Cd. Clark et Mz. Hoffman, ION-PAIRING CONTROL OF EXCITED-STATE ELECTRON-TRANSFER REACTIONS - QUENCHING, CHARGE RECOMBINATION, AND BACK ELECTRON-TRANSFER, Journal of physical chemistry, 100(18), 1996, pp. 7526-7532
The rate constants for the oxidative quenching of Ru(bpy)(3)(2+) by M
V(2+) (k(q)) and the charge recombination reaction between Ru(bpy)(3)(
3+) and MV(.+) in bulk solution (k(rec)) and the cage escape yields of
the redox products (eta(ce)) were determined as a function of added e
lectrolytes (Na+ salts of oxyanions and halides) and temperature (10-6
0 degrees C) in aqueous solution. At 25 degrees C and constant [anion]
, k(q) and k(rec) are a function of the specific anion, decreasing in
the order ClO4- >> SO42- similar to HPO42- > H2PO4- similar to CH3CO2-
and I- > Br- > Cl- > F-. Activation energies for k(q) and k(rec) for
ClO4- are similar to 30% lower than the average values for the other s
alts. Values of eta(ce) show anion-specific trends opposite to those f
or k(q) and k(rec). The reactant cations are extensively ion-paired by
the dominant anions in bulk solution, and a similar situation is prop
osed to exist within the quenching solvent cage. The electron-transfer
component of quenching for ion-paired species (ki,) was extracted fro
m k(q) by use of the Olson-Simonson treatment; Delta H-double dagger (
activation enthalpy) and lambda (solvent reorganization energy) were e
valuated for k(ip) and back electron transfer within the solvent cage
(k(bt)) and were found to be smallest for ClO4- and I-. The correlatio
n that exists between k(ip) or k(bt) and the standard free energy of h
ydration (Delta G degrees(hyd)) of the anions indicates that the rates
of electron transfer between the cationic reactants are greatest in t
he presence of anions that have the most weakly-held hydration sphere
and the greatest ability to break the water structure. The rate consta
nts of quenching and charge recombination and the yields of redox prod
ucts can be fine-tuned through the variation of the supporting electro
lyte as well as the variation of temperature and ionic strength.