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

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 E1/2(1) = 0.280 V and E1/2(2) = 0.556 V with ΔE (Eox − Ere) = 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 pKb constant of the salt is 5.93, corresponding to pKa(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.