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.