Formal Redox Potentials of Organic Molecules in Ionic Liquids on the Basis of Quaternary Nitrogen Cations as Adiabatic Electron Affinities

Abstract

Formal redox potentials E°' involving neutral species R and radical anions R(•-) in ionic liquids (ILs) composed of ammonium, pyridinium, and imidazolium cations are discussed from the point of view of the adiabatic electron affinity as a molecular property. The dependence of the 1,4-benzoquinone (BQ)/BQ(•-) redox process in CH2Cl2 and CH3CN is primarily investigated over a wide concentration range of ILs as the supporting electrolyte. A logarithmic relationship involving a positive shift of E°' with increasing concentration is obtained when the concentration is changed from 0.01 to 1.0 M. The relationship of E°' at IL concentrations greater than 1.0 M gradually reaches a plateau and remains there even for the neat ILs. It is found that the E°' values in the neat ILs are not influenced by the measurement conditions, and that they remain considerably dependent on the nature and concentration of the electrolyte when measured using the traditional method involving molecular solvents combined with a supporting electrolyte (0.1-0.5 M). The difference in the E°' values observed in the ammonium and pyridinium ILs is only several millivolts. In addition, ESR and self-consistent isodensity polarized continuum model calculation results reveal that the potential shift toward positive values upon the transition from molecular solvents containing ILs to neat ILs is adequately accounted for by changes in the electrostatic interaction of R(•-) taken into the cavity composed of the solvent and IL. On the other hand, the first reduction waves of quinones, electron-accepting molecules, and polynuclear aromatic hydrocarbons are reversibly or quasi-reversibly observed in the ILs. The electrochemical stability of the ILs is exploited in the facile measurement of these quasi-reversible waves at quite negative potentials, such as for the naphthalene (NP)/NP(•-) couple. Notably, the E°' values obtained in the ammonium ILs correlate well with the calculated standard redox potentials and are linearly fitted with high correlation over all classes of compounds using a single regression equation based on Koopmans' theorem.

Hurdles to organic quinone flow cells. Electrode passivation by quinone reduction in acetonitrile Li electrolytes

“…These adducts could be anionic ([QQ] 2-, [QQ] -) or radical species ([QQ] • ) 6,[11][12][13] that modify the mechanism leading to an irregular reversibility of the second redox process. The variable redox behaviour of quinones with solvent polarity 14,15 is due to the selective stabilization of some of these adduct species in different solvents. Similarly, the support electrolyte modulates the redox mechanism due to the interaction between the electrolyte cation and the different quinone anions [13][14][15][16][17] .…”

Building and testing correlations for the estimation of one‐electron reduction potentials of a diverse set of organic molecules

“…As an alternative to the Born-Haber cycle methods, linear relationships between molecular orbital energies and the ability of a molecule to accept or donate an electron are among the earliest relationships [25,26] that were considered in the literature and have been used extensively for specific families [27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43] of molecules. The use of a linear correlation of calculated frontier orbital energies (highest occupied molecular orbital (HOMO) energy for oxidation, lowest unoccupied molecular orbital (LUMO) energy for reduction) of the singlet ground state molecules with their experimentally measured redox potentials is an even simpler way to estimate the redox potentials of unknown molecules.…”

Study on Redox Properties and Cytotoxicity of Anthraquinone Derivatives to Understand Antitumor Active Anthracycline Substances

“…Further details of our electrochemical measurements are given in a previous paper. 5,9) Quantum chemical calculations were performed at Hartree-Fock (HF) and density functional theory (DFT) levels as implemented in the Gaussian 03 program. Geometry optimization for AQ and its derivatives are performed by employing the standard split-valence double-ζ 6-31G basis sets augmented by the polarization d and diffusion orbitals 6-31 + G(d).…”