Ab initio studies on the radiationless decay mechanisms of the lowest excited singlet states of 9H-adenine

The mechanisms that are responsible for the rapid deactivation of the ¹nπ* and ¹ππ* excited singlet states of the 9H isomer of adenine have been investigated with multireference ab initio methods (complete-active-space self-consistent-field (CASSCF) method and second-order perturbation theory based on the CASSCF reference (CASPT2)). Two novel photochemical pathways, which lead to conical intersections of the S ₁ excited potential-energy surface with the electronic ground-state surface, have been identified. They involve out-of-plane deformations of the six-membered aromatic ring via the twisting of the N₃C₂ and N₁C₆ bonds. These low-lying conical intersections are separated from the minimum energy of the lowest (¹nπ*) excited state in the Franck-Condon region by very low energy barriers (of the order of 0.1 eV). These properties of the S₁ and S₀ potential-energy surfaces explain the unusual laser-induced fluorescence spectrum of jet-cooled 9H-adenine, showing sharp structures only in a narrow energy interval near the origin, as well as the extreme excess-energy dependence of the lifetime of the singlet excited states. It is suggested that internal-conversion processes via conical intersections, which are accessed by out-of-plane deformation of the six-membered ring, dominate the photophysics of the lowest vibronic levels of adenine in the gas phase, while hydrogen-abstraction photochemistry driven by repulsive ¹πσ* states may become competitive at higher excitation energies. These ultrafast excited-state deactivation processes provide adenine with a high degree of intrinsic photostability.