Combining excited state ab-initio molecular dynamics and time resolved vibrational analysis to understand nuclear photorelaxation processes

Understanding at atomistic level the mechanism of photoinduced chemical reactions in terms of electronic/nuclear motions responding to the external perturbation is a challenging task from both experimental and theoretical point of view. Time resolved vibrational spectroscopies, based on using ultrashort laser pulses, were shown to be a very powerful tool to investigate and follow in real time the nuclear motion of photoexcited chromophores, even in complex matrix. The interpretation of the often complex experimental spectra can benefit from the employment of a theoretical-computational approach. In particular, ab-initio molecular dynamics methods allow to simulate and accurately reproduce the behavior of chromophore in solution in both equilibrium and far for equilibrium regime, i.e. after the interaction with light. A rational analysis of the signals extracted from dynamics may be crucial to create a direct link between electronic/nuclear structure and spectroscopic properties. In this contribution, we will discuss computational strategies including ab-initio molecular dynamics and time resolved vibrational analysis based on Wavelet Transform (WT), to address this challenging issue. WT makes possible to perform a multiresolution analysis, namely any signal extracted from excited state trajectory is localized in both time and frequency domain. Therefore, we can follow the activation and relaxation of the vibrational signals of photoexcited chromophores and to reproduce their reaction pathway. The computational approach is applied to the photoreactivity of a popular photoacid molecule, i.e 8- hydroxypyrene-1,3,6-trisulfonic acid or pyranine, in pure water solution and in presence of acetate molecule. Moreover, we will discuss the photoinduced behavior of a diarylethene photochromic molecule, that in excited state undergoes an opening ring reaction. The time resolved vibrational analysis is here extended to unveil the anharmonic coupling between vibrational modes.

Dr. M. Gabriella Chiariello was born in Naples, Italy in 1990. She received her Master degree cum laude in Chemical Sciences in 2014 from University of Naples under the supervision of Prof. Nadia Rega. During the internship, she focused on the simulation of the photophysical and photochemical properties of a fluorescent protein. In 2014 Dr. Chiariello got a position as a Ph.D. student at the Chemical Science Department of the University Federico II of Naples in the group of Prof. Nadia Rega to set up and test a computational protocol for the study of ultrafast nuclear processes in photochemistry. Her Ph.D. thesis, entitled "Models and Methods to understand ultrafast dynamics in photochemistry", was successfully defended in 2018. Her scientific interests focus on hybrid QM/MM models combined with ab-initio molecular dynamics methods to describe the excited state behavior of molecular system in condensed phase.