Convergent synthesis of internally branched PAMAM dendrimers

[reaction: see text] A series of aliphatic internally branched poly(amido amine) dendrons and dendrimers has been synthesized. The internal branching unit was 1,2-propanediamine and a series of PAMAM-type dendrons of the types AB2, AB4, and AB8 were built. These were anchored on a core molecule containing four carboxylic acid moieties and the 1,2-propanediamine unit resulted in PAMAM dendrimers with 4, 8, 16, and 32 end groups.     

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 photo- and electroactive stilbenoid dendrimers carrying dibutylamino peripheral groups

[structure: see text]. A novel convergent synthetic route for the preparation of functionalized and fluorescent stilbenoid dendrons built on the 1,3,5-benzene core and endowed with a periphery of dibutylamino groups has been developed. Long alkyl chains have been incorporated on the peripheral amino moieties to increase the solubility of the final products. Good donor ability of the new dendrimers has been observed by cyclic voltammetry measurements as a result of the presence of the peripheral dibutylaniline moieties.     

The synthesis and properties of iridium(III)-cored dendrimers with carbazole peripherally functionalized beta-diketonato dendrons

A simple convergent synthetic approach has been developed for the synthesis of iridium(III)-cored dendrimers with carbazole peripherally functionalized beta-diketonato dendrons. The zeroth- to third-generation green-emitting dendrimers were synthesized by reacting the corresponding beta-diketonato dendrons with iridium(III) dimer under mild conditions with good yields, respectively. This approach proved to be modular, and could be used to prepare blue-green-emitting and red-emitting dendrimers with the same beta-diketonato dendrons only by using different cyclometallating ligands. The resulting dendritic ligands and iridium(III)-cored dendrimers were well characterized. Their photoluminescent properties both in solution and in the solid state were tested. It was found that all the dendrimers retained the photophysical properties of the corresponding small analogues with high emission quantum yields (0.06-0.30). Preliminary results indicated that these dendrimers functionalized carbazole units exhibited distinct light-harvesting potential, resulting in a strong intense emission from the iridium core of the dendrimers.     

Multifunctional Supramolecular Dendrimers with an s‐Triazine Ring as the Central Core: Liquid Crystalline,Fluorescence and Photoconductive Properties

Novel liquid crystal (LC) dendrimers have been synthesised by hydrogen bonding between an s‐triazine as the central core and three peripheral dendrons derived from bis(hydroxymethyl)propionic acid. Symmetric acid dendrons bearing achiral promesogenic units have been synthesised to obtain 3:1 complexes with triazine that exhibit LC properties. Asymmetric dendrons that combine the achiral promesogenic unit and an active moiety derived from coumarin or pyrene structures have been synthesised in order to obtain dendrimers with photophysical and electrochemical properties. The formation of the complexes was confirmed by IR and NMR spectroscopy data. The liquid crystalline properties were investigated by differential scanning calorimetry, polarising optical microscopy and X‐ray diffractometry. All complexes displayed mesogenic properties, which were smectic in the case of symmetric dendrons and their complexes and nematic in the case of asymmetric dendrons and their dendrimers. A supramolecular model for the lamellar mesophase, based mainly on X‐ray diffraction studies, is proposed. The electrochemical behaviour of dendritic complexes was investigated by cyclic voltammetry. The UV/Vis absorption and emission properties of the compounds and the photoconductive properties of the dendrons and dendrimers were also investigated     

Synthesis of novel dendrimers containing pyrimidine units

A novel convergent approach to dendritic macromolecules is described in which 4,6-dichloro-2-(4-methoxyphenyl)-pyrimidine is used as the building block. The nucleophilic aromatic substitution reaction at this AB₂-monomer was used as the key step in the propagation of the dendrons. Different core reagents were used to form the dendrimers, including a 5,15-bis(pyrimidyl)porphyrin core. Fourth-generation dendrons and third-generation dendrimers could be synthesized. The presented dendrimers are promising candidates to be used in applications where a more rigid structure and a larger resistance towards the applied conditions is required.     

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.     

Searching for new host compounds: synthesis and characterization of novel crown ether-functionalized dendrimers

Novel lipophilic dendrimers as host compounds, that is, 7-15, containing crown ether moieties with different sizes as the core, surrounded by first, second or third generation poly(aromatic ether) wedges, were synthesized by the use of bis(bromomethyl)-substituted crown ethers and Fréchet-type poly(benzyl ether) dendrons as building blocks. The compounds were fully characterized.     

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.     

Azide‐functionalized nanoparticles as quantized building block for the design of soft–soft fluorescent polystyrene core—PAMAM shell nanostructures

This work deals with the covalent coupling of azide‐functionalized polymeric nanoparticles as a reactive core and amino‐terminated PAMAM dendrons as a reactive shell. The nanoassemblies thereby obtained could be modified after the dendronization step by grafting an alkynyl Bodipy dye on the unreacted azide moieties. Only a few steps are required to attain nanoassemblies that could mimic dendrimers of high generation with sizes of nano‐objects beyond those of dendrimers. The structure of the nanoassemblies are composed of a polystyrene core, an inner shell including the Bodipy dyes along with the internal branches of the PAMAM, and the terminal amino groups from the outer shell. The dendritic shell acts as a protective layer that prevents NP from aggregation in a surfactant free aqueous solution. The nano‐objects display absorption and emission maxima above 500 nm with brightness that are the same order of magnitude than Qdots. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 115–126     

Photoswitchable flexible and shape-persistent dendrimers: comparison of the interplay between a photochromic azobenzene core and dendrimer structure

Two analogous classes of dendrimers with a single azobenzene moiety at the core have been prepared. Flexible benzyl aryl ether dendrimers 1a-e were obtained in good yields by direct alkylation of diphenolic azobenzene 3 with benzyl aryl ether dendrons [G-n]-Br (n = 0-4). In rigid dendrimers 2a-e, the azobenzene configurational switch was linked to phenylacetylene dendrons through acetylenic linkages to maintain the shape-persistent nature of these dendrimers. A comparison of these two different classes of dendrimers with azobenzene cores reveals a difference in the properties of the photochromic moiety upon dendritic incorporation as well as a significant difference in the photomodulation of dendrimer properties. The E --> Z photoisomerization quantum yield decreased markedly with increasing generation for dendrimers 1a-e but only slightly for dendrimers 2a-e. However, increasing generation did not significantly alter thermal isomerization kinetics or activation barriers. The hydrodynamic volumes of azobenzene-containing dendrimers 2b-e were significantly modulated when the azobenzene unit is subjected to irradiation, while those of dendrimers 1b-e were only slightly affected.     

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.     

Designer dendrimers: branched oligosulfonimides with controllable molecular architectures

The synthesis of "designer" dendrimers and dendrons with sulfonimide units at every branching point is reported. The synthesis is based on a series of (regio)selective functionalization reactions of amines and sulfonamides allowing precise control of the dendrimers' shape, the number of branches in each generation, and their peripheral decoration with functional groups. In principle, structurally different branches can be incorporated at any position within the dendrimer structure at will. Structurally perfect symmetrical and two-faced "Janus"-type dendrimers, as well as dendrimers and dendrons with intended interstices were synthesized on a preparative scale and fully characterized. Oligosulfonimide dendrons of various generations bearing an aryl bromide functional group at their focal points were attached to a p-phenylene core with the aid of Suzuki cross-coupling reactions resulting in dendrimers with photoactive terphenyl cores. The structure and the high purity of all dendritic sulfonimides were confirmed by means of (1)H and (13)C NMR, electrospray ionization mass spectrometry (ESI-MS), and elemental analysis. The utility of MALDI-TOF mass spectrometry for the analytical characterization of these dendrimers was evaluated in comparison to electrospray ionization. Two model branched oligosulfonimides were characterized in the solid state by single-crystal X-ray analysis. Reaction selectivities and conformation of sulfonimide branching points were rationalized by DFT calculations.     

Synthesis and photophysical properties of triphenylamine-based dendrimers with 1,3,5-triphenylbenzene cores

Two new conjugated dendrimers bearing a triphenylamine moiety as dendrons and 1,3,5-triphenylbenzene as a core have been synthesized through a convergent synthetic strategy. These conjugated dendrimers have high fluorescence quantum yields and exhibit similar absorption and emission behaviors in solutions and in solid films, which demonstrate that these dendrimers form good amorphous states.     

Controlling solubility and modulating peripheral function in dendrimer encapsulated dyes

The synthesis of large dendrons and dendrimers with site-isolated dyes at their core has been explored. The dyes selected for this work were coumarin 343 and pentathiophene, as energy transfer processes prevail when the two dyes are intimately mixed but each should behave independently of the other if site-isolation is achieved. Because the two dyes have very different functional characteristics, a protocol involving orthogonal protecting groups and allowing the use of a single family of electroactive dendrons for their encapsulation had to be developed. The synthetic protocol must balance the need to incorporate electroactive groups at the periphery of the dendrons with the requirement for high solubility and a size sufficient to fully encapsulate the central dye. Because of their poor solubility and tendency to crystallize, dendrons with uniform triarylamine substitution proved unsatisfactory leading to the development of new unsymmetrical dendrons with alternating branched alkyl groups and triarylamine moieties at their periphery. These dendrons, which show excellent solubility and no tendency to crystallize, were assembled into large dendrimers using a modular protocol with the light emitting dye at their core. It is expected that the large size of the dendritic shell will provide effective site-isolation for the encapsulated central dyes enabling them to exhibit their intrinsic emission properties with minimal energy transfer between neighboring core fluorophores when processed in bulk thin films.     

Dendrimers based on a three-dimensionally disposed AB4 monomer

[reaction: see text] Design and synthesis of a novel class of dendrons based on an AB(4) monomer are described. These dendrons have been evaluated by using dendritic encapsulation of a redox active core. The electrochemical properties of symmetric ferrocene-cored dendrimers show that significant alterations in redox potential and heterogeneous electron-transfer rate constants could be achieved even at lower generations.     

Expanding the structural diversity of self-assembling dendrons and supramolecular dendrimers via complex building blocks

The design and synthesis of the first examples of AB4 and AB5 dendritic building blocks with complex architecture are reported. Structural and retrostructural analysis of supramolecular dendrimers self-assembled from hybrid dendrons based on different combinations of AB4 and AB5 building blocks with AB2 and AB3 benzyl ether dendrons demonstrated that none of these new hybrid dendrons exhibit the previously encountered conformations of libraries of benzyl ether dendrons. These hybrid dendrons enabled the discovery of some highly unusual tapered and conical dendrons generated by the intramolecular back-folding of their repeat units and of their apex. The new back-folded tapered dendrons have double thickness and self-assemble into pine-tree-like columns exhibiting a long-range 7/2 helical order. The back-folded conical dendrons self-assemble into spherical dendrimers. Non-back-folded truncated conical dendrons were also discovered. They self-assemble into spherical dendrimers with a less densely packed center. The discovery of dendrons displaying a novel crown-like conformation is also reported. Crown-like dendrons self-assemble into long-range 5/1 helical pyramidal columns. The long-range 7/2 and 5/1 helical structures were established by applying, for the first time, the helical diffraction theory to the analysis of X-ray patterns obtained from oriented fibers of supramolecular dendrimers.     

Optically active dendrimers with a binaphthyl core and phenylene dendrons: light harvesting and enantioselective fluorescent sensing

Optically active dendrimers containing a 1,1'-binaphthyl core and cross-conjugated phenylene dendrons were synthesized and characterized. The chiral optical properties of these phenylene-based dendrimers are different from the previously reported phenyleneethynylene-based dendrimers probably because of the increased steric interaction between the adjacent phenylene units. UV and fluorescence spectroscopic studies demonstrate that the energy harvested by the periphery of the dendrimers can be efficiently transferred to the more conjugated core, generating much enhanced fluorescence signal at higher generation. The fluorescence of these dendrimers can be quenched both efficiently and enantioselectively by chiral amino alcohols. The energy migration and light-harvesting effects of the dendrimers make the higher generation dendrimer more sensitive to fluorescent quenchers than the lower ones. Thus, the dendritic structure provides a signal amplification mechanism. These materials are potentially useful in the enantioselective recognition of chiral organic molecules.     

Synthesis of 1,2,4-triazole dendrimers

A convergent synthetic strategy towards novel 1,2,4-triazole dendrimers, in which 3,5-dichloro-4-(4-methoxyphenyl)-4H-1,2,4-triazole was used as the heterocyclic building block, was successfully explored. Nucleophilic aromatic substitution at this novel AB₂-monomer was used as the key step in the propagation of the heterocyclic dendrons and these dendrons were attached to both a 1,3,5-triazine and a methylene core. The peripheral 1,2,4-triazole could be varied not only by nucleophilic aromatic substitution but also by Suzuki cross-coupling. The presented dendrimers are promising candidates to be used in applications where the large number of heteroatoms can be exploited or a better resistance to the applied conditions is required.     

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.