Luminescent alkynylplatinum(II) terpyridyl metallogels stabilized by Pt...Pt, pi-pi, and hydrophobic-hydrophobic interactions

Supramolecular Ni(II)-Selective Gel Assembly toward Construction of a Schottky Barrier Diode

A mechanically stable and thermo-irreversible supramolecular Ni(II)-selective gel (MG) has been developed by utilizing the N,O-donor Schiff base (E)-1-((4-(diethylamino)phenylimino)-methyl)naphthalen-2-ol (HL) gelator and Et3N in binary THF:CH3OH (1:1) solutions at room temperature (rt). Metallogel MG has been characterized by spectral and analytical techniques, i.e., ESI-MS, FT-IR, NMR (1H & 13C), powder-XRD, FE-SEM, and rheological analysis. Further, noncovalent interactions responsible for the gelation mechanism have been illustrated with the aid of powder-XRD and FE-SEM analysis. The toughness, viscoelasticity, and flow behavior of MG were explored through rheological studies. Rheological and compressive measurements showed higher values of storage modulus and rigidity of MG; however, the flow property along with enrichment of toughness in MG can be an analytical metric for various engineering and industrial applications. Eventually, a Schottky barrier diode (SBD) was successfully constructed to mimic the functionality of MG-based metal-semiconductor (MS) junction devices for possible application in electrical engineering.

Induced Self-Assembly and Disassembly of Alkynylplatinum(II) 2,6-Bis(benzimidazol-2'-yl)pyridine Complexes with Charge Reversal Properties: "Proof-of-Principle" Demonstration of Ratiometric Förster Resonance Energy Transfer Sensing of pH

A series of pH-responsive alkynylplatinum(II) 2,6-bis(benzimidazol-2'-yl)pyridine (bzimpy) complexes with charge-reversal properties was synthesized, and the supramolecular assemblies between conjugated polyelectrolyte, PFP-OSO₃^-, and [Pt{bzimpy(TEG)₂}{C≡C-C₆H₃-(COOH)₂-3,5}]Cl (1) have been studied using UV-vis absorption, emission, and resonance light scattering (RLS) spectroscopy. An efficient Förster resonance energy transfer (FRET) from PFP-OSO₃^- donor to the aggregated 1 as acceptor with the aid of Pt(II)···Pt(II) interactions has been presented, which leads to a growth of triplet metal-metal-to-ligand charge transfer (³MMLCT) emission in the low-energy red region. The two-component PFP-OSO₃^--1 ensemble was then exploited as a "proof-of-principle" concept strategy for pH sensing by tracking the ratiometric emission changes. With the aid of judicious molecular design on the pH-driven charge-reversal property, the polyelectrolyte-induced self-assembly and the FRET from PFP-OSO₃^- to the platinum(II) aggregates have been modulated. Together with its excellent reversibility and photostability, the extra stability provided by the Pt(II)···Pt(II) and π-π stacking interactions on top of the electrostatic and hydrophobic interactions existing in polyelectrolye-complex assemblies has led to a selective and sensitive pH sensing assay.

A Wide Bandgap Semiconducting Magnesium Hydrogel: Moisture Harvest, Iodine Sequestration, and Resistive Switching

Water harvesting from the ubiquitous moisture is pivotal for delivering fresh water to earth's arid/semiarid regions, and sequestration of iodine from the solution is crucial for environmental safety due to its severe effect on human metabolic processes. In this context, herein, a multifunctional supramolecular metallohydrogel (Mg@TAEA) is synthesized through direct mixing of magnesium nitrate hexahydrate and the low molecular weight gelator tris(2-aminoethyl)amine. Electron microscopy reveals that Mg@TAEA is sculptured in vertically grown well-oriented micrometer-sized flakes. The porous crystalline material (52 m²/g) was found to be an efficacious host matrix for water harvesting from moisture (847 mg/g). Mg@TAEA shows effective (513 mg/g) iodine sequestration from solution and adsorption of carbon dioxide (15 mg/g). The wide bandgap semiconducting Mg@TAEA (3.6 eV) material is a potential candidate for building memory devices, and the I_on/I_off ratio of the device based on the indium tin oxide (ITO)/Mg@TAEA/Ag heterostructure was found to be ∼62. We further extended our work by analyzing the charge transport properties of the system and found space charge limited conduction (SCLC) and trap-filled SCLC to be responsible for the nonlinear transport behavior observed in the device.

Molecular Engineering onto RuII Bis(1,2-diphenylphosphinoethane) Synthon: Toward an Original Organometallic Gelator

In this article, we report the successful molecular engineering of Ru bis-acetylides that led for the first time to a gelator and more specifically in aromatic solvents. By means of a nonlinear ligand and an extended aromatic platform, the bulky Ru bis-acetylides were able to self-assemble into lamellar structures as evidenced by scanning electron microscopy (SEM) in benzene, toluene, and o- and m-xylene, which in turn induced gelation of the solution with a critical gelation concentration of 30 mg/mL. Nuclear magnetic resonance (NMR), variable temperature (VT)-NMR, and Fourier transform infrared (FT-IR) spectroscopies evidenced that hydrogen bonds are mainly responsible for the self-organization. VT-NMR and small-angle X-ray scattering (SAXS) have also suggested that the pro-ligand and the complex stack in different ways.

Recent advances in coordination-driven polymeric gel materials: design and applications

Recently, research attention has been directed towards the coordination driven synthesis of gels, including coordination polymer gels (CPGs) and metal-organic cage based gels, which have shown applications in diverse fields, including optoelectronics, catalysis, sensing, gas-storage, and self-healing. A wide variety of CPGs and metal-organic cage based gels have been reported, to date, by choosing the right combination of metal ions and rationally designed organic linkers. In this article, we focused on recent developments in CPGs and metal-organic cage based gels and their applications.

Self-healing, luminescent metallogelation driven by synergistic metallophilic and fluorine-fluorine interactions

Square planar platinum(ii) complexes are attractive building blocks for multifunctional soft materials due to their unique optoelectronic properties. However, for soft materials derived from synthetically simple discrete metal complexes, achieving a combination of optical properties, thermoresponsiveness and excellent mechanical properties is a major challenge. Here, we report the rapid self-recovery of luminescent metallogels derived from platinum(ii) complexes of perfluoroalkyl and alkyl derivatives of terpyridine ligands. Using single crystal X-ray diffraction studies, we show that the presence of synergistic platinum-platinum (PtPt) metallopolymerization and fluorine-fluorine (FF) interactions are the major driving forces in achieving hierarchical superstructures. The resulting bright red gels showed the presence of highly entangled three-dimensional networks and helical nanofibres with both (P and M) handedness. The gels recover up to 87% of their original storage modulus even after several cycles under oscillatory step-strain rheological measurements showing rapid self-healing. The luminescence properties, along with thermo- and mechanoresponsive gelation, provide the potential to utilize synthetically simple discrete complexes in advanced optical materials.

Self-Assembly of Copper Polypyridyl Supramolecular Metallopolymers to Achieve Enhanced Anticancer Efficacy

Self‐assembled functional supramolecular metallopolymers have demonstrated application potential in cancer therapy. Herein, a copper polypyridyl complex was found able to self‐assemble into a supramolecular metallopolymer driven by the intermolecular interactions, which could enhance the uptake in cancer cells through endocytosis, and thus effectively inhibiting tumor growth in vivo without damaging to the major organs. This study provides a facile way to achieve enhanced anticancer efficacy by using self‐assembled metallopolymers.

Controlling the supramolecular polymerization and metallogel formation of Pt(ii) complexes via delicate tuning of non-covalent interactions

Direct influence of noncovalent ionic and hydrogen bonding interactions on supramolecular polymerization mechanisms and their impact on gel formation of luminescent platinum complexes have been comprehensively investigated.

Directional Self-Assembly and Photoinduced Polymerization of Diacetylene-Containing Platinum(II) Terpyridine Complexes

A series of newly designed and synthesized diacetylene-containing platinum(II) terpyridine complexes exhibited intriguing self-assembly properties. Facilitated by Pt⋅⋅⋅Pt, π-π stacking, hydrogen-bonding and hydrophobic-hydrophobic interactions, these complexes are preorganized to readily undergo topochemical polymerization reactions upon photoirradiation. The in situ polymerization of the diacetylene units to form polydiacetylene, indicated by the UV/Vis spectral changes, gel permeation chromatography and dynamic light scattering, was found to alter their assembly behaviours, as revealed by TEM images.

Supramolecular alternate donor-acceptor copolymers mediated by PtPt metal-metal interactions and their photocatalytic applications

Precise arrangement of the aromatic donor-acceptor units is of paramount importance to dictate the performance of multi-component π-functional materials. Herein a novel strategy has been developed toward alternate donor-acceptor copolymers, by incorporating Pt(ii)Pt(ii) metal-metal interactions for the hetero-complexation process. The proximity of Pt atoms endows the resulting supramolecular copolymers with metal-metal-to-ligand charge-transfer transitions in the visible/NIR region. The signals have been further applied for low-energy visible-light photo-catalysis that is unattainable for the individual species. More interestingly, "on-demand" photo-catalytic efficiency can be achieved by manipulating the reversibility of the supramolecular copolymerization process. Hence, the current work demonstrates the efficiency of fabricating multi-component π-functional materials via the elaborate manipulation of non-covalent driving forces.

Supramolecular Assembly of Phosphole Oxide Based Alkynylplatinum(II) 2,6-Bis(N-alkylbenzimidazol-2'-yl)pyridine Complexes-An Interplay of Hydrophobicity and Aromatic π-Surfaces

A new class of phosphole oxide based alkynylplatinum(II) 2,6-bis(N-alkylbenzimidazol-2'-yl)pyridine (bzimpy) complexes were synthesized and characterized. Their self-assembly was driven by hydrophobic-hydrophobic and π-π stacking interactions. The self-assembly properties were also investigated by UV/Vis absorption spectroscopy, which revealed that the alkyl-chain length of the bzimpy moiety and the π-surface area of the alkynyl ligand have significant influence on the overall self-assembly process. The alkyl-chain length also affected the morphological structures of the aggregates, which were studied by transmission electron microscopy and scanning electron microscopy.

Dimensional Control and Morphological Transformations of Supramolecular Polymeric Nanofibers Based on Cofacially-Stacked Planar Amphiphilic Platinum(II) Complexes

Square-planar platinum(II) complexes often stack cofacially to yield supramolecular fiber-like structures with interesting photophysical properties. However, control over fiber dimensions and the resulting colloidal stability is limited. We report the self-assembly of amphiphilic Pt(II) complexes with solubilizing ancillary ligands based on polyethylene glycol [PEG_n, where n = 16, 12, 7]. The complex with the longest solubilizing PEG ligand, Pt-PEG₁₆, self-assembled to form polydisperse one-dimensional (1D) nanofibers (diameters <5 nm). Sonication led to short seeds which, on addition of further molecularly dissolved Pt-PEG₁₆ complex, underwent elongation in a "living supramolecular polymerization" process to yield relatively uniform fibers of length up to ca. 400 nm. The fiber lengths were dependent on the Pt-PEG₁₆ complex to seed mass ratio in a manner analogous to a living covalent polymerization of molecular monomers. Moreover, the fiber lengths were unchanged in solution after 1 week and were therefore "static" with respect to interfiber exchange processes on this time scale. In contrast, similarly formed near-uniform fibers of Pt-PEG₁₂ exhibited dynamic behavior that led to broadening of the length distribution within 48 h. After aging for 4 weeks in solution, Pt-PEG₁₂ fibers partially evolved into 2D platelets. Furthermore, self-assembly of Pt-PEG₇ yielded only transient fibers which rapidly evolved into 2D platelets. On addition of further fiber-forming Pt complex (Pt-PEG₁₆), the platelets formed assemblies via the growth of fibers selectively from their short edges. Our studies demonstrate that when interfiber dynamic exchange is suppressed, dimensional control and hierarchical structure formation are possible for supramolecular polymers through the use of kinetically controlled seeded growth methods.

Self-Assembled Architectures of Alkynylplatinum(II) Amphiphiles and Their Structural Optimization: A Balance of the Interplay Among Pt···Pt, π-π Stacking, and Hydrophobic-Hydrophobic Interactions

A series of alkynylplatinum(II) terpyridine complexes with triethylene glycol units was synthesized, and their self-assembly properties were investigated in solution by UV-vis absorption, emission, and ¹H NMR spectroscopy. The aggregation behaviors of several water-soluble complexes were investigated in aqueous media. Some of them were found to give rise to uniform fibers, suggesting the important role that triethylene glycol units has in regulating their self-assembly properties. Further modifications of these structures through the incorporation of alkyl chains and changes in counter-anions have rendered the complexes more amphiphilic in nature, and the effect of their alkyl chain lengths was studied and optimized. The distinguishable color and spectral changes upon variations in solvent compositions might have potential applications in developing colorimetric and luminescent probes for the detection of microenvironment change. Furthermore, an optimum chain length, i.e., n-butyl chain, is required for the formation of stable and ordered nanostructures. This represents a delicate balance among Pt···Pt, π-π stacking, and hydrophobic-hydrophobic interactions and provides guiding principles into the construction of supramolecular materials with practical applications.

Binuclear alkynylplatinum(ii) terpyridine complexes with flexible bridges behave as organogelators for several organic solvents

New binuclear alkynylplatinum(ii) terpyridyl complexes with flexible bridges have been synthesized and characterized by ¹H NMR, mass spectra and elemental analysis.

Parallel folding topology-selective label-free detection and monitoring of conformational and topological changes of different G-quadruplex DNAs by emission spectral changes via FRET of mPPE-Ala-Pt(ii) complex ensemble

The formation of supramolecular assemblies between [Pt(bzimpy-Et){C[triple bond, length as m-dash]CC6H4(CH2NMe3-4)}]Cl2 (1) and mPPE-Ala and the FRET properties of the ensemble have been revealed from the UV-vis absorption, steady-state emission and time-resolved emission decay studies. The two-component mPPE-Ala-1 ensemble has been employed in a "proof-of-principle" concept for label-free detection of G-quadruplex DNAs with the intramolecular propeller parallel folding topology, such as c-myc, in aqueous buffer solution. By the modulation of the aggregation/deaggregation of the polymer-metal complex aggregates and hence the FRET from the mPPE-Ala donor to the aggregated 1 as acceptor, the ensemble has been demonstrated for sensitive and selective label-free detection of c-myc via the monitoring of emission spectral changes of the ensemble. Ratiometric emission of the ensemble at 461 and 662 nm has been shown to distinguish the intramolecular propeller parallel G-quadruplex folding topology of c-myc from other G-quadruplex-forming sequences of different folding topologies, owing to the strong and specific interactions between c-myc and 1 as suggested by the UV-vis absorption and UV melting studies. In addition, the formation of high-order intermolecular multimeric G-quadruplexes from c-myc under molecular crowding conditions has been successfully probed by the ratiometric emission of the ensemble. The conformational and topological transition of human telomeric DNA from the mixed-hybrid form to the intramolecular propeller parallel form, as observed from the circular dichroism spectroscopy, has also been monitored by the ratiometric emission of the ensemble. The ability of the ensemble to detect these conformational and topological transitions of G-quadruplex DNAs has been rationalized by the excellent selectivity and sensitivity of the ensemble towards the intramolecular propeller parallel G-quadruplex DNAs and their high-order intermolecular multimers, which are due to the extra stabilization gained from Pt···Pt and π-π interactions in addition to the electrostatic and hydrophobic interactions found in the polymer-metal complex aggregates.

Coordination-induced gelation of an l-glutamic acid Schiff base derivative: the anion effect and cyanide-specific selectivity

Three metallogels, ZnG, CuG and Zn-CuG, were prepared in the presence of specific anions, with their efficacy linked to the Hofmeister series. Importantly, Zn-CuG gel could fluorescently detect CN⁻ with specific selectivity.

Solvent induced helical aggregation in the self-assembly of cholesterol tailed platinum complexes

Three alkynylplatinum(ii) bipyridyl complexes in which two cholesterol groups are combined with a bipyridyl group via alkyl chains and amido bonds were designed and synthesized. The complexes have different lengths of ethylene glycol chains at the para-position of 1-phenylethyne. All three complexes can self-assemble to gel networks in DMSO, while only the morphology of 1a without an ether chain shows a well-defined right-handed helical structure in layer packing mode. However, 1c with long ethylene glycol chains forms perfect regular left-handed helical structures in aqueous ethanol solution while the volume percentage of water is less than 5% (v/v). As the ratio of water increases, the chirality changes from a left-handed helix to a right-handed helix and the packing mode alters from a monolayer structure to a hexagonal structure. As the ratio of water further increases to greater than 50% (v/v), the structure of the assembly finally transforms into bilayer vesicles. The process of the morphology transition is traced by circular dichroism spectra, powder X-ray diffraction, SEM and TEM images. The result indicates that a polar solvent (water) acts as a trigger to change the self-assembly of the chiral structures of the complex due to the strong hydrophobic interaction between cholesterol groups and the balance of the hydrophobicity and hydrophilicity of the solvent environment.

Morphology-driven absorption and emission colour changes in liquid-crystalline, cyclometallated platinum(ii) complexes

Platinum(ii) complexes of 1,3-bis(2-pyridyl)benzene containing two alkyl chains are unexpectedly mesomorphic and capable of changing absorption and emission colour depending on the phase obtained after thermal treatment.

Supramolecular assemblies of amide-derived organogels featuring rigid π-conjugated phenylethynyl frameworks

Organogels, being an important class of soft materials, have evolved to be one of the most attractive subjects bridging supramolecular chemistry and material sciences due to their structural diversity and associated physical properties. Myriad applications in fields such as optoelectronics, light harvesting, environmental science, and regenerative medicine are being envisaged. Supramolecular gels usually are formed through self-aggregation of small-molecule gelators to form entangled self-assembled fibrillar networks through a combination of non covalent interactions such as hydrogen bonding, π-π stacking, electrostatic forces, donor-acceptor interactions, metal coordination, solvophobic forces, and van der Waals interactions. This feature article discusses recent and current state of research on amide derived organogelators bearing rigid conjugated phenylethynyl building blocks. Selective examples from our works along with some closely related examples from literature have been highlighted to showcase the structural diversity and their potential applications in supramolecular chemistry and materials science.