Synthesis and Characterization of Isoindigo[7,6-g]isoindigo-Based Donor–Acceptor Conjugated Polymers

Four donor (D)–acceptor (A) conjugated polymers with isoindigo[7,6-g]isoindigo ([3E,8E]-3,8-bis(2-oxoindolin-3-ylidene)-6,8-dihydroindolo[7,6-g]indole-2,7(1H,3H)-dione, DIID) as A-unit and thiophene derivatives as D-units were synthesized by Stille polycondensation. Optical and electrochemical properties of the polymers were studied by UV–vis–NIR absorption spectrometer and cyclic voltammetry. Compared with isoindigo-based analogues, the polymers display much broader absorption spectra (covering 400–950 nm) and remarkably lower bandgaps (ca. 1.3 eV). All polymers showed ambipolar transport properties as evaluated by bottom-gate/top-contact (BGTC) and top-gate/bottom-contact (TGBC) organic thin film transistors (OTFTs) in air. Gate-voltage-dependent hole mobility (μh) was observed for BGTC devices, while the mobility of TGBC devices exhibited weak gate-voltage dependence. P3 delivered the best device performance. At the optimized thermal annealing temperature (200 °C), a μh of 1.79 cm2/(V s) and an electron mobility (μe) of 0.087 cm2/(V s) were demonstrated with BGTC devices, and μh calculated from the higher VGS region is decreased to 0.35 cm2/(V s). The relatively balanced hole and electron mobilities were observed for TGBC devices based on P3, which were 0.45 and 0.16 cm2/(V s), respectively.