Application of spectroelectrochemistry in elucidation of electrochemical mechanism of azoquinoline dye 2-methyl-5-[(E)-phenyldiazenyl]quinolin-8-ol

In situ spectroelectrochemical detection of reaction intermediates was used as a decisive method for elucidation of a rather complex redox mechanism of azoquinoline dye 2-methyl-5-[(E)-phenyldiazenyl]quinolin-8-ol (R-N=N-Ph; where Ph = phenyl, R = 2-methyl-8-hydroxyquinoline fragment). Electrochemical properties were studied in non-aqueous solution by cyclic voltammetry, UV-Vis and IR spectroelectrochemistry and high pressure liquid chromatography with diode array detector. Oxidation and reduction mechanisms involve coupled electron and proton transfers. Oxidation proceeds primarily on hydroxyl group at quinoline moiety and (E)-5-(phenyldiazenyl)quinoline-7,8-diol as the main oxidation product has been suggested. The electrochemically active site for reduction is the azo group. This was proved by in situ UV-Vis and IR spectroelectrochemical data. Detailed analysis of the effects of the presence of acids and bases evidenced the presence of two species in equilibrium: HOR(NH+)-N=N-Ph and HOR-N=N-Ph. The compound containing the hydrazo group (5-(2-phenylhydrazinyl)quinolin-8-ol (R-N=N-Ph) is the main reduction product. Molecular orbital calculations and DFT calculations of IR spectra support the experimental results. In situ IR spectroelectrochemical experiments proved that no reaction of R-N=N-Ph anion or dianion with the solvent acetonitrile was observed during the reduction of the azodye.