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.

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Publication costs assisted by Consiglio Nazionale delle RicercheThe photochemistry of Co(NH3)5N022+ has been investigated in water-ethanol solvent using 11 different wavelengths of irradiation in the 254-530-nm range, covering the charge-transfer and ligand field regions of the spectrum. In agreement with previous reports, both redox decomposition and nitro-nitrito linkage isomerization photoreactions occurred with constant ratio at all of the irradiation wavelengths used. The redox quantum yields underwent a continuous decrease as the irradiation wavelength was increased within the charge-transfer region of the spectrum, showing definite variations within each charge-transfer absorption band. The quantum yield sharply decreased in going from the charge-transfer region to the ligand field one, but then it remained constant throughout the whole ligand field band. The results indicate that thermal equilibration prior to reaction is efficient in the singlet ligand field excited state but not in the charge-transfer singlet states. The reasons for this behavior are discussed on the basis of proposed potential energy surfaces for the singlet excited states involved.

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