Nonradiative and radiative deactivation of singlet molecular oxygen (O2(a1Δg)) in micellar media and microemulsions

The effects of microheterogeneous media (micelles and microemulsions) on the lifetime and, to our knowledge for the first time, on the emission of singlet molecular oxygen (O2 (a1Δg), denoted as 1O2) were investigated. Micellar media and various types of microemulsions based on anionic (sodiumdodecyl sulfate), cationic (cetyltrimethylammonium chloride) or nonionic (Triton X-100) surfactants were formulated for this purpose. The nonradiative and radiative deactivation rate constants (kd and ke, respectively) were determined in selected microheterogeneous media and in the pure solvents used for their formulation, by combining steady-state and time-resolved 1O2 luminescence detection techniques. We have shown that a simple additive model, as used in homogeneous mixtures of solvents, was inadequate for predicting values of kd and ke in organized media. In contrast, both 1O2 lifetimes (τΔ=1/kd) and ke in the microheterogeneous systems investigated could be predicted with good precision from the composition of the media and the τΔ and ke values in the pure solvents, using a two-pseudophase kinetic model for the 1O2 distribution. Such a model takes into account the average times spent by 1O2 in the aqueous and lipophilic pseudo-phases of the organized media, the corresponding equilibrium constant (Keq) depending on the nature of the system.