Knowledge Bank

The ultraviolet absorption spectra of methyl and phenyl derivatives of s-triazine were measured in both polar and non-polar solvents at room temperature and $77\circ$K. In contrast to s-triazine and the methyl triazines where a low energy ($\sim 37,000cm.-1$), relatively intense $(ϵ\sim 7001./molecm)n\to \pi \ast$ transition is observed, the phenyl triazines show a very intense $(ϵ>2.5\times 1041./molecm.)n\to \pi \ast$ transition at $\sim 38,500cm.-1$ which is very similar to biphenyl and the angular polyphenyls. At $33,000cm.-1$ there is an indication of a shoulder which is obscured in alcohol solvent. It is tempting to attribute this weak feature to an $n\to \pi \ast$ transition, but the evidence is not very convicining. The polarization of the phosphorescence of the phenyl derivatives of s-triazine was determined in rigid glass solutions at $77\circ K$, by the method of ``Photoselection''. Excitation spectra of the polarized phosphorescence of these molecules has permitted an unambiguous assignment of the lowest energy singlet transition as an $n\to \pi \ast$ orbital type.