Dendron Generation Research Articles

Synthesis and Characterization of Controlled Dendritic Architectures by Association of Two Phosphorus Dendrons Through a Metallic Center

The reactions of RuCl 2 (PPh 3 ) 3 with either a small amino diphosphine or a first-generation phosphorus-containing dendron possessing an aminodiphosphine as the core induce the formation of complexes in which two amino diphosphines are associated through the metallic center. Determination of the structure of the smallest complex by X-ray diffraction shows that the metal is surrounded by a square-based bi-pyramid defined by the four phosphorus atoms of both diphosphines in a single plane, with the chlorine atoms at both summits. The first generation dendrons behave in the same way as the small compound, and allow us to isolate an original bis dendron, associated through the ruthenium atom.

Peripherally Dimethyl Isophthalate-Functionalized Poly(benzyl ether) Dendrons: A New Kind of Unprecedented Highly Efficient Organogelators

A series of poly(benzyl ether) dendrons, up to the fourth generation, decorated in their periphery with dimethyl esters were divergently synthesized and fully characterized. These dendrons were found to be unprecedented highly efficient organogelators toward various aromatic and polar organic solvents with the critical gelator concentration (CGC) approaching 2.2 mg/mL. The gelation ability was found to be highly dependent on the nature of the peripheral groups, dendron generation, and the dendritic architecture. The large monodisperse dendron G(4) with a globular shape could also form stable gels in several aromatic solvents with relatively high CGCs. A number of experiments (SEM, TEM and AFM imaging, X-ray crystal structure analysis, concentration- and temperature-dependent (1)H NMR spectroscopy, fluorescence spectroscopy, and powder X-ray diffraction) confirmed the self-aggregation of these dendrons, despite the lack of any conventional gelating motifs such as amides, long alkyl side chains, and steroidal groups. The multiple strong pi-pi stacking interactions due to the peripheral dimethyl isophthalate rings and the internal benzyl rings are found to be the key contributor in forming the self-assembled gel.

Solution and Thin‐Film Aggregation Studies of Octasubstituted Dendritic Phthalocyanines

AbstractThe synthesis and solution and thin‐film characterization of eight octasubstituted dendritic phthalocyanines (Pcs) and their zinc complexes are reported. The Pc chromophore was substituted in the 2,3,9,10,16,17,23,24‐positions with three generations of benzylaryl ether dendrons with either a benzyl (3a‐3c) or 3,5‐di‐t‐butylbenzyl periphery (3d‐3f). Visible spectra in solution (CH2Cl2‐EtOH mixtures, toluene, THF, dioxane, acetone, and EtOAc) indicated a varying degree of chromophore aggregation that depended on solvent, dendrimer generation, and whether the Pc was metallated. Variable‐concentration visible spectroscopic studies were analyzed using a nonlinear least‐squares fitting procedure giving Kd values. These values further quantitated the observations that the t‐butyl‐substituted den‐drimers 3d‐3f were all less prone to aggregation in solution than the unsubstituted dendrimers 3a‐3c, with a monotonic decrease in Kd across the series 3a → 3b → 3c → 3d → 3e → 3f. Second‐generation t‐butyl‐substituted dendrimer 3f showed little to no aggregation in all solvents studied. Thin‐film studies indicated that the largest members of the two dendrimer groups, third‐generation 3c and second‐generation 3f, were largely monomeric as evidenced by split Q‐bands, similar to that seen in dilute CH2Cl2 solution when deposited via spin‐coating onto glass slides. The metallated zinc Pcs 4a‐4f all exhibited significantly less tendency toward aggregation in both solution and thin‐films than their unmetallated analogues.

Synthesis of multiple shaped gold nanoparticles using wet chemical method by different dendritic peptides at room temperature

Gold nanoparticles of various shapes including pentagonal, hexagonal, prismatic, branched (multipods), spherical and oval have been prepared by using different generations of peptidic dendrons as templates at room temperature without adding any external reducing or stabilizing agents. Each peptide based dendron molecule (G1, G2 or G3) contains a redox active tyrosine (Tyr) moiety and a free –NH2group at the N-terminus and as a result of that each peptide-dendron molecule can be used to reduce AuCl4− to Au0 and stabilize the nascent gold nanoparticles (GNPs) by the NH2group present in the molecule at room temperature. Not only the shapes but also the sizes of these particles can be tuned by using this chemical methodology involving different peptidic dendrons in water-methanol (4:1) at pH 11. The first generation dendritic peptide produces various shaped GNPs, while the second-generation peptide-dendron molecule leads to the exclusive formation of hexagonal gold nanoparticles and the third generation dendritic peptide gives rise to branched nanoparticles at a certain dendron concentration (5.88 × 10−6 M). In this procedure no external reducing agents or stabilizing agents nor relatively high temperature are required to produce various anisotropic gold nanoparticles. Nanoparticles with different shapes exhibit their characteristic surface plasmon resonance peaks. TEM images vividly demonstrate the specific morphology of gold nanoparticles with different shapes obtained by using different generations of peptide-based dendrons. Corresponding dark field luminescence spectra provide an insight into the shape dependence of the optical properties.

Self-organization of dendrons with focal pyrene moiety and diacetylene-containing periphery

We have synthesized the amide dendrons with diacetylene-containing periphery and the focal pyrene moiety to investigate their self-organization behavior in aqueous phase. We investigated the effect of the assembled structure of the dendron on the polymerizability of the peripheral diacetylene moiety and the possibility of the supramolecular transformation due to host-guest interactions between the dendron and β-CD. The second generation dendron with a focal pyrene group self-assembled into the fibrous aggregates in water. We observed a low degree of polymeization of diacetylene units upon photopolymerization from the fibrous aggregates possibly due to the amorphous nature of the hydrophobic core. After addition of β-CD into the aqueous solution of dendron 7, only a fraction of pyrenes of fibrous bundles could be included in β-CDs with maintaining a fibrous structure. We are currently carrying out a comprehensive study to understand the detailed mechanism of formation of different supramolecular structures with the modified amide dendrons and their supramolecular transformation so that we can provide a clear scope of the functional nanostructures which can be derived from selfassembly of the amide dendrons.

Superhydrophobic waxy-dendron-grafted polymer films via nanostructure manipulation

To imitate the superhydrophobicity of salient epicuticular wax on lotus leaves (hereafter “Lotus effect”), waxy dendrons were synthesized and subsequently grafted on amine-containing polystyrenes. To achieve a low surface energy and a specific surface morphology, the waxy dendron design is composed of two parts—the focal part possessing plenty of hydrogen bonding sites, and the peripheral part rich in van der Waals forces. The enhanced van der Waals force accompanied with increasing generation of dendrons helps induce self-assembly and phase separation in the preparation process of the polymer films. By different coating processes, three different films (thin film, honeycomb-like film, and three-dimensional rod-co-valley-like film) were obtained with contact angles of 95°, 130°, and 165°, respectively. The three-dimensional rod-co-valley film samples were able to imitate the superhydrophobic property (i.e. Lotus effect), as well as utilize the built-in strong hydrogen bonds to adhere water droplets on surfaces or substrates.

Elucidation of Architectural Requirements from a Spacer in Supported Proline‐Based Catalysts of Enantioselective Aldol Reaction

AbstractIn order to delineate the properties of the spacer architecture responsible for the strong positive dendritic effect exhibited by polymer‐supported proline‐based catalysts, we prepared two series of polystyrene‐bound model catalysts. The first series was based on a linear and partially dendritic spacers (of reduced branching and valency) imitating the length of the second generation spacer, while the second series was based on the first generation dendron spacer with one functional (proline‐terminated) and one non‐functional arm. Comparative studies of the model and original (fully dendritic) catalysts in the asymmetric aldol reaction of aromatic aldehydes with acetone disclose the features characteristic to the dendritic architecture, such as proximity between the terminal catalytic units and enhanced branching, as crucial for inducing higher yield and enantioselectivity in catalysis.

Nonlinear optical polyimides consisting of chromophore‐containing dendrons with site‐isolation effect

AbstractTo investigate the dendritic structure effects on the electro‐optical (EO) coefficients and thermal stability of the nonlinear optical (NLO) active materials, a bifunctional compound, IDD (4‐isocyanato‐4′(3,3‐dimethyl‐ 2,4‐dioxo‐acetidino)‐diphenylmethane) was used as a building block to synthesize a series of novel NLO chromophore‐containing dendritic structures including Generation 0.5 (G0.5) to Generation 3 (G3). The glass transition temperatures (Tg) of G1–G3 dendrons were in the range of 76–116°C, whereas only the G0.5 dendron exhibited a melting temperature (Tm), 98°C. Moreover, a series of NLO‐active guest–host systems ranging from polyimide‐G0.5 (PI‐G0.5) to polyimide‐G3 (PI‐G3) were prepared by blending 20 wt% chromophore‐containing dendron with a high Tg polyimide. EO coefficients ranged from 6.1 to 12.9 pm/V. The r33/dye content ratio increased with increasing generation of dendron‐containing polyimide samples. Particularly, the improvement in r33/dye content ratio of PI‐G2.5 sample tripled as compared to that of the guest–host sample with Disperse Red 1. Excellent temporal stability of PI‐G0.5 and PI‐G1.5 at 80°C was obtained. Moreover, waveguide properties for NLO polymers containing higher generation dendrons (3.1–3.6 dB/cm at 830 nm) were also obtained. Copyright © 2008 John Wiley & Sons, Ltd.

A Convergent Approach to Biocompatible Polyglycerol “Click” Dendrons for the Synthesis of Modular Core–Shell Architectures and Their Transport Behavior

Dendrimers are an important class of polymeric materials for a broad range of applications in which monodispersity and multivalency are of interest. Here we report on a highly efficient synthetic route towards bifunctional polyglycerol dendrons on a multigram scale. Commercially available triglycerol (1), which is highly biocompatible, was used as starting material. By applying Williamson ether synthesis followed by an ozonolysis/reduction procedure, glycerol-based dendrons up to the fourth generation were prepared. The obtained products have a reactive core, which was further functionalized to the corresponding monoazido derivatives. By applying copper(I)-catalyzed 1,3-dipolar cycloaddition, so-called "click" coupling, a library of core-shell architectures was prepared. After removal of the 1,2-diol protecting groups, water-soluble core-shell architectures 24-27 of different generations were obtained in high yields. In the structure-transport relationship with Nile red we observe a clear dependence on core size and generation of the polyglycerol dendrons.

Water‐soluble dendritic‐conjugated polyfluorenes: Synthesis, characterization, and interactions with DNA

AbstractThree generation of Boc‐protected dendritic‐conjugated polyfluorenes (Boc‐PFP‐G0‐2) were synthesized by Suzuki coupling 1,4‐phenyldiboronic ester with dendritic monomers that were synthesized through generation‐by‐generation approach. The gel permeation chromatography (GPC) analyses showed that the weight‐average molecular weight (Mw) of Boc‐PFP‐G0‐2 was in the range of 11,400–20,400 Da with the polydispersity index (PDI) in the range of 1.32–1.96. Treatment of Boc‐protected polymers with 6 M HCl in dioxane yielded cationic dendritic‐conjugated polyfluorenes (PFP‐G0‐2). They were soluble in common polar solvents such as DMSO, DMF, and water with absorption maxima between 345 and 379 nm. The solutions of PFP‐G0‐2 in water were highly fluorescent with emission maxima between 416 and 425 nm. Because higher generation dendrons could prevent the formation of π‐stacking aggregates of backbones of conjugated polymer, the fluorescence quantum efficiencies (QEs) of PFP‐G0‐2 enhance as the dendritic generation grew. The interactions between 25 mer double‐stranded DNA (dsDNA) and PFP‐G0‐2 were studied using ethidium bromide (EB) as fluorescent probe. The electrostatic bindings of PFP‐G0‐2 with dsDNA/EB complex result in displacement of EB from DNA double helix to the solution accompanying by a quenching of EB fluorescence. The PFP‐G2 with highest generation of dendritic side chains possessed a highest charge density and could form most stable complex with dsDNA. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7462–7472, 2008

Iterative Synthesis of 1,3,5-Polyphenylene Dendrons via C−H Activation

An iterative synthesis of 1,3,5-polyphenylene dendrons via C-H activation/Suzuki-Miyaura coupling up to a third generation dendron is described. C-H bonds at the focal points of the dendrons are selectively borylated via iridium-catalyzed borylation, eliminating the need for reactive protecting groups. Sequential additions of low catalyst loadings efficiently borylate higher-generation dendrons, whereas higher initial catalyst loadings are less efficient.

Solution‐Processible Red Iridium Dendrimers based on Oligocarbazole Host Dendrons: Synthesis, Properties, and their Applications in Organic Light‐Emitting Diodes

AbstractA series of novel red‐emitting iridium dendrimers functionalized with oligocarbazole host dendrons up to the third generation (red‐G3) have been synthesized by a convergent method, and their photophysical, electrochemical, and electroluminescent properties have been investigated. In addition to controlling the intermolecular interactions, oligocarbazole‐based dendrons could also participate in the electrochemical and charge‐transporting process. As a result, highly efficient electrophosphorescent devices can be fabricated by spin‐coating from chlorobenzene solution in different device configurations. The maximum external quantum efficiency (EQE) based on the non‐doped device configuration increases monotonically with increasing dendron generation. An EQE as high as 6.3% was obtained as for the third generation dendrimer red‐G3, which is about 30 times higher than that of the prototype red‐G0. Further optimization of the device configuration gave an EQE of 11.8% (13.0 cd A−1, 7.2 lm W−1) at 100 cd m−2 with CIE coordinates of (0.65, 0.35). The state‐of‐the‐art performance indicated the potential of these oligocarbazole‐based red iridium dendrimers as solution processible emissive materials for organic light‐emitting diode applications.

Janus-type dendrimer-like poly(ethylene oxide)s.

A straightforward and original methodology allowing the synthesis of Janus-type dendrimer-like poly(ethylene oxide)s (PEOs) carrying orthogonal functional groups on their surface is described. The use of 3-allyloxy-1,2-propanediol (1) as a latent AB2-type heterofunctional initiator of anionic ring-opening polymerization (AROP) of ethylene oxide (EO) and of selective branching agents of PEO chain ends served to construct the two dendrons of these dendrimer-like PEOs, following a divergent pathway. Thus, the first PEO generation of the first dendron was grown by AROP from 1 followed by the reaction of the corresponding alpha-allyl,omega,omega'-bishydroxy- heterofunctional PEO derivative with 2-(3'-chloromethybenzyloxymethyl)-2-methyl-5,5-dimethyl-1,3-dioxane (2) used as a branching agent. This afforded the dendron A with four latent peripheral hydroxyls protected in the form of two ketal rings. The remaining alpha-allylic double bond of the PEO thus prepared was transformed into two hydroxyl groups using OsO4 in order to create the first PEO generation of the dendron B by AROP of EO. Allyl chloride (3) was then used as another (latent) branching agent to react with the terminal hydroxyl of the corresponding PEO chains. Deprotection under acidic conditions of the ketal groups of dendron A, followed by AROP of EO, afforded the second PEO generation on this face. This alternate and divergent procedure, combining AROP of EO and selective branching of PEO branches, could be readily iterated, one dendron after the other up to the generation six, leading to a Janus-type dendrimer-like PEO exhibiting a total mass of around 300 kg/mol and possessing 64 peripheral groups on each face. The possibility of orthogonal functionalization of the surfaces of such Janus-type dendritic PEOs was exploited. Indeed, a dendron of generation 4 was functionalized with hydroxyl functions at its periphery, whereas the other was end-capped with either tertiary amino or disulfide groups. In a variant of this strategy, azido groups and acetylene could also be orthogonally introduced at the periphery of the fourth generation Janus-type dendrimer-like PEO and subjected to polycondensation by a 1,3-dipolar cycloaddition reaction. This afforded a necklace-like covalent assembly of dendrimer-like PEOs through the formation of stable [1,2,3]-triazole linkages.

Solution Behavior of Dendrimer-Coated Rodlike Coordination Polymers

We report here the synthesis and self-assembly behavior of a series of three coordination polymers (1−3) insulated with different generations and lengths of oligoether dendrons. Their self-assembly behavior in solutions was investigated according to the variation in generation of dendrons and solvent polarity by using spectroscopic measurements, molecular calculation, and transmission electron microscopy (TEM). In contrast, polymer 1 based on first-generation dendrons with diethylene oxide unit and polymers 2 and 3 based on higher volume fractions of dendrons are readily soluble in common organic solvents and H2O. Polymer 2 based on first-generation dendrons with triethylene oxide unit was observed to self-assemble into nanorods consisting of two π-stacked polymer backbones in both nonpolar CHCl3 solvent and film, whereas 2 exists as isolated single polymer chains in polar solvents. Polymer 3 based on second-generation dendrons with tetraethylene oxide units also exists as isolated single polymer chains in both solution and film states. Notably, the CHCl3 solution of 2 at concentrations above 2 wt % was observed to be a nematic fluid, resulting from the formation of rigid polymer backbone driven by π-stacking, whereas the THF solution of 2 undergoes gelation at concentrations above 2 wt %. The primary driving force for this unique self-assembling behavior is proposed to be the solvation ability of solvent molecules to silver salts and the coverage ability of dendrons to insulate the conjugated backbones.

Synthesis, characterization, and optoelectronic properties of hyperbranched polyfluorenes containing pendant benzylether dendrons

AbstractLinear polyfluorenes with low generation of side benzylether dendrons (PLG0, PLG1) and hyperbranched polyfluorenes with 1,3,5‐benzene branch unit (PHG0 and PHG1) were prepared by the Suzuki coupling reaction to investigate the structural effect on optoelectronic properties. Their optical properties, both in solution and film state, were investigated using absorption and photoluminescence (PL) spectra. The excimer emission of polyfluorene at about 530 nm, induced by thermal annealing, was completely suppressed by the hyperbranched structure, but the suppression was not obvious by the side benzylether dendrons. The optoelectronic performance of the EL devices (ITO/PEDOT:PSS/polymer/Ca/Al) was strongly dependent upon chemical structures of the emitting polyfluorenes. The hyperbranched PHG0 with zero generation of benzylether side groups revealed the best device performance, with maximal luminance and maximal luminance efficiency of 2350 cd/m2 and 0.33 cd/A, respectively. The results suggest that incorporation of branch units with low generation of benzylether dendrons is an effective way to improve annealing stability and EL performance of the polyfluorenes. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5945–5958, 2008

Cooperativity and Tunable Excited State Deactivation: Modular Self-Assembly of Depsipeptide Dendrons on a Hamilton Receptor Modified Porphyrin Platform

A series of novel supramolecular architectures were built around a tin tetraphenyl porphyrin platform 6--functionalized by a 2-fold 1-ethyl-3-3-(3-dimethylaminopropyl)carbodiimide (EDC) promoted condensation reaction--and chiral depsipeptide dendrons of different generations 1-4. Here, implementation of a Hamilton receptor provided the necessary means to keep the constituents together via strong hydrogen bonding. Characterization of all architectures has been performed, including 4 which is the fourth generation, on the basis of NMR and photophysical methods. In particular, several titration experiments were conducted suggesting positive cooperativity, an assessment that is based on association constants that tend to be higher for the second binding step than for the first step. Importantly, molecular modeling calculations reveal a significant deaggregation of the intermolecular network of 6 during the course of the first binding step. As a consequence, an improved accessibility of the second Hamilton receptor unit in 6 emerges and, in turn, facilitates the higher association constants. The features of the equilibrium, that is, the dynamic exchange of depsipeptide dendrons 1-4 with fullerene 5, was tested in photophysical reference experiments. These steady-state and time-resolved measurements showed the tunable excited-state deactivations of these complexes upon photoexcitation.

Protein Resistance of PEG-Functionalized Dendronized Surfaces: Effect of PEG Molecular Weight and Dendron Generation

Dendronized surfaces were prepared by chemisorption of poly(ethylene glycol) monothiol (HS-PEG650-OH) onto gold-coated silicon wafers followed by functionalization of the PEG terminal OH group with aliphatic polyester dendrons, generation 1−4, using divergent dendron growth. PEG monomethyl ether (PEG-OMe) chains of various molecular weight (MW) were covalently attached to the peripheral hydroxyl groups of the dendronized surfaces via EDC coupling and investigated for protein adsorption. Protein adsorption studies were carried out using fibrinogen (Fg) and lysozyme (Lys) as model proteins from phosphate buffered saline (PBS) (Fg, Lys) and plasma (Fg). In the first part of this study, the effect of functionalization of the peripheral hydroxyl groups with PEG-OMe oligomers (Mn = 350 Da) on protein adsorption was investigated. Results showed that adsorption of both Fg and Lys was reduced when dendronized surfaces were grafted with PEG-OMe oligomers. To investigate the effect of molecular weight on protein adsorption, PEG-OMe chains of greater length (750, 2000, and 5000 Da) were coupled to first generation dendronized surfaces (Au-G1(OH)). Results showed that protein adsorption decreased with increasing PEG-OMe MW up to 2000 Da. To further investigate the effect of dendron generation on protein resistance, dendronized surfaces of generation 1−4 were coupled with PEG2000-acid. Subsequent protein studies showed a decrease in Fg and Lys adsorption with increasing dendron generation.

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.

Photoinduced energy transfer in bichromophoric co-assemblies based on amphiphilic poly(amidoamine) dendrons

Spherical co-assemblies were generated from an equivalent-molar-ratio mixture of amphiphilic poly(amidoamine) dendrons that were modified with focal carbazole and/or pyrene chromophores from the first to the third generation (D nCz I and D nPy, where n = 1 , 2, 3 denotes the generation of dendrons) in water at a concentration of 3.0 × 10 −6 M , lower than the individual critical aggregation concentrations (CAC) of either D nCz I or D nPy. UV–vis absorption and fluorescence emission spectra indicated that fluorescence resonance energy transfer (FRET) from carbazole to pyrene units occurred efficiently within the co-assemblies. Further study implied that part of the carbazole and/or pyrene units stacked homogeneously in the co-assemblies via π – π stacking and the dendritic branches served as connectors due to the hydrogen-bonding interaction.

Segment Block Dendrimers via Diels−Alder Cycloaddition

Segment block dendrimers consisting of polyester and polyaryl ether dendrons were synthesized using reagent free Diels-Alder cycloaddition reactions. Three generations of furan functionalized polyaryl ether dendrons were reacted with maleimide functionalized polyester dendrons of the same generation to obtain segment block dendrimers in good yields. The thermoreversible nature of these macromolecules was investigated by subjecting them to elevated temperatures in the presence of anthracene as a scavenger diene.