The Microscopic Structure-Property Relationship of Metal-Organic Polyhedron Nanocomposites

Emergent Research Trends on the Structural Relaxation Dynamics of Molecular Clusters: From Structure-Property Relationship to New Function Prediction

ConspectusMolecular clusters (MCs) are monodispersed, precisely defined ensembles of atom collections featured with shape-persistent architectures that can deliver certain functions independently. Their molecular compositions and surface functionalities can be tailored feasibly in a predefined manner, and they can be applied as basic structural units to be engineered into materials with desirable hierarchical structures and enriched functions. The chemical systems also offer great opportunities for the design and fabrication of soft structural materials without the chain topologies of polymers. The bulks of MC assemblies demonstrate viscoelasticity that is used to be considered as the unique feature of polymers, while the MC systems are distinct from polymers since their elasticities are resilient even at temperatures 100 K above their glass transition temperatures. The understanding of their anomalous viscoelasticity and the extended studies of general structure-property relationships are desired for the development of new chemical systems for emergent functions and the possibilities to resolve the intrinsic trade-offs of traditional materials.Meanwhile, general macroscopic functions or properties of materials are related to the transportation of mass, momentum, and/or energy, and they are basically realized or directed by the motions of structural units at different length scales. Structural relaxation dynamics research is critical in quantifying motions ranging from fast bond deformation, bond break/formation, and diffusion of ions and particles to the cooperative motions of structure units. Due to the advancement of measurement technology for relaxation dynamics (e.g., quasi-elastic scattering and broadband dielectric spectroscopy), the structural relaxation dynamics of MC materials have been probed for the first time, and their multiple relaxation modes across several temporal scales were systematically studied to bridge the correlation between molecular structures and macroscopic functions. The fingerprint information from dynamics studies, e.g., the temperature dependence of relaxation time and certain property, e.g., ion conductivity, was proposed to quantify the structure-property relationship, and the microscopic mechanism on the mechanical properties, ion conduction, and gas absorption and separation of MC materials can be fully understood.In this Account, to elucidate the uniqueness of MC materials, especially in comparison with polymers, four topics are mainly summarized: structural features, relaxation dynamics characterization techniques, relaxation dynamics characteristics, and quantified understanding of the structure-property relationship. The capability for new function prediction from relaxation dynamics studies is also introduced, and the typical example in impact resistant materials is provided. The Account aims to prove the significance of relaxation dynamics characterization for material innovation, while it also confirms the potential of MCs for functional material fabrications.

Retrospective Analysis of Structure-Property Relationship of Emergent Metallo-Supramolecular Polymer Networks

Metallo-supramolecular polymer networks (MSPNs) are fabricated from the crosslinking of polymers by discrete supramolecular coordination complexes. Due to the availability of various coordination complexes, e. g., 2D macrocycles and 3D nanocages, the MSPNs have been recently developed with broadly tunable visco-elasticity and enriched functions inherited from the coordination complexes. The coordination complexes possess enriched topologies and unique structural relaxation dynamics, rendering them the capability to break the traditional tradeoffs of polymer systems for the design of materials with enhanced mechanical performance. The structure-property relationship studies are critical for the material-by-design of MSPNs, while the spatiotemporal investigations are desired for the exploration of dynamics information. The work summarizes recent studies on the unique ligand-exchange kinetics and the multi-level structural relaxation dynamics of MSPNs. The MSPNs' mechanical properties can be quantitatively correlated with the dynamics for understanding the structure-property relationship. This concept will not only serve to attract more researchers to engage in the study of the structure-activity relationship of MSPNs but also inspire innovative research findings pertaining to the application of MSPNs.

Supramolecular Synthesis of Star Polymers

Supramolecular polymers, in which monomers are assembled via intermolecular interactions, have been extensively studied. The fusion of supramolecular polymers with conventional polymers has attracted the attention of many researchers. In this review article, the recent progress in the construction of supramolecular star polymers, including regular star polymers and miktoarm star polymers, is discussed. The initial sections briefly provide an overview of the conventional classification and synthesis methods for star polymers. Coordination-driven self-assembly was investigated for the supramolecular synthesis of star polymers. Star polymers with multiple polymer chains radiating from metal-organic polyhedra (MOPs) have also been described. Particular focus has been placed on the synthesis of star polymers featuring supramolecular cores formed through hydrogen-bonding-directed self-assembly. After describing the synthesis of star polymers based on host-guest complexes, the construction of miktoarm star polymers based on the molecular recognition of coordination capsules is detailed.

Exploration of Molecular Nanoparticles as Soft Structural Materials and Their Structure-Property Relationship

The search for alternative chemical systems other than polymers with chain topologies for soft structural materials raises general interests in fundamental materials and chemical sciences. It is also appealing from an engineering perspective for the urgent need to resolve the typical trade-offs of polymer systems. Herein, a subnanometer molecular cluster, polyhedral oligomeric silsesquioxanes, is assembled into molecular nanoparticles (MNPs) with star topology. Broadly tunable viscoelasticity can be realized by fine-tuning the MNPs' deformability. Being analogous to polymeric systems, the hierarchical structural relaxation dynamics can be observed, and their relaxation time and temperature dependence are dominated by the linker flexibilities. This not only provides microscopic understanding on MNP's unique viscoelasticity but also offers enormous opportunities for modulating their mechanical properties via linker engineering. Our work proves the possibility of applying structural units with particle topologies for the design of soft structural materials.

Porous and Meltable Metal-Organic Polyhedra for the Generation and Shaping of Porous Mixed-Matrix Composites

Here, we report the synthesis of BCN-93, a meltable, functionalized, and permanently porous metal-organic polyhedron (MOP) and its subsequent transformation into amorphous or crystalline, shaped, self-standing, transparent porous films via melting and subsequent cooling. The synthesis entails the outer functionalization of a MOP with meltable polymer chains: in our model case, we functionalized a Rh(II)-based cuboctahedral MOP with poly(ethylene glycol). Finally, we demonstrate that once melted, BCN-93 can serve as a porous matrix into which other materials or molecules can be dispersed to form mixed-matrix composites. To illustrate this, we combined BCN-93 with one of various additives (either two MOF crystals, a porous cage, or a linear polymer) to generate a series of mixed-matrix films, each of which exhibited greater CO2 uptake relative to the parent film.

Spatiotemporal Studies of Soluble Inorganic Nanostructures with X-rays and Neutrons

This Review addresses the use of X-ray and neutron scattering as well as X-ray absorption to describe how inorganic nanostructured materials assemble, evolve, and function in solution. We first provide an overview of techniques and instrumentation (both large user facilities and benchtop). We review recent studies of soluble inorganic nanostructure assembly, covering the disciplines of materials synthesis, processes in nature, nuclear materials, and the widely applicable fundamental processes of hydrophobic interactions and ion pairing. Reviewed studies cover size regimes and length scales ranging from sub-Ångström (coordination chemistry and ion pairing) to several nanometers (molecular clusters, i.e. polyoxometalates, polyoxocations, and metal-organic polyhedra), to the mesoscale (supramolecular assembly processes). Reviewed studies predominantly exploit 1) SAXS/WAXS/SANS (small- and wide-angle X-ray or neutron scattering), 2) PDF (pair-distribution function analysis of X-ray total scattering), and 3) XANES and EXAFS (X-ray absorption near-edge structure and extended X-ray absorption fine structure, respectively). While the scattering techniques provide structural information, X-ray absorption yields the oxidation state in addition to the local coordination. Our goal for this Review is to provide information and inspiration for the inorganic/materials science communities that may benefit from elucidating the role of solution speciation in natural and synthetic processes.

Modulating Ligand-Exchange Dynamics on Metal-Organic Polyhedra for Reversible Sorting and Hybridization of Miktoarm Star Polymers

The precise synthesis of miktoarm star polymers (MSPs) remains one of the great challenges in synthetic chemistry due to the difficulty in locating appropriate structural templates and polymer grafting/growing strategies with high selectivity and efficiency. Herein, ≈2 nm metal-organic polyhedra (MOPs), constructed from the coordination of isophthalic acid (IPA) and Cu2+ , are applied as templates for the precise synthesis of 24-arm MSPs for their unique logarithmic ligand-exchange dynamics. Six different polymers are prepared with IPA as an end group and they further coordinated with Cu2+ to afford the corresponding 24-arm star homo-polymers. MSPs can be obtained by mixing targeted homo-arm star polymers in solutions upon thermal annealing. The compositions of MSPs can be facilely and precisely tuned by the recipe of the star polymer mixtures used. Interestingly, the obtained MSPs can be sorted into homo-arm star polymers through a typical solvent extraction procedure. The hybridization and sorting process can be reversibly conducted through the cycle of thermal annealing and solvent treatment. The complex coordination framework not only opens new avenues for the facile and precise synthesis of MSPs and MOPs with hybrid functionalities, but also provides the capability to design sustainable polymer systems.

Functional Molecular Granular Materials: Advances and Perspectives

Molecular granular materials (MGMs) are constructed with sub-nanoscale molecular clusters (MCs) as the building units and they have recently been observed to possess enriched functionalities distinct from granular materials of colloid nanoparticles. Herein, the birth and recent research advances in MGMs are summarized with the topics covering the precise synthesis of MC assemblies with target topologies, the hierarchical relaxation dynamics and tuneable viscoelasticity, impact-resistant capacity, and proton conductivity performance. The extremely small size of MC renders them two features: bulk diffusive dynamics with energy scale close to thermal fluctuation energy and the dominant volume fraction of surface structures. This finally leads to the hierarchical relaxation dynamics and broadly tuneable viscoelasticity of MGMs although the structural units are with small sizes and low M_w . Therefore, MGMs have been applied as impact resistant materials and proton conductors for the highly tuneable relaxation dynamics.

Model2SAS: software for small-angle scattering data calculation from custom shapes

To meet the challenges in resolving the complex morphologies of emergent nanoparticles, a program with a user-friendly graphical user interface has been developed for calculating small-angle scattering curves from custom shapes. The software allows STL-format 3D models, models defined by mathematical functions or combinations of the two as initial input. As a transitional stage, lattice models are generated and the orientation-averaged small-angle scattering data can be calculated using typical spherical harmonics expansion. The validity of the protocol is verified by demonstration models with Protein Data Bank structures and known scattering functions. The software is applied to successfully calculate the scattering curves of a porous spherical shell model where traditional mathematical derivation fails.

Surface Functionality-Regulated and Entropy-Driven Thermodynamics of the Formation of Coordination Nanocages

Coordination nanocages (CNCs) are under intense research in nanoscience and supramolecular chemistry for their enriched surface functionalities and micro-porosity; however, the understanding of their formation mechanism is still poor due to the difficulty in probing their solution structures. Herein, the CNC formation process from the coordination complexation of the macromolecular isophthalic acid (IPA) ligand and Cu²⁺ is studied via isothermal titration calorimetry, and its entropy-driven feature is revealed to be originated from the collapse of solvation layers of the assembly units. The CNC formation is thermodynamically less favored with smaller binding constants when the sizes of macromolecular IPA ligands are larger, which originated from the space crowding of macromolecules of the ligands on CNC surfaces and the resulting entropy loss of polymer chain conformations. Meanwhile, the chemical equilibrium of CNC formation can be tuned upon altering the Cu²⁺/IPA ratio, and the yield of CNCs, suggested from size exclusion chromatography studies, decreases when excessive Cu²⁺ is applied, providing guidelines for CNC design and synthesis.

Robust Heterogeneous Photocatalyst for Visible-Light-Driven Hydrogen Evolution Promotion: Immobilization of a Fluorescein Dye-Encapsulated Metal-Organic Cage on TiO2

The design of artificial photocatalytic devices that simulates the ingenious and efficient photosynthetic systems in nature is promising. Herein, a metal-organic cage [Pd₆(NPyCzPF)₁₂]¹²⁺ (MOC-PC6) integrating 12 organic ligands NPyCzBP and 6 Pd²⁺ catalytic centers is designed, which is well defined to include organic dye fluorescein (FL) for constructing a supramolecular photochemical molecular device (SPMD) FL@MOC-PC6. Photoinduced electron transfer (PET) between MOC-PC6 and the encapsulated FL has been observed by steady-state and time-resolved emission spectroscopy. FL@MOC-PC6 is successfully heterogenized with TiO₂ by a facile sol-gel method to achieve a robust heterogeneous FL@MOC-PC6-TiO₂. The close proximity between the Pd²⁺ catalytic site and FL included in the cage enables PET from the photoexcited FL to Pd²⁺ sites through a powerful intramolecular pathway. The photocatalytic hydrogen production assessments of the optimized 4 wt % FL@MOC-PC6-TiO₂ demonstrate an initial H₂ production rate of 2402 μmol g^-1 h^-1 and a turnover number of 4356 within 40 h, enhanced by 15-fold over that of a homogeneous FL@MOC-PC6. The effect of the MOC content on photocatalytic H₂ evolution (PHE) is investigated and the inefficient comparison systems, such as MOC-PC6, MOC-PC6-TiO₂, FL-sensitized MOC-PC6/FL-TiO₂, and analogue FL/MOC-PC6-TiO₂ with free FL, are evaluated. This study provides a creative and distinctive approach for the design and preparation of novel heterogeneous SPMD catalysts based on MOCs.

Modulating Polymer Dynamics via Supramolecular Interaction with Ultrasmall Nanocages for Recyclable Gas Separation Membranes with Intrinsic Microporosity

The engineering of mixed-matrix membranes is severely hindered by the trade-off between mechanical performance and effective utilization of inorganic fillers' microporosity. Herein, we report a feasible approach for optimal gas separation membranes through the fabrication of coordination nanocages with poly(4-vinylpyridine) (P4VP) via strong supramolecular interactions, enabling the homogeneous dispersion of nanocages in polymer matrixes with long-term structural stability. Meanwhile, suggested from dynamics studies, the strong attraction between P4VP and nanocages slows down polymer dynamics and rigidifies the polymer chains, leading to frustrated packing and lowered densities of the polymer matrix. This effect allows the micropores of nanocages to be accessible to external gas molecules, contributing to the intrinsic microporosity of the nanocomposites and the simultaneous enhancement of permselectivities. The facile strategy for supramolecular synthesis and polymer dynamics attenuation paves avenues to rational design of functional hybrid membranes for gas separation applications.

Polymer Topology Reinforced Synergistic Interactions among Nanoscale Molecular Clusters for Impact Resistance with Facile Processability and Recoverability

The intrinsic conflicts between mechanical performances and processability are main challenges to develop cost-effective impact-resistant materials from polymers and their composites. Herein, polyhedral oligomeric silsesquioxanes (POSSs) are integrated as side chains to the polymer backbones. The one-dimension (1D) rigid topology imposes strong space confinements to realize synergistic interactions among POSS units, reinforcing the correlations among polymer chains. The afforded composites demonstrate unprecedented mechanical properties with ultra-stretchability, high rate-dependent strength, superior impact-resistant capacity as well as feasible processability/recoverability. The hierarchical structures of the hybrid polymers enable the co-existence of multiple dynamic relaxations that are responsible for fast energy dissipation and high mechanical strengths. The effective synergistic correlation strategy paves a new pathway for the design of advanced cluster-based materials.

Unique Dynamics of Hierarchical Constrained Macromolecular Ligands on Coordination Nanocage Surface Promotes Facile and Precise Assembly of Polymers

With access to the solution structures of nanocomposites of coordination nanocages (CNCs) via scattering and chromatography techniques, their mysterious solution dynamics have been, for the first time, resolved, and interestingly, the surface macromolecules can be substituted by extra free macromolecules in solutions. Obvious exchange of macromolecules can be observed in the solution mixtures of CNC nanocomposites at high temperatures, revising the understanding of the dynamics of CNC nanocomposites. Being distinct from nanocomposites of a simple coordination complex, the quantified solution dynamics of CNC nanocomposites indicates a typical logarithmic time dependence with the dissociation of surface macromolecules as the thermodynamically limiting step, suggesting strongly coupled and hierarchically constrained dynamics among the surface macromolecules. Their dynamics can be activated only upon application of high temperature or selected solvents, and therefore, the rational design of polymer assemblies, for example, hybrid-arm star polymers with precisely controlled compositions and reprocessable, robust CNC-cross-linked supramolecular polymer networks, is facilitated.

When polymerization meets coordination-driven self-assembly: metallo-supramolecular polymers based on supramolecular coordination complexes

Polymers have greatly changed and are still changing the way we live ever since, and the construction of novel polymers as functional materials remains an attractive topic in polymer science and related areas. During the past few years, the marriage of discrete supramolecular coordination complexes (SCCs), including two-dimensional (2D) metallacycles and three-dimensional (3D) metallacages, and polymers gave rise to two novel types of metallo-supramolecular polymers, i.e., metallacycle/metallacage-cored star polymers (MSPs) and metallacycle/metallacage-crosslinked polymer networks (MPNs), which has attracted increasing attention and emerged as an exciting new research direction in polymer chemistry. Attributed to their well-defined and diverse topological architectures as well as the unique dynamic features of metallacycles/metallacages as cores or crosslinks, these novel polymers have shown extensive applications. In this review, aiming at providing a practical guide to this emerging area, the introduction of synthetic strategies towards MSPs and MPNs will be presented. In addition, their wide applications in areas such as functional materials, molecular sieving, drug delivery, bacterial killing and bioimaging are also discussed.