Models and Methods to understand ultrafast dynamics in photochemistry

The PhD project was devoted to the development and the validation of a general computational protocol for simulating and understanding ultrafast nuclear relaxation processes of chromophores upon interaction with light. Ab-initio molecular dynamics was employed in both the ground and excited electronic states of interest, to monitor the molecular response to the photoexcitation in real-time. Once representative trajectories have been collected, a multiresolution vibrational analysis based on the wavelet transform is performed. The far-from-equilibrium nuclear relaxation is therefore rationalized and understood in the frequency domain, maintaining the temporal information and providing a direct comparison with advanced time resolved vibrational spectroscopy experiments. The photoinduced relaxation of the pyranine photoacid molecule in aqueous solution was considered as first case study. As main finding, low frequency vibrational modes promoting photoreactivity are effectively individuated and correlated to key structural parameters of the relaxation process. Moreover, the modified reactivity of pyranine in presence of acetate was also investigated. We simulated the excited state proton transfer reactions (ESPT) involving pyranine-acetate and pyranine-water-acetate clusters in aqueous solution. The study has revealed interesting correlations between the structure of the reactive cluster and the ESPT kinetics, improving our understanding of the driving forces at the origin of ESPT photoreactivity. The last part of the thesis concerned the photoinduced reactivity of a diarylethene molecule, a well-known photo-switch system. In this case, the multiresolution vibrational analysis has been extended and tested to unveiling the anharmonic coupling between vibrational modes.