Theoretical and experimental evidence on the structure of benzotriazole in its first triplet electronic state

Theoretical data reveal that benzotriazole in its first triplet electronic state, T1, exhibits two energy minima. One is of Cs symmetry, corresponds to the 1H tautomer which has its 0–0 component at 398 nm; the other lacks symmetry, corresponds to the 2H tautomer which has its 0–0 component at 473 nm. The 2H tautomer is intrinsically more stable (by ca. 11 kcal/mol). tent with the previous theoretical inferences, experimental evidence for polycrystalline benzotriazole at 77 K, which consists of 100% of the 1H tautomer in the ground state, only exhibits the phosphorescence for the 2H tautomer.