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 (kq′) 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 (kq) thus estimated are seen to be much smaller than those reported in the homogeneous solutions (e.g., in acetonitrile), indicating that the electron transfer in the micellar media is inherently inefficient. Correlation of the observed kq values in the micellar solutions with the free-energy changes (ΔG0) for electron transfer reactions show an inversion in the observed rates as predicted by Marcus’ outer sphere electron transfer theory at exergonicities more that ∼0.65 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.