Radiational Deuterium Effect in the Triplet—Singlet Transition of Benzene

Abstract

Detailed phosphorescence spectra of C₆H₆ and C₆D₆ in an argon matrix (20°K) were analyzed and found to contain emissions from several crystalline sites. Benzene's triplet decay time is found to be 11.7 and 17.5 sec in the two major sites. These are increased in the deuterated molecule to 14.5 and 29.5 sec. It is shown that the lifetime difference caused by deuteration, which had been ascribed entirely to different rates of radiationless T₁—S₀ decay for the two molecules, actually is due substantially to changes in the vibronic coupling of the radiational decay pathway as indicated by the lower emission intensity in certain C₆D₆ vibrational progressions relative to those in C₆H₆. The ratio of the quantum yields of phosphorescence of C₆D₆ to C₆H₆ is found to be $\text{1.40 ± 0.03}$ while the ratio of radiative triplet decay rates is $\text{0.82 ± 0.04}$ ..