Dynamic orientation control of bimolecular electron transfer at charged micelle surfaces

Visible light excitation of the neutral complex [Ru^II(phen)₂(bps)]⁰ (phen = 1,10-phenanthroline, bps = 4,7-biphenylsulfonate-1,10-phenanthroline) results in the formation of a triplet metal-to-ligand charge transfer excited state with a lifetime, τ_o, of 4.6 µs, where the promoted electron is localized on the bps ligand, ^3*[Ru^III(phen)₂(bps^•-)]⁰. The complex is dynamically quenched by di-n-heptyl-viologen, C₇C₇V²⁺, in solution and when the acceptor is embedded into negatively charged and neutral micelles. Addition of NaCl to solutions containing C₇C₇V²⁺ bound to negatively charged dodecyl sulfate sodium dodecyl sulfate micelles results in a monotonic increase in the quenching rate constant from k_q = 6.0 × 10⁷ to 1.7 × 10⁹ M^-1 s^-1. In contrast, k_q was independent of [NaCl] and diffusion limited in water and neutral micellar solution. Activated rate constants, k_act, revealed that electron transfer was slowed by a factor of 450 when occurring in negatively charged micelle solution relative to neutral octaethylene glycol monododecyl ether (C12E8) micelles. In the ^3*[Ru^III(phen)₂(bps^•-)]⁰ excited state, the bps ligand is oriented away from the anionic micelle surface potential, -141 ≤ ψ ≤ -67 mV, due to a Frumkin effect operative in the deceleration of k_act. Frumkin corrected rate constants were within a factor of three of those measured in C12E8 solution. Distance-dependent reorganization energies resulting from the orientation vary from 0.47 eV to 0.35 eV, while electronic coupling decreases by a factor of 10. The collective data show that orientation control over bimolecular rate constants in micellar solution can be achieved by screening micellar surface charges.