Fluorescence and Triplet State of Hexafluorobenzene

Abstract

The photochemistry of hexafluorobenzene in the vapor phase has been studied to compare the heavy‐atom substituted molecule with the unsubstituted molecule of the same symmetry. Results show that at all wavelengths below 2800 Å, absorption produces two excited singlet states, one of which is quickly equilibrated to a fluorescent state, the other more slowly. Some process occurs from the latter state with a rate such that collisional loss of energy is barely competitive. This process may be an isomerization. The use of the biacetyl sensitization and Cundall techniques at various wavelengths indicated that the lifetime of the hexafluorobenzene triplet was short, and of the order of 10⁻⁷ sec leading to difficulties in absolute measurement of the triplet‐state yields. The quenching of biacetyl phosphorescence excited by 4358‐Å radiation by hexafluorobenzene is observed, and the effect of exciting wavelength upon the fluorescence yields of hexafluorobenzene is discussed. A simple mechanism has been proposed and rate constants and quenching cross sections calculated.