A two-step ICT process for solvatochromic betaine pyridinium revealed by ultrafast spectroscopy, multivariate curve resolution, and TDDFT calculations†
Abstract
This work deals with the photophysics of a pyridinium betaine, 2-pyridin-1-yl-1H-benzimidazole (SBPa), based on a combination of steady-state, femtosecond photoionization (gas phase) and femtosecond transient absorption (solution) spectroscopic measurements, supported by (LR)-PCM-(TD)DFT calculations. Preliminary and new electrochemical results have revealed a strongly negative solvatochromic charge transfer (CT) absorption due to a S0 → S2 vertical transition and a weakly-solvatochromic emission due to S1 → S0 transition. Advanced TDDFT optimizations of the Franck–Condon states S2(FC) and S1(FC) led to two additional CT levels with planar geometry, S2(CT) and S1(CT), respectively, allowing prediction of a two-step photoinduced ICT process, i.e., S0 → S2(FC) and S2(CT) → S1(CT), separated by a S2(FC) → S2(CT) back charge transfer relaxation. While the pyridinium ring is the acceptor group in both steps, two different donor groups, the