Benzodithiophene–Thiadiazoloquinoxaline as an Acceptor for Ambipolar Copolymers with Deep LUMO Level and Distinct Linkage Pattern

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Abstract

Two new conjugated copolymers, PBDTTQ-1 and PBDTTQ-2, with a distinct linked pattern between benzodithiophene–thiadiazoloquinoxaline (BDTTQ) as acceptor and bithiophene as donor were synthesized and characterized. The difference in the linkage between donor and acceptor exerts great influence on the optoelectronic properties of the two polymers. The optical band gap decreases from 1.18 eV for PBDTTQ-1 to 1.03 eV for PBDTTQ-2, due to the lower LUMO energy level (−4.01 eV) of the latter. Moreover, density functional theory calculations demonstrate that the electron density is mainly confined on the acceptor unit in both HOMO and LUMO of PBDTTQ-1, while the electronic densities almost delocalize along the entire backbone of PBDTTQ-2, which facilitates the charge transport within the polymer chain. In contrast to PBDTTQ-1 missing any field-effect characteristics, PBDTTQ-2 exhibits ambipolar charge transporting behavior with mobilities of 1.2 × 10–3 cm2/(V s) for holes and 6.0 × 10–4 cm2/(V s) for electrons.

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