Theoretical study of the photophysics of adenine in solution: Tautomerism, deactivation mechanisms, and comparison with the 2-aminopurine fluorescent isomer

The lower singlet excited states of isolated and solvated tautomers of adenine and of the fluorescent isomer 2-aminopurine have been studied with different quantum mechanical methods: geometries have been obtained at CIS level, while calculations of electronic structures have been performed using a multireference perturbed CI method and (for excitations) a TDDFT approach. In all calculations, the solvent (e.g., water) has been described within the IEF polarizable continuum model (IEF-PCM). The results have been successfully compared to experimental absorption and emission spectra. The same approach has been applied to the analysis of possible deactivation mechanisms acting on adenine but not on 2-aminopurine: both effects due to the proximity of pi --> pi* and n --> pi* states and the chance of an intramolecular twisting of the amino group have been considered.