On the mechanisms of triplet excited state population in 8-azaadenine.

The photophysics of 8-azaadenine (8-AA) has been studied with the CASPT2//CASSCF protocol and ANO-L double-ζ basis sets. Stationary equilibrium structures, surface crossings, minimum energy paths, and linear interpolations have been used to study possible mechanisms to populate the lowest triplet state, T(1)(3)(ππ*), capable of sensitizing molecular oxygen. Our results show that two main mechanisms can occur after photoexcitation to the S(2)(1)(ππ*) state. The first one is through the S(2)/S(1) conical intersection (((1)ππ*/(1)nπ*)(CI)), leading to the S(1) ((1)nπ*) state minimum, (S(1) ((1)nπ*))(min), where a singlet-triplet crossing, ((1)nπ*/(3)ππ*)(STC), is accessible. The second one starts with the ((1)ππ*/(3)nπ*)(STC) at the (S(2)((1)ππ*))(min), from which the system can evolve to the (T(2) ((3)nπ*))(min), with subsequent population of the T(1) excited electronic state, due to the ((3)nπ*/(3)ππ*)(CI) conical intersection.