Tetra-2,3-pyrazinoporphyrazines with externally appended thienyl rings: synthesis, UV-visible spectra, electrochemical behavior, and photoactivity for the generation of singlet oxygen

A series of pyrazinoporphyrazine macrocycles carrying externally appended 2-thienyl rings, represented as [Th(8)TPyzPzM], where Th(8)TPyzPz = tetrakis-2,3-[5,6-di(2-thienyl)pyrazino]porphyrazinato anion and M = Mg(II)(H(2)O), Zn(II), Co(II), Cu(II), or 2H(1), were prepared and isolated as solid air-stable hydrated species. All of the compounds, completely insoluble in water, were characterized by their UV-visible spectra and electrochemical behavior in solutions of dimethylformamide (DMF), dimethyl sulfoxide, and pyridine. Molecular aggregation occurs at concentrations of ca. 10(-4) M, but monomers are formed in more dilute solutions of 10(-5) M or less. The examined octathienyl compounds [Th(8)TPyzPzM] behave as electron-deficient macrocycles, and UV-visible spectral measurements provide useful information about how the peripheral thienyl rings influence the electronic distribution over the entire macrocyclic framework. Cyclic voltammetric and spectroelectrochemical data confirm the easier reducibility of the compounds as compared to the related phthalocyanine analogues, and the overall redox behavior and thermodynamic potentials for the four stepwise one-electron reductions of the compounds are similar to those of the earlier examined octapyridinated analogues [Py(8)TPyzPzM]. Quantum yields (Φ(Δ)) for the generation of singlet oxygen, (1)O(2), the cytotoxic agent active in photodynamic therapy (PDT), and fluorescence quantum yields (Φ(F)) were measured for the Zn(II) and Mg(II) complexes, [Th(8)TPyzPzZn] and [Th(8)TPyzPzMg(H(2)O)], and the data were compared to those of corresponding octapyridino macrocycles [Py(8)TPyzPzZn] and [Py(8)TPyzPzMg(H(2)O)] and their related octacations [(2-Mepy)(8)TPyzPzZn](8+) and [(2-Mepy)(8)TPyzPzMg(H(2)O)](8+). These measurements were carried out in DMF and in DMF preacidified with HCl (ca. 10(-4) M). All of the examined Zn(II) compounds behave as excellent photosensitizers (Φ(Δ) = 0.4-0.6) both in DMF and DMF/HCl solutions, whereas noticeable fluorescence activity (Φ(F) = 0.36-0.43) in DMF/HCl solutions is shown by the Mg(II) derivatives; these data might provide perspectives for applications in PDT (Zn(II)) and imaging response and diagnosis (Mg(II)).