Phenylazomethine dendrimers with soft aliphatic units as metal-storage nanocapsules and their self-assembled structures

New dendritic poly(phenylazomethine)s (DPAs) with dodecyl end groups (C12DPA) have been synthesized. C12DPA showed stepwise radial complexation with metals as a metal-storage nanocapsule. Through modification with dodecyl, the properties of environmental responsiveness and self-assembly become apparent for C12DPA as a π-conjugated soft material. The dodecyl-modified DPAs (C12DPAG4) were synthesized up to the fourth generation dendrimer, for the first time, which is a nanocapsule with a basic atmosphere for metal complexes in a hydrophobic environment. In addition, we found a suitable structure and conditions for the fibrous self-assembly of DPA through the precise design of its dendritic structure. C12DPA, with an asymmetric structure, showed a fibrous assembly by a solvent drop-casting. The metal-complexed C12DPA showed an assembly different from the fibrous one through metal complexation on DPA imines. Interestingly, the vesicular assembly structure of C12DPA has been observed by the complexation of CuCl(2) in toluene. In this study, we investigated the unique properties of C12DPA as a novel π-conjugated soft material.     

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 (R(h) = 22 A 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.     

Dendrimer complexes: Fine control of metal assembly in macromolecules

New phenylazomethine dendrimers have been developed, with each dendrimer having a unique single structure. From ultraviolet–visible absorption spectra, we determined that stannous chloride added to dendritic polyphenylaz omethine (DPA) was assembled in a stepwise manner from the core shell to the periphery. The selective binding was achieved by the electron density gradient formed in the dendrimer, and it was confirmed by shell‐selective imine reduction, transmission electron microscopy, and NMR measurements. The complex formation constants of the respective shells were estimated to be considerably different by a simulation analysis of ultraviolet–visible absorption spectra. The reversible assembly and release of iron into and out of the dendrimer were possible by electrochemical oxidation and reduction, respectively. DPA with a porphyrin at the core was found to function as an effective reduction catalyst of carbon dioxide. DPA with aryl amine at the core was demonstrated to function as a hole‐transporting material in electroluminescent devices. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3719–3727, 2005     

Novel phenylazomethine dendrimers: synthesis and structural properties

The novel dendrimers consisting of a pi-conjugated backbone, the dendritic polyphenylazomethines (DPAs), were synthesized by the convergent method via the dehydration of aromatic ketones with aromatic amines in the presence of titanium(IV) tetrachloride. The obtained DPAs have a high solubility unlike the conventional linear polyphenylazomethines. NMR studies revealed the conformational rigidity of DPA G4. DPAs having many azomethine groups as the coordination site for metal ions are expected to be novel ligands.     

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 SnCl(2) molecules, because the imine group is complexed with SnCl(2) at a ratio of 1:1. During addition of 30 equiv of SnCl(2) to DPA G4, four shifts in the isosbestic point were observed in the UV-vis spectra, and the amount of SnCl(2) 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 SnCl(2) 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 (13)C 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.     

Deposition of gallium oxide nanodots prepared from metal‐assembling dendrimer molecules isolated by the spacing of the nonmetal‐assembling dendrimer molecules in the two‐dendrimers mixture monolayer

Phenylazomethine dendrimers (DPA) can precisely incorporate metal chlorides onto the imine sites in a stepwise fashion. Such precise dendrimer–metal complexes allow the preparation of size‐controlled subnanometer metal particles. We now propose a novel approach for the fabrication of size‐controlled subnanometer metal oxide dots isolated on a substrate using two different‐type dendrimers. One is a fourth‐generation phenylazomethine dendrimer (DPAG4) and the other is a benzylether dendrimer (BzEG3) with a zinc porphyrin core. Even though the diameter of BzEG3 corresponds to that of DPAG4, BzEG3 has no metal‐complexing site. Upon dip coating on a highly oriented pyrolytic graphite substrate by the mixed solution of the metal chloride‐assembling DPAG4 molecules and BzEG3 molecules, the dendrimer monolayer was immobilized on the substrate. The concentration of the dendrimer mixture was determined in order to separate each DPAG4–metal chloride complex molecule by BzEG3. Monodispersed metaloxide nanodot arrays could be obtained from the dendrimer monolayer in which DPAG4–metal chloride complex molecule is well isolated by the BzEG3 as a spacer after the hydrolysis of metal chlorides followed by the complete removal of dendrimers. Copyright © 2013 John Wiley & Sons, Ltd.     

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.     

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.     

苯丙氨酸改性树枝状聚赖氨酸的合成及表征

以双芴甲氧羰基赖氨酸(Di- Fmoc -Lysine)为支化单元采用固相多肽合成技术制备了一种树枝状聚赖氨酸.该聚合物表面活性氨基通过与芴甲氧羰基苯丙氨酸(Fmoc- Phe)反应获得表面完全替代的苯丙氨酸改性树枝状聚赖氨酸.产物的分子量和结构经MS和NMR进行了表征.研究结果表明,利用该方法得到的产物分子量单一,是一种潜在的理想非病毒治疗载体.     

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.     

Ferrocene encapsulated within symmetric dendrimers: a deeper understanding of dendritic effects on redox potential

Ferrocene has been encapsulated within a symmetric ether-amide dendritic shell and its redox potential monitored in a variety of solvents. The dendritic effect generated by the branched shell is different in different solvents. In less polar, non hydrogen bond donor solvents, attachment of the branched shell to ferrocene increases its E(1/2), indicating that oxidation to ferrocenium (charge buildup) becomes thermodynamically hindered by the dendrimer, a result explained by the dendrimer providing a less polar medium than that of the surrounding electrolyte solution. The effect of electrolyte concentration on redox potential was also investigated, and it was shown that the concentration of "innocent" electrolyte has a significant effect on the redox potential by increasing the overall polarity of the surrounding medium. Dendritic destabilization of charge buildup is in agreement with the majority of reported dendritic effects. A notable exception to this is provided by the asymmetric ferrocene dendrimers previously reported by Kaifer and co-workers, in which the branching facilitated oxidation, and it is proposed that in this case the dendritic effect is generated by a different mechanism. Interestingly, in methanol, the new symmetric ferrocene dendrimer exhibited almost no dendritic effect, a result explained by the ability of methanol to interact extensively with the branched shell, generating a more open superstructure. By comparison of all the new data with other reports, this study provides a key insight into the structure-activity relationships which control redox processes in dendrimers and also an insight into the electrochemical process itself.     

"Pocket dendrimers" as nanoscale receptors for bimolecular guest accommodation

A new series of dendrimer receptors was prepared by combining a (tetraphenylporphinato)zinc(II) core and benzyl ether type dendritic substituents. Since one direction of the (tetraphenylporphinato)zinc(II) was not substituted by a dendritic residue, the resulting unsymmetrical dendrimers have "pockets" available for access of external substrates. Molecular modeling, NMR measurements, and zinc-coordination experiments revealed that the third-generation dendrimer of this type exhibited characteristic inclusion of coordinative pyridine guests. When diamidopyridine moiety was introduced into the dendrimer pocket, a thymine derivative was bound through complementary hydrogen bonding. Two different kinds of substrates, pyridine and thymine derivatives, were simultaneously accommodated in the nanoscale pocket and bimolecular guest accommodation was realized with the designed dendrimer receptor.     

Shape persistence and bistability of planar three-fold core polyphenylene dendrimers: a molecular dynamics study

We present an atomistic molecular dynamics investigation of the structural time evolution of isolated polyphenylene dendrimers, carbon based dendrimers with a planar core formed by a 1,3,5 trisubstituted benzene ring. Simulations are carried out at low (80 K) and room temperature. A general classification of the conformations (core conformations) assumed by the three dendrimer branches with respect to the planar core is presented. It is found that out of the six possible core conformations only four are stable, the remaining two being unstable for steric reasons. For second generation dendrimers, two of the four accessible core conformations are associated with an open arrangement of the three branches attached to the planar 3-fold core of the dendrimer, whereas the remaining two are associated with a collapsed arrangement of two branches. At low temperature the initial conformation is generally conserved whereas at room temperature jumps among the four possible core conformations are observed in the nanosecond time range. For second generation dendrimers the core conformation jumps are associated with an oscillation between two global shape states: open and collapsed. The computed bistability of the global shape suggests additional possible functional uses for some of these carbon based dendrimers.     

Photocurrent generation in layer-by-layer assembled dendrimers with ruthenium tris-bipyridine peripheral groups and a viologen-like core

The photophysical and photoelectrochemical properties of first- and second-generation dendrimers with ruthenium tris-bipyridine peripheral groups and a tri-viologen like core (Ru3V3 and Ru6V3) were investigated in solution and when embedded within assembled films. The stepwise assembly of these dendrimers on quartz and ITO surfaces utilizing the layer-by-layer approach was investigated. The amount of the assembled dendrimers was found to increase on going to the higher generation dendrimer. This dendrimer generation effect was evident from the UV-vis, atomic force microscopy, and electrochemical measurements of the dendrimers in either solution phase or when embedded in films. The anodic and cathodic photocurrent generation was seen upon visible light irradiation, with higher photocurrents for Ru6V3 than Ru3V3. This observation was attributed to better light-harvesting properties, thicker films, and slower charge recombination processes in Ru6V3 when compared to Ru3V3.     

Paramagnetic cobalt(II) as an NMR probe of dendrimer structure: mobility and cooperativity of dendritic arms

Cobalt(II) has been utilized as an external paramagnetic (1)H NMR probe for the study of the structure of dendrimers that possess specifically located metal recognition sites. The hyperfine-shifted (1)H NMR signals of the Co(II) complexes of several 2,6-diamidopyridine-containing dendrimers have been fully assigned by means of 1D and 2D NMR techniques, including NOE difference, EXSY, COSY, and TOCSY. Temperature-dependent T(1) values of the hyperfine-shifted signals were used to conclude that the Co(II)-dendrimer complexes are in the "liquidlike" regime, indicative of a shell-like structure instead of a "dense-core" structure. The presence of sizable cavities within the dendrimers was observed including a loosely packed conformation for the 2,6-diamidopyridino moiety to bind to potential guest molecules. Cooperativity among the dendritic arms in metal binding is also observed, whereby two dendritic arms bind to the metal center at the same time. In the case of dendrimers with the metal binding site located near the surface of the molecule, such binding cooperativity is still observed despite the large degree of freedom of the metal-binding moiety. Cooperativity among the dendritic arms can thus be considered an intrinsic property, which has to be taken into consideration in future design of functional dendrimers for the purpose of specific recognition and catalysis. The hydrodynamic radii of these dendrimers have been determined by means of nuclear Overhouser effect at low temperature. The study offers a method for the study of the dynamics of dendrimers in solution under different conditions and upon ligand binding and recognition. The study also provides a tool for monitoring systematic variation of the metal binding site in different dendrimer frameworks for specific applications, such as catalysis and molecular recognition.     

手性树状分子膦配体在不对称催化氢化中的应用

树状大分子作为一类组成精确的超支化结构大分子,近十多年来引起了科学家们的广泛关注.作为一类新型可溶性载体应用于均相催化剂,尤其是手性均相催化剂的负载化研究是树状大分子的重要应用领域之一.本文主要介绍了手性树状大分子膦配体,包括膦配体位于树状分子核心、末端和表面的几种类型,重点对它们与金属配合物形成的催化剂在不对称催化氢化反应中的应用研究进行总结,同时对负载催化剂的分离与回收、树状分子载体的结构和体积对催化剂性能的影响进行了讨论.     

Host-guest complexes between an aromatic molecular tweezer and symmetric and unsymmetric dendrimers with a 4,4'-bipyridinium core

We have investigated the spectroscopic and electrochemical behavior of symmetric and unsymmetric first-, second-, and third-generation dendrimers comprising an electron-acceptor 4,4'-bipyridinium core (viologen type) and electron-donor 1,3-dimethyleneoxybenzene (Fréchet-type) dendrons. The quite strong fluorescence of the symmetrically and unsymmetrically disubstituted 1,3-dimethyleneoxybenzene units of the dendrons is completely quenched as a result of donor-acceptor interactions that are also evidenced by a low-energy tail in the absorption spectrum. In dichloromethane solution, the 4,4'-bipyridinium cores of the investigated dendrimers are hosted by a molecular tweezer comprising a naphthalene and four benzene components bridged by four methylene units. Host-guest formation causes the quenching of the tweezer fluorescence. The association constants, as measured from fluorescence and (1)H NMR titration plots, (i) are of the order of 10(4) M(-1), (ii) decrease on increasing dendrimer generation, and (iii) are slightly larger for the unsymmetric than for the symmetric dendrimer of the same generation. The analysis of the complexation-induced shifts of the temperature-dependent (1)H NMR signals of the host and guest protons confirms that the bipyridinium core is positioned inside the tweezer cavity and allows the conclusions that (i) shuttling of the tweezer from one to the other pyridinium ring is fast (DeltaG < 10 kcal/mol), (ii) in the case of the unsymmetric dendrimers, the less substituted pyridinium ring is preferentially complexed in apolar solvents, and (iii) complexation of the 4,4'-bipyridinium core proceeds by clipping for the symmetric dendrimers and by threading in the case of unsymmetric ones. Host-guest formation causes a displacement of the first reduction wave of the 4,4'-bipyridinium unit toward more negative potential values, whereas the second reduction wave is unaffected. These results show that the host-guest complexes between the tweezer and the dendrimers are stabilized by electron donor-acceptor interactions and can be reversibly assembled/disassembled by electrochemical stimulation.     

A recyclable dendritic osmium catalyst for homogeneous dihydroxylation of olefins

A series of osmate (OsO₄²⁻) core dendrimers was prepared by an ion-exchange technique through the mixing of K₂OsO₄ and a bis(quaternary ammonium bromide) core dendrimer, which consisted of poly(benzyl ether) dendron. By employing an osmate core dendrimer as a homogeneous catalyst, dihydroxylation reactions of olefins proceeded rapidly, and the dendritic osmium catalyst was recovered by reprecipitation and then reused. Furthermore, a dendritic effect on the recyclability of a catalyst was observed. In the case of asymmetric dihydroxylation reactions, the corresponding diol was obtained in a high chemical yield with a fair enantiomeric excess (ee). In this case, not only the dendritic osmium catalyst but also the chiral ligand could be recovered by reprecipitation and reused efficiently up to five times.     

A peptide dendrimer enzyme model with a single catalytic site at the core

Catalytic esterase peptide dendrimers with a core active site were discovered by functional screening of a 65,536-member combinatorial library of third-generation peptide dendrimers using fluorogenic 1-acyloxypyrene-3,6,8-trisulfonates as substrates. In the best catalyst, RMG3, ((AcTyrThr)(8)(DapTrpGly)(4)(DapArgSerGly)(2)DapHisSerNH2), ester hydrolysis is catalyzed by a single catalytic histidine residue at the dendrimer core. A pair of arginine residues in the first-generation branch assists substrate binding. The catalytic proficiency of dendrimer RMG3 (kcat/KM = 860 M(-1) min(-1) at pH 6.9) per catalytic site is comparable to that of the multivalent esterase dendrimer A3 ((AcHisSer)(8)(DapHisSer)(4)(DapHisSer)2DapHisSerNH2) which has fifteen histidines and five catalytic sites (Delort, E. et al. J. Am. Chem. Soc. 2004, 126, 15642-15643). Remarkably, catalysis in the single site dendrimer RMG3 is enhanced by the outer dendritic branches consisting of aromatic amino acids. These interactions take place in a relatively compact conformation similar to a molten globule protein as demonstrated by diffusion NMR. In another dendrimer, HG3 ((AcIlePro)(8)(DapIleThr)(4)(DapHisAla)(2)DapHisLeuNH2) by contrast, catalysis by a core of three histidine residues is unaffected by the outer dendritic layers. Dendrimer HG3 or its core HG1 exhibit comparable activity to the first-generation dendrimer A1 ((AcHisSer)(2)DapHisSerNH2). The compactness of dendrimer HG3 in solution is close to that a denatured peptide. These experiments document the first esterase peptide dendrimer enzyme models with a single catalytic site and suggest a possible relationship between packing and catalysis in these systems.     

Synthesis and characterization of a ligand‐substituted poly(amidoamine) dendrimer with external terpyridine units and its iron(II) complexes

The ligand‐substituted poly(amido amine) dendrimer 2 containing terpyridine ligands located exclusively at the peripheries of the dendrimer and its iron‐containing dendrimer 3 have been synthesized using modification reactions. Cyclic voltammetric studies indicate that the external iron centers on the outer surface of the dendrimer are electrochemically equivalent. The treatment of 2 with Fe²⁺ ions leads to a dendrimer assembly due to the formation of iron complexes among the ligand‐substituted dendrimers.