Observation of a new type of aggregation-induced emission in nanoclusters

Superlattice Assembly for Empowering Metal Nanoclusters

ConspectusAtomically precise metal nanoclusters, serving as an aggregation state of metal atoms, display unique physicochemical properties owing to their ultrasmall sizes with discrete electronic energy levels and strong quantum size effects. Such intriguing properties endow nanoclusters with potential utilization as efficient nanomaterials in catalysis, electron transfer, drug delivery, photothermal conversion, optical control, etc. With the assistance of atomically precise operations and theoretical calculations on metal nanoclusters, significant progress has been accomplished in illustrating their structure-performance correlations at the single-molecule level. Such research achievements, in turn, have contributed to the rational design and customization of functional nanoclusters and cluster-based nanomaterials.Most previous studies have focused on investigating structure-property correlations of nanocluster monomers, while the exploration of electronic structures and physicochemical properties of hierarchical cluster-based assembled structures was far from enough. Indeed, from the application aspect, the nanoclusters with controllably assembly states (e.g., crystalline assembled materials, host-guest hybrid materials, amorphous powders, and so on) were more suitable for performance expression relative to those in the monomeric state and more directed to downstream solid-state applications. In this context, more attention should be paid to the state-correlated property variations of metal nanoclusters occurring in their aggregating and assembling processes for better applications in accordance with their aptitude.Crystalline aggregates are crucial in the structural determination of metal nanoclusters, also acting as a cornerstone to analyze the structure-property correlations by affording atomic-level information. The regular arrangement, uniform composition, and close intermolecular distance of the cluster molecules in their supercrystal lattices are beneficial for property retention and amplification from the molecule itself as a monomeric state. Besides, for these nanoparticles with strong quantum size effects, the intercluster distances in the supercrystal lattices are still located at the nanoscale level, wherein the quantum size effect is highly likely to take effect with additional intermolecular synergistic effects. Accordingly, it is expected that novel performances might occur in the crystalline aggregates of nanoclusters that are completely different from those in the monomolecular state.In this Account, we emphasize our efforts in exploring the performance enhancement of atomically precise metal nanoclusters in their crystalline aggregate states, such as thermal stability, photoluminescence, optical activity, and an optical waveguide. Such performance enhancements further supported the practical uses of metal nanoclusters in structure determination, a polarization switch, an optical waveguide device, and so on. We also demonstrated that the differences in physicochemical properties between crystalline aggregates and monomers of metal nanoclusters might be attributed to the change in electronic structures during the crystalline aggregation processes in the superlattice. The "superlattice assembly" is intended to customize the function of cluster-based aggregates for downstream solid-state applications.

Luminescent metal nanoclusters and their application in bioimaging

Owing to their unique optical properties and atomically precise structures, metal nanoclusters (MNCs) constitute a new generation of optical probe materials. This mini-review provides a brief overview of luminescence mechanisms and modulation methods of luminescent metal nanoclusters in recent years. Based on these photophysical phenomena, the applications of cluster-based optical probes in optical bioimaging and related sensing, disease diagnosis, and treatment are summarized. Some challenges are also listed at the end.

Au25 Cluster-Based Atomically Precise Coordination Frameworks and Emission Engineering through Lattice Symmetry

The atomically precise metal nanoclusters (NCs) have attracted significant attention due to their superatomic behavior originating from the quantum confinement effect. This behavior makes these materials suitable for various photoluminescence-based applications, including chemical sensing, bioimaging, and phototherapy, owing to their intriguing optical properties. Especially, the manipulation of inter- or intracluster interaction through cluster-assembled materials (CAMs) presents significant pathways for modifying the photophysical properties of NCs. Herein, two distinct CAMs, Au25-Zn-Hex and Au25-Zn-Rod, were synthesized via forming a coordination bond between [Au25(p-HMBA)18]- (p-H2MBA = 4-mercaptobenzoic acid) and Zn2+. Au25-Zn-Rod exhibited a 6-fold higher luminescence intensity in the near-infrared region compared to Au25-Zn-Hex, attributed to synergistic inter- and intracluster interactions that induce exciton delocalization and structure rigidification at the atomic scale. This study highlights the potential of diverse lattice symmetries in cluster-based frameworks for tuning the photophysical properties, contributing to a deeper understanding of the structure-property relationship in Au NCs.

Size disproportionation among nanocluster transformations

Controllable transformation is a prerequisite to the in-depth understanding of structure evolution mechanisms and structure-property correlations at the atomic level. Most transformation cases direct the directional evolution of nanocluster sizes, i.e., size-maintained, size-increased, or size-reduced transformation, while size disproportionation was rarely reported. Here, we report the Au-doping-induced size disproportionation of nanocluster transformation. Slight Au-doping on the bimetallic (AgCu)43 nanocluster produced its trimetallic derivative, (AuAgCu)43, following a size-maintained transformation. By comparison, the (AgCu)43 nanocluster underwent a size-disproportionation transformation under heavy Au alloying, leading to the formation of size-reduced (AuAgCu)33 and size-increased (AuAgCu)56 nanoclusters simultaneously. Such a size disproportionation among the nanocluster transformations was verified by the thin-layer chromatography analysis. This work presented a novel nanocluster transformation case with a size disproportionation characteristic, expected to provide guidance for the understanding of cluster size evolutions.

Regulating Arrhenius Activation Energy and Fluorescence Quantum Yields of AuNCs-MOF to Achieve High Temperature Sensitivity in a Wide Response Window

Luminescent thermometry affords remote measurement of temperature and shows huge potential in future technology beyond those possible with traditional methods. Strategies of temperature measurement aiming to increase thermal sensitivity in a wide temperature response window would represent a pivotal step forward, but most thermometers cannot do both of them. Herein, we propose a balancing strategy to achieve a trade-off between high Arrhenius activation energy (Ea), which could stretch the temperature response windows, and fluorescence quantum yields (QYs) in a manner that will increase thermal sensitivity in a wide response window. In particular, a luminescent thermometer composed of AuNCs-MOF is prepared via a facile impregnation process to enhance QYs and Ea, responsible for high relative sensitivity (Sr) (15.6% K-1) and ultrawide temperature linearity range (from 83 to 343 K), respectively. Taking fluorescence intensity and lifetime as multiple parameters, the maximum Sr can reach 22.4% K-1 by multiple linear regression. The maximum Sr and temperature response range of the proposed thermometer outperform those of the most recent luminescent thermometers. The strategy of balancing Sr and thermal response range by regulating Ea and QYs enables the construction of ultra-accurate thermal sensors in the age of artificial intelligence.

A Phosphorescent P/N/S Hybrid Ligand Stabilized Au2Cu Complex Selectively Senses Ammonia and Amines

Reaction of a P/N/S hybrid ligand dpppyatc (N,N-bis((diphenylphosphaneyl)methyl)-N-(pyridin-2-yl)-amino-thiocarbamide) with Au(tht)Cl (tht=tetrahydrothiophene) and [Cu(MeCN)4]BF4 afforded cluster complex [Au2Cu(dpppyatc)2](BF4)2Cl (1). Upon excitation at 480 nm, 1 emitted orange phosphorescence at 646 nm, which was red-shifted to ~698 nm selectively in the presence of ammonia or amine vapor. This chromic photoluminescent response toward ammonia was sensitive and reversible. Complex1 could detect ammonia in aqueous solution down to concentrations of 2 ppm (w/w).

Assembly of anionic silver nanoclusters with controlled packing structures through site-specific ionic bridges

The assembly of metal nanoclusters (NCs) into crystalline lattice structures is of interest in the development of NC-based functional materials. Here we demonstrate that the assembled structures of tri-anionic tetrahedral symmetric [Ag29(BDT)12]3- (Ag29 NC, BDT: 1,3-benzenedithiol) NCs are controlled into a polyethylene-like zigzag chain and a "poly-ring-fused-cyclohexane"-like honeycomb arrangement through ionic interactions with alkali metal cations such as K+ and Cs+. The site-specific binding of alkali metal ions on the tetrahedrally arranged binding sites of Ag29 NCs successfully connects the adjacent NCs into various packing modes. The number and type of bridges between NCs determine the Ag29 NC packing structures, which are affected by the solvent species, enabling the transformation of packing modes in the single-crystalline state. The photoluminescence (PL) properties of the crystals responded to the packing modes of the NCs in terms of anisotropy and bridge linkage style inducing a varied degree of relaxation of the excited state depending on the relocation mobility of alkali metal ions in the crystals.

Chiral Ligand-Concentration Mediating Asymmetric Transformations of Silver Nanoclusters: NIR-II Circularly Polarized Phosphorescence Lighting

Near infrared (NIR) emitter with circularly polarized phosphorescence (CPP), known as NIR CPP, has emerged as a key part in the research of cutting-edge luminescent materials. However, it remains a challenge to obtain nanoclusters with NIR CPP activity. Here, we propose an asymmetric transformation approach to efficiently synthesize two pairs of chiral silver nanoclusters (R/S-Ag29 and R/S-Ag16) using an achiral Ag10 nanocluster as starting material in the presence of different concentration chiral inducer (R/S)-1,1'-binaphthyl-2,2'-diyl hydrogenphosphate (R/S-BNP). R/S-Ag29, formed in the low-concentration R/S-BNP, exhibits a unique kernel-shell structure consisting of a distorted Ag13 icosahedron and an integrated cage-like organometallic shell with a C3 symmetry, and possesses a superatomic 6-electron configuration (1S2|1P4). By contrast, R/S-Ag16, formed in the high-concentration R/S-BNP, features a sandwich-like pentagram with AgI-pure kernel. Profiting from the hierarchically chiral structures and superatomic kernel-dominated phosphorescence, R/S-Ag29 exhibits infrequent CPP activity in the second near-infrared (975 nm) region, being the first instance of NIR-II CPP observed among CPL-active metal nanoclusters. This study presents a new approach to reduce the difficulty of de novo synthesis for chiral silver nanomaterials, and facilitates the design of CPP-active superatomic nanoclusters in NIR region.

Fluorescence resonance energy transfer in atomically precise metal nanoclusters by cocrystallization-induced spatial confinement

Understanding the fluorescence resonance energy transfer (FRET) of metal nanoparticles at the atomic level has long been a challenge due to the lack of accurate systems with definite distance and orientation of molecules. Here we present the realization of achieving FRET between two atomically precise copper nanoclusters through cocrystallization-induced spatial confinement. In this study, we demonstrate the establishment of FRET in a cocrystallized Cu8(p-MBT)8(PPh3)4@Cu10(p-MBT)10(PPh3)4 system by exploiting the overlapping spectra between the excitation of the Cu10(p-MBT)10(PPh3)4 cluster and the emission of the Cu8(p-MBT)8(PPh3)4 cluster, combined with accurate control over the confined space between the two nanoclusters. Density functional theory is employed to provide deeper insights into the role of the distance and dipole orientations of molecules to illustrate the FRET procedure between two cluster molecules at the electronic structure level.

Rethinking the stability of metal nanoclusters: the individual versus the collective

Research on the stability of metal nanoclusters and their molecular/supramolecular chemistry has proceeded significantly independently thus far. We herein have demonstrated that the stability of a nanocluster-based system should be assessed from both the cluster individual aspect (i.e., the energy of the molecular conformer) and the cluster collective aspect (i.e., the energy of the supramolecular lattice). A pair of Au2Cu6 cluster polymorphs, including Au2Cu6-triclinic and Au2Cu6-trigonal, was developed here to reveal the energy and stability contributions of both cluster conformers and crystalline lattices to their total systems. This work hopefully promotes a comprehensive understanding of the stability of cluster-based nano-systems which is beneficial for their downstream applications.

Chemical Flexibility of Atomically Precise Metal Clusters

Ligand-protected metal clusters possess hybrid properties that seamlessly combine an inorganic core with an organic ligand shell, imparting them exceptional chemical flexibility and unlocking remarkable application potential in diverse fields. Leveraging chemical flexibility to expand the library of available materials and stimulate the development of new functionalities is becoming an increasingly pressing requirement. This Review focuses on the origin of chemical flexibility from the structural analysis, including intra-cluster bonding, inter-cluster interactions, cluster-environments interactions, metal-to-ligand ratios, and thermodynamic effects. In the introduction, we briefly outline the development of metal clusters and explain the differences and commonalities of M(I)/M(I/0) coinage metal clusters. Additionally, we distinguish the bonding characteristics of metal atoms in the inorganic core, which give rise to their distinct chemical flexibility. Section 2 delves into the structural analysis, bonding categories, and thermodynamic theories related to metal clusters. In the following sections 3 to 7, we primarily elucidate the mechanisms that trigger chemical flexibility, the dynamic processes in transformation, the resultant alterations in structure, and the ensuing modifications in physical-chemical properties. Section 8 presents the notable applications that have emerged from utilizing metal clusters and their assemblies. Finally, in section 9, we discuss future challenges and opportunities within this area.

Triplet-triplet annihilation-based photon upconversion using nanoparticles and nanoclusters

The phenomenon of photon upconversion (UC), generating high-energy photons from low-energy photons, has attracted significant attention. In particular, triplet-triplet annihilation-based UC (TTA-UC) has been achieved by combining the excitation states of two types of molecules, called the sensitizer and emitter (or annihilator). With TTA-UC, it is possible to convert weak, incoherent near-infrared (NIR) light, which constitutes half of the solar radiation intensity, into ultraviolet and visible light that are suitable for the operation of light-responsive functional materials or devices such as solar cells and photocatalysts. Research on TTA-UC is being conducted worldwide, often employing materials with high intersystem crossing rates, such as metal porphyrins, as sensitizers. This review summarizes recent research and trends in triplet energy transfer and TTA-UC for semiconductor nanoparticles or nanocrystals with diameters in the nanometer range, also known as quantum dots, and for ligand-protected metal nanoclusters, which have even smaller well-defined sub-nanostructures. Concerning nanoparticles, transmitter ligands have been applied on the surface of the nanoparticles to efficiently transfer triplet excitons formed inside the nanoparticles to emitters. Applications are expanding to solid-state UC devices that convert NIR light to visible light. Additionally, there is active research in the development of sensitizers using more cost-effective and environmentally friendly elements. Regarding metal nanoclusters, methods have been established for the evaluation of excited states, deepening the understanding of luminescent properties and excited relaxation processes.

Construction of novel Ag(0)-containing silver nanoclusters by regulating auxiliary phosphine ligands

Controlled synthesis of metal clusters through minor changes in surface ligands holds significant interest because the corresponding entities serve as ideal models for investigating the ligand environment's stereochemical and electronic contributions that impact the corresponding structures and properties of metal clusters. In this work, we obtained two Ag(0)-containing nanoclusters (Ag17 and Ag32) with near-infrared emissions by regulating phosphine auxiliary ligands. Ag17 and Ag32 bear similar shells wherein Ag17 features a trigonal bipyramid Ag5 kernel while Ag32 has a bi-icosahedral interpenetrating an Ag20 kernel. Ag17 and Ag32 showed a near-infrared emission (NIR) of around 830 nm. Benefiting from the rigid structure, Ag17 displayed a more intense near-infrared emission than Ag32. This work provides new insight into the construction of novel superatomic silver nanoclusters by regulating phosphine ligands.

Fluorometric and colorimetric dual-mode sensing of α-glucosidase based on aggregation-induced emission enhancement of AuNCs

Herein, a fluorometric and colorimetric dual-mode assay platform used for α-glucosidase (α-Glu) activity sensing based on aggregation-induced emission enhancement (AIEE) of AuNCs was developed for the first time. The quantum yield (QY) and fluorescence lifetime of AuNCs were successfully ameliorated by Ce3+-triggered AIEE (Ce@AuNCs). Subsequently, on the basis of the inner filter effect (IFE) and dynamic quenching effect (DQE) between 2,6-dichlorophenolindophenol (DCIP) and Ce@AuNCs as well as the reduction of DCIP by ascorbic acid (AA) generated from α-Glu-catalyzed hydrolysis of L-ascorbic acid-2-O-α-D-glucopyranosyl (AA2G), the marriage of fluorometric and colorimetric modes applied for α-Glu activity monitoring was achieved. Besides, the feasibility of this dual-mode sensing system was confirmed by the assays versus potential interfering substances and in real samples. In particular, this system was further applied to evaluate natural α-Glu inhibitors (AGIs) including luteolin, apigenin, and hesperidin. Overall, the multi-mode optical sensor newly designed here has the potential for the accurate discovery of natural anti-diabetes drugs and the therapy of diabetes.

Effects of bromine-containing counterion salts in directing the structures of medium-sized silver nanoclusters

The preparation and structural determination of silver nanoclusters (especially the medium-sized Ag clusters) remain more challenging relative to those of their gold counterparts because of the comparative instability of the former. In this work, three medium-sized Ag clusters were controllably synthesized and structurally determined, namely, [Ag52(S-Adm)30Br4H20]2- (Ag52 for short), Ag54(S-Adm)30Br4H20 (Ag54 for short), and [Ag58(S-Adm)30Br4(NO3)2H22]2+ (Ag58 for short) nanoclusters. Specifically, the introduction of PPh4Br gave rise to the generation of Ag52 and Ag54 nanoclusters with homologous compositions and configurations, while the TOABr salt selected Ag58 as the sole cluster product, whose geometric structure was completely different from those of Ag52 and Ag54 nanoclusters. In addition, the optical absorptions and emissions of the three medium-sized silver nanoclusters were compared. The findings in this work not only provide three uniquely medium-sized nanoclusters to enrich the silver cluster family but also point out a new approach (i.e., changing the counterion salt) for the preparation of new nanoclusters with novel structures.

Symmetry breaking of highly symmetrical nanoclusters for triggering highly optical activity

Developing new approaches to fulfill the enantioseparation of nanocluster racemates and construct cluster-based nanomaterials with optical activity remains highly desired in cluster science, because it is an essential prerequisite for fundamental research and extensive applications of these nanomaterials. We herein propose a strategy termed "active-site exposing and partly re-protecting" to trigger the symmetry breaking of highly symmetrical nanoclusters and to render cluster crystals optically active. The vertex PPh3 of the symmetrical Ag29(SSR)12(PPh3)4 (SSR = 1, 3-benzenedithiol) nanocluster was firstly dissociated in the presence of counterions with large steric hindrance, and then the exposed Ag active sites of the obtained Ag29(SSR)12 nanocluster were partly re-protected by Ag+, yielding an Ag29(SSR)12-Ag2 nanocluster with a symmetry-breaking construction. Ag29(SSR)12-Ag2 followed a chiral crystallization mode, and its crystal displayed strong optical activity, derived from CD and CPL characterizations. Overall, this work presents a new approach (i.e., active-site exposing and partly re-protecting) for the symmetry breaking of highly symmetrical nanoclusters, the enantioseparation of nanocluster racemates, and the achievement of highly optical activity.

Stepwise construction of Ag29 nanocluster-based hydrogen evolution electrocatalysts

Although several silver-based nanoclusters have been controllably prepared and structurally determined, their electrochemical catalytic performances have been relatively unexplored (or showed relatively weak ability towards electro-catalysis). In this work, we accomplished the step-by-step enhancement of the electrocatalytic hydrogen evolution reaction (HER) efficiency based on an Ag29 cluster template. A combination of atomically precise operations, including the kernel alloying, ligand engineering, and surface activation, was exploited to produce a highly efficient Pt1Ag28-BTT-Mn(10) nano-catalyst towards HER, derived from both experimental characterization and theoretical modelling. The precision characteristic of the Ag29-based cluster system enables us to understanding the correlations between nanocluster structures and HER performances at the atomic level. Overall, the findings of this work will hopefully provide more opportunities for the customization of new cluster-based nano-catalysts with enhanced electrocatalytic capacities.

Restriction of intramolecular rotation for functionalizing metal nanoclusters

The restriction of intramolecular rotation has been extensively exploited to trigger the property enhancement of nanocluster-based materials. However, such a restriction is induced mainly by intermolecular aggregation. The direct restriction of intramolecular rotation of metal nanoclusters, which could boost their properties at the single molecular level, remains rarely explored. Here, ligand engineering was applied to activate intramolecular interactions at the interface between peripheral ligands and metallic kernels of metal nanoclusters. For the newly reported Au4Ag13(SPhCl2)9(DPPM)3 nanocluster, the molecule-level interactions between the Cl terminals on thiol ligands and the Ag atoms on the cluster kernel remarkably restricted the intramolecular rotation, endowing this robust nanocluster with superior thermal stability, emission intensity, and non-linear optical properties over its cluster analogue. This work presents a novel case of the restriction of intramolecular rotation (i.e., intramolecular interaction-induced property enhancement) for functionalizing metal clusters at the single molecular level.

Structural water molecules dominated p band intermediate states as a unified model for the origin on the photoluminescence emission of noble metal nanoclusters: from monolayer protected clusters to cage confined nanoclusters

In the past several decades, noble metal nanoclusters (NMNCs) have been developed as an emerging class of luminescent materials due to their superior photo-stability and biocompatibility, but their luminous quantum yield is relatively low and the physical origin of the bright photoluminescence (PL) of NMNCs remain elusive, which limited their practical application. As the well-defined structure and composition of NMNCs have been determined, in this mini-review, the effect of each component (metal core, ligand shell and interfacial water) on their PL properties and corresponded working mechanism were comprehensively introduced, and a model that structural water molecules dominated p band intermediate state was proposed to give a unified understanding on the PL mechanism of NMNCs and a further perspective to the future developments of NMNCs by revisiting the development of our studies on the PL mechanism of NMNCs in the past decade.

Excited State Engineering in Ag29 Nanocluster through Peripheral Modification with Silver(I) Complexes for Bright Near-Infrared Photoluminescence

The optical property of an ionic metal nanocluster (NC) is affected by the ionic interaction with counter ions. Here, we report that the modification of trianionic [Ag₂₉(BDT)₁₂(TPP)₄]^3- NC (BDT: 1.3-benzenedithiol; TPP: triphenylphosphine) with silver(I) complexes led to the intense photoluminescence (PL) in the near-infrared (NIR) region. The binding of silver(I) complexes to the peripheral region of Ag₂₉ NC is confirmed by the single-crystal X-ray diffraction (SCXRD) measurement, which is further supported by electrospray ionization mass spectrometry (ESI-MS) and nuclear magnetic resonance (NMR) spectroscopy. The change of excited-state dynamics by the binding of silver(I) complexes is discussed based on the results of a transient absorption study as well as temperature-dependent PL spectra and PL lifetime measurements. The modification of Ag₂₉ NCs with cationic silver(I) complexes is considered to give rise to a triplet excited state responsible for the intense NIR PL. These findings also afford important insights into the origin of the PL mechanism as well as the possible light-driven motion in Ag₂₉-based NCs.

Au10Ag17(TPP)10(SR)6Cl5 nanocluster: structure, transformation and the origin of its photoluminescence

Nanocluster photoluminescence (PL) has important practical applications and its rationalization is therefore of significant interest. Here, we report the synthesis, structure determination and photoluminescence of Au₁₀Ag₁₇(TPP)₁₀(SR)₆Cl₅ (TPP = triphenylphosphine, SR = 3, 5-bis(trifluoromethyl)thiophenol, denoted as Au10Ag17). Au10Ag17 exhibited a low photoluminescence quantum yield (PLQY) of 2.8%, which could be increased 15-fold by removing the two terminal silver atoms to give Ag_xAu_25-x(SR)₅(TPP)₁₀Cl₂²⁺ (x = 11-13, SR = 2-phenylethylmercaptan, abbrev. Au12Ag13). The discovery of such a PL switch constitutes an interesting opportunity to further understand the origin of fluorescence in nanoclusters.

Ligand effects on the photoluminescence of atomically precise silver nanoclusters

Photoluminescence (PL) is one of the most exciting properties of atomically precise metal nanoclusters (NCs), making them a prime choice for various applications, from sensing to bio-imaging. While there are several advantages of metal NCs for PL-based applications, their PLQY is significantly low compared to other PL-active nanomaterials or organic dyes. It is essential to understand the PL mechanism in detail to tune the PLQY of NCs. There are numerous reports on gold NCs with a known structure where the origin of PL has been explored, and it was found that ligands play a vital role in their PL properties along with the kernel (core). Reports on understanding the ligand effects on PL properties are also evolving for the case of atomically precise silver NCs. This mini-review will summarize the ligands' role in PL of 29 atom Ag NCs, the most reported NCs with diversity in the silver family. The ligands were classified as primary and secondary, and their effects on tuning the PL properties were explained. The review will also address some of the answers to open questions for AgNCs, such as the origin of PL, dynamics, and the tunability of PLQY using ligand modifications.

Viewing Aggregation-Induced Emission of Metal Nanoclusters from Design Strategies to Applications

Aggregation-induced emission (AIE)-type metal nanoclusters (NCs) represent an innovative type of luminescent metal NCs whose aggregates exhibit superior performance over that of individuals, attracting wide attention over the past decade. Here, we give a concise overview of the progress made in this area, from design strategies to applications. The representative design strategies, including solvent-induction, cation-induction, crystallization-induction, pH-induction, ligand inheritance, surface constraint, and minerals- and MOF-confinement, are first discussed. We then present the typical practical applications of AIE-type metal NCs in the various sectors of bioimaging, biological diagnosis and therapy (e.g., antibacterial agents, cancer radiotherapy), light-emitting diodes (LEDs), detection assays, and circularly polarized luminescence (CPL). To this end, we present our viewpoints on the promises and challenges of AIE-type metal NCs, which may shed light on the design of highly luminescent metal NCs, stimulating new vitality and serving as a continuous boom for the metal NC community in the future.

Engineering Coinage Metal Nanoclusters for Electroluminescent Light-Emitting Diodes

Coinage metal nanoclusters (MNCs) are a new type of ultra-small nanoparticles on the sub-nanometer (typically < three nm) scale intermediate between atoms and plasmonic nanoparticles. At the same time, the ultra-small size and discrete energy levels of MNCs enable them to exhibit molecular-like energy gaps, and the total structure involving the metal core and surface ligand together leads to their unique properties. As a novel environmentally friendly chromophore, MNCs are promising candidates for the construction of electroluminescent light-emitting diodes (LEDs). However, a systematic summary is urgently needed to correlate the properties of MNCs with their influences on electroluminescent LED applications, describe the synthetic strategies of highly luminescent MNCs for LEDs’ construction, and discuss the general influencing factors of MNC-based electroluminescent LEDs. In this review, we first discuss relevant photoemissions of MNCs that may have major influences on the performance of MNC-based electroluminescent LEDs, and then demonstrate the main synthetic strategies of highly luminescent MNCs. To this end, we illustrate the recent development of electroluminescent LEDs based on MNCs and present our perspectives on the opportunities and challenges, which may shed light on the design of MNC-based electroluminescent LEDs in the near future.

Bright and Photostable Fluorescent Metal Nanocluster Supraparticles from Invert Emulsions

Fluorescent supraparticles of gold, silver and copper nanoclusters are synthesized by simply drying of invert emulsions, resulting in a dozen-fold increase in photoluminescence quantum yield (up to ≈80 %) and a significant improvement in photostability. The inhibition of the ligand twisting during the intramolecular charge transfer is found to be responsible for the enhancement, especially for the gold nanocluster supraparticles. This research provides a general, flexible, and easy method for producing highly luminescent and photostable metal nanocluster-based materials that promise practical applications in white-light-emitting diodes.

Gold nanocluster with AIE: A novel photodynamic antibacterial and deodorant molecule

Designing long-lasting yet high-efficiency antimicrobial and deodorant agents is an everlasting goal for environmental and public health. Here we present the design of AIE-featured Au nanoclusters (NCs) for visible-light-driven antibacterial and deodorant applications. Owing to the intriguing AIE traits, the good harvest of visible-light, and rich surface chemistry, the AIE-featured Au NCs unprecedentedly exhibit excellent visible-light-driven antibacterial activities against gram-positive (≥98.5%) and gram-negative bacteria (≥99.94%), which is resulted from their photodynamic producibility of abundant reactive oxygen species including O₂^•-, •OH and H₂O₂ via O₂ reduction and subsequent H₂O₂ oxidation. In addition, the Au NCs are demonstrated to be biocompatible, and easy to be deployed for downstream antibacterial and deodorant applications. For example, the Au NCs-modified domestic materials (e.g., latex, ceramic glaze, organic fiber, and clothings) achieve long-lasting antibacterial efficiency of 99% and deodorant efficiency of >97.9% under visible-light irradiation. This work may shed light on designing novel AIE-featured metal NCs with photodynamic antibacterial and deodorant functions, enabling metal NCs and corresponding downstream materials to step into the photodynamic antibacterial and deodorant era.

Fluorescence or Phosphorescence? The Metallic Composition of the Nanocluster Kernel Does Matter

It remains challenging to manipulate the nature of photoluminescence as either fluorescence or phosphorescence for a correlated cluster series. In this work, two correlated nanoclusters, Au₅ Ag₁₁ (SR)₈ (DPPOE)₂ and Pt₁ Ag₁₆ (SR)₈ (DPPOE)₂ with comparable structure features, were synthesized and structurally determined. These two alloy nanoclusters displayed distinct photoluminescent nature-the Au₅ Ag₁₁ nanocluster is fluorescent, whereas the Pt₁ Ag₁₆ nanocluster is phosphorescent. The decay processes of the excited electrons in these two nanoclusters have been explicitly mapped out by both experimental and theoretical approaches, disclosing the mechanisms of their fluorescence and phosphorescence. Specifically, the metallic compositions of the nanocluster kernels mattered in determining their photoluminescent nature. The results herein provide an intriguing nanomodel that enables us to grasp the origin of photoluminescence at the atomic level, which further paves the way for fabricating novel nanoclusters or cluster-based nanomaterials with customized photophysical properties.

Assembling Atomically Precise Noble Metal Nanoclusters Using Supramolecular Interactions

Supramolecular chemistry (SC) of noble metal nanoclusters (NMNCs) is one of the fascinating areas of contemporary materials science. It is principally concerned with the noncovalent interactions between NMNCs, as well as between NMNCs and molecules or nanoparticles. This review focuses on recent advances in the supramolecular assembly of NMNCs and applications of the resulting structures. We have divided the topics into four distinct subgroups: (i) SC of NMNCs in gaseous and solution phases, (ii) supramolecular interactions of NMNCs in crystal lattices, (iii) supramolecular assemblies of NMNCs with nanoparticles and NMNCs, and (iv) SC of NMNCs with other molecules. The last explores their interactions with fullerenes, cyclodextrins, cucurbiturils, crown ethers, and more. After discussing these topics concisely, various emerging properties of the assembled systems in terms of their mechanical, optical, magnetic, charge-transfer, etc. properties and applications are presented. SC is seen to provide a crucial role to induce new physical and chemical properties in such hybrid nanomaterials. Finally, we highlight the scope for expansion and future research in the area. This review would be useful to those working on functional nanostructures in general and NMNCs in particular.

Carboranethiol-Protected Propeller-Shaped Photoresponsive Silver Nanomolecule

We report the synthesis, structural characterization, and photophysical properties of a propeller-shaped Ag₂₁ nanomolecule with six rotary arms, protected with m-carborane-9-thiol (MCT) and triphenylphosphine (TPP) ligands. Structural analysis reveals that the nanomolecule has an Ag₁₃ central icosahedral core with six directly connected silver atoms and two more silver atoms connected through three Ag-S-Ag bridging motifs. While 12 MCT ligands protect the core through metal-thiolate bonds in a 3-6-3-layered fashion, two TPP ligands solely protect the two bridging silver atoms. Interestingly, the rotational orientation of a silver sulfide staple motif is opposite to the orientation of carborane ligands, resembling the existence of a bidirectional rotational orientation in the nanomolecule. Careful analysis reveals that the orientation of carborane ligands on the cluster's surface resembles an assembly of double rotors. The zero circular dichroism signal indicates its achiral nature in solution. There are multiple absorption peaks in its UV-vis absorption spectrum, characteristic of a quantized electronic structure. The spectrum appears as a fingerprint for the cluster. High-resolution electrospray ionization mass spectrometry proves the structure and composition of the nanocluster in solution, and systematic fragmentation of the molecular ion starts with the loss of surface-bound ligands with increasing collision energy. Its multiple optical absorption features are in good agreement with the theoretically calculated spectrum. The cluster shows a narrow near-IR emission at 814 nm. The Ag₂₁ nanomolecule is thermally stable at ambient conditions up to 100 °C. However, white-light illumination (lamp power = 120-160 W) shows photosensitivity, and this induces structural distortion, as confirmed by changes in the Raman and electronic absorption spectra. Femtosecond and nanosecond transient absorption studies reveal an exceptionally stable excited state having a lifetime of 3.26 ± 0.02 μs for the carriers, spread over a broad wavelength region of 520-650 nm. The formation of core-centered long-lived carriers in the excited state is responsible for the observed light-activated structural distortion.

Recent progress of carbon dot fluorescent probes for tetracycline detection

We made a detailed discussion about TCs and CDs, including the synthetic methods, doping strategies and promising prospects.

Role of Ligand on Photophysical Properties of Nanoclusters with fcc Kernel: A Case Study of Ag14(SC6H4X)12(PPh3)8 (X = F, Cl, Br)

The structure-property correlation of a series of silver nanoclusters (NCs) is essential to understand the origin of photophysical properties. Here, we report a series of face-centered cubic (fcc)-based silver NCs by varying the halogen atom in the thiolate ligand to investigate the influence of the halide atoms on the electronic structure. These are {Ag₁₄(FBT)₁₂(PPh₃)₈·(solvent)_x} (NC-1), Ag₁₄(CBT)₁₂(PPh₃)₈ (NC-2), and Ag₁₄(BBT)₁₂(PPh₃)₈ (NC-3), where 4-fluorothiophenol (FBT), 4-chlorothiophenol (CBT), and 4-bromothiophenol (BBT) have been utilized as thiolate ligands, respectively. Interestingly, the optical and electrochemical bandgap values of these NCs nicely correlated with the electronic effect of the halides, which is governed by the intracluster and interclusters π-π interactions. These clusters are emissive at room temperature and the luminescence intensity increases with the lowering of temperature. The short lifetime data suggest that the emission is predominantly originating due to the interband relaxation (d → sp) of the Ag cores. Femtosecond transient absorption (TA) spectra revealed similar types of decay profiles for NC-2 and NC-3 and longer decay time for NC-2. The relaxation dominates the decay profile to the surface states and most of the excited-state energy dissipates via this process. This supports the molecular-like dynamics of these series of NCs with an fcc core. This overview shed light on an in-depth understanding of ligand's role in luminescence and transient absorption spectra.

Intrinsic-to-extrinsic emission tuning in luminescent Cu nanoclusters by in situ ligand engineering

Enhancement of the emission quantum yield and expansion of the emission tunability spectrum are the key aspects of an emitter, which direct the evolution of future generation light harvesting materials. In this regard, small molecular ligand-protected Cu nanoclusters (SLCuNCs) have emerged as prospective candidates. Herein, we report the broadband emission tunability in a SLCuNC system, mediated by in situ ligand replacement. 1,6-Hexanedithiol-protected blue emissive discrete Cu nanoclusters (CuNCs) and red emissive CuNC assemblies have been synthesized in one pot. The red emissive CuNC assemblies were characterized and found to be covalently-linked nanocluster superstructures. The blue emissive CuNC was further converted to a green-yellow emissive CuNC over time by a ligand replacement process, which was mediated by the oxidized form of the reducing agent used for synthesizing the blue emissive nanocluster. Steady-state emission results and fluorescence dynamics studies were used to elucidate that the ligand replacement process not only modulates the emission color but also alters the nature of emission from metal-centered intrinsic to ligand-centered extrinsic emission. Moreover, time-dependent blue to green-yellow emission tunability was demonstrated under optimized reaction conditions.

Source of Bright Near-Infrared Luminescence in Gold Nanoclusters

Gold nanoclusters with near-infrared (NIR) photoluminescence (PL) have great potential as sensing and imaging materials in biomedical and bioimaging applications. In this work, Au₂₁(S-Adm)₁₅ and Au₃₈S₂(S-Adm)₂₀ are used to unravel the underlying mechanisms for the improved quantum yields (QY), large Stokes shifts, and long PL lifetimes in gold nanoclusters. Both nanoclusters show decent PL QY. In particular, the Au₃₈S₂(S-Adm)₂₀ nanocluster shows a bright NIR PL at 900 nm with QY up to 15% in normal solvents (such as toluene) at ambient conditions. The relatively lower QY for Au₂₁(S-Adm)₁₅ (4%) compared to that of Au₃₈S₂(S-Adm)₂₀ is attributed to the lowest-lying excited state being symmetry-disallowed, as evidenced by the pressure-dependent antispectral shift of the absorption spectra compared to PL, yet Au₂₁(S-Adm)₁₅ maintains some emissive properties due to a nearby symmetry-allowed excited state. Furthermore, our results show that suppression of nonradiative decay due to the surface "lock rings", which encircle the Au kernel and the surface "lock atoms" which bridge the fundamental Au kernel units (e.g., tetrahedra, icosahedra, etc.), is the key to obtaining high QYs in gold nanoclusters. The complicated excited-state processes and the small absorption coefficient of the band-edge transition lead to the large Stokes shifts and the long PL lifetimes that are widely observed in gold nanoclusters.

Reversible transformation between Au14Ag8 and Au14Ag4 nanoclusters

Although several approaches have been exploited to trigger the structural transformation of metal nanoclusters, most cases are assigned to the unidirectional conversion, while the reversible conversion of nanoclusters remains challenging. In this work, the reversible conversion between two Au-Ag alloy nanoclusters, Au₁₄Ag₈(Dppm)₆(CN)₄Cl₄ and Au₁₄Ag₄(Dppm)₆Cl₄, has been accomplished, which was tracked by UV-vis and ESI-MS spectroscopy. The condition of the nanocluster reversible conversion has been meticulously mapped out. Our results provide some new insights into the cluster transformation, which will benefit the future preparation of metalloid clusters with customized structures and properties.

Ligand Effects on Intramolecular Configuration, Intermolecular Packing, and Optical Properties of Metal Nanoclusters

Surface modification has served as an efficient approach to dictate nanocluster structures and properties. In this work, based on an Ag22 nanocluster template, the effects of surface modification on intracluster constructions and intercluster packing modes, as well as the properties of nanoclusters or cluster-based crystallographic assemblies have been investigated. On the molecular level, the Ag22 nanocluster with larger surface steric hindrance was inclined to absorb more small-steric chlorine but less bulky thiol ligands on its surface. On the supramolecular level, the regulation of intramolecular and intermolecular interactions in nanocluster crystallographic assemblies rendered them CIEE (crystallization-induced emission enhancement)-active or -inactive nanomaterials. This study has some innovation in the molecular and intramolecular tailoring of metal nanoclusters, which is significant for the preparation of new cluster-based nanomaterials with customized structures and enhanced performances.

Synthesis and Luminescence Properties of Two-Electron Bimetallic Cu-Ag and Cu-Au Nanoclusters via Copper Hydride Precursors

The synthesis, structural characteristics, and photophysical properties of luminescent Cu-rich bimetallic superatomic clusters [Au@Cu₁₂(S₂CNⁿPr₂)₆(C≡CPh)₄]⁺ (1a⁺), [Au@Cu₁₂{S₂P(OR)₂}₆(C≡CPh)₄]⁺ (2⁺), (2a⁺ = ⁱPr; 2b⁺ = ⁿPr), [Au@Cu₁₂{S₂P(C₂H₄Ph)₂}₆(C≡CPh)₄]⁺ (2c⁺), and [Ag@Cu₁₂{S₂P(OⁿPr)₂}₆(C≡CPh)₄]⁺ (3⁺) were studied. Compositionally uniform clusters 1⁺-3⁺ were isolated from the reaction of dithiolato-stabilized, polyhydrido copper clusters with phenylacetylene in the presence of heterometal salts. By using X-ray diffraction, the structures of 1a⁺, 2a⁺, 2b⁺, and 3⁺ were able to be determined. ESI-mass spectrometry and elemental analysis confirmed their compositions and purity. The structural characteristics of these clusters are similar with respect to displaying gold (or silver)-centered Cu₁₂ cuboctahedra surrounded by six dithiocarbamate/dithiophosph(in)ate and four alkynyl ligands. The doping of Au and Ag atoms into the polyhydrido copper nanoclusters significantly enhances their PL quantum yields from Ag@Cu₁₂ (0.58%) to Au@Cu₁₂ (55%) at ambient temperature in solution. In addition, the electrochemical properties of the new alloys were investigated by cyclic voltammetry.

Cocrystals of Atomically Precise Noble Metal Nanoclusters

Cocrystallization is a phenomenon involving the assembly of two or more different chemical entities in a lattice, occurring typically through supramolecular interactions. In this concept, recent advancements in the cocrystallization of atomically precise noble metal clusters and their potential future directions are presented. Different strategies to create coassemblies of thiolate-protected noble metal nanoclusters are presented first. An approach is the simultaneous synthesis, and cocrystallization of two clusters having similar structures. A unique pair of clusters found recently, namely Ag₄₀ and Ag₄₆ with same core but different shell are taken to illustrate this. In another category, the case of the same core is presented, namely Ag₁₁₆ with different shells, as in a mixture of Ag₂₁₀ and Ag₂₁₁ . Next, an intercluster reaction is presented to create cocrystals through selective crystallization of the reaction products. The coexistence of competing effects, magic sizes, and magic electron shells in a coassembly of alloy nanoclusters is discussed next. Finally, an assembly strategy for nanoclusters using electrostatic interactions is described. This concept is concluded with a future perspective on the emerging possibilities of such solids. Advancements in this field will certainly help the development of novel materials with exciting properties.

Impact of Ligands on Structural and Optical Properties of Ag29 Nanoclusters

A ligand exchange strategy has been employed to understand the role of ligands on the structural and optical properties of atomically precise 29 atom silver nanoclusters (NCs). By ligand optimization, ∼44-fold quantum yield (QY) enhancement of Ag₂₉(BDT)_12-x(DHLA)_x NCs (x = 1-6) was achieved, where BDT and DHLA refer to 1,3-benzene-dithiol and dihydrolipoic acid, respectively. High-resolution mass spectrometry was used to monitor ligand exchange, and structures of the different NCs were obtained through density functional theory (DFT). The DFT results from Ag₂₉(BDT)₁₁(DHLA) NCs were further experimentally verified through collisional cross-section (CCS) analysis using ion mobility mass spectrometry (IM MS). An excellent match in predicted CCS values and optical properties with the respective experimental data led to a likely structure of Ag₂₉(DHLA)₁₂ NCs consisting of an icosahedral core with an Ag₁₆S₂₄ shell. Combining the experimental observation with DFT structural analysis of a series of atomically precise NCs, Ag_29-yAu_y(BDT)_12-x(DHLA)_x (where y, x = 0,0; 0,1; 0,12 and 1,12; respectively), it was found that while the metal core is responsible for the origin of photoluminescence (PL), ligands play vital roles in determining their resultant PLQY.

A photoluminescent Au(I)/Ag(I)/PNN coordination complex for relatively rapid and reversible alcohol sensing

Trinuclear complex [Au2Ag(dppmaphen)2(CN)2]PF6 photoluminesces on exposure to low molecular weight alcohols. This emission is likely due to C-Hπ interactions between the analyte and -PPh2 group, that inhibits non-radiative relaxation of the photoexcited state. Photoluminescene was quenched by removing the analyte under a stream of N2 or replacing it with H2O. This on/off switching was clearly visible, relatively rapid and recyclable.

Controlling the Crystallographic Packing Modes of Pt1Ag28 Nanoclusters: Effects on the Optical Properties and Nitrogen Adsorption-Desorption Performances

We herein report the manipulation of the crystallographic packing modes of Pt₁Ag₂₈(S-Adm)₁₈(PPh₃)₄ nanoclusters by altering counterions as different polyoxometalates (POMs). Specifically, the Cl^- anion of the presynthesized Pt₁Ag₂₈ nanocluster was substituted by POM anions including [Mo₆O₁₉]^2-, [W₆O₁₉]^2-, or [PW₁₂O₄₀]^3-. The crystal lattices of these Pt₁Ag₂₈ nanoclusters with diverse anions showed distinct packing modes and thus manifested remarkably distinguishable crystalline-state optical properties and nitrogen adsorption-desorption performances. Overall, the combination of intercluster control in this work and intracluster control reported previously (the control over metal-ligand within the nanocluster framework) accomplished a more comprehensive manipulation over the M₂₉(SR)₁₈(PR'₃)₄ nanocluster system, which enables us to further grasp the structure-property correlations at the atomic level.