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 NhtH(2)Pc, where Nht indicates naphthoxy-4-sulfonic acid sodium salt, was synthesized and its aggregation, electrochemical and spectro-electrochemical properties were investigated in non-aqueous solutions. The aggregation study of NhtH(2)Pc showed that NhtH(2)Pc 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. NhtH(2)Pc displayed three reversible one-electron reductions waves, assigned to Pc3-/Pc2-, Pc4-/Pc3- and Pc5-/Pc4- 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)(4)H2Pc]. The electrochemical studies showed that the electron-withdrawing sulfonated-naphthoxy groups on the macrocycle core made the reduction processes of NhtH(2)Pc easier in DMSO solution. The well-defined UV-Vis spectra of the electro-reduced species [NhtH(2)PC](-) 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. (c) 2006 Elsevier Ltd. All rights reserved.