Alternating narrow band gap copolymers derived from indeno[1,2-b]fluorene and thiophene-cored-thieno[3,4-b]pyrazine derivatives-Synthesis, characterization and comparative studies of photochemical stability

Alternating narrow band gap (NBG) conjugated polymers derived from 6,6′,12,12′-tetraoctylindeno[1,2-b]fluorene (IF) and 2,3-dimethyl-5,7-dithien-2-yl-thieno[3,4-b]pyrazine (DTTP), 2,3-diphenyl-5,7- dithien-2-yl-thieno[3,4-b]pyrazine (DPTP) or 2,3-dioctyl-5,7-dithien-2-yl- thieno[3,4-b]pyrazine (DOTP), named as PIF-DTTP, PIF-DPTP, and PIF-DOTP, respectively, were synthesized by Suzuki coupling reaction and characterized. The photochemical stabilities of the copolymers and copolymer derived from IF and 5,7-dithien-2-yl-thieno[3,4-b]pyrazine (DTP) were investigated by the UV absorptions, PL spectra, FT-IR spectra, and photovoltaic properties of the copolymers as a function of UV irradiation time. The studies revealed that the degradation of thieno[3,4-b]pyrazine (TP) ring under UV irradiation can be retarded or eliminated by introducing phenyl group into the 2,3-positions of TP ring, and indicated that 2,3-diphenylthieno[3,4-b]pyrazine could be used as durable electron deficient moiety to achieve donor-acceptor NBG-conjugated polymers.