Relative Interaction Radii for Quenching of Triplet-State Molecules

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Abstract
The relative efficiencies with which O2, NO, and Xe enhance the transition from the excited triplet state to the ground state in naphthalene have been determined from static experiments. Essentially, the experiments consisted of the observation of the steady-state phosphorescent emission intensity and the intensity of the electron spin resonance (ESR) signal of the triplet-state moelcules as functions of added quencher. All measurements were made in 3-methylpentane (3-MeP) glass solutions at 77°K, where material diffusion is minimized. The data were analyzed in terms of the expression N / N0 = exp{ − [0.0025(Rq)3Cq]}, where N and N0 are the steady-state naphthalene triplet-state concentrations in the presence and absence of added quencher, respectively, Rq is an effective interaction distance for the quenching process, and Cq is the concentration of added quencher. This equation has previously been shown to be an adequate approximation for interaction mechanisms that are controlled by electron exchange overlap integrals. The values of Rq derived from the experimental data are 10.5 ± 0.3, 13.1 ± 0.2, and 5.8 ± 0.4 Å for O2, NO, and Xe, respectively. Corresponding values for quenching of phenanthrene triplet-state molecules by naphthalene and for naphthalene-naphthalene energy transfer are 13.6 ± 0.1 and 12 ± 1 Å, respectively. Analysis of the emission and ESR data shows that the mechanisms of quenching by NO and O2 are essentially radiationless while the Xe perturbation affects mainly the radiative transition probability. The derived results are discussed in terms of triplet-state quenching by energy transfer to the quencher and by the enhancement of intramolecular triplet-singlet intersystem crossing. Since NO does not have the necessary energy levels for energy transfer from naphthalene to proceed, the result that the value for Rq for NO is larger than that for O2 indicates that energy transfer probably does not occur in the O2 case either.

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