Knowledge Bank

Recently there has been considerable interest in radiationless transitions in aromatic molecules. We have studied the phosphorescent lifetimes of benzene and various methylated and deuterated derivatives in highly purified solvents such as cyclohexane and borazine. We find a radiationless intersystem crossing rate which is a function of the following variables: 1) temperature; 2) solvent; 3) solvent crystal phase; 4) solute deuteration; and 5) solvent deuteration. The exponential decay data fit rather well a radiationless rate constant of the form $k1=k10+A\exp (-Ea/kT)$. The activation energies are of the order of skeletal molecular vibrations ($500-1500cm-1$) and the pre-exponential factors are typically $104\sec -1$. Both are solute and solvent dependent, being smaller for $C6H6$ than for $C6D6$. The temperature at which the lifetime begins to shorten is also very solute and solvent dependent, occurring below $77\circ K$ for many systems, especially those containing $C6H6$. A completely satisfactory explanation of these results cludes us at present. Nevertheless, it is clear that the solvent and temperature play a much larger role in the mechanism of radiationless transitions than heretofore has been imagined, especially for benzene.