Phosphorescence and the True Lifetime of Triplet States in Fluid Solutions

Abstract

Phosphorescence has been observed in a highly purified fluid solution of naphthalene in 3‐methylpentane (3‐MP). The phosphorescence lifetime of C₁₀H₈ in 3‐MP at −45°C was found to be 0.49 ± 0.07 sec, while that of C₁₀D₈ under identical conditions is 0.64 ± 0.07 sec. At this temperature 3‐MP has the same viscosity (0.65 CP) as that of benzene at room temperature. It is believed that even these long lifetimes are dominated by impurity‐quenching mechanisms. Therefore it seems that the radiationless decay times of the lowest triplet states of simple aromatic hydrocarbons in liquid solutions are sensibly the same as those in the solid phase. A slight dependence of the phosphorescence lifetime on solvent viscosity was observed in the temperature region, −60° to −18°C. This has been attributed to the diffusion‐controlled quenching of the tiplet state by a residual impurity, perhaps oxygen. Bimolecular depopulation of the triplet state was found to be of major importance over a large part of the triplet decay. The lifetime of triplet C₁₀H₈ at room temperature was also measured in highly purified benzene by means of both phosphorescence and triplet–triplet absorption. The lifetime was estimated to be at least 10 times shorter than that in 3‐MP. This is believed to be due not only to residual impurities in the solvent but also to small amounts of impurities produced through unavoidable irradiation by the excitation source. In agreement with this idea, lifetime shortening caused by intense flashes of light is readily observed. This latter result suggests that experiments employing flash lamp techniques are not suitable for these kinds of studies.