Excited-state dynamics of thiophene substituted betaine pyridinium compounds

Abstract

This work deals with the photophysics of novel pyridinium betaine based on 2-pyridin-1-yl-1H-benzimidazole (SBPa) substituted symmetrically by mono- (Th₂SBPa) and bi-thiophene fragments (Th₄SBPa). The study is based on a combination of steady-state, femtosecond transient absorption spectroscopic measurements supported by PCM–(TD)DFT calculations. It is found that the two step ICT process (S₀ → S₂ excitation followed by S₂(CT) → S₁(CT) internal conversion) occurring for the parent molecule remains unaffected for Th₂SBPa while the situation is less clear for Th₄SBPa. Actually, for both molecules, a new decay route involving the π-electron system localized in thiophenic groups is responsible for the production of triplet states. Involvement of this new route in the parallel production of S₁(CT) is strongly suspected.