Photochemistry of diflunisal in nonionic (Brij-35) Micelles: Influence of the microenvironment on photoionization, electron trapping, and persistent radical effect

The photochemistry of diflunisal (DF) in nonionic micelles of polyoxyethylene lauryl ether (Brij-35) has been investigated by combining steady-state and time-resolved spectroscopic techniques. Photophysical and photochemical measurements account for an efficient aggregation of the drug with the micellar system. Shortening of the singlet and lengthening of the triplet lifetimes of DF is observed upon incorporation into micelles, respectively. The former effect is responsible for changes in the relative weight of the mono- and biphotonic pathways of photoionization. The quenching constant related to the trapping by the ground state of DF of the hydrated electrons photoejected is markedly reduced in the presence of Brij-35. The exit dynamic of DF from the micellar cage is the almost exclusive rate-controlling step for the electron-trapping process. Both nature of the photoionization and efficiency of the electron trapping provide basic information about the localization of DF in the micellar cage. The "persistent radical effect" responsible for the cross-combination reaction between phenoxy and sigma aryl radicals in aqueous solution is suppressed in the presence of micelle. Mobility and different reactivity in the micellar environment of the two main radical species generated upon DF photolysis are proposed as a rationale for this observation. The relation of the overall results to the phototoxic effects displayed by the drug is also commented upon briefly. RI Sortino, Salvatore/E-4684-2011