Takane Imaoka

Controlled storage of ferrocene derivatives as redox-active molecules in dendrimers

Dendritic polyphenylazomethines (DPA) could encapsulate ferroceniums by complexation of the electron-donating skeleton of the DPA imines. Upon addition of ferroceniums to a series of dendritic polyphenylazomethines (DPAGX, where X is the generation number, X = 1-4), the UV-vis spectra showed changes in a manner similar to that observed for the complexation of metal ions with DPAGX. Stepwise shifts in the isosbestic point were consistently observed with the number of imine groups in the first and second layers of the generation-4 dendrimer (DPAG4). DPAG2 and DPAG3 were also found to trap 6 equiv of ferroceniums. To investigate the complexation, UV-vis spectroscopy, (57)Fe Mossbauer spectroscopy, electrospray ionization-mass spectroscopy (ESI-MS), cyclic voltammetry (CV), and fluorescence spectroscopy were performed. We confirmed that neutral ferrocenes cannot complex with the imine group while ferroceniums can. Utilizing the redox property of ferrocenes, we were able to electrochemically control the encapsulation and release of ferrocenes into the DPA in a manner similar to redox-responsive proteins such as ferritin. In addition to ferrocenes, oligoferrocenes could also be trapped in the DPA. The biferrocene cation(1+) was particularly suitable for electrochemical switching due to its stable mixed valence condition. The terferrocene dication(2+) encapsulated into DPAG4 could be fabricated into a thin film, which exhibited the near-infrared absorption of an intervalence charge-transfer (IV-CT) band, pointing the way toward the use of such systems in material science.

Finding the Most Catalytically Active Platinum Clusters With Low Atomicity

On a subnanometer scale, an only one-atom difference in a metal cluster may cause significant transitions in the catalytic activity due to the electronic and geometric configurations. We now report the atomicity-specific catalytic activity of platinum clusters with significantly small atomicity, especially below 20. The atomic coordination structure is completely different from that of the larger face-centered cubic (fcc) nanocrystals. Here, an electrochemical study on such small clusters, in which the atomicity ranged between 12 and 20, revealed Pt19 as the most catalytically active species. In combination with a theoretical study, a common structure that leads to a high catalytic performance is proposed.

Formation of a Pt12 Cluster by Single‐Atom Control That Leads to Enhanced Reactivity: Hydrogenation of Unreactive Olefins

A platinum subnanocluster catalyst composed of 12 atoms was synthesized using a phenylazomethine dendrimer, which can assemble twelve PtCl4 units by stepwise complexation, followed by reduction to Pt(0). Unreactive olefins that were not activated by conventional 2 nm Pt nanoparticles were successfully hydrogenated by the subnanocluster. EWG = electron-withdrawing group.

Macromolecular semi-rigid nanocavities for cooperative recognition of specific large molecular shapes

Molecular shape recognition for larger guest molecules (typically over 1 nm) is a difficult task because it requires cooperativity within a wide three-dimensional nanospace coincidentally probing every molecular aspect (size, outline shape, flexibility and specific groups). Although the intelligent functions of proteins have fascinated many researchers, the reproduction by artificial molecules remains a significant challenge. Here we report the construction of large, well-defined cavities in macromolecular hosts. Through the use of semi-rigid dendritic phenylazomethine backbones, even subtle differences in the shapes of large guest molecules (up to ~2 nm) may be discriminated by the cooperative mechanism. A conformationally fixed complex with the best-fitting guest is supported by a three-dimensional model based on a molecular simulation. Interestingly, the simulated cavity structure also predicts catalytic selectivity by a ruthenium porphyrin centre, demonstrating the high shape persistence and wide applicability of the cavity.

Enhancing the photoelectric effect with a potential-programmed molecular rectifier.

Dendrimer-based electron rectifiers were applied to photoconducting devices. A remarkable enhancement of the photocurrent response was observed when a zinc porphyrin as the photosensitizer was embedded in the dendritic phenylazomethine (DPA) architecture. The dendrimer-based sensitizer exhibited a 20-fold higher current response than the non-dendritic zinc porphyrin. In sharp contrast, a similar application of the dendrimer with poly(vinylcarbazole) as the electron donor resulted in a decreased response. This is consistent with the idea that the DPA facilitates electron transfer from the core to its periphery along a potential gradient, as predicted by density functional theory calculations.

Electrochemistry of Co–Ru hetero-dinuclear porphyrin complex in a Nafion matrix

The metallo porphyrins possessing four ionic substituents of (meso-tetrakis(N-methyl-4-pyridiniumyl)porphyrinato)ruthenium(II) (RuTMPyP) and (meso-tetrakis(4-sulfonatophenyl)porphyrinato)cobalt(II) (CoTPPS) spontaneously associate. The equilibrium constant of dissociation (Kd) of the Co–Ru porphyrin was estimated to be 5 × 10−7 mol dm−3 by Jobs titration. A single redox wave based on a 2-electron transfer was observed at 0.19 V s. SCE using cyclic voltammetry in a Nafion matrix. Potential-step chronocoulometry (PSCC) and potential-step chronoamperometry (PSCA) determined the kinetics of the electron transfer (k0 = 1.47 cm s−1) from the electrode to the complex in the Nafion film, and the apparent diffusion coefficient of electrons (Dapp = 8 × 10−8 cm2 s−1) in the Nafion film. Dimerization of the Co and Ru porphyrins results in an accelerated 2-electron transfer because the electrons are transferred ia overlapping π orbitals on the porphyrin ring. The film containing the dinuclear porphyrins shows sufficient catalytic activity toward dioxygen with a 4-electron reduction, based on successive electron transfer from the electrode to dioxygen.

Solution-phase synthesis of Al 13 − 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.Superatoms—clusters that exhibit some of the properties of elemental atoms—could serve as building blocks for functional materials, but their synthesis outside of the gas phase is highly challenging. Here, the authors use a dendrimer template to successfully produce Al13− in solution.

Preparation of Carbon Nanotubes Using Iron Oxide(III) Nanoparticles Size-Controlled by Phenylazomethine Dendrimers

Dendritic polyphenylazomethine (DPAG4) can precisely incorporate any number of metals onto the imine sites. After spin casting of the solution of the complex between DPAG4 and iron trichloride(III) (FeCl3) on the substrate, iron oxide(III) nano dots could be prepared by hydrolysis of the complex fabricated on the substrate. The diameters and heights of the iron oxide could be controlled based on the molar amount of incorporated FeCl3 in DPAG4. Using these iron oxide nano dots as the catalyst, we attempted to fabricate carbon nanotubes on the bare silicon substrate by reacting with a mixed gas at high temperature.

Control of Single Molecular Nanodot Patterns of Phenyl Azomethine Dendrimers by Statistical Simulation

Precisely synthesized subnanometer particles of metals or metal oxides can be prepared using dendritic polyphenyl azomethines as the template. With a goal of their arrays to a surface using a simple and quick process, such as spin-casting, statistical analyses were applied to a nanodot array of the dendrimers to obtain the relationship between the experimental condition and the results such as size, spacing, or its standard deviations. The dot patterns of a single molecular dendrimer on a substrate were able to be predicted with numerical values of the experimental parameters associated with the spin coat (concentration of the dendrimer, physical properties of solvent, the spin coating recipe, temperature of the solution, relative humidity (RH)) as the inputs for the statistical analysis.

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.