Nature of s-tetrazine emission spectra

Abstract

Fluorescence is established as the unique emission spectrum of s-tetrazine and dimethyl s-tetrazine in several ways-notably by polarization measurements (out of plane) and life-time measurements of the emitting state (4X10_-9 sec for dimethyl s-tetrazine at 300° K). The vibrational features (which indicate an allowed transition) are not fully understood on the basis of emission solely from the lowest singlet state. Experiments involving H-bonding solvents suggest that the hydrogen bond exists in the (n, ∏) singlet state in s-tetrazine as contrasted to the situation in the (n, ∏) triplet state in pyrazine and pyrimidine, where the H bond is broken. Polarization measurements on the emission as a function of the exciting wavelength confirms Mason's observation of a second n→∏ band system at about 3200 Å. The polarization of the 3200-Å band system is out of plane, thereby supporting Mason's assignment of this band as an allowed n→∏ transition. Different viewpoints on the mechanism of the fluorescence are summarized. The localized exciton splitting prediction of a forbidden lowest singlet state is contradicted by the experimental evidence. It seems probable that the lo west excited state is a triplet.