Theoretical study toward understanding ultrafast internal conversion of excited 9H-adenine

The CASPT2/CASSCF method with the 6-311G* basis set and an active space up to (14, 11) was used to explore the ultrafast internal conversion mechanism for excited 9H-adenine. Three minima, two transition states, and seven conical intersections were obtained to build up the two deactivation pathways for the internal conversion mechanism. Special efforts were made to explore the excited-state potential energy surfaces near the Franck-Condon region and determine the various barriers in the processes of deactivation. The barrier required from the (1)pi pi* (L-1(a)) state to deactivate nonradiatively is found to be lower than that required from the (1)pi pi* (L-1(b)) state. On 250 nm excitation, the (1)pi pi* (L-1(a)) state is populated, and the transition from (1)pi pi* (L-1(a)) to the lowest (1)n pi* state involves very low barriers, which may account for the observed short (< 50 fs) lifetime of the (1)pi pi* excited state. The deactivation of the lowest (1)n pi* state is required to overcome a barrier of 3.15 kcal/mol, which should be responsible for the 750 fs lifetime of the n pi* excited state. On 267 nm excitation, the vibrationally active (1)pi pi* (L-1(b)) state is populated. Excitation at 277 nm prepares the (1)pi pi* (L-1(b)) state without much excessive vibrational energy, which may be responsible for the observed > 2 ps lifetime.