Key Role of a Threefold State Crossing in the Ultrafast Decay of Electronically Excited Cytosine

A three-state conical intersection between the ground state and the (π,π*) and ( n O,π*) singlet excited states of cytosine is the topological feature that dominates the ultrafast decay of singlet excited cytosine. The three-state intersection is associated with seams of intersection between pairs of states ( S 1 / S 0 and S 2 / S 1, respectively), and the resulting topology has been mapped out with CASSCF and CAS-PT2 calculations. The minimum-energy path for the optically active (π,π*) state lies on the S 1 surface, and decay to the ground state takes place at the S 1 / S 0 seam. On the other hand, the region of the S 2 / S 1 seam must be traversed before accessing the conical intersection with the ground state and recrossing to S 2 becomes possible. Another feature associated with the three-state degeneracy is vibronic coupling between the (π,π*) and ( n O,π*) excited states (proximity effect), which lowers the barrier to the S 1 / S 0 seam. From a mechanistic point of view, then, the decay is the outcome of the interaction between the three states. The results also suggest that the experimental excited-state lifetime is the effect of two factors, an energetically accessible region of S 1 / S 0 degeneracy and a region where the decay can be slowed because of recrossing to S 2 .