Concentration Depolarization of the Phosphorescence Emission

Abstract

The degree of polarization (P) of the phosphorescence for several organic molecules is studied as a function of concentration (C) in the 10⁻⁴−10⁻¹M range in rigid medium. For all of the molecules examined, P remains unchanged until C reaches 10⁻²M; then P starts declining. The decline of P is shown to arise neither from the decrease in solvent rigidity nor from crystallization of the molecules studied. Two possibilities that might explain the decline of P are discussed: (a) the formation of a charge‐transfer complex that perturbs the direction of the emitting oscillator and (b) T—T energy transfer. The degree of polarization of some of the compounds studied does not change during the phosphorescence decay, a result that might favor the first possibility. The sensitivity of the value of the slope of the P–C curve to the phosphorescence lifetime provides evidence in support of the T—T energy‐transfer mechanism. At concentrations below 10⁻²M, nonradiative S–S transfer seems to be either absent or relatively inefficient in depolarizing the phosphorescence emission in the systems studied. External heavy‐atom effects on the degree of polarization of the phosphorescence of aromatic hydrocarbons are found to be much greater when caused by haloaromatics than by ethyl iodide. This observation might be due to either a larger association constant or stronger spin—orbit perturbation for the former compounds than for the latter.