Temperature Dependence of Triplet-Exciton Dynamics in Anthracene Crystals

Abstract

The component of the triplet-exciton diffusion along the crystal $a$ axis was measured in anthracene crystals at 118, 160, 298, and 371 °K, and found to be $D_{\mathrm{aa}}=(4.0\mathrm{}\pm 0.5),(2.5\pm 0.3),(1.5\pm 0.2)$, and (1.6 ± 0.3) × ${10}^{-4\mathrm{}}$ ${\mathrm{cm}}^2$ ${\sec }^{-1\mathrm{}}$, respectively. The polarized-excitation spectra (0, 0 line) for delayed fluorescence have also been measured. The inferred values for the Davydov splitting $\Delta$ and the effective scattering rate $\Gamma$ at the corresponding temperatures are $\Delta =18\mathrm{}\pm 2,18\pm 6,17\pm 3$, and 19 ± 2 ${\mathrm{cm}}^{-1\mathrm{}}$ and $\Gamma =14\mathrm{}\pm 1,30\pm 2,51\pm 1$, and 65 ± 2 ${\mathrm{cm}}^{-1\mathrm{}}$. 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 $\pm \frac{1}{2}(a\pm b)$, is estimated to be ∼ 0.1 ${\mathrm{cm}}^{-1\mathrm{}}$, 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.