Spectroscopic and electrochemical evaluation of salt effects on electron-transfer equilibria between donor/acceptor and ion-radical pairs in organic solvents.

Addition of "inert" tetrabutylammonium hexafluorophosphate (Bu(4)NPF(6)) to a solution of TMDO/DDQ in dichloromethane (where TMDO=2,2,6,6-tetramethylbenzo[1,2-d;4,5-d]bis[1,3]-dioxole, donor, and DDQ=diclorodicyano-p-benzoquinone, acceptor) is accompanied by drastic changes in the electronic spectrum, which are related to the appearance of the DDQ(-.) and TMDO(+.) ion radicals and a decrease in the concentration of the neutral molecules and the charge-transfer complex [TMDO,DDQ]. These changes point to a considerable rise (of about three orders of magnitude) in the apparent electron-transfer equilibrium constant (K(ET)) for this donor/acceptor pair upon increasing the electrolyte concentration from 0 to 0.5 M. Accordingly, the ion-radical fractions and K(ET) values are higher in dichloromethane, at high electrolyte concentrations, than in acetonitrile (where the effect of Bu(4)NPF(6) is less pronounced). Similar trends of the apparent equilibrium constants are observed for the tetramethyl-p-phenylenediamine/tetracyanoethylene pair. Electron-transfer equilibrium constants for both donor/acceptor dyads obtained from spectral measurements are related to those derived from the redox potentials of the reactants. The effects of media variations on the electron-transfer equilibria are discussed within the ion-pairing and ionic-activity frameworks.