Deactivation of pyrene derivatives by nitroxides in AOT reverse micelles. Dependence of quenching efficiency on the probe and quencher location in the microaggregates

The quenching of the fluorescence of pyrene derivatives with different hydrophobicity by a series of nitroxides has been measured in AOT reverse micelles. When both donor and quencher are predominantly located in the organic pseudophase (i.e., 1-methylpyrene and 2,2,6,6-tetramethylpiperidine-N-oxyl, TEMPO) the quenching process is not affected by the presence of the microaggregates. On the other hand, when one of the partners is located at the interface (pyrenesulfonate or 4-amino-2,2,6,6-tetramethylpiperidine-N-oxyl, TEMPAMINE(+)), the rate of the process is slower than that in the bulk solvent, being determined by the rate of access to the interface. The restriction imposed by the interface changes with the water content of the micelle and the location of the bound species. When both species are bound to the interface, the quenching process presents static and dynamic components. The slow rate of the dynamic process indicates a highly restricted intra-interface mobility, particularly for quencher and/or donors that are counterions of the surfactant. The efficiency as quencher of 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl, TEMPOL, decreases with the AOT concentration. From this dependence is derived the partition of TEMPOL between the bulk solvent, the interface, and the water pool.