TRIPLET-TRIPLET ANNIHILATION AND DELAYED FLUORESCENCE IN ISOTOPIC MIXED CRYSTALS OF BENZENE

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.