Collective Effects of Multiple Fluorine Atoms Causing π-philic Characteristic within a Caged Polyoxometalate Framework.

Perfluorination brings about distinctive properties arising from the unusual nature of the F element, which have been extensively developed in materials science and chemistry. Herein we report that the construction of F-rich inner space within a hollowed Mo132 O372 cage ([Mo132 O372 (OCOR)30 (H2 O)72 ]42- ) leads to the emergence of unique guest binding activities in encapsulation. Prominently, the trifluoroacetate-modified cage (R=CF3 , 2) having as many as 90 F groups inside favors trapping cyclopentadiene (Cp), which is hardly trapped by the non-fluorinated counterpart (R=CH3 , 1). Systematic studies using related hydrocarbons show that the amount of the encapsulated guest is correlated with the unsaturation degree of the guests, implying the involvement of the attractive interaction of the CF3 -modified interior wall with the guest π-electron clouds. Control experiments using the semi-fluorinated analogues (R=CF2 H, CFH2 ) reveal that the perfluorination is a critical factor to facilitate the Cp encapsulation by 2, indicating that collective effects of polar C-F bonds spreading over the interior surface, rather than the polarity of the individual C-F bonds, are responsible. We also provide a successful example of the physical molecular confinement within the cage through the "ship-in-a-bottle" Diels-Alder reaction between trapped diene and dienophile.

In situ Microinflammation Detection Using Gold Nanoclusters and a Tissue-clearing Method.

Microinflammation enhances the permeability of specific blood vessel sites through an elevation of local inflammatory mediators, such as interleukin (IL)-6 and tumor necrosis factor (TNF)-α. By a two-dimensional immunohistochemistry analysis of tissue sections from mice with experimental autoimmune encephalomyelitis (EAE), an animal model for multiple sclerosis (MS), we previously showed that pathogenic immune cells, including CD4+ T cells, specifically accumulate and cause microinflammation at the dorsal vessels of the fifth lumbar cord (L5), resulting in the onset of disease. However, usual pathological analyses by using immunohistochemistry on sections are not effective at identifying the microinflammation sites in organs. Here, we developed a new three-dimensional visualization method of microinflammation using luminescent gold nanoclusters (AuNCs) and the clear, unobstructed brain/body imaging cocktails and computational analysis (CUBIC) tissue-clearing method. Our protocol is based on the detection of leaked AuNCs from the blood vessels due to an enhanced vascular permeability caused by the microinflammation. When we injected ultrasmall coordinated Au13 nanoclusters intravenously (i.v.) to EAE mice, and then subjected the spinal cords to tissue clearing, we detected Au signals leaked from the blood vessels at L5 by light sheet microscopy, which enabled the visualization of complex tissue structures at the whole organ level, consistent with our previous report that microinflammation occurs specifically at this site. Our method will be useful to specify and track the stepwise development of microinflammation in whole organs that is triggered by the recruitment of pathogenic immune cells at specific blood vessels in various inflammatory diseases.

Anion-π interaction inside the polyanionic Mo132O372 cage with hydrophobic inner space.

In this paper, we provide experimental evidence to indicate that the polyanionic Mo132O372 cage with a hydrophobic inner nanospace has a unique capability to participate in anion-π interactions by showing a preference for electron-deficient mono-substituted benzenes over non-electron-deficient guests in inclusion.

Aggregation-induced chirality amplification of optically active fluorescent polyurethane and a cyclic dimer in the ground and excited states.

Optically active linear polyurethane and a cyclic dimer were synthesized from 2,7-diisocyanatofluorene and 2,2'-dihydroxy-1,1'-binaphthyl. The circular dichroism (CD) spectral intensity of the polymer was amplified at a higher concentration through aggregate formation, while circularly polarized light (CPL) emission was not enhanced. The cyclic dimer's CPL emission was largely amplified (glum 1.1 × 10-2) due to intermolecular excimer formation through aggregation, while the CD intensity was not affected.

Diarsine- vs diphosphine-protected Au13 clusters: Effect of subtle geometric differences on optical property and electronic structure.

In the design of ligand-protected metal clusters, the choice of protecting ligands is a critical factor because they can profoundly affect the nuclearity, geometry, and electronic structures to afford a diverse range of cluster compounds. Here, we report the synthesis of two novel diarsine-protected Au13 clusters ([Au13L5Cl2]3+, L = diarsine) and compare these clusters with diphosphine analogs in terms of the core geometry and optical properties. In the crystal structure, the cluster bearing C3-bridged diarsines {[Au13(dpap)5Cl2]3+, 3} had an apparently identical icosahedral Au13 core to [Au13(dppe)5Cl2]3+ (1) with C2-bridged diphosphines, but slight structural differences associated with the bridging unit of the ligands were found. Despite similar icosahedral Au13 cores 1 and 3, their absorption and photoluminescence profiles were evidently different. Theoretical calculations revealed that the subtle deformation of the Au13 icosahedron, rather than the coordinating atoms (As or P), notably influences the electronic structure to cause the difference in the absorption profiles.

Self-promoted solid-state covalent networking of Au25(SR)18 through reversible disulfide bonds. A critical effect of the nanocluster in oxidation processes.

Covalent crosslinking of ligand-protected gold nanoclusters offers interesting platforms to investigate the properties associated with the synergetic effects of multiple nanoclusters. In this paper, we report the synthesis of covalent networks of [Au25(SR)18]- nanoclusters using reversible disulfide linkages, which was facilitated by the unique capabilities of the nanocluster to mediate oxidation processes. The conventional Au25 synthesis using 1,6-hexanedithiol afforded a soluble oligodisulfide-appended [Au25(SR)18]- monomer possessing uncoordinated anionic thiolate sites at the terminal ends. Upon exposure to O2, the monomer spontaneously underwent intercluster crosslinking in the solid state to give free-standing transparent films, in which the nanoclusters were condensed with the retention of the original Au25 framework. Through studies combined with model experiments, the Au25 cluster was found to be involved in the O2-mediated radical reactions, promoting the formation of intercluster disulfide linkages. The composition of the films implied the involvement of reversible exchange reactions between disulfide and thiyl radicals, from which it was suggested that solid-state crosslinking occurred in adaptive manners under the control of dynamic covalent chemistry. We also demonstrate that the nanocluster film can serve as a robust and efficient heterogeneous photosensitizer to mediate the generation of singlet oxygen. This work demonstrates a unique aspect of the Au25(SR)18-type nanocluster to mediate oxidation processes as well as the utility of the concept of dynamic covalent chemistry in the bottom-up construction of nanomaterials, which would widen the potential of ligand-protected nanoclusters.

Chiroptical activity of Au13 clusters: experimental and theoretical understanding of the origin of helical charge movements.

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 transitions remain unexplored. Herein, we perform experimental and theoretical studies on the structures and chiroptical properties of Au13 clusters protected by mono- and di-phosphine ligands. Based on the experimental reevaluation of diphosphine-ligated Au13 clusters, we show that these surface ligands slightly twist the Au13 cores from a true icosahedron to generate intrinsic chirality in the gold frameworks. Theoretical investigation of a monophosphine-ligated cluster model reproduced the experimentally observed circular dichroism (CD) spectrum, indicating that such a torsional twist of the Au13 core, rather than the surrounding chiral environment by helically arranged diphosphine ligands, contributes to the appearance of the chiroptical response. We also show that the calculated CD signals are dependent on the degree of asymmetry (torsion angle between the two equatorial Au5 pentagons), and provide a visual understanding of the origin of helical charge movements with transition-moment and transition-density analyses. This work provides novel insights into the chiroptical activities of ligand-protected metal clusters with intrinsically chiral cores.

Terahertz Raman Spectroscopy of Ligand-Protected Au8 Clusters.

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 ultralow-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, THz). 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 can be utilized for metal nanoclusters to visualize core structural changes and Au···π interactions, which cannot be captured by single crystal X-ray analysis.

Unusual Attractive Au-π Interactions in Small Diacetylene-Modified Gold Clusters.

It is well known that alkynes act as π-acids in the formation of complexes with metals. We found unprecedented attractive Au-π interactions in diacetylene-modified [core+exo]-type [Au8 ]4+ clusters. The 4-phenyl-1,3-butadiynyl-modified cluster has unusually short Au-Cα distances in the crystal structure, revealing the presence of attractive interactions between the coordinating C≡C moieties and the neighboring bitetrahedral Au6 core, which is further supported by IR and NMR spectra. Such weak interactions are not found in mono-acetylene-modified clusters, which indicates that they are specific for diacetylenic ligands. The attractive Au-π interactions are likely associated with the low energy of the π* orbital in the diacetylenic moieties, into which the valence electrons of the gold core may be back donated. The [Au8 ]4+ clusters show clear red-shifts of >10 nm with respect to the corresponding mono-acetylenic clusters in UV/Vis absorption bands, which indicates substantial electronic perturbation effects of the Au-π interactions.

Phosphine-Ligated Gold Clusters with Core+ exo Geometries: Unique Properties and Interactions at the Ligand-Cluster Interface.

Over recent years, research on the structures and properties of ligand-protected gold cluster molecules has gained significant interest. The crystal structure information accumulated to date has revealed the structural preference to adopt closed polyhedral geometries, but the use of multidentate ligands sometimes leads to the formation of exceptional structures. This Account describes results of our studies on diphosphine-coordinated [core+ exo]-type gold clusters featuring extra gold atoms outside the polyhedral cores, highlighting (1) their distinct optical properties due to the unique electronic structures generated by the exo gold atoms and (2) electronic/attractive ligand-cluster interactions that cause definite perturbation effects on the cluster properties. Subnanometer gold clusters with [core+ exo]-type geometries (nuclearity = 6, 7, 8, and 11) commonly displayed single absorption bands in the visible region, which are distinct in patterns from those of conventional polyhedral-only homologues. Theoretical studies demonstrated that the exo gold atoms are critically involved in the generation of unique electronic structures characterized by the HOMO-LUMO transitions with dominant oscillator strengths, leading to the appearance of the isolated absorption bands. On the basis of the frontier orbital distributions, the HOMO and LUMO were shown to be localized around the polyhedral cores and exo gold atoms, respectively. Therefore, the HOMO-LUMO transitions responsible for the visible absorptions occur in the core → exo direction. The HOMO-LUMO gap energies showed no clear trends with respect to the nuclearity (size), indicating that the individual geometric features of the inorganic framework primarily govern the clusters' electronic structures and properties. Systematic studies using octagold clusters bearing various anionic coligands revealed that electronic or attractive interactions between the gold framework and ligand functionalities, such as π-electron systems and heteroatoms, cause substantial perturbations of the wavelength of the visible absorption band due to the HOMO-LUMO transitions. Especially, significant red shifts were observed as a result of the electronic coupling with specific π-resonance contributors. It was also found that the orientation of aromatic rings around the inorganic framework is a factor that affects the cluster photoluminescence. These findings demonstrate the utility of the ligand moieties surrounding the gold frameworks for fine-tuning of the optical properties. During these studies, unusual but definite attractive interactions between the gold framework and C-H groups of the diphosphine ligand were found in the hexagold clusters. On the basis of careful crystallographic and NMR analyses, these interactions were deemed as a certain kind of M···H hydrogen bonds, which critically affect the maintenance of the cluster framework. Such unique interaction activities are likely due to the valence electrons in the gold framework, which serve as the hydrogen-bond acceptor for the unfunctionalized C-H groups. Overall, these observations imply the uniqueness of the ligand-cluster interface associated with the partially oxidized gold entities, which may expand the scope of ligand-protected clusters toward various applications.

Preparation of Carbodiimides with One-Handed Axial Chirality.

The axial chirality of carbodiimide was proposed in 1932, but the synthesis of carbodiimide with one-handed axial chirality has not been achieved because of the low barrier of racemization. This work presents a strategy to use a conformationally restrained cyclic structure for creating carbodiimides whose biases of the axial chirality (labeled as SNCN/ RNCN) are higher than 100:1, as determined by vibrational circular dichroism spectroscopy and density functional theory calculations.

An Inherently Chiral Au24 Framework with Double-Helical Hexagold Strands.

2,3-bis(diphenylphosphino)butane enantiomers (chiraphos, L) used as chiral auxiliaries results in the preferential formation of an unprecedented Au24 framework with inherent chirality. The crystal structure of [Au24 L6 Cl4 ]2+ (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 Au24 frameworks possess unique chiroptical properties. The Au24 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.

Aggregation-Induced Fluorescence-to-Phosphorescence Switching of Molecular Gold Clusters.

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 [Au8]4+ 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.

Hydrogen bonds to Au atoms in coordinated gold clusters.

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.Many transition metals can form hydrogen bonds to organic species, but experimental evidence for Au is still lacking. Here, the authors obtain crystallographic and NMR spectroscopic evidence of hydrogen bonding between C-H groups and Au atoms of gold clusters, suggesting that non-covalent interactions may play a role in gold cluster catalysis.

Ligand-Based Toolboxes for Tuning of the Optical Properties of Subnanometer 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 Au8 and Au13 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.

Hexanuclear Platinum(II) Thiolate Macrocyclic Host: Charge-Transfer-Driven Inclusion of a Ag(I) Ion Guest.

The inclusion of a Ag(I) ion by a hexanuclear platinum(II) thiolate macrocycle in solution was demonstrated, and the inclusion structure was determined by X-ray crystallography. Unique host-guest intermetallic interactions driven by charge transfer were elucidated by optical absorption spectroscopy and theoretical calculations.

Facile modulation of optical properties of octagold clusters through the control of ligand-mediated interactions.

In the recent development of structurally defined ligand-stabilized gold clusters, it has been revealed that not only the inorganic units but also the surrounding organic ligands substantially affect their electronic/optical properties. In this work, a series of core + exo type Au8 clusters decorated by dppp (Ph2P(CH2)3PPh2) and arylthiolate ligands ([Au8(dppp)4(SR)2](2+), 1-5) were synthesized, and their optical properties were studied in order to gain insights into the perturbation effects of the organic ligands. 1-5 showed visible absorption and photoluminescence emission bands at longer wavelengths compared to their chloro- and acetylide-modified analogues, suggesting the contribution of weak non-bonding interactions of the Au framework with the ligand heteroatoms. Upon acid treatment, 2- and 4-pyridinethiolate clusters (R = Py, 2 and 4) showed larger red shifts of the absorption and emission bands than the 3-pyridyl isomer (3), implying the involvement of the resonance structures of the SPy units. On the other hand, all regioisomers (2-4) showed large photoluminescence enhancements upon pyridine protonation. X-ray crystallographic and NMR analyses of 4 and its protonated form (4') showed that the electron-deficient pyridinium rings of 4' form π-stacks with neighbouring phenyl groups of dppp, suggesting that the orientation of the surface aromatics is a plausible factor governing the emission efficiency. These observations provide examples of successful modulation of optical properties of small gold clusters through the electronic and/or steric perturbation by the proximal organic ligands, highlighting the importance of the ligand design in the fine tuning of cluster properties directed for optical chemosensors and luminescent materials.

Facile Diastereoseparation of Glycosyl Sulfoxides by Chiral Stationary Phase.

Separation of the diastereomers of glycosyl sulfoxides differing in the sulfur chirality has been difficult. This article presents a fast and scalable method for their diastereoseparation using a chiral stationary phase. The usefulness of this method was demonstrated in a 500-mg scale separation within 20 min, and in the separation of trisaccharyl sulfoxide diastereomers. Chirality 28:534-539, 2016. © 2016 Wiley Periodicals, Inc.

Unexpected electronic perturbation effects of simple PEG environments on the optical properties of small cadmium chalcogenide clusters.

Poly(ethylene glycol) (PEG) has been widely used for the surface protection of inorganic nanoobjects because of its virtually 'inert' nature, but little attention has been paid to its inherent electronic impacts on inorganic cores. Herein, we definitively show, through studies on optical properties of a series of PEG-modified Cd(10)Se(4)(SR)(10) clusters, that the surrounding PEG environments can electronically affect the properties of the inorganic core. For the clusters with PEG units directly attached to an inorganic core (R = (CH(2)CH(2)O)nOCH(3), 1-PEGn, n = 3, ∼7, ∼17, ∼46), the absorption bands, associated with the low-energy transitions, continuously blue-shifted with the increasing PEG chain length. The chain length dependencies were also observed in the photoluminescence properties, particularly in the excitation spectral profiles. By combining the spectral features of several PEG17-modified clusters (2-C(m)-PEG17 and 3) whose PEG and core units are separated by various alkyl chain-based spacers, it was demonstrated that sufficiently long PEG units, including PEG17 and PEG46, cause electronic perturbations in the cluster properties when they are arranged near the inorganic core. These unique effects of the long-PEG environments could be correlated with their large dipole moments, suggesting that the polarity of the proximal chemical environment is critical when affecting the electronic properties of the inorganic cluster core.

Cluster-π electronic interaction in a superatomic Au13 cluster bearing σ-bonded acetylide ligands.

An organometallic Au13 cluster having two σ-bonded acetylide ligands was synthesized and its structure was determined by X-ray crystallography. Absorption spectral studies indicated the presence of electronic coupling between the superatomic Au13 core and the acetylide π-orbitals, which was supported by theoretical considerations.