Thermal effects on triplet exciton processes in anthracene crystals

Energy transfer and fusion annihilation of the triplet exciton were studied in anthracene crystals using delayed fluorescence. The decay rates of the exciton via monomolecular and bimolecular processes (β and γ, respectively) were measured at temperatures between 90-400 K. As predicted by kinetic analysis including the exciton-trapping effect, the same temperature dependence was obtained on β and γ by eliminating the following thermal effects on the diffusion and fusion-annihilation rates. The energy transfer to traps was diffusion limited, and the diffusion rate decreased by a factor of about 3 on heating from 90 to 400 K. The fusion-annihilation rates between free and trapped excitons, which were about 10⁵ times larger than that between two free excitons, increased as the temperature was raised. The mechanism of the free-trapped fusion must be dominated by some long-range interaction.