Deciphering Low Energy Deactivation Channels in Adenine

The radiationless decay paths of 9H-adenine in its lowest excited states ¹nπ*, ¹L_b(¹ππ*), and ¹L_a(¹ππ*) and in dissociative ¹πσ* states have been mapped in vacuo at the CASPT2//CASSCF resolution. The minimum energy path (MEP) of the ¹L_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 S₀/¹L_a conical intersection (CI). The vertically excited ¹nπ* and ¹L_b states are found to relax to the respective minima and to require some energy to reach CIs with S₀. This picture suggests that ¹L_a alone is responsible of both components of the ultrafast biexponential decay (with τ₁ < 0.1 ps and τ₂ < 1 ps) recently observed in time-resolved pump−probe resonant ionization and fluorescence spectroscopy, and that the ¹nπ* and ¹L_b states do not act as important intermediates in the ¹L_a decay process. We find that the ¹L_a→¹πσ_N9H* internal conversion can be followed by N₉−H photocleavage, albeit with tiny quantum yield. The amino N₁₀−H bond photocleavage is hindered by the high barrier encountered along the N₁₀−H bond-breaking path in the ¹πσ_N10H* state.