| Abstract
| - Photoinduced electron transfer (ET) reactions between amines and a series of coumarins have been investigatedusing fluorescence−quenching measurements in aqueous P123 triblock copolymer micellar solutions.Fluorescence spectral characteristics and fluorescence anisotropy measurements indicated a nearly similarmicroenvironment for all of the coumarins used in P123 micelles. Substantial quenching of coumarinfluorescence in the presence of amines has been observed. The quenching rates () are largely reduced inthe P123 micelle as compared to those in other micelles (sodium dodecyl sulfate (SDS), Triton-X 100 (TX-100), cetyl trimethyl ammonium bromide (CTAB), and dodecyl trimethyl ammonium bromide (DTAB)),which is probably due to larger coumarin−amine separations in the micellar phase. Thevalues, whenplotted against free energy changes (ΔG°), follow a Marcus predicted bell-shaped correlation. The estimatedactivation energy for the ET reactions follow an inverse bell-shaped correlation with ΔG°. Present resultsindicate that the appearance of Marcus inversion is primarily related to the changes in the activation barrier,as predicted from the Marcus ET theory. As thevalues are higher than the estimated bimoleculardiffusional rate constant, the role of reactant diffusion on the quenching kinetics in the P123 micelle is negligible.The appearance of Marcus inversion at unexpectedly lower exergonicity has been rationalized on the basis ofslow solvent relaxation and by the application of the two-dimensional ET (2DET) theory. Critical analysis ofthe present results establishes that the inversion in the ET rates at high exergonicity is not due to the alterationin the diffusion parameters of the reactants, as has been suggested in some recent reports. Instead, it is evidentthat the inversion in quenching rates at high exergonicity is due to the alteration in the activation barrier forthe ET reactions.
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