An ab initio theoretical study at the CASPT2 level is reported on minimum energy reaction paths, state minima, transition states, reaction barriers, and conical intersections on the potential energy hypersurfaces of two tautomers of adenine: 9H- and 7H-adenine. The obtained results led to a complete interpretation of the photophysics of adenine and derivatives, both under jet-cooled conditions and in solution, within a three-state model. The ultrafast subpicosecond fluorescence decay measured in adenine is attributed to the low-lying conical intersection (gs/ππ* La)CI, reached from the initially populated 1(ππ* La) state along a path which is found to be barrierless only in 9H-adenine, while for the 7H tautomer the presence of an intermediate plateau corresponding to an NH2-twisted conformation may explain the absence of ultrafast decay in 7-substituted compounds. A secondary picosecond decay is assigned to a path involving switches towards two other states, 1(ππ* Lb) and 1(nπ*), ultimately leading to another conical intersection with the ground state, (gs/nπ*), with a perpendicular disposition of the amino group. The topology of the hypersurfaces and the state properties explain the absence of secondary decay in 9-substituted adenines in water in terms of the higher position of the 1(nπ*) state and also that the 1(ππ* Lb) state of 7H-adenine is responsible for the observed fluorescence in water. A detailed discussion comparing recent experimental and theoretical findings is given. As for other nucleobases, the predominant role of a ππ*-type state in the ultrafast deactivation of adenine is confirmed.
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