Delayed Fluorescence of Solid Solutions of Polyacenes. II. Kinetic Considerations

Abstract

Delayed fluorescence has been observed at 77°K from rigid‐glass solutions of some aromatic hydrocarbons and their halo derivatives. The intensity of the delayed fluorescence is proportional to the square of the phosphorescence intensity. The decay of delayed fluorescence in all cases may be represented as the sum of two first‐order processes; the lifetime of the longer‐lived component varies between 1/2↔1/5 that of the phosphorescence while the lifetime of the shorter‐lived component varies between 1/10↔1/50 that of phosphorescence. The intensity of delayed fluorescence observed from a solution of a given compound varies as the ratio φ_fφ_p²/k₃^*2, where φ_f and φ_p are fluorescence and phosphorescence quantum yields, respectively, and where k₃^* is the emissive phosphorescence rate constant. Phosphorescence decay exhibits a slight nonexponentiality only in the case of those solutions where the delayed fluorescence is especially strong. Delayed fluorescence is spectrally identical, within the limits of spectrograph resolution available to us, with ordinary fluorescence. The above facts may only be interpreted by a kinetic mechanism which involves a triplet—triplet annihilation process, this mutual annihilation resulting in the eventual production of the singlet excited state of one of the partners. Some ancillary hypotheses are required to interpret the shorter‐lived component of the delayed fluorescence, and a kinetic flowsheet representative of the energy‐transfer processes involved in the production of delayed fluorescence is proposed.