Quantitative generation of cation radicals and dications using aromatic oxidants: effect of added electrolyte on the redox potentials of aromatic electron donors

Abstract

Redox titrations using robust aromatic oxidants allow for a quantitative analysis of redox and optical properties of organic electron donors in their oxidized states. Unlike spectroelectrochemistry, redox titrations can be performed without added electrolyte in relatively nonpolar solvents, affording quick access to the redox and optical properties of a given electron donor without the need of a complex electrochemical setup. However, the redox potentials obtained by the two methods are not the same. To establish the direction and magnitude of this discrepancy, we have performed a systematic case study using a set of tetraarylethylene donors and a tetrasubstituted hydroquinone ether cation radical () as a stable aromatic oxidant. We show that redox potentials (especially second and higher oxidation potentials) measured by electrochemical methods are systematically lower compared with the redox potentials obtained by redox titrations in the absence of electrolyte, because of the enhanced stabilization of dications and polycations by electrolyte. We have also uncovered that the smaller cation radicals (e.g., a para-hydroquinone or ortho-hydroquinone ether cation radicals) are much more effectively stabilized when compared with the cation radicals in which charge is delocalized over larger area (e.g., tetraarylethylene cation radicals) in the presence of electrolyte because of increased ionic strength of the solution.

Unraveling the Coulombic Forces in Electronically Decoupled Bichromophoric Systems during Two Successive Electron Transfers

“…Figure 5C). [11,18] It is well known that electrostatic interactions among chargeds pecies are modulated by solvation;p olar solvents shield the charge and reduce the effective Coulombic interactions amongt he charged species. The experimental/computational demonstration of severely diminished electrostatic interactions between the cationicf luorenes separated by 8.9 in 1sh 2 + + agreesw ell with the observation of ac omplete absence of the electrostatic interactions amongst circularly arrayed electro-active groups in hexaarylbenzene derivatives (B and C in Figure1), which bear separations of about8 .8 .W hile an approximate distance of 9 seems reasonable based on experimental data (Figure 1a nd Figure 5), the characteristic interchromophoric distance where electrostatic interactions are absent is expected to dependo n the size, electronic structure of the chromophore, the solvation, and ion pairing.…”

An Electron‐Rich Calix[4]arene‐Based Receptor with Unprecedented Binding Affinity for Nitric Oxide

“…The cation radical of 3 was generated in solution by means of quantitative redox titrations using [ THEO +. SbCl 6 − ] ( THEO =1,2,3,4,5,6,7,8‐octahydro‐9,10‐dimethoxy‐1,4:5,8‐dimethano‐anthracene, E red =0.67 V vs. Fc/Fc + , λ max =518 nm, ϵ max =7300 cm −1 m −1 ) as oxidant.…”

“…A complete consumption of the oxidant (i.e., THEO +. ) after the addition of 1 equiv of neutral 3 , was determined by a quantitative deconvolution of the component spectra of various species present at each titration point. A plot of the mole fractions of THEO +.…”

Isolation of a chiral anthracene cation radical: X-ray crystallography and computational interrogation of its racemization

“…C : Plot of the mole fractions of THEO + • (burgundy) and S ANT + • (blue) against the added equivalents of neutral S ANT . Symbols represent experimental points, while the solid lines show best-fit to experimental points using ΔG 1 = E red ( THEO + • ) – E ox ( S ANT ) = 0.2 V. 20 …”

“…A complete consumption of the oxidant (i.e. THEO +• ) and clean formation of S ANT +• , after the addition of 1 equivalent of neutral S ANT , was ascertained by a quantitative deconvolution 20 of the component spectra of various species present at each titration point (Fig. 2B).…”