Environmental Effects on Phosphorescence. II. “Activation Volumes” for Triplet Decay of Aromatic Hydrocarbons

The rate of triplet decay $k$ has been measured to 32 kbar for 20 aromatic hydrocarbons at room temperature. The measured lifetimes are generally shortened by high pressure. The slope of $\ln \mathrm{k‐}\mathrm{vs}\mathrm{‐P}$ plots were all linear above 15 kbar. This slope is positive except for terphenyl, and can be related to “activation volumes” ${\mathrm{\Delta V}}^{‡}$ for this kinetic process. The values ${\text{−1.0\hspace{0.17em}<\hspace{0.17em}ΔV}}^{‡}{\text{\hspace{0.17em}<\hspace{0.17em}0.2 Å}}^3\mathrm{\hspace{0.17em}/\hspace{0.17em}}\mathrm{molecule}$ are found for this class of compounds. The perdeuterated compounds have a larger negative “activation volume” than the corresponding perprotonated aromatics. Hence, environmental perturbations are most effective for slow processes which arise from weak intramolecular perturbations in the solute.