Internal conversion and intersystem crossing in fluoropyridine vapors

Abstract

Fluorescence, fluorescence excitation, biacetyl-sensitized phosphorescence excitation and absorption spectra of three fluoropyridines (2-fluoropyridine, 6-fluoropyridine and 2,6-difluoropyridine) have been measured in the vapor phase. The quantum yields of the S1→T1 intersystem crossing of these fluoropyridines have been determined by means of a sensitized biacetyl phosphorescence method based on intermolecular T–T energy transfer. It is shown that the main nonradiative process in these fluoropyridines is the internal conversion S1→S0, which may include possible photochemical decomposition processes. It is shown that the internal conversion rates increase almost exponentially with increasing excitation energy, while the increase of the S1→T1 intersystem crossing rates is not significant.