Radiationless decay of excited states of uracil through conical intersections

The electronically excited singlet states of uracil have been studied theoretically using ab initio multireference configuration interaction methods focusing on the mechanism for radiationless decay back to the ground state. The first excited state with a significant oscillator strength is calculated to be the S-2 state, corresponding to an excitation pi --> pi*, while a dark state,S-1 (n(O) --> pi*), exists below S-2. Conical intersections have been located between the S-2 and S-1 states 0.9 eV below the vertical excitation to S-2 and between the S-1 and the ground state, ca. 1.8 eV below the vertical excitation energy to S-2. These conical intersections are connected with each other and the Franck Condon region by pathways that exhibit no barriers and provide for a nonradiative decay to the around state.