Ab initio study of the excited-state coupled electron-proton-transfer process in the 2-aminopyridine dimer

The low-lying ¹ππ* excited states of the 2-aminopyridine dimer have been investigated with multi-reference ab initio methods (CASSCF and MRMP2). The 2-aminopyridine dimer can be considered as a mimetic model of Watson-Crick DNA base pairs. The reaction path and the energy profile for single proton transfer in the lowest ¹ππ* inter-monomer charge-transfer state have been obtained. A weakly avoided crossing of the ¹ππ* surface with the electronic ground-state surface has been found near the single-proton-transfer minimum of the ¹ππ* surface. From the splitting of the adiabatic surfaces at the avoided crossing, an internal-conversion lifetime of the excited state of <100 ps has been estimated. The potential relevance of these results for the rationalization of radiation-induced mutations and the photostability of the genetic code is briefly discussed.