Photochemical Processes of Benzophenone in Microheterogeneous Systems

The benzophenone triplet has been observed upon laser flash photolysis of the ketone solubilized in perfluorated micelles; its lifetime is found to be similar (7.87 X 10^-6 s) to the value known from experiments in perfluorobenzene and perfluoromethylcyclohexane. Its E_TT(λ_max 520 nm) of 2660 ● 380 L mol^-1 cm^-1 in sodium perfluorooctanoate (SPFO) and of 2460 ± 350 L mol^-1 cm^-1 in potassium perfluorooctylsulfonate (PPFOS) have been determined by using the method of Lachish et al. and assuming ɸ_isc = 1. Its phosphorescence is observed in perfluorated micelles (T_ph = (7 ± 3) X 10^-6 s), water (T_ph = (12 ± 4) X 10^-6 s), isooctane (T_ph = (1.78 ± 0.06) X 10^-6 s), and Freon 112 (T_ph = (132 ± 79) X 10^-6 s) with a striking resemblance of the weakly structured spectra (λ_max 445 nm) in the first two systems. The emission is quenched by nonfluorated surfactants such as sodium laurylsulfate (SLS) or cetyltrimethylammonium chloride (CTAC), presumably due to hydrogen abstraction; phosphorescence, triplet-triplet, and ketyl radical absorption transients are observed at concentrations of SLS above CMC, where the ketone should be nearly completely solubilized in a highly reactive micellar environment. Under micellar conditions, hydrogen abstraction occurs within the duration of our laser pulse (~30 ns). The kinetics of the triplet decay in perfluorated micelles as well as the decay of the ketyl radical, which results from hydrogen abstraction in SLS micelles, is strictly first order. Experiments in mixed micelles of SLS and SPFO indicate a faster decay of benzophenone triplets and a greater optical density of the signal component representing the corresponding ketyl radical when the mean occupancy number of SDS in SPFO is increased. The kinetics show a faster rate constant of hydrogen abstraction in those modelized micelles than in solutions of hydrocarbons. The solubilized ketyl radical may be deprotonated by an alkaline aqueous phase, and a cationic micelle clearly catalyzes this process. The lifetimes of ketyl radical and deprotonated ketyl radical anion are both longer in micellar systems than in homogenous solutions due to their isolation in surfactant aggregates.