Direct Arylation Polycondensation of β-Fluorinated Bithiophenes to Polythiophenes: Effect of Side Chains in C–Br Monomers
Xuwen Zhang, Yibo Shi, Yanfeng Dang, Ziqi Liang, Zhongli Wang, Yunfeng Deng, Yang Han, Wenping Hu, Yanhou Geng
Abstract
Six polythiophenes (PThs) were synthesized via direct arylation polycondensation (DArP) with β-fluorinated bithiophenes, that is, 4,4′-difluoro-2,2′-bithiophene (2FBT) and 3,3′,4,4′-tetrafluoro-2,2′-bithiophene (4FBT), as the C–H monomers and 5,5′-dibromo-2,2′-bithiophenes DCBT-2Br, DABT-2Br, and DTBT-2Br that carry alkoxycarbonyl, alkyl, and alkylthienyl side chains at 4,4′-positions, respectively, as the C–Br monomers. Both 2FBT and 4FBT are highly reactive for DArP, and high molecular weight PThs with unobservable chain defects were successfully synthesized from 2FBT/DCBT-2Br, 2FBT/DABT-2Br, 2FBT/DTBT-2Br, and 4FBT/DTBT-2Br. However, the DArP of 4FBT with DCBT-2Br and DABT-2Br produced PThs containing C–H/C–H homo-coupling defects along with relatively lower molecular weights. Detailed theoretical analysis suggests that the occurrence of C–Br reduction-provoked C–H/C–H homo-coupling for 4FBT/DCBT-2Br and 4FBT/DABT-2Br monomer pairs originated from the poor selectivity between the cis- and trans-forms of transition states in the concerted metalation–deprotonation (CMD) process is responsible for the formation of structural defects and depressed molecular weights. This research reveals that a C–H monomer with high reactivity and selectivity is not sufficient for the synthesis of well-defined conjugated polymers via DArP, and the structure of the C–Br monomer also has a significant impact on the results of DArP via influencing the catalytic cycles.