Effect of Solvent on Excimer Fluorescence Yields

It was observed that certain solvents enhanced the relative amount of fluorescence from the excimer of an excited solute molecule. The ratio of intensity of excimer fluorescence $I_D$ to intensity of monomer fluorescence $I_M$ for solutions of equal concentration of 2,5‐diphenyl‐1,3‐oxazole, PPO was approximately four times greater in cyclohexane and ethanol than in the aromatic solvents benzene, toluene, p‐xylene, and 1,2,4‐trimethylbenzene. Also at room temperature 24°C excimer fluorescence of 2‐(4‐isopropylphenyl)‐5‐(4‐biphenylyl)‐1,3,4‐oxadiazole (i‐p‐PBD) was observed only from concentrated solutions in cyclohexane. The effect of temperature on the intensity of excimer fluorescence indicated the role of the solvent in excimer formation and dissociation.