Competing Hydrogen-Bonding, Decomposition, and Reversible Dimerization Mechanisms during the One- and Two-Electron Electrochemical Reduction of Retinal (Vitamin A)

Abstract

Retinal (R) can be sequentially voltammetrically reduced in CH3CN in two one-electron processes to form first the anion radical (R(•-)) at -1.75 (±0.04) V vs Fc/Fc(+) (Fc = ferrocene) then the dianion (R(2-)) at -2.15 (±0.04) V vs Fc/Fc(+). The anion radical undergoes a reversible dimerization reaction to form the dianion (R2(2-)) with a forward dimerization rate constant k(dim) = 8 × 10(2) L mol(-1) s(-1) and a reverse monomerization rate constant k(mon) = 2 × 10(-2) s(-1) at 295 K. All three anion species (anion radical, dianion, and dimer dianion) undergo hydrogen-bonding interactions with water that is present at millimolar levels in the solvent. As the water content of the solvent increases, the fate of the reduced compounds is determined by chemically irreversible hydrolysis reactions with H2O and decomposition reactions of the highly charged R(2-). Bulk-controlled potential electrolysis experiments combined with NMR analysis of the reaction solutions indicate that the reduction occurs at the aldehyde group of retinal. The electrochemical data obtained under a range of experimental conditions (varying voltammetric scan rates, temperatures, H2O content of solutions, and retinal concentrations) were modeled by digital simulation techniques to determine the kinetic and thermodynamic parameters associated with all of the homogeneous reactions.

Spin Manipulation by Creation of Single-Molecule Radical Cations

“…As switching to states that exhibit a Kondo resonance was only observed at negative sample voltages, this is specifically the formation of a positively charged radical (ReA* + ). This interpretation is consistent with the findings for solvated ReA, where addition of an electron leads to dissociation via deprotonation [41], whereas electron removal leads to an equally stable radical cation under acidic and alkaline conditions [42].…”

Electrochemical Methods to Evaluate the Antioxidant Activity and Capacity of Foods: A Review

“…A mirable example is provided by Makhotkina and Kilmartin, i. e. using digital simulation to confirm the EC mechanism of the oxidation of polyphenols in the presence of sulfur dioxide [164]. Other relevant examples are also offered by Webster et al., that investigated the redox behavior of retinal (vitamin A) under a range of experimental conditions (scan rates, temperatures, H 2 O content of solutions, and reactant concentrations) [165].…”

“…retinal (vitamin A) under a range of experimental conditions (scan rates, temperatures, H 2 O content of solutions, and reactant concentrations)[165].…”

Reversible Redox Switching of Chromophoric Phenylmethylenepyrans by Carbon–Carbon Bond Making/Breaking

“…Supramolecular approaches involving large amplitude motion of their inner components, such as observed in mechanically interlocked molecular (MIM) switches, have been widely developed over the past years, leading to a wide range of functionalized systems including coordination metal complexes and purely organic systems. − Alternatively, organic chromophores displaying low-amplitude motion have also drawn interest when fast switching time is requested. Devices based on the photoactivated switching between two isomers, such as azobenzenes, have been well developed over the past years. , Redox-triggered switches displaying reversible π- or σ-dimerization from electrochemically or photochemically generated radical anions or cations have also drawn interest. − In particular, intermolecular σ C–C dimerization of radical cations of conjugated aromatic systems, such as stilbene, triarylamines, , pyridine, cyanins, dialkoxyanthracene, spiropyran, , tripyrrolinidobenzene, bis­(benzodithiolyl)­bithienyl, and dithiafulvene − were reported. It is noteworthy that recent investigations have focused on the immobilization of this family of redox switches on solid surfaces for the development of smart multiple responsive materials. ,− …”