Direct Arylation Polycondensation of β-Fluorinated Bithiophenes to Polythiophenes: Effect of Side Chains in C–Br Monomers

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.