Excited State Deactivation Mechanism in Protonated Uracil: New Insights from Theoretical Studies.

We have conducted here a theoretical exploration, discussing the distinct excited state lifetimes reported experimentally for the two lowest lying protonated isomers of uracil. In this regard, the first-principal computational levels as well as the nonadiabatic surface hopping dynamics have been employed. It has been revealed that relaxation of the ππ* state of enol-enol form (EE) to the ground is barrier-free via out-of-plane coordinates, resulting in an ultrashort S lifetime of this species. For the second most stable isomer (EK), however, a significant barrier predicted in the CASPT2 S potential energy profile along the twisting coordinate has been proposed to explain the relevant long lifetime reported experimentally.