On the triplet energy transfer mechanism in isotopic mixed molecular crystals

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Abstract
The transfer of triplet excitation energy between isotopic traps in benzene isotopic mixed crystals has been investigated using both steady state and pulsed excitation sources. It has been demonstrated for the first time that the excitation–tunneling transfer mechanism [H. Sternlicht, G. C. Nieman, and G. W. Robinson, J. Chem. Phys. 38, 1326 (1963)] predicts the correct dependence of the transfer rate on trap depth. Furthermore, it is shown that the Perrin approximation, which grossly simplifies the treatment of the experimental data, is a reasonable albeit not perfect approximation for this system. A large fraction of the energy transfer is completed during times much shorter than the benzene triplet state lifetime. This is consistent with the strong distance dependence of the transfer rate constant predicted by a tunneling mechanism. Using the Perrin approximation, it has been shown that the use of a linear chain tunneling mechanism predicts the excitation exchange integral to be of the correct magnitude, a conclusion that is also consistent with a strong distance dependence of the transfer rate constant. (Energy can be transferred through 2–3 intervening host molecules.) In this system there is no evidence for the existence of the intermolecular triplet energy relaxation mechanism recently shown to be operative in chemically mixed crystals [H. C. Brenner and C. A. Hutchison, Jr., Chem. Phys. 58, 1328 (1973)].

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