Metallo-supramolecular Gels Research Articles

Manipulating Assemblies in Metallosupramolecular Gels, Driven by Isomeric Ligands, Metal Coordination, and Adaptive Binary Gelator Systems.

Metallosupramolecular gel (MSG) is a unique combination of metal-ligand coordination chemistry and supramolecular gel chemistry with extraordinary adaptivity and softness. Such materials find broad uses in industry, pharmaceutical and biomedical sectors, and in technology generation among many others. Pyridyl-appended bis(urea) gelator systems have been extensively studied as potential MSG-forming materials in the presence of various metal ions. The previous molecular engineering approaches depicted competitive intermolecular and intramolecular binding modes involving urea and pyridyl groups and further fine-tuned by the presence of various molecular spacers. In those studies, formation of intermolecular hydrogen bonding among urea moieties to form urea tape was found to be the key factor in one-dimensional assembly and gel formation. In the present study, we show how two isomeric pyridyl-appended bis(urea) ligands can be designed appropriately to essentially eliminate the interference of competitive factors, leaving the intermolecular urea assembly practically unaffected even in the presence of metal ions. We found that one of the two ligands (L2) and the mixed ligand (L1 + L2) assemblies formed gel in the presence and absence of various metal ions. A metal ion with a linear coordination geometry significantly strengthened the gels. Moreover, an inherently weak L1 + L2 assembly appears to be more adaptive in accommodating larger metal ions especially with nonlinear coordination geometry preferences. Small-angle neutron scattering and rheological, spectroscopic, and morphological characterizations, collectively, capture a detailed interplay among ligand assembly, metal-ligand coordination, and adaptivity, driven by the pure versus mixed ligand assemblies. The knowledge gathered from the present study would be highly beneficial in engineering the metallosupramolecular polymeric assemblies toward their functional applications.

Rapid self-healing and anion selectivity in metallosupramolecular gels assisted by fluorine-fluorine interactions.

Simple ML2 [M = Fe(ii), Co(ii), Ni(ii)] complexes obtained from a perfluoroalkylamide derivative of 4-aminophenyl-2,2',6,2'-terpyridine spontaneously, yet anion selectively, self-assemble into gels, which manifest an unprecedented rapid gel strength recovery, viz. self-healing, and thermal rearrangement in aqueous dimethyl sulfoxide. The key factor for gelation and rheological properties emerges from the fluorine-fluorine interactions between the perfluorinated chains, as the corresponding hydrocarbon derivative did not form metallogels. The perfluoro-terpyridine ligand alone formed single crystals, while its Fe(ii), Co(ii) or Ni(ii) complexes underwent rapid gelation leading to highly entangled fibrillar networks visualized by electron microscopy. The thermodynamic parameters of gelation based on variable temperature NMR 1H and 19F resonances showed that gelation was enthalpically favourable and entropically disfavourable. The step strain rheological experiments revealed that the gels undergo rapid self-healing and the morphological features, thermal stability and mechanical properties were found to depend on the nature of the metal ion.

Self-organized dysprosium-directed alginate hydrogels and its chemical features

Rational use of self-organized materials may contribute in developing new structures and devices in practical technology. Synthetic metallo-supramolecular gels are generally designed with transitional metal-directed process. However, the assembly of both lanthanide and sodium alginate in macromolecular systems would find a new way of utilizing its physical properties. The stimuli-responsive molecule (alginate) could firmly form stable hydrogels upon the encapsulation of dysprosium ions. In addition, the immobilization of YVO4: Eu3+ nanoparticle in the soft matrix has been achieved and it has never been explored in the fabrication of phosphor-incorporated luminescent alginate gels. The key feature of the present soft matter is that its red emission could be switched off in the presence of sodium ascorbate and the results may have a tremendous impact on the extension of photophysical application based on soft nanoscale devices.

Synthesis, structural, photophysical and electrochemical studies of various d-metal complexes of btp [2,6-bis(1,2,3-triazol-4-yl)pyridine] ligands that give rise to the formation of metallo-supramolecular gels

2,6-Bis(1,2,3-triazol-4-yl)pyridine (btp) is a terdentate binding motif that is synthesised modularly via the CuAAC reaction. Herein, we present the synthesis of ligands 1 and 2 and the investigation of the coordination chemistry, photophysical behaviour and electrochemistry of complexes of these with a number of d-metal ions (e.g. Ru(II), Ir(III), Ni(II) and Pt(II)). The X-ray crystal structures of ligand 1 and the complexes [Ru·2(2)](PF6)Cl, [Ni·1(2)](PF6)Cl and [Ir·1Cl3] are also presented. All of the complexes displayed non-classical triazolyl C-H···Cl(-) hydrogen bonding. All but one complex showed no metal-based luminescence at room temperature, while all of the Pt(ii) complexes displayed luminescence at 77 K. The electrochemistry of the Ru(II) complexes was also studied and these complexes were found to have higher oxidation potentials than analogous compounds. The redox behaviour of [RuL2](2+) complexes with both 1 and 2 was nearly identical, while [Ru·1Cl2(DMSO)] was oxidised at significantly lower potential. We also show that the Ru(II) complex of 2, [Ru·2(2)](PF6)Cl, gave rise to the formation of a metallo-supramolecular gel, the morphology of which was studied using scanning electron and helium ion microscopy.

Multi-responsive self-healing metallo-supramolecular gels based on “click” ligand

Metallo-supramolecular gels were prepared using a ligand macromolecule containing the tridentate 2,6-bis(1,2,3-trizol-4-yl)pyridine (BTP) ligand unit synthesized via CuAAC “click” chemistry in the main chain, together with transition metal ions and/or lanthanide ions. The gelation and gel properties, e.g. swelling, emission, rheological properties, thermo- and chemo-responsive properties, can be tuned by the careful selection of metal ions and their combinations, solvents, concentration, etc. Most interestingly, the gels exhibited repeatable autonomic healing ability. Moreover, the repeatable self-healing ability of the gels found practical application in the repairing of metal coatings.

Metallo-Supramolecular Gels Based on a Multitopic Cyclam Bis-Terpyridine Platform

The domain of soluble metallopolymers (i.e., polymers spontaneously obtained from the self-assembly of metal ions and polytopic bridging ligands) is promising for the construction of novel materials with perspectives in magnetic, redox, optical, electrochromic, or mechanical properties. We report the preparation and characterization of metallo-supramolecular gels based on a multitopic cyclam bis-terpyridine platform (CHTT) that are representative of a second generation of molecular gels exhibiting "intelligent" properties. The systems were characterized by UV-visible spectroscopy, cyclic voltammetry, viscosimetry, rheology, and small-angle neutron scattering (SANS). A basic analysis of the viscosity taking into account the interplay of hydrodynamic versus Brownian motions of 1D species is proposed. SANS data demonstrate the formation of rod-like species whose radius R and aspect ratio f = L/R were extracted. Rheological features are typical of weak gels having storage moduli of more than 1 order of magnitude weaker than for ordinary molecular organogels. These new molecular materials exhibit interesting properties: (i) chemosensitivity, with the gelation being dependent on the type of metal, stoichiometry M/CHTT, solvent, and counterion; (ii) electro-sensitivity: the Co(II)/CHTT system can be electrochemically and reversibly commuted between the gel (red) and liquid (green) states; and (iii) mechanical sensitivity, with the system being able to cycle from weakly to highly viscous states upon fast application/suppression of a shearing stress.

Metallosupramolecular Gels Made of Four-armed Poly(ethylene glycol)s Having Terpyridine Termini

Four-armed poly(ethylene glycol) having 2,2′:6′,2″-terpyridine) termini was synthesized as a component unit and further utilized for preparation of metallosupramolecular gels by the addition of FeCl2. The formed gels collapsed by the addition of NH3.

Control of Gel Morphology and Properties of a Class of Metallo-Supramolecular Polymers by Good/Poor Solvent Environments

Through the utilization of a mixed solvent system, consisting of a good solvent (DMSO) with either a nonsolvent (water) or a poor solvent (ethylene glycol), the morphology and properties of a class of stimuli-responsive metallo-supramolecular gels can be systematically tailored. These gels are formed via self-assembly of a ditopic ligand, consisting of a 2,6-bis(1′-methylbenzimidazolyl)-4-oxypyridine moiety attached to either end of a penta(ethylene glycol) core, in the presence of a transition metal ion (ZnII). In each solvent system, a composition window is located in which gels, varying in turbidity from highly opaque (water- or ethylene glycol-rich) to highly transparent (DMSO-rich), are formed. Morphological observations suggest gelation occurs by a common mechanism throughout the composition range, i.e., via the flocculation of semicrystalline colloidal particles. Increase of DMSO content leads to a reduction in the particle size, accompanied by an increase in sol concentration. WAXD and SAXS analys...

Structural Origin of the Thixotropic Behavior of a Class of Metallosupramolecular Gels.

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Structural origin of the thixotropic behavior of a class of metallosupramolecular gels

Complexation, between a ditopic ligand, consisting of a 2,6-bis-(1′-methylbenzimidazolyl)-4-oxypyridine moiety (O-Mebip) attached to either end of a penta(ethylene glycol) core, with transition metal and lanthanide ions, results in the formation of metallosupramolecular polymers, soluble in acetonitrile at high temperatures. Cooling the hot sol to room temperature causes phase separation and crystallization, and produces mechanically-strong gels, which exhibit a highly thixotropic behavior. Optical microscopy indicates that the gel morphology consists of spherulitic particles, which are easily broken by mechanical shear. Reproducible gel properties are produced when the gel is formed by cooling in a sonication bath, which produces a finely-divided globular morphology, and increases the modulus of the gels. Wide angle X-ray diffraction study shows that the crystalline structures of the gels are strongly dependent on the thermal history of gel formation and the nature of the metal ion. The gel properties are a result of the interactions between the colloidal particles produced by the phase separation and crystallization process. These interactions, which may reflect electrostatic forces and possibly metal–ligand binding, in addition to the usual van der Waals interactions, give rise to the formation of a network structure. The disruption of this network by mechanical shear, and its facile reformation when shear is removed, are the origin of the pronounced thixotropic behavior of the gels.

Understanding the Mechanism of Gelation and Stimuli-Responsive Nature of a Class of Metallo-Supramolecular Gels

Utilizing metal-ligand binding as the driving force for self-assembly of a ditopic ligand, which consists of a 2,6-bis-(1'-methylbenzimidazolyl)-4-oxypyridine moiety attached to either end of a penta(ethylene glycol) core, in the presence of a transition metal ion (Zn(II)) and a lanthanide metal ion (La(III)), we have achieved formation of stimuli-responsive metallo-supramolecular gels. We describe herein a series of experimental studies, including optical and confocal microscopy, dynamic light scattering, wide-angle X-ray diffraction, and rheology, to explore the properties of such gels, as well as the nature of the gelation mechanism. Morphological and X-ray diffraction observations suggest gelation occurs via the flocculation of semicrystalline colloidal particles, which results in the gels exhibiting pronounced yielding and thixotropic behavior. Application of mechanical stress results in a decrease in the particle size, which is accompanied by an increase in gel strength after removal of the stress. Moreover, studies show that the presence of lanthanide(III) perchlorate increases the mechano-responsiveness of the gels, as a consequence of reduced crystallinity of the colloidal particles, presumably due to the different coordination ability of lanthanide(III) and zinc(II), which changes the nature of the self-assembly in these materials.

Rheological Behavior of Shear-Responsive Metallo-Supramolecular Gels

Rheological Behavior of Shear-Responsive Metallo-Supramolecular Gels