Excited-State Deactivation Pathways in Uracil versus Hydrated Uracil: Solvatochromatic Shift in the 1nπ* State is the Key

Excited-state deactivation mechanisms of uracil are investigated using spin-flip time-dependent density functional theory. Two important minimum-energy crossing points are located, for both gas-phase and hydrated uracil, and optimized relaxation pathways connecting the most important critical points on the (1)n pi* and (1)pi pi* potential energy surfaces are determined. An ultrafast decay time constant, measured via femtosecond spectroscopy, is assigned to direct (1)pi pi* -> S-0 deactivation, while a slower decay component is assigned to indirect (1)pi pi* -> (1)n pi* -> S-0 deactivation. The shorter lifetime of the dark (1)n pi* state in aqueous solution is attributed to a decrease in the energy barrier along the pathway connecting the (1)n pi* minimum to a (1)pi pi*/S-0 conical intersection. This barrier arises due to hydrogen bonding between uracil and water, leading to a blue-shift in the S-0 -> (1)n pi* excitation energy and considerable modification of energy barriers on the (1)n pi* potential surface. These results illustrate how hydrogen bonding to the chromophore can significantly impact excited-state dynamics and also highlight that relaxation pathways can be elucidated using low-cost methods based on density functional theory.