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 S2 state, corresponding to an excitation π → π*, while a dark state, S1 (nO → π*), exists below S2. Conical intersections have been located between the S2 and S1 states 0.9 eV below the vertical excitation to S2 and between the S1 and the ground state, ca. 1.8 eV below the vertical excitation energy to S2. 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 ground state.