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

The 000 bands in the S1-S0 electronic absorption systems of lH-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 S1-S0 electronic system is assigned as Ã1A′(ππ*)-X̃1A′. 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 S1-S0 transitions in the indole derivatives previously analyzed were shown to be π*-π, it seems likely that the nature of the S1-S0 electronic transition is the same in all the molecules of this kind. © 1993 Academic Press, Inc.