this field and what is not.

Strategies for writing the literature review

Find a focus.

A literature review, like a term paper, is usually organized around ideas, not the sources themselves as an annotated bibliography would be organized. This means that you will not just simply list your sources and go into detail about each one of them, one at a time. No. As you read widely but selectively in your topic area, consider instead what themes or issues connect your sources together. Do they present one or different solutions? Is there an aspect of the field that is missing? How well do they present the material and do they portray it according to an appropriate theory? Do they reveal a trend in the field? A raging debate? Pick one of these themes to focus the organization of your review.

Convey it to your reader

A literature review may not have a traditional thesis statement (one that makes an argument), but you do need to tell readers what to expect. Try writing a simple statement that lets the reader know what is your main organizing principle. Here are a couple of examples:

The current trend in treatment for congestive heart failure combines surgery and medicine. More and more cultural studies scholars are accepting popular media as a subject worthy of academic consideration.

Consider organization

You’ve got a focus, and you’ve stated it clearly and directly. Now what is the most effective way of presenting the information? What are the most important topics, subtopics, etc., that your review needs to include? And in what order should you present them? Develop an organization for your review at both a global and local level:

First, cover the basic categories

Just like most academic papers, literature reviews also must contain at least three basic elements: an introduction or background information section; the body of the review containing the discussion of sources; and, finally, a conclusion and/or recommendations section to end the paper. The following provides a brief description of the content of each:

• Introduction: Gives a quick idea of the topic of the literature review, such as the central theme or organizational pattern.
• Body: Contains your discussion of sources and is organized either chronologically, thematically, or methodologically (see below for more information on each).
• Conclusions/Recommendations: Discuss what you have drawn from reviewing literature so far. Where might the discussion proceed?

Organizing the body

Once you have the basic categories in place, then you must consider how you will present the sources themselves within the body of your paper. Create an organizational method to focus this section even further.

To help you come up with an overall organizational framework for your review, consider the following scenario:

You’ve decided to focus your literature review on materials dealing with sperm whales. This is because you’ve just finished reading Moby Dick, and you wonder if that whale’s portrayal is really real. You start with some articles about the physiology of sperm whales in biology journals written in the 1980’s. But these articles refer to some British biological studies performed on whales in the early 18th century. So you check those out. Then you look up a book written in 1968 with information on how sperm whales have been portrayed in other forms of art, such as in Alaskan poetry, in French painting, or on whale bone, as the whale hunters in the late 19th century used to do. This makes you wonder about American whaling methods during the time portrayed in Moby Dick, so you find some academic articles published in the last five years on how accurately Herman Melville portrayed the whaling scene in his novel.

Now consider some typical ways of organizing the sources into a review:

• Chronological: If your review follows the chronological method, you could write about the materials above according to when they were published. For instance, first you would talk about the British biological studies of the 18th century, then about Moby Dick, published in 1851, then the book on sperm whales in other art (1968), and finally the biology articles (1980s) and the recent articles on American whaling of the 19th century. But there is relatively no continuity among subjects here. And notice that even though the sources on sperm whales in other art and on American whaling are written recently, they are about other subjects/objects that were created much earlier. Thus, the review loses its chronological focus.
• By publication: Order your sources by publication chronology, then, only if the order demonstrates a more important trend. For instance, you could order a review of literature on biological studies of sperm whales if the progression revealed a change in dissection practices of the researchers who wrote and/or conducted the studies.
• By trend: A better way to organize the above sources chronologically is to examine the sources under another trend, such as the history of whaling. Then your review would have subsections according to eras within this period. For instance, the review might examine whaling from pre-1600-1699, 1700-1799, and 1800-1899. Under this method, you would combine the recent studies on American whaling in the 19th century with Moby Dick itself in the 1800-1899 category, even though the authors wrote a century apart.
• Thematic: Thematic reviews of literature are organized around a topic or issue, rather than the progression of time. However, progression of time may still be an important factor in a thematic review. For instance, the sperm whale review could focus on the development of the harpoon for whale hunting. While the study focuses on one topic, harpoon technology, it will still be organized chronologically. The only difference here between a “chronological” and a “thematic” approach is what is emphasized the most: the development of the harpoon or the harpoon technology.But more authentic thematic reviews tend to break away from chronological order. For instance, a thematic review of material on sperm whales might examine how they are portrayed as “evil” in cultural documents. The subsections might include how they are personified, how their proportions are exaggerated, and their behaviors misunderstood. A review organized in this manner would shift between time periods within each section according to the point made.
• Methodological: A methodological approach differs from the two above in that the focusing factor usually does not have to do with the content of the material. Instead, it focuses on the “methods” of the researcher or writer. For the sperm whale project, one methodological approach would be to look at cultural differences between the portrayal of whales in American, British, and French art work. Or the review might focus on the economic impact of whaling on a community. A methodological scope will influence either the types of documents in the review or the way in which these documents are discussed. Once you’ve decided on the organizational method for the body of the review, the sections you need to include in the paper should be easy to figure out. They should arise out of your organizational strategy. In other words, a chronological review would have subsections for each vital time period. A thematic review would have subtopics based upon factors that relate to the theme or issue.

Sometimes, though, you might need to add additional sections that are necessary for your study, but do not fit in the organizational strategy of the body. What other sections you include in the body is up to you. Put in only what is necessary. Here are a few other sections you might want to consider:

• Current Situation: Information necessary to understand the topic or focus of the literature review.
• History: The chronological progression of the field, the literature, or an idea that is necessary to understand the literature review, if the body of the literature review is not already a chronology.
• Methods and/or Standards: The criteria you used to select the sources in your literature review or the way in which you present your information. For instance, you might explain that your review includes only peer-reviewed articles and journals.

Questions for Further Research: What questions about the field has the review sparked? How will you further your research as a result of the review?

Begin composing

Once you’ve settled on a general pattern of organization, you’re ready to write each section. There are a few guidelines you should follow during the writing stage as well. Here is a sample paragraph from a literature review about sexism and language to illuminate the following discussion:

However, other studies have shown that even gender-neutral antecedents are more likely to produce masculine images than feminine ones (Gastil, 1990). Hamilton (1988) asked students to complete sentences that required them to fill in pronouns that agreed with gender-neutral antecedents such as “writer,” “pedestrian,” and “persons.” The students were asked to describe any image they had when writing the sentence. Hamilton found that people imagined 3.3 men to each woman in the masculine “generic” condition and 1.5 men per woman in the unbiased condition. Thus, while ambient sexism accounted for some of the masculine bias, sexist language amplified the effect. (Source: Erika Falk and Jordan Mills, “Why Sexist Language Affects Persuasion: The Role of Homophily, Intended Audience, and Offense,” Women and Language19:2).

Use evidence

In the example above, the writers refer to several other sources when making their point. A literature review in this sense is just like any other academic research paper. Your interpretation of the available sources must be backed up with evidence to show that what you are saying is valid.

Be selective

Select only the most important points in each source to highlight in the review. The type of information you choose to mention should relate directly to the review’s focus, whether it is thematic, methodological, or chronological.

Use quotes sparingly

Falk and Mills do not use any direct quotes. That is because the survey nature of the literature review does not allow for in-depth discussion or detailed quotes from the text. Some short quotes here and there are okay, though, if you want to emphasize a point, or if what the author said just cannot be rewritten in your own words. Notice that Falk and Mills do quote certain terms that were coined by the author, not common knowledge, or taken directly from the study. But if you find yourself wanting to put in more quotes, check with your instructor.

Summarize and synthesize

Remember to summarize and synthesize your sources within each paragraph as well as throughout the review. The authors here recapitulate important features of Hamilton’s study, but then synthesize it by rephrasing the study’s significance and relating it to their own work.

Keep your own voice

While the literature review presents others’ ideas, your voice (the writer’s) should remain front and center. Notice that Falk and Mills weave references to other sources into their own text, but they still maintain their own voice by starting and ending the paragraph with their own ideas and their own words. The sources support what Falk and Mills are saying.

Use caution when paraphrasing

When paraphrasing a source that is not your own, be sure to represent the author’s information or opinions accurately and in your own words. In the preceding example, Falk and Mills either directly refer in the text to the author of their source, such as Hamilton, or they provide ample notation in the text when the ideas they are mentioning are not their own, for example, Gastil’s. For more information, please see our handout on plagiarism .

Revise, revise, revise

Draft in hand? Now you’re ready to revise. Spending a lot of time revising is a wise idea, because your main objective is to present the material, not the argument. So check over your review again to make sure it follows the assignment and/or your outline. Then, just as you would for most other academic forms of writing, rewrite or rework the language of your review so that you’ve presented your information in the most concise manner possible. Be sure to use terminology familiar to your audience; get rid of unnecessary jargon or slang. Finally, double check that you’ve documented your sources and formatted the review appropriately for your discipline. For tips on the revising and editing process, see our handout on revising drafts .

Works consulted

We consulted these works while writing this handout. This is not a comprehensive list of resources on the handout’s topic, and we encourage you to do your own research to find additional publications. Please do not use this list as a model for the format of your own reference list, as it may not match the citation style you are using. For guidance on formatting citations, please see the UNC Libraries citation tutorial . We revise these tips periodically and welcome feedback.

Anson, Chris M., and Robert A. Schwegler. 2010. The Longman Handbook for Writers and Readers , 6th ed. New York: Longman.

Jones, Robert, Patrick Bizzaro, and Cynthia Selfe. 1997. The Harcourt Brace Guide to Writing in the Disciplines . New York: Harcourt Brace.

Lamb, Sandra E. 1998. How to Write It: A Complete Guide to Everything You’ll Ever Write . Berkeley: Ten Speed Press.

Rosen, Leonard J., and Laurence Behrens. 2003. The Allyn & Bacon Handbook , 5th ed. New York: Longman.

Troyka, Lynn Quittman, and Doug Hesse. 2016. Simon and Schuster Handbook for Writers , 11th ed. London: Pearson.

You may reproduce it for non-commercial use if you use the entire handout and attribute the source: The Writing Center, University of North Carolina at Chapel Hill

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A systematic exploration of scoping and mapping literature reviews

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• Published: 23 May 2024

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• Eirini Christou   ORCID: orcid.org/0000-0001-6928-1013 1 ,
• Antigoni Parmaxi   ORCID: orcid.org/0000-0002-0687-0176 1 &
• Panayiotis Zaphiris   ORCID: orcid.org/0000-0001-8112-5099 1  

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Systematic literature mapping can help researchers identify gaps in the research and provide a comprehensive overview of the available evidence. Despite the importance and benefits of conducting systematic scoping and mapping reviews, many researchers may not be familiar with the methods and best practices for conducting these types of reviews. This paper aims to address this gap by providing a step-by-step guide to conducting a systematic scoping or mapping review, drawing on examples from different fields. This study adopts a systematic literature review approach aiming to identify and present the steps of conducting scoping and mapping literature reviews and serves as a guide on conducting scoping or mapping systematic literature reviews. A number of 90 studies were included in this study. The findings describe the steps to follow when conducting scoping and mapping reviews and suggest the integration of the card sorting method as part of the process. The proposed steps for undertaking scoping and mapping reviews presented in this manuscript, highlight the importance of following a rigorous approach for conducting scoping or mapping reviews.

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Avoid common mistakes on your manuscript.

1 Introduction

An essential component of academic research is literature review. A systematic literature review, also known as a systematic review, is a method for locating, assessing, and interpreting all research related to a specific research question, topic, or phenomenon of interest [ 1 ].

Scoping and mapping reviews are variations of systematic literature mapping [ 2 ]. Both mapping and scoping reviews can help researchers to understand the scope and breadth of the literature in a given field, identify gaps in the research, and provide a comprehensive overview of the available evidence. Systematic literature mapping purposely focuses on a narrower but more general academic or policy issue and does not try to synthesize the results of research to address a particular subject. The scoping review is exploratory in nature, whereas the mapping review can be conclusive in describing the available evidence and identifying gaps. Mapping review includes a thorough, systematic search of a wide field. It identifies the body of literature that is currently available on a subject and points out any glaring gaps in the evidence [ 3 ].

1.1 Rationale

Despite the importance and benefits of conducting systematic scoping and mapping reviews, many researchers may not be familiar with the methods and best practices for conducting these types of reviews. This paper aims to address this gap by providing a step-by-step guide to conducting a systematic scoping or mapping review, drawing on examples from different fields.

This study adopts a systematic literature review approach aiming to identify and present the differences and the steps of conducting scoping and mapping literature review. The paper provides practical guidance on how to address common challenges in conducting systematic scoping or mapping reviews, such as dealing with the volume of studies identified, managing the data extraction and synthesis process, and ensuring rigor and reproducibility in the review methodology. The main research questions that guide this study are:

RQ1: What is a systematic scoping review and how is it conducted?

RQ2: What is a systematic mapping review and how is it conducted?

RQ3: What are the main differences between systematic scoping and systematic mapping reviews?

Overall, this paper will be a valuable resource for researchers who are interested in conducting a systematic scoping or mapping review. By providing clear guidance and practical examples, the paper aims to promote best practices in systematic scoping and mapping review methodology. The study is organized as follows: The following section presents the methodology of the study, followed by the results showing the process of the scoping and mapping literature review and presenting some examples. Finally, suggestions on how to plan and perform a quality scoping and mapping review are presented.

2 Methodology

The methodology of this paper was adopted by Xiao and Watson [ 4 ].

2.1 Literature search

The search was conducted in two well-known online databases, Web of Science and EBSCOHost, across various disciplines. The searched terms combined keywords related to the performance of scoping and mapping literature review, such as “systematic literature review”, “methodology”, “map”, “mapping” and “scoping”. The title of each manuscript was used to determine its initial relevance. If the content of the title suggested that it would explain the method of the literature review process, we obtained the full reference, which included the author, year, title, and abstract, for additional analysis.

2.2 Initial search results

The query string used for the database search is the following: systematic literature review AND methodology AND (“map” OR “mapping” OR “scoping”). Abstract search was conducted in both databases for the last 10 years (2013–2022). A search on EBSCOHost revealed 643 results of which 291 were duplicated and automatically removed. After applying the database filters to limit the articles to peer-reviewed academic journal articles written in English, a number of 102 papers were excluded. Additional 109 papers were duplicated and removed manually. After an initial screening of the titles, a total of 13 studies were identified as relevant to the methodology of the scoping and mapping literature review. A search on Web of Science, revealed 888 results of which 9 were duplicate and removed, and 157 were found to be related to the methodology of scoping or mapping literature reviews after the first title screening. Last search was conducted on the 2nd of November 2022. Both sources revealed 161 related studies, excluding 9 duplicates that were removed.

2.3 Inclusion and exclusion criteria

Only studies that provide instructions on how to perform a scoping or mapping review were included in this paper. Reviews of the literature on certain subjects and in languages other than English were excluded. The study is limited to papers published within the last 10 years, aiming to collect recent information for performing scoping or mapping reviews. Inclusion and exclusion criteria can be found in Table  1 .

2.4 Screening

To further assess the 161 studies’ applicability to the study topic, their abstracts were reviewed. The manuscripts were evaluated independently and in parallel by two researchers. The researchers’ differing opinions were discussed and settled. Then the full-text of a total of 20 studies was acquired for quality evaluation.

2.5 Eligibility and quality evaluation

To further assess the quality and relevance of the studies, the full-text papers were reviewed. Journal articles and books published by prominent publishers were included in the review as they contained high-quality research. Because there is no peer review procedure, the majority of technical reports and online presentations were excluded.

Two researchers worked independently and simultaneously on evaluating eligibility and quality. Any disagreements between them were discussed and resolved. A total of ten (10) studies were excluded after careful review: one study was excluded because it lacked instructions on how the scoping or mapping review methodology was conducted, three studies were excluded because the methodology was not related to scoping or mapping review, while five studies were disregarded because they focused on a particular subject. One of the studies’ full text couldn’t be accessed. This resulted in ten (10) eligible for full-text analysis.

2.6 Iterations

Through backward and forward searching, additional 18 studies were discovered, from which only 10 met the inclusion criteria. The forward and backward search was also used to find manuscripts that applied scoping or mapping literature review methodology. After finding the article that established the scoping or mapping review methodology, articles that had cited the methodology paper to find instances of best practices in different fields were searched. Following consideration of examples’ adherence to the methodology, preference was given to planning-related articles. In total, 90 studies were analyzed in this study, i.e. 10 methodological papers that describe the application of scoping or mapping review, as well as 80 papers that demonstrate the application of the scoping and mapping methodology in different fields, that are used as examples. The PRISMA flow diagram (see Fig.  1 ) depicts the process of the search strategy [ 5 ].

PRISMA flow diagram

2.7 Extraction and analysis of data

Data were extracted in the process of scoping literature reviews, including information with regards to formulating the problem, establishing and validating the review procedure, searching the literature, screening for inclusion, evaluating quality, extracting data, analyzing and synthesizing data, and reporting the findings (Xiao & Watson, 2019). NVivo software was used for all data extraction and coding procedures. Initially, two researchers each took information from articles for cross-checking. The two researchers reached an agreement on what to extract from the publications after reviewing a few articles together. Then the first author classified the data based on the research questions.

In this section we present the findings of our review.

3.1 Defining “Scoping” and “mapping” review

According to [ 2 ], scoping and mapping reviews are variations of systematic literature mapping that focus on narrower but more general academic or policy issues. A scoping review is exploratory in nature, seeking to identify the nature and extent of research on a particular topic, and can be used to identify gaps in the literature. An example of a research question suitable for a scoping review is “What engagement strategies do educators use in classroom settings to facilitate teaching and learning of diverse students in undergraduate nursing programs?“ [ 6 ]. A mapping review, on the other hand, is a thorough and systematic search of a wide field of literature that aims to identify the body of literature currently available on a subject and point out any glaring gaps in the evidence. An example of a research question suitable for a mapping review is “What are the currently available animal models for cystic fibrosis” [ 3 ]. Overall, each type of review has its own specific aims and can be useful for different types of research questions.

3.1.1 Defining scoping review

There is no single definition for scoping reviews in the literature. According to [ 7 ], scoping review is a type of knowledge synthesis that uses a systematic process to map the evidence on a subject and identify key ideas, theories, sources, and knowledge gaps. The goal of a scoping review is to include all relevant information that is available, including ‘grey’ literature, which includes unpublished research findings, therefore including all available literature and evidence, but the reviewers can decide what type of publications they would like to include [ 8 , 9 , 10 , 11 ].

Scoping review process is sometimes used as a preliminary step before a systematic literature review, in cases where the topic or research area in focus has not yet been extensively reviewed or is complicated or heterogeneous in nature and the types of evidence available remain unclear [ 3 ]. For example, while a scoping review might serve as the foundation for a full systematic review, it does not aim to evaluate the quality of the evidence like systematic reviews do [ 8 ]. Moreover, scoping review is also referred to as a “pilot study” [ 12 ], that can be used as a “trial run” of the entire systematic map; it helps to mold the intended approach for the review and inform the protocol development.

Rapid and scoping meta-reviews were also referred as types of scoping reviews. A “rapid review” is a particular kind of scoping review, which aims to provide an answer to a particular query and can shorten the process compared to a full systematic review [ 3 ]. The “scoping meta-review” (SMR) is a scoping evaluation of systematic reviews that offers researchers a flexible framework for field mapping and a way to condense pertinent research activities and findings, similar to a scoping review [ 13 ].

Almost all of the scoping studies identified in the corpus, draw from previews scoping review frameworks, such as the one proposed by [ 14 , 15 ] and the authors’ manual provided by the Joanna Briggs Institute [ 11 , 16 , 17 , 18 ].

3.1.2 Defining mapping review

A mapping review, also referred to as a “systematic map”, is “a high-level review with a broad research question” [ 3 ](p.133). The mapping review includes a thorough, systematic search of a wide field. It identifies the body of literature that is currently available on a subject and points out any gaps in the evidence. The mapping review can be conclusive in describing the available evidence and identifying gaps, whereas the scoping review is exploratory in nature [ 3 ].

The term “mapping” is used to describe the process of synthesizing and representing the literature numerically and thematically in tables, figures, or graphical representations, which can be thought of as the review output. Mapping enables researchers to pinpoint potential areas for further study as well as gaps in the literature [ 19 ].

Systematic mapping uses the same strict procedures as systematic reviews do. However, systematic mapping can be used to address open or closed-framed questions on broad or specific topics, because it is not constrained by the requirement to include fully specified and defined key elements [ 12 ]. Systematic mapping is especially useful for broad, multifaceted questions about an interesting topic that might not be appropriate for systematic review because they involve a variety of interventions, populations, or outcomes, or because they draw on evidence that is not just from primary research [ 12 ].

3.2 Process of conducting mapping and scoping reviews

As noted in the previous sections, mapping reviews and scoping reviews both aim to provide a broad overview of the literature, but the former focuses on the scope of the literature while the latter focuses on the nature and extent of available evidence on a specific research question or topic. In understanding the process for conducting mapping and scoping reviews, we adopted the eight steps proposed by Xiao and Watson [ 4 ] that are common for all types of reviews: (1) Formulate the problem; (2) Establishing and validating the review procedure; (3) Searching the literature; (4) Screening for inclusion; (5) Evaluating quality; (6) Extracting data; (7) Analyzing and synthesizing data; (8) Reporting the findings. The steps are explained in detail below and describe the methodology for both scoping and mapping reviews, distinguishing their differences where applicable. A summary of the review types along with their characteristics and steps as identified from the literature are presented in Table  2 .

3.2.1 Step 1 formulate the problem

The first step for undertaking a mapping or a scoping review is to formulate the problem by setting the research question that should be investigated, taking into account the topic’s scope [ 12 ]. It is important to clearly state the review objectives and specific review questions for the scoping review. The objectives should indicate what the scoping review is trying to achieve [ 10 , 20 ].

In mapping reviews, it can be helpful to create a conceptual framework or model (visual or textual) to describe what will be explored by the map when determining the mapping review’s scope. It should also be determined whether the topic’s scope is broad, specific, or likely to be supported by a substantial body of evidence [ 12 ].

3.2.1.1 Defining the research question(s)

Prior to beginning their search and paper selection process, the authors should typically define their research question(s) [ 3 ]. There are specific formats that are recommended for structuring the research question(s), as well as the exclusion and inclusion criteria of mapping and scoping reviews [ 21 ] (see Table  3 ).

PCC (Population, Concept, and Context) and PICO format (Population, Intervention, Comparator and Outcome) are often used in scoping and mapping reviews. It is recommended that research questions for scoping reviews follow the PCC format and include all of its components [ 17 , 18 , 21 ]. Information about the participants (e.g. age), the principal idea or “concept,” and the setting of the review, should all be included in the research question. The context should be made explicit and may take into account geographical or locational considerations, cultural considerations, and particular racial or gender-based concerns [ 10 ].

Researchers use the PCC format in order to determine the eligibility of their research questions, as well as to define their inclusion criteria (e.g [ 22 , 23 , 24 , 25 , 26 ]). Most scoping reviews have a single main question, but some of them are better served by one or more sub-questions that focus on specific PCC characteristics [ 8 , 18 ].

3.2.2 Step 2. Establishing and validating the review procedure

A protocol is crucial for scoping and mapping reviews because it pre-defines the scoping review’s goals and procedures [ 11 , 17 , 18 , 20 ], it clearly states all methodological decisions since the very beginning [ 2 ], and it also specifies the strategy to be used at each stage of the review process [ 12 ]. Similar to all systematic reviews, scoping reviews start with the creation of an a-priori protocol that includes inclusion and exclusion criteria that are directly related to the review’s objectives and questions [ 7 , 11 , 17 , 18 , 20 ]. In order to decrease study duplication and improve data reporting transparency, the use of formalized, registered protocols is suggested [ 18 , 19 ]. The international prospective register of systematic reviews, known as PROSPERO, states that scoping reviews (and literature reviews) are currently ineligible for registration in the database. While this could change in the future, scoping reviews can currently be registered with the Open Science Framework ( https://osf.io/ ) or Figshare ( https://figshare.com/ ), and their protocols can be published in select publications, including the JBI Evidence Synthesis [ 18 ].

Scoping and mapping reviews should require at least two reviewers in order to minimize reporting bias, as well as to ensure consistency and clarity [ 3 , 16 , 17 , 18 ]. The reviewers should include a plan for the results presentation during the protocol development, such as a draft chart or table that could be improved at the end when the reviewers become more familiar with the information they have included in the review [ 17 , 18 ].

3.2.3 Step 3. Searching the literature

Searching the literature requires to prepare a search strategy, decide on search terms, search databases or journals, and perform a manual search [ 27 ]. For example, deciding on search terms, can follow an iterative process that is further explained in the sub-section below. Thinking about searching in terms of broader to narrower strategies is helpful. Fewer databases and/or journals will be checked out in narrower searches (search only in the title, keywords, and abstract fields), which are frequently used in scoping reviews, while multiple databases can be checked for mapping reviews [ 2 ].

Search strategy

Mapping and scoping review search should aim to be as thorough as possible [ 12 ] to find both published and unpublished evidence. An inclusive approach is frequently preferred for scoping reviews to prevent potential omission of crucial information [ 10 , 17 , 18 ].

According to JBI, there should be a three-step search process for scoping reviews [ 10 , 17 , 18 ]. The first step is a quick search of at least two databases followed by a text word check of the article’s title, abstract, and body of text that are then analyzed. All determined index terms and keywords are used in the second stage across all included databases. In the third stage, additional studies should be looked up in the identified reports and articles’ reference lists [ 10 , 11 , 18 ]. The reviewers may look solely at the reference lists of the studies that were chosen from the full-text and/or included in the review, or they may look at the reference lists of all identified studies. In any case, it needs to be made very clear which group of studies will be looked at [ 8 , 11 , 18 ]. As reviewers gain more familiarity with the body of available evidence, new keywords, sources, and possibly helpful search terms may be found and incorporated into the search strategy, hence the search for a scoping review may be quite iterative. If so, it is crucial that the entire search process and the outcomes are open to scrutiny and audit [ 11 , 18 ].

In the same line, it is recommended for mapping reviews to search multiple databases [ 2 ] in all pertinent searchable fields (e.g., title, abstract, keywords, etc.) [ 3 ]. Thematic keywords, along with all of their synonyms and regional/temporal variations, are joined together to form Boolean strings using Boolean signs. Building looser, multiple Boolean strings instead of well-targeted ones (for example, using OR instead of AND, NOT, and exact phrases, respectively) is preferable. The latter runs the risk of omitting crucial references, whereas the former may return a sizable sample of sparsely relevant references [ 2 ]. Focusing the search on a specific component and then filtering all the results can be more effective for mapping reviews [ 3 ].

3.2.4 Step 4. Screening for inclusion

Screening and choosing the studies to be included in a review are the main objectives of this phase. According to [ 27 ], there are two levels of screening. Titles and abstracts are scanned in the first level to limit the range of the studies to be included, while full texts are scanned in the second level to re-examine the relevance of the studies and to settle disagreements between reviewers regarding the study selection. Discussions, meetings, consulting a third reviewer, and determining inter-rater reliability/agreement (using Cohen’s kappa coefficient or intraclass correlation coefficient) are the most typical ways to resolve disagreements. Soaita et al. (2020) [ 2 ] also support that the sample of retrieved references should be ‘cleaned-up’ once it has been finalized and duplicates have been automatically removed.

Different methodological approaches, including primary research articles, summary articles, opinion pieces, and grey literature, can all be included in the literature that scoping reviews identify and analyze [ 7 , 18 , 19 ], but they may also serve as an exclusion criterion [ 2 ]. Peters, Godfrey, et al. (2020) [ 18 ] advice against limiting source inclusion based on language unless there are compelling justifications for doing so (such as practical considerations).

According to the PRISMA extension for scoping reviews (PRISMA-ScR), a description of the study selection process must be provided in both a narrative and flow diagram format. Including the date of the most recent literature search, enables the reader to assess how current the scoping review is [ 7 ].

3.2.4.1 Inclusion and exclusion criteria

Inclusion criteria offer a framework on which the reviewers can decide which sources to include in the scoping review. To ensure transparency and replicability, the exclusion and inclusion criteria need to be documented [ 7 , 8 , 10 , 11 , 17 ]. Authors should specify any limitations by year, language, publication status, or other factors, and explain why each one was put in place [ 7 ].

When it comes to mapping reviews, criteria should be created whenever possible with participation from stakeholders. Depending on the type of research questions, stakeholders may include practitioners, designers, policy makers, scientists and research funding bodies, but attention should be paid to avoid bias [ 12 ].

3.2.5 Step 5. Evaluating quality

Scoping and mapping reviews are not concerned with quality assessment as a criterion for inclusion [ 2 ]. Assessments of reporting quality and bias risk are typically outside the scope. Although it is possible to extract study characteristics that reflect study and reporting quality, bias cannot be assessed against a specific hypothesis if a mapping review is exploratory [ 3 ].

3.2.6 Step 6. Extracting data

The process of data extraction for a scoping review is also known as “charting the results”. A draft charting table or form needs to be created to capture the key details about the relevance of the included studies to the review question, as well as the characteristics of the included studies. The data extraction process can be iterative, with the charting table being constantly updated.

The reviewers should become familiar with the source results and test the extraction form on two or three studies to ensure that all relevant results are extracted [ 7 , 8 , 10 , 11 , 17 , 18 , 28 ]. In order to increase reporting transparency, authors should explain the main revisions with a justification if the charting process was iterative (i.e., the form was continuously updated). If appropriate, details about the procedures used to collect and verify information from the researchers of the included sources of evidence should be provided [ 7 ]. Author(s), year of publication, source origin, country of origin, objectives, purpose, study population, sample size, methodology, intervention type and comparator, concept, duration of the intervention, how outcomes are measured, and key findings that are related to the review question are all types of information that may be extracted [ 7 , 8 , 10 , 11 , 17 ].

When it comes to data extraction for mapping reviews, it is restricted to important study characteristics and outcomes due to the size of a mapping review [ 3 ]. The process of mapping is intended to produce a practical and organized resource that provides enough detail about studies to be helpful in further work [ 12 ].

To move beyond a straightforward list of citations, it is crucial to maintain a high level of clarity throughout any databases that are created. Studies that are discussed in several papers or that seem to be connected should be marked in the database. In the future, this helps prevent the double counting of research findings in syntheses that might overlook connections between study lines in the databases [ 12 ].

Aiming to capture the key characteristics of the included studies in the scoping and mapping reviews, we suggest the use of a guiding table for extracting data (see Supplementary_Material_1_Guiding_Table).

3.2.7 Step 7. Analyzing and synthesizing data

Authors may extract results and map descriptively. Simple frequency counts of concepts, populations, characteristics, or other fields of data will suffice for many scoping reviews [ 17 , 18 ]. In-depth analysis of quantitative data is not typically required in scoping reviews, although in some cases the authors may take into consideration a more advanced analysis depending on the nature and purpose of their review. A meta-analysis or interpretive qualitative analysis is probably not necessary in scoping reviews [ 17 , 18 ].

When it comes to mapping reviews, no results synthesis is carried out [ 12 ]. Different analytical approaches can be used to map chronological, geographical, conceptual, and thematic trends, which include some form of coding, once the sample has been limited to the pertinent references [ 2 ]. It is possible to identify correlations, trends, gaps, and clusters using simple numerical accounts of frequencies in each category (for example, the number of studies looking at a specific species) and more complex cross-tabulations (for instance, the number of studies looking at the effectiveness of a specific intervention, in a particular farming system, for a named species). Users have access to the map and can query it to find information pertaining to any chosen combinations of the subsets of the meta-data [ 12 ].

3.2.8 Step 8. Reporting the findings

Authors should specify exactly how the evidence will be presented, whether it be in a narrative format, table, or visual representation, such as a map or diagram [ 7 ].

In scoping reviews, a summary of all the relevant information gathered can be presented [ 8 ] using a logical and descriptive summary of the findings based on the research questions [ 10 , 11 , 17 ]. The distribution of studies by year or period of publication, countries of origin, field of intervention, and research methodologies, may be displayed in the tables and charts accompanied with a narrative summary that explains how the results relate to the review’s objectives [ 7 , 11 , 17 , 18 ].

The conclusions should be consistent with the review objective or question based on the findings of the scoping review [ 10 ]. Following the conclusions, specific recommendations for future research based on gaps in knowledge identified by the review results can be presented. Because of the lack of a methodological quality appraisal, recommendations for practice may be unable to be developed; however, suggestions based on the conclusions may be made [ 10 ].

A scoping review’s results section should include a PRISMA flow diagram and details the outcomes of the search strategy and selection procedure [ 7 , 17 ] outlining the grounds for exclusion at the full-text level of screening [ 7 ]. For example, a study [ 29 ] used the PRISMA-ScR extension for scoping reviews to ensure all important sections have been covered in their review.

Mapping reviews may place more emphasis on describing the evidence. The use of pivot tables and pivot charts is helpful for quickly visualizing the amount (and quality, if it is measured) of evidence across a variety of meta-data variables [ 12 ]. Such visualizations can display the quantity of research, the conclusions of a critical appraisal, the sample size across nations, outcomes, populations, or variables. These visualizations can contain categorical variables as additional dimensions. The geographic distribution of study effort and type may be particularly important in mapping reviews with a global or large-scale reach [ 12 ].

4 Discussion

This systematic literature review aimed to describe the process of conducting mapping and scoping literature reviews. In summary, the main difference between the two types of reviews is in their focus and scope. Mapping reviews provide a comprehensive overview of the literature while scoping reviews identify gaps and inconsistencies in the literature and outline potential areas for future research.

A lot of the methodological papers included in this systematic literature review (e.g [ 10 , 19 , 28 ]), referred to the “consultation process” as an additional, optional step that has been suggested by [ 14 ]. In this stage, subject experts or potential review users like practitioners, consumers, and policymakers may be consulted [ 8 ]. Researchers argue that this step should be mandatory [ 15 , 28 ]. In agreement with Levac et al.’s [ 15 ] choice, Daudt et al. [ 28 ] encourage the use of the consultation stage whenever it is practical because it adds richness to the entire research process and, consequently, the findings. Despite the fact that stakeholder consultations can make scoping review planning and execution more difficult, they guarantee that the findings are pertinent to educational practice and/or policies [ 19 ].

Scoping and mapping reviews should require more than one author to eliminate bias and ensure their quality. The card-sorting technique is suggested to be employed within the review process as a means for resolving discrepancies between the stakeholders and come to an agreement on the categorization and evaluation of the data to be included. Other studies (e.g [ 30 , 31 , 32 , 33 ]), propose the card-sorting technique as a method for resolving disagreements between people’s disparities, as well as to evaluate and verify extracted themes from datasets. Card sorting is a quick and reliable sorting method that finds patterns in how users would expect to find content or functionality. Due to the patterns and insights it exposes about how people organize and categorize content, card sorting is a successful approach for resolving categorization disagreements [ 34 ]. According to Wood and Wood [ 35 ], the majority of card sorting projects involve an open sort, where participants receive a list of items and are asked to organize them in the most appropriate way. However, in some cases, a pre-existing set of categories is given to the participants, the so-called closed card sorting project. This assumes that the existing categories are already well-organized, and the goal is to make minor adjustments. Wood and Wood [ 35 ], suggest that it’s best to start with an open sort and analyze the data before conducting a closed sort for validation. If a closed sort is necessary, it should be kept simple, and the results may not be optimal. For example, in a study [ 30 ] that aimed to review the use of makerspaces for educational purposes, the card sorting technique was used for the development of the coding scheme. A three-member academic committee, consisting of three professors took part in the card sorting exercise where they went through the abstracts of the relevant papers and were asked to categorize each manuscript after discussion. They then categorized the manuscripts in the three major themes and 11 subcategories that emerged during the card sorting exercise [ 30 ]. Similarly, the authors of [ 31 ] employed the card sorting technique in their research in order to agree on the main categorization and sub-categorization of the articles identified for inclusion in their review. Card sorting can be integrated as an additional step when conducting scoping and mapping reviews, as it provides useful insights from the experts’ perspective and makes the mapping process more inclusive (see Fig.  2 ).

Proposed steps for conducting scoping and mapping reviews

5 Conclusion

Scoping and mapping reviews need a methodological framework that is rigorous, consistent, and transparent, so that the results can be trusted and the review replicated. This provides enough information for the readers to evaluate the review’s accuracy, relevance, and thoroughness [ 8 ]. Scoping reviews should be carried out in accordance with established methodological guidance and reported using reporting standards (like PRISMA-ScR) guidelines [ 36 ]. The proposed steps for undertaking scoping and mapping reviews presented in this manuscript, highlight the importance of following a rigorous approach for conducting scoping or mapping reviews. Overall, this paper is a valuable resource for researchers who are interested in conducting a systematic scoping or mapping review in different fields and are looking to apply these review methods to their own research questions.

5.1 Limitations and future work

This study does not lack limitations. As specific keywords and specific databases were searched, not all relevant work is included. The study was also limited to the past 10 years, letting out methodologies and frameworks for scoping and mapping literature reviews that were not published within the specific timeframe. The fact that the number of methodological papers identified for inclusion are limited to ten, makes it difficult to clarify the differences between mapping and scoping reviews. Therefore, further research is encouraged in order to clarify and verify the differences and similarities between the two. The application of the proposed process for conducting systematic scoping and mapping reviews on specific topics will verify the process.

Data availability

The data that support the findings of this study are available from the corresponding author, [EC], upon reasonable request.

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Recent advances in structurally elaborate triptycenes, triptycene-containing polymers and assemblies: structures, functions and applications

• Fumitaka Ishiwari   ORCID: orcid.org/0000-0002-0200-4510 1 , 2   nAff4 ,
• Yoshiaki Shoji 1 , 2 , 3 ,
• Colin J. Martin 1 &
• Takanori Fukushima   ORCID: orcid.org/0000-0001-5586-9238 1 , 2 , 3  

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• Polymer synthesis
• Self-assembly

Triptycene, a rigid propeller-shaped molecule, was first synthesized in the early 1940s. More recently, many triptycene-containing polymers and molecular assemblies have been developed for a wide range of applications, including guest recognition, material transport, separation, catalysis, and as device components. The advantages of triptycenes lie in their ability to introduce a variety of functional groups on their three-dimensional backbone, with changes in substitution patterns as well as the type of substituents present having a significant impact on the material properties. In this review, we describe the synthesis of triptycene derivatives and polymers, detailing selected examples of triptycene-containing functional polymers. We also focus on the construction of triptycene-based two-dimensional assemblies and polymers, where space-filling designs based on rigid propeller-shaped skeletons are essential. Through a thorough literature survey, future directions and possibilities for the development of triptycene-containing functional materials are discussed.

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Introduction.

Triptycene is a rigid molecule with a three-dimensional (3D) skeleton consisting of three phenylene rings arranged with interblade angles of 120° (Fig.  1a ) [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 ]. Triptycene derivatives have been utilized in many fields, including supramolecular chemistry and polymer and materials science, and researchers have taken advantage of the structural features of triptycenes in the development of functional molecules, polymers, and assemblies. For example, a number of triptycene-containing polymers (trip-polymers) have been reported [ 2 , 10 ], many of which exhibit microporosity, can serve as low dielectric materials, and show specific assembly properties arising from the large free volumes present around the rigid triptycene skeleton. Importantly, the properties of trip-polymers largely depend on their substitution patterns, the symmetry of the constituent triptycene units, and the type of substituents present. To date, synthetic methods for accessing triptycene derivatives have been intensely developed, allowing the introduction of various substituents at different positions (Fig.  1b ). These structurally elaborate triptycene derivatives are useful for the design of various functional polymers (Fig.  1c–f ). Our group has also been engaged in the development of trip-polymers that exhibit unique assembly properties, for which triptycene units with particular substitution patterns [ 11 ], different from the majority of previously reported trip-polymers, are essential ( vide infra ).

a Molecular structure of triptycene with carbon numbering. b Synthesis and ( c – f ) schematic drawings of trip-polymers with ( c ) network, ( d ) linear, ( e ) linear ladder, and ( f ) hyperbranched structures

In this review, based on a thorough literature survey, we describe the synthesis and structures of triptycene derivatives and trip-polymers with various substitution patterns while summarizing recent research trends. To the best of our ability, we attempted to avoid including examples of trip-polymers that have already been described in the book and review by C-F. Chen et al. [ 1 , 8 ]. We also describe the formation of two-dimensional (2D) assemblies and related polymers using particular types of triptycenes. Triptycene derivatives, when appropriately functionalized, self-assemble into either porous or dense (nonporous) 2D structures through intermolecular π-stacking and/or nested packing of their propeller-blade moieties. These triptycene-based 2D assemblies are useful for the construction of polymeric materials featuring long-range structural order. We also describe various applications of triptycene-based 2D assemblies, including the development of polymer materials with enhanced properties for high-performance organic electronic devices.

Literature survey

Our manual survey of academic papers (other than patents) using Google Scholar ® in March 2024 with the keywords “triptycene + polymer” and “iptycene + polymer” revealed that 364 research publications reported a total of 923 triptycene- and iptycene-containing polymers from 1968 to the present. Note that (i) when polymers with the same structure were reported in different publications, we included them multiple times in the count, (ii) copolymers were counted separately, and (iii) metal-organic frameworks were excluded. Figure  2 shows histograms of the numbers of published papers (Fig.  2a ) and trip-polymers (Fig.  2b ) reported in these papers, where the colors in Fig.  2b corresponds to the substitution patterns of the constituent triptycene units. Figure  2c shows the percentage of polymers with each substitution pattern of triptycene relative to all trip-polymers; linear polymers linked at the 1,4-positions (338 examples) are the most common, followed by network polymers linked at the 2,7,14(15)-positions (189 examples), linear ladder polymers linked at the 2,3,6,7-positions (118 examples), linear polymers linked at the 2,6(7)-positions (92 examples) and network polymers linked at the 2,3,6,7,14,15-positions, including 2D/3D covalent organic frameworks (COFs) (107 examples) [ 12 ]. These polymers account for ca . 90% of the total number of trip-polymers. Both the reported number of papers and the number of trip-polymers rapidly increased around 2010. In the following, we describe the research trends related to triptycene-based polymers and assemblies in chronological order.

a Histogram of the number of annual publications on trip-polymers. b Histogram and distribution of the number and substitution patterns of trip-polymers reported per year. c Summary of the distribution of substitution patterns in reported trip-polymers

Early-stage examples including engineering polymers, fluorescent sensors, and low-dielectric materials

The first trip-polymer was reported in 1968 [ 13 ]. In that period, colorless, heat-resistant engineering plastics were the focus of attention, and structurally rigid triptycene derivatives with a small π-conjugated system were predicted to be potent building blocks in this area [ 14 ]. In 1998, Swager et al. reported a seminal paper describing the detection of explosives (TNT; trinitrotoluene) using π-conjugated polymers containing pentiptycene in the main chain [ 15 ]. Since then, a series of related papers have been published. Swager et al. also developed low- k polymers that serve as insulating interlayer materials for high-density electronic devices [ 16 ]. Trip-polymers with large free volumes around their polymer backbones tend to have low relative permittivity.

PIMs, COFs and 2D polymers (2000s~)

In 2004, McKeown et al. reported ladder polymers with rigid and contorted main chains that yielded microporous freestanding membranes featuring very high specific surface areas [ 17 ] and named them “polymers of intrinsic microporosity (PIMs)” [ 9 , 18 , 19 ]. Since then, PIMs have been studied extensively as membranes for gas separation and storage. Structurally rigid triptycenes, to which a variety of substituents can be introduced, are excellent building blocks for PIMs. Yaghi et al. and others introduced the concept of covalent organic frameworks (COFs) using similar porous solid materials in 2005 [ 12 , 20 ]. Propeller-shaped triptycenes also serve as useful building blocks for 2D and 3D COFs, and an increasing number of triptycene-based COFs have been reported since 2013. The synthesis of exfoliable triptycene-based 2D polymers with single-crystal-like structural order has been achieved using triptycene derivatives consisting of extended propeller blades [ 21 , 22 ].

Ion-conducting membranes (2010s~)

In 2011, a trip-polymer that serves as an excellent proton exchange membrane for fuel cells was first reported [ 23 ]. Then, triptycene-based anion exchange membranes for use as alkaline fuel cells were reported in 2017 [ 24 ]. While the initial focus in the development of these ion-exchange membranes was on introducing dense ionic functional groups on the triptycene moieties, later works shifted toward designs that take advantage of the microporous nature of trip-polymers.

Self-assembled materials (2010s~)

The abovementioned examples of trip-polymers all exhibit porosity and utilize micro- and nanopores, e.g., material transport and separation. In 2015, our research group reported that triptycene derivatives with alkoxy groups at the 1,8,13-positions self-assemble to fill the free volume between neighboring triptycene molecules, forming a 2D nested hexagonal packing arrangement, which has been applied in the development of polymers with new functionalities [ 11 ].

Chiral polymers (2010s~)

Triptycenes substituted at two or more positions, such as 2,6- or 1,5-substituted derivatives, are inherently chiral [ 7 ]. However, until recently, such chiral triptycenes have not been actively explored. Recently, chiral polymers utilizing this triptycene chirality have been reported, and their functions have attracted increased amounts of attention.

In the following sections, we will discuss general synthetic methods for trip-polymers, and their structures and applications. This review builds upon the examples found in the 2013 book of C-F. Chen et al., in which they describe a chapter on trip-polymers [ 1 ]. As such, we will focus on results published after this date.

Synthetic routes for accessing triptycene derivatives and triptycene-containing polymers

As shown in Fig.  1b , there are two major ways to synthesize trip-polymers: direct polymerization of unsubstituted triptycene or polymerization using a triptycene monomer (trip-monomer) substituted with a reactive functional group. Unsubstituted triptycene ( 1 ) easily reacts at its 2,7,14(,15)-positions under Friedel-Crafts-type conditions. Although monomer ( 1 ) can be easily prepared, this polymerization method does not yield network polymers containing triptycenes linked at the 2,7,14(,15)-positions. Trip-polymers linked at other substitution positions require the synthesis of trip-monomers, in which reactive substituents are introduced at the desired positions in advance. One method is to introduce substituents at the 2(,3)(,6),7,14(,15)-positions by Friedel–Crafts-type reactions with 1 . The main trip-monomers synthesized starting with this method are shown in Fig.  3 . The Friedel–Crafts conditions used include nitration with nitric acid [ 25 , 26 ], acylation using Lewis acid catalysts such as aluminum chloride or tin chloride, and formylation [ 27 ]. For nitration, 2-monosubstituted ( 2 ), 2,6- ( 3 ) or 2,7-disubstituted ( 4 ) and 2,6,14- ( 5 ) or 2,7,14-trisubstituted ( 6 ) nitro triptycene derivatives have been reported. The ratios of formation can be controlled somewhat selectively by varying the amounts of reagents used, and importantly, all isomers can be separated using silica gel column chromatography (except for optical isomers). This has allowed for the synthesis of trip-monomers with the desired number and patterns of nitro substituents. Amino-substituted triptycenes ( 7 – 11 ), obtained by reducing nitro moieties, are frequently used as monomers in the synthesis of polyamides, polyimides, and Tröger’s base-containing PIMs [ 9 , 19 ]. Sandmeyer-type reactions via diazotization of the amino forms have led to the insertion of halo substituents, yielding bromo ( 12 – 16 ) and iodo ( 17 – 21 ) triptycenes [ 26 , 28 ] along with an azide-substituted trip-monomer ( 22 ) [ 29 , 30 ], which have been used in various transition-metal-catalyzed couplings and click polymerizations. In addition, 2,6-diaminotriptycene ( 8 ) [ 31 ] and its derivatives [ 32 ] are relatively easy to separate optically using chiral HPLC, and chiral polymers with interesting optically active functions have been obtained, which will be described in detail later. Although most of the trip-monomers shown here are chiral, many optically active monomers and polymers are expected to be developed in the future.

Synthesis of trip-monomers from unsubstituted triptycene 1

Baeyer–Villiger oxidation of acylated trip-monomers ( 23 – 32 ) with m CPBA and subsequent hydrolysis have led to hydroxy trip-monomers for use in polyester and polyether synthesis ( 33 – 37 ) [ 27 , 33 ]. These compounds have also been synthesized by other methods [ 34 ] and will be described later. Nitration and reduction of 2,6(,7)-dihydroxytriptycene ( 34,35 ) yields a trip-monomer with two hydroxy and two amino groups ( 38 ) [ 33 ]. This type of monomer is important in the preparation of polybenzimidazoles [ 35 ] through thermal rearrangement to the corresponding hydroxy group-containing polyimides, which have been shown to act as excellent gas-separation membranes [ 36 ]. Another monomer related to gas separation membranes, the di- ortho -brominated 2,6(,7)-diaminomonomer ( 39 ), which is useful in the synthesis of polyimides with reduced free rotation, has recently been reported [ 37 , 38 ]. A route to synthesize triaminotriptycenes via Beckmann-rearrangement ( 40 ) from acetylated triptycenes ( 23 – 32 ) is also known [ 27 ].

Amino- and/or bromo-substituted monomers can be synthesized by reacting with unsubstituted triptycene 1 to yield, for example, 2,3,6,7,14,15-hexasubstituted trip-monomers. For the di-amination ( 42 ), tetra-amination ( 43 ) and hexa-amination ( 44 ) of triptycenes, a multistep synthesis via protection, nitration ( 41 ), deprotection, and reduction of triaminotriptycene ( 10, 11 ) was originally reported [ 39 ]. Later, Mastalerz et al. reported that amino-substituted derivatives can be synthesized as the corresponding hydrochloride salts in two steps from 1 using fuming nitric acid [ 40 ]; however, the yield from 1 is only 18%. On the other hand, for the synthesis of hexabromotriptycene ( 45 ), bromination of 1 using Br 2 in the presence of iron proceeded in relatively high yield [ 41 ]. Very recently, the addition of I 2 was shown to increase the yield to 90% [ 42 ]. It has been reported that hexaaminotriptycene ( 44 ) can be synthesized efficiently by palladium-catalyzed Buchwald–Hartwig amination of hexabromide with benzophenone imine, followed by deprotection with hydrochloric acid [ 43 ]. This hexa-aminated trip-monomer is widely used as a building block for COFs and network polymers. Sonogashira couplings with di-, tri-, and hexa-brominated monomers have led to ethynyl-substituted trip-monomers ( 46 – 48 ) that can be used in further Sonogashira couplings and click polymerizations. 2,3,6,7,14,15-Hexaethynyltriptycene ( 48 ) can be used for Masamune–Bergman polymerizations, resulting in interesting porous network polymers [ 44 , 45 ], which will be discussed in detail later. Hexa-hydroxylated trip-monomers ( 50 ) can also be synthesized by treating the hexa-brominated trip-monomers with sodium methoxide to introduce methoxy groups ( 49 ) and then removing the methyl group using BBr 3 [ 46 ]. This approach has been used in the preparation of COFs and PIMs. In addition to the synthetic methods described thus far, it is also possible to prepare amino substituents from acyl groups by Beckmann rearrangement and hydrolysis [ 27 ] and hydroxyl substituents from amino groups by Sandmeyer-type reactions [ 25 ], but as of yet, no polymers have been obtained using monomers in these ways. Moreover, direct borylation of 1 using an iridium catalyst and bis(pinacolato)diboron has been reported to yield 2,6,14(15)-tris[(pinacolato)boryl]triptycene as a mixture of regioisomers [ 47 ]. The propeller blades of these triboryltriptycenes can be extended using Suzuki coupling and dehydrogenative cyclization. Thus, triboryltriptycenes could be useful as building blocks for network polymers.

Another method to synthesize trip-monomers is to introduce functional groups into anthracenes, benzynes, or quinones, which are common precursors in the synthesis of triptycenes. In addition to simple modification reactions, anthracene derivatives with various functional groups can be derived from commercially available anthraquinone. Anthracenes can also be synthesized using aromatic electrophilic substitution reactions between relatively electron-rich benzenes and reagents such as aldehydes or dichloromethane. A number of commercially available substituted benzyne precursors, including anthranilic acids and derivatives with triflate and trimethylsilyl groups, have been used.

The synthesis of the most common 1,4-substituted triptycene derivatives is shown in Fig.  4 . para -Benzoquinone ( 51 ) has been used as a dienophile with anthracenes ( 52, 53 ) in Diels-Alder reactions, followed by aromatization to the 1,4-dihydroxy trip-monomer ( 54, 55 ) [ 16 , 17 ]. From this, a perfluorosulfonated intermediate ( 56 ) can be formed and converted into a 1,4-diethynyl trip-monomer ( 57 ) by Sonogashira coupling [ 48 ]. From the 1,4-dihydroxy trip-monomer ( 54 ), a para -benzoquinone derivative ( 59 ) can be obtained by oxidation, and from the imine derivative ( 60 ), it can be prepared through condensation with hydroxylamine, which can be subsequently reduced to yield the 1,4-diamino trip-monomer ( 61 ) [ 49 , 50 ]. Compound 61 can undergo a Sandmeyer-type reaction to yield the 1,4-diiodo trip-monomer ( 62 ) [ 48 ], which has been used in a further Sonogashira reaction to synthesize the 1,4-diethynyl trip-monomer ( 57 ) [ 51 , 52 ]. The para -benzoquinone form of pentiptycene ( 64 ) can be synthesized using an excessive amount of anthracene in a Diels–Alder reaction with para -benzoquinone or by allowing the para -benzoquinone form of triptycene to undergo a second Diels–Alder reaction with anthracene in acetic acid [ 16 , 48 ]. A reduction of 64 yields the 1,4-dihydroxypentiptycene monomer ( 63 ) [ 52 ], whereas a nucleophilic attack of 64 with a TMS-acetylide ( 65 ) and subsequent oxidation ( 66 ) and deprotection yields the 1,4-diethynylpentiptycene monomer ( 58 ) [ 16 , 48 ]. However, it has been noted that the equivalent 1,4-diaminopentiptycene cannot be synthesized by a similar route [ 4 , 48 , 53 ].

Synthesis of 1,4-substituted trip-monomers

It has been reported that functionalized triptycenes and pentiptycenes can be obtained by reacting substituted anthracenes with benzoquinone 51 . The reaction of 1,8-dimethoxyanthracene ( 53 ) with 51 yields 1,4-dihydroxytriptycene ( 55 ), which has two hydroxy substituents protected by methyl groups [ 54 ]. In 2021, a highly functionalized pentiptycene monomer was synthesized by Guo et al. [ 55 ]. They synthesized 2,3-dimethoxyanthracene ( 71 ) from phthalic anhydride ( 67 ) and dimethoxybenzene ( 68 ) by stepwise Friedel–Crafts reactions ( 69 ) and subsequent reduction/aromatization ( 70, 71 ). The reaction of 71 with para -benzoquinone 51 gave a pentiptycene ( 72 ) bearing upper and lower methoxy groups as a mixture of syn- and anti -isomers. These isomers can be separated by means of silica gel column chromatography. The central para -benzoquinone moiety of 72 can be reduced ( 73 ) and etherified ( 74 ). The use of ceric ammonium nitrate (CAN) for the reaction with 74 resulted in the selective oxidation of the dimethoxybenzene moieties to yield the ortho -quinone pentiptycene ( 75 ). This was reduced via a hydrogenation reaction to yield a tetrahydroxy derivative ( 76) [ 55 ], which has been used in the preparation of PIMs [ 9 ].

Figure  5 shows the synthetic pathway for accessing triptycenes with hydroxyl, carboxyl, carboxylic acid chloride, isocyanate, and amino groups as polymerizable substituents at the 9,10-positions. The 9,10-positions of unsubstituted anthracene ( 77 ) were chloromethylated ( 78 ) and converted to acetoxy groups via nucleophilic substitution ( 79 ), followed by a Diels–Alder reaction with benzyne generated from anthranilic acid ( 80 ) and amyl nitrite to yield 9,10-diacetoxymethyltriptycene ( 81 ). The acetyl group can then be deprotected to the 9,10-dihydroxymethyl trip-monomer ( 82 ) [ 14 , 15 ]. It is also possible to synthesize a 9,10-dihydroxyethyl trip-monomer ( 86 ) through the introduction of a cyano group ( 83 ) via nucleophilic substitution of the chloromethyl group of 78 and subsequent functional group transformation followed by a Diels–Alder reaction ( 84, 85 ) [ 15 ]. Oxidation of the 9,10-dihydroxymethyl trip-monomer ( 82 ) with chromium trioxide yields the 9,10-dicarboxyl trip-monomer ( 87 ), and further treatment with thionyl chloride yields the 9,10-dichlorocarbonyl trip-monomer ( 88 ). This synthetic method, reported in 1968 and 1969 [ 13 , 14 ], is still used today. For example, compound 87 has been utilized for the synthesis of MOFs [ 56 ]. 9,10-Diamino Trip-monomer ( 91 ) [ 14 , 15 ] can be synthesized from 88 through a reaction with sodium azide, a thermal Curtius rearrangement of the resulting benzoyl azide ( 89 ), and the subsequent hydrolysis of a diisocyanate derivative ( 90 ). 9,10-Functionalized triptycene derivatives such as 87 and 91 have been used as building blocks for polyesters and polyamides. Moreover, an interesting synthetic method has been developed for 9,10-dihydroxylated derivatives ( 94, 95 ). For example, anthracene substituted with hydroxyl and ethynyl groups at the 9- and 10-positions ( 92 ) reacts with norbornadienes or terminal alkynes in the presence of a rhodium catalyst in a [2+2+2] cycloaddition to form 9,10-dihydroxytriptycene 94 [ 57 , 58 ]. Cycloaddition with terminal alkynes or norbornadienes affords derivatives with or without a substituent at the 14-position, respectively. This synthetic protocol can be applied to the synthesis of a highly substituted, 1,4-dibrominated derivative ( 95 ) from a corresponding precursor ( 93 ) [ 57 ]. Sonogashira coupling polymerization using 95 yields polyphenylene ethynylenes, which can be transformed to polyphenylene vinylenes through intramolecular hydrooxygenation of alkyne moieties.

Synthesis of 9,10-functionalized trip-monomers

Figure  6 summarizes the trip-monomers that can be synthesized from functionalized anthraquinones. The chloro groups of 1,5-dichloroanthraquinone ( 96 ) can be converted to cyano groups ( 97 ) using copper cyanide and then to carboxylic acids ( 98 ) by hydrolysis [ 59 ]. The anthraquinone skeleton is reduced to anthracene by treatment with zinc under basic conditions ( 99 ), which is followed by methylation of the carboxylic acid moieties to yield the methyl ester derivative ( 100 ). Subsequently, 100 reacts with benzyne generated from anthranilic acid ( 80 ) and isoamyl nitrite to yield a triptycene with ester groups at the 1,5-positions ( 101 ). Hydrolysis of 101 yields the 1,5-dicarboxytriptycene monomer ( 102 ). Here, the 1,5-diaminotriptycene monomer ( 106 ) can be synthesized through acid chloride ( 103 ), benzoyl azide ( 104 ), and isocyanate ( 105 ) intermediates, similar to the case of 91 in Fig.  5 [ 59 ]. 1,5-Dicarboxytriptycene ( 102 ) is chiral, and its enantiomeric separation has been achieved by the formation of diastereomeric salts with the naturally occurring optically active alkaloids synconidine or synconine. Thus, ( R,R )- and ( S,S )-1,5-dicarboxytriptycene have been obtained using synconidine and synconine, respectively [ 59 ]. The use of these 1,5-dicarboxytriptycenes results in the formation of the corresponding optically active 1,5-diamino trip-monomers ( 106 ), which can be further transformed into optically active 1,5-dihydroxy trip-monomers ( 107 ) by reaction with water via a diazonium salt [ 59 ].

Synthesis of trip-monomers from functionalized anthraquinones

The synthesis of 1,5-dihydroxy trip-monomer ( 107 ) has also been reported using the scheme shown in the lower part of Fig.  6 [ 60 ]. A sodium salt of anthracene-1,5-disulfonic acid ( 109 ), synthesized from the corresponding anthraquinone ( 108 ), is subjected to alkali fusion, affording 1,5-dihydoxytriptycene ( 110 ). After reacting with acetic anhydride, the resulting 1,5-diacetoxyanthracene ( 111 ) reacts with benzyne, followed by hydrolysis to yield the 1,5-dihydroxy trip-monomer ( 107 ) [ 60 ]. For 2,6-dihydroxytriptycene ( 34 ) [ 34 ], 2,6-dihydroxyanthraquinone ( 113 ) has been used as a starting material. Compound 113 is converted to 2,6-dimethoxyanthracene ( 116 ) via methylation and reduction (in no particular order) [ 61 , 62 ], and then 116 reacts with benzyne followed by demethylation using boron tribromide to yield 2,6-dihydroxytriptycene 34 . Therefore, 1,5-dihydroxyanthraquinone ( 117 ) may provide an alternative synthetic route to 1,5-dihydroxytriptycene 107 , which does not involve alkali fusion. It has been reported that 1,5-dimethoxyanthracene ( 119 ) derived from 117 , upon reaction with para -benzoquinone ( 51 ), yields 1,5-dimethoxytriptycene monobenzoquinone ( 120 ) [ 63 ]. para -Benzoquinone is useful in the synthesis of benzoquione derivatives of triptycene [ 64 ]. 2,6-Diaminoanthracene ( 122 ), obtained by the reduction of 2,6-diaminoanthraquinone ( 121 ) [ 65 ], has been reacted with 1,4-epoxy-1,4-dihydronaphthalene ( 123 ) under Diels–Alder conditions to form 124 , followed by dehydration using perchloric acid to yield 2,6-diaminobenzotriptycene ( 125 ) [ 66 ]. The Diels–Alder reaction using 123 , which can be carried out even in the presence of amino functionalities, is useful for the synthesis of benzotriptycene derivatives. Compound 125 has been used for the synthesis of microporous polyimides and PIMs consisting of Tröger’s base moieties [ 66 ].

Figure  7 summarizes the synthesis of highly functionalized trip-monomers using multisubstituted anthracene derivatives. These triptycene derivatives have attracted much attention in recent years as monomers for use in the preparation of ladder polymer-based PIMs and microporous polyimides. The Friedel–Crafts reaction of 1,2-dimethoxybenzene ( 68 ) with various aldehydes ( 126 ) in the presence of sulfuric acid yields the corresponding hexa-substituted 9,10-dialkyl-2,3,6,7-tetramethoxyanthracenes ( 127 ). By subjecting these anthracenes to a Diels–Alder reaction with benzyne generated from unsubstituted or dimethoxy anthranilic acid ( 80 or 128 ), triptycene derivatives methoxy-substituted at the 2,3,6,7- ( 129 ) or 2,3,6,7,14,15-positions ( 130 ) are obtained [ 46 , 67 , 68 , 69 ]. Deprotection of these methoxy groups using boron tribromide yields the corresponding triptycene 2,3,6,7-tetrahydroxy ( 131 ) or 2,3,6,7,14,15-hexahydroxy ( 132 ) derivatives [ 46 , 67 , 68 , 69 , 70 ]. Related hexahydroxytriptycene ( 50 ) devoid of alkyl substituents at the 9- and 10-positions (i.e., R 1 = H for 132 ) was prepared from 2,3,6,7,14,15-hexabromotriptycene 45 (Fig.  3 ) [ 46 ]. The reaction of furan ( 136 ) with benzyne derived from the lithiation of substituted benzenes ( 133 – 135 ) yields 1,4-epoxy-1,4-dihydronaphthalene derivatives ( 137 ). These compounds undergo Diels–Alder reactions with hexa-substituted anthracenes ( 127 ) to yield epoxy products ( 138 ), which, through dehydration and demethylation by acid treatment, are transformed into highly substituted 2,3,6,7-tetrahydroxybenzotriptycene ( 139 ) [ 69 , 70 ]. 2,3,6,7-Tetracyanotriptycene derivatives ( 141 ) were obtained by palladium-catalyzed cyanation of the corresponding triflated compound ( 140 ) derived from 131 [ 71 ]. Hydrolysis of the cyano groups of 141 yields 2,3,6,7-tetracarboxyltriptycene ( 142 ). The corresponding acid anhydride derivatives ( 143 ) were obtained by reacting with acetic anhydride [ 71 ]. An acid anhydride derivative devoid of alkyl substituents at the 9- and 10-positions (R 1 = H for 142 ) can be synthesized by oxidation of 2,3,6,7-tetramethyltriptycene ( 147 ) using potassium permanganate [ 72 ]. 2,3,6,7-Tetramethylanthracene ( 145 ), the precursor of 147 , is obtained by a Friedel–Crafts reaction using ortho -xylene ( 144 ), dichloromethane, and aluminum chloride. The Diels–Alder reaction between 145 and benzyne yields 147 . Similarly, 2,3,6,7,14,15-hexamethyltriptycene ( 148 ) can be obtained using anthranilic acid with two methyl groups ( 146 ) instead of anthranilic acid ( 80 ) as a benzyne precursor. Compound 148 can be transformed to tri-acid anhydride 150 through successive oxidation ( 149 ) and intramolecular condensation [ 73 ]. 1,3,6,8-Tetramethylanthracene ( 152 ) can be prepared through a Friedel–Crafts reaction of meta -xylene ( 151 ) in dichloromethane [ 74 ]. 1,3,6,8-Tetramethyltriptycene ( 153 ) [ 74 ] and its monobenzoquinone derivative ( 154 ) [ 75 ] are obtained by the reaction of 152 with benzyne generated from anthranilic acid ( 80 ) and para -benzoquinone ( 51 ), respectively. Both 153 and 154 can be selectively nitrated at the 2,7-positions flanked by methyl groups ( 155 [ 74 ] and 156 [ 75 ]) using a mixture of potassium nitrate and trifluoroacetic acid. Reduction of these nitrated products using hydrazine and metal catalysts yields 1,3,6,8-tetramethyl-2,7-diaminotriptycene ( 157 ) and 13,16-dihydroxy-1,3,6,8-tetramethyl-2,7-diaminotriptycene ( 158 ). The latter has been used as a building block for redox-active PIMs [ 75 ].

Synthesis of trip-monomers from multifunctionalized anthracenes

Figure  8 summarizes the synthesis of the 1,8,13-substituted triptycene derivatives. Our group reported that, through the Diels–Alder reaction of 1,8-dimethoxyanthracene ( 53 ) and benzyne generated from a methoxy-substituted precursor ( 162 ), 1,8,13-trimethoxytriptycene ( 165 ) can be prepared along with its 1,8,16-isomer as a minor product (1,8,13/1,8,16 = 2/1) [ 11 ]. Compounds 53 and 162 are synthesized in two steps from 1,8-dihydroxyanthraquinone ( 163 ) and in four steps from guaiacol ( 159 ), respectively. Similarly, 1,8-dimethoxytriptycene ( 164 ) can be obtained using 53 and benzyne [ 76 ]. By reacting these methoxy-substituted triptycenes with boron tribromide, we have shown that 1,8-dihydroxy and 1,8,13-trihydroxytriptycenes ( 166, 167 ) can be synthesized [ 11 , 77 ]. A wide variety of substituents can be introduced to these hydroxylated derivatives via ether linkages, and the resulting alkoxytriptycenes have been found to exhibit particular 2D assembly properties [ 11 , 77 , 78 , 79 , 80 , 81 ], which will be described in detail later. The various 1,8(,13)-substituted derivatives developed thus far include those with terminal olefin moieties ( 168, 171, 172 ) [ 77 , 80 , 81 ] and acryloyl groups ( 169 ) [ 78 ]. Compounds 171 and 172 are used as end modifiers for polydimethylsiloxane (PDMS) [ 80 , 81 ]. We also prepared a 1,8,13-triptycene initiator with hydroxy termini ( 170 ) [ 79 ]. Mastalerz et al. also reported a synthetic method for 1,8,13-trihydroxytriptycene 167 at approximately the same time and showed that the ortho -positions of its hydroxy groups can be formylated ( 173 ) by the reaction using hexamethylene tetraamine in the presence of trifluoroacetic acid [ 82 ]. From hexa-substituted monomer 173 , a triptycene-containing 3D COF was synthesized through the formation of a salphen complex [ 83 ].

Synthesis of 1,8,13-substituted trip-monomers

The synthesis of bridgehead-substituted 1,8,13-triptycenes, which involves selective functionalization of the 10-position of 1,8-substituted anthracenes, requires somewhat laborious, multiple protection/deprotection steps [ 84 ]. Converting the methoxy groups of 53 to acetoxy groups ( 174 ) allows selective bromination at the 10-position using N -bromosuccinimide (NBS). The acetoxy groups of the resulting 175 were then converted back to methoxy groups. The synthesized 10-bromo-1,8-dimethoxyanthracene ( 176 ) is lithiated at the 10-position using n -butyllithium and subsequently treated with N , N -dimethylformamide (DMF), yielding 10-formyl-1,8-dimethoxyanthracene ( 177 ). A bridgehead-substituted 1,8,13-triptycene skeleton ( 179 ) can be synthesized by protecting the formyl group of 177 with ethylene glycol ( 178 ) and then treating it with in situ generated methoxybenzyne. Successive deprotection of 179 using hydrochloric acid and boron tribromide yields 10-formyl-1,8,13-trihydroxytriptycene ( 181 ) [ 84 ]. The formyl group of 181 can be converted to an ethynyl group ( 182 ) at this stage using the Ohira–Bestmann reagent [ 84 ]. Moreover, terminal ethynyl groups at this position are available for click reactions using copper(I) iodide and triethylamine [ 85 ]. The hydroxyl groups of 182 can be functionalized via etherification, and the remaining ethynyl group can be functionalized by nucleophilic substitution through acetylide or Sonogashira coupling. This protocol allows for the synthesis of bifunctional derivatives ( 183 ) carrying azide and terminal ethynyl groups [ 86 ]. This AB 3 -type monomer was found to undergo polymerization in the assembled state, which will be discussed later. 1,8,13-Substituted triptycenes are relatively recently developed derivatives, and their synthesis has been thoroughly reviewed by Shindo et al. [ 87 ].

Recent progress in triptycene-containing polymers

Linear polymers.

Here, we present some recent examples of triptycene-containing linear polymers with unique structures and interesting properties. Swager et al. reported the synthesis of polyethersulfone ( poly-1 ) by polycondensation using 13,16-dihydroxy-1,8-dimethoxytriptycene ( 55 ), bisphenol A ( 184 ) and difluorodiphenylsulfone ( 185 ) (Fig.  9 ) [ 54 ]. The methoxy groups of poly-1 were deprotected with boron tribromide to form poly-2 with hydroxyl substituents, and bulky pyrazolium chloride ( 186 and 187 ) was attached to the hydroxyl groups to form a cross-linked network polymer ( poly-3 ) with a structure in which the pyrazolium ions are densely aggregated (referred to as an ionic highway). Films of poly-3 are reported to show both anion-conducting properties and high stability resulting from suppressed swelling due to its cross-linked nature.

Highly conductive and stable triptycene-containing anion-exchange polymer with an “ionic highway”

Swager et al. synthesized a simple triptycene-containing polyethersulfone ( poly-4 ) from 1,4-dihydroxytriptycene ( 54 ) and 185 , chloromethylated its triptycene moieties, and then treated the resulting poly-5 with N -methylimidazole ( 188 ) to obtain poly-6 with N -methylimidazolium chloride pendants (Fig.  10 ) [ 24 ]. This polymer not only forms anion exchange membranes for alkaline fuel cells but also provides a scaffold for metal nanoparticles [ 24 , 88 , 89 ]. For example, poly-6 was composited with poly(4-vinylpyridine) and single-walled carbon nanotubes (SWCNTs), and the counter anion (Cl – ) was exchanged with tetrachloroaurate (AuCl 4 – ). Upon treatment with sodium borohydride, small (<5 nm) gold nanoparticles (Au NPs) were formed in the composite film (Fig.  10 ). A field-effect transistor (FET) incorporating a Au NP composite layer was reported to serve as a chemo FET sensor for the detection of carbon monoxide and other gaseous molecules [ 88 ]. Moreover, bimetallic Pd/Pt nanoparticles (PdPt NPs) with a size of 1 nm can be synthesized within the poly-6 film (Fig.  10 ). PdPt NPs react with hydrogen gas with high sensitivity in an oxygen atmosphere, and it has been reported that these materials can be applied as very sensitive hydrogen gas sensors [ 89 ]. In these systems, the use of triptycene units allows for the dense accumulation of functional groups as well as the formation of microporous structures.

a Synthesis of triptycene-containing anion exchange polymers and their application in ( b ) chemo-FET sensors (e.g., CO gas) and ( c ) H 2 gas sensors. b Adapted with permission [ 88 ] Copyright 2019, American Chemical Society. c Adapted under terms of the CC-BY 4.0 license [ 89 ] Copyright 2020, Elsevier

Conjugated polymers derived from the 9,10-diethynylpentiptycene monomer 58 have been used as components of highly responsive chemosensors due to their porous nature [ 15 ]. Pentiptycene-containing poly-7 with a benzothiadiazole unit composited with SWCNTs has been reported to serve as a sensor for the chemiresistive detection of solvent vapors such as benzene, toluene and xylene (i.e., BTX) (Fig.  11 ) [ 90 ]. Pentiptycene-containing poly-8 with semiperfluoroalkyl side chains has been reported to function as a fluorescent polymer sensor for the detection of poly(fluoroalkyl) substances (PFAS) (Fig.  11 ) [ 91 ].

Pentiptycene-containing π-conjugated polymers applied for chemosensing

Swager et al. reported that a palladium-catalyzed reaction between phenolic hydroxyl groups and aryl halides can be used in condensation polymerization to obtain a variety of polyaryl ethers that cannot be synthesized through standard aromatic nucleophilic substitution methods (Fig.  12 ) [ 92 ]. Typically, 1,4-dihydroxy-6,14-di- tert -butyl-triptycene ( 189 ), 2,7-dibromospirobifluorene ( 190 ) and aromatic dibromides with various photocatalytically active sites ( 191 – 196 ) were reacted in the presence of a palladium catalyst ( 197 ) and a monophosphine ligand ( 198 ) to afford the corresponding polyethers ( poly-9 – poly-16 ). The obtained polymers are solution-processable and form microporous films that exhibit photocatalytic abilities, as shown in Fig.  12 [ 93 ]. A ruthenium catalyst can also be used to introduce perfluoroalkyl groups into the spirobifluorene moiety. The resulting polymer carrying fluorophilic side chains ( poly-9-C 17 F 35 ) can be used for the modification of the surface inside perfluoroalkoxy alkane (PFA) tubes. Using these modified PFA tubes, photoreactions can be conducted allowing the reagent to flow while irradiating with light [ 93 ].

Triptycene-containing solution-processable porous photocatalysts obtained by palladium-catalyzed carbon–oxygen bond formation

Triptycene is widely used as a constituent in PIMs due to its rigidity and high intramolecular free volume (Fig.  13 ) [ 9 , 18 ]. McKeown et al. reported dioxane-forming ladder polymerization using tetrafluoroterephthalonitrile ( 199 ) and 2,3,6,7-tetrahydroxybenzotriptycene with very bulky substituents ( 139 ) [ 69 , 70 ]. The resulting poly-17 was reported to exhibit highly selective gas permeability, significantly outperforming the 2008 Robeson upper bounds for O 2 /N 2 , H 2 /N 2 , CO 2 /N 2 , H 2 /CH 4 and CO 2 /CH 4 selectivities. Using a similar polymerization method, Guo et al. reported the synthesis of a ladder-type polymer ( poly-18 ) with pentiptycene in the main chain from a tetrahydroxylated pentiptycene ( 76 ), a tetramethyl spirobindan tetraol ( 200 ) and 201 [ 55 ]. High CO 2 /CH 4 permselectivity was reported for poly-18 . McKeown et al. also reported ladder polymerizations by the reaction of dimethoxymethane with 2,6-diaminotriptycene ( 8 ) and 2,6-diaminobenzotriptycene ( 125 ) to yield Tröger’s base-containing PIMs poly-19 [ 94 ] and poly-20 [ 66 ], respectively. Condensation of a trip-monomer with two hydroxy groups and two amino groups ( 38 ) and 4,4’-(hexafluoroisopropylidene)diphthalic anhydride ( 201 ) in the presence of isoquinoline yielded polyimide poly-21 , which, after thermal treatment, was transformed into thermally rearranged polybenzoxazole poly-22 [ 36 ]. Li, Yi, Ma et al. reported that a polyimide membrane ( poly-23 ) obtained from 2,6-diaminomonomer ( 39 ) with ortho -bromo groups and pyromellitic anhydride ( 202 ) showed ca . 8 times greater gas permeability than the corresponding polyimide devoid of bromo groups [ 38 ]. Moreover, when the poly-23 membrane was heated to 550 °C, debromination occurred yielding a carbon molecular-sieve membrane, which displayed an almost 9-fold increase in gas permeability while maintaining permselectivity, with CO 2 /N 2 and CO 2 /CH 4 selectivities of 29.2 and 30.9, respectively, and an unprecedentedly high CO 2 permeability coefficient of 20639 barrer [ 38 ].

Triptycene-containing PIMs useful in gas-separation membranes

Gong, McKeown et al. developed a triptycene-containing polyimide poly-24 with a hydroquinone moiety by reacting 13,16-dihydroxy-1,3,6,8-tetramethyl-2,7-diaminotriptycene ( 158 ) with pyromellitic dianhydride ( 202 ) in the presence of isoquinoline (Fig.  14 ) [ 75 ]. This polymer has a rigid and randomly contorted main-chain structure in which the rotation of the diimide unit is inhibited by the methyl groups on the triptycene moiety. Accordingly, poly-24 shows excellent solubility in aprotic polar solvents and can be easily cast into freestanding films. Upon treatment with CAN, poly-24 is oxidized to form the para -benzoquinone derivative poly-25 , which undergoes a four-step redox process with excellent reversibility (Fig.  14 ). Lithium-ion battery cells using easily solution-processable poly-24 as a component of the cathode material have been reported to exhibit stable cycling performance [ 75 ].

Triptycene-containing redox-active PIMs useful as electrodes for lithium-ion batteries

Despite the fact that many trip-monomers have chirality, there are only a few examples of optically active triptycene polymers [ 7 , 31 , 32 ]. Examples of the synthesis of such systems are shown in Fig.  15 . The polycondensation of optically active 2,6-diaminotriptycene ( 8 ), which can be obtained by means of chiral HPLC, with acid anhydride 4,4’-(hexafluoroisopropylidene)diphthalic anhydride ( 201 ) in the presence of isoquinoline yields triptycene-containing polyimide poly-27 [ 31 ]. This polymer serves as an optically active PIM, and its freestanding film can be used for enantioselective membrane separation. Chen et al. synthesized a chiral triptycene with dihydroacridine blades ( 203 ) from optically active 8 [ 95 ]. Compound 203 was then reacted with bis(4-bromophenyl)sulfone ( 204 ) in the presence of a palladium catalyst to produce optically active poly-28 . This polymer shows thermally activated delayed fluorescence (TADF) properties, where the triptycene and sulfone moieties serve as electron donors and acceptors, respectively, and emits circularly polarized luminescence (CPL) with a dissymmetry factor ( g lum ) on the order of 10 –3 [ 95 ]. Using poly-28 , circularly polarized organic light-emitting diodes (OLEDs) were successfully fabricated by means of solution processing. Ikai et al. synthesized optically pure 2,6-diiodotriptycene ( 18 ) from the corresponding optical isomer of 8 [ 96 ]. This was then used in a Suzuki coupling polymerization with ethynyl-substituted diboronic ester ( 205 ) to yield optically active poly-29 . Upon treatment with trifluoroacetic acid, poly-29 undergoes a geometrically selective Friedel–Crafts reaction, yielding the ladder polymer poly-30 , whose conformation is fixed in a one-handed helical structure [ 96 ]. It has been reported that poly-30 exhibits CPL and can be used as a stationary phase for chiral column chromatography.

Optically active triptycene-containing polymers

Network polymers

Triptycene can be easily tri- and hexa-functionalized by Friedel–Crafts-type reactions (Fig.  3 ). This approach is also useful for the synthesis of triptycene-containing network polymers; however, the resulting polymers are usually insoluble and devoid of regular structures. Many examples of this type of polymer have been described in a recent review by C-F. Chen et al. [ 8 ], so only selected examples are presented here (Fig.  16 ). McKeown et al. reported that porous 3D network polymers ( poly-31 ) with a Brunauer–Emmett–Teller (BET) specific surface area (S BET ) of 1750 m 2 /g can be obtained by the reaction of unsubstituted triptycene ( 1 ) with dichloromethane in the presence of aluminum trichloride (Fig.  16a , top) [ 97 ]. Furthermore, poly-31 can be easily functionalized with nitro ( poly-32 ), amino ( poly-33 ), or sulfo ( poly-34 ) groups [ 98 ]. McKeown et al. synthesized a Tröger’s base-containing network polymer ( poly-35 ) by the reaction of 2,6,14- or 2,7,14-triaminotriptycene ( 10, 11 ) with dimethoxymethane in the presence of trifluoroacetic acid (Fig.  16a , middle). They found that poly-35 serves as a base catalyst for the Knoevenagel condensation of benzaldehyde and malonitrile [ 99 ]. Moreover, the use of various diamines for copolymerization with triaminotoriptycene allows for tuning of the pore size and catalytic activity of the resulting network polymers [ 100 ]. Baek et al. reported a very unique solid-state polymerization of triptycene derivatives [ 45 ]. They found that 2,3,6,7,14,15-hexaethynyltriptycene ( 48 ) forms stable crystalline materials that incorporate acetone and water molecules, but rapid heating causes desorption of the solvent. This triggers Masamune–Bergman cyclization of the ortho -diethynyl moiety to generate highly reactive radicals, leading to explosive reactions and the formation of poly-36 with a porous structure (Fig.  16b, c ).

a Selected examples of triptycene-containing nonregular network polymers. b , c Explosive reactions of 48 forming poly-36 triggered by solvent desorption. b , c Adapted under terms of the CC-BY 4.0 license [ 45 ] Copyright 2017, Springer Nature

Porous 2D polymers

Triptycene derivatives that undergo porous 2D hexagonal packing typically have structures with blades laterally extended through the 2,3-, 6,7- and 14,15-positions (Fig.  1 ). In 2014, King et al. reported a triptycene derivative ( 206 ) with photoreactive tetrafluoroanthraceno blades (Fig.  17a , left and center) [ 101 ]. This extended triptycene derivative forms porous hexagonal packing in the crystalline state, with a configuration in which the tetrafluoroanthraceno blades are intermolecularly π-stacked (Fig.  17b , left). Then, light irradiation was used to induce [4+4] cycloadditions between the blades, with photoirradiation (460 nm) of a single-crystal sample at 223 K leading to dimerization ( 207 ) (Fig.  17b , center) and further photoirradiation at 400 nm forming a 2D polymer ( poly-37 ) (Fig.  17b , right). This two-step photochemical reaction proceeds in a single-crystal-to-single-crystal manner. It has been reported that crystalline samples of this 2D polymer can be exfoliated to monolayer sheets by heating to 50 °C in N -methyl-2-pyrrolidone (NMP). Before this study, the same group reported in 2013 that light irradiation of single crystals of a derivative with nonfluorinated anthraceno blades ( 208 ) produced a 2D polymer (Fig.  17a , right) [ 102 ]. However, the photopolymerization of 208 was reported to be “not single-crystal to single-crystal transformation”. In 2021, Lackinger et al. reported that vacuum deposition of 206 on a hexacosane-passivated graphite substrate followed by thermal annealing produced a crystalline monolayer film (with a domain size of up to 500 nm) consisting of a porous hexagonal lattice similar to that of a single crystal [ 103 ]. When this monolayer sample is irradiated with light using a high-power LED, on-surface photocycloadditions proceed, yielding a 2D polymer.

a Molecular structures of 206 and 208 , which feature photoreactive tetrafluoroanthraceno and anthraceno blades, respectively. b Schematic illustrations of the two-step single-crystal-to-single-crystal transformations of 206 into its 2D polymer poly-37

Based on a similar molecular design, King et al. reported the formation of Langmuir–Blodgett (LB) films at the air/water interface using triptycene derivative 209 , which contains a hydrophilic diethylene glycol moiety at the bridgehead position that serves as an anchor for the water layer (Fig.  18a ) [ 104 ]. A chloroform/hexane solution of 209 was spread on the air/water interface and compressed at 1 °C using an LB trough, and the mean molecular area (MMA) was calculated from the compression isotherm. Phase changes were observed at approximately 155–135 and 80 Å 2 , and the former MMA was assigned to porous hexagonal p 6 packing. STM measurements of a LB film of 209 transferred onto a HOPG substrate show a porous structure that is consistent with the simulated pattern (Fig.  18b ). Upon irradiation with 365 nm light, the LB film of 209 underwent photopolymerization, and the obtained thin-film polymer was found to be mechanically hard enough to deform a paper Wilhelmy plate during the Langmuir experiment.

a Formation of a LB film composed of amphiphilic propeller-shaped 209 , which carries a diethylene glycol chain that anchors the molecule to the water surface. b STM image (left) and simulated p 6 lattice of 209 (right). c Structures of a series of amphiphilic propeller-shaped molecules ( 210, 211, and 212 ) designed for the formation of structurally well-defined LB films and their crosslinked 2D polymers. d Schematic illustration of plasmon-induced [4+4]-cycloaddition/2D polymerization of LB films 210 and 211 . b Adapted with permission [ 104 ] Copyright 2015, American Chemical Society. Adapted under terms of the CC-BY 4.0 license [ 107 ] Copyright 2021, Springer Nature

Similar to 209 , several amphiphilic extended triptycene derivatives have been reported to form highly ordered LB films with porous hexagonal structures ( 210 [ 105 ], 211 [ 106 ] and 212 [ 106 ], Fig.  18c ). Schlüter et al. reported the synthesis of 2D polymers using 211, 212 and a 1:1 mixture of 211 and 212 using the LB technique and light irradiation [ 106 ]. After being transferred onto a Au(111) substrate, the obtained 2D polymers were characterized by tip-enhanced Raman spectroscopy (TERS), which allowed for the evaluation of the conversion number ( X ) of the crosslinked structures (i.e., anthracene dimer moieties). The results showed high conversion numbers, with averages of X  = 87.7 ± 1.8 ( 211 ), 92.0 ± 1.7 ( 212 ), and 94.1 ± 2.1% (copolymer). Based on these values, along with the random-growth model, the calculated crystallinities of the 2D polymers were 95.8 ± 1.2, 98.2 ± 1.1 and 99.0 ± 1.4% for 211, 212 and 211/212 (1/1), respectively. In 2021, Shao, Lan, Zenobi, and coworkers performed TERS measurements on LB films of 210 and 211 transferred onto Au(111) substrates using silver chips and found that visible laser irradiation (633 nm) caused intermolecular [4+4] cycloadditions between the blades in the 2D assembly as plasmon-induced chemical reactions (PICRs) (Fig.  18d ) [ 107 ].

Nonporous 2D assemblies and polymers

Triptycene derivatives that form dense and nonporous 2D hexagonal packing are characterized by structures in which substituents are introduced at either the 1,8,13- or 4,5,16-positions (Fig.  1 ). In derivatives with these substitution patterns, the free volume around the triptycene backbone can be retained, which is favorable for the formation of 2D nested packing. In fact, most triptycene derivatives reported to form nonporous 2D hexagonal structures in crystals or in the liquid-crystalline state have these substitution patterns [ 87 , 108 ]. It has been found that 1,8,13-substituted triptycene derivatives can exhibit 2D assembly ability even when another substituent is introduced at the bridgehead position ( vide infra ). Note that unsubstituted triptycene ( 1 ) does not form nested packing in the crystal or on the substrate surface [ 109 , 110 ]. Therefore, it is essential to choose appropriate substituents and substitution patterns to realize dense 2D hexagonal assemblies.

With the aim of constructing large-area, highly ordered organic thin films through molecular self-assembly, our group developed a tripodal triptycene ( 213 ) with long-chain alkoxy groups introduced at the 1,8,13-positions (Fig.  19a ) [ 11 ]. In the bulk state, this compound forms a “2D nested hexagonal packing + 1D layer stacking” structure (Fig.  19b ). The assembly of 213 on a solid substrate yields oriented thin films with 2D hexagonal sheets stacked parallel to the substrate (Fig.  19b ) on a variety of inorganic and polymer substrates simply by, e.g., spin-coating or vacuum deposition [ 11 , 111 , 112 ]. Importantly, in thin films of 213 , the ordered structure formed on nanometer-length scales can propagate to macroscopic length scales. For example, in through-view XRD measurements of a vacuum-deposited film of 213 (50 nm in thickness) formed on a sapphire substrate (2.0 cm in diameter), the in-plane azimuthal angle dependence of the peak intensity originating from the 2D hexagonal lattice does not change when the measurement position on the film is changed [ 11 ]. This observation indicates that the orientation of the 2D hexagonal lattice is aligned across the entire film.

a Molecular structures of 1,8(,13)-substituted triptycene derivatives 213 – 215 having alkoxy side chains. b Schematic illustrations of a “2D nested hexagonal packing + 1D layer stacking” structure formed by the self-assembly of 213 – 215

Derivatives with different alkoxy chain lengths and those with only two or one long-chain alkoxy group also form 2D+1D structures similar to that of 213 , whereas their thermal stability largely depends on the triptycene substitution pattern (Fig.  19a ) [ 11 , 113 ]. For example, the 2D+1D assembly of 1,8-bis(dodecyloxy)tripycene 214 (m.p. 134 °C) shows significantly lower thermal stability than that of 213 (m.p. 211 °C). However, the assembly of 1,8-bis(dodecyloxy)-13-methoxytriptycene 215 , in which one of the long-chain alkoxy groups in 213 is replaced by a methoxy group, exhibits greater relative thermal stability (m.p. 231 °C). These differences are directly reflected in the properties of the soft materials incorporating the triptycene units ( vide infra ).

Tripodal triptycenes are also useful as components in organic electronic devices [ 113 , 114 , 115 , 116 , 117 ]. We have found that the performance of flexible organic thin-film transistor devices can be significantly improved by forming several layers of 215 on the surface of a parylene-based polymer dielectric layer [ 114 , 115 , 116 , 117 ]. This triptycene-based surface modifier covers structural defects on the surface of the polymer substrate and lowers and homogenizes the surface energy. This leads to improved crystallinity of the organic semiconductor on the dielectric layer and overall performance of the transistors. Although SAMs can be used for the surface coating of inorganic substrates, they cannot be applied to polymer substrate surfaces. Tripodal triptycenes, which can form highly ordered and perfectly oriented films regardless of substrate type, provide a powerful tool for improving the performance of organic electronic devices.

Importantly, tripodal triptycenes exhibit excellent 2D assembly ability even when various functional groups are incorporated [ 84 , 86 ]. For example, compound 216 , in which a sterically bulky spherical C 60 is introduced via an ethynyl group at the bridgehead position, as well as its precursor 217 with a terminal ethynyl group (Fig.  20a ), also forms 2D+1D structures in the bulk state [ 84 ]. The spin-coated thin films of 216 are oriented similarly to those of 215 , with a 2D+1D structure consisting of 2D layers of densely arranged C 60 units (Fig.  20b ). Time-resolved microwave conductivity (TRMC) measurements of these thin films revealed the anisotropic conduction of photocarriers in the in-plane direction. Thus, tripodal triptycenes are useful supramolecular scaffolds [ 118 ] that promote the 2D assembly of various functional groups and molecular units, making it possible to improve their anisotropic properties. Face-selective introduction of an azide group and three terminal ethynyl groups into a tripodal triptycene ( 183 ) also leads to assembly into a 2D+1D structure both in the bulk state and on solid substrates (Fig.  20c ) [ 86 ]. When this assembly is heated, intermolecular uncatalyzed Huisgen cycloadditions maintain the assembly structure. The resulting polymer is insoluble and possibly composed of a highly entangled polycatenane-like cyclic structure (Fig.  20c ). The solid-phase polymerization of the oriented film of 183 proceeded while retaining the surface nanoterraced structure.

a Molecular structure of C 60 -appended tripodal triptycene 216 and its precursor 217 . b Schematic illustrations of the 2D+1D structure formed by the self-assembly of 216 . c Molecular structure (left) and schematic assembly structures of 183 before and after catalyst-free thermal Huisgen cycloaddition

The incorporation of 1,8,(13)-substituted triptycenes into polymers induces long-range ordered structures through 2D assembly of the triptycene units, greatly enhancing the mechanical properties of the polymers [ 77 , 78 , 79 , 80 ]. Such specific 2D assembly can be achieved by incorporating the triptycene units at various sites of polymers, including at both ends [ 80 , 81 ], in the main chain [ 77 ], side chains [ 78 ] and at various branch points [ 79 ]. Figure  21a shows the chemical structures of poly(dimethylsiloxane) (PDMS) derivatives ( poly-38 – poly-40 ) with 1,8(,13)-triptycene units at both ends ( poly-38 and poly-39 ) along with a 1,4-substituted derivative ( poly-40 ). For example, hydrosilylation of olefin-appended 1,8-substituted triptycene 171 (Fig.  8 ) with hydride-terminated PDMS (PDMS-H, M n = 18 or 24 kDa) affords poly-38 [ 80 ]. In sharp contrast to liquid PDMS-H, the telechelic polymer poly-38 forms a highly viscous solid that exhibits birefringence (Fig.  21b ). Rheological measurements revealed that the complex viscosity of poly-38 (approximately 10 5  Pa·s) is 10 4 times greater than that of PDMS-H (approximately 10 1  Pa·s). Small- and wide-angle XRD measurements revealed that poly-38 forms a 2D+1D structure with long layer spacings of 18–20 nm, in which 2D sheets of terminal triptycene units are stacked one-dimensionally via PDMS domains (Fig.  21c ). Telechelic poly - 39 synthesized from 1,8,13-substituted triptycene 172 (Fig.  8 ) also forms a 2D+1D structure but has an even greater mechanical strength than poly-38 and behaves as a thermoplastic, resulting in a freestanding film without any covalent cross-linking (Fig.  21b ) [ 81 ]. It has also been shown that poly-39 films exhibit self-healing properties. The only structural difference between poly-39 and poly-38 is the presence or absence of a methoxy group at the 13-position of the terminal triptycene units, and it is thus surprising that such a tiny change in substituent relative to the entire polymer has a significant impact on the mechanical and thermal properties. Moreover, the structural and physical properties of PDMS are largely unchanged in poly-40 (Fig.  21a, b , right), where 1,4-substituted triptycene units without 2D assembly ability are introduced at both ends of the PDMS.

a 1,8(,13)-Substituted triptycene-containing telechelic polymers poly-38 and poly-39 and 1,4-substituted triptycene-containing poly-40 . b Photographs of bulk samples of PDMS-H and the telechelic polymers. c Schematic illustrations of the assembly structures of the telechelic polymers

Ring-closing metathesis of 1,8-olefin-appended 168 (Fig.  8 ) yields macrocyclic olefin monomer 218 , which has been used for the synthesis of main chain-type triptycene-containing polymers (Fig.  22 ) [ 77 ]. We synthesized the homopolymer poly-41 by ring-opening metathesis polymerization of 218 and the copolymer poly-42 by copolymerization of 218 with cyclooctene [ 77 ]. Both poly-41 and poly-42 form 2D hexagonal sheet structures with triptycene units and lamellar structures filled with polymer chain domains, which leads to a marked improvement in the mechanical properties of the polymers. In poly-43 without 2D assembly ability, no improvement in physical properties was observed [ 77 ]. While the triptycene polymers poly-44 and poly-45 , which have oligosiloxane- and ester-based main chains, respectively, also form 2D+1D structures, the urethane-containing poly-46 does not (Fig.  22 ) [ 77 ]. Presumably, the hydrogen bonding ability of the urethane group may be superior to the assembling ability of the triptycene, hampering structural ordering of the polymer.

Polymers incorporating 1,8-substituted triptycene units ( poly-41, poly-42, poly-44 – poly-46 ) in the main chains with a 1,4-substituted unit ( poly-43 )

We synthesized diblock ( poly-47 ), ABA triblock ( poly-48 ) and random ( poly-49 ) copolymers by reversible addition-fragmentation chain-transfer (RAFT) polymerization of 1-acryloyl-8-methoxytriptycene 169 (Fig.  8 ) and n-butyl acrylate (Fig.  23a ) [ 78 ]. The thermal stability of these triptycene-containing copolymers is comparable (2% weight loss temperature: ca . 300 °C) to that of the precursor polymer ( poly-50 ) without triptycene units. However, while poly( n -butyl acrylate) derivatives, including poly-50 , are liquids, the copolymers are all viscous solids. For diblock poly-47 , both the melting ( T m ) and crystallization ( T c ) temperatures shift toward higher values with increasing triptycene content. While their triptycene contents are comparable, diblock poly-47 has slightly greater T m and T c values than triblock poly-48 , whereas the enthalpy changes associated with the phase transitions are in the same range. XRD measurements revealed that both diblock and triblock copolymers form 2D+1D structures with long-range ordering. In contrast, the random copolymer poly-49 does not form a 2D+1D structure and shows no clear phase transition. It is worth noting that rheological measurements show that diblock poly-47 and triblock poly-48 exhibit complex viscosities 10 4 –10 5 times greater than that of random poly-49 over a wide temperature range. Therefore, in the design of side-chain-type triptycene polymers, the presence of block segments is essential for achieving long-range ordering. Accordingly, we designed poly-51 with triptycenes at both ends of a poly( N -isopropylacrylamide) (PNIPAM) segment (Fig.  23b ) [ 78 ]. This triblock copolymer yields hydrogels (60 wt% water content) without chemical cross-linking due to the formation of a 2D+1D structure with a 2D triptycene sheet and a hydrated PNIPAM domain. The hydrogel became turbid at 60 °C, and the PNIPAM domain exhibited a lower critical solution temperature (LCST) upon increasing the temperature. VT-XRD measurements of the hydrogel revealed that the interlayer distances decreased with increasing temperature while maintaining the 2D structure. These findings are expected to be applied to the future development of soft actuators that exhibit anisotropic motion in response to stimuli.

a Molecular and schematic structures of diblock poly-47 , ABA triblock poly-48, and random poly-49 carrying 1,8-substituted triptycene-based side chains as well as poly-50 , a precursor of poly-48 . b Molecular structure of ABA triblock polymer poly-51 with a PINIPAM segment and ( c ) photographs of its hydrogel. c Adapted with permission [ 78 ] Copyright 2023, American Chemical Society

Conclusions

As outlined in this review, triptycene is a versatile scaffold that can be exploited by introducing a number of substituents in various patterns at its blades and bridgehead positions. It is also possible to impart characteristic redox properties to the blade sites. One of the main topics in the development of triptycene-based functional materials, diverse polymers, and molecular assemblies is the design of nanospaces (microporosity) in the assembly state, which is being explored for use in guest recognition, material transport, separation, and catalysis. Rigid three-bladed propeller molecules are suitable for forming porous or nonporous 2D lattices, and they provide excellent building blocks for forming 2D sheet structures that are not easily accessible. Their ability to form dense 2D sheets stems from their characteristic space-filling structure, which enables precise control over the structures and properties of surfaces and interfaces. It is expected that in the future, the development of materials for the realization of unique electronic states that reflect the two-dimensional nature and topology of assemblies will accelerate, leading to new functionalities. From various perspectives, triptycenes will continue to drive the development of functional materials that cannot be realized using planar molecules. Organic synthesis technologies will be the driving force to enable the precise design of such triptycene derivatives.

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Acknowledgements

This work was supported by JSPS KAKENHI (JP21H05024 and JP21H04690 for TF) and a Grant-in-Aid for Transformative Research Areas (A) “Condensed Conjugation” (JP20H05868 for TF). This work was also supported in part by the Research Program of “Five-Star Alliance” in “NJRC Mater. & Dev.”.

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Fumitaka Ishiwari

Present address: Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka, 565-0871, Japan

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Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, 226-8501, Japan

Fumitaka Ishiwari, Yoshiaki Shoji, Colin J. Martin & Takanori Fukushima

Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, 226-8501, Japan

Fumitaka Ishiwari, Yoshiaki Shoji & Takanori Fukushima

Research Center for Autonomous Systems Materialogy (ASMat), Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, 226-8501, Japan

Yoshiaki Shoji & Takanori Fukushima

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Ishiwari, F., Shoji, Y., Martin, C.J. et al. Recent advances in structurally elaborate triptycenes, triptycene-containing polymers and assemblies: structures, functions and applications. Polym J (2024). https://doi.org/10.1038/s41428-024-00920-x

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Received : 29 March 2024

Revised : 24 April 2024

Accepted : 29 April 2024

Published : 29 May 2024

DOI : https://doi.org/10.1038/s41428-024-00920-x

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