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.     

Synthesis of poly(propyl ether imine) dendrimers and evaluation of their cytotoxic properties

In this paper, we report the synthesis of several poly(propyl ether imine) dendrons and dendrimers. These dendrons and dendrimers were constructed by involving an ether as the linker component and an imine as the branching component. The divergent syntheses of dendrons and dendrimers were established with the aid of two alternate Michael addition reactions and two alternate reduction reactions in a four-step iterative synthetic sequence. Dendrons up to three generations were synthesized and some of the dendrons were attached to a benzenoid core so as to obtain dendrimers up to two generations containing 12 carboxylic acids at the periphery. Divergent synthesis involving ether as the core was found to be more facile, and dendrimers up to three generations having 16 carboxylic acids at the periphery were achieved in good to excellent yields in each individual step. The adopted synthetic sequence allows us to install either alcohol, an amine, or a carboxylic acid at their peripheries. The carboxylic acid-terminated dendrons and dendrimers were evaluated as to their cytotoxic properties, and while most dendrons and dendrimers did not exhibit any measurable cytotoxicity, even up to 100 microg/mL, the second-generation dendrimer with the benzenoid core exhibited a mild toxicity at concentrations above 30 microg/mL.     

Synthesis of nanoparticle-cored dendrimers by convergent dendritic functionalization of monolayer-protected nanoparticles

This article presents a synthesis method for nanoparticle-cored dendrimers (NCDs), which have dendritic architectures around a monolayer-protected gold nanoparticle. The synthesis method is based on a strategy in which the synthesis of monolayer-protected nanoparticles is followed by adding dendrons on functionalized nanoparticles by a single coupling reaction. NMR spectroscopy, IR spectroscopy, and thermogravimetric analysis (TGA) characterizations confirmed the successful coupling reaction between dendrons with different generations ([G1], [G2], and [G3]) and COOH-functionalized nanoparticles ( approximately Au201L71). The dendrimer wedge density also could be controlled by reacting nanoparticles having different loading of COOH groups ( approximately 60 and approximately 10% COOH of the 71 ligands per gold nanoparticle) with functionalized dendrons. Transmission electron microscope results showed that this synthesis strategy maintains the average size of the nanoparticle core during dendron coupling reactions. This control over the composition and core size makes the systematic study of NCDs with different generations possible. The chemical stability of NCDs was found to be affected by dendron generation around the nanoparticle core. The current-potential response of NCD films on microelectrode arrays exhibited better electrical conductivity for NCDs with lower dendron generation.     

Effects of benzyl ether type dendrons as hole-harvesting antennas, and shielding for the neutralization of stilbene core radical cations with chloride ion during two-photon ionization of stilbene dendrimers having stilbene core and benzyl ether type dendrons

The two-photon ionization (TPI) process (308 and 266 nm) of stilbene dendrimers having a stilbene core and benzyl ether type dendrons has been investigated in an acetonitrile and 1,2-dichloroethane mixture (3:1) in order to elucidate the dendrimer effects. The quantum yield of the formation of stilbene core radical cation during the 308-nm TPI was independent of the dendron generation of the dendrimers, whereas a generation dependence of the quantum yield of the radical cation was observed during the 266-nm TPI, where both the stilbene core and benzyl ether type dendron were ionized, suggesting that the subsequent hole transfer occurs from the dendron to the stilbene core, and that the dendron acts as a hole-harvesting antenna. The neutralization rate of the stilbene core radical cation with the chloride ion, generated from the dissociative electron capture by 1,2-dichloroethane, decreased with the increase in the dendrimer generation, suggesting that the dendron is an effective shield of the stilbene core radical cation against the chloride ion.     

Core Functionality and Scaling Behavior of Lysine Dendrimers

Summary: A scaling exponent to describe the dependence of the hydrodynamic radius as a characteristic length of the molecule on the molecular weight, has been determined for low generation dendrimers with a thiacalixarene core and lysine dendrons. The hydrodynamic radius has been calculated from the diffusion coefficient measured by pulsed‐field‐gradient NMR spectroscopy. Scaling exponents of 2.0 for the lysine monodendron, 2.3 for a dendrimer with a bifunctional core, and 3.9 for a dendrimer with a tetrafunctional core have been determined. For a given structure of the dendrons, the scaling exponent reflects the functionality of the core of the dendrimers.

Hydrodynamic radius of lysine dendrons as a function of molar mass.     

Molecular batteries: ferrocenylsilylation of dendrons, dendritic cores, and dendrimers: new convergent and divergent routes to ferrocenyl dendrimers with stable redox activity

The ferrocenylsilylation of the phenol triallyl dendron 2, of the phenol nonaallyl dendron 4, and of the 9-, 27-, 81-, and 243-allyl dendrimers 7-10 (monitored by the disappearance of the signals of the olefinic protons in 1H NMR spectra) has been achieved using ferrocenyldimethylsilane 1 and Karstedt's catalyst in diethyl ether at 40 degrees C, yielding the corresponding ferrocenyl dendrons and dendrimers. An alternative convergent synthesis of the nonaferrocenyl dendron 5 was carried out by reaction of the triferrocenyl dendron 2 with a protected triododendron followed by deprotection. Reaction of the nonaferrocenyl dendron 5 with hexakis(bromomethyl)benzene gave the 54-ferrocenyl dendron 6. All the ferrocenyl dendron and dendrimers produce a chemically and electrochemically reversible ferrocenyl oxidation wave at seemingly the same potential. Stable platinum electrodes modified with the high ferrocenyl dendrimers were fabricated. The soluble orange-red ferrocenyl dendrimers can also be oxidized in CH2Cl2 by [NO][PF6] to the insoluble deep blue polyferrocenium dendrimers. For instance, the 243-ferrocenium dendrimer has been characterized by its Mossbauer spectrum, which is of the same type as that of ferrocenium itself. The ferrocenium dendrimers can be reduced without any decomposition back to the ferrocenyl dendrimer, indicating that these multielectronic redoxstable dendrimers behave as molecular batteries.     

Size and branching effects on the fluorescence of benzylic dendrimers possessing one apigenin fluorophore at the core

Different generations of dendrimers incorporating one fluorescent core of apigenin and three Fréchet benzylic dendrons have been prepared. The chief geometric features of these dendrimers have been obtained by Molecular Dynamics simulations. These computational data suggest that the asphericities of dendrimers belonging to the third and fourth generations are considerably larger than those associated with lower radii of gyration. Fluorescence spectra of high generation dendrimers evolve along time and quantum yields show an appreciable lowering for the fourth generation dendrimer. All these data suggest aggregation phenomena and lower quantum yields for nonspheric dendrimers in solution.     

Synthesis and properties of a novel phenylazomethine dendrimer with a tetraphenylmethane core

[structure: see text] A new type of phenylazomethine dendrimer with a tetraphenylmethane core was synthesized by a convergent method. The properties of the dendrimer were confirmed by thermal, rheological, TEM, and AFM measurements. A stepwise radial complexation was clearly observed with SnCl(2).     

Efficient synthesis of poly(phenylazomethine) dendrons allowing access to higher generation dendrimers

We have developed a novel synthetic method of phenylazomethine dendrons that uses 4,4'-methylenedianiline instead of 4,4'-diaminobenzophenone to synthesize the precursor of the phenylazomethine dendron and then oxidized the precursor to the next-generation dendron. For this method, the productivity of the dendrons has been significantly increased. Furthermore, as the synthesis of high-generation dendrons becomes easier, synthesis of DPA G5 was achieved. [structure--see text]     

Molecular force field study concerning the host properties of carbosilane dendrimers

The effect of structural variations at the core of carbosilane dendrimers has been studied using Molecular Dynamics. Several derivatives of the parent dendrimer have been modeled and characterized with respect to the dimensions of the inner cavities which lie on the order of 5–15 Å. The “denseness” of the outer shell has been studied by calculating solvent accessible surfaces and excluded volumes as a function of the radius of the probe sphere. The higher generation dendrimers were found to possess dense outer shells with holes of the order of 2–3 Å. Dendron separation especially with respect to clefts in the molecule is discussed. It was found that in low generation carbosilane dendrimers the dendrons are clearly separated while at higher generations a substantial amount of interpenetration occurs. In addition, results are presented showing that carbosilane dendrimer surfaces are fractal only over a narrow range of length scales. In this range, the fractal dimensions of the surfaces are of the order of 2.1.     

Effect of structure on the reduction potentials of films of constitutional isomers of iron-sulfur cluster core dendrimers

The thermodynamic redox potentials of films of constitutional isomers of iron-sulfur cluster core dendrimers were measured and compared. It was determined that the primary structure of the dendrimer influences its reduction potential. Dendrimers containing so-called backfolded linkages were more difficult to reduce than their extended analogues. This behavior is rationalized by suggesting that the backfolded isomers pack more tightly around the iron-sulfur cluster, creating a more hydrophobic local microenvironment. Also, all of these molecules are easier to reduce in the film than in dimethyl formamide solution. The variation in redox potential between film and solution environment was compared to that of dendrimers of differing generations and correlated with the amount of hydrophobic dendron surrounding the cluster.     

Relationship between incoherent excitation energy migration processes and molecular structures in zinc(II) porphyrin dendrimers

Multiporphyrin dendrimers are among the most promising architectures to mimic the oxygenic light-harvesting complex because of their structural similarities and synthetic convenience. The overall geometries of dendrimers are determined by the core structure, the type of dendron, and the number of generations of interior repeating units. The rigid core and bulky volume of exterior porphyrin units in multiporphyrin dendrimers give rise to well-ordered three-dimensional structures. As the number of generations of interior repeating units increases, however, the overall structures of dendrimers become disordered and randomized due to the flexibility of the repeating units. To reveal the relationship between molecular structure and processes of excitation-energy migration in multiporphyrin dendrimers, we calculated the molecular structure and measured the time-resolved transient absorption and fluorescence anisotropy decays for various hexaarylbenzene-anchored polyester zinc(II) porphyrin dendrimers along with three types of porphyrin dendrons as references. We found that the congested two-branched type dendrimers exhibit more efficient energy migration processes than one- or three-branched type dendrimers because of multiple energy migration pathways, and the three-dimensional packing efficiency of dendrimers strongly depends on the type of dendrons.     

Synthesis of novel phenylazomethine dendrimers having a cyclam core and their zinc complex

[structure: see text] Novel phenylazomethine dendrimers having a cyclam core were synthesized by the convergent method. The dendrimers showed selective coordination with zinc chloride on the cyclam ring and with tin chloride on the imine groups. The metal cyclam exhibits a metal-assembling function to provide a multinuclear hetero metal complex.     

A new approach to heterofunctionalized dendrimers: a versatile triallyl chloride core

A reactive triallyl chloride moiety was prepared as a suitable core for aliphatic polyether dendrons. In addition to allowing access to fourth generation dendrimers, this core contains three alkene functionalities that can be modified after dendrimer formation and used for further dendritic growth.     

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.     

Synthesis and catalytic antibody functionalization of dendrimers

Antibody 38C2 catalyzed a retro-aldol process upon dendritic modified aliphatic polyesters. This catalytic system was studied in detail and displayed rate enhancements, k(cat)/k(uncat), of greater than 10(6). These antibody-catalyzed reactions took place in a stepwise manner yielding partially modified aldol-dendrimers until a fully substituted aldehyde dendrimer was formed. The catalytic antibody 38C2 only reacted with surface-exposed aldol moieties and did not significantly interact with the core groups for dendrons 4 and 8. For a higher generation dendron 8 the rate of unmasking slightly decreased presumably due to steric crowding of the aldol functionalities. In addition, catalytic antibody 38C2 was able to selectively differentiate block-hybrid dendrons and was regiospecific in the retro-aldol reaction of dendron 21. This is an inaugural report of a catalytic antibody utilizing dendrimers as substrates and suggests that antibodies could be used as selective catalysts for the controlled release and activation of specific molecules attached to biodegradable polymeric materials. Furthermore, this is the first example of catalytic antibody 38C2 displaying regioselectivity on a multifunctional aldol substrate. Important for synthetic applications is the antibody's ability to selectively differentiate regions on dendritic substrates and produce partly aldol functionalized dendrons under conditions mild enough to avoid beta-elimination.     

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.     

Luminescence properties of soluble 2,2′,7,7′‐Tetrakis(2‐(9‐hexyl‐9H‐carbazol‐3‐yl)‐vinyl)‐9,9′‐spirobifluorene‐labeled dendrimers: Photoluminescence and electroluminescence

New light emitting dendrimers were synthesized by reacting 3,5‐bis‐(3,5‐bis‐benzyloxy‐benzyloxy)‐benzoic acid or 3,5‐bis‐[3,5‐bis‐(3,5‐bis‐benzyloxy‐benzyloxy)‐benzyloxy]‐benzoic acid with a carbazolyl vinyl spirobifluorene moiety. A blue‐emitting core dye was encapsulated by multibenzyloxy dendrons, and two dendrimers having different densities of dendrons were prepared. Photoluminescence (PL) studies of the dendrimers demonstrated that at the higher density of benzyloxy dendrons, the featureless vibronic transitions were improved, causing lesser excimer emission. The similarity of the solution and solid emission spectra of the larger dendrimer, 10 , revealed the suppression of molecular aggregation in the solid film, which is attributed to the presence of the bulky benzyloxy dendrons. The electroluminescence spectra of multilayered devices made using 10 predominantly exhibited blue emissions; similar emission was observed in the PL spectra of its thin film. The multilayered devices made using 3 , 9 , and 10 showed luminances of 1021 cd m^?2 at 5 V, 916 cd m^?2 at 6 V, and 851 cd m^?2 at 6.5 V, respectively. The largest dendrimer, 10 , bearing a greater number of benzyloxy dendrons, exhibited a blue‐like emission with CIE 1931 chromaticity coordinates of x = 0.16 and y = 0.13, which is due to the influence of a higher shielding effect. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 501–514, 2008     

Synthesis and Characterization of Gold‐Nanoparticle‐Cored Dendrimers Stabilized by Metal–Carbon Bonds

Synthesis and characterization of gold‐nanoparticle‐cored dendrimers (NCDs), in which the dendrons are attached to the gold core through gold–carbon bonds, are described. Synthesis of these materials involved the simultaneous reduction of HAuCl₄ and a Fréchet‐type dendron with a diazonium group at the focal point, all in an organic solvent such as toluene. These materials possess a nanometer‐sized gold core surrounded by a shell of polyaryl ether dendrons, which are connected radially to the core. The NCDs were characterized by TEM, thermogravimetric analysis (TGA), and IR, UV, and NMR spectroscopic techniques. Average particle diameter of the NCDs ranged from 4.7 to 5.5 nm for the different generations. All NCDs exhibit the characteristic plasmon absorption of gold nanoparticles at 520 nm. Average numbers of dendrons per NCD in AuG_n were calculated using results from TGA and TEM studies. Multiple layering of the dendrons is proposed as a possible reason for the high dendron/NCD value.