Phosphorescence Enhancement in Phenyl-Substituted Methanes

Experimental evidence is presented which shows that the natural phosphorescence lifetimes, ${\tau }_P^{°},$ decrease with increased phenylation in the molecular series: toluene, diphenylmethane, triphenylmethane. This indicates that a spin—orbit enhancement mechanism is operative. Natural fluorescence lifetime data support the exciton molecular model for diphenylmethane and triphenylmethane and show that the phenyl groups have little electronic overlap. The experimentally observed natural fluorescence lifetimes, ${\tau }_F^{°},$ at 77°K suggest that the exciton levels have similar transition moments with small forbidden character for the low‐energy component. The dramatic enhancement of phosphorescence in the series is due to several factors, namely, an increase in intersystem crossing rate constant k_isc, an increase in phosphorescence rate constant k_P, and a decrease in radiationless triplet decay constant k_P′. Although the electronic overlap is small enough for the exciton model to be a valid approximation to explain singlet absorption intensities and fluorescence lifetimes, the overlap must be taken into consideration to account for the enhancement of spin—orbit coupling down the series. The molecular geometry of diphenylmethane consistent with the fluorescence lifetime data and the observed exciton splitting is described.