Electronic excitation energy transfer in organic liquid scintillators

Abstract

The rate constant for transfer of electronic excitation energy from the first excited singlet state of the solvent toluene to two indoles, 1-p-chlorophenyl-5-benzyloxy-2-phenylindole and 5-methyl-3-phenylindole-1-propionitrile, as solutes in three types of solvent media, pure toluene, 1:9 mixture of toluene and cyclohexane, and 1:9 mixture of toluene and liquid paraffin is determined as a function of temperature in the range 293–353 K under oxygen-free conditions. The diffusion coefficients of the interacting molecules are measured in situ at ambient temperature. The rate constant is found to vary, in the case of each of the six systems, linearly with the sum of the diffusion coefficients of the two interacting molecules; from this linear variation the contributions of molecular diffusion and excitation energy migration to the rate constant are quantitatively estimated. The results indicate that energy migration involves weak multipole–multipole interaction between an excited and an unexcited solvent molecule, and not successive excimer formation and dissociation. Unique values of the energy migration coefficient and the effective distance at which the solvent–solute energy transfer takes place in the final step, are evaluated in the case of all the six systems; these values are model independent. The final step in energy transfer essentially involves long-range interaction. Dependence of energy migration coefficient and the effective energy transfer distance on environment is discussed.