Knowledge Bank

Emission with a low quantum yield ($<$1%) has been observed after exciting each of several bands in the electric dipole forbidden 3200 \AA absorption system $(1E\prime \prime ⟵1A1)$ of s-triazine vapor. This now leaves pyridine as the only one of the six well known azabenzenes from which vapor emission has not been observed. Emission from 0.5 torr of s-triazine is fluorescence only, and its structure is consistent with that expected from the single vibronic level in the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}E\prime \prime$ state pumped in excitation. The fluorescence spectrum from the level $61$ is particularly simple. The intensity is induced principally by the $e\prime$ ring distortion $\nu 6$ appearing in the vibronic origins $601$ and $621$ This mirrors the activity in vapor absorption, but other significant vibronic origins seen in absorption are of reduced importance. Long progressions in the symmetric ring breathing mode $\nu 12$ are built on the $\nu 6$ fluorescence origins giving information on the Franck-Condon development which in absorption is obscured by conjestion. The fluorescence is quenched by added gases such as isopentane. By analogy with the known behavior of glyoxal, pyrazine, and other molecules, this is indicative of collision-induced triplet formation. However, no phosphorescence can be observed after fluorescence quenching. If triplet formation does occur, this suggests that the lowest triplet in the vapor is the $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}E\prime \prime$ state predicted by Hochstrasser to be nonemitting.