Excited-state dynamics of thiophene substituted betaine pyridinium compounds

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 (S-0 -> S-2 excitation followed by S-2(CT) -> S-1(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 pi-electron system localized in thiophenic groups is responsible for the production of triplet states. Involvement of this new route in the parallel production of S-1(CT) is strongly suspected.