Tetra-2,3-pyrazinoporphyrazines with peripherally appended pyridine rings. 23. Synthesis, physicochemical properties and photoactivity studies of pentanuclear porphyrazines carrying externally 1,2-dithiolate-o-carborane groups

This work has been developed on a new class of homo- and heteropentanuclear porphyrazine complexes of formula [{Pd(CBdT)}4Py8TPyzPzM]⋅xH2O (Py8TPyzPz = tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazinato anion, CBdT = 1,2-dithiolate-o-carborane, and M = MgII(H2O), ZnII, PdII). All the species were prepared by reaction of the respective pentanuclear complexes [(PdCl2)4Py8TPyzPzM] with 1,2-dithiol-o-carborane. Physicochemical characterization of the compounds was carried out by elemental analysis, IR and XRD-P spectra and 1H/13C/11B NMR measurements. UV–visible spectral characterization performed in DMSO, DMF and pyridine provided information about the stability of the new species. NMR data and UV-visible studies indicate that external coordination of the Pd(CBdT) units to the mononuclear [Py8TPyzPzM] species causes only a modest change of the π electron distribution within the internal macrocyclic chromophore. The reported new pentanuclear complexes behave in DMF and/or DMF/HCl solution as active photosensitizers for singlet oxygen production, 1O2, the main cytotoxic agent in Photodynamic Therapy (PDT), and have quantum yield values (ΦΔ = 0.58–0.61) higher than those found on average for the related tetrapyrazinoporphyrazine analogues (ΦΔ = 0.4–0.6). Due to their high boron content perspectives are also open of their possible use in the field of Boron Neutron Capture Therapy (BNCT), this announcing presumable potential applications in bimodal PDT/BNCT anticancer treatments.