Knowledge Bank

Generally aromatic molecules having a lowest singlet excited state of $(n,\pi \ast )$ assignment show phosphorescence and no fluorescence. An exception to this general rule is found in the case of s-tetrazine, and its dimethyl and diphenyl derivatives. The singlet nature of the emitting excited state is proved by measurement of life-time and polarization. The vibrational sequences in the emission spectrum are found to be closely related to those in absorption spectrum. Like absorption spectrum the fluorescence spectrum shows blurring and blue-shift in hydroxylic solvents. Possible reasons for the lack of intersystem crossing to the triplet state are also discussed.