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 (pi,pi*) and (n(O),pi*) singlet excited states of cytosine is the topological feature that dominates the ultrafast decay of singlet excited cytosine. The threestate 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 minimumenergy path for the optically active (pi,pi*) state lies on the S, 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 (pi,pi*) and (pi(O),pi*) 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.