title: On the mechanisms of triplet excited state population in 8-azaadenine.
abstract: 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.
authors.affiliation: Instituto de Química, Universidade de São Paulo, Brazil. jpgobbo@iq.usp.br
authors.forename: João Paulo
authors.initials: JP
authors.lastname: Gobbo
authors.forename: Antonio Carlos
authors.initials: AC
authors.lastname: Borin
mesh_terms: D000225:Adenine / Q000031:analogs & derivatives* / Q000737:chemistry; D008958:Models, Molecular; D010100:Oxygen / Q000737:chemistry; D010777:Photochemistry; D011789:Quantum Theory
chemical_list: D000225:Adenine; D010100:Oxygen; C006654:8-azaadenine