Excited-state potential energy surface for the photophysics of adenine

The decay paths on the singlet excited-state surface of 9H-adenine and the associated energy barriers have been calculated at the CAS-PT2//CASSCF level. There are three fundamental paths for the photophysics: two paths for the L-1(b) state which are virtually barrierless at the present level of theory and correspond to formation of the (n,pi*) intermediate and direct decay to the ground state and a third path for ground-state decay of the (n,pi*) state with an activation barrier of approximately 0.1 eV. The L-1(a) state, which has the largest oscillator strength, either decays directly to the ground state or contributes indirectly to the excited-state lifetime by populating the two other states. The results are used to interpret the photophysics in terms of an excited-state plateau for the L-1(b) state that corresponds to the short-lived excited-state component (approximately 0.1 ps) and a well (i.e., a proper minimum) for the (n,pi*) state that gives rise to the long component (1 ps or more). The direct decay to the ground state of the 1Lb state is probably the decay channel invoked to explain the experimental wavelength dependence of the relative amplitudes of the two components. In addition to that, the excited-state component in the nanosecond range detected in the time-resolved photoelectron spectrum is proposed to be a triplet (pi,pi*) state formed after intersystem crossing from the singlet (n,pi*) state.