Electrochemical behaviour of anthraquinone dyes in non aqueous solvent solution. Part I. Medium effect on the electrochemical behaviour

Abstract

The conditions under which alizarin, purpurin, carminc acid, quinalizarin and alizarin red S in the solution ofseveral organic solvents can be electrochemically transformed are analysed. The electrochemical activity ofthese molecules with quinone and phenolic moieties is relatively easy detected by cyclic voltammetry. Numerouselectro-reduction/oxidation processes without and with added acid or base are observed. The number,the current intensity and the peak potential of the charge transfer processes are rationalized on the basis of thehomogeneous dissociation equilibriums of the phenolic-OH groups. These equilibria are significantly alteredby the medium. A supporting electrolyte unexpected and important effect on the electrochemical behaviour isobserved. Alkaline cation of the supporting electrolyte promotes the changes in the electrochemical behaviourby coordinating with opposite charge species. A confusion published in the literature that the electro-reductionof the hydroxyquinones proceeds only through the formation of a radical-anion and a dianion is revealed.Solvent effect is explained by the stabilization of charge species, which is related to some solvent properties(Gutmann acceptor/donor number and dielectric constant). Solvents with strong acceptor properties favour thedissociation of the phenolic-OH groups. The dissociation equilibriums of phenolic-OH groups in each solventare confirmed by UV-vis spectra of each dye in solution. Alizarin in DMA solvent with added base shows aparticular electrochemical behaviour.For alizarin/DMF system a reaction mechanism is proposed, and alternative mechanisms are suggested forother dyes.