Dendritic Molecules Research Articles

Supramolecular architectures from bent-core dendritic molecules.

Control of the self-assembly of small molecules to generate architectures with diverse shapes and dimensions is a challenging research field. We report unprecedented results on the ability of ionic, bent dendritic molecules to aggregate in water. A range of analytical techniques (TEM, SEM, SAED, and XRD) provide evidence of the formation of rods, spheres, fibers, helical ribbons, or tubules from achiral molecules. The compact packing of the bent-core structures, which promotes the bent-core mesophases, also occurs in the presence of a poor solvent to provide products ranging from single objects to supramolecular gels. The subtle balance of molecule/solvent interactions and appropriate molecular designs also allows the transfer of molecular conformational chirality to morphological chirality in the overall superstructure. Functional motifs and controlled morphologies can be combined, thereby opening up new prospects for the generation of nanostructured materials through a bottom-up strategy.

Supramolecular Architectures from Bent‐Core Dendritic Molecules

AbstractControl of the self‐assembly of small molecules to generate architectures with diverse shapes and dimensions is a challenging research field. We report unprecedented results on the ability of ionic, bent dendritic molecules to aggregate in water. A range of analytical techniques (TEM, SEM, SAED, and XRD) provide evidence of the formation of rods, spheres, fibers, helical ribbons, or tubules from achiral molecules. The compact packing of the bent‐core structures, which promotes the bent‐core mesophases, also occurs in the presence of a poor solvent to provide products ranging from single objects to supramolecular gels. The subtle balance of molecule/solvent interactions and appropriate molecular designs also allows the transfer of molecular conformational chirality to morphological chirality in the overall superstructure. Functional motifs and controlled morphologies can be combined, thereby opening up new prospects for the generation of nanostructured materials through a bottom‐up strategy.

Elucidating factors leading to acidolytic degradation of sterically strained oligoether dendrons.

While steric hindrance can be particularly large in dendritic molecules, it is usually implicated with difficulties in the synthesis of higher generation structures and restricted access of reagents, including bond-cleaving agents, to the dendritic interior. A different situation, where the steric hindrance is translated into a steric strain within the dendritic molecule and, consequently, causes enhanced decomposition of the dendron-containing structure, has only occasionally been reported and exclusively for dendronized polymers. In this work we describe post-synthetic cleavage of sterically congested third-generation oligoether dendrons from solid supports, followed by their disassembly into monomeric building blocks under acidic conditions. This disassembly was monitored by H NMR, revealing the intermediate fragment structures and the exact order of bond cleavage within the dendritic molecule. These conclusions were supported by MS analysis of the cleavage mixtures. Though distinguishing between steric and electronic reasons of molecular disassembly can be a challenging task, we were able to analyze these factors separately by monitoring the dendron disassembly and comparing the rates of "decay" of parent dendrons and their fragments. This comparison reveals that while the electron donation of the steric congestion-inducing alkyl substituents is a prerequisite for the disassembly of the structures, this disassembly is very strongly accelerated in the sterically crowded dendrons and intermediates, where both steric and electronic factors contribute in a synergistic way to the disassembly phenomenon.

Potassium ion-mediated synthesis of highly water-soluble dendritically functionalized melanins

A series of melanin derivatives has been prepared by functionalization with dendritic azobenzene molecules under basic conditions with (K2CO3) and without (Et3N) a potassium ion. The dendritic melanins treated with K2CO3 show excellent solubility in water and tetrahydrofuran. This may be due to the multivalent chelating properties between the heteroatoms of melanin and potassium ions. These highly soluble dendritic melanins were coupled with TiO2 nanotube arrays easily for enhanced photocurrent generation.

Advanced Monte Carlo Model for Arborescent Polyisobutylene Production in Batch Reactor

An advanced Monte Carlo (MC) model is developed to predict the molecular weight distribution and branching level for arborescent polyisobutylene produced in a batch reactor via carbocationic copolymerization of isobutylene and an inimer. This new MC model uses differential equations and random numbers to determine the detailed structure of dendritic polymer molecules. Results agree with those from a traditional MC model for the same system, but the proposed model requires considerably less computational effort. The proposed MC model is also used to obtain information about polymer segments between branch points and dangling polymer segments.

Modular Construction of Dynamic Nucleodendrimers

AbstractIsoguanosine‐containing dendritic small molecules self‐assemble into decameric nucleodendrimers as observed by 1D NMR spectroscopy, 2D DOSY, and mass spectrometry. In particular, apolar building blocks readily form pentameric structures in acetonitrile while the presence of alkali metals promotes the formation of stable decameric assemblies with a preference for cesium ions. Remarkably, co‐incubation of guanosine and isoguanosine‐containing nucleodendrons results in the formation of decameric structures in absence of added salts. Further analysis of the mixture indicated that guanosine derivatives facilitate the formation, but are not involved in decameric structures; a process reminiscent of molecular crowding. This molecular system provides a powerful canvas for the rapid and modular assembly of polyfunctional dendritic macromolecules.

Modular Construction of Dynamic Nucleodendrimers

Isoguanosine-containing dendritic small molecules self-assemble into decameric nucleodendrimers as observed by 1D NMR spectroscopy, 2D DOSY, and mass spectrometry. In particular, apolar building blocks readily form pentameric structures in acetonitrile while the presence of alkali metals promotes the formation of stable decameric assemblies with a preference for cesium ions. Remarkably, co-incubation of guanosine and isoguanosine-containing nucleodendrons results in the formation of decameric structures in absence of added salts. Further analysis of the mixture indicated that guanosine derivatives facilitate the formation, but are not involved in decameric structures; a process reminiscent of molecular crowding. This molecular system provides a powerful canvas for the rapid and modular assembly of polyfunctional dendritic macromolecules.

Dynamic coupling of ALCAM to the actin cortex strengthens cell adhesion to CD6

At the immunological synapse, the activated leukocyte cell adhesion molecule (ALCAM) on a dendritic cell (DC) and CD6 molecules on a T cell contribute to sustained DC-T-cell contacts. However, little is known about how ALCAM-CD6 bonds resist and adapt to mechanical stress. Here, we combine single-cell force spectroscopy (SCFS) with total-internal reflection fluorescence microscopy to examine ALCAM-CD6-mediated cell adhesion. The combination of cells expressing ALCAM constructs with certain cytoplasmic tail mutations and improved SCFS analysis processes reveal that the affinity of ALCAM-CD6 bonds is not influenced by the linking of the intracellular domains of ALCAM to the actin cortex. By contrast, the recruitment of ALCAM to adhesion sites and the propensity of ALCAM to anchor plasma membrane tethers depend on actin cytoskeletal interactions. Furthermore, linking ALCAM to the actin cortex through adaptor proteins stiffens the cortex and strengthens cell adhesion. We propose a framework for how ALCAMs contribute to DC-T-cell adhesion, stabilize DC-T-cell contacts and form a mechanical link between CD6 and the actin cortex to strengthen cell adhesion at the immunological synapse.

Ambipolar Freestanding Triphenylamine/Fullerene Thin-film by Electrochemical Deposition and Its Read-Writable Properties by Electrochemical Treatments

Novel dendritic triphenylamine substituted fullerene as a rigid scaffold for electropolymerization have been prepared through cycloaddition reactions. Oxidative electrochemical polymerization of the dendritic molecules leads to thick freestanding fullerene/polytriphenylamine composite films. Due to their porous properties that allow n-Bu4NBF4 and n-Bu4NClO4 migrating in and out of the electrochemical deposition (ECD) layers, the films are highly electrochemically active with amphoteric properties. However, the fullerene reduction can be swept out and become silent after a high reduction potential of -2.4V being applied to the film. If one considers the status before being treated by a high reduction potential as “1” that could be read out by the fullerene reduction currents, and the status after being treated as “0”, in which the reduction current is silent, information of “1” or “0” could then be written on the plate through electrochemistry. Since the channel of anodic oxidation of the polytriphenylamine component still maintains electrochemically active after the reduction shock, the anodic oxidation current can be used as reference for alignment.

Branching Defects in Dendritic Molecules: Coupling Efficiency and Congestion Effects

An analytical model supplemented by Monte Carlo simulations specifies the statistics of branching defects in dendritic molecules as a function of the generation g as well as the maximal g for which defect-free synthesis is possible, gmax. The defects arise because of (i) imperfect coupling efficiency characterized by a constant fraction P ≤ 1 of successful add-on reactions in the absence of excluded volume effects and (ii) packing constraints associated with steric congestion at high g when the maximal density is approached. The model specifies ng, the number of junctions, and the number of defects for both g ≤ gmax and g > gmax, as well as gmax and its dependence on P. The branching polydispersity is characterized by the average number of junction–junction bonds, Xgeff. For g < gmax and efficient synthesis Xgeff is weakly reduced with respect to X, its value in defect-free molecules, and ng ∼ (Xeff – 1)g increases exponentially. In the congested regime, at g > gmax, branching is strongly reduced, and Xgeff slowly approaches 2 as Xgeff – 2 ∼ 1/g while ng eventually exhibits power law growth: ng ∼ g3 for dendrimers and ng ∼ g2 for dendronized polymers. The branching defects can be interrogated by different forms of end-group analysis utilizing the theory framework proposed.

The synthesis and study of fluorescent PAMAM-based dendritic molecules

New water soluble and fluorescent PAMAM-based dendritic molecules based on the naphthalimide derivative 7H-benz[de]benzimidazo[2,1-a]isoquinoline-7-one as the fluorescent unit, have been prepared and their photophysical properties studied in aqueous solution over a wide pH range. The dendrons are all strongly fluorescent through an intramolecular charge transfer (ICT) excited state, but this can be modulated by photoinduced electron transfer (PeT) processes, which increases with higher PAMAM dendron generation.

Synthetic regimes due to packing constraints in dendritic molecules confirmed by labelling experiments

Classical theory predicts that branching defects are unavoidable in large dendritic molecules when steric congestion is important. Here we report first experimental evidence of this effect via labelling measurements of an extended homologous series of generations g = 1...6 of dendronized polymers. This system exhibits a single type of defect interrogated specifically by the Sanger reagent thus permitting to identify the predicted upturn in the number of branching defects when g approaches g(max) and the polymer density approaches close packing. The average number of junctions and defects for each member of the series is recursively obtained from the measured molar concentrations of bound labels and the mass concentrations of the dendritic molecules. The number of defects increases at g=5 and becomes significant at g=6 for dendronized polymers where the g(max) was estimated to occur at 6.1 ≤ g(max) ≤ 7.1. The combination of labelling measurements with the novel theoretical analysis affords a method for characterizing high g dendritic systems.

Dendritic Chemistry Applied to the Construction of Tailored Functional Nanomaterials: Synthesis and Characterization of Gold Nanoparticle-Cored Dendrimers (NCDs)

A family of Newkome-type dendritic molecules bearing a disulfide anchor group, different peripheral groups (-COOtBu or - COOH) and sizes or generations (Gn=0-2) was synthesized and used as ligand for the synthesis of gold nanoparticle-cored dendrimers (NCDs). These hybrid materials were characterized by means of UV-vis spectroscopy, TEM, and IR and NMR spectroscopies, finding that the capping molecules of the organic shell determine the solubility and stability of the different NCDs, as well as the characteristics of the inorganic core through a dendritic control characteristic for Newkome-type ligands. Remarkably, a combination of detailed IR and NMR studies allows for studying the ligand-ligand and ligand-core interactions that take place in these materials. It is demonstrated that concepts, techniques and methods normally used in organic and dendritic chemistry, such as the synthesis of precisely controlled architectures and their rigorous spectroscopic characterization, can be efficiently applied to engineer novel organic-inorganic hybrid materials with enhanced properties. Keywords: Dendritic control, IR spectroscopy, Nanoparticle-cored dendrimers, NMR spectroscopy, Organic-inorganic hybrid nanomaterials, Tailored properties.

Optical and electro-optical properties of silicon-contaning thiophene derivatives of star-shaped and dendritic structure

Star-thiophene derivatives with a silicon atom as the branching center were investigated by absorption spectroscopy and electro-optical Kerr effect in solutions at variations in a number and chemical structure of branches. The star-shaped oligomers were compared with dendritic analogues containing silicon atoms at the points of branching. It is shown that thiophene-containing moieties determine both spectral and electrooptical properties of the molecules. Molecular parameters of the star-shaped oligomers of various structure vary identically with increasing the number of branches. The absorption of star-shaped oligomers is additive due to the autonomy of the absorption of radiation by the separate branches. For dendritic molecules the additive nature of the absorption is kept, but their electro-optical properties are independent of a generation number. It was shown that the latter is a consequence of the manifestation by dendrimers of deformation flexibility, which is not peculiar to the starshaped derivatives.

Astrocytes inhibit microglial surface expression of dendritic cell‐related co‐stimulatory molecules through a contact‐mediated process

Murine microglia cultured in isolation were treated sequentially with granulocyte/monocyte colony-stimulating factor (GM-CSF) (5days) and lipopolysaccharide (LPS) (2days) to elicit a mature dendritic cell-like (DC-like) phenotype. Examined by flow cytometry microglia thus isolated show high surface expression of CD11c together with the co-stimulatory molecules CD40, CD80, and CD86 that are necessary for T-cell activation. In contrast, microglia co-cultured with astrocytes fail to achieve a mature DC-like phenotype. Contact with the astrocytic environment is necessary for the inhibition. Failure was not because of a more rapid degradation of protein. Bone marrow-derived cells, like microglia, were prevented by astrocytes from attaining a mature DC phenotype. Although GM-CSF pre-treatment substantially increases mRNA of co-stimulatory molecules and major histocompatibility complex (MHC) Class II in isolated microglia, co-cultured microglia await treatment with LPS to up-regulate them. In contrast, western blot and immunocytochemical analysis revealed that it is not a failure of transcription or translation, nor is it a more rapid degradation of mRNA that is responsible for the low surface expression; rather microglia co-cultured with astrocytes produce mRNA and protein but do not traffic the protein onto the cell surface.

Dendrigraft with citric acid on acrylonitrile/acrylic acid copolymer electrospun fibres

Acrylonitrile/acrylic acid copolymers were synthesized with different mole fractions (1, 2, 5 and 10 mol%) of acrylic acid (AA) in the feed by aqueous suspension polymerization, and bead-free fibres (295–375 nm in diameter) were made from the copolymers in dimethylformamide solutions by electrospinning. In a heterogeneous system containing electrospun fibre mats, dendritic molecules were grown by reaction between carboxylic acid of AA and –OH groups of citric acid activated by dicyclohexylcarbodiimide. The products were analysed using 1H NMR, 13C NMR and Fourier transform infrared spectroscopies, density determination, scanning electron microscopy, optical microscopy, differential scanning calorimetry and pH response properties. With decreasing AA content, the isotacticity of the copolymers decreases with a more random distribution of the co-monomers which leads to higher percentage conversions of dendrigraft due to reduced steric hindrance. On the formation of dendrigraft, the percentage conversion of the reactions decreases with an increase of generation number and AA content. A reduction of density for the first generation and then an increase with increasing generation are observed. During oxidation stabilization of fibres with a decrease of AA content and an increase in generation number, the amount of liberated heat increases. Fibres containing more carboxylic groups show significantly greater amounts of swelling/de-swelling in basic and acidic media, respectively. To be used as nanocarbon fibre precursors, or as active particles for loading with guest molecules, or as pH actuators, the first generation of dendrigrafted fibres are expected to have the greatest potential among the various samples examined. © 2013 Society of Chemical Industry

Dendritic Amphiphiles Strongly Affect the Biophysical Properties of DPPC Bilayer Membranes

Molecular dynamics (MD) simulations were used to gain insight on the molecular interactions in a model biological membrane comprised of a bilayer with DPPC (dipalmitoylphosphotidylcholine) and antimicrobial dendritic amphiphile molecules [RCONHC(CH(2)CH(2)COOH)(3), where R is the saturated hydrocarbon tail (R = n-C(n)H(2n+1)), to be abbreviated as 3CAmn]. This study analyzes different biophysical properties of the equilibrated mixed bilayers, at 300 and 325 K, to determine how the presence of the 3CAmn, in varying concentrations and tail lengths, affects the lipid bilayer. Lipid tail order parameter data, bilayer thickness trends, and qualitative lipid tail tilt observations suggest that a molar ratio of 0.2 3CAm19/DPPC is sufficient to induce a phase transition in the bilayer from gel to liquid crystalline at 300 K. These results also imply that the phase transition temperature of the mixed bilayer decreases upon incorporation of higher concentrations of 3CAm19. Hydrogen bonding takes place between the 3CAmn and DPPC at specific sites, as evidenced by the radial distribution function. Increased hydrogen bonding and the smaller headgroup size of the 3CAmn molecule result in a decrease in the total lateral area with higher concentrations of 3CAm19. Diffusion constants of 3CAmn varied with concentration and tail length; diffusion constants of DPPC and 3CAm19 increased with increasing 3CAm19 concentration at 300 K and shorter 3CAmn tails had higher diffusion constants at both temperatures. These computational studies provide a comprehensive understanding of the biophysical changes to model biological membranes by the association of 3CAmn.

N‐Boc‐Protected 1,2‐Diphenylethylenediamine‐Based Dendritic Organogels with Multiple‐Stimulus‐Responsive Properties

A new class of poly(benzyl ether) dendrimers, decorated in their cores with N-Boc-protected 1,2-diphenylethylenediamine groups, were synthesized and fully characterized. It was found that the gelation capability of these dendrimers was highly dependent on dendrimer generation, and the second-generation dendrimer (R,R)-G(2)DPENBoc proved to be a highly efficient organogelator. A number of experiments (SEM, TEM, FTIR spectroscopy, (1)H NMR spectroscopy, rheological measurements, UV/Vis absorption spectroscopy, CD, and XRD) revealed that these dendritic molecules self-assembled into elastically interpenetrating one-dimensional nanostructures in organogels. The hydrogen bonding, π-π, and solvophobic interactions were found to be the main driving forces for formation of the gels. Most interestingly, these dendritic organogels exhibited smart multiple-stimulus-responsive behavior upon exposure to environmental stimuli such as temperature, anions, and mechanical stress.

Multivalent dendritic molecules as broad spectrum bacteria agglutination agents.

This study reports the first set of synthetic molecules that act as broad spectrum agglutination agents and thus are complementary to the specific targeting of antibodies. The molecules have dendritic architecture and contain multiple copies of zinc(II)-dipicolylamine (ZnDPA) units that have selective affinity for the bacterial cell envelope. A series of molecular structures were evaluated, with the number of appended ZnDPA units ranging from four to thirty-two. Agglutination assays showed that the multivalent probes rapidly cross-linked ten different strains of bacteria, regardless of Gram-type and cell morphology. Fluorescence microscopy studies using probes with four ZnDPA units indicated a high selectivity for bacteria agglutination in the presence of mammalian cells and no measurable effect on the health of the cells. The high bacterial selectivity was confirmed by conducting in vivo optical imaging studies of a mouse leg infection model. The results suggest that multivalent ZnDPA molecular probes with dendritic structures have great promise as selective, broad spectrum bacterial agglutination agents for infection imaging and theranostic applications.

Amidoamine dendron-based co-adsorbents: improved performance in dye-sensitized solar cells

Three dendritic molecules EG0, EG1 and EG2 were synthesized and separately used as co-adsorbents in dye-sensitized solar cells (DSSCs) to explore their effect on controlling interfacial charge recombination. It was found that the resulting device with EG2 as the co-adsorbent (device EG2) displayed superior open-circuit voltage (VOC), improved short-circuit current density (JSC) and a higher power conversion efficiency (PCE) of 7.50%, as compared to the control device (N719) and the devices with EG0 or EG1 as the co-adsorbent (devices EG0 and EG1). Such improved performance is ascribed to the increase of the interfacial distance upon the introduction of EG2, and the formation of a more compact monolayer of the dye N719 and co-adsorbent EG2 at the interface, which shields the acceptors in the electrolyte from contacting the TiO2 surface, hence reducing the back current and increasing the VOC. On the other hand, the aggregation of dye molecules is restrained due to the competitive adsorption between dye N719 and co-adsorbent EG2, thus leading to the minimization of energy loss and the enhancement of JSC. As a co-adsorbent, EG2 was also introduced to DSSCs using either the iodide electrolyte without the additive 4-tert-butylpyridine (TBP) or the cobalt electrolyte. The results showed that EG2 also displayed effective capability of suppressing interfacial charge recombination and reducing intermolecular energy transfer.