Solvent effect on conical intersections in excited-state 9H-adenine: Radiationless decay mechanism in polar solvent

The ground- and excited-state free energy minima and the conical intersections among these states of 9H-adenine in aqueous and acetonitrile solutions are studied theoretically to elucidate the mechanism of radiationless decay. We employ the recently proposed linear-response free energy (LRFE) to locate the energy minima and conical intersections in solution. The LRFE is calculated by using the reference interaction site model self-consistent field method. The geometry optimizations are carried out at the complete active space self-consistent field level, and the dynamic electron correlation energies are estimated by the multireference Mlller-Plesset method. We find that the conical intersection between the L-1(a) and L-1(b) states in aqueous solution occurs at a wide area of the free energy surface, indicating a strong vibronic coupling between them. On the other hand, the (1)n pi* state is largely blue-shifted at planar geometries in solution, which suggests that the nonadiabatic transition to this state is suppressed. The importance of the 1 pi sigma* channel is also examined in both the gas phase and solution. Based on the free energy characteristics obtained by the calculations, we intend to explain the experimental observations that the excited state of 9H-adenine decays monoexponentially with shorter lifetimes in polar solvents than that in the gas phase.