Combined spectroscopic studies on post-functionalized Au25 cluster as an ATR-FTIR sensor for cations

Understanding the decomposition process of the Pt1Ag24(SPhCl2)18 nanocluster at the atomic level

We report the decomposition of the Pt1Ag24(SPhCl2)18 nanocluster into a crown-like Pt1Ag4(SR)8 (SR = 2,4-SPhCl2 and 4-SPhBr) complex. UV-vis spectra and single crystal X-ray diffraction were used to track the structure-conversion process. Based on the total structure, the differences in ligand exchange rates at different sites and the effects on the stability were mapped out. This work can not only help us understand the ligand exchange behavior of the clusters, but also provide experimental support for the design of stable metal clusters.

Size Growth of Au4Cu4: From Increased Nucleation to Surface Capping

The size conversion of atomically precise metal nanoclusters is fundamental for elucidating structure-property correlations. In this study, copper salt (CuCl)-induced size growth from [Au₄Cu₄(Dppm)₂(SAdm)₅]⁺ (abbreviated as [Au₄Cu₄S₅]⁺) to [Au₄Cu₆(Dppm)₂(SAdm)₄Cl₃]⁺ (abbreviated as [Au₄Cu₆S₄Cl₃]⁺) (SAdmH = 1-adamantane mercaptan, Dppm = bis-(diphenylphosphino)methane) was investigated via experiments and density functional theory calculations. The [Au₄Cu₄S₅]⁺ adopts a defective pentagonal bipyramid core structure with surface cavities, which could be easily filled with the sterically less hindered CuCl and CuSCy (i.e., core growth) (HSCy = cyclohexanethiol) but not the bulky CuSAdm. As long as the Au₄Cu₅ framework is formed, ligand exchange or size growth occurs easily. However, owing to the compact pentagonal bipyramid core structure, the latter growth mode occurs only for the surface-capped [Au₄Cu₆(Dppm)₂(SAdm)₄Cl₃]⁺ structure (i.e., surface-capped size growth). A preliminary mechanistic study with density functional theory (DFT) calculations indicated that the overall conversion occurred via CuCl addition, core tautomerization, Cl migration, the second [CuCl] addition, and [CuCl]-[CuSR] exchange steps. And the [Au₄Cu₆(Dppm)₂(SAdm)₄Cl₃]⁺ alloy nanocluster exhibits aggregation-induced emission (AIE) with an absolute luminescence quantum yield of 18.01% in the solid state. This work sheds light on the structural transformation of Au-Cu alloy nanoclusters induced by Cu(I) and contributes to the knowledge base of metal-ion-induced size conversion of metal nanoclusters.

Crown Ether-Capped Gold Nanoclusters as a Multimodal Platform for Bioimaging

The distinct polarity of biomolecule surfaces plays a pivotal role in their biochemistry and functions as it is involved in numerous processes, such as folding, aggregation, or denaturation. Therefore, there is a need to image both hydrophilic and hydrophobic bio-interfaces with markers of distinct responses to hydrophobic and hydrophilic environments. In this work, we present a synthesis, characterization, and application of ultrasmall gold nanoclusters capped with a 12-crown-4 ligand. The nanoclusters present an amphiphilic character and can be successfully transferred between aqueous and organic solvents and have their physicochemical integrity retained. They can serve as probes for multimodal bioimaging with light (as they emit near-infrared luminescence) and electron microscopy (due to the high electron density of gold). In this work, we used protein superstructures, namely, amyloid spherulites, as a hydrophobic surface model and individual amyloid fibrils with a mixed hydrophobicity profile. Our nanoclusters spontaneously stained densely packed amyloid spherulites as observed under fluorescence microscopy, which is limited for hydrophilic markers. Moreover, our clusters revealed structural features of individual amyloid fibrils at a nanoscale as observed under a transmission electron microscope. We show the potential of crown ether-capped gold nanoclusters in multimodal structural characterization of bio-interfaces where the amphiphilic character of the supramolecular ligand is required.

Circularly polarized luminescence from Tb(III) interacting with chiral polyether macrocycles

A straightforward two-step synthesis protocol affords a series of chiral amide-based bis-pyridine substituted polyether macrocycles. One ligand is particularly able to complex terbium(III) ions spontaneously. Upon complexation, interesting chiroptical properties are observed both in absorbance (ECD) and in fluorescence (CPL). In ligand-centered electronic circular dichroism, a sign inversion coupled with a signal enhancement is measured; while an easily detectable metal-centered circularly polarized luminescence with a glum of 0.05 is obtained for the main 5D4 → 7F5 terbium transition. The coordination mode and structure of the complex was studied using different analysis methods (NMR analysis, spectrophotometric titration and solid-state elucidation).

Control over Ligand-Exchange Positions of Thiolate-Protected Gold Nanoclusters Using Steric Repulsion of Protecting Ligands

Organic ligands on gold nanoclusters play important roles in regulating the structures of gold cores. However, the impact of the number and positions of the protecting ligands on gold-core structures remains unclear. We isolated thiolate-protected Au₂₅ cluster anions, [Au₂₅(SC₂Ph)₁₇(Por)₁]^- and [Au₂₅(SC₂Ph)₁₆(Por)₂]^- (SC₂Ph = 2-phenylethanethiolate), obtained by ligand exchange of [Au₂₅(SC₂Ph)₁₈]^- with one or two porphyrinthiolate (Por) ligands as mixtures of regioisomers. The ratio of two regioisomers in [Au₂₅(SC₂Ph)₁₇(Por)₁]^- as measured by ¹H NMR spectroscopy revealed that the selectivity could be controlled by the steric hindrance of the incoming thiols. Extended X-ray absorption fine structure studies of a series of porphyrin-coordinated gold nanoclusters clarified that the Au₁₃ icosahedral core in the Au₂₅ cluster was distorted through steric repulsion between porphyrin thiolates and phenylethanethiolates. This paper reveals interesting insights into the importance of the steric structures of protecting ligands for control over core structures 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.