Zinc(II) complexes of tetrakis-(6,7-quinoxalino)porphyrazine bearing externally appended 2-pyridyl rings: Synthesis, UV-visible spectral behavior and photoactivity for singlet oxygen generation

The synthesis and physicochemical properties of the ZnII complexes, [Py8QxPzZn] (Py8QxPz = tetra-2,3-[6,7-di(2-pyridyl)quinoxalino]porphyrazinato dianion), the corresponding octacation [(2-Mepy)8QxPzZn]8+ (neutralized by I- ions), and the heteropentanuclear complex [(PdCl2)4Py8QxPzZn] are presented and discussed. According to their UV-visible spectral data in non aqueous solvents (CHCl3, pyridine, dimethyl sulfoxide, dimethylformamide), the species exhibit intense absorptions in the Q-band region at 750–770 nm, in line with parallel spectral data observed for the free base [Py8QxPzH2], also reported, and the previously investigated MgII complex [Py8QxPzMg(H2O)]. The observed Q-band position for this series of “quinoxalinoporphyrazine” macrocycles is about 100 nm to the red with respect to the position of the same absorption for the series of the already reported “pyrazinoporphyrazine” analogs. The spectral effects induced by the peripherally appended methylpyridinium groups in the octacation [(2-Mepy)8QxPzZn]8+ or by exocyclic coordination in the complex [(PdCl2)4Py8QxPzZn] are also considered. Of the investigated ZnII complexes, [Py8QxPzZn] and the related octacation [(2-Mepy)8QxPzZn]8+ are active in dimethylformamide solution as singlet oxygen photosensitizers, which is of interest for applications in the photodynamic therapy (PDT) of cancer