Deciphering low energy deactivation channels in adenine.

The radiationless decay paths of 9H-adenine in its lowest excited states (1)npi*, (1)L(b)((1)pipi*), and (1)L(a)((1)pipi*) and in dissociative (1)pisigma* states have been mapped in vacuo at the CASPT2//CASSCF resolution. The minimum energy path (MEP) of the (1)L(a) state, which shows the strongest absorption below 5 eV, is found to decrease monotonically along the puckering coordinate from the vertical excitation to a S(0)/(1)L(a) conical intersection (CI). The vertically excited (1)npi* and (1)L(b) states are found to relax to the respective minima and to require some energy to reach CIs with S(0). This picture suggests that (1)L(a) alone is responsible of both components of the ultrafast biexponential decay (with tau(1) < 0.1 ps and tau(2) < 1 ps) recently observed in time-resolved pump-probe resonant ionization and fluorescence spectroscopy, and that the (1)npi* and (1)L(b) states do not act as important intermediates in the (1)L(a) decay process. We find that the (1)L(a)-->(1)pisigma(N9H)* internal conversion can be followed by N(9)-H photocleavage, albeit with tiny quantum yield. The amino N(10)-H bond photocleavage is hindered by the high barrier encountered along the N(10)-H bond-breaking path in the (1)pisigma(N10H)* state.