Phenyleneethynylene- and Thienyleneethynylene-Based π-Conjugated Polymers with Imidazolium Units in the Main Chain

Phenyleneethynylene-based and thienyleneethynylene-based π-conjugated polymers with imidazolium cationic units in the main chain and their model compounds were synthesized and characterized. The imidazolium-containing cationic polymers were transferred to an ionic liquid by the spontaneous anion-exchange from iodide to bis(trifluoromethanesulfonyl)imide (TFSI). The fluorescence quantum yield of phenyleneethynylene-based π-conjugated polymer in the ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (emimTFSI), was higher than that of polymer based on iodide anion in pyridine because of a low degree of aggregation of the main chain and/or the absence of an external heavy-atom effect of the iodide counteranion. UV−vis and fluorescence spectra of N-methylated polymer and model compound with TFSI counteranion exhibited a significant blue shift with respect to the parent neutral molecules. Density functional theory (DFT) calculations for the model compounds showed a decrease in π-conjugation by quaternization of the N-position at the imidazole unit, supporting the blue shift observed for UV−vis and fluorescence spectra. The quaternization of the N-position at the imidazole unit in the thienyleneethynylene-based π-conjugated polymers and model compounds resulted in a red shift of the UV−vis absorption and fluorescence spectra compared with those of neutral polymers and model compounds. The DFT results suggest that the red shift comes from the contribution of coplanarization in the quaternized π-conjugated compounds.