Photostability of 9H-adenine: mechanisms of the radiationless deactivation of the lowest excited singlet states

Three different mechanisms of radiationless deactivation of the excited singlet states of 9H-adenine have been studied with CASSCF/CASPT2 methods. Out-of-plane deformation of the six-membered ring, hydrogen abstraction from the amino and azine groups, as well as opening of the five-membered ring have been considered. All these photochemical reaction paths lead to low-lying S0-S1 conical intersections and thus provide channels for effective radiationless deactivation of the electronic excitation. The internal conversion processes associated with these channels open at different excitation energies. Taken together, they provide adenine with high level of intrinsic photostability over a broad range of UV irradiation.