Ab initio studies of the photophysics of 2-aminopurine

Abstract

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*(L_b), ¹ππ*(L_a) 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.

Supporting information

Cartesian coordinates of the local minimum (LM), saddle point (SP) and conical intersection (CI) structures.

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Acknowledgements

This work has been supported by grant no. 3 T09A 160 28 of the Polish Ministry of Science and Information and through a travel grant from the COST action P9 for S. P.