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.

Progress in Molecular Nanoarchitectonics and Materials Nanoarchitectonics

Although various synthetic methodologies including organic synthesis, polymer chemistry, and materials science are the main contributors to the production of functional materials, the importance of regulation of nanoscale structures for better performance has become clear with recent science and technology developments. Therefore, a new research paradigm to produce functional material systems from nanoscale units has to be created as an advancement of nanoscale science. This task is assigned to an emerging concept, nanoarchitectonics, which aims to produce functional materials and functional structures from nanoscale unit components. This can be done through combining nanotechnology with the other research fields such as organic chemistry, supramolecular chemistry, materials science, and bio-related science. In this review article, the basic-level of nanoarchitectonics is first presented with atom/molecular-level structure formations and conversions from molecular units to functional materials. Then, two typical application-oriented nanoarchitectonics efforts in energy-oriented applications and bio-related applications are discussed. Finally, future directions of the molecular and materials nanoarchitectonics concepts for advancement of functional nanomaterials are briefly discussed.

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.

Composition-defined nanosized assemblies that contain heterometallic early 4d/5d-transition-metals

The controlled assembly of early transition metals remains a challenging research target, especially with respect to the generation of heterometallic molecules and nanomaterials. In this study, metal chlorides of the early 4d/5d-transition-metals, i.e., ZrCl₄, NbCl₅, MoCl₅, HfCl₄, TaCl₅, and WCl₆, were stoichiometrically introduced into a tetraphenylmethane-core dendritic-phenylazomethine generation 4 dendrimer in the presence of an optimal amount of organic ligands such as pyridine and 3-chloropyridine. The coordinative interactions between the metal chlorides and the imines in the dendrimers indicated a positive correlation for the Lewis acidity of the metals. Moreover, it was clearly demonstrated for the first time that heterometallic assemblies of defined composition contain four kinds of early 4d/5d-transition-metals, such as Ta^V, Nb^V, Mo^V, and Zr^IV, which was confirmed by UV-vis titration, XPS, and HAADF-STEM/EDS measurements. The results of this study should provide access to new routes to produce nanomaterials composed of heterometallic early 4d/5d-transition metals.

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 potential gradient along the layer-by-layer architecture for electron transfer rectification

Electrochemical and photochemical measurements demonstrated that dendritic phenylazomethines, which can make complexes with SnCl2 by a stepwise process, only permit an outbound electron transfer. The unique dendrimer effect allows efficient production of photo-generated radical ion pairs by suppressing their charge recombination. In sharp contrast, the phenylenevinylene or benzylether dendrimers, which lack a heteroatom or π-conjugation, did not exhibit such non-symmetric characters.

Photoactive Dendrimer for Water Photoreduction: A Scaffold to Combine Sensitizers and Catalysts

We report on the synthesis and characterization of platinum nanoparticles (PtNps) inside the cavities of a PAMAM dendrimer decorated with [Ru(bpy)3](2+) units at the periphery. The phosphorescent ruthenium complexes are used as signaling units of the Pt(2+) complexation in the dendritic architecture and as photosensitizer units in the photocatalytic production of H2 from water. This is the first example of water photoreduction in which the catalyst and the sensitizer are anchored on a dendritic molecular scaffold. This study provides a new outlook in the design of new supramolecular systems and materials for developing artificial photosynthesis.

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.

Catalytic dehalogenation enhanced by appending dendritic ligands on cobalt porphyrins

The dehalogenation activities of cobalt porphyrin catalysts with dendritic phenylazomethine units on the meso-positions were studied. Tetrachloroethylene was used as a substrate. By applying samarium iodide as a reducing agent, the tetrachloroethylene conversion into trichloroethylene and dichloroethylene was observed in the presence of the cobalt porphyrin catalyst. The turnover frequency was 1.70 min−1 when a cobalt porphyrin connected with dendritic phenylazomethines (3 generations) was employed as the catalyst. While the turnover frequency was almost the same as that for a non-dendritic porphyrin catalyst (1.57 min−1 by cobalt tetraphenylporphyrin), a significant increase in the turnover frequency was observed when samarium (3.43 min−1) or terbium trifluoromethane sulfonate (8.57 min−1) binds to the phenylazomethine units of the dendrimer. Because these metal ions ( Sm 3+ or Tb 3+) are positioned in a space between the encapsulated catalytic center and the solvent phase, they can act as electron mediators that relay electrons from the reducing agent (samarium iodide) to the catalytic center (cobalt porphyrin). This facile electron transfer would provide an enhancement in the catalytic activity for the tetrachloroethylene reduction leading to dehalogenation.

Modulation of the aerobic oxidative polymerization in phenylazomethine dendrimers assembling copper complexes

The aerobic oxidative polymerization of phenol derivatives can provide poly(phenylene oxide)s, which are known as engineering plastics. This oxidation can be carried out with atmospheric oxygen molecules as the oxidizing reagent in the presence of copper complexes as the catalyst; however, stoichiometric or excess amounts of bases are also generally required. By using a phenylazomethine dendrimer complexed with several equivalent amounts of copper chloride, the additive (base)-free polymerization of 2,6-difluorophenol was successful with a very small amount of the catalyst (0.7 mol% of copper for the monomer) because the dendrimer was composed of many Schiff base units, affording a base and catalyst (copper complex) condensed reaction field. The resulting polymer was nearly linear and the molecular weight was very high. When the equimolar amount of the copper complex in one dendrimer molecule was increased, the polymer obtained under this reaction condition was rather branched, resulting in a higher glass transition temperature.

Dendrimers as therapeutic agents: a systematic review

Objectives

Dendrimers by virtue of their therapeutic value have recently generated enormous interest among biomedical scientists. This review describes the therapeutic prospects of the dendrimer system.

Key findings

Their bioactivity suggests them to be promising therapeutic agents, especially in wound healing, bone mineralisation, cartilage formation and tissue repair, and in topical treatments to prevent HIV transmission. Findings also demonstrate their potential as anti-prion, anti-Alzheimer's, anticoagulant, antidote, anti-inflammatory and anticancer agents. One of the dendrimer-based formulations with activity against herpes simplex virus (VivaGel from Starpharma) has successfully completed phase I clinical trials and is expected to be available on the market soon.

Summary

All reports cited in this review demonstrate the use of dendrimers as medical therapeutics in different ailments. The review focuses on the current state of therapeutic potential of the dendrimer system.

Size-specific catalytic activity of platinum clusters enhances oxygen reduction reactions

Colloidal platinum nanoparticles with diameters of 2-5 nm on carbon supports are currently regarded as the best catalysts for the oxygen reduction reaction. However, the particle size is limited by the conventional preparation methods that are used to synthesize small platinum particles; the inherent activity of ultrasmall nanoparticles has not yet been revealed. We present a practical synthesis for ultrafine subnanometre platinum clusters using a spherical macromolecular template with no disorder in molecular weight or structure. The template, a phenylazomethine dendrimer, offers control of the number of metal complexes in an assembly through stepwise complexation, allowing the complexes to accumulate in discrete nano-cages. Subsequent reduction of Pt(IV) chloride to Pt(0) results in the formation of platinum clusters composed of a defined number of atoms. As a result of exceptionally small particle size, the clusters exhibit very high catalytic activity for the four-electron reduction of oxygen molecules.

Hexadecameric self-assembled dendrimers built from 2'-deoxyguanosine derivatives

Herein we describe the construction of hexadecameric self-assembled dendrimers (SADs) using a series of dendronized 8-(m-acetylphenyl)-2′-deoxyguanosine (mAG) subunits. The azido-substituted mAG subunits were covalently linked to alkynyl polyester dendrons using a copper-catalyzed 1,3-dipolar cycloaddition reaction. Discrete SADs are formed with high fidelity and thermal stability even with the increased steric hindrance offered by the dendrons.

Investigation of a Molecular Morphology Effect on Polyphenylazomethine Dendrimers; Physical Properties and Metal-Assembling Processes

A series of novel dendritic polyphenylazomethines (DPA) with asymmetric morphologies was synthesized. Their physical properties, such as encapsulating effect, molecular dynamics, and metal assembly, are strongly dependent on the entire conformation of the molecules. The most important property is layer-by-layer metal assembly in the dendrimer structure from the core to the outside. Bis- and tris-substituted DPAs of the fourth generation also act as frameworks for stepwise assembly of a metal component (SnCl2), like the fully substituted symmetric DPA. However, extensive investigation of metal assembly in specific DPAs revealed that they do not follow the stepwise process. The molecular density calculated from the experimental hydrodynamic volume indicated that bis- and tris-substituted DPAs with asymmetric morphology still retain a free space similar to that of fully substituted symmetric DPA. The monosubstituted DPA, however, displayed a slightly higher density (smaller space) than the other DPAs. The experimental results suggest a bent conformation of the dendrimer in which the core moiety is folded into the dendron structure. In addition, the molecular dynamics were probed by means of the 1H NMR signals of the porphyrin core. It was demonstrated that the conformation is not fixed at room temperature in solvated DPAs, especially in monosubstituted DPA. A similar observation was for the smaller DPAs (third generations) with asymmetric morphologies. These dendrimers do not follow the stepwise complexation process. The structures of bis- and tris-substituted dendrimers which accurately follow the stepwise process are fixed. These observations provide a new insight into the finely controlled metal-assembly chemistry of dendritic macromolecules, and a rigid and fixed conformation is one of important factors for their unique properties.