The Decay from the Dark n/pi* Excited State in Uracil: An Integrated CASPT2/CASSCF and PCM/TD-DFT Study in the Gas Phase and in Water

By integrating the results of MS-CASPT2/CASSCF and TD-PBE0 calculations, we propose a mechanism for the decay of the excited dark state in pyrimidine, fully consistent with all the available experimental results. An effective conical intersection (CI-n?) exists between the spectroscopic ?/?* excited state (S?) and a dark n/?* state (Sn), and a fraction of the population decays to the minimum of Sn (Sn-min). The conical intersection between Sn and the ground-state is not involved in the decay mechanism, because of its high energy gap with respect to Sn-min. On the other hand, especially in hydrogen bonding solvents, the energy gap between Sn-min and CI-n? is rather small. After thermalization in Sn-min, the system can thus recross CI-n? and then quickly proceed on the S? barrierless path toward the conical intersection with the ground state.