A water-soluble derivative of tetrathiafulvalene exhibiting pH sensitive redox properties

Abstract

The properties of a water-soluble TTF dicarboxyl salt, Na₂[C₁₀H₆O₄S₆]·3H₂O, have been explored. The cyclic voltammogram of the salt shows two quasi-reversible redox waves at E_1/2(1) = 0.280 V and E_1/2(2) = 0.556 V with ΔE (E^ox − E^re) = 78 and 68 mV, respectively, in H₂O–CH₃CN (1∶1 v/v). The redox potential of this compound is strongly dependent on the proton concentration. An ECEC square reaction, a redox-coupled association/dissociation of proton, was found for the second redox cycle. The redox active salt forms a mono-protonated anion in the pH range of 2–5. The calculated pK_b constant of the salt is 5.93, corresponding to pK_a⁽²⁾ = 8.07 of the original binary acid. A single crystal structure of Bu₄N[C₁₀H₇O₄S₆] was also obtained, which confirms that the mono-protonated compound has a stable conjugated structure.