Diffusion of Singlet Excitons in Tetracene Crystals

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Abstract
The diffusion length ls of singlet excitons in crystalline tetracene was measured in the temperature range 293 to 160 °K utilizing the quenching effect exerted on the crystal fluorescence by exciton traps located at the crystal surface. It is found that ls = 120 ± 10 Å at room temperature. It increases exponentially with decreasing temperature until it approaches a constant value of 580 ± 50 Å below 190 °K. The temperature dependence of the singlet exciton lifetime determined by exciton fission can fully account for the observed temperature dependence of the diffusion length. Therefore energy localization, like excimer formation, cannot be an effective process. The exciton diffusion coefficient is by a factor of 4 higher than in anthracene crystals. This can be explained in terms of the increase in the Davydovsplitting.