Ab initiostudies of the photophysics of 2-aminopurine

The low-lying excited singlet states of 2-aminopurine, an isomer of adenine, have been investigated with multi-reference ab initio methods (complete-active-space self-consistent-field (CASSCF) method and second-order perturbation theory based on the CASSCF reference (CASPT2)). In particular, conical intersections of the S1 potential-energy surface with the S0 surface and the reaction paths leading from the Franck–Condon region to these conical intersections have been explored. It is shown that the twisting deformation of CN bonds of the six-membered ring leads to two low-lying S0–S1 conical intersections. The potential-energy profiles of the 1ππ*(Lb), 1ππ*(La) and 1nπ* excited states as well as of the ground state have been determined along these reaction paths. The results are discussed in comparison with analogous results for 9H-adenine [S. Perun, A. L. Sobolewski, and W. Domcke, J. Am. Chem. Soc. 127, 6275 (2005)]. The computational data provide a convincing explanation of the strikingly different fluorescence properties of 9H-adenine and 2-aminopurine.