Temperature Dependence of Triplet-Exciton Dynamics in Anthracene Crystals

The component of the triplet-exciton diffusion along the crystal a axis was measured in anthracene crystals at 118, 160, 298, and 371 ^, and found to be D₀₀= (4. 00. 5), (2. 50. 3), (1. 50. 2), and (1. 6 0. 3) 10^-4 cm^2 sec^-1, respectively. The polarized-excitation spectra (0, 0 line) for delayed fluorescence have also been measured. The inferred values for the Davydov splitting and the effective scattering rate at the corresponding temperatures are =182, 186, 173, and 19 2 cm^-1 and =141, 302, 511, and 65 2 cm^-1. The simultaneous measurement of these parameters, in conjunction with an expression for diffusion derived from a phenomenological model of triplet-exciton scattering, allows the assessment of the relative importance of local vs nonlocal scattering mechanisms in the triplet-exciton motion. The nonlocal scattering rate, due to fluctuations in the exciton-transfer matrix elements between molecules separated by 12 (a), is estimated to be 0. 1 cm^-1, and appears to be temperature insensitive. The local scattering mechanism is dominant, but the nonlocal fluctuation rates can make a sizable contribution to the rate of triplet transport. The spectroscopic measurements show that the hopping model for transport is applicable in the temperature range studied.