Photosensitization by Benzene Vapor: Biacetyl. The Triplet State of Benzene

Abstract

Biacetyl strongly quenches the fluorescence of benzene vapor. There is a resulting dissociation of biacetyl which occurs presumably because the second excited singlet state of biacetyl is produced by energy transfer from the benzene. Emission by biacetyl also occurs, but the ratio of phosphorescence to fluorescence is very large and may be infinity. The triplet state of biacetyl seems to be produced preferentially by energy transfer from a triplet state of benzene. Emission efficiencies are such that nearly every singlet‐state molecule of benzene which does not fluoresce must undergo an intersystem crossover to the triplet state. Since this crossover predominates over fluorescent emission, the life of the singlet state of benzene is presumably determined mainly by the crossover. The effective cross section for self‐quenching of the singlet state of benzene is about 0.036×10⁻¹⁶ cm², while the effective cross section for quenching by biacetyl is about 2.5×10⁻¹⁶ cm². Since the life of the triplet state of benzene in the gas phase is not known, cross sections for reactions of this state may not be calculated.