Ab Initio Studies on the Radiationless Decay Mechanisms of the Lowest Excited Singlet States of 9H-Adenine

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Abstract
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 S1 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.

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