| Abstract: | The γ-ray induced trans → cis isomerization of stilbene was investigated in solutions in the frozen organic solvents benzene, toluene, naphthalene, dibenzyl, and cyclohexane, and compared with fluid solutions in the same solvents, at appropriate temperatures. In dilute (up to 10?3M) solutions in benzene isomerization takes place in the frozen system at a higher rate than in the corresponding fluid system, with the ratio between these rates increasing with decreasing stilbene concentration. These results indicate that energy transfer in frozen benzene, and to stilbene embedded in it, takes place by a more efficient mechanism than the diffusional one operative in fluid solutions. It is suggested that singlet or triplet excitons are responsible for energy transfer within the microcrystals of the frozen benzene, and to stilbene molecules situated in specific (possibly surface) sites. At stilbene concentrations in the range 5 × 10?5 to 10?1M G(trans → cis) in the frozen solutions is approximately constant (? 0.7), corresponding to G(excited stilbene) ? 1.4. This result is compatible with a mechanism involving efficient “harvesting” of all triplet benzene molecules in the solid under these conditions. Dilute solutions in toluene behave in a similar way, but no radiolytic isomerization takes place in solutions in solid naphthalene or dibenzyl, and also not in solid trans stilbene. |
