Triplet exciton transfer and fusion in isotopically mixed molecular crystals

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Abstract
We present experimental results on triplet energy transfer from traps to super‐traps and on triplet exciton fusion in substitutionally disordered naphthalene crystals, monitored by the phosphorescence and by the delayed fluorescence of the traps and of the super‐traps. The concentration dependence of these emissions at fixed temperatures has been discussed abundantly by other authors. In this work, we present new results on their dependence on temperature and on the source of excitation, which have, respectively, been little or not at all reported on before. Comparison of experiments done with selective and with nonselective excitation of the traps shows that the results depend on the method of preparation of the trap excitons. The existence of threshold temperatures for these emissions is interpreted as evidence of phonon assisted processes, for which mechanisms are proposed. A significant drop in transport efficiency at high temperatures is observed for trap concentrations from 4% to 8%. This drop with trap concentration is discussed in relation to the random distribution of traps and to the likely importance of clusters of ’’conducting’’ sites for energy transport in random systems.