Highly water-soluble three-redox state organic dyes as bifunctional analytes

Dyes such as indigo and anthraquinone derivatives that find extensive use in industrial colorant applications contain electron density donor (-NH- or-OH) and acceptor (>C=O) groups linked by conjugated bonds in their molecular structure, which are responsible for their deep shades. Departing from this stabilized redox state (push-pull'') requires substantial energy and we found that with a proper choice of aqueous electrolyte these dyes are able to both reversibly oxidize and reduce, developing two-sets of fast two proton-electron transfers separated by similar to 1 V. These large flat molecules are poorly soluble due to extensive pi'' stacking, and the classical strategy of appending a sulphonate (-SO3Na) group appeared insufficient to reach approximate to 1 M. Herein, we have further boosted the solubility of these dyes in water up to 1.6 M by replacing Na+ with an organic cation such as tetrakis(hydroxyethyl)-ammonium [N(CH2CH2OH)(4)](+) or tetrakis(hydroxymethyl)-phosphonium [P(CH2OH)(4)](+) that are non-chelatable and very hygroscopic while maintaining a reversible charge transfer and a large electrochemical potential window in dilute protic acids (0.1 M HClO4), or in neutral pH buffered aqueous solutions (Britton-Robinson buffer). The corresponding pure salts melt even near ambient temperature. Our findings imply that the same electroactive material could be used as a catholyte and an anolyte in a 1 V symmetrical redox flow cell at neutral or close to neutral pH, with all its advantages in terms of cell corrosion and membrane requirements.