Ab initio studies 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 S ₁ potential-energy surface with the S 0 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 S 0 -S 1 conical intersections. The potential-energy profiles of the 1 pp*(L b ), 1 pp*(L a ) and 1 np*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.