ENHANCEMENT OF THE SENSITIVITY OF RADIATIVE AND NONRADIATIVE DETECTION TECHNIQUES IN THE STUDY OF PHOTOSENSITIZATION BY WATER-SOLUBLE SENSITIZERS USING A REVERSE MICELLE SYSTEM

The study of photosensitization processes in aqueous solution is complicated by the unsuitability of water as solvent for the commonly used techniques for monitoring radiative and non-radiative relaxation processes. The infrared luminescence method is hampered by the relatively weak intensity of emission and short lifetime of singlet molecular oxygen, O2(1-DELTA-g), in water. Photoacoustic and photothermal detection of the dominant non-radiative relaxation process for O2(1-DELTA-g) is also inhibited by the unfavorable physico-chemical properties of water which reduce the sensitivities of these techniques for the study of sensitizers in aqueous with respect to non-aqueous media. Such problems have been alleviated by incorporating the sensitizer in the aqueous core of aerosol OT reverse micelles. It is demonstrated that the sensitivity of both radiative and non-radiative detection depends not on the local environment of the sensitizer but on the overall composition of the medium, which, in the case of reverse micelle solutions, is predominantly hydrocarbon. This produces a 4.5 and 6-fold enhancement of the sensitivity of luminescence and optoacoustic detection, respectively, in comparison to purely aqueous solution. The utility of the method is demonstrated for Rose Bengal, where quantum yields of intersystem crossing (PHI-isc) of 0.78 (LIOAC) and singlet oxygen generation (PHI-DELTA) of 0.81 (LIOAC) and 0.80 (TRLD) were measured. Phenazine was also studied for comparative purposes and to corroborate values obtained for Rose Bengal.