This thesis represents a step towards the detailed comprehension of the photoisomerization processes and their exploitation. State-of-the-art ab initio quantum chemical calculations will be used to achieve various objectives. When a molecule undergoes an excitation to an higher electronic energy level, one of the main features to study is the way followed by the molecule to relax back on the electronic fundamental state. This implies the computation of the molecular structures where radiationless deactivation is most probable: conical intersections and singlet/triplet crossings. Such crossings provide effective channels for the radiationless deactivation to the ground state. During the last fifteen years the quantum chemical methods mentioned above have been successfully applied to the investigation of the mechanism of both the internal conversion and photochemical transformation of isolated molecules. First objective of this thesis is about retinal chromophores and the relationship between their structure and the ultrafastness and stereospecificity of the photoisomerization in the protein environment. The acquired experience will be used to investigate for possible applications of molecules that can undergo a cis-trans photoisomerization. In fact, the second objective of this thesis is the study of a chromophore, N-methylthioacetammide (NMTAA), model for a thio-substituted peptide bond. The last part of the thesis, and its third objective, is committed to the development of new tools for the geometry optimization of potential energy surfaces crossings, with high level ab initio computations.
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