Building-up Remarkably Stable Magnesium Porphyrin Polymers Self-Assembled via Bidentate Axial Ligands: Synthesis, Structure, Surface Morphology, and Effect of Bridging Ligands

A series of supramolecular architectures of magnesium tetranitrooctaethylporphyrins mediated by several bidentate axial ligands have been synthesized in excellent yields and structurally characterized. Six conjugated axial ligand with increasing chain lengths have been utilized in the present investigations in which the Mg···Mg nonbonding distance between successive ions also increases from 0.73 to 2.70 nm in the series. To the best of our knowledge, this is the first report where stable metallo-porphyrin polymers with such long spacers have been synthesized in one pot so easily. Linear one-dimensional (1D) polymeric chains were observed in the X-ray structure of the six-coordinated complexes in which porphyrin units are aligned parallel to each other to have so-called "shish kebab" like architectures to maintain offset-stacked overlap. However, after an optimum Mg···Mg nonbonding distance, these 1D chain do not continue, rather they form five-coordinated porphyrin dimers with "wheel-and-axle" like architectures which are then self-aggregated by π-π interactions in a perpendicular manner to fill space created by large bridging ligands more effectively which consequently results in spherical structures. The structures of the molecules in solution and their surface patterns on highly ordered pyrolytic graphite (HOPG) have also been investigated.