Nonradiative Decay Processes of the Triplet State of Aromatic Compounds: Benzene

Abstract

The observed phosphorescence decay times of C₆H₆, ¹³CC₅H₆, C₆H₅D, C₆HD₅ and C₆D₆ have been measured in reproducibly treated rigid media of cyclohexane, 3‐methylpentane, and isopropanol at 77°K. The lifetimes are in the approximate ratio 1:1:1.06:1.5:2, respectively. The precision is ∼1% and the results show solvent dependence. The CH motion is clearly responsible for a large increase in the nonradiative transition probability on passing from C₆D₆ to C₆HD₅. The relative effect of further substituent hydrogens is less, on the average, as the H/D ratio increases. It appears that some nontotally symmetric CH stretching normal mode(s) contribute less to the radiationless depopulation of the ${}^3{B}_{\text{1u}}$ state than does the CH stretch corresponding to ${\nu }_2{\mathrm{(a}}_{\text{1g}})$ in C₆H₆. The discussion is based upon Franck–Condon considerations. The theory proposed by Siebrand [J. Chem. Phys. 47, 2411 (1967)] is considered in light of the results.