Bimolecular Electron Transfer Quenching of Neutral *Ru(phen)₂bps by 4,4‘-Diheptyl Viologen in Water and Bound to SDS Micelles

A donor/acceptor system was designed to measure the bimolecular electron transfer (ET) rate constant from a donor in the aqueous phase to an acceptor anchored to the micellar surface utilizing a simple kinetic formalism. In this system, the donor, Ru(phen)2bps (phen = 1,10-phenanthroline, bps = disulfonated 4,7-diphenyl-1,10-phenanthroline), possesses an overall zero charge and does not associate with anionic or neutral surfactants, whereas the C7C7V2+ (1,1‘-diheptyl 4,4‘-viologen) acceptor is anchored to the micellar surface through the heptyl chains. In water, the formation of a ground-state aggregate between Ru(phen)2bps and C7C7V2+ results in biexponential decay of the emission of the probe in the presence of quencher. The bimolecular quenching of Ru(phen)2bps by C7C7V2+ takes place with kq = 3.0 × 109 M-1 s-1, in agreement with that measured for the related C1C1V2+ (methyl viologen) 4.8 × 109 M-1 s-1, where no aggregation was observed. Within the Ru(phen)2bps−C7C7V2+ aggregate the excited state of the complex is quenched with a rate of ∼6.3 × 106 s-1. The emission decay of *Ru(phen)2bps remains monoexponential in the presence of anionic SDS (sodium dodecyl sulfate) micelles, but becomes biexponential upon addition of C7C7V2+. The short- and long-lifetime components have been interpreted as the reaction of aqueous *Ru(phen)2bps with C7C7V2+ either residing in the aqueous phase or bound to SDS (kq = 8.0 × 107 M-1 s-1). The bimolecular quenching by C7C7V2+ bound to the surface of SDS micelles is ∼ 30 times slower than in water and bound to neutral C12E8 (n-dodecyl octaoxyethylene glycol monoether) micelles (2.6 × 109 M-1 s-1). Several possible explanations for the difference in the observed rates when the acceptor is bound to the SDS micellar surface are provided.