BENZOPHENONE PHOSPHORESCENCE AT $77^{\circ}K$ AND 30 KBAR

Author Institution: Department of Chemistry, University of CaliforniaThe phosphorescence spectrum of benzophenone has been measured as a function of pressure (0-30 kbar) in rigid media at $77^{\circ}K$. The 0,0 band of the $^{3}n\pi^{\ast}$ emission is located at progressively higher energies in the order benzene, polyethylene, polymethylmethacrylate, and EPA. The red shift ($150-350 cm^{-1}$ at 30 kbar) also decreases in the more polar solvents, which is expected for $n\pi^{\ast}$ bands. From the pressure-induced shift, which is similar in magnitude to that observed for aromatic hydrocarbons, it is concluded that the triplet energy is lowered by $\sim 1%$ at 30 kbar. The total phosphorescence intensity increases roughly 20% near 10 kbar and then decreases with increasing pressure to as much as 50% of the 1 atm intensity. In contrast, the triplet lifetimes shorten monotonically, yielding a 20-30% reduction at 30 kbar. The corresponding activation volumes for triplet decay are ---0.050 and ---$0.065 {\AA}^{3}$/molecule in polymethylmethacrylate and EPA, respectively. These values are similar to those observed for aromatic hydrocarbons at this temperature, even though the relative contributions of radiative and radiationless transition rates to the measured lifetime are very different for these two classes of compounds