A new water-soluble metal-free phthalocyanine substituted with naphthoxy-4-sulfonic acid sodium salt. Synthesis, aggregation, electrochemistry and in situ spectroelectrochemistry

A new water-soluble metal-free phthalocyanine, 2,9,16,23-tetrakis(4-(1-naphthoxy-4-sulfonic acid sodium salt))phthalocyanine NhtH₂Pc, where Nht indicates naphthoxy-4-sulfonic acid sodium salt, was synthesized and its aggregation, electrochemical and spectroelectrochemical properties were investigated in non-aqueous solutions. The aggregation study of NhtH₂Pc showed that NhtH₂Pc had both aggregated and non-aggregated mono phthalocyanine forms in the case of the 1:1 ratio of methanol and water, while it exhibited only the characteristic UV-Vis absorption of monomeric phthalocyanine in methanol and DMSO. NhtH₂Pc displayed three reversible one-electron reductions waves, assigned to Pc^3-/Pc^2-, Pc^4-/Pc^3- and Pc^5-/Pc^4- couples, respectively. The electrochemical half-wave potentials of the reduction processes were located at E_1/2 = -0.510, -0.924 and -1.24 V, respectively while the anodic potential of the oxidation process was displayed at E_1/2 = 0.590 V versus pseudo Ag/AgCl. The half-wave potentials of the first and second reductions were positively shifted by 0.150 and 0.136 V compared with those of the unsubstituted metal-free phthalocyanine (H₂Pc). These shift values are almost the same as those observed for [(SO₃)₄H₂Pc]. The electrochemical studies showed that the electron-withdrawing sulfonated-naphthoxy groups on the macrocycle core made the reduction processes of NhtH₂Pc easier in DMSO solution. The well-defined UV-Vis spectra of the electro-reduced species [NhtH₂Pc]^- were obtained with an applied potential (E_app = -0.70 V) in a thin-layer cell. The spectroelectrochemical results showed that the first reduction product exhibited characteristic spectral changes corresponding to mono-anionic species of metal-free phthalocyanines, having long-term stability during the reduction process.