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

Abstract

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.

Structural studies on inclusion compounds and solvent sorption behavior of gradually elongated wheel-and-axle-type diol hosts featuring lateral benzo[b]thiophene units

“…general. This is in line with findings for gradually elongated porphyrin systems [8,9]. The most efficient solvent inclusion has been found for a biphenyl spacer showing an inclusion ratio comparable to previously discussed molecules having phenyl or an ethynlene spacer.…”

“…Hence, it is convincingly shown that the covalently linked axis of a conventional wheeland-axle host molecule can be replaced by a system of hydrogen bonds or by coordination of ligands generating inclusions alike. Moreover, derived from the existing findings, the length of the central axis is demonstrated to decisively control the void dimensions as well as the mode of aggregation in the crystal including, e.g., wheel-and-axle or shish-kebab type shapes of supramolecular scaffolds [8,9], aside from the kind of subunits that compose the molecular wheels exercising a similar effect. Only recently, steroidal wheel-and-axle-type hosts [11] as well as the exchange of the lateral aryl groups of respective host molecules against heterocycles [12] opened up a further new way of structural modification, showing high potential to control inclusion selectivity of organic guest species.…”

“…Owing to their irregular shape, corresponding hosts do not pack efficiently and thus they tend to yield inclusion compounds in the presence of neutral guest molecules. In a modification of this proven design, wheel-and-axle type geometries have also been created bipartitely via an association of bulky carboxylic acid fragments [7] or using ligands of varying length for bridging the metal centers of porphyrins [8,9] or other metal containing molecules [10]. Hence, it is convincingly shown that the covalently linked axis of a conventional wheeland-axle host molecule can be replaced by a system of hydrogen bonds or by coordination of ligands generating inclusions alike.…”

Porphyrin polymers and organic frameworks

“…The polymers form linear structures with porphyrin units perpendicular to the ligands, but with increasing length of the axial ligands, some porphyrin units also fill the spaces by aligning parallel to the ligands. 57 Cao et al published two papers in 2013 concerning selfassembly of side-chain-coordinated Zn and Hg porphyrin MOFs studied for solar photovoltaic cells (Fig. 22).…”

Axial coordination of pyridyl-containing pentacenes to porphyrins

“…Axial complexation of pyridyl moieties to metalloporphyrin centers can lead to supramolecular assemblies such as cages [5], oligomers [6], polymers [7], wires [8], or tweezers [9]. Complementing the intriguing properties of porphyrins are those of polycyclic aromatic hydrocarbons such as pentacenes.…”