Temperature dependence of the triplet exciton yield in fission and fusion in tetracene

The increase in quantum efficiency for luminescence with increasing excitation intensity is measured as a function of temperature in tetracene. The results indicate that the quantum efficiency for the production of triplets from an excited singlets decreases markedly below 300 °K. This decrease is accounted for by a process which competes with exciton fission with a rate K = (1.3±0.2)×10⁸ sec^-1. This value is shown to be inconsistent with a singlet trapping interpretation for the nonexponential decays found by others in the prompt fluorescence. The results are consistent with a nearly temperature independent value for the fraction of exciton fusion events which lead to excited singlets f. The relative constancy of f above 300 °K implies that fusion resulting in a triplet proceeds via the first excited triplet. This in turn provides a qualitative explanation for the apparent lack of a relationship between the measured value of f and the rudimentary value inferred from spin statistics.