Dynamic orientation control of bimolecular electron transfer at charged micelle surfaces

摘要

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, tau (o), of 4.6 mu s, where the promoted electron is localized on the bps ligand, (3*)[Ru-III(phen)(2)(bps(center dot-))](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 x 10(7) to 1.7 x 10(9) 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(center dot-))](0) excited state, the bps ligand is oriented away from the anionic micelle surface potential, -141 <= psi <= -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.