Ab initio study on deactivation pathways of excited 9H-guanine

The complete active space with second-order perturbation theory/complete active space self-consistent-field method was used to explore the nonradiative decay mechanism for excited 9H-guanine. On the (1)pi pi(*) (L-1(a)) surface we determined a conical intersection (CI), labeled (S-0/pi pi(*))(CI), between the (1)pi pi(*) (L-1(a)) excited state and the ground state, and a minimum, labeled (pi pi(*))(min). For the (1)pi pi(*) (L-1(a)) state, its probable deactivation path is to undergo a spontaneous relaxation to (pi pi(*))(min) first and then decay to the ground state through (S-0/pi pi(*))(CI), during which a small activation energy is required. On the (1)n(N)pi(*) surface a CI between the (1)n(N)pi(*) and (1)pi pi(*) (L-1(a)) states was located, which suggests that the (1)n(N)pi(*) excited state could transform to the (1)pi pi(*) (L-1(a)) excited state first and then follow the deactivation path of the (1)pi pi(*) (L-1(a)) state. This CI was also possibly involved in the nonradiative decay path of the second lowest (1)pi pi(*) (L-1(b)) state. On the (1)n(O)pi(*) surface a minimum was determined. The deactivation of the (1)n(O)pi(*) state to the ground state was estimated to be energetically unfavorable. On the (1)pi sigma(*) surface, the dissociation of the N-H bond of the six-membered ring is difficult to occur due to a significant barrier. (c) 2006 American Institute of Physics.