EPR characterization of gadolinium(III)-containing-PAMAM-dendrimers in the absence and in the presence of paramagnetic probes

Gd(III)-containing dendrimers are promising contrast agents for magnetic resonance imaging (MRI). An important issue in the effectiveness and toxicity of a Gd(III) based MRI contrast agent is knowledge of the relative locations and concentrations of Gd(III) in dendrimer drug delivery hosts. In order to provide experimental information on this issue, we have investigated the electron paramagnetic resonance (EPR) of a stable Gd(III) complex with diethylenetriaminepentaacetic acid (DTPA) in various polyammidoamine (PAMAM) dendrimers as a function of dendrimer generation (G2, G4, and G6), dendrimer core (ethylenediamine = EDA, and cystamine = cys), and dendrimer surface functionality (NH(2), 5-oxo-3-pyrrolidinecarboxylic acid methyl ester = pyr, and tris(hydroxymethyl) methylamine = tris). The dendrimer systems were investigated in the presence and absence of paramagnetic probes, that is, Cu(II) and nitroxide radicals (4-(trimethylammonium and dodecyl-dimethylammonium) 2,2,6,6-tetramethylpiperidine 1-oxyl bromide = CAT1 and CAT12, respectively). The analysis of the EPR spectra revealed anisotropic locations of Gd-DTPA inside the dendrimer. Computer analysis of the EPR spectra of the probes identified the interactions of the Gd-dendrimers with ions and organic molecules. The interaction between the probes and the dendrimer internal and external surface depends on the type of core, the composition of the external surface and the generation of the dendrimer. The negatively charged Gd-DTPA complex attracts the positively charged species and this provokes spin-spin interactions between Gd and the probes, which increases with a decrease in generation, mainly from G6 to G4, and with an increase in both the Gd-dendrimer concentration and the probe concentration. The cys core increases the internal volume and decreases the packing of the branches.     

Valence isomerization in dendrimers by photo-induced electron transfer and energy transfer from the dendrimer backbone to the core

A series of poly(aryl ether) dendrimers with a norbornadiene (NBD) group attaching to the core (Gn-NBD), generations 1–4, were synthesized and characterized, and their photophysical and photochemical properties were examined. The fluorescence of the dendrimer backbone is quenched by the norbornadiene group as a result of the electron transfer and energy transfer from the dendrimer backbone to the norbornadiene group in Gn-NBD. Selective excitation of the dendrimer backbone results in an isomerization of the norbornadiene group to the quadricyclane (QC) group. The intramolecular electron transfer and energy transfer efficiencies are ca. 0.93, 0.73, 0.54, 0.30 in dichloromethane, and ca. 0.90, 0.70, 0.55, 0.34 in tetrahydrofuran for generations 1–4, respectively, with the rate constant ca. 10¹⁰ s⁻¹. The light-harvesting ability of these dendritic molecules is demonstrated by the enhanced valence isomerization rate of NBD to QC with increasing generation.     

Assemblies of dendrimers and proteins on carbon and gold electrodes

Dendritic macromolecules of two adjacent (G3.5 and G4) generations have been used to modify gold or carbon electrodes. The structure and stability of deposited films have been explored by quartz crystal microbalance (QCM), Surface Plasma Resonance (SPR) and electrochemistry. Dendrimers have been shown to adsorb spontaneously on electrode materials as compressed macromolecular films. They are able to inhibit (G3.5) or promote (G4) electroactive anionic species such as Fe(CN)(6)(3-/4-) used as a probe system. Mixed protein/dendrimer assemblies have been constructed with proteins differing in charge, nature of the prosthetic groups and sizes such as lysozyme, cytochrome c, polyhemic cytochrome c(3) or glucose oxidase. Generally, the stability of adsorbed films seems to be limited to one dendrimer/protein bilayer. Owing to the satisfactory stability of composite cytochrome c(3)/G3.5 or glucose oxidase/G4 films, biosensing applications are described for metal bioremediation and glucose detection, respectively.     

Surface characterization, modification chemistry, and separation performance of polyimide and polyamidoamine dendrimer composite films

6FDA-polyimide films modified by polyamidoamine (PAMAM) dendrimers with generations of 0, 1, and 2 are reported in this article. The actual molecular conformation and bulk size of these three generation dendrimers immobilized on polyimide surface were characterized by atomic force microscopy. After comparing with the results of dynamic simulation, we believe that the disk-shape cluster structure of dendrimers has been developed on the polymer surfaces. The amidation and cross-linking reaction between dendrimers and polyimide were examined and quantified by X-ray photoelectron spectroscopy, attenuated total reflection Fourier transform infrared spectroscopy, and gel content measurements. Modification time and the generations of PAMAM dendrimer have been verified as two important factors in determining the properties of modified polyimide films. These modified polyimide films exhibit excellent gas separation performance. The ideal selectivity of He/N(2) increases tremendously to about 200% as compared to that of the original polyimide film. Particularly, the separation performance of CO(2)/CH(4) gas pair can be improved beyond the upper bond limit possibly due to the strong interactions of dendrimer molecules with CO(2), which was verified by sorption tests.     

Structure of poly(propyl ether imine) dendrimer from fully atomistic molecular dynamics simulation and by small angle x-ray scattering

We study the structure of carboxylic acid terminated neutral poly(propyl ether imine) (PETIM) dendrimer from generations 1-6 (G1-G6) in a good solvent (water) by fully atomistic molecular dynamics (MD) simulations. We determine as a function of generation the structural properties such as radius of gyration, shape tensor, asphericity, fractal dimension, monomer density distribution, and end-group distribution functions. The sizes obtained from the MD simulations have been validated by small angle x-ray scattering experiment on dendrimer of generations 2-4 (G2-G4). A good agreement between the experimental and theoretical value of radius of gyration has been observed. We find a linear increase in radius of gyration with the generation. In contrast, Rg scales as approximately Nx with the number of monomers. We find two distinct exponents depending on the generations, x=0.47 for G1-G3 and x=0.28 for G3-G6, which reveal their nonspace filling nature. In comparison with the amine terminated poly(amidoamine) (PAMAM) dendrimer, we find that Rg of Gth generation PETIM dendrimer is nearly equal to that of (G+1)th generation of PAMAM dendrimer as observed by Maiti et al. [Macromolecules 38, 979 (2005)]. We find substantial back folding of the outer subgenerations into the interior of the dendrimer. Due to their highly flexible nature of the repeating branch units, the shape of the PETIM dendrimer deviates significantly from the spherical shape and the molecules become more and more spherical as the generation increases. The interior of the dendrimer is quite open with internal cavities available for accommodating guest molecules, suggesting the use of PETIM dendrimer for guest-host applications. We also give a quantitative measure of the number of water molecules present inside the dendrimer.     

Cobaltocenium-functionalized poly(propylene imine) dendrimers: redox and electromicrogravimetric studies and AFM imaging

The first four generations of cobaltocenium-functionalized, diaminobutane-based poly(propylene imine) dendrimers DAB-dend-Cb,(PFb)x (x = 4, 8, 16, and 32; Cb=[Co(eta5-C5H4CONH)(eta5-C5H5)] (1-4) have been synthesized and characterized. The redox activity of the cobaltocenium centers in 1-4 has been characterized by using cyclic voltammetry and the electrochemical quartz-crystal microbalance (EQCM). All of the dendrimers exhibit reversible redox chemistry associated with the cobaltocenium/cobaltocene redox couple. Upon reduction. the dendrimers exhibit a tendency to electrodeposit onto the electrode surface, which is more pronounced for the higher generations. Pt and glassy carbon electrodes could be modified with films derived from 1-4,exhibiting a well-defined and persistent electrochemical response. EQCM measurements show that the dendrimers adsorb, at open circuit, onto platinum surfaces at monolayer or submonolayer coverage. Cathodic potential scanning past -0.75 V at which the cobaltocenium sites are reduced, gave rise to the electrodeposition of multilayer equivalents of the dendrimers. The additional material gradually desorbs upon re-oxidation so that only a monolayer equivalent remains on the electrode surface. Changes in film morphology as a function of dendrimer generation and surface coverage were studied by using admittance measurements of the quartz-crystal resonator on the basis of its electrical equivalent circuit, especially in terms of its resistance parameter. In general, we find that films of the lower dendrimer generation 1 behave rigidly, whereas those of the higher generation 4 exhibit viscoelastic behavior with an intermediate behavior being exhibited by 2 and 3. Using tapping-mode atomic force microscopy (AFM). we have been able to obtain molecularly resolved images of dendrimer 4 adsorbed on a Pt(111) electrode.     

PAMAM树枝状高分子自组装膜的微摩擦性能研究

自组装超薄膜的研究是近年来的热点课题，其制备与应用已深入到各个领域．研究表明，以高分子为构筑单元的有序结构超薄膜具有优良的微摩擦性能．在微摩擦学研究中，用C60作刚性基团，提高薄膜润滑材料的承载能力，已有不少报道．Tsukruk和Perry等分别研究了由C60组装形成的单层膜的微摩擦行为．薛群基等曾尝试在脂     

Construction of giant dendrimers using a tripodal building block

Giant pentane-soluble organo-silicon dendrimers have been synthesized using a triallylphenol brick according to a new divergent construction that uses a hydrosilylation-nucleophilic substitution sequence up to the ninth generation (G(9)). All the reactions were monitored by (1)H, (13)C, and (29)Si NMR until G(9), indicating that they were clean at the NMR accuracy until this last generation. MALDI TOF mass spectra were recorded for G(1) to G(4) and show the nature and amounts of defects that are intrinsic to the divergent construction. This technique and SEC (recorded up to G(5)) confirm the monodispersity (1.00 to 1.02) from G(1) to G(5). HRTEM and AFM images recorded for the high generations disclose the expected smooth dendrimer size progression and the globular shape. At G(9), the theoretical number of termini (TNT) is 177 407 branches (abbreviation: G(9)-177 047). It is estimated that more than 10(5) terminal branches are actually present in the G(9) dendrimer, far beyond the De Gennes "dense-packing" limit (6000 branches), and it is believed that the branch termini turn inside the dendrimer toward the core. This is corroborated by lower reaction rates and yields for the highest generation numbers presumably due to intradendritic reactions. It is probable that the dendritic construction is limited by the density of branches inside the dendrimer, i.e., far beyond the dense-packing limit.     

聚酰胺-胺型树枝状高分子水溶液的表面活性

树枝状化合物是一类三维的、高度有序的新型高分子.与传统高分子相比,这类化合物在合成时,可以在分子水平上严格控制,设计分子大小、形状、结构和功能基团,产物一般高度对称,单分散性好.因而具有广泛的潜在用途^[1,2].近年来,已引起广大化学工作者的重视.     

苯丙氨酸改性树枝状聚赖氨酸的合成及表征

以双芴甲氧羰基赖氨酸(Di- Fmoc -Lysine)为支化单元采用固相多肽合成技术制备了一种树枝状聚赖氨酸.该聚合物表面活性氨基通过与芴甲氧羰基苯丙氨酸(Fmoc- Phe)反应获得表面完全替代的苯丙氨酸改性树枝状聚赖氨酸.产物的分子量和结构经MS和NMR进行了表征.研究结果表明,利用该方法得到的产物分子量单一,是一种潜在的理想非病毒治疗载体.     

Benzophenone and zinc(II) phthalocyanine dichromophores-labeled poly (aryl ether) dendrimer: synthesis,characterization and photoinduced energy transfer

A series of benzophenone chromospheres and zinc(II) phthalocyanine dichromophores labeled poly (aryl benzyl ether) dendrimer (G_n-DZnPc(BP)_8n, n = 1?2) were synthesized. Their structures were characterized by elemental analysis, ¹H NMR, IR, UV–vis and matrix-assisted laser desorption/ionization time-of-flight spectrometry (MALDI-TOF MS). Their photophysical properties were examined by steady-state and time-resolved fluorescence methods. Both the poly (aryl benzyl ether) dendrimer and BP terminal chromophores had a significant effect on photophysical properties of the zinc(II) phthalocyanine core. Time-resolved spectroscopic measurements indicated that the lifetime of benzophenone (donor) chromophore was longer than that of the zinc(II) phthalocyanine (acceptor). The fluorescence of the peripheral benzophenone chromophores was quenched by the phthalocyanine group attached to the focal point. All of these observations suggest that an intramolecular singlet energy transfer occurs in G_n-DZnPc(BP)_8n molecules. The light-harvesting abilities of these molecules increased with generations due to an increase in the number of benzophenone chromophores. The energy transfer efficiencies were ca. 0.49 and 0.68 for generations 1 and 2, respectively, and the rate constants of the singlet-singlet energy transfer were ca. 10⁸ s^?1. The rate constants changed inconspicuously with increase of dendron generations. The intramolecular singlet-singlet energy transfer is proposed to proceed mainly via a Förster-type interaction mechanism involving the dendrimer backbone as a scaffold to hold the peripheral benzophenone chromophores and the phthalocyanine core together. This dendrimer was an effective new energy transmission complex with high efficiency and could be used as a potential light-harvesting system.     

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.     

Single-component films of different generations of dendrimers bearing a diphenylanthracene core

Different generations of carboxyl-terminated poly(aryl ether) dendrimers bearing a diphenylanthracene core were designed and synthesized. It is interesting to see that not only two-generation dendrimers but also one-generation dendrimers can be fabricated into thin films by self-deposition. Fluorescence spectra indicate that increasing the generation number of a dendrimer can effectively control the quenching of the fluorescence. Moreover, the fluorescence property of the diphenylanthracene core of the dendrimers in a solid film is quite similar to that of one in a solution, which is important for designing light-emitting materials.     

不同代硝基芳基苄醚树枝配体轴向取代硅(Ⅳ)酞菁光物理性质的比较

在二氯硅酞菁轴向位置引入硝基芳基苄醚树枝配体是一种减少酞菁配合物聚集体形成,提高其光动力活性的有效方法。本文采用UV/Vis、稳态和瞬态荧光光谱法比较了1-3代硝基芳基苄醚树枝配体轴向取代硅(Ⅳ)酞菁的光物理性质。研究结果表明,随着轴向树枝配体代数的增加,Q带最大吸收峰强度增大,酞菁核荧光强度增强,荧光量子产率降低,荧光寿命增长。研究结果将为开发轴向取代硅(Ⅳ)酞菁配合物作为新型光敏剂提供重要的理论依据。     

New Liquid Crystalline Materials Based on Two Generations of Dendronised Cyclophosphazenes

A divergent approach was used for the synthesis of dendritic structures based on a cyclotriphosphazene core with 12 or 24 hydroxyl groups, by starting from [N₃P₃(OC₆H₄OH‐4)₆] and using an acetal‐protected 2,2‐di(hydroxymethyl)propionic anhydride as the acylating agent. Hydroxyl groups in these first‐ and second‐generation dendrimers, G1‐(OH)₁₂ or G2‐(OH)₂₄ , were then condensed in turn with mono‐ or polycatenar pro‐mesogenic acids to study their ability to promote self‐assembly into liquid crystalline structures. Reactions were monitored by using ³¹P{¹H} and ¹H NMR spectroscopy and the chemical structure of the resulting materials was confirmed by using different spectroscopic techniques and mass spectrometry (MALDI‐TOF MS). The results were in accordance with monodisperse, fully functionalised cyclotriphosphazene dendrimers. Thermal and liquid crystalline properties were studied by using optical microscopy, differential scanning calorimetry and X‐ray diffraction. The dendrimer with 12 4‐pentylbiphenyl mesogenic units gives rise to columnar rectangular organisation, whereas the one with 24 pentylbiphenyl units does not exhibit mesomorphic behaviour. In the case of materials that contain polycatenar pro‐mesogenic units with two aromatic rings ( A4 vs. A5 ), the incorporation of a short flexible spacer connected to the periphery of the dendron (acid A5 ) was needed to achieve mesomorphic organisation. In this case, both dendrimer generations G1 A5 and G2 A5 exhibit a hexagonal columnar mesophase.     

Investigating carbazole jacketed precursor dendrimers: sonochemical synthesis, characterization, and electrochemical crosslinking properties

Precursor carbazole terminated dendrons and dendrimers up to generation four (G4-D) were synthesized using a convergent approach. Sonication as a means of facilitating organic reactions in dendrimer chemistry was explored resulting in very facile and very fast (up to 50x) reaction times compared to those using traditional reflux conditions. The limits of peripheral group functionality were explored as a function of generation. The electrochemical cross-linking of the dendrimers as thin films revealed unusual cyclic voltammetry (CV) behavior depending upon the generations, which were significantly different from their linear counterpart, Poly(N-vinylcarbazole) (PVK). G1-D showed a higher extent of intermolecular cross-linking while G4-D showed a higher extent of intramolecular cross-linking. The formed films were optically clear and possess superior energy band gap properties making them an alternative candidate over PVK for future hole-transport layer materials in electro-optical devices.     

Gradient shape-persistent pi-conjugated dendrimers for light-harvesting: synthesis, photophysical properties, and energy funneling

A new class of pi-conjugated dendrimers G0, G1, and G2 was developed through a double-stage divergent/convergent growth approach, in which 5,5,10,10,15,15-hexahexyltruxene was employed as the node and oligo(thienylethynylene)s (OTEs) with different lengths as the branching moieties. The dendrimers were fully characterized by (1)H and (13)C NMR, elemental analysis, gel permeation chromatography, and MALDI-TOF MS. Also, by using atomic force microscopy, it was observed that dendrimer G2 laid nearly flat on the mica surface as a single molecule. Dynamic light scattering results showed that the molecule retained its relatively flat shape in solution. To our best knowledge, dendrimer G2, with a radius approaching 10 nm and a molecular weight of 27 072 Da, was the largest among reported second generation dendrimers. The energy gradient in G2 was constructed by linking OTEs of increasing effective conjugation lengths from the dendritic rim to the core. The intramolecular energy transfer process was studied using steady-state UV-vis absorption and photoluminescent spectroscopies, as well as time-resolved fluorescence spectroscopy. Our structurally extended dendrimers showed an excellent energy funneling ability (their energy transfer efficiencies were all over 95%). All results demonstrate that these dendrimers are promising candidates as light-harvesting materials for optoelectronic devices.     

Thermal decomposition of generation-4 polyamidoamine dendrimer films: decomposition catalyzed by dendrimer-encapsulated Pt particles

The thermal decomposition of hydroxyl-terminated generation-4 polyamidoamine dendrimer (G4OH) films deposited on Au surfaces has been compared with decomposition of the same dendrimer encapsulating an approximately 40-atom Pt particle (Pt-G4OH). Infrared absorption reflection spectroscopy studies showed that, when the films were heated in air to various temperatures up to 275 degrees C, the disappearance of the amide vibrational modes occurred at lower temperature for the Pt-G4OH film. Dendrimer decomposition was also investigated by thermogravimetric analysis (TGA) in both air and argon atmospheres. For the G4OH dendrimer, complete decomposition was achieved in air at 500 degrees C, while decomposition of the Pt-G4OH dendrimer was completed at 400 degrees C, leaving only platinum metal behind. In a nonoxidizing argon atmosphere, a greater fraction of the G4OH decomposed below 300 degrees C, but all of the dendrimer fragments were not removed until heating above 550 degrees C. In contrast, Pt-G4OH decomposition in argon was similar to that in air, except that decomposition occurred at temperatures approximately 15 degrees C higher. Thermal decomposition of the dendrimer films on Au surfaces was also studied by temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) under ultrahigh vacuum conditions. Heating the G4OH films to 250 degrees C during the TPD experiment induced the desorption of large dendrimer fragments at 55, 72, 84, 97, 127, 146, and 261 amu. For the Pt-G4OH films, mass fragments above 98 amu were not observed at any temperature, but much greater intensities for H(2) desorption were detected compared to that of the G4OH film. XPS studies of the G4OH films demonstrated that significant bond breaking in the dendrimer did not occur until temperatures above 250 degrees C and heating to 450 degrees C caused dissociation of C=O, C-O, and C-N bonds. For the Pt-G4OH dendrimer films, carbon-oxygen and carbon-nitrogen bond scission was observed at room temperature, and further decomposition to atomic species occurred after heating to 450 degrees C. All of these results are consistent with the fact that the Pt particles inside the G4OH dendrimer catalyze thermal decomposition, allowing dendrimer decomposition to occur at lower temperatures. However, the Pt particles also catalyze bond scission within the dendrimer fragments so that decomposition of the dendrimer to gaseous hydrogen is the dominant reaction pathway compared to desorption of the larger dendrimer fragments observed in the absence of Pt particles.     

Polyelectrolyte behavior and interpolyelectrolyte matching of AstramolTM poly(propylene imine) dendrimers

Polyelectrolyte behavior of Astramol^TM poly(propylene imine) dendrimers of five generations, G1‐G5, namely DAB‐dendr‐(NH₂)_x (where x is equal to 4, 8, 16, 32 or 64) was studied by means of potentiometric titration in salt‐free water solutions and also in the presence of a shielding low molecular electrolyte (NaCI). In addition to x outer primary amine groups the dendrimer molecule contains x‐2 inner tertiary amine groups. The repeating unit, the core molecule and the fifth generation dendrimer structure are shown in the following Scheme.     

Single-Crystalline Optical Microcavities from Luminescent Dendrimers

Microcrystallites are promising minute mirrorless laser sources. A variety of luminescent organic compounds have been exploited along this line, but dendrimers have been inapplicable owing to their fragility and extremely poor crystallinity. Now, a dendrimer family that overcomes these difficulties is presented. First-, second-, and third-generation carbazole (Cz) dendrimers with a carbon-bridged oligo(phenylenevinylene) (COPV2) core (GnCOPV2, n=1–3) assemble to form microcrystals. The COPV2 cores align uni/bidirectionally in the crystals while the Cz units in G2- and G3COPV2 align omnidirectionally. The dendrons work as light-harvesting antennas that absorb non-polarized light and transfer it to the COPV2 core, from which a polarized luminescence radiates. Furthermore, these crystals act as laser resonators, where the lasing thresholds are strongly coupled with the crystal morphology and the orientation of COPV2, which is in contrast with the conventional amorphous dendrimers.