Assignment of the S1- S0 Electronic Absorption Spectra of 1 H-Benzotriazole and of 1,2,4-Triazolo[1,5- a]Pyrimidine as π*-π by Rotational Band Contour Analysis

Abstract

The 0 0 0 bands in the S 1- S 0 electronic absorption systems of l H-benzotriazole at 286 nm and of 1,2,4-triazolo[l,5- a]pyrimidine at 315 nm have been analyzed by computer simulation of their rotational contours. The results have shown that the benzotriazole hand is an almost pure type- B band, while in the case of 1,2,4-triazolo[l,5- a] pyrimidine it is a pure type- A band, so each S 1- S 0 electronic system is assigned as à 1 A′(ππ*)- X̃ 1 A′. In both of these molecules, the transition moment is in the molecular plane, almost parallel to the b-inertial axis in benzotriazole. while it is parallel to the a-inertial axis in 1,2,4-triazolo[1,5- a]pyrimidine. Since the S 1- S 0 transitions in the indole derivatives previously analyzed were shown to be π*-π, it seems likely that the nature of the S 1- S 0 electronic transition is the same in all the molecules of this kind.