beta-Carboline Photosensitizers. 3. Studies on Ground and Excited State Partitioning in AOT/Water/Cyclohexane Microemulsions

Abstract

The photophysical behavior of the P-carbolines norharmane, hmane, and harmine has been studied in waterin- oil (w/o) microemulsions in AOT/cyclohexane. With pure solvents, solubility and partitioning studies indicate that these molecules dissolve preferentially in the organic phase but that their solubility in the aqueous phase increases upon decreasing pH as a consequence of protonation. However, in w/o microemulsions these molecules appear to be located predominantly at the surfactant/water interface. Again, decreasing pH increases partitioning into the water region. From absorption and fluorescence spectral studies with the microemulsion system, differences are observed in the predominant species at any “pH’ compared with that in pure water. From analysis of fluorescence decays, four kinetically distinguishable species have been observed in the excited state, namely, neutral species in cyclohexane, neutral species in the microemulsion droplets, cation, and zwitterion. For harmine, the effect of waterhfactant ratio (WO) on the photophysical behavior and distribution of these species was studied. Partitioning was observed between aqueous and organic subphases. A study has been carried out on the effect of pH on these processes. In agreement with other studies on “pH’ in microemulsions, the cation is observed up to apparent pH values much higher than those for pure aqueous solutions. The results are interpreted in terms of the compartmentalization of the hydroxide ions between the water pools. Effects of changing wo on the decay of the various species are interpreted in terms of “quenching” of the neutral species in the cyclohexane subphase by its trapping within the microemulsion droplets. A kinetic model for this is presented. The effect of changing the polarity (by addition of methanol) upon the partitioning behavior has been studied, and it is shown that addition of methanol favors partitioning into the organic phase.