Thermal and Sample-Size Effects on the Fluorescence Lifetime and Energy Transfer in Tetracene-Doped Anthracene

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Abstract
The fluorescence decay time of anthracene crystals was studied for crystals of various sizes at numerous temperatures between 4.2 and 300 °K. The decrease in the lifetime observed between 300 and ∼ 140 °K is attributed to changes in the amount of reabsorption, while the decrease observed between ∼ 75 and 25 °K is consistent with fluorescence emission from two types of levels with different intrinsic lifetimes whose populations are in thermal equilibrium. The time evolution of the anthracene and tetracene fluorescence intensities was also investigated for samples of various sizes and at numerous temperatures. The results cannot be explained by exciton diffusion theory and are consistent with the predictions of long-range-interaction theory only if an anomalously large value is assumed for the strength of the interaction. This discrepancy is not yet understood, but it is shown that it is not due to the effects of reabsorption.