Band structure engineering for low band gap polymers based on 3-hexylthiophene and thienopyrazine

Four new low band conjugated copolymers, named as P1-P4, consisting of electron-donating units (3-hexylthiophene, 3HT) and electron-accepting units (2,3-diethylthieno[3,4-b]pyrazine, ETP) in different molar ratios (from 1:1 to 4:1); as well as a conjugated copolymer comprising of 9,9'-dioctylfluorene, 3HT and ETP (denoted as PBF) were synthesized via Suzuki coupling. The chemical structures, molecular weights, thermal stability as well as photophysical, electrochemical and photovoltaic properties were studied. All polymers exhibited bimodal optical absorptions and low optical band gaps. Theoretical calculations based on density function theory with the B3LYP functional and the 6-31G* basis set were performed to retrieve the information of geometric structures, electronic energy levels and molecular orbitals of these copolymers. It is found that the incorporation of ETP would significantly lower the LUMO and slightly elevate the HOMO. The break of coplanarity along the main chain had a strong effect on HOMO but appeared trivial to LUMO in this system.