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Spectroscopic and Electrochemical Evaluation of Salt Effects on Electron‐Transfer Equilibria between Donor/Acceptor and Ion‐Radical Pairs in Organic Solvents
Author(s) -
Rosokha Sergiy V.,
Sun Duoli,
Fisher Jeffrey,
Kochi Jay K.
Publication year - 2008
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.200800470
Subject(s) - chemistry , tetracyanoethylene , acceptor , electron transfer , dichloromethane , electron acceptor , electrochemistry , hexafluorophosphate , acetonitrile , equilibrium constant , photochemistry , ion , electrolyte , inorganic chemistry , radical ion , ionic bonding , electron donor , ionic liquid , solvent , organic chemistry , electrode , physics , condensed matter physics , catalysis
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.