Direct Arylation Polycondensation of Bithiazole Derivatives with Various Acceptors

Novel pi-conjugated polymers with a bithiazole unit and various acceptor units were synthesized by polycondensation via Pd-catalyzed direct arylation. The results of polycondensation reactions depend on chalcogen elements in the monomers. The presence of a Se atom inhibited polymerization, presumably owing to the coordination of the Se moiety to the Pd center; the issue was resolved by increasing the reaction temperature (120 degrees C). The chalcogen elements also affected the interchain interactions of the obtained polymers. The strong interchain interactions led to low solubility, high crystallinity, and a large red-shift of absorption in the film state compared with that in the solution state. Absorption spectra and DFT calculations revealed that the bithiazole units served as weak donor units in case that the bithiazole units were directly connected to strong acceptor units. The combination of the bithiazole units with the strong acceptor units afforded deep HOMO-LUMO levels.