Imide-Functionalized Thiazole-Based Polymer Semiconductors: Synthesis, Structure–Property Correlations, Charge Carrier Polarity, and Thin-Film Transistor Performance

Imide-functionalized arenes, exemplified by naphthalene diimides (NDIs), perylene diimides (PDIs), and bithiophene imides (BTIs), are the most promising building blocks for constructing high-performance n-type polymers. In order to reduce the steric hindrance associated with NDI- and PDI-based polymers and to address the high-lying LUMO issue of BTI-based polymers, herein a highly electron-deficient imide-functionalized bithiazole, N -alkyl-5,5′-bithiazole-4,4′-dicarboximide ( BTzI ), was successfully synthesized via an efficient C–H activation. Single crystal of BTzI model compound showed a planar backbone with close π-stacking distances (3.2–3.3 Å). The N, N ′-bis(2-alkyl)-2,2′-bithiazolethienyl-4,4′,10,10′-tetracarboxdiimide ( DTzTI ) was also used for constructing polymer semiconductors. Compared to DTzTI, BTzI is more electron-deficient, rendering it highly appealing for enabling n-type polymers. On the basis of BTzI and DTzTI, a series of polymers, including acceptor–acceptor homopolymers, and donor–acceptor and donor–acceptor–acceptor copolymers, were synthesized, which feature different contents of acceptor units in polymeric backbones. As imide content increases, the polymer FMO levels were gradually lowered, yielding a transition of charge carrier from ambipolarity to unipolar n-type in organic thin-film transistors (OTFTs). The acceptor–acceptor homopolymer PBTzI possesses the deepest LUMO/HOMO level of −3.94/-6.17 eV, enabling minimal off-current ( I off ) of 10 –10 –10 –11 A in OTFTs. The highest electron mobility of 1.61 cm 2 V –1 s –1 accompanied by small I off of 10 –10 –10 –11 A and high on-current/off-current ratio ( I on / I off ) of 10 7 –10 8 was achieved from OTFTs using PDTzTI homopolymer, showing the pronounced advantages of acceptor-acceptor homopolymer approach for developing unipolar n-type polymer semiconductors. The correlations between the FMO levels and the transistor performances underscore the significance of FMO tuning for enabling unipolar electron transport. The results demonstrate that imide-functionalized thiazoles are excellent units for constructing high-performance n-type polymers. Moreover, the synthetic routes to these highly electron-deficient imide-functionalized thiazoles and the polymer structure–property correlations developed here are informative for materials invention in organic electronics.