Trifluoromethylated thieno[3,4-b]thiophene-2-ethyl carboxylate as a building block for conjugated polymers

Two new conjugated polymers based on trifluoromethylated thieno[3,4-b]thiophene-2-ethyl carboxylate have been synthesized. The incorporation of a trifluoromethyl group at the 3-position of a thieno[3,4-b]thiophene-2-ethyl carboxylate unit has shown pronounced steric and electronic effects and significantly altered the optical and electrochemical properties of polymers. Trifluoromethylation reduces both the HOMO and the LUMO levels of new polymers compared with their non-trifluoromethylated counterparts. A relatively large trifluoromethyl group on the 3-position of thieno[3,4-b]thiophene can result in a significant twist of the conjugated main-chains from planarity and thus disrupt the effective conjugation in polymers with aryl repeating units. This steric hindrance can be relieved by using an alkynyl linker. Solar cell devices with PPDA-TTCF3 as a donor and PC61BM as an acceptor have been fabricated and evaluated.