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

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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 B1u3 state than does the CH stretch corresponding to ν2(a1g) 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.