C−H Arylation Reaction: Atom Efficient and Greener Syntheses of π‑Conjugated Small Molecules and Macromolecules for Organic Electronic Materials Ken Okamoto, †,‡ Junxiang Zhang, § Jeremy B. Housekeeper, ‡,∥ Seth R. Marder,* ,§ and Christine K. Luscombe* ,†,‡ † Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195-2120, United States ‡ Molecular Engineering and Sciences Institute, University of Washington, Seattle, Washington 98195-1652, United States § School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States ∥ Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, United States ABSTRACT: π-Conjugated small molecules, oligomers, and macromolecules are being used in the fabrication of a wide variety of organic electronic devices such as organic field- effect transistors (OFETs), organic photovoltaic (OPV) devices, and organic light-emitting diodes (OLEDs). Efficient syntheses involving fewer steps, fewer toxic reagents, and highly reactive compounds are needed to lower the cost of materials in a manner that is fundamentally more eco-friendly. Additionally, synthetic approaches for π-conjugated materials with more functional group tolerance are desirable to expand the range of properties that can be realized in such materials. Developing new synthetic routes to materials can both broaden the scope of science that can be explored and increase the probability that interesting materials can be developed in an economically viable manner for inclusion in consumer products. One such synthetic strategy that can impact all of these issues is carbon−hydrogen bond activation and subsequent carbon−carbon bond formation (C−H functionalization). While the C−H functionalizations represented by direct arylation-based methods are not as developed as the widely used Stille and Suzuki methods at this stage, they allow for the use of readily accessible halogenated aromatic substances and can negate the need for toxic organotin reagents. They also hold promise of allowing for the synthesis of previously inaccessible materials. In this Perspective, our goal is to provide an overview of the current status in this challenging field by highlighting (1) the history of preparing π-conjugated small molecules and macromolecules via cross-coupling reactions, (2) advances in preparation of versatile π-conjugated small molecules and macromolecules via transition-metal-catalyzed direct arylation, and (3) the scope, limitations, and challenges for materials science. 1. INTRODUCTION Since the seminal work on the conductivity of polyacetylene by Heeger, MacDiarmid, and Shirakawa was published in the 1970s, 1,2 the field of organic electronics has grown exponentially. We have now reached a stage where the quantum efficiencies of OLEDs outperform those of inorganic LEDs, 3,4 the highest charge mobilities obtained for polymers reach 8.5 cm 2 /(V s), 5,6 and OPVs now have a power conversion efficiency (PCE) of 7.6−7.7% (single cell) and 10.6% (tandem cells), which are derived from a low-band-gap polymer bearing BDTT or DTP as donor units. 7−12 The advances made in organic electronics have been driven by the syntheses of π-conjugated molecules with increasingly complex structures. For example, one polymer in the BDTT series, PBDTTT-CF, provides a PCE of 6.8% (National Renewable Energy Laboratory (NREL) certified value) and has an estimated cost of over $400/g, requiring 10 steps to synthesize starting from basic commercial materials. 13 The cost is approximately 25 times greater than that of poly(3-hexylthio- phene-2,5-diyl) (P3HT) synthesis. For organic electronic devices, especially OPVs, to become economically viable, it is important that simple and efficient synthetic strategies are developed to mitigate both the financial and environmental costs associated with the syntheses of organic electronic materials. 13 For this to be achieved in materials science the following challenges should be addressed: (1) develop living polymer- ization techniques such that semiconducting polymers can be synthesized in a reproducible manner with limited defects, (2) develop sequence specific polymerizations that will allow chemists to synthesize semiconducting copolymers in a single step, and (3) develop cross-coupling techniques that eliminate or reduce the use of organometallic reagents. A number of reviews have already been written about the first challenge. 14−16 Sequence specific polymerizations have been addressed by other chemists although not specifically for the synthesis of semiconducting polymers. 17−21 There are a growing number of studies focusing on direct arylation for π-conjugated polymers. Recent reviews have looked at possible mechanisms of the direct arylation polymerization and have highlighted the types of Received: June 10, 2013 Revised: August 7, 2013 Published: August 20, 2013 Perspective pubs.acs.org/Macromolecules © 2013 American Chemical Society 8059 dx.doi.org/10.1021/ma401190r | Macromolecules 2013, 46, 8059−8078