Synthetic Methods in Organic Electronic and Photonic Materials

A Practical Guide

Timothy Parker, Seth Marder

• Publisher: Royal Society of Chemistry
• Year: 2019
• ISBN:
  9781788018142
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Physical & Theoretical Chemistry

With the development of courses on materials synthesis and the need to carry out specific chemical transformations in the laboratory, good practical advice will be needed for those requiring more detail on conjugated materials synthesis. The purpose of this book is to give researchers and students an introduction and reference that efficiently provides general information for each important synthetic method category and a number of examples from the literature to convey practically important variations. It is useful as an outline for advanced organic and materials science courses as well as a good introduction and desk reference for new and experienced researchers in the field.

CHAPTER 1

Introduction

1.1 HISTORICAL PERSPECTIVE

Organic electronics and photonics as a research field encompasses small molecule and polymer theory, materials synthesis, processing, and device design and fabrication. How does one define “organic electronics and photonics”? In the broadest sense, it can be defined as the study of devices and processes that operate through phenomena arising from electrons and/or photons interacting with materials that contain small molecule organics or organic polymers. For the practical purposes of this book, we will define organic electronics and photonics as a collection of technologies that include: (1) organic light emitting diodes (OLED); (2) organic photovoltaics (OPV); (3) dye sensitized solar cells (DSSC); (4) organic field effect transistors (OFET); (5) 2nd order nonlinear optical (NLO) chromophores and electro-optic (EO) polymers; (6) photo-refractive polymers; (7) electrochromic molecules and polymers; (8) two-photon absorption (TPA) molecules and polymers; (9) optical limiting molecules and polymers; and (10) 3rd order NLO polymers. Although this may seem like a long list of technologies to cover in one book, from a synthetic standpoint, this can be done because, fundamentally, the vast majority of organic electronic and photonic materials are π-conjugated molecules and polymers that have properties modified and tuned through organic synthesis. Throughout this book, many of the synthetic methods discussed for conjugated small molecules and polymers will have multiple practical examples cited from a variety organic electronic and photonic literature.

The field of organic electronics and photonics has grown extensively during the past twenty years. The driving force for this growth has been mostly the increasing importance of microelectronics, computers and mobile phones, and high bandwidth telecommunications in our daily lives. Research on the semiconducting properties of organic molecules was conducted as early as the 1910s, and the potential to modify such properties through organic synthesis,¹ as well as applications in “micromodule electronic components (printed and evaporated circuits)”,² have been recognized since at least the 1960s. Perhaps the best way to illustrate the growing research interest and intensity in the field is by examining the number of scholarly articles that are cataloged as either “organic electronics” or “polymer photonics.” In Figure 1.1, the number of articles found on Web of Science covering these topics shows some activity in the 1980s followed by growth during the 1990s and annual growth on average of greater than 20% until 2014. The foundation for much of this growth was largely laid down in the mid-to-late 1980s and 1990s by seminal work in the field including: a small molecule, double layer OLED,³ a conjugated polymer light emitting diode⁴ (PLED), and a phosphorescence-based light emitting diode⁵ (PhOLED); an OPV with a small molecule donor layer and a small molecule acceptor layer⁶ and then a polymer donor–fullerene acceptor bulk heterojunction⁷ (BHJ) OPV; an efficient DSSC fabricated with low-cost processes and medium purity materials;⁸ a polymer OFET,⁹ and an all-polymer OFET fabricated through printing techniques;¹⁰ a poled EO polymer,¹¹ a high EO activity polymer composite,¹² and a low drive voltage (V_π) optical modulator;¹³ the observation of the photorefractive effect in polymer films;¹⁴ a strongly nonlinear optical limiting material;¹⁵ chromophores with large TPA cross section;¹⁶ and more recently molecules with large third order NLO susceptibility (χ³) and low nonlinear losses for all optical switching.¹⁷ Thus, the seeds were sown for the impressive increase in research in organic electronics and photonics during the early part of the 21st century, which itself lays the foundation for further hypotheses and questions. It is strongly recommended that students and researchers read the foundational publications cited above both to appreciate how far the field has advanced and to understand the scientific motivations for new lines of inquiry.

Figure 1.1 Growth of research in the field of organic electronics and photonics over the past 30 years. Seminal work is labeled briefly next to the bar for the year its journal publication appeared (references given in text).

1.2 THE IMPORTANCE OF ORGANIC SYNTHESIS

Organic electronic and photonic materials are typically highly polarizable π-conjugated molecules and polymers. As such, they often have design parameters that may be similar in certain aspects but different in others. For example, for NLO applications, in most cases intramolecular delocalization is highly desirable and in many cases intermolecular interactions are undesirable, as they tend to introduce excited state due to electronic coupling (interactions) between molecules, which can lead to unwanted linear or nonlinear absorptive properties, such as optical loss. In contrast, for organic semiconductors, in many cases intramolecular delocalization is essential and strong intermolecular interactions are desirable so that charges can move efficiently from molecule to molecule, or polymer chain to polymer chain. However, the factors that affect the π-conjugation within organic semiconductors and photonic materials are often fundamentally similar. For example, increased length of the polymer or molecular π-system typically leads to lower energy absorption of light; but the extent of the shift to lower energy depends not only on length but also on the degree to which different building blocks lead to delocalization—or disrupt delocalization—along the π chain axis. As such, one would the not expect the same absorption for similar-length materials that are differently comprised of triple bonds, double bonds, benzene, or thiophene rings. Furthermore, substitution of either the end of the π-system, or one of the backbone units of the π-system, with electron donors and/or acceptors can also further modulate the extent of delocalization along the chain in a dramatic fashion. Concomitant with expected changes in absorption properties induced by the aforementioned structural changes will come changes to the electron affinity and ionization potential, which are closely related to electrochemical reduction and oxidation potentials. Thus, by starting with simple π-conjugated groups one can build up complex and highly polarizable molecules with tunable properties related to electrical applications (such as ionization potential, electron affinity, charge mobility) and tunable optical properties such as the strength and position of the absorption band.

It is difficult to quantify exactly how much a role organic synthesis has played in both small molecule and polymer advances in the field. One way to at least illustrate this is by comparing “then and now” structures of materials in some of the seminal publications mentioned above to those in more recent noteworthy publications (Figure 1.2). The p-channel transport material dihexyl oligo-6-thiophene (1) used for printed OFET in 1994¹⁰ has a thiophene as the only functional unit and straight hexyl chains as solubilizing groups; however, a more recent p-channel polymer¹⁸ (4) has two more structurally complex functional units in the polymer chain (diketopyrrolopyrrole and thienothiophene) while also having two branched solubilizing groups (2-octyldodecyl) for each subunit. Another example is a recent small molecule n-channel transport material (5) used in ink-jet printed OFETs,¹⁹ which also demonstrates increased structural complexity compared to 1 in that there are three functional units in the molecule (the peripheral naphthylene diimides, the core tetrazine, and the bridging thiophenes) instead of one. Another example is the 1995 BHJ OPV⁷ polymer 2 having a single phenyl ring and alkene in the repeating unit compared to a more recent BHJ OPV example (6) that has a bifunctional benzodithiophene-fluorinated thienothiophene²⁰ subunit with multiple branched chain 2-ethylhexyl solubilizing groups and an additional n-octyl ester solubilizing group. A final example would be the 1986 poled EO polymer chromophore¹¹ (3) compared to a chromophore reported in 2000¹³ (7): (1) the “donor” 2-hydroxyethyl aniline moiety in 3 is a protected bis(2-hydroxyethyl) aniline in 7 with t-butyldimethylsilyl (TBDMS) groups used as solubilizing substituents; (2) the “acceptor” 4-nitrophenyl moiety in 3 is a dicyanomethylene cyanofuran in 7; and (3) the alkene “π-bridge” in 3 is a tetraene with a dimethylcyclohexene unit in 7. Although these are just three examples, there are, of course, many other structural variations in the various aspects of organic electronics and photonics that have been explored. Some of these variations have been used to test hypotheses and discover important fundamental principles, and others have been used to map out structure–property relationships in a more Edisonian manner. Regardless of the research approach, organic synthesis has played a central role in moving these materials from promising demonstrations in 1980s and 1990s to the current state of some commercial applications (OLEDs, EO polymers) and continued intense development. Indeed, without organic synthesis, the structural variations required to validate hypotheses or discover new phenomenon would not be possible.

Figure 1.2 Chemical structures of materials appearing in early organic photonic and electronic publications (1–3) and structures appearing in more recent notable publications (4–7).

1.3 INTENT OF THIS BOOK AND RECOMMENDED READING

The materials chemist attempting to design materials for organic electronics and photonics needs to develop knowledge of how chemical and structural properties relate to the electrical and optical properties at both the molecular and materials level. There are numerous books and research papers that can serve as useful starting points to begin to understand the detailed interplay of structure and properties (many of which are still not well understood and so provide a still fertile ground for research), and many researchers involved in the development of organic materials often have the skills to explore structure/property relationships, or the ability to assemble complex molecules and materials; but only a small fraction of the community can do both effectively as the field is strongly interdisciplinary. As such, while we fully acknowledge that everyone cannot be expert in all things, years of experience suggest to us that even non-synthetic chemists involved in studying materials can benefit from having a rudimentary knowledge of how molecules and polymers are assembled, and certainly students and researchers entering the field can profit from having an understanding of the common reactions and strategies employed by organic synthetic chemists to synthesize π-conjugated molecules and polymers with tunable properties. Herein, we have chosen a variety of common building blocks whose structures are found in a diverse set of...