Depth-dependent solvent relaxation in reverse micelles: A fluorescence approach

We report here depth-dependent solvent relaxation effects in the reverse micellar assembly using the deeply embedded probe NBD-cholesterol, a fluorescent cholesterol analogue in which the 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) group is covalently attached to the flexible acyl chain of cholesterol. Because of its deeper location, the NBD group of NBD-cholesterol is capable of reporting solvation dynamics in the deeper regions of the organized molecular assembly in which it is incorporated. NBD-cholesterol exhibits red edge excitation shift (REES) when incorporated into reverse micelles formed by sodium bis(2-ethylhexyl) sulfosuccinate (ACT) in isooctane with varying [water]/[surfactant] molar ratio (w(0)). Interestingly, the extent of REES increases with increasing w(0) implying that the overall motional restriction experienced by the reorienting solvent molecules is increased with increasing hydration. This is in contrast to the behavior of interfacially localized probes. In addition, our results show that with increasing w(0), the NBD group of NBD-cholesterol experiences increased polarity as evidenced by the decrease in fluorescence lifetime and other fluorescence parameters such as fluorescence intensity. NBD-cholesterol could prove to be a useful probe for monitoring depth-dependent dynamics in organized molecular assemblies.