Three new conjugated polymers based on benzo[2,1-b:3,4-b′]dithiophene: synthesis, characterization, photoinduced charge transfer and theoretical calculation studies

Three new conjugated polymers have been designed and synthesized via the alternate copolymerization of the electron-donating monomer benzodithiophene (BDT) and three different electron-accepting monomers: 5,8-dibromoquinoxaline, 5,8-dibromo-2,3-diphenylquinoxaline, 10,13-dibromodibenzo[a,c]phenazine. The synthesized polymers show good solubility in common organic solvents as well as broader absorptions in the visible region and narrower optical band gaps compared to homopolymers made from BDT units. It is found that the absorptions of the copolymers are red-shifted by increasing the electron-withdrawing ability of the co-monomer. In particular, the absorption edge of the P3 film extends to 778 nm, whereas that of the P1 film is only at 636 nm. Theoretical calculation studies of structure-property relations indicated that polymer chains with better coplanarity or pi-pi* delocalization will lead to lower band gaps. Photoinduced charge transfer (PCT) studies disclosed that exciton dissociation of photoexcited PX (X = 1, 2, 3) via electron-transfer may occur at the interface with CHL-C-60, enabling the photogeneration of electrons and holes. The P3/CHL-C-60 blended film was chosen as a representative sample to test the applicability of the polymer P3 for energy conversion, a 1.76% PCE was finally obtained.