Torsionally Responsive Tropone-Fused Conjugated Polymers

Torsionally responsive molecular systems can change their electronic properties according to the dihedral angles and can be utilized as sensory materials. We have designed and synthesized novel tropone-fused conjugated polymers PBTr, PBTr-T, and PBTr-Tz that showed interesting dihedral-angle-dependent variations in UV-vis absorptions. Tropone-fused thiophene derivatives were prepared from one-step condensation of thiophene-3,4-dialdehyde and aliphatic ketones via a modular, facile, and high-yielding method. Subsequent halogenation and Stille cross-coupling polymerization with a bis(stannyl)benzodithiophene resulted in a tropone-fused conjugated polymer PBTr. We were also able to prepare thiophene- and thiazole-bridged polymers, PBTr-T and PBTr-Tz, respectively, using similar synthetic methods. Electronic absorptions of the newly synthesized PBTrs were measured in solutions and in films states. Substantial red-shifts occurred in the case of thiophene-bridged PBTr-T, whereas almost no shift was observed for thiazole-bridged PBTr-Tz. We attributed this to the substantial change in the torsional angle between the tropone-fused thiophene moiety and thiophene, which was further supported by density functional theory (DFT) calculations. Similar spectral changes of UV-vis absorptions were observed when a poor solvent (methanol) was introduced to a chloroform solution of PBTr-T. Reverse torsional angle variations were realized with initially planar PBTr-Tz by introducing steric hindrance through protonation on the thiazole rings. We believe that torsionally responsive tropone-fused conjugated polymers are promising as novel platforms for sensory applications.