Synthesis, electro-spectroelectrochemical characterization and electrocatalytic behavior towards dioxygen reduction of a new water-soluble cobalt phthalocyanine containing naphthoxy-4-sulfonic acid sodium salt

A new water-soluble cobalt phthalocyanine, 2(3), 9(10), 16(17), 23(24)-tetrakis(4-(1-naphthoxy-4-sulfonic acid sodium salt)) phthalocyaninato cobalt(II) NhtCoPc, where Nht indicates naphthoxy-4-sulfonic acid sodium salt, was synthesized and its electro-spectroelectrochemical and electrocatalytic behavior towards oxygen reduction reaction were investigated in details. The complexation reaction was monitored with the UV-vis spectral changes of NhtH₂Pc in methanol solution. The reaction was completed when the main Q-band was observed in maximum intensity. The electrochemical studies showed that the cobalt complex exhibited two reversible one-electron reductions with the corresponding anodic wave and an irreversible oxidation reaction in DMSO solution. These reduction processes were assigned to Co(2+)Pc(2-)/Co(+)Pc(2-) and Co(+)Pc(2-)/Co(+)Pc(3-) couples, respectively. The well-defined UV-vis spectra of the mono-anionic species [NhtCo(+)Pc(2-)]^-, di-anionic species [NhtCo(+)Pc(3-)]^2-, and mono-cationic species [NhtCo(3+)Pc(2-)]⁺, were obtained by the applied potentials (E_app = -0.60, -1.40, and 0.70 V, respectively) in a thin-layer cell. NhtCoPc was incorporated into the conductive polyaniline (PAni) films as a dopant-anion during electropolimerization in acid medium, and thus formed the Pt/PAni-NhtCoPc electrode. This modified electrode was characterized by scanning electron microscopy (SEM), as well. The addition of NhtCoPc to the polymerization solution changes markedly the morphology of the films obtained and increases the redox-activity towards oxygen reduction of the PAni film formed compared to those of Pt/PAni and bare platinum electrodes in the same experimental conditions.