Nature of s-Tetrazine Emission Spectra

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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 (4×10—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, π*) singlet state in s‐tetrazine as contrasted to the situation in the (n, π*) 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→π* 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→π* 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 lowest excited state is a triplet.

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