Deciphering Low Energy Deactivation Channels in Adenine

The radiationless decay paths of 9H-adenine in its lowest excited states (1)n pi*, (1)Lb((1)pi pi*), and L-1(a)((1)pi pi*) and in dissociative (1)pi sigma* states have been mapped in vacuo at the CASPT2//CASSCF resolution. The minimum energy path (MEP) of the L-1(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/La-1 conical intersection (CI). The vertically excited (1)n pi* and L-1(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 L-1(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)n pi* and L-1(b) states do not act as important intermediates in the L-1(a) decay process. We find that the L-1(a) -> (1)pi sigma(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)pi sigma(N10H)* state.