Trap–Trap Triplet Energy Transfer in Isotopic Mixed Benzene Crystals

Abstract

The phosphorescence and fluorescence spectra of three-component isotopic mixed benzene crystals, containing two different guests in a C6D6 host crystal, are studied as a function of guest concentration, excitation lamp intensity, and temperature. The energy relaxation processes in these systems are discussed, and it is established that few, if any, host-triplet excitons are produced during the interconversion of “trapped” singlet excitation into “trapped” triplet excitation. Triplet–triplet excitation transfer from one guest (trap) to a lower-energy guest (supertrap) is observed at temperatures where the host–guest ΔE is greater than 30KT. The concentration dependence of the trap–supertrap triplet energy transfer is interpreted in terms of a Perrin model and indicates that the energy can be transferred over no more than ∼20 Å in this system. At high lamp intensities, the phosphorescence intensity is found to depend upon the square root of lamp intensity. This is discussed in terms of excitation-annihilation models. Evidence is also presented for the photogeneration of impurities at these high lamp intensities.