Alkoxy-Functionalized Bithiophene/thiazoles: Versatile Building Blocks for High-Performance Organic and Polymeric Semiconductors

CONSPECTUS: Organic electronics has experienced substantial advances in the past decade, driven by the development of high-performance organic semiconductors (OSCs) in combination with device engineering. While the pursuit of new aromatic building blocks has been a central topic in OSC innovation, the installation of novel side chains is also of significance for accessing high-performance solution-processable OSCs due to their great impact on (macro)molecular conformation/configuration, energy levels, intra/intermolecular interaction, and packing motifs, as well as film morphology of the materials. Compared to tuning the length, branching point, anchoring position, and terminal group of alkyl side chains, alkoxy functionalization can afford multifaceted advantages by modulating the properties of both pi-conjugated main chains and side chain substituents. For instance, the oxygen atom in alkoxy chains not only greatly decreases the steric hindrance between adjacent aromatic rings due to its reduced van der Waal radius (similar to 1.4 angstrom) versus that of a CH2 moiety (similar to 2.0 angstrom) but also induces intramolecular noncovalent interaction for improving backbone coplanarity and charge transport properties. The highly electron-donating alkoxy chains can also greatly facilitate the intramolecular charge transfer (ICT), and hence the resulting semiconductors can yield absorption beyond the infrared region, which is essential for full coverage of solar absorption in photovoltaic devices. Additionally, the high polarity of oligo(ethylene glycol)-type alkoxy side chains can improve the miscibility of the OSCs with molecular dopants and ions, thus enabling them as a key part of the OSCs for emerging applications such as organic thermoelectrics and organic electrochemical transistors.In this Account, we summarize our pioneering and systematic efforts on the rational design, synthesis, and applications of novel alkoxy-functionalized head-to-head (HH)-linked bithiophene/thiazole-based building blocks and related organic/polymeric semiconductors. First, starting with a brief retrospective to the long pursuit of regioregular polythiophenes to avoid HH linkages for accessing highly planar polymers with high mobility, we introduce the basic design guidelines for developing alkoxy-functionalized bithiophene/thiazole-based building blocks via various molecular engineering strategies, including noncovalent interaction incorporation, symmetry-breaking, fluorination, cyanation, esterification, etc.In this part, the merits of HH-linked alkoxy-functionalized bithiophene/thiazoles in constructing high-performance OSCs will be elaborated. Then the principles of designing organic and polymeric semiconductors based on these building blocks toward applications in different optoelectronic devices will be further discussed. Afterward, we present recent examples of alkoxy-functionalized bithiophene/thiazoles-based materials which have delivered state-of-the-art performance in various optoelectronic devices, showing how the judicious structure tailoring of these building blocks can optimize the materials properties and device performance. Finally, we offer our insights into the further development of alkoxy-functionalized bithiophene/thiazoles-based derivatives and semiconductors for high-performance optoelectronic devices.