Phosphorescence lifetimes of cationic dyes: Concentration and temperature effects

Abstract

Experiments have been performed to determine the effect of the concentration of various anions on the phosphorescence lifetimes of cationic dyes in ethanol solution. The three dyes studied were 1,1′‐diethyl 2,2′‐cyanine; 3,3′‐diethyl thiacyanine; and acridine orange. At 77 °K in a glassy solvent the chloride form of the dye salts decays exponentially. In two cases the decay time is independent of concentration; however, the lifetime of the thiacyanine dye increases from (0.35 ± 0.02) sec at infinite dilution up to a maximum of (0.55 ± 0.02) sec at 10⁻⁴M. In the presence of heavy ions the decay becomes nonexponential and the lifetime decreases in the order Cl⁻ > Br⁻ > I⁻. The relative strengths of the perturbation within this series of anions are nearly the same for all three dyes. Below its melting point, ethanol can be prepared either as a supercooled liquid (and glass) or in a crystalline state. The study of phosporescence lifetimes as a function of temperature reveals two different behaviors, depending only on the phase of the low‐temperature solution. The phosphorescence lifetime can be used to detect the glass transition and also the development of crystallization in the supercooled liquid.