A new pathway for the rapid decay of electronically excited adenine

Combined density functional and multireference configuration interaction methods have been used to calculate the electronic spectrum of 9H-adenine, the most stable tautomer of 6-aminopurine. In addition, constrained minimum energy paths on excited potential energy hypersurfaces have been determined along several relaxation coordinates. The minimum of the first (1)[n ->pi(*)] state has been located at an energy of 4.54 eV for a nuclear arrangement in which the amino group is pyramidal whereas the ring system remains planar. Close by, another minimum on the S-1 potential energy hypersurface has been detected in which the C-2 center is deflected out of the molecular plane and the electronic character of S-1 corresponds to a nearly equal mixture of (1)[pi ->pi(*)] and (1)[n ->pi(*)] configurations. The adiabatic excitation energy of this minimum amounts to 4.47 eV. Vertical and adiabatic excitation energies of the lowest n ->pi(*) and pi ->pi(*) transitions as well as transition moments and their directions are in very good agreement with experimental data and lend confidence to the present quantum chemical treatment. On the S-1 potential energy hypersurface, an energetically favorable path from the singlet n ->pi(*) minimum toward a conical intersection with the electronic ground state has been identified. Close to the conical intersection, the six-membered ring of adenine is strongly puckered and the electronic structure of the S-1 state corresponds to a pi ->pi(*) excitation. The energetic accessibility of this relaxation path at about 0.1 eV above the singlet n ->pi(*) minimum is presumably responsible for the ultrafast decay of 9H-adenine after photoexcitation and explains why sharp vibronic peaks can only be observed in a rather narrow wavelength range above the origin. The detected mechanism should be equally applicable to adenosine and 9-methyladenine because it involves primarily geometry changes in the six-membered ring whereas the nuclear arrangement of the five-membered ring (including the N-9 center) is largely preserved.