Novel Supramolecular Thixotropic Metallohydrogels Consisting of Rare Metal-Organic Nanoparticles: Synthesis, Characterization, and Mechanism of Aggregation

Even without obvious sticky sites, novel supramolecular thixotropic metallohydrogels consisting of rare metal-organic nanoparticles (MNPs) have been readily accessible from simple structured pincer-type terpyridine Cu(II) complexes at the gelator concentration as low as 0.25 wt %. The obtained soft materials have been fully characterized by using a combination of experimental techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), dynamic light scattering (DLS), X-ray diffraction (XRD), high-resolution mass spectrometry (HR-MS), and theology measurements. Based on these studies, hypothesized molecular assembly mechanisms for distinctly different morphologies observed in different solvents were proposed. Besides general heating-cooling gel preparation procedures, surprisingly, the metallohydrogel was formed by simply stirring the mixture of pincer ligand and Cu salts in water directly, further indicating thixotropic property and self-healing ability of the resulting metallohydrogel under external stress. This stirring approach is highly anion and phase selective with various potential applications. Furthermore, the infrequent observation of the crystal growth from metallogels in situ readily reavealed the self-assembly mechanism that g-stacking and metal-metal interactions along with hydrogen-bonding interactions between gelator and guest molecules are responsible for the gel formation, which is further confirmed by the control gel collapse experiment via external ligand substitution. All these results indicated that pincer organometallic complexes not only can function as a new type of hydrogelators but also are readily to fabricate useful thixotropic materials with various applicability based on their morphologies and assembly mechanism studies.