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.

Poly-phenylene jacketed tailor-made dendritic phenylazomethine ligand for nanoparticle synthesis

Synthesizing metal clusters with a specific number of atoms on a preparative scale for studying advanced properties is still a challenge. The dendrimer templated method is powerful for synthesizing size or atomicity controlled nanoparticles. However, not all atomicity is accessible with conventional dendrimers. A new tailor-made phenylazomethine dendrimer (DPA) with a limited number of coordination sites (n = 16) and a non-coordinating large poly-phenylene shell was designed to tackle this problem. The asymmetric dendron and adamantane core four substituted dendrimer (PPDPA16) were successfully synthesized. The coordination behavior confirmed the accumulation of 16 metal Lewis acids (RhCl₃, RuCl₃, and SnBr₂) to PPDPA16. After the reduction of the complex, low valent metal nanoparticles with controlled size were obtained. The tailor-made dendrimer is a promising approach to synthesize a variety of metal clusters with desired atomicity.

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.

One-Pot, In-Situ Synthesis of 8-Armed Poly(Ethylene Glycol)-Coated Ag Nanoclusters as a Fluorescent Sensor for Selective Detection of Cu2

Fluorescent nanomaterials, such as quantum dots, have developed rapidly in recent years and have been significantly developed. Herein, we demonstrate a facile, one-pot, and in-situ synthesis strategy to obtain fluorescent silver nanoclusters (AgNCs) coated with eight-armed poly (ethylene glycol) polymers (8PEG-AgNCs) via a direct gel-mediated process. During the synthesis, ammonium (NH₃) served as the crosslinker for the gel formation via a amine-type Michael addition reaction. This hydrogel can be used as a template to synthesize AgNCs using its volume-limiting effect. The in-situ generation of AgNCs takes place inside the nanocages of the formed gels, which guarantees the homogenous distribution of AgNCs in the gel matrix, as well as the efficient coating of PEG on the nanoclusters. After the degradation of gels, the released 8PEG-AgNCs nanohybrids showed strong blue fluorescence and exhibited long-term stability in aqueous solution for nearly one year. Results showed that the fabricated sensor revealed excellent fluorescent sensitivity for the selective detection of Cu²⁺ with a detection limit of 50 nM and a wide linear detection range of 5–100 μM. It is proposed that the greater cross-linking density leads to smaller gel pores and allows the synthesis of AgNCs with fluorescent properties. These results indicate that this novel hydrogel with certain biodegradation has the potential to be applied as a fluorescent sensor for catalytic synthesis, fluorescence tracing in cells, and fluorescence detection fields. Meanwhile, the novel design principle has a certain versatility to accelerate the development and application of other kinds of metal nanoclusters and quantum dots.

Superatomic Gallium Clusters in Dendrimers: Unique Rigidity and Reactivity Depending on their Atomicity

Superatoms have been investigated due to their possible substitution for other elements. The solution-phase synthesis of superatoms has attracted attention to realize the availability of superatoms. However, the previous approach is basically limited to the formation of a single cluster. Here, superatoms are investigated and the number of valence electrons in these superatoms is changed by designing the number of gallium atoms present. Based on the dendrimer template method, clusters consisting of 3, 12, 13, and other numbers of atoms have been synthesized. The halogen-like superatomic nature of Ga₁₃ is structurally and electrochemically observed as completely different to the other clusters. The gallium clusters of 13 and 3 atoms, which can fill the 2P and 1P superatomic orbitals, respectively, exhibit different reactivities. The 3-atom gallium cluster is suggested as being reduced to Ga₃ H₂ ^- due to the lower shift of energy levels in the unoccupied orbitals. The results for these gallium clusters provide candidates for superatoms.

Atom-hybridization for synthesis of polymetallic clusters

The chemistry of metal clusters on the sub-nanometer scale is not yet well understood because metal clusters, especially multimetallic clusters, are difficult to synthesize with control over size and composition. The template synthesis of multimetallic sub-nanoclusters is achieved using a phenylazomethine dendrimer as a macromolecular template. Its intramolecular potential gradient allows the precise uptake of metal precursor complexes containing up to eight elements on the template. The usefulness of this method is demonstrated by synthesizing multimetallic sub-nanoclusters composed of five elements (Ga₁In₁Au₃Bi₂Sn₆). The size and composition of this cluster can be precisely controlled and the metals involved are alloyed with each other. This approach provides the ability to easily blend different metals in various combinations to create new materials on the sub-nanometer scale, which will lead to the development of a new area in the field of chemistry.

Multimetallic clusters are difficult to synthesize with control over elemental composition and organization. Here, the authors use dendrimers to precisely template the formation of five-element sub-nanoclusters, providing an elegant route to otherwise-inaccessible multinary compounds.

Solution-phase synthesis of Al13- using a dendrimer template

Superatoms, clusters that mimic the properties of elements different to those of which they are composed, have the potential to serve as building blocks for unprecedented materials with tunable properties. The development of a method for the solution-phase synthesis of superatoms would be an indispensable achievement for the future progress of this research field. Here we report the fabrication of aluminum clusters in solution using a dendrimer template, producing Al13-, which is the most well-known superatom. The Al13- cluster is identified using mass spectrometry and scanning transmission electron microscopy, and X-ray photoelectron spectroscopy is used to measure the binding energies. The superatomic stability of Al13- is demonstrated by evaluating its tendency toward oxidation. In addition, the synthesis of Al13- in solution enables electrochemical measurements, the results of which suggest oxidation of Al13-. This solution-phase synthesis of Al13- superatoms has a significant role for the experimental development of cluster science.

Nanoparticle characterization based on STM and STS

Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) can characterize intriguing nanoparticle properties towards solid-state nanodevices.

Precision synthesis of subnanoparticles using dendrimers as a superatom synthesizer

Classical metal-based nanomaterials come in two prominent types: a mononuclear or multinuclear complex chemically stabilized by organic ligands or a nanoparticle (also called a nanorod, nanosheet, or nanocrystal) physically stabilized by inorganic or polymer supports. Over the last decade, a class of superatoms that lies between these categories of materials has attracted attention because their properties are dramatically different from those typically ascribed to their component elements. Typically the superatoms include a specific, low number of metallic atoms. Because a one-atom difference can alter the properties of these superatoms, their synthesis must be ultraprecise, requiring one-atom resolution. To date, researchers have only been able to prepare monodisperse superatoms using gas-phase synthesis followed by purification through a flight tube. Though this technique provides monodisperse superatoms, it does not allow researchers to produce them in large quantites. Other researchers have proposed ligand-assisted liquid-phase synthesis as an alternative, but this technique is only useful for a few stable "magic number" clusters. Recently researchers have developed a new approach for the synthesis of superatoms that employs a novel class of molecular templates, which can define the number of metal ions or salts precisely. As a result, researchers can now synthesize nanoparticles or even subnanoparticles successfully. A dendrimer-type template has proven to be especially useful for ultraprecise control of the atomicity of the product, but it works with a full range of metal elements. In this Account, we highlight recent advances in the precise preparation of metal-assembling complexes using the dendrimer as a template. Next we discuss the selective assembly of subnanoparticles that utilize the dendrimer as a superatom synthesizer. The resulting subnanoparticles are almost monodisperse, and as a result, some of them exhibited distinctive characteristics based on their atomicity. For example, because of the quantum-size effect, the reduction in particle size of TiO2 and other metal-oxide subnanoparticles led to a significant shift in the band-gap energy. In addition, a miniaturized platinum particle less than 1 nm in diameter showed unexpectedly high catalytic activity for the oxygen reduction reaction (ORR) and other related reactions. Of particular note, in all these examples, this substantial change in their properties arose out of a single-atom difference in the atomicity. These results suggest that next-generation subnanoparticle design could play an important role in new materials and offer an additional palette of physical and chemical properties for new applications.

Magic number Pt13 and misshapen Pt12 clusters: which one is the better catalyst?

A relationship between the size of metal particles and their catalytic activity has been established over a nanometer scale (2-10 nm). However, application on a subnanometer scale (0.5-2 nm) is difficult, a possible reason being that the activity no longer relies on the size but rather the geometric structure as a cluster (or superatomic) compound. We now report that the catalytic activity for the oxygen reduction reaction (ORR) significantly increased when only one atom was removed from a magic number cluster composed of 13-platinum atoms (Pt13). The synthesis with an atomic-level precision was successfully achieved by using a dendrimer ligand as the macromolecular template strictly defining the number of metal atoms. It was quite surprising that the Pt12 cluster exhibited more than 2-fold catalytic activity compared with that of the Pt13 cluster. ESI-TOF-mass and EXAFS analyses provided information about the structures. These analyses suggested that the Pt12 has a deformed coordination, while the Pt13 has a well-known icosahedral atomic coordination as part of the stable cluster series. Theoretical analyses based on density functional theory (DFT) also supported this idea. The present results suggest potential activity of the metastable clusters although they have been "missing" species in conventional statistical synthesis.

New Protocols for the Synthesis of Stable Ag and Au Nanocluster Molecules

"Catching" metals in the nonmetallic form in solution, as they grow to bulk, is one of the most exciting areas of contemporary materials research. A new kind of stabilization to catch the nonmetallic form of noble metals with small thiols has evolved as an exciting area of synthesis during the past decade. Gold clusters stay in the frontline of this research, yielding new "molecules" composed of a few to several hundreds of atoms. By taking guidelines from gold cluster research, various new protocols for silver nanoclusters were developed. In this Perspective, we highlight the recent advances on the synthesis of atomically precise silver, gold, and their alloy clusters with a special emphasis on silver. As a result of intense efforts of the recent past, clusters such as Ag7,8(SR)7,8, Ag7(-S-R-S-)4, Ag9(SR)7, Ag32(SR)19, Ag44(SR)30, Ag140(SR)53, Ag280(SR)140, and Ag152(SR)60 (SR and S-R-S refer to thiolate and dithiolate ligands, respectively) were added to the literature. Moreover, "silver-covered" and "gold-covered" alloy clusters have also been synthesized. Early reports of the crystallization of such clusters are available. Several of these clusters are shown to act as sensors, catalysts, and pesticide degradation agents, which suggests that these materials may find applications in daily life in the foreseeable future.