The journey from light-induced excitation of a photoactive molecule to the formation of a stable photoproduct unfolds through a sequence of complex transformations within the molecular system. These transformations are governed by ultrafast processes and short-lived intermediates or transient species, which are crucial to the photoprocess. Studying these intermediates is vital not only for understanding the fundamental light-matter interactions that underlie natural optical phenomena but also for enabling the design of photochemical and photophysical reactions across diverse fields of application. Optical spectroscopic methods are commonly used to gather experimental evidence of these phenomena. Although these methods identify the intermediates based on their energy signatures, a proper characterization of the electronic structure remains elusive.

This thesis delves into sophisticated quantum chemical frameworks, supported by multiple studies, to elucidate the mechanism and architecture involved in characterizing the short-lived molecular transients. The studies addressed in the thesis highlight the rich photochemistry of heterocyclic compounds through theoretical dynamical modeling and spectral simulations. In the first part, the non-adiabatic effects in photoexcited thiopyridone are explored. A collective theoretical and experimental investigation describes the molecular evolution during the ultrafast process and the observed variations across its constitutional isomers. The possibility of formation of a molecular excimer is also explored. The second part focuses on imidazole to simulate the electronic and structural changes that lead to proton transfer, with particular attention given to the local effects that influence the overall characteristics.