Trap–Trap Triplet Energy Transfer in Isotopic Mixed Benzene Crystals

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 $\mathrm{\Delta E}$ is greater than $\text{30KT}$ . 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.