Ion-pairing control of excited-state electron-transfer reactions. Quenching, charge recombination, and back electron transfer
- Boston Univ., MA (United States)
The rate constants for the oxidative quenching of {sup *}Ru(bpy){sub 3}{sup 2+} by MV{sup 2+} (k{sub q}) and the charge recombination reaction between Ru(bpy){sub 3}{sup 3+} and MV{sup .+} in bulk solution (k{sub rec}) and the cage escape yields of the redox products ({eta}{sub ce}) were determined as a function of added electrolytes (Na{sup +} salts of oxyanions and halides) and temperature (10-60{degree}C) in aqueous solution. At 25{degree}C and constant [anion], k{sub q} and k{sub rec} are a function of the specific anion. Activation energies for k{sub q} and k{sub rec} for ClO{sub 4}{sup -} are nearly 30% lower than the average values for the other salts. Values of {eta}{sub ce} show anion-specific trends opposite to those for k{sub q} and k{sub rec}. The reactant cations are extensively ion-paired by the dominant anions in bulk solution, and a similar situation is proposed to exist within the quenching solvent cage. {Delta}H{sup dagger} (activation enthalpy) and {lambda} (solvent reorganization energy) were evaluated for k{sub ip} and back electron transfer within the solvent cage (k{sub bt}) and were found to be smallest for ClO{sub 4}{sup -} and I{sup -}. The correlation that exists between k{sub ip} or k{sub bt} and the standard free energy of hydration ({Delta} G{degree}{sub hyd}) of the anions indicates that the rates of electron transfer between the cationic reactants are greatest in the presence of anions that have the most weakly-held hydration sphere and the greatest ability to break the water structure. 33 refs., 7 figs., 3 tabs.
- OSTI ID:
- 249667
- Journal Information:
- Journal of Physical Chemistry, Journal Name: Journal of Physical Chemistry Journal Issue: 18 Vol. 100; ISSN JPCHAX; ISSN 0022-3654
- Country of Publication:
- United States
- Language:
- English
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