Synthesis and functionalities of FeSn12 superatom prepared by single atom introduction with a dendrimer template.

Superatoms are promising as new building block materials that can be designed by precise controlling of the constituent atoms. Stannaspherene (Sn12 2-) is a rigid cage-like cluster with icosahedral symmetry, for which one-atom encapsulation was theoretically expected and detected in the gas phase. Here, a single-atom introduction method into stannaspherene using a dendrimer template with polyvinylpyrrolidone (PVP) protection is demonstrated. This advanced solution-phase synthesis allows not only the selective doping of one atom into the cluster cage, but also enable further detail characterization of optical and magnetic properties that were not possible in the gas-phase synthesis. In other words, this liquid-phase synthesis method has enabled the adaptation of detailed analytical methods. In this study, FeSn12 was synthesized and characterized, revealing that a single Fe atom introduction in the Sn12 2- cage result in the appearance of near-infrared emission and enhancement in the magnetism.

Unique Functions and Applications of Rigid Dendrimers Featuring Radial Aromatic Chains.

Dendrimers, which are highly branched polymers and regarded as huge single molecules, are interesting substances from the aspect of not only polymer chemistry but also molecular chemistry. Various applications in materials science and life science have been investigated by taking advantage of the radially layered structures and intramolecular nanospaces of dendrimers. Most dendrimers have flexible structures that originate from their organic chains which contain many sp3-type atoms, while relatively rigid dendrimers composed only of sp2-type atoms have rarely been reported. It has been recently clarified that such rigid dendrimers exhibit a specific aromatic property not found in other materials. Dendritic phenylazomethines (DPAs), as one of the rigid dendrimers, have only sp2-type C and N atoms and possess a radially branched π-conjugation system in their own macromolecular chains. Such geometric and electronic structures heighten the electron density at the core of the dendrimer and induce an intramolecular potential gradient, which affords unique reactivities that lead to extraordinary functions. This unique property of the rigid dendrimers can be regarded as a new atypical electronic state based on radial aromatic chains not found in conventional aromatic compounds containing spherical aromaticity, Möbius aromaticity, metal aromaticity, and conductive polymers. Therefore, this as-yet-unknown characteristic is expected to contribute to the further development of fundamental and materials chemistry.In this Account, we highlight the rigid DPA dendrimers and their peculiar atomically precise and selective assembly behaviors that originate from the radial aromatic chains. One of the most noteworthy attainments based on the radial aromatic chains is the precise synthesis of a multimetallic multinuclear complex of a dendrimer containing a total of 13 elements. Next, we describe the electrochemical and catalytic functionalization of such multinuclear dendrimer complexes and the construction of supramolecular nanoarchitectures by the polymerization of DPAs. These complexes exhibit encapsulation-release switching of guests and additive-free catalytic ability similar to proteins and enzymes. Such selective and accurate control of the intramolecular assembly of guests and the intermolecular arrangement of hosts realized by the radial aromatic chains of dendrimers will enable supramolecular chemistry and biochemistry to be linked from a new aspect. In addition, the multimetallic multinuclear complexes of dendrimers afford a novel approach to precisely synthesize sub-nanoparticles with ultrasmall particle sizes (1 nm) that have been technically difficult to obtain by conventional nanotechnology. We discuss the method for the synthesis of these sub-nanoparticles with well-controlled atomicity and composition using DPA complexes as a template and recent advances to reveal their specific physical and chemical properties. These results suggest that the unique electronic states induced in such radial aromatics could play an important role in the development of next-generation chemistry.

New Horizon of Nanoparticle and Cluster Catalysis with Dendrimers.

Among various approaches synthesizing metal nanoparticles and tiny clusters, a template method using dendrimers has significant advantages over other chemical approaches with respect to their synthetic precision and the scalability. A dendrimer of polydentate ligands assembles metal ions or salts into the interior allowing production of metal nanoparticles in the dendrimer. The dendrimer-encapsulated nanoparticles (DENs) exhibit unique and remarkable catalytic properties depending on the size and elemental formula. Recent advances in dendrimer chemistry even enabled the atom precise synthesis of subnanometer metal clusters that have been impossible to prepare by wet chemical methods. In addition, not only for the synthesis of metal nanoparticles and clusters, the dendrimer itself can also provide the modulation of activity and selectivity in the catalysis. In this review, we summarized the most relevant research in which the dendrimer was employed as the template, modulator, or stabilizer for nanoparticle synthesis for catalytic applications.

Controlled Synthesis of Au25 Superatom Using a Dendrimer Template.

Superatoms are promising materials for their potential in elemental substitution and as new building blocks. Thus far, various synthesis methods of thiol-protected Au clusters including an Au25 superatom have been investigated. However, previously reported methods were mainly depending on the thermodynamic stability of the aimed clusters. In this report, a synthesis method for thiol-protected Au clusters using a dendrimers template is proposed. In this method, the number of Au atoms was controlled by the stepwise complexation feature of a phenylazomethine dendrimer. Therefore, synthesis speed was increased compared with the case without the dendrimer template. Hybridization for the Au25 superatoms was also achieved using the complexation control of metals.

Highly Accurate Synthesis of Quasi-sub-nanoparticles by Dendron-assembled Supramolecular Templates.

Quasi-sub-nanomaterials (1-3 nm) have been predicted to exhibit unique properties originating from the gray structures considered both bulk solids and molecules, while their synthesis is extremely difficult. The present study describes a new template synthesis method for quasi-sub-nanosized materials using a combination of coordination chemistry and polymer chemistry. Utilizing self-assembly of guest basic phenylazomethine dendron units onto host acidic core units with six tritylium cations, the dendron-assembled supramolecules were constructed easily and quantitatively without costly techniques. This huge supramolecular capsule accumulating multiple acidic rhodium salts in its basic ligands enabled a precise synthesis of rhodium particles via formation of multinuclear complexes. The obtained particles (Rh84 , ≈1.5 nm) have particle sizes within 1-3 nm range and were larger than conventional sub-nanoparticles (Rh14 , ≈0.85 nm), therefore the precise template synthesis of quasi-sub-nanoparticles was successfully demonstrated.

Alloying at a Subnanoscale Maximizes the Synergistic Effect on the Electrocatalytic Hydrogen Evolution.

Bonding dissimilar elements to provide synergistic effects is an effective way to improve the performance of metal catalysts. However, as the properties become more dissimilar, achieving synergistic effects effectively becomes more difficult due to phase separation. Here we describe a comprehensive study on how subnanoscale alloying is always effective for inter-elemental synergy. Thirty-six combinations of both bimetallic subnanoparticles (SNPs) and nanoparticles (NPs) were studied systematically using atomic-resolution imaging and catalyst benchmarking based on the hydrogen evolution reaction (HER). Results revealed that SNPs always produce greater synergistic effects than NPs, the greatest synergistic effect was found for the combination of Pt and Zr. The atomic-scale miscibility and the associated modulation of electronic states at the subnanoscale were much different from those at the nanoscale, which was observed by annular-dark-field scanning transmission electron microscopy (ADF-STEM) and X-ray photoelectron spectroscopy (XPS), respectively.

Highly Dispersed Molybdenum Oxycarbide Clusters Supported on Multilayer Graphene for the Selective Reduction of Carbon Dioxide.

Molybdenum oxycarbide clusters are novel nanomaterials that exhibit attractive catalytic activity; however, the methods for their production are currently very restrictive. This work represents a new strategy for the creation of near-subnanometer size molybdenum oxycarbide clusters on multilayer graphene. To adsorb Mo-based polyoxometalates of the type [PMo12 O40 ]3- as a precursor for Mo oxycarbide clusters, the novel tripodal-phenyl cation N,N,N-tri(4-phenylbutyl)-N-methylammonium ([TPBMA]+ ) is synthesized. [TPBMA]+ exhibits superior adsorption on multilayer graphene compared to commercially available cations such as tetrabutylammonium ([nBu4 N]+ ) and tetraphenylphosphonium ([PPh4 ]+ ). Using [TPBMA]+ as an anchor, highly dispersed precursor clusters (diameter: 1.0 ± 0.2 nm) supported on multilayer graphene are obtained, as confirmed by high-resolution scanning transmission electron microscopy. Remarkably, this new material achieves the catalytic reduction of CO2 to selectively produce CO (≈99.9%) via the reverse water-gas-shift reaction, by applying carbothermal hydrogen reduction to generate Mo oxycarbide clusters in situ.

Low-Temperature H2 Reduction of Copper Oxide Subnanoparticles.

Invited for the cover of this issue is Kimihisa Yamamoto and co-workers at Tokyo Institute of Technology and International Christian University. The image depicts enhanced reactivity of the copper oxide subnanoparticles under low-temperature conditions. Read the full text of the article at 10.1002/chem.202100508.

Low-Temperature H2 Reduction of Copper Oxide Subnanoparticles.

Subnanoparticles (SNPs) with sizes of approximately 1 nm are attractive for enhancing the catalytic performance of transition metals and their oxides. Such SNPs are of particular interest as redox-active catalysts in selective oxidation reactions. However, the electronic states and oxophilicity of copper oxide SNPs are still a subject of debate in terms of their redox properties during oxidation reactions for hydrocarbons. In this work, in situ X-ray absorption fine structure (XAFS) measurements of Cu28 Ox SNPs, which were prepared by using a dendritic phenylazomethine template, during temperature-programmed reduction (TPR) with H2 achieved lowering of the temperature (T50 =138 °C) reported thus far for the CuII →CuI reduction reaction because of Cu-O bond elongation in the ultrasmall copper oxide particles.

Development of Highly Sensitive Raman Spectroscopy for Subnano and Single-Atom Detection.

Direct detection and characterisation of small materials are fundamental challenges in analytical chemistry. A particle composed of dozens of metallic atoms, a so-called subnano-particle (SNP), and a single-atom catalyst (SAC) are ultimate analysis targets in terms of size, and the topic is now attracting increasing attention as innovative frontier materials in catalysis science. However, characterisation techniques for the SNP and SAC adsorbed on substrates requires sophisticated and large-scale analytical facilities. Here we demonstrate the development of an ultrasensitive, laboratory-scale, vibrational spectroscopic technique to characterise SNPs and SACs. The fine design of nano-spatial local enhancement fields generated by the introduction of anisotropic stellate-shaped signal amplifiers expands the accessibility of small targets on substrates into evanescent electromagnetic fields, achieving not only the detection of isolated small targets but also revealing the effects of intermolecular/interatomic interactions within the subnano configuration under actual experimental conditions. Such a development of "in situ subnano spectroscopy" will facilitate a comprehensive understanding of subnano and SAC science.

Silver in the Center Enhances Room-Temperature Phosphorescence of a Platinum Sub-nanocluster by 18 Times.

There has been controversy surrounding the roles of the metal core (metal-metal interaction) and the shell (metal-ligand interaction) in photoluminescence of ligand-protected metal nanoclusters. We have discovered aggregation-induced room-temperature phosphorescence of a platinum-thiolate complex and its silver ion inclusion complex (a silver-doped platinum sub-nanocluster). The inclusion of silver ion boosted the photoluminescent quantum yield by 18 times. Photophysical measurements indicate that the rate of nonradiative decay was slower for the silver-doped platinum sub-nanocluster. DFT calculations showed that the LUMO, which had the main contribution from Ag s-orbital and Pt d-orbitals, played a critical role in suppressing the structural distortion at the excited state. This work will hopefully stimulate more research on designing strategies based on molecular orbitals of atomicity-precise luminescent multimetallic nanoclusters.

Quantum Materials Exploration by Sequential Screening Technique of Heteroatomicity.

Subnanoparticles (SNPs) exhibit unique properties and functions due to their extremely small particle sizes which extend into the quantum scale. Although the synthesis of SNPs requiring precise control of atomicity and composition has not been accomplished, we recently developed an atom-hybridization method (AHM) that realizes such atomic-level control using a macromolecular template. As a next step in the quest for innovative quantum materials, the practical creation of functional subnanomaterials will become a central subject. In this study, we established a new screening technique for functional SNPs by focusing on the simple indium-tin binary system with sequential compositions using the latest AHM. As a result, it was revealed that a thermodynamically unstable indium species was produced only at a certain composition leading to a durable luminescent function. Such a phenomenon in subnanosized substances will play an important role in the development of the as-yet-unknown field of quantum materials.

Metallic Tungsten Nanoparticles That Exhibit an Electronic State Like Carbides during the Carbothermal Reduction of WCl6 by Hydrogen.

Carbothermal hydrogen reduction (CHR) is a unique dry chemical process used to fabricate metals and carbides on carbon supports. In this study, a stepwise CHR of WCl6 on a graphite support is demonstrated for the first time. Powder X-ray diffraction studies revealed that, at 773 K, metallic tungsten nanoparticles are produced, whereas, at 1073 K, the metastable W2C phase is generated rather than the thermodynamically stable WC phase. X-ray photoelectron spectroscopy and X-ray absorption near edge structure studies showed that the chemical state of the W nanoparticles simultaneously exhibits metallic W(∼0) and carbide W(δ+) character. The obtained results suggest that, although electronic interactions exist between the metallic W atoms and the graphite support, the body-centered cubic structure of the metallic tungsten is maintained, confirmed by the extended X-ray absorption fine structure. In addition, high-resolution scanning transmission electron microscopy observations revealed that the W nanoparticles exhibit a thin flattened shape on the support. These results support the notion that the mechanism for the formation of the W nanoparticles during the CHR is influenced by the electronic interactions between the W nanoparticles and the graphite support. Our work thus suggests that the combination of early-transition-metal atoms and carbon-based supports would afford modulatable electronic systems though the electronic interactions.

Selective Hydroperoxygenation of Olefins Realized by a Coinage Multimetallic 1-Nanometer Catalyst.

The science of particles on a sub-nanometer (ca. 1 nm) scale has attracted worldwide attention. However, it has remained unexplored because of the technical difficulty in the precise synthesis of sub-nanoparticles (SNPs). We recently developed the "atom-hybridization method (AHM)" for the precise synthesis of SNPs by using a suitably designed macromolecule as a template. We have now investigated the chemical reactivity of alloy SNPs obtained by the AHM. Focusing on the coinage metal elements, we systematically evaluated the oxidation reaction of an olefin catalyzed by these SNPs. The SNPs showed high catalytic performance even under milder conditions than those used with conventional catalysts. Additionally, the hybridization of multiple elements enhanced the turnover frequency and the selectivity for the formation of the hydroperoxide derivative. We discuss the unique quantum-sized catalysts providing generally unstable hydroperoxides from the viewpoint of the miniaturization and hybridization of materials.

Single-Crystalline Optical Microcavities from Luminescent Dendrimers.

Microcrystallites are promising minute mirrorless laser sources. A variety of luminescent organic compounds have been exploited along this line, but dendrimers have been inapplicable owing to their fragility and extremely poor crystallinity. Now, a dendrimer family that overcomes these difficulties is presented. First-, second-, and third-generation carbazole (Cz) dendrimers with a carbon-bridged oligo(phenylenevinylene) (COPV2) core (GnCOPV2, n=1-3) assemble to form microcrystals. The COPV2 cores align uni/bidirectionally in the crystals while the Cz units in G2- and G3COPV2 align omnidirectionally. The dendrons work as light-harvesting antennas that absorb non-polarized light and transfer it to the COPV2 core, from which a polarized luminescence radiates. Furthermore, these crystals act as laser resonators, where the lasing thresholds are strongly coupled with the crystal morphology and the orientation of COPV2, which is in contrast with the conventional amorphous dendrimers.

Solution Phase Mass Synthesis of 2D Atomic Layer with Hexagonal Boron Network.

Borophene and the analogs are attractive 2D-materials showing unique mechanical and electronic properties. In this study, the bottom-up synthesis of an atomic boron network possessing a completely planar skeleton was achieved from KBH4. The borophene-analog was stabilized by oxygen atoms positioned on the same plane, providing holes and the anionic state of the layer. Potassium cations between the layers enabled crystalline stacking of the layers, as well as dissolution in solvents as atomically thin layers. The conductivity measurements revealed the electronic feature. Unlike the interplane conducting property, almost zero activation energy like a metal was suggested from the in-plane measurement.

Aerobic Toluene Oxidation Catalyzed by Subnano Metal Particles.

Subnanocatalysts (SNCs) containing various noble metals (Cu, Ru, Rh, Pd, or Pt) with sizes of approximately 1 nm were synthesized using dendritic poly(phenylazomethine)s as a macromolecular template. These materials exhibit high catalytic performance during toluene oxidation without the use of harmful solvents or explosive oxidants, resulting in the formation of valuable organic products, including benzoic acid as the major product. In particular, Pt19 SNC with a narrow particle size distribution exhibits extraordinary catalytic activity, with a turnover frequency of 3238 atom-1 h-1 , which is 1700 times greater than that obtained by commercial Pt/C catalysts.

Epitaxially Grown Ultra-Flat Self-Assembling Monolayers with Dendrimers.

Mono-molecular films formed by physical adsorption and dendrimer self-assembly were prepared on various substrate surfaces. It was demonstrated that a uniform dendrimer-based monolayer on the subnanometer scale can be easily constructed via simple dip coating. Furthermore, it was shown that an epitaxially grown monolayer film reflecting the crystal structure of the substrate (highly ordered pyrolytic graphite (HOPG)) can also be formed by aligning specific conditions.

A Polyaromatic Molecular Clip That Enables the Binding of Planar, Tubular, and Dendritic Compounds.

By the covalent linkage of two bent bisanthracene amphiphiles with a biphenyl spacer bearing hydrophilic pendants, we synthesized a new molecular clip with a C-shaped conformation. The molecular clip provides an acyclic, open cavity surrounded by four anthracene panels in water. In contrast to previous clip- and tweezers-like compounds as well as cage-shaped compounds, the C-shaped polyaromatic cavity displays unusual wide-ranging capturing abilities toward not only planar perylene-based pigments and cylindrical single-walled carbon nanotubes but also highly branched macromolecules (carbazole dendrimers).

Reducing Capsule Based on Electron Programming: Versatile Synthesizer for Size-Controlled Ultra-Small Metal Clusters.

Controlled reducing capsules with a specific number of reducing electrons were achieved by appropriately placed BH3 units in the dendritic polyphenylazomethines (DPAs). Using the 1:1 coordination fashion on their basic branches with radius affinity gradient, the 4th generation DPA (DPAG4) possessing four BH3 units in the central positions was prepared as a template synthesizer for size-controlled ultra-small metal clusters. This was well-demonstrated by reduction of Ag, Pt, and other metal ions resulting in monodispersed ultra-small clusters.