Light–matter interactions drive fundamental processes across chemistry, biology, and materials science, where the fate of photoexcited molecules is determined by ultrafast excited-state dynamics. Understanding how structural and electronic factors control relaxation pathways such as internal conversion, intersystem crossing, fragmentation, or bond rearrangement, is essential for interpreting natural phenomena. In this thesis, we investigate the ground- and excited-state behavior of adamantane, uracil and their substituted derivatives using a combination of adiabatic and nonadiabatic molecular dynamics simulations. Methodologically, the work combines density functional theory and semiempirical quantum-chemical approaches with Born–Oppenheimer molecular dynamics, surface hopping dynamics, and spectroscopic modeling to capture both energetic landscapes and time-resolved processes. In the first part, we focus on adamantane, a prototypical cage-like hydrocarbon that serves as a model system for studying rigid molecular frameworks. Through adiabatic dynamics we explore its fragmentation possibilities at different excess energies, revealing threshold-dependent dissociation channels. Using nonadiabatic surface hopping simulations we show relaxation times and fragmentation possibilities after internal conversion. Building on this foundation, we extend the study to functionalized derivatives: amantadine and cyanoadamantane; where electron-donating and electron-withdrawing substituents significantly alter excitation energies, spectral profiles, and fragmentation dynamics. Comparative analysis highlights the crucial role of excited-state energetics and nonadiabatic couplings in governing internal conversion processes. The second part of the thesis turns to uracil and its derivatives, which are of biological relevance due to their roles in photochemistry and photodamage. First, we discuss about 2-thiouracil and 4-thiouracil. Using adiabatic simulations, we map the dissociative photoionization pathways of the isomers and reveal how the position of the thione substitution controls the fragmentation outcomes. Additional analysis identifies that formation of secondary structures like tautomers can also influence fragmentation products. In the end, we investigate uracil itself, combining nonadiabatic surface hopping simulations with time-resolved near-edge X-ray absorption spectroscopy (TR-NEXAFS). This integrated approach links electronic population dynamics with atom-specific structural signatures, showcasing how carbonyl distortions and local functional groups guide relaxation frombright ππ^∗ to dark nπ^∗ states. By bridging spectroscopic observables with molecular dynamics, we provide atomistic insight into population trapping and nonradiative decay in uracil nucleobase. Together, these studies shed light on how molecular architecture, chemical substitution, and topology influence ground- and excited-state dynamics of considered molecules.
Bonasree Roy (Thu,) studied this question.