Salt effects on nearly diffusion controlled electron-transfer reactions: bimolecular rate constants and cage escape yields in oxidative quenching of tris(2,2'-bipyridine)ruthenium(II)

Abstract

The rate constants for the quenching of excited Ru(bpy)32+ by methylviologen (MV2+) and Ru(NH3)5py3+ have been studied in aqueous solution as a function of the concentration (0.01-1 M) and type (NaCl, NaC104, CaCl2) of added electrolyte. With MV2+ as quencher, the yield of products escaping cage recombination and the rate constant of their back electron transfer reaction have also been studied as a function of the concentration of added NaCl. The results have been compared with predictions based on expressions available in the literature for the ionic strength dependence of diffusional parameters kd and k-¿. With uni-univalent electrolytes, the Debye and Eigen equations appear to be adequate for the calculation of kd and k-¿, respectively, provided that the appropriate numerical integration over the interreactant distance is performed. Approximations leading to more tractable expressions (such as, e.g., those leading to a Bronsted-Bjerrum ionic strength dependence of kd) give rise to serious disagreement with experiments. Specific counterion effects (C104" faster than Cl-) are observed that can be best interpreted in terms of changes in the rate of the unimolecular electron-transfer step within an encounter complex including the counterion. Also, counterion concentration rather than ionic strength better represents (Olson-Simonson effect) the salt effects obtained with the CaCl2 electrolyte.

Comparison of Nitrogen Core and Ethylenediamine Core Starburst Dendrimers through Photochemical and Spectroscopic Probes

“…[13][14][15] In homogeneous aqueous solutions, a bimolecular rate constant of 4.0 × 10 8 M -1 s -1 was observed, which is in good agreement with previously published values. 7,16 Since MV 2+ is positively charged, it possesses, as does the metal complex, a strong electrostatic affinity to the surface of the negatively charged n.5-SBD. When both Ru(bpy) 3 2+ and MV 2+ are adsorbed on the same dendrimer molecule, photoinduced electron-transfer quenching is expected to be enhanced compared to that in the homogeneous solution, because the dendrimer acts as a host which organizes the system and allows Ru(bpy) 3 2+ and MV 2+ to behave as a correlated reactive pair on a single surface.…”

Diffusion-Controlled Charge Transfer from Excited Ru(bpy)32+ into Nanosized TiO2 Colloids Stabilized with EDTA

“…where the value of k ce can be formulated by the Eigen equation [35][36][37][38][39] where k is the Boltzmann constant, T is the absolute temperature, R is the gas constant, r is the encounter distance of the geminate species Ru(bpy) 3 3+ ...e -(TiO 2 ), η is the viscosity of the bulk solution, N is Avogadro's number, Z a is the ionic charge of Ru(bpy) 3 3+ , Z b is the apparent surface charge of the particle, e is the electron charge, is the static dielectric constant of the solvent, and µ is the ionic strength. In the case of the photoinduced charge transfer between Ru(bpy) 3 2+ and MV 2+ , the back electron transfer involving the geminate redox species Ru(bpy) 3 3+ ...MV •+ was believed to be only slightly dependent on ionic strength.…”

Theoretical Treatment of Magnetic Field Dependent In-cage Backward Electron Transfer During Photooxidation of Ru(II) Complexes

“…By applying the dynamic theory described in the last section, one should be able to account for the observed net yield of cage escape [5] [5,26] when estimating the translational diffusion coefficient. A value of Dr = 0.5 ns -1 would be consistent with such a radius.…”