Photoinduced intermolecular electron transfer from aromatic amines to coumarin dyes in sodium dodecyl sulphate micellar solutions

Abstract

Photoinduced intermolecular electron transfer interaction between coumarin dyes and aromatic amines has been investigated in sodium dodecyl sulphate micellar solutions using steady-state and time-resolved fluorescence quenching measurements. Steady-state fluorescence quenching of the coumarin dyes by the amine quenchers always shows a positive deviation from linear Stern-Volmer relationship, which arises due to the localized high quencher concentrations at the micellar Stern layer. In time-resolved fluorescence measurements, the analysis of the fluorescence decays following a micellar quenching kinetics model assuming a unified quenching constant (k q Ј) per quencher occupancy does not give satisfactory results, especially for the higher quencher concentrations used. The observed fluorescence decays are, however, seen to fit reasonably well following a bi-exponential analysis for all the quencher concentrations used. The average fluorescence lifetimes of the coumarin dyes in the micellar solution as estimated from the bi-exponential decay analysis are seen to undergo a systematic reduction with the effective mean quencher concentrations. The bimolecular quenching constants (k q ) thus estimated are seen to be much smaller than those reported in the homogeneous solutions ͑e.g., in acetonitrile͒, indicating that the electron transfer in the micellar media is inherently inefficient. Correlation of the observed k q values in the micellar solutions with the free-energy changes (⌬G 0 ) for electron transfer reactions show an inversion in the observed rates as predicted by Marcus' outer sphere electron transfer theory at exergonicities more that ϳ0.65 eV. To the best of our knowledge this is the first report on the Marcus inverted region observed for the electron transfer reactions in micellar solution.

Ultrafast Electron Transfer Dynamics in Micellar Media Using Surfactant as the Intrinsic Electron Acceptor

“…7,9,11,15 Moreover, it has been shown that the inversion region of the Marcus correlation curve can be suitably tuned by changing the characteristics of the micellar media. 10,88 For different coumarin dyes in CPC micelle, the correlation of the fastest ET rates, i.e., the inverse of the first component of the ET times (k et ≈ τ 1 -1 ) with the ∆G 0 values of the ET reactions, is shown in Figure 3.…”

“…Thus, it helps us to observe the bell-shaped Marcus correlation curve for intermolecular ET reactions easily in microheterogeneous media, without increasing the reaction exergonicity (-∆G 0 ) to an unreasonably high value. 7 Additionally, in organized assemblies, the tethering of the reactants within the surfactant chains causes a large retardation in the rate of reactant diffusion, making the ET reactions to occur effectively under nondiffusive condition. [7][8][9][10][11]15 In one of our recent studies, it was observed that, for intermolecular ET reactions in different noninteracting micelles, the fastest ET time corresponding to the maxima of the Marcus correlation curves is only in the range of about 4 ps.…”

“…7 Additionally, in organized assemblies, the tethering of the reactants within the surfactant chains causes a large retardation in the rate of reactant diffusion, making the ET reactions to occur effectively under nondiffusive condition. [7][8][9][10][11]15 In one of our recent studies, it was observed that, for intermolecular ET reactions in different noninteracting micelles, the fastest ET time corresponding to the maxima of the Marcus correlation curves is only in the range of about 4 ps. 10 Note that for intermolecular ET in homogeneous media under nondiffusive condition, i.e., when the electron acceptor dyes are dissolved directly in neat electron-donating solvents (cf.…”

Photoinduced Electron Transfer Reactions of Ruthenium(II) Phenanthroline Complexes with Dimethylaniline in Aqueous and Micellar Media

“…DMA is a very good electron donor, neutral in nature and sparingly soluble in aqueous solution [38,[54][55][56][57][58][59][60]. However, in the presence of CTAB micelles, the solubility of DMA has drastically increased.…”

Photoinduced electron transfer from N,N-dimethylaniline to 7-amino Coumarins in protein-surfactant complex: Slowing down of electron transfer dynamics compared to micelles

“…[27][28][29] Several groups also reported fluorescence quenching electron transfer in micelles. [30][31][32][33] It is well known that fluorescence quenching process of Coumarin dyes by DMA occurs due to the photoinduced electron transfer from DMA to the excited Coumarin dyes. [34][35][36][37][38][39][40] In the present study we observed a slower fluorescence quenching electron transfer rate due to the formation of proteinsurfactant complex as compared to the pure micellar solution.…”