'Excimer’ fluorescence VI. Benzene, toluene, p -xylene and mesitylene

Observations were made of the fluorescence spectra of cyclohexane and n-hexane solutions of benzene, toluene, p-xylene and mesitylene and of the pure liquids as a function of concentration and temperature under standard excitation conditions. Excimer fluorescence is observed in benzene, toluene and mesitylene, and there is evidence for excimer formation in p-xylene. The data are analysed to determine the fluorescence quantum efficiencies, the rate parameters and other properties of the monomer-excimer systems. The following values are obtained for the excimer binding energy B; benzene, 0.22 eV; toluene, 0.17 eV; p-xylene, > 0.11 eV; and mesitylene, 0.12 eV. The results are consistent with the excimer potential model, and the reduction in B in the methyl derivatives is attributed to steric hindrance.