The kinetics of phosphorescence and delayed fluorescence decay for aromatic hydrocarbons in liquid paraffin

Abstract

The decay constantskTfor phosphorescence andkDfor delayed fluorescence have been measured for the same outgassed solutions of acenaphthene, pyrene, 1,2-benzanthracene and fluoranthene in liquid paraffin over a range of temperatures between – 70 and 30 °C. In all cases the luminescence decay is exponential and the intensity of delayed fluorescence is found to vary as the second power of incident light intensity; under these conditions the relationkD= 2kTrequired by the triplet-triplet annihilation origin of delayed fluorescence is established over the temperature ranges in which delayed fluorescence and phosphorescence are exhibited simultaneously. Diffusional quenching of the triplet state3Aby a solute impurityQis believed to be responsible for the temperature (viscosity) dependence ofkTandkDsince (a)kTandkDare linear functions of solute concentration where this is examined, (b) by taking account of the reversibility of the quenching process3A+Q⇌A+3Qat higher temperatures (lower viscosities) the virtual independence ofkon temperature exhibited for 1,2-benzanthracene and fluoranthene under these conditions may be explained. The resumed temperature dependence ofkDfor fluoranthrene at still higher temperatures is attributed to endothermic quenching of the triplet state by an impurity with a higher triplet state than that of the solute.