Tetra-2,3-Pyrazinoporphyrazines With Peripherally Appended Pyridine Rings. 20. Mono- And Pentanuclear Al-Iii And Ga-I(Ii) Complexes: Synthesis And Physicochemical And Photoactivity Studies

The autocyclotetramerization of the precursor 2,3-dicyano-5,6-di(2'-pyridyl)pyrazine [(CN)(2)Py(2)Pyz] in the presence of MCl3 compounds (M = Al-III, Ga-III) leads to the formation of the new mononuclear porphyrazine complexes [Py(8)TPyzPzMCl]center dot xH(2)O (Py(8)TPyzPz = tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazinato anion). From these species, the hydroxide analogues [Py(8)TPyzPzMOH]center dot xH(2)O were obtained by contact with hot water, and their corresponding pentanuclear species [(PdCl2)(4)[Py(8)TPyzPzMCl]center dot xH(2)O could be easily formed by the reaction with PdCl2. Physicochemical characterization of the mono- and pentanuclear macrocycles was based on elemental analysis, and mass, powder X-ray, and IR spectra. UV-vis spectral studies of the mononuclear species in solutions of DMF, DMSO, or pyridine (c approximate to 10(-5)-10(-6) M) indicate (mainly in DMF and DMSO) the initial presence of aggregation, in some cases accompanied by the concomitant occurrence for the complex of a one-electron reduction. While disaggregation into a single species evolves spontaneously over time, the -1 charged species, eventually also found present, can be brought back to its neutral form by addition of a slight amount of HCl, the final spectrum showing the presence of the starting neutral species in its pure monomeric form. Similar aspects were faced also for the parent pentanuclear complexes. Cyclic voltammetry experiments, conducted for the mono-/ pentanuclear complexes in DMF and DMSO (c approximate to 10(-4) M), exhibit progressive one-electron reductions (1 -> 4) characterized by E-1/2 values (V vs SCE) positioned to significantly less negative values than those known for the phthalocyanine (Pc) analogues, these data confirming the previously already proven higher electron-deficient character of the M-II derivatives of the Py(8)TPyzPz macrocycle with respect to Pc. The role of the present new series of Al-III and Ga-III macrocyclic species to act as photosensitizers for the generation of singlet oxygen, O-1(2), the cytotoxic agent in the anticancer treatment known as photodynamic therapy (PDT), has been explored in DMF/HCl. Among the quantum yields Phi(Delta), the value found for the Ga-III complex [Py(8)TPyzPzGaCl] (0.68), practically coincident with that observed for the TTDPz analogue (0.69), is well above those of most porphyrazines analogues (Phi(Delta) = 0.4-0.6), a result encouraging further research work for potential applications in the biochemical field.