Temperature Effects on the Phosphorescence of Benzene and Toluene

Phosphorescence lifetimes have been measured as a function of temperature in the region 2–160°K for the following polycrystalline mixtures: C6H6 and C6D6 in cyclohexane and in cyclopentane; toluene-h8, toluene-α-d3, and toluene-d8 in methylcyclohexane; and toluene-h8 in perdeuteromethylcyclohexane. In each case the results can be reasonably fit by a formula of the form 1 / τP = K0 + K1exp(− ΔE / kT), with the values of K0, ΔE, and K1 depending on both the solute and the solvent. An unexpected increase in lifetime at very low temperatures has been observed for benzene. For C6H6 and C6H6 in cyclohexane the ratio ΦP / ΦF = kpkISCτP / kF has been determined by numerical integration of the corrected emission spectra. Plots of ΦP / ΦF vs τP yield straight lines.