Phonon assisted trap–trap triplet energy migration in the 0 °K limit in crystalline benzene

Abstract

The transfer of triplet excitation energy between isotopic traps in three component (host, trap, and supertrap) benzene isotopic mixed crystals has been studied. The trap–host energy difference was kept constant for the systems investigated while the trap–supertrap difference was varied. The transfer of triplet excitation energy was determined by measuring the trap‐to‐supertrap phosphorescence intensity ratio as a function of concentration. The results demonstrate the expected dependence of off‐resonance trap–supertrap excitation energy transfer on the lattice (phonon) Franck–Condon factor. The efficiency is found to go through a maximum at a trap–supertrap energy separation of ∼50 cm⁻¹, decreasing rapidly at higher and lower values. The observed inefficiency of trap–trap transfer (as compared to trap–supertrap transfer) is attributed to small differences in trap energies caused by crystal inhomogeneities.