Spectroscopic and Electrochemical Evaluation of Salt Effects on Electron‐Transfer Equilibria between Donor/Acceptor and Ion‐Radical Pairs in Organic Solvents

Abstract

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.

Selectively Altering the Reactivity of Transient Organic Radical Ions via Their Solvation Environment

“…If we consider any of these processes and compare the alteration to its rate as a function of salt concentration in the different alkyl alcohols, the largest change is always observed in PrOH, followed by EtOH, and MeOH shows the smallest change. These observations of more pronounced changes in the dynamics of radical ions upon the addition of salt to lower polarity solvents are in agreement with salt-induced changes in the electron-transfer equilibrium constant of bimolecular electron donor–acceptor complexes in solution …”

“…These observations of more pronounced changes in the dynamics of radical ions upon the addition of salt to lower polarity solvents are in agreement with salt-induced changes in the electron-transfer equilibrium constant of bimolecular electron donor−acceptor complexes in solution. 46 Empirical parameters and simplified linear solvation energy relationship models provide a useful starting point to identify the key variables determining the equilibration and reactivity of photoexcited radical ion pairs; however, it is relevant to connect these observations to the molecular-level interactions that underpin them. To begin, local solvent structure and dynamics around each solute species play an important role in their reactivity.…”

Optical, Magnetic, Electrochemical, and Electrical Properties of 8-Hydroxyquinoline-Based Complexes with Al3+, Cr3+, Mn2+, Co2+, Ni2+, Cu2+, and Zn2+

“…This would have been the case in molecules that have several electroactive sites in resonance. − Nevertheless, when the supporting electrolyte is changed for others containing smaller cations, only the third peak shifts (see Figure b). This indicates that the occurrence of the first two processes is independent of ion-pair interactions, which lets us conclude that the first wave corresponds to the couple Cr 3+ /Cr 2+ . − The second and third waves are electron-transfer reactions involving the quinoline system. Providing the ligands are equivalent, their electrochemical reduction should occur at the same potential value.…”

Investigation of Iron(III) Tetraphenylporphyrin as a Redox Flow Battery Anolyte: Unexpected Side Reactivity with the Electrolyte

“…Tetraalkylammonium salts are some of the most commonly used cations in support electrolytes in nonaqueous systems due to their resistance to reduction and relatively high molar conductivity in organic solvents. − The reactivity of the ions that make up support electrolytes is often ignored over the large potential windows they enable but should not be overlooked. Hexafluorophosphate salts with Lewis acidic cations, such as NaPF 6 and LiPF 6 , have been known to hydrolyze in the presence of trace water to form HF. , Alternatively, tetraalkylammonium salts are known to form ion pairs with reduced species that can affect thermal and photochemical processes . When paired with halides, they are also known to undergo nucleophilic reactions to form neutral trialkylamines as well as alkenes and alkanes through radical and disproportionation reactions .…”