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 pi) exists between the spectroscopic pi/pi* excited state (S pi) and a dark n/pi* state (Sn), and a fraction of the population decays to the minimum of Sri (Sn-min). The conical intersection between Sri 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 pi is rather small. After thermalization in Sn-min, the system can thus recross CI-n pi and then quickly proceed on the S pi barrierless path toward the conical intersection with the ground state.