Ab Initio Studies on the Radiationless Decay Mechanisms of the Lowest Excited Singlet States of 9H-Adenine

摘要

The mechanisms that are responsible for the rapid deactivation of the ~1npi~* and ~1pipi~* excited singlet states of the 9H isomer of adenine have been investigated with multireference ab initio methods (complete-active-space self-consistent-field (CASSCF) method and second-order perturbation theory based on the CASSCF reference (CASPT2)).Two novel photochemical pathways,which lead to conical intersections of the Si excited potential-energy surface with the electronic ground-state surface,have been identified.They involve out-of-plane deformations of the six-membered aromatic ring via the twisting of the N_3C_2 and N_1C_6 bonds.These low-lying conical intersections are separated from the minimum energy of the lowest (~1npi~*) excited state in the Franck-Condon region by very low energy barriers (of the order of 0.1 eV).These properties of the S_1 and So potential-energy surfaces explain the unusual laser-induced fluorescence spectrum of jet-cooled 9H-adenine,showing sharp structures only in a narrow energy interval near the origin,as well as the extreme excess-energy dependence of the lifetime of the singlet excited states.It is suggested that internal-conversion processes via conical intersections,which are accessed by out-of-plane deformation of the six-membered ring,dominate the photophysics of the lowest vibronic levels of adenine in the gas phase,while hydrogen-abstraction photochemistry driven by repulsive ~1pisigma~* states may become competitive at higher excitation energies.These ultrafast excited-state deactivation processes provide adenine with a high degree of intrinsic photostability.