Photoexcitation and relaxation kinetics of molecular systems in solution: towards a complete in silico model

In this study, we have modelled, through a theoretical–computational approach based on classical molecular dynamics simulations and quantum-chemical calculations, the complete relaxation process of a photo-excited ionic stilbene-like compound termed as DASPMI in solution. Starting from the absorption spectrum we have reconstructed the entire process of the excited-state relaxation involving the intra- molecular charge-transfer and eventually leading to the charge-recombination regenerating the ground state. The results obtained, well reproducing the experimental time-resolved emission spectra and kinetic observables, show that the relaxation process is essentially driven by the internal conformational transitions of the chromophore with the solvent almost instantaneously relaxed for each chromophore conformation. This study represents the first attempt, carried out using our theoretical–computational approach, of modelling a complete experiment involving the overposition of relaxation kinetics ranging from hundreds of femtoseconds to nanoseconds on the time scale.