Electron-Transfer Reactions in Micelles:  Dynamics of Psoralen and Coumarin Radical Cations

Abstract

Radical cations of several psoralens and coumarins have been generated by photoionization in aqueous micellar solution for a comparison of their dynamic behavior with results observed previously in aqueous solution. The photoionization efficiencies are significantly higher in anionic sodium dodecyl sulfate (SDS) micelles than in aqueous solution as a result of favorable electrostatic effects that lead to rapid ejection of the electron into the aqueous phase. By contrast, much lower quantum yields are measured in neutral and cationic micelles. Experiments with anionic quenchers that are soluble only in the aqueous phase demonstrate that exit of the radical cations from anionic SDS micelles is too slow to measure (<105 s-1) under our conditions. Exit rate constants from neutral Triton X-100 micelles are approximately an order of magnitude faster. Comparison of equilibrium constants for both triplet and radical cations of 4,5‘,8-trimethylpsoralen demonstrates that electrostatic effects lead to an order of magnitude enhancement of the affinity of the radical cation for the anionic vs neutral micelles, as compared to the triplet of the same substrate. The slow exit of the radical cation from the micelle facilitates the measurement of rate constants for reaction of the micelle-incorporated radical cations with both water soluble quenchers and species for which exchange between micelle and aqueous phases is rapid on the time scale of the radical cation lifetime. This work provides some of the first kinetic data for dynamics of exit of reactive radical cations from micelles and indicates that these species may provide useful probes for studying electron-transfer dynamics in micellar media.

Temperature Effect on the Fluroescence Anisotropy Decay Dynamics of Coumarin‐153 Dye in Triton‐X‐100 and Brij‐35 Miscellar Solutions¶

“…Since the micellar core is highly nonpolar (similar to hydrocarbon solvents) and the bulk water outside the micelle is highly polar, the estimated ɛ values in the present systems indicate that the dye preferentially resides in the micellar Palisade layer, having an intermediate polarity (21). Our earlier work (5, 6) and also other reported literature (33, 34) suggest that the Coumarin dyes mainly reside in the micellar Palisade layer. The higher ɛ value for the BJ‐35 micelle than that for the TX‐100 micelle suggests that the Palisade layer of the former micelle is more hydrated than that of the latter micelle, which was not unexpected considering that BJ‐35 has more oxyethylene units in the surfactant chain than does TX‐100 ( cf Chart 1).…”

Psoralen and Coumarin Photochemistry in HSA Complexes and DMPC Vesicles†

“…Psoralen‐HSA complexes show larger blue shifts than do the coumarin complexes, consistent with the larger effects of solvent polarity for psoralens in homogeneous solution. As an example, 4,5′, 8‐trimethylpsoralen (TMP) has fluorescence maxima at 368 nm in decane, 422 nm in acetonitrile, 468 nm in aqueous solution and 439 nm in SDS micellar solution (16). The observed fluorescence maximum of 438 nm for the HSA complex (Fig.…”

Temperature dependence of water dynamics at an aqueous micellar surface: Atomistic molecular dynamics simulation studies of a complex system

“…It is clear that the rates of electron transfer reactions can be affected by this slow component. 11 Work in these directions is under progress.…”

“…The slow second component appears to be unique to water in complex systems and may play an important role in many chemical reactions such as electron transfer. 11 The ori-gin of this slow component has been attributed to the existence of a dynamic equilibrium between the ''quasibound'' and the free water molecules. 2, 12,13 Recently, we have presented atomistic molecular dynamics simulation studies of solvation 14 and orientational 15 dynamics at the surface of an aqueous micelle at 350 K. The micelle simulated was pentadecafluorooctanoate, with cesium being the counterion, commonly referred to as CsPFO.…”