Mail:
Dept. of Chemistry
Ohio State University
100 W. 18th Ave.
Columbus, OH 43210
Office:
412 CBEC
Email:
herbert@
chemistry.ohio-state.edu
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 1nπ* and 1ππ* potential energy surfaces are determined. An ultrafast decay time constant, measured via femtosecond spectroscopy, is assigned to direct 1ππ* → S0 deactivation, while a slower decay component is assigned to indirect 1ππ* → 1nπ* → S0 deactivation. The shorter lifetime of the dark 1nπ* state in aqueous solution is attributed to a decrease in the energy barrier along the pathway connecting the 1nπ* minimum to a 1ππ*/S0 conical intersection. This barrier arises due to hydrogen bonding between uracil and water, leading to a blue-shift in the S0 → 1nπ* excitation energy and considerable modification of energy barriers on the 1nπ* 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.