A two-step ICT process for solvatochromic betaine pyridinium revealed by ultrafast spectroscopy, multivariate curve resolution, and TDDFT calculations

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 S-0 -> S-2 vertical transition and a weakly-solvatochromic emission due to S-1 -> S-0 transition. Advanced TDDFT optimizations of the Franck-Condon states S-2(FC) and S-1(FC) led to two additional CT levels with planar geometry, S-2(CT) and S-1(CT), respectively, allowing prediction of a two-step photoinduced ICT process, i.e., S-0 -> S-2(FC) and S-2(CT) -> S-1(CT), separated by a S-2(FC) -> S-2(CT) back charge transfer relaxation. While the pyridinium ring is the acceptor group in both steps, two different donor groups, the benzene ring and the imidazole bridge, are involved in the excitation and internal conversion processes, respectively. Femtosecond transient absorption experiments supported by MCR-ALS decomposition confirmed indeed the contribution of two distinct CT states in the photophysics of SBPa: following excitation to the S-2(CT) state, ultrafast production of the emissive S-1 state (the only channel observable in the gas phase) was observed to occur in competition with a further ICT process toward the S-1(CT) state, with a time constant ranging from 300 fs to 20 ps depending on the solvent. While in aprotic media this ICT process was found to be purely solvent controlled (double polarity and viscosity dependency), in protic solvents, the influence of the hydrogen bond network has to be taken into account. Comparison with data obtained for a pre-twisted SBPa analogue led us to exclude the presence of any large-amplitude geometrical change during ICT. Analyzing the solvent dependency using the power law approach, we concluded that the S-1(CT) state decays essentially through IC in the 3-40 ps time range whereas the emissive S-1 state decays within 130-260 ps via IC, ISC and fluorescence.