EXPLORATORY-STUDY OF THE APPLICATION OF TRANSMISSION AND DIFFUSE-REFLECTANCE LASER TECHNIQUES IN THE STUDY OF FREE-RADICAL PROCESSES IN VESICLES

Diphenylmethyl radicals generated photochemically from 1,1,3,3-tetraphenylacetone (TPA) or 1,1-diphenylacetone (DPA) have been examined in vesicles using laser flash photolysis techniques. Experiments in small vesicles generated by sonication show large magnetic field effects on the ground-state radical decay when TPA is the precursor, but not in the case of DPA. The results are interpreted in terms of rapid separation of the geminate triplet radical pair when DPA is the radical precursor. No magnetic field effect was observed on the decay of the excited radical pair formed from TPA, an observation that contrasts with results in micelles. In large (injected) vesicles, experiments with DPA and TPA demonstrate that time-resolved diffuse reflectance can be a very powerful technique in the study of opaque solutions and this technique could possibly be applied to cell suspensions. Contrary to small vesicles, no magnetic field effect was observed for the ground-state radical decay when TPA was the precursor. These differences are suggested to be related to the higher degree of rigidity of the large vesicles. Oxygen scavenging experiments suggest that access to radicals in the bilayer is more facile in the case of the small vesicles, as compared with large vesicles prepared by injection, where radical trapping by oxygen is slower than in homogeneous solution.