Molecular design for tuning electronic structure of π-conjugated polymers containing fused dithienobenzimidazole units

Three dithienobenzimidazole derivative monomers (M1, M2, and M3) were prepared, where M3 was obtained by the oxidation of M1 and identified by the X-ray crystallographic analysis. pi-Conjugated homopolymers (P1-0, P2-0, and P3-0) and copolymers (P1-2, P3-1, and P3-2) were synthesized by the palladium-catalyzed coupling polymerizations of M1, M2, and M3. The absorption spectra of the reference compounds (fused R1 and non-fused R2), in conjunction with the optimized ground state structure, certified the importance of the fused dithienobenzimidazole skeleton to increase the effective conjugation length of the polymers. On the basis of the absorption and emission spectra of the pi-conjugated polymers in CHCl3, the influence of the thiophene-S, S-dioxide as well as the comonomer structure were investigated to find out that P3-0 and P3-2 exhibited peak maxima at the relatively longer wavelength region due to the donor-acceptor interaction. In addition, the protonation of the imidazole imine group further tuned the optical properties of the it-conjugated polymers by promoting the charge transfer interaction along the polymer main chain, which was supported by the theoretical calculations in detail. (C) 2016 Elsevier Ltd. All rights reserved.