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 $[n→π*]1$ state has been located at an energy of $4.54eV$ for a nuclear arrangement in which the amino group is pyramidal whereas the ring system remains planar. Close by, another minimum on the $S1$ potential energy hypersurface has been detected in which the $C2$ center is deflected out of the molecular plane and the electronic character of $S1$ corresponds to a nearly equal mixture of $[π→π*]1$ and $[n→π*]1$ configurations. The adiabatic excitation energy of this minimum amounts to $4.47eV$⁠. Vertical and adiabatic excitation energies of the lowest $n→π*$ and $π→π*$ 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 $S1$ potential energy hypersurface, an energetically favorable path from the singlet $n→π*$ 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 $S1$ state corresponds to a $π→π*$ excitation. The energetic accessibility of this relaxation path at about $0.1eV$ above the singlet $n→π*$ 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 $N9$ center) is largely preserved.