Preliminary measurements of the triplet diffusivity tensor in cryStalline pyrene at 300 K are reported. Maximum diffusiGily occus along the b axis and L+,b = (1.25 f 0.3) X lO+ cm' s-' _ Diffusion in the UC' p!ane is nearly isotropic with D J (0.3 * 0.1) X lb* cm' s-' . These results are tentatively interpreted as diffusion dominated by nonlocal scattering.'
l.Introduction
In early theoretical calculations on triplet exkiton' ., diffusion in aromatic hydrocarbons, or on related properties such as Davydov splitting, the molecules were assumed to be immobile at their equilibrium positions in the crystal, and "static" transfer matrix elements were calculated. In the cask of anthracene and naphthalene, experimental triplet Davydovsplittitigs could be accounted for sntisfactorily in this way -P1
. *** Exciton-phonon interaction should however be included in any theoretical treatment of exciton transport. This was attempted in the case of the triplet exciton by introducing the concep't of "non.local" .scattering [2,3 J, where fltictuations of transfer matrix eizments due to vibration or rotation of molecules about their equilibrium positions assist triplet diffusion, as they do for cha'rge carriers in the.so-called "phonon- assisted tramp&t? regime 141. Measurement of the temperature dependence of triplet evciton.dynamics in Gthracene indeed shcwed the l&al scattering