Aggregation-induced optical responses are ubiquitous among a wide range of organic and inorganic compounds. Here, we demonstrate an unprecedented effect of aggregation on the photoluminescence (PL) profiles of [core + exo]-type [Au] clusters, which displayed a change in the dominant PL emission mode from fluorescence to phosphorescence-type upon aggregation. In solvents in which cluster molecules are highly soluble and exist as monomers, they displayed single PL bands at ∼600 nm at ambient temperatures. However, in solvents in which cluster molecules are less soluble and cluster aggregation is induced, a new PL band at ∼700 nm also emerged. Lifetime measurements revealed that the PL emissions at ∼600 and ∼700 nm had fluorescence and phosphorescence characters, respectively. Studies of the excitation spectra suggested that organized cluster assemblies were responsible for the lower-energy emission at ∼700 nm and had exceptionally high emission activity. Accordingly, intense phosphorescence-type emissions were observed in the solid state in which the quantum efficiencies were higher by two orders of magnitude than those of the corresponding monomeric forms in solution. This work provides an example of the critical effects of cluster aggregation events on their optical properties and shows the potential of such effects in the design of cluster-based materials with unique functions and properties.

2,3-bis(diphenylphosphino)butane enantiomers (chiraphos, L) used as chiral auxiliaries results in the preferential formation of an unprecedented Au framework with inherent chirality. The crystal structure of [Au L Cl ] (1) has a square antiprism-like octagold core twinned by two helicene-like hexagold motifs, where the inherent chirality is associated with the helical arrangement. The clusters carrying (R,R)- and (S,S)- diphosphines had right- and left-handed strands, respectively. Circular dichroism spectra showed peaks in the visible to near-IR region, some of which did not coincide with absorption bands, suggesting the enantiomeric Au frameworks possess unique chiroptical properties. The Au frameworks were thermally robust, which could be attributed to the superatomic concept (18 e system) and the steric constraint effects of the bridging ligand units.

An organometallic Au 13 cluster having two σ--bonded acetylide ligands was synthesized and its structure was determined by X--ray crystllography. Absorption spectral studies indicated the presence of the electronic coupling between the superatomic Au13 core and the acetylide π--orbitals, which was supported by theoretical considerations.Gold-acetylide sigma bonds have been of general interest as they bring about the emergence of unique optical properties and reactive intermediates in catalysis. [1][2][3][4][5][6][7][8][9] For simple metal complexes, rich chemistry has been explored from both functional and structural perspectives. On the other hand, although numerous ligand-protected gold clusters have been synthesized, 10-14 examples of alkynyl-ligated clusters have been quite rare to date, and the nature of the bonding and electronic interaction between a cluster kernel and a C≡C π-system has not been experimentally characterized. [15][16][17][18] Tsukuda et al. have observed no appreciable electronic interaction in the optical absorption spectra of alkyne-protected gold clusters of diameter > 1 nm. 15,16 We have also demonstrated a similar phenomenon for a structurally precise Au 8 cluster with an anisotropic core+exo-type structure. 17 Herein we report the first experimental evidence of electronic interaction between a polyhedral gold core and a σ-bonded π-unit in the absorption spectrum of a phenylethynyl-modified Au 13 cluster with an icosahedral geometry and superatomic 8-electron system. [19][20][21][22] We have also investigated its electronic structure based on DFT calculations, which support the presence of electronic interaction between the superatomic gold core and π-conjugated units.Regioselective introduction of two alkynyl ligands on the surface of the icosahedral Au 13 skeleton was achieved by the ligand-exchange reaction of a dichloro-substituted Au 13 cluster cation ([Au 13 (dppe) 5 Cl 2 ](PF 6 ) 3 , 1·(PF 6 ) 3 ), which was synthesized according to the HCl-mediated post-synthetic method. 20 The reaction cleanly proceeded by employing excess amounts of terminal alkynes and base (sodium methoxide), and the complete ligand exchange was verified by electrospray ionization mass spectrometry (ESI-MS) analysis of the reaction mixture. After workup, the crude product was recrystallized from acetonitrile and diethyl ether to give the pure dialkynylsubstituted cluster as its hexafluorophosphate salt ([Au 13 (dppe) 5 (C≡CPh) 2 ](PF 6 ) 3 , 2·(PF 6 ) 3 ), which was thoroughly characterized by ESI-MS analyses, elemental analyses, X-ray crystallography, and 1 H and 31 P NMR spectroscopies. For instance, the ESI mass spectrum of 2·(PF 6 ) 3 showed a set of signals around m/z 1585, in perfect agreement with the calculated isotope pattern for [Au 13 (dppe) 5 (C≡CPh) 2 ] 3+ (Fig. S2). Single-crystal X-ray analysis of 2·(PF 6 ) 3 revealed that the cluster core adopts an icosahedral geometry. Two alkynyl ligands are σ-coordinated to the two diagonal apexes (Au1 and Au1′) of the icosahedron from the trans positio...

It is well known that various transition elements can form M···H hydrogen bonds. However, for gold, there has been limited decisive experimental evidence of such attractive interactions. Herein we demonstrate an example of spectroscopically identified hydrogen bonding interaction of C–H units to Au atoms in divalent hexagold clusters ([Au6]2+) decorated by diphosphine ligands. X-ray crystallography reveals substantially short Au–H/Au–C distances to indicate the presence of attractive interactions involving unfunctionalized C–H moieties. Solution 1H and 13C NMR signals of the C–H units appear at considerably downfield regions, indicating the hydrogen-bond character of the interactions. The Au···H interactions are critically involved in the ligand-cluster interactions to affect the stability of the cluster framework. This work demonstrates the uniqueness and potential of partially oxidised Au cluster moieties to participate in non-covalent interaction with various organic functionalities, which would expand the scope of gold clusters.

Recent advances in the crystal structure determination of ligand-protected metal clusters have revealed that their electronic structures and optical features are essentially governed by the nuclearity and geometries of the inorganic frameworks. In this Perspective, we point out the definite effects of the exterior ligand moieties on the properties of small gold clusters. On the basis of systematic experimental studies on the optical properties of Au and Au clusters with various anionic ligands, it was shown that not only the "through-bond" electronic effects of coordinating atoms but also the nonbonding interaction with neighboring heteroatoms and the electronic coupling with π-systems cause substantial perturbations. We also suggest that the steric rigidity of the ligand environments affects their photoluminescence efficiencies. These findings imply the feasibility of the facile modulation of the cluster properties through the appropriate choice of ligand modules, which may lead to the evolution of novel cluster-based materials with unique properties and functions.

Ligand-protected gold clusters with an asymmetric nature have emerged as a novel class of chiral compounds, but the origins of their chiroptical activities associated with helical charge movements in electronic...

For ligand-protected gold clusters, geometrical differences of gold cores and/or the presence of secondary gold core-ligand interactions influence their unique optical and electronic properties and can, in principle, be detected by spectral changes of gold core vibrations (phonon modes) in ultra-low-frequency Raman spectroscopy. We report experimental and theoretical Raman spectra of Au8 clusters protected by phosphine ligands particularly in the "gold cluster fingerprint" region from 50 to 150 cm -1 Raman shift (1.5 to 4.5 terahertz). A characteristic core breathing mode observed at ca. 123 cm -1 was sensitive to differences of core geometries. A new band was found at ca. 150 cm -1 originating from a local strain on a polyhedral gold core caused by weak Au•••π interactions. THz Raman spectroscopy would be utilized for metal nanoclusters to visualize core structural changes and Au•••π interactions, which cannot be captured by single crystal X-ray analysis.

2,3-bis(diphenylphosphino)butane enantiomers (chiraphos,L )u sed as chiral auxiliaries results in the preferential formation of an unprecedented Au 24 framework with inherent chirality.T he crystal structure of [Au 24 L 6 Cl 4 ] 2+ (1)h as as quare antiprism-like octagold core twinned by two helicene-like hexagold motifs,w here the inherent chirality is associated with the helical arrangement. The clusters carrying (R,R)-and (S,S)-diphosphines had right-and left-handed strands,respectively.Circular dichroism spectra showed peaks in the visible to near-IR region, some of which did not coincide with absorption bands,s uggesting the enantiomeric Au 24 frameworks possess unique chiroptical properties.T he Au 24 frameworks were thermally robust, which could be attributed to the superatomic concept (18 e À system) and the steric constraint effects of the bridging ligand units.Supportinginformation and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.