Abstract
This thesis adresses the study and control of the photo-reactivity of molecular switches, here the photo-isomerisation of spiropyrans. This theoretical work has been achieved in close collaboration with the experimental team PFL within the ICB in Dijon. Non-adiabatic quantum dynamics simulations were carried out so as to reproduce and rationalise the experimental results from time-resolved transient absorption spectroscopy. Such experiments have demonstrated ultra-fast photo-reactivity (~ 100 fs) following excitation by an ultra-short LASER pulse. It is interpreted as an internal conversion mechanism between the first singlet excited eletronic state and the ground state via a conical intersection. The theoretical study used the ring-opening reaction of benzopyran as a model. Developments were made regarding: (1) The exploration of the reaction mechanism and the computation of potential energy surfaces with perturbative, post-CASSCF quantum chemistry methods (XMCQDPT2). This investigation showed that results changed significantly compared to those reported in the literature with lower-level calculations. (2) The generation of a diabatic Hamiltonian based on ab initio XMCQDPT2 data. Owing to the significant anharmonicity in the ground electronic state, we designed a new effective approach, quite different from the previous studies. (3) The production of non-adiabatic quantum dynamics simulations using the MCTDH method. The results thus obtained are in excellent agreement with the experimental ones. Including explicitly the LASER pulse allowed us to reproduce and rationalise the action of pulse shaping on control observed in experiments. The present work thus made possible the succesful implementation of a theoretical/experimental collaboration.