Influence of surface effects and temperature on the interaction of triplet excitons in molecular crystals

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Abstract
The anisotropy of the triplet-exciton annihilation rate in a magnetic field has been investigated in naphthalene and in anthracene as a function of temperature and of the penetration depth of the incident light. In the region between the well-known level-crossing resonances, dips in the delayed fluorescence intensity are observed which are much more pronounced if the triplet excitons are generated near the surface of the crystal than in the volume and which disappear at lower temperatures. It is shown that this difference between the surface and the volume triplet annihilation rate and its temperature dependence is due to the triplet spin-relaxation rate which influences the effective lifetime of the triplet pairs. The anisotropy of prompt and delayed fluorescence in anthracene and of the delayed fluorescence in naphthalene, measured for different ways of exciton excitation, are fitted according to Suna's theory but with the anisotropic spin relaxation taken into account. As a result we get some new information about the temperature dependence of the triplet spin-relaxation rate and its dependence on the molecular environment of the triplet excitons in the volume and at the surface of the crystals.