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Probing Stepwise Complexation in Phenylazomethine Dendrimers by a Metallo-Porphyrin Core

2005.09.14


T. Imaoka, R. Tanaka, S. Arimoto, M. Sakai, M. Fujii, K. Yamamoto
J. Am. Chem. Soc. 2005, 127, 13896-13905. Probing Stepwise Complexation in Phenylazomethine Dendrimers by a Metallo-Porphyrin Core A series of dendritic phenylazomethines (DPA), which have a meso-substituted zinc porphyrin core (DPAGX-ZnP, X = 1−4), were synthesized. Structural studies of these dendrimers were carried out using Tri-SEC (triple detection after size exclusion chromatography), intrinsic viscosity analysis, TEM (tunneling electron microscopy), and molecular modeling calculations by AM1. As a result, a sphere-like structure within a single-nanometer scale (Rh = 22 Å for DPAG4-ZnP) was observed. In addition, encapsulating effects by the DPA shell in the larger dendrimers were confirmed as fundamental properties, based on the UV−vis abosorption spectra, cyclic voltammograms, and 1H NMR spin−lattice relaxation times (T1). The DPAGX-ZnP acts as a multi-metal ion reservoir for SnCl2 and FeCl3. The generation-4 dendrimer (DPAG4-ZnP) can take up to 60 molar amounts of metal complexes around the porphyrin core. A quantitative study of the metal assembling reaction by UV−vis titration revealed stepwise layer-by-layer complexations from the inner imines nearest to the core to the surface. The redox behavior and fluorescence of the zinc porphyrin in these metal-assembled dendrimers also support the stepwise complexation of the metal ion. These analyses suggest that the finely assembled metal complexes in a dendrimer architecture strongly affect the electronic status of the porphyrin core. Results from transient absorption measurements strongly indicate a very fast electron transfer on a subpicosecond time scale between the core and assembled metal complexes.

Metal-Assembling Dendrimers with a Triarylamine Core and Their Application to a Dye-Sensitized Solar Cell

2005.08.26


N. Satoh, T. Nakashima, K. Yamamoto
J. Am. Chem. Soc. 2005, 127, 13030-13038. Metal-Assembling Dendrimers with a Triarylamine Core and Their Application to a Dye-Sensitized Solar Cell A series of charge-separable and hole-transporting phenylazomethine dendrimers with a triarylamine core are prepared and evaluated for use as a charge separator in dye-sensitized solar cells (DSSCs). Triphenylamine with dendric phenylazomethine (TPA-DPA) is prepared by synthesizing up to five generations of dendrons using a convergent method. The resultant dendrimer has a rigid sphere structure similar to globular protein, with a hydrodynamic radius of 2.43 nm. Electrochemical oxidation of the TPA core reveals that the dendron units in the dendrimer have 0.35 of the attenuation factor (β) in the electron transfer. Complexation of TPA-DPA with SnCl2 proceeds in stepwise fashion from the core to the terminal imine following the basicity gradient among imine groups in each dendron shell. DSSCs prepared by casting these dendrimers onto dye-sensitized TiO2 film exhibited a higher open-circuit voltage than the bare film through the suppression of back electron transfer. The generational growth of dendrons increases the radius of the dendrimer, resulting in a stronger association with I3- and higher open-circuit voltage with an increasing number of generations. Complexation with SnCl2 reduces the resistance of TPA-DPA and improves the fill factor. The energy conversion efficiency of the DSSC prepared using fifth-generation TPA-DPA is 21% higher than that for the bare film and, when complexed with SnCl2, provides a 34% improvement.

Fine Control of the Release and Encapsulation of Fe Ions in Dendrimers through Ferritin-like Redox Switching

2004.01.27


R. Nakajima, M. Tsuruta, M. Higuchi, K. Yamamoto
J. Am. Chem. Soc. 2004, 126, 1630-1631. Fine Control of the Release and Encapsulation of Fe Ions in Dendrimers through Ferritin-like Redox Switching Numerous dendrimers incorporating metal ions or clusters have received much attention as catalytic and drug delivery materials. We expanded the variety of metal ions that complex with DPA through a radial stepwise complexation to create novel organic−inorganic hybrid materials. As one of the most common and significant iron ions, Fe3+ was used. It was confirmed that iron ions, FeCl3, are coordinated to the imine groups of a spherical phenylazomethine dendrimer (DPA) in a stepwise radial fashion, which should make it possible to control the number and location of the Fe3+ ions incorporated into the dendrimers. Iron possesses very interesting properties such as magnetism, redox chemistry, and catalysis and is also one of the essential elements of our body. Here, we show the first successful attempt to control the biomimetic switching of iron ions' release/encapsulation in the dendrimer driven by their redox response of the Fe2+/Fe3+ couple, which might find uses as a drug delivery system.

Control of Stepwise Radial Complexation in Dendritic Polyphenylazomethines

2003.07.25


M. Higuchi, M. Tsuruta, H. Chiba, S. Shiki, K. Yamamoto
J. Am. Chem. Soc. 2003, 125, 9988-9997. Control of Stepwise Radial Complexation in Dendritic Polyphenylazomethines The fourth generation of a dendritic polyphenylazomethine (DPA G4) has 2, 4, 8, and 16 imine groups in the first, second, third, and fourth shells, respectively (total, 30 imine groups). DPA G4 can trap 30 equiv of SnCl2 molecules, because the imine group is complexed with SnCl2 at a ratio of 1:1. During addition of 30 equiv of SnCl2 to DPA G4, four shifts in the isosbestic point were observed in the UV−vis spectra, and the amount of SnCl2 added in each step is in agreement with the number of imine groups in each shell of DPA G4. This result shows that the complexation of the imine groups in DPA G4 with SnCl2 occurs stepwise in the order of the first, second, third, and fourth shells. The unique stepwise complexation was also observed in DPA G2 and G3 as two and three shifts of the isosbestic point, respectively. The stepwise complexation was supported by TEM, NMR, and a novel shell-selective reduction (SSR) method for imines. An expansion in the molecular size of DPA G4 by the complexation was revealed by molecular modeling and TEM measurements. The stepwise complexation is caused by the different basicity of the imine groups between the shells, which was supported by the chemical shifts of the peaks attributed to the imine carbons in the 13C NMR spectra. The gradients in the basicity were controlled by the introduction of electron-withdrawing or -releasing groups to the core of the dendrimers; the core imines were complexed last in DPAs having a 2,3,5,6-tetrafluoro or 2,5-dichlorophenyl core due to the low basicity of the core imines. The different complexation pattern was also clearly confirmed by the SSR method.

Novel Triarylamine Dendrimers as a Hole-Transport Material with a Controlled Metal-Assembling Function

2003.06.17


N. Satoh, J. Cho, M. Higuchi, K. Yamamoto
J. Am. Chem. Soc. 2003, 125, 8104-8105. Novel Triarylamine Dendrimers as a Hole-Transport Material with a Controlled Metal-Assembling Function A series of phenylazomethine dendrimers with a triarylamine core (TPA−DPA) were synthesized by dehydration using TiCl4. The complexation of the fourth genereration (G4) TPA−DPA with SnCl2 proceeds in not a random but a stepwise fashion from the core to the terminal imines of the G4 dendrimer. The molecular size of TPA−DPA G4 is larger than that of DPA G4 in THF solution and has a rigid sphere structure like a globular protein. Organic light-emitting diodes (OLEDs) were fabricated, and the EL performances of the devices using the TPA−DPA−metal complexes as the hole-transport materials are drastically increased (ca. 20 times) by metal complexation.

Metal Assembly in Novel Dendrimers with Porphyrin Cores

2002.12.13


T. Imaoka, H. Horiguchi, K. Yamamoto
J. Am. Chem. Soc. 2003, 125, 340-341. Metal Assembly in Novel Dendrimers with Porphyrin Cores A series of phenylazozmethine (DPA) dendrimers with a porphyrin core (PnH2) were synthesized by dehydration using TiCl4 from meso-tetrakis(4-aminophenyl)porphyrin and the DPA dendrons. The addition of SnCl2 to a dichloromethane/acetonitrile solution of dendritic cobalt porphyrin resulted in a stepwise spectral change. By using UV−vis spectroscopy to monitor the complexation of the P4CoIIICl until an equimolar amount of SnCl2 has been added, four changes in the position of the isosbestic point were observed during the addition of SnCl2. Titration results suggest that four different complexes are successively formed upon the SnCl2 addition and that the complexation proceeds in, not a random, but a stepwise fashion from the core imines to the terminal imines of P4CoIIICl. The electrochemical study reveals that their dendrimers with Tb ion act as a multielectron mediator in CO2 reduction at high applied potential on the electrode.

Stepwise radial complexation of imine groups in phenylazomethine dendrimers

2002.01.31


K. Yamamoto, M. Higuchi, S. Shiki, M. Tsuruta, H. Chiba
Nature 2002, 415, 509-511.
Dendrimers are highly branched organic macromolecules with successive layers or ‘generations’ of branch units surrounding a central core. Organic–inorganic hybrid versions have also been produced, by trapping metal ions or metal clusters within the voids of the dendrimers. The unusual, tree-like topology endows these nanometre-sized macromolecules with a gradient in branch density from the interior to the exterior, which can give rise to an energy gradient that directs the transfer of charge and energy from the dendrimer periphery to its coreHere we show that tin ions, Sn2+, complex to the imine groups of a spherical polyphenylazomethine dendrimer in a stepwise fashion. This behaviour reflects a gradient in the electron density associated with the imine groups, with complexation in a more peripheral generation proceeding only after complexation in generations closer to the core has been completed. By attaching an electron-withdrawing group to the dendrimer core, we are able to change the complexation pattern, so that the core imines are complexed last. By further extending this strategy, it should be possible to control the number and location of metal ions incorporated into dendrimer structures, which might find uses as tailored catalysts or building blocks for advanced materials.

First Synthesis of Phenylazomethine Dendrimer Ligands and Structural Studies

2001.03.21


M. Higuchi, S. Shiki, K. Ariga, K. Yamamoto
J. Am. Chem. Soc. 2001, 123, 4414-4420. First Synthesis of Phenylazomethine Dendrimer Ligands and Structural Studies Novel dendritic polyphenylazomethines (DPAs), which consist of a π-conjugated backbone, were synthesized up to the fourth generation by the convergent method via dehydration of aromatic ketones with aromatic amines in the presence of titanium(IV) tetrachloride. The obtained dendrimers, DPA G1−4 (designated as GX, where X is the generation number), show high thermostability (Td10% 521 °C in DPA G4) and high solubility for the common solvents such as chloroform, THF, and DMSO unlike the conventional linear polyphenylazomethines, which have very low solubilities. The DPA G4 molecule was confirmed to have a spherelike structure by GPC measurement and a molecular model based on the crystal structure of DPA G2. Crystal data for DPA G2:  monoclinic space group P21/a, a = 25.352(4) Å, b = 8.577(2) Å, c = 16.151(2) Å, β = 106.25(1)°, V = 3371.6(10) Å3, Z = 2, Dcalc = 1.168 g/cm3, μ(Cu Kα) = 0.536 cm-1, final R = 0.089, and Rw = 0.287. The molecular modeling reveals that a DPA G4 molecule has a spherelike structure, in which the height, width, and depth are 2.3, 2.9, and 2.5 nm, respectively. The TEM and AFM pictures show the DPA G4 molecules to have a spherelike structure (the diameter:  2.3 nm) and are regularly assembled on a plate by casting. The occupied area of one DPA G4 molecule in a monolayer on water was estimated by π−A measurements to be 3.8−4.2 nm2 (the calculated diameter 2.2−2.3 nm, which agreed with the TEM result). NMR studies (1H NMR at 130 °C and T1 measurements) supported a conformational rigidity of DPA G4 in solution.