Modelling of spectroscopic observables in molecular chromophores: static and dynamic approaches

The ability to reproduce ultrafast photo-induced processes is pivotal for advancing our understanding of molecular systems and their applications in technology. This thesis explores how these reactions unfold, focusing on the critical role of conical intersections, which provide ultrafast pathways for energy dissipation. By using cutting-edge computational methods such as CASPT2 and TD-DFT, our aim is to model the dynamics of molecules in excited states, providing insights that align with experimental results. The ability to reproduce and interpret experimental time-resolved spectra is particularly significant for understanding the transition pathways and photochemical reaction mechanisms. Throughout this research, I have focused on modelling various molecular systems, both in the gas phase and in explicit solvents, to investigate their static and dynamic behavior following photoexcitation. The overall objective has been to assign experimental spectral features to specific structural changes, electronic states, and photoreaction pathways. The aforementioned modellistic strategies have been successfully applied to interpret the photophysical behaviour of a novel molecular switch azodicarboxamide-based, supported by the simulation of the experimental UV pump / IR probe time-resolved spectrum; and to disentangle a long-standing debate concerning the de-activation mechanism of Thymidine in water following UV irradiation. Also in this case, our interpretations have been corroborated by the simulation of the UV pump / XUV probe time-resolved photoelectron spectrum to validate the quality of our simulations. Finally, the electronic structure modelling, at multi-reference 2nd-order perturbation level of theory, of the Tris(2-Phenylpyridine)Iridium transition metal complex is presented. Here, the extended number of molecular orbitals involved and the high density of close-lying excited states of different spin multiplicities pose formidable theoretical challenges aimed at obtaining a reasonable balance between accuracy and computational cost. By accurately simulating time-resolved spectra and energy pathways, these models demonstrate their robustness and predictive power, offering a reliable framework for studying ultrafast photochemical processes.