Thermochromism and piezochromism of an atomically precise high-nuclearity silver sulfide nanocluster

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.

Thermotropic Structure Phase Transitions and Two Types of Thermochromic Behaviors in a Bromoargentate Cluster [Pr-dabco]2Ag4Br6

Organic-inorganic silver halide hybrids show abundant phase transitions and thermochromism. However, it is very rare that silver halides exhibit thermochromism related to thermotropic structure phase transition. Herein, a bromoargentate hybrid, [Pr-dabco]2Ag4Br6 (1) (Pr-dabco+ = 1-propyl-1,4-diazabicyclo-[2.2.2]octan-1-ium), with tetranuclear [Ag4Br6]2- clusters was prepared and characterized by microanalysis, ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy, and thermogravimetric (TG) and differential scanning calorimetry (DSC) techniques. Interestingly, 1 undergoes an irreversible structure phase transition at ∼436 K in the first heating process, which is accompanied by an abrupt color change from colorless to yellow; however, a reversible color change between pale yellow and yellow is observed in the next heating-cooling cycles. Notably, DSC measurement revealed that a reversible phase transition is associated with the change in color between pale yellow and yellow, while the powder X-ray diffraction (PXRD) patterns corresponding to pale yellow and yellow phases are quite similar to each other. These observations demonstrate that thermochromism in the next heating-cooling runs is associated with a reversible structure phase transition, which perhaps concerns the disorder-order transformation of alkyl chains in the cationic ligand [Pr-dabco]+, and relevant to the anharmonic fluctuations of the Ag-Br and Ag-N bonds, a strong electron-phonon coupling effect is seen within the bromoargentate cluster.

A Multiple Stimuli-Responsive Ag/P/S Complex Showing Solvochromic and Mechanochromic Photoluminescence

The reaction of CF₃COOAg, 3-bdppmapy (N,N-bis(diphenylphosphanylmethyl)-3-aminopyridine) and HTZ (1,2,4-triazole-3-thiol) in CH₂Cl₂/MeOH resulted in a dinuclear Ag/P/S complex [Ag₂(TZ)₂(3-bdppmapy)₂]·xSol (1·xSol). Crystals of 1·xSol converted to 1·2MeOH in air at room temperature and further to 1 under vacuum upon heating. The solid-state, room-temperature photoluminescent emission of 1·xSol (510 nm) shifted to 494 nm (1·2MeOH) and 486 nm (1). Grinding solids of 1·2MeOH in air resulted in amorphous 1G characterized by solid-state emission at 468 nm, which converted to 1GR with 513 nm emission upon MeOH treatment. Grinding 1GR in air returned 1G, and this interconversion was reproducible over five cycles. The solid-state photoluminescence of 1G changed in response to vapors containing low-molecular weight alcohols but remained unchanged after exposure to other volatile organic compounds (VOCs) or to water vapor. Test papers impregnated with 1G could detect methanol in vapors from aqueous solutions at concentrations above 50%. Complex 1G is, therefore, an example of a stimuli-responsive molecular sensor for the detection of alcohols.

Organic-inorganic haloargentate hybrids of [Me-dabco]Ag2X3 (X = I or Br) with halide ions manipulating the crystal structure, phase transition, and dielectric behavior

Two haloargentate hybrids, [Me-dabco]Ag₂X₃ (Me-dabco = 1-methyl-1,4-diazabicyclo-[2.2.2]octan-1-ium, X = I (1) or Br (2)), with the same formula but different structures have been synthesized by a slow evaporation method and characterized by microanalysis, infrared spectroscopy, thermogravimetric, and powder X-ray diffraction techniques. Hybrid 1 consists of completely isolated [Ag₄I₆]^2- clusters, while hybrid 2 exhibits a complicated one-dimensional (1D) chain structure formed by four different configurations of neutral chains and two dissimilar configurations of anionic chains. Hybrid 2 undergoes two reversible order-disorder phase transitions, while hybrid 1 displays one reversible and one irreversible structural phase transition. Both 1 and 2 displayed step-like dielectric anomalies in the vicinity of the phase transition temperature. The corresponding dielectric constants in the high dielectric states are approximately 13 and 6 times higher than those in the low dielectric states for 1 and 2, respectively. Interestingly, the subtle change of halides from I^- to Br^- significantly affects the aggregated structure of haloargentate, the phase transition, and dielectric behaviors, revealing the typical 'butterfly effect' with the ion radii of halides in these two haloargentate hybrids.

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.

Superatomic Au25(SC2H5)18 Nanocluster under Pressure

The past decade has witnessed significant advances in the synthesis and structure determination of atomically precise metal nanoclusters. However, little is known about the condensed matter properties of these nanosized metal nanoclusters packed in a crystal lattice under high pressure. Here using density functional theory calculations, we simulate the crystal of a representative superatomic gold cluster, Au₂₅(SR)₁₈ ⁰ (R = C₂H₅), under various pressures. At ambient conditions, Au₂₅(SC₂H₅)₁₈ ⁰ clusters are packed in a crystal via dispersion interactions; being a 7e superatom, each cluster carries a magnetic moment of 1 μ_B or one unpaired electron. Upon increasing compression (from 10 to 110 GPa), we observe the formation of intercluster Au-Au, Au-S, and S-S covalent bonds between staple motifs, thereby linking the clusters into a network. The pressure-induced structural change is accompanied by the vanishment of the magnetic moment and the semiconductor-to-metal transition. Our work shows that subjecting crystals of atomically precise metal nanoclusters to high pressures could lead to new crystalline states and physical properties.

Self-assembly patterns of non-metalloid silver thiolates: structural, HR-ESI-MS and stability studies

Screening of AgNO₃/AgS^tBu solutions in DMF, DMSO and NMP resulted in the isolation of three novel nanosized silver/thiolate complexes with a torus-like {Ag₂₀(S^tBu)₁₀} core. The structures of [NO₃@Ag₂₀(S^tBu)₁₀(NO₃)₉(DMF)₆] (1) and [NO₃@Ag₂₀(^tBuS)₁₀(NO₃)₈(NMP)₈][NO₃@Ag₁₉(^tBuS)₁₀(NO₃)₈(NMP)₆]₂(NO₃) (2) were studied by single crystal X-ray diffraction (SCXRD). The self-assembly process leading to 1 can be switched to a different outcome using Br^-, resulting in [Br@Ag₁₆(S^tBu)₈(NO₃)₅(DMF)₃](NO₃)₂ (3), which is the one of the few genuine host-guest complexes in the silver/thiolate systems. Solutions of the individual complexes in CH₃CN were studied by HR-ESI-MS techniques, which revealed a dynamic behavior for each complex, driven by a redistribution of the {AgNO₃} units. This dynamics results in the appearance of both cationic and anionic species, based on unchanged silver-thiolate cores. Daylight causes degradation of 3 with the formation of a composite material based on defective orthorhombic Ag₂S with a porous morphology, as observed using the SEM technique. The electrocatalytic HER activity of such a material was studied.

Molecular copper iodide clusters: a distinguishing family of mechanochromic luminescent compounds

Mechanochromic luminescent materials displaying switchable luminescence properties in response to external mechanical force are currently attracting wide interest because of their multiple potential applications. In the growing number of mechanochromic luminescent compounds, mechanochromic complexes based on copper present appealing features with a large variety of mechanochromic properties and economical advantages over other metals. Among Cu-based compounds, molecular copper iodide clusters of cubane geometry with formula [Cu₄I₄L₄] (L = organic ligand) stand out. Indeed, they can exhibit multiple luminescent stimuli-responsive properties, being particularly suitable for the development of multifunctional photoactive systems. This perspective describes the survey of these mechanochromic luminescent cubane copper iodide clusters. Based on our investigations, their mechanochromic luminescence properties are presented along with the study of the underlying mechanism. Establishment of structure-property relationships supported by various characterization techniques and associated with theoretical investigations permits gaining insights into the mechanism at play. Studies of other researcher groups are also described and illustrate the interest shown by these mechanochromic compounds. Mechanically responsive films are reported, demonstrating their potential use in a range of applications of such copper-based stimuli-responsive materials. Current challenges faced by the development of technological applications are finally outlined.

Trinuclear cationic silver nanoclusters based-on bis-(phosphine) ligands and stabilized by CF3SO3− anions

Three trinuclear cationic silver nanoclusters based-on bis-(phosphine) ligands and stabilized by CF₃SO₃⁻ anions, displayed excellent photocurrent responses and electrochemical properties.