AB3 building blocks for the synthesis of polyester dendrimers

Syntheses of several examples of a new type of trivalent building blocks for the preparation of aliphatic polyester dendrimers are presented. Starting from the well-known mono-O-benzylidenepentaerythritol, AB₃ type acid dendrons can be obtained in high yield in only two steps. Other triprotected bis-2,2-(hydroxymethyl)-3-hydroxypropanoic acid derivatives with varying protecting groups were also synthesized readily. This type of dendron was used in combination with 2,2′-bis(hydroxymethyl) propanoic acid (bis-HMPA) divalent dendrons to produce low generation mixed polyester dendrimers with increased number of branching points.     

Self-host yellow iridium dendrimers based on carbazole dendrons: synthesis,characterization and application in solution-processed organic light-emitting diodes

On the basis of different generation carbazole dendrons, a series of self-host yellow Ir dendrimers (Y-G0, Y-G1 and Y-G2) have been successfully synthesized and characterized in detail. It is found that the peripheral dendrons can effectively reduce the intermolecular interactions between emissive Ir cores, as verified by the increased photoluminescence quantum yields and film lifetimes. Among these dendrimers, Y-G2 bearing the second generation dendrons shows the best non-doped device performance, revealing a peak luminous efficiency of 20.2 cd/A. The value is nearly twice that of Y-G0 without any dendrons, which could be further improved to 32.1 cd/A by dispersing Y-G2 into a host matrix. We believe that this work will shed light on the development of highly efficient yellow phosphorescent dendrimers with a self-host strategy.     

Synthesis and properties of polyaromatic dendrimers possessing a repetitive amide-ester coupling sequence

A series of novel polyaromatic dendrimers that feature tris-(2-ethylamino)amine as the central core unit has been synthesized up to the third generation by employing a convergent growth strategy. The building blocks 1,3-diamino-2-hydroxypropane and 4-carboxybenzaldehyde were used for dendron construction, a process that involved the cyclic repetition of esterification, oxidation and selective amidation steps. Molecular modelling of this class of dendrimers has been used to predict potential solution state conformations employing molecular mechanics and molecular dynamic simulations. In addition, the results of preliminary metal binding studies using the first generation dendritic system are also outlined.     

Synthesis and Characterization of New Conjugated Fluorenyl‐Porphyrin Dendrimers for Optics

A new family of conjugated meso‐tetraphenylporphyrin‐based dendrimers with four ( TPP1 , TPP2 ), eight ( TPP3 , TPP4 , TPP5 ) and up to sixteen ( TPP6 ) fluorenyl groups has been synthesized and fully characterized. These tetraphenylporphyrin‐cored dendrimers present peripheral alkynyl π‐conjugated dendrons with fluorenyl termini. The meso‐aryl rings of these porphyrins are functionalized either in para‐ ( TPP1 , TPP2 , and TPP3 ) or meta‐positions ( TPP4 , TPP5 , and TPP6 ). Their detailed luminescence properties are discussed in reference to two porphyrins lacking fluorenyl dendrons ( TPP ‐ H_1,2,3 and TPP ‐ H_4,5,6 ). A strong dependence of their luminescence quantum yield and lifetime on their structures is stated, their nonlinear optical properties were also discussed.     

Second-harmonic properties of dendritic polymers skeleton-constructed with azobenzene moiety used for nonlinear optical materials

A series of dendrons and dendrimers skeleton-constructed with azobenzene moiety were synthesized and doped in a high T_g polycarbonate (PC) as a host for nonlinear optical (NLO) materials. The optimal loading density and poling conditions were investigated using UV-vis spectral and second-harmonic generation (SHG) measurements. The results showed that the dendrons and dendrimers have good solubility in PC host, which increased with the generation increased. Moreover, the SHG measurements indicated that the dendrons possess higher nonlinearity than the dendrimers. The chromophores of lower generation dendrons were easier to orient along the poling electric field and gave a cone shape with the azobenzene branching units, which coherently contributed to the molecular hyperpolarizability and resulted in the higher SHG intensity. The temporal stability of the dendrons with 15% loading density was also investigated, and showed that the decaying in nonlinearity was slower for higher generation dendrons.     

Clickable dendronized copolymers for introducing structural heterogeneity

A new class of shape-persistent dendronized copolymers 1[Gn]4–5(a–d) (n = 1, 2) with an alternating structure was synthesized via a combination of conventional radical copolymerization and Cu(I)-catalyzed click reaction (i.e. CuAAC). Firstly, azide-functionalized dendronized copolymers 1[Gn]4-N₃ that could be used as “clickable” single molecular platforms were synthesized through two steps: (i) the preparation of methanesulfonate-modified copolymers by a radical copolymerization of styrene derivatives pendant with Percec-type polyether dendron and methanesulfonate-containing maleimide; (ii) the replacement reaction of methanesulfonate unit with sodium azide. Then a series of functional molecules bearing an alkyne group, respectively, such as anthracene and mini-dendritic wedges, were attached onto each maleimide unit of the copolymers through CuAAC. Moreover, a postmodification of the denpols bearing anthracene has been conducted using Diels–Alder cycloaddition between the attached anthracenes and the maleimide dendron, demonstrating that more complicated denpols can be fabricated. These novel dendronized copolymers have been characterized by nuclear magnetic resonance spectroscopy, atom force microscopy, Fourier transform infrared spectroscopy, UV–vis spectroscopy.     

Synthesis of fluorescent dendritic 8-hydroxyquinoline ligands and investigation on their coordinated Zn(II) complexes

A series of luminescent PAMAM dendrons emanating from 8-hydroxyquinoline have been synthesized and their coordination with Zn(II) was investigated for the first time. The obtained dendritic Zn(II) complexes were soluble in common organic solvents. It was found that the luminescence intensity of G2 dendron 6 was higher than that of G1 dendron 4. Furthermore, when they were coordinated with Zn(II), red-shift was observed and the intensities of the coordinated Zn(II) complexes were higher than that of the corresponding ligands.     

Synthesis of new cores and their use in the preparation of polyester dendrimers

Six dendrimer and dendron cores terminated by hydroxyl groups that are neither phenolic nor cleavable by hydrogenolysis have been prepared in a consistent one-pot manner from terminal allyl groups by reduction of the product of reductive ozonolysis. Some of the terminal allyl derivatives are new and others have been prepared by new methods. The well-known O-benzylidene derivative of 2,2′-bis(hydroxymethyl)propanoic acid was shown to be the cis-stereoisomer. A new AB₃-type anhydride, tris(benzyloxymethyl)acetic anhydride has been prepared. It was demonstrated that these cores and dendrons could be assembled into first and second generation homo- and mixed polyester dendrimers.     

Silole‐core phenylacetylene dendrimers and their application in detecting picric acid

Silole‐core phenylacetylene dendrimers were designed and synthesized, among them, the model compound (n = 0) and the first generation of the dendrimer (n = 1) were obtained by the reaction of 2,5‐dibromosilole with corresponding terminal alkynes, the second generation of the dendrimers (n = 2) was synthesized from 2,5‐diiodosilole. These compounds indicated the absorptions of both phenylacetylene dendrons (250–350 nm) and silole core (400–500 nm). The first generation displayed efficient energy transfer from phenylacetylene dendrons to silole core, whose energy transfer efficiency was as high as 80%. These compounds were used as chemical sensors to probe explosive, for picric acid (PA), the Stern–Volmer constants of model compound and the first generation are 7120 and 5490M^?1, respectively. J. Heterocyclic Chem., (2012).     

Hydrodynamic, conformational, and electrooptical properties of cylindrical dendrimers with dendron ionic addition in solutions

Cylindrical dendrimers of first, second, and third generations with side dendrons based on L-aspartic acid which are attached to poly(styrenesulfonic acid) chains via ionic bonds are studied by molecular hydrodynamic, optical, and electrooptical measurements. Macromolecules of the said dendrimers demonstrate significant kinetic rigidity in external electric and hydrodynamic fields simultaneously with moderate equilibrium chain rigidity. The comparative analysis of the experimental data on various cylindrical dendrimers with the polyvinyl backbone and L-aspartic acid-based dendrons is performed. It has been shown that, in solvents not disturbing intramolecular hydrogen bonds between side dendrons, the conformational and dynamic properties of various cylindrical dendrimers are similar. The molecular characteristics of cylindrical dendrimers in non-dissociating solvents are insignificantly affected by the mode of dendron attachment to the backbone (covalent or ionic binding).     

Fluorenyl dendrimer porphyrins: synthesis and photophysical properties

New symmetrical dendrimeric type super-porphyrin bearing sixteen fluorenyl donor groups sixteen fluorenylporphyrin SOFP (1) have been synthesized and characterized. Preliminary photophysical properties are reported; in comparison to the references first generation dendrimer tetrafluorenylporphyrin TOFP (2) bearing four peripheral fluorenyl and second generation dendrimer octafluorenylporphyrin OOFP (3) bearing eight peripheral fluorenyl, the luminescence properties are slightly improved. It is found that the excitation energy transfer occurs from the sixteen fluorenyl units to the porphyrin core, following what the porphyrin emits intense red light.     

Water-soluble azobenzene dendrimers

Water-soluble azobenzene-cored dendrimers 4-6 were synthesized as potential photoresponsive micelles to which the aggregation is triggered by photoirradiation. In the absorption spectra, the π- absorption band of the dendrimers largely blue-shifted with increasing the generation due to the shielding effect of surrounding dendron on the interaction between the core azobenzene and solvent water. Not only the surrounding dendron groups suppressed the E-to-Z photoisomerization, but also the rate of thermal Z-to-E isomerization depended on the generation of the dendrimer. Furthermore, we have observed a novel temperature dependence of the rate constant of the thermal isomerization.     

Synthesis,Structural Characterization,and Thermoresponsivity of Hybrid Supramolecular Dendrimers Bearing a Polyoxometalate Core

A series of cationic dendrons bearing triethylene glycol monomethyl ether terminal groups of different generations have been synthesized and used to encapsulate an inorganic polyanionic cluster [K_12.5Na_1.5(NaP₅W₃₀O₁₁₀)] through electrostatic interactions. The resulting dendritic cation–encapsulated polyoxometalate (POM) complexes, cluster–dendrimers, are soluble in water and exhibit lower critical solution temperatures (LCST). The thermoresponsivities of these complexes in aqueous solutions were studied by turbidimetry and variable‐temperature ¹H NMR spectroscopy. The observed cloud points show a remarkable dependence on the generation of the dendrons. Complexes composed of first‐generation dendrons exhibit no obvious thermoresponsive properties, but for complexes bearing second‐generation dendrons, the LCST decreases as the number of dendritic cations around the POM cluster increases. Complexes composed of third‐generation cations underwent reversible aggregation and disaggregation upon heating and cooling, respectively. This thermally induced self‐aggregation was characterized by DLS and TEM. In addition, the effects of salt and solvent on the LCST were investigated. This research demonstrates a new type of thermoresponsive dendritic organic–inorganic hybrid complex and provides a general route to the endowment of POMs with temperature‐sensitive properties through electrostatic interactions.     

Crowned dendron: ion-responsive flexibility of macromolecules induced by integrated crown ether moieties

Polybenzyl ether type dendrons bearing the crown ether moieties at the periphery, namely, crowned dendrons were synthesized, and the effect of complex formation on their flexibility with metal-ion binding properties was examined. Upon addition of Na⁺, ¹H NMR spectra of the crowned dendrons in CD₃CN were significantly broadened, reflecting the flexibility restriction of the crowned dendrons by the complex formation with Na⁺. Such a significant flexibility restriction was observed only with Na⁺, although ESI-MS studies revealed that the crowned dendrons formed 1:2 complexes (a metal ion:the crown ether moiety) regardless of the kind of metal ions. The flexibility restriction became significant with increasing dendron generation on the basis of ¹H NMR spectra and spin-lattice relaxation time (T₁) measurements. Binding constants of the crowned dendrons with metal ions in CD₃CN decreased with the increase of the dendron generation, reflecting an influence of the charge repulsion as well as a dendrimer effect to cause the steric hindrance. The examination of UV-vis absorption spectra for complexes of the crowned dendron with metal picrates in THF displayed the formation of a loose ion-pair complex with Na⁺, namely, a typical sandwich type complex. However, in CH₃CN, all metal picrates were solvated to be in a loose ion-pair even without complex formation. These results suggested that the control of macromolecular flexibility with metal ions is feasible by the integration of crown ether moieties with a dendritic structure.     

Synthesis of Novel Biodegradable Cationic Dendrimers

Summary: Dendrons and dendrimers with cationic amino groups at their periphery were successfully synthesized up to the third and second generation, respectively. The results obtained by ¹H NMR spectroscopy and gel permeation chromatography analysis supported the formation of the targeted dendrons and dendrimers. The dendrons were grown via ester linkages, which endowed them with biodegradability in D₂O at 37 °C. The degradation rate depends upon the steric hindrance and reactivity caused by the bulkiness and compact structure of the dendrons. All of the synthesized dendrons were degraded within a month, while 60% of the ester groups in the sterically crowded dendrimers were degraded over the same time period. The cytotoxicity of the dendrons was evaluated by the MTT assay on a 293T cell line which indicated that the obtained dendrons were completely non‐toxic. These non‐toxic, biodegradable cationic dendrons and dendrimers are believed to have potential applications in the biomedical field.

Synthetic procedure of dendrons and dendrimers.     

Synthesis and electrochemical properties of dendrimers containing meta-terphenyl peripheral groups and a 4,4′-bipyridinium core

Fréchet-type poly(arylether) dendrons carrying m-terphenyl peripheral groups were synthesized up to second generation by convergent methodology. Simple quarternisation of 4,4′-bipyridine with the dendritic bromides afforded the corresponding dendrimers containing a 4,4′-bipyridine core. The electrochemical parameters were obtained for all the dendrimers and the half-wave potentials of both the first and second redox processes shift to less-negative values as the dendrimer generation increases.     

Dendritic encapsulation of active core molecules

Specific properties of several series of dendrimers have been systematically investigated as a function of the generation number. The encapsulation of a redox-active unit, namely a bis(phenanthroline) copper(I) complex, has been evidenced by the attenuation of the electron transfer rate with increasing molecular size. On the other hand, photophysical studies of dendrimers with a fullerene core have shown that the shielding effect of the dendritic shell has a dramatic effect on the lifetime of the first triplet excited state of the core unit. Actually, the fullerene is a very sensitive probe and lifetime measurements in different solvents can be used to evaluate the degree of isolation of the central C₆₀ moiety from external contacts. Finally, the inclusion abilities of dendrophanes with a cyclotriveratrylene (CTV) core for fullerenes have shown that the dendritic architecture is not only able to isolate a central functional core but can also modulate its binding properties by means of the size and the nature of the surrounding dendrons. To cite this article: J.-F. Nierengarten, C.R. Chimie 6 (2003).     

Synthesis and opto‐electrical properties of dendron‐containing poly(2,3‐diphenyl‐1,4‐phenylenevinylene) derivatives

A new series of poly(2,3‐diphenyl‐1,4‐phenylenevinylene) derivatives containing dendritic side groups were synthesized. Different generations of dendrons were integrated on the pendant phenyl ring to investigate their effect on optical and electrical properties of final polymers. Homopolymers can not be obtained via the Gilch polymerization because of sterically bulky dendrons. By controlling the feed ratio of different monomers during polymerization, dendron‐containing copolymers with high molecular weights were obtained. The UV–vis absorption and photoluminescent spectra of the thin films are pretty close; however, quantum efficiency is significantly enhanced with increasing the generation of dendrons. The electrochemical analysis reveals that hole‐injection is also improved by increasing dendritic generation. Double‐layer light‐emitting devices with the configuration of ITO/PEDOT:PSS/polymer/Ca/Al were fabricated. High generation dendrons bring benefit of improved device performance. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3440–3450, 2007     

Novel first-generation dendrimers on calix[4]resorcinol core equipped with multiple triazole units

Novel first-generation dendrimers on the calix[4]resorcinol core with four branches each containing multiple 1,2,3-triazole units have been synthesized in one-step by acid catalyzed condensation of resorcinols with a new aldehyde dendron, namely, 4-{3,5-bis[(1-benzyl-1H-1,2,3-triazol-4-yl)- methoxy]benzyloxy}benzaldehyde (obtained by alkyne–azide cycloaddition). The reaction proceeds stereoselectively to form rccc-diastereoisomers in high yields.     

A dioxotetraamine fluorenyl ligand and its nickel(II) complex — crystal structure and fluorescent sensing properties in aqueous solution

A new ligand consisting of fluorenyl and dioxotetraaza units, namely, 2,10-diamino-6-(9H-fluoren-9-yl)-4,8-diazaundecane-5,7-dione (L), and its nickel(II) complex were synthesized. The crystal structure of [Ni(H₋₂L)] showed that Ni(II) is in a square planar coordination environment. Studies of solution chemistry of the ligand and its nickel(II) complex indicate that the ligand can form a stable complex with Ni²⁺ ions and fluorescence quenching of the ligand is accompanied by the coordination of the metal ion to the dioxotetraaza unit and electron transfer from the Ni(II) center to fluorenyl in aqueous solution.