Elucidating deactivation and reaction paths of photosensitive organic systems through computational methods

Identifier:  TDX:2261

Handle: https://hdl.handle.net/20.500.11797/TDX2261

Authors:  Casellas Soler, Josep

Abstract:
This thesis develops a computational work to study the processes that explain the photochemical properties of some organic compounds. Multiconfigurational ab initio methods, CASSCF and CASPT2 have been used to determine the topography of the potential energy surfaces of the low energy states and locate reaction paths. We have studied first the mechanism of photoisomerization of azobenzene and its derivative phenylazopyridine. The reaction mechanism obtained explains satisfactorily why the isomerization quantum yield depends on the initial excitation and on the degree of the restriction on the internal rotation of these systems. We have also analysed the performance of a computational protocol to reproduce the absorption spectrum of flexible systems in solution, using phenylazopyridine as target system. We have also studied 9-phenylphenalenone. Its parent system, phenalenone, shows a high quantum yield in the process of singlet oxygen sensitization, but the quantum yield of this phenyl derivate for the same process is very low. Our study shows that this difference is due to a side reaction induced by the initial photoexcitation, which produces a metastable photoproduct by cyclisation that turns back quickly to the initial reactant. This study has been a challenge for the computational methods used due to its big size. Finally, we have studied dehidrosqualene, a carotenoid involved in light harvesting processes in vegetables. Our study determines the structure of the low energy excited states and the influence of the flexibility of this compound in the energy of these states. We show that to use only the most symmetric conformation on this flexible compound to calculate the energy of the excited states, give results that qualitatively agree with the experimental observations but, if quantitative agreement is required, the molecular flexibility must be taken into account.