Triplet–Triplet Annihilation and Delayed Fluorescence in Isotopic Mixed Crystals of Benzene

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Abstract
The behavior of triplet excitations of a single isotopic guest in a benzene crystal (C6H6 in C6D6) is investigated as a function of temperature from 2° to 15°K by detailed measurements of the phosphorescence. It is found that even in dilute systems there is significant (∼20% for 0.2% guest) temperature‐independent annihilation below 8.0°K. Above 8.0°K a thermally activated annihilation process dominates the phosphorescence quenching in an ultrapure sample. Thus while the first‐order decay rate, which includes impurity quenching, increases by less than a factor of 2, the second‐order annihilation rate increases by five orders of magnitude when the temperature is raised from 4° to 12°K with an activation energy equal to the known trap depth. The difficulty of fitting a nonexponential decay to the combination of first‐ and second‐order decays is discussed and a new method is suggested which accomplishes this. Measurements of the temperature dependence of the phosphorescence decay, the steady‐state phosphorescence intensity, and the steady‐state delayed fluorescence intensity provide independent evaluations of the annihilation rate which are in good agreement with one another at all temperatures. The low delayed‐fluorescence quantum yield in benzene is discussed.