Matrix Coordination Induced Emission in a Three-Dimensional Silver Cluster-Assembled Material

A comprehensive review of atomically precise metal nanoclusters with emergent photophysical properties towards diverse applications

Atomically precise metal nanoclusters (MNCs) composed of a few to hundreds of metal atoms represent an emerging class of nanomaterials with a precise composition. With the size approaching the Fermi wavelength of electrons, their energy levels are well-separated, leading to molecule-like properties, like discrete single electronic transitions, tunable photoluminescence (PL), inherent structural anisotropy, and distinct redox behavior. Extensive synthetic efforts and electronic structure revelation have expanded applicability of MNCs in catalysis, optoelectronics, and biology. This review highlights the intriguing photophysical and electrochemical behaviors of MNCs and their regulatory parameters and applications. Initially, we present a brief discussion on the evolution of MNCs from gas-phase naked metal clusters to monolayer ligand-protected MNCs along with representative studies on their electronic structure. Due to their quantized molecular orbitals, they often exhibit PL, which can be regulated based on their capping ligands, number of atoms, crystal packing, presence of heterometal, and surrounding environment. Apart from PL, the relaxation pathways of MNCs on an ultrafast time scale have been extensively studied, which significantly differ from that of plasmonic metal nanoparticles. Moreover, their interaction with high-intensity light results in unique non-linear optical properties. The synergy between MNCs in a hierarchical self-assembled structure has been exploited to enhance their PL by precisely tuning their non-covalent interactions. Moreover, several NC-based hybrids have been designed to exhibit efficient electron or energy transfer in the photoexcited state. In the next section, we briefly focus on the redox behavior of NCs and facile electron transfer to suitable substrates, which result in enzyme-like catalytic activity. Utilizing these photophysical and electrochemical behaviors, NCs are widely employed in catalysis, optical sensing, and light-harvesting applications, which are also discussed in this review. In the final section, conclusions and open questions for the NC research community are included. This review will provide a comprehensive view of the emerging physicochemical properties of MNCs, thereby enabling an understanding for their precise modulation in future.

A silver cluster-assembled material as a matrix for enzyme immobilization towards a highly efficient biocatalyst

Silver cluster-assembled materials (SCAMs) epitomize well-defined extended crystalline frameworks that combine the ingenious designability at the atomic/molecular level and high structural robustness. They have captivated the interest of the scientific fraternity because of their modular construction which enables to systematically tailor their functions, and their capacity to not only inherit the characteristics of component building units but also introduce their uniqueness in endowing the final material with extraordinary properties. Herein, we demonstrate the synthesis of a novel (3,6)-connected two-dimensional (2D) SCAM [Ag12(StBu)6(CF3COO)6(THIT)6]n (described as TUS 5, THIT = 2,4,6-tri(1H-imidazol-1-yl)-1,3,5-triazine) composed of Ag12 cluster nodes and tritopic imidazolyl linkers. We have leveraged, for the first time, this precisely architected extended SCAM structure as a support matrix for enzyme immobilization. The electrostatic attraction between the negatively charged amano lipase PS and positively charged TUS 5 as well as the surface hydrophobicity of TUS 5 catered to great binding of lipase onto the TUS 5 matrix, in addition to boosting the activity of lipase via interfacial activation. Capitalizing on the cooperative benefits of organic and inorganic support matrices wherein organic supports impart with cost-efficiency, biocompatibility, and improved enzyme stability and reusability and inorganic supports confer high thermal, mechanical and microbial resistance, we have utilized the immobilized lipase on TUS 5 SCAM (lipase@TUS 5) for the kinetic resolution of (R,S)-1-phenylethanol by transesterification reaction. Importantly, lipase@TUS 5 could attain appreciably higher conversion into (R)-1-phenylethyl acetate, besides featuring superior thermal stability, solvent tolerance and recyclability, over the native lipase.

Structural diversity of copper(I) alkynyl cluster-based coordination polymers utilizing bifunctional pyridine carboxylic acid ligands

The utilization of bifunctional ligands, specifically pyridine carboxylic acids, endowed with dual coordination sites, has been instrumental in the assembly of polymer materials. The ambidentate characteristics of these ligands play a crucial role in shaping the structure and framework of cluster-based polymers. In this study, we have synthesized a diverse array of multidimensional copper(I) alkynyl cluster-based polymers (CACPs) by employing four distinct pyridine carboxylic acids - namely, isonicotinic acid (INA), 6-isoquinolinecarboxylic acid (IQL), 4-pyridin-4-yl-benzoic acid (4-PyBA), and 3-pyridin-4-yl-benzoic acid (3-PyBA) - as linking ligands. These pyridine carboxylic acids not only serve as protective ligands but also act as pivotal linkers in constructing the cluster-based framework materials, exerting significant influence on the overall framework structures. Furthermore, the incorporation of auxiliary ligands has been shown to markedly impact the structural integrity and framework architecture of the CACPs. This study elucidates the indispensable role of pyridine carboxylic acids in the construction and stabilization of cluster-based framework materials, thereby advancing the frontier of research in metal cluster-based framework material synthesis.

Recent progress in atomically precise silver nanocluster-assembled materials

In the dynamic landscape of nanotechnology, atomically precise silver nanoclusters (Ag NCs) have emerged as a novel and promising category of materials with their fascinating properties and enormous potential. However, recent research endeavors have surged towards stabilizing Ag-based NCs, leading to innovative strategies like connecting cluster nodes with organic linkers to construct hierarchical structures, thus forming Ag-based cluster-assembled materials (CAMs). This approach not only enhances structural stability, but also unveils unprecedented opportunities for CAMs, overcoming the limitations of individual Ag NCs. In this context, this review delves into the captivating realm of atomically precise nitrogen-based ligand bonded Ag(I)-based CAMs, providing insights into synthetic strategies, structure-property relationships, and diverse applications. We navigate the challenges and advancements in integrating Ag(I) cluster nodes, bound by argentophilic interactions, into highly connected periodic frameworks with different dimensionalities using nitrogen-based linkers. Despite the inherent diversity among cluster nodes, Ag(I) CAMs demonstrate promising potential in sensing, catalysis, bio-imaging, and device fabrication, which all are discussed in this review. Therefore, gaining insight into the silver nanocluster assembly process will offer valuable information, which can enlighten the readers on the design and advancement of Ag(I) CAMs for state-of-the-art applications.

The structure and application portfolio of intricately architected silver cluster-assembled materials

Silver Cluster-Assembled Materials (SCAMs) represent a new frontier of crystalline extended solids hallmarked by their customizable structures, commendable stabilities, and unique physical/chemical properties. Since their discovery in 2017, the diversity of organic linkers has endowed SCAMs with ingenious architectures and the application scenario has expanded beyond photoluminescence sensing to environmental sustainability and biomedical applications. It is critically important to chronicle these recent key advances and review the progress of SCAMs that can enable translating the material discoveries into real implementation. Herein, we provide a succinct overview of the trajectory of SCAM research, with crucial insights into atomic-level structural correlations with the phenomena at the nanoscale and discuss the gaps and opportunities that are still open in addition to charting a roadmap for future research directions.

Chiral lanthanide-silver(I) cluster-based metal-organic frameworks exhibiting solvent stability, and tunable photoluminescence

Due to the lack of effective synthetic strategies, the preparation of chemically stable chiral Ag(I) cluster-based materials for assembly remains challenging. Here, we have developed an approach to synthesize three pairs of chiral Ln-Ag(I) cluster-based metal-organic frameworks (MOFs) named l-LnAg5-3D (Ln = Gd for 1-L, Eu for 2-L, and Tb for 3-L) and d-LnAg5-3D (Ln = Gd for 1-D, Eu for 2-D, and Tb for 3-D) by employing a chiral Ag(I) cluster ({Ag5S6}) as the node and Ln3+ ion as the inorganic linker. Structural analysis revealed that the chiral ligands induced chirality through the entire structure, resulting in a chiral helix arrangement of the C3-symmetric chiral {Ag5S6} nodes and Ln3+ ions. These compounds showed high solvent stability in various polar organic solvents. The solid-state circular dichroism (CD) spectra of compounds l-LnAg5-3D and d-LnAg5-3D exhibited obvious mirror symmetrical peaks. The emission spectra in the solid state revealed that compound 1-L only exhibited the emission peak of {Ag5S6}, while compounds 2-L and 3-L exhibited overlapping peaks of Ln3+ and {Ag5S6} at different excitation wavelengths. This demonstrates the tunable photoluminescence from {Ag5S6} to Ln3+ by introducing different Ln3+ ions and manipulating the excitation wavelengths. The study underscores the enhanced stability of Ag(I) cluster-based MOFs achieved through the incorporation of Ln3+ ions and establishes chiral Ln-Ag(I) cluster-based MOFs as promising candidates for advanced materials with tunable photoluminescence.

Recent developments in the investigation of driving forces for transforming coinage metal nanoclusters

Metal nanoclusters serve as an emerging class of modular nanomaterials. The transformation of metal nanoclusters has been fully reflected in their studies from every aspect, including the structural evolution analysis, physicochemical property regulation, and practical application promotion. In this review, we highlight the driving forces for transforming atomically precise metal nanoclusters and summarize the related transforming principles and fundamentals. Several driving forces for transforming nanoclusters are meticulously reviewed herein: ligand-exchange-induced transformations, metal-exchange-induced transformations, intercluster reactions, photochemical transformations, oxidation/reduction-induced transformations, and other factors (intrinsic instability, pH, temperature, and metal salts) triggering transformations. The exploitation of transforming principles to customize the preparations, structures, physicochemical properties, and practical applications of metal nanoclusters is also disclosed. At the end of this review, we provide our perspectives and highlight the challenges remaining for future research on the transformation of metal nanoclusters. Our intended audience is the broader scientific community interested in metal nanoclusters, and we believe that this review will provide researchers with a comprehensive synthetic toolbox and insights on the research fundamentals needed to realize more cluster-based nanomaterials with customized compositions, structures, and properties.

Silver cluster-assembled materials for label-free DNA detection

Herein, we report two newly synthesized silver cluster-assembled materials (SCAMs), [Ag₁₄(S^tBu)₁₀(CF₃COO)₄(bpa)₂]_n (bpa = 1,2-bis(4-pyridyl)acetylene) and [Ag₁₂(S^tBu)₆(CF₃COO)₆(bpeb)₃]_n (bpeb = 1,4-bis(pyridin-4-ylethynyl)benzene) composed of Ag₁₄ and Ag₁₂ chalcogenolate cluster cores, respectively, bridged by acetylenic bispyridine linkers. The linker structures and electrostatic interaction between positively charged SCAMs and negatively charged DNA confer the SCAMs with the ability to suppress the high background fluorescence of single-stranded (ss) DNA probes with SYBR Green I nucleic acid stain, leading to high signal-to-noise ratio for label-free target DNA detection.

Solvent-Dependent Self-Assembly of a Pillar[5]arene-Based Poly-Pseudo-Rotaxane Linked and Threaded by Silver(I) Trifluoroacetate: A Double Role

In the supramolecule area, the fabrication of a new concept called polyrotaxanes or poly-pseudo-rotaxanes remains challenging. We herein report the formation of a poly-pseudo-rotaxane in which the same salt-type guest serves both linking and threading in the resulting structure. The combination of A1/A2-thiopyridyl pillar[5]arene (L) and silver(I) trifluoroacetate in CHCl₃/CH₃OH afforded a one-dimensional (1D) poly-pseudo-rotaxane. In this structure, to our surprise, the AgCF₃CO₂ guest not only links the di-armed L ligands via an infinite -L-Ag-L-Ag- arrangement but also threads into a pillar[5]arene cavity in a dimer form, (AgCF₃CO₂)₂. In contrast, the same reaction in CH₂Cl₂/CH₃OH yielded a simple 1D coordination polymer because an included CH₂Cl₂ molecule in the pillar[5]arene cavity prevents the threading of the silver(I) trifluoroacetate guest. Comparative ¹H- and ¹⁹F-NMR studies support the solvent-dependent poly-pseudo-rotaxane formation at a lower concentration of L.

Self-Assembly of Silver Clusters into One- and Two-Dimensional Structures and Highly Selective Methanol Sensing

The development of new materials for the design of sensitive and responsive sensors has become a crucial research direction. Here, two silver cluster-based polymers (Ag-CBPs), including one-dimensional {[Ag22(L1)8(CF3CO2)14](CH3OH)2} n chain and two-dimensional {[Ag12(L2)2(CO2CF3)14(H2O)4(AgCO2CF3)4](HNEt3)2} n film, are designed and used to simulate the human nose, an elegant sensor to smells, to distinguish organic solvents. We study the relationship between the atomic structures of Ag-CBPs determined by x-ray diffraction and the electrical properties in the presence of organic solvents (e.g., methanol and ethanol). The ligands, cations, and the ligated solvent molecules not only play an important role in the self-assembly process of Ag-CBP materials but also determine their physiochemical properties such as the sensing functionality.

Epitaxial coordination assembly of a semi-conductive silver-chalcogenide layer-based MOF

Using a carboxylic acid linker, this work achieved the epitaxially coordinated assembly of a Ag-S layer into a three-dimensional semi-conductive framework, with high thermal stability, as well as an interesting temperature-dependent luminescence response. This work provides a new avenue to prepare semi-conductive metal-chalcogenide layer-based materials in electricity-related applications.

Self-Assembly of Precision Noble Metal Nanoclusters: Hierarchical Structural Complexity, Colloidal Superstructures, and Applications

Ligand protected noble metal nanoparticles are excellent building blocks for colloidal self-assembly. Metal nanoparticle self-assembly offers routes for a wide range of multifunctional nanomaterials with enhanced optoelectronic properties. The emergence of atomically precise monolayer thiol-protected noble metal nanoclusters has overcome numerous challenges such as uncontrolled aggregation, polydispersity, and directionalities faced in plasmonic nanoparticle self-assemblies. Because of their well-defined molecular compositions, enhanced stability, and diverse surface functionalities, nanoclusters offer an excellent platform for developing colloidal superstructures via the self-assembly driven by surface ligands and metal cores. More importantly, recent reports have also revealed the hierarchical structural complexity of several nanoclusters. In this review, the formulation and periodic self-assembly of different noble metal nanoclusters are focused upon. Further, self-assembly induced amplification of physicochemical properties, and their potential applications in molecular recognition, sensing, gas storage, device fabrication, bioimaging, therapeutics, and catalysis are discussed. The topics covered in this review are extensively associated with state-of-the-art achievements in the field of precision noble metal nanoclusters.

Thiolate-Protected Metal Nanoclusters: Recent Development in Synthesis, Understanding of Reaction, and Application in Energy and Environmental Field

Metal nanoclusters (NCs), which are composed of about 250 or fewer metal atoms, possess great potential as novel functional materials. Fundamental research on metal NCs gradually started in the 1960s, and since 2000, thiolate (SR)-protected metal NCs have been the main metal NCs actively studied. The precise and systematic isolation of SR-protected metal NCs has been achieved in 2005. Since then, research on SR-protected metal NCs for both basic science and practical application has rapidly expanded. This review describes this recent progress in the field of SR-protected metal NCs in three areas: synthesis, understanding, and application. Specifically, the recent study of alloy NCs and connected structures composed of NCs is highlighted in the "synthesis" section, recent knowledge on the reactivity of NCs in solution is highlighted in the "understanding" section, and the applications of NCs in the energy and environmental field are highlighted in the "application" section. This review provides insight on the current state of research on SR-protected metal NCs and discusses the challenges to be overcome for further development in this field as well as the possibilities that these materials can contribute to solving the problems facing modern society.

Assembly of Chiral Cluster-Based Metal-Organic Frameworks and the Chirality Memory Effect during their Disassembly

Many metal clusters are intrinsically chiral but are often synthesized as a racemic mixture. By taking chiral Ag₁₄(SPh(CF₃)₂)₁₂(PPh₃)₄(DMF)₄ (Ag₁₄) clusters with bulky thiolate ligands as an example, we demonstrate herein an interesting assembly disassembly (ASDS) strategy to obtain the corresponding, optically pure crystals of both homochiral enantiomers, R-Ag_14m and S-Ag_14m. The ASDS strategy makes use of two bidentate linkers with different chiral configurations, namely, (1R,2R,N¹E,N²E)-N¹,N²-bis(pyridin-3-ylmethylene)cyclohexane-1,2-diamine (LR) and the corresponding chiral analogue LS. For comparison, we also use the racemic mixture of equimolar of LR and LS (LRS). Three three-dimensional (3D) Ag₁₄-based metal-organic frameworks (MOFs) were characterized by X-ray crystallography to be [Ag₁₄(SPh(CF₃)₂)₁₂(PPh₃)₄(LR)₂]_n (Ag₁₄-LR), [Ag₁₄(SPh(CF₃)₂)₁₂(PPh₃)₄(LS)₂]_n (Ag₁₄-LS), and [Ag₁₄(SPh(CF₃)₂)₁₂(PPh₃)₄(LRS)₂]_n (Ag₁₄-LRS), respectively. As expected, the building blocks in Ag₁₄-LR or Ag₁₄-LS are homochiral R-Ag₁₄ or S-Ag₁₄, respectively. In contrast, Ag₁₄-LRS is achiral and crystallizes with a diamond-like structure containing alternate R-Ag₁₄ and S-Ag₁₄ clusters. During the assembly process, the racemic Ag₁₄ clusters were converted to homochiral building blocks, namely, R-Ag₁₄ for Ag₁₄-LR and S-Ag₁₄ for Ag₁₄-LS. Subsequently, the chiral linkers were removed from the crystals of Ag₁₄-LR and Ag₁₄-LS via hydrolysis with water, and from the disassembled solid material Ag₁₄-DR and Ag₁₄-DS, optically pure enantiomers R-Ag_14m and S-Ag_14m were obtained. It is hoped that this simple assembly strategy can be used to construct cluster-based chiral assemblage materials and that the subsequent disassembly protocol can be used to obtain optically pure chiral cluster molecules from as-prepared racemic mixtures.

Shell engineering to achieve modification and assembly of atomically-precise silver clusters

This tutorial review focuses on the modification and assembly of atomically-precise silver clusters by changing shell layers for more stability and functionalities, especially for brighter luminescence.

One-, Two-, and Three-Dimensional Self-Assembly of Atomically Precise Metal Nanoclusters

Metal nanoclusters (NCs), which consist of several, to about one hundred, metal atoms, have attracted much attention as functional nanomaterials for use in nanotechnology. Because of their fine particle size, metal NCs exhibit physical/chemical properties and functions different from those of the corresponding bulk metal. In recent years, many techniques to precisely synthesize metal NCs have been developed. However, to apply these metal NCs in devices and as next-generation materials, it is necessary to assemble metal NCs to a size that is easy to handle. Recently, multiple techniques have been developed to form one-, two-, and three-dimensional connected structures (CSs) of metal NCs through self-assembly. Further progress of these techniques will promote the development of nanomaterials that take advantage of the characteristics of metal NCs. This review summarizes previous research on the CSs of metal NCs. We hope that this review will allow readers to obtain a general understanding of the formation and functions of CSs and that the obtained knowledge will help to establish clear design guidelines for fabricating new CSs with desired functions in the future.

A hydrophobic semiconducting metal–organic framework assembled from silver chalcogenide wires

Silver chalcogenide wires are for the first time assembled into a rigid framework structure using a fluorinated carboxylate ligand, where electron transfer along Ag–S chains enables semi-conduction.

Sky-blue thermally activated delayed fluorescence (TADF) based on Ag(i) complexes: strong solvation-induced emission enhancement

Remarkable solvation-induced emission enhancement is discovered on a new Ag(i) complex showing sky-blue thermally activated delayed fluorescence (TADF).

Photoluminescence of Ag(i) complexes with a square-planar coordination geometry: the first observation

First examples of square-planar Ag(i) complexes showing MLCT emission are reported. They demonstrate an interesting thermochromic luminescence with the nano- and microsecond lifetime components.