Quenching of Naphthalene Luminescence by Oxygen and Nitric Oxide

Abstract

The quenching of naphthalene-h8 and naphthalene-d8 luminescence by O2 and NO in the absence of molecular diffusion is investigated. By measurement of the dependence of fluorescence and phosphorescence yields and lifetimes on quencher concentration, it is possible to separate the effects of singlet and triplet electronic state quenching and to determine the critical quenching radii R0. No static quenching of naphthalene fluorescence is observed for O2 concentrations of up to 0.25M. Thus, the corresponding R0 value in the Perrin formula is less than or equal to 7.1 Å. Conversely, the concentration dependence of naphthalene-h8 and naphthalene-d8 triplet state quenching by O2 is in excellent agreement with the Perrin formula, and the corresponding R0 values are 9.9 ± 0.3 and 9.8 ± 0.3 Å, respectively. Contrary to the Perrin model, but in agreement with the theory for quenching by an exchange interaction, the quenching process is competitive with the normal decay rate, and the phosphorescence decay curves become nonexponential with increasing O2 concentration. The rate constant for naphthalene triplet-state quenching by O2 decreases upon the deuteration of naphthalene. This suggests that enhanced intersystem crossing, and not energy transfer, is the dominant triplet-state quenching mechanism, at least for naphthalene-O2 separations ≈ 10 Å. The concentration dependence of naphthalene-h8 and naphthalene-d8 fluorescence quenching by NO is in excellent agreement with the Perrin formula, and the corresponding R0 values are 12.3 ± 0.3 Å and 13.0 ± 0.3 Å, respectively. Fluorescence quenching by NO (80% decrease at 0.3M NO) has no effect on the fluorescence lifetime of naphthalene, which indicates that quenching occurs in less than 5 nsec. Possible causes for the greater efficiency of fluorescence quenching by NO than by O2 are discussed. The value of R0 for naphthalene triplet-state static quenching by NO cannot be determined with certainty because of the possibility that the excited singlet state is quenched directly to the ground state. But the value of R0 for naphthalene triplet-state quenching must be less than or equal to that for fluorescent singlet-state quenching.