Dendrimers as photochemical reaction media. Photochemical behavior of unimolecular and bimolecular reactions in water-soluble dendrimers

Synthesis and studies of poly(alkyl aryl ether) dendrimers, possessing carboxylic acid functionalities at their peripheries, are reported. 5-Bromopentyloxy methylisophthalate was utilized as the monomer to O-alkylate the phenolic hydroxyl groups of poly(alkyl aryl ether) dendrimers. Dendrimers of first, second, and third generations, possessing 6, 12, and 24 carboxylic acids, respectively, were thus prepared. These dendrimers were soluble in alkaline aqueous solutions, and the ensuing microenvironmental properties of the aqueous solutions were assessed by pyrene solubilization studies. Upon establishing the presence of nonpolar microenvironments within the dendritic structures, solubilizations of few organic substrates were conducted and their photochemical behaviors were assessed. Specifically, the photolysis of 1-phenyl-3-p-tolyl-propan-2-one and benzoin ethyl ether and photodimerization of acenaphthylene were conducted. These studies revealed that the product distribution and the "cage effect" were more distinct and efficient for the third generation dendrimer, than for the first and second generation dendrimers. The photochemical studies of carboxylic acid functionalized dendrimers were compared to that of hydroxyl group terminated poly(alkyl aryl ether) dendrimers.     

Synthesis of large generation poly(propyl ether imine) (PETIM) dendrimers

Large generation poly(propyl ether imine) (PETIM) dendrimers are synthesized in iterative synthetic cycles of two reductions and two Michael addition reactions. Dendrimers up to sixth generation, containing up to 128 peripheral functionalities, are synthesized. Growth of the PETIM dendrimers, possessing a tertiary amine as the branch juncture and an ether as the linker component, is assessed systematically by routine spectroscopic methods. The peripheries of these dendrimers possess either alcohols, amines, carboxylic acids, esters, or nitriles, thereby opening up possibilities for varied studies involving PETIM dendrimers.     

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.     

The chirality of dendrimer-based supramolecular complexes

Boc-protected L-phenylalanine has been connected to a spacer-armed ureido-acetic acid derivative, which can form multiple supramolecular complexes with urea-adamantyl modified poly(propylene imine) dendrimers in chloroform. Complexes of this guest with several generations of urea-adamantyl dendrimers were prepared. The dendrimer-guest complexes were characterized in detail by (1)H-NMR, (1)H-(1)H-NOESY spectroscopy and mass spectrometry to prove their formation. Optical rotation experiments performed on different generations of dendrimer-guest complexes showed a constant positive value. These observations indicate that, though guest molecules decrease the flexibility at the periphery of the dendrimer upon binding, the amino acid at the terminus of the guest molecule retains its high local conformational freedom. This is in agreement with values found for covalently modified spacer-armed dendrimers.     

以尼罗红为探针对新型两亲性树枝形聚合物微环境的研究

本工作合成了一系列外围以三缩四乙二醇单甲醚修饰的烷基芳醚骨架两亲性树枝形聚合物Gn(n=0—3),化合物通过了¹H-NMR,IR和MALDI-TOF-MS的表征.利用吸收光谱,稳态和时间分辨荧光光谱研究了水溶液中Gn对尼罗红分子的增溶作用以及Gn内部微环境的极性.研究结果表明,高代数树枝形聚合物Gn对尼罗红具有更好的增溶效果.1—3代树枝形聚合物Gn内部疏水孔腔微环境极性随代数增加而逐渐降低,G1和G2树枝形聚合物具有相似的微环境极性,而由于构象的变化使G3具有更加疏水的微环境.     

Inherent photoluminescence properties of poly(propyl ether imine) dendrimers

Hydroxyl group terminated poly(propyl ether imine) dendrimers of 1 to 5 generations absorb in the region of 260-340 nm, in MeOH and aqueous solutions. Excitation of a solution of the dendrimers at 330 nm led to an emission at approximately 390 nm. The emission intensities increased under acidic pH and in more viscous solvents. The presence of air did not affect the emission profiles, as also aging of a dendrimer solution for prolonged periods. Lifetime measurements show at least two species responsible for the emission. Anions perchlorate, periodate, nitrite, and pyridinium methyliodide quenched the fluorescence efficiently, among several anions tested.     

Guest–host chemistry with dendrimers: Stable polymer assemblies by rational design

A new type of guest has been designed and synthesized for the exo‐type supramolecular functionalization of adamantyl‐urea‐terminated poly(propylene imine) dendrimers. This new type of guest motif features a uriedo methane sulfonic acid moiety that binds very selectively to the surfaces of dendrimers via a combination of noncovalent interactions forming well‐defined complexes. The guest–host properties have been examined for a fifth‐generation adamantyl‐urea‐functionalized poly(propylene imine) dendrimer capable of binding 32 guest molecules and for a model host molecule that can bind only one guest molecule. The guest–host chemistry has been studied with ¹H NMR spectroscopy, nuclear Overhauser enhancement spectroscopy NMR spectroscopy, T₁‐relaxation NMR experiments, and IR spectroscopy. The 1:32 ratio with the dendrimer has been confirmed unambiguously from ¹H NMR spectra of the complex after size exclusion chromatography. Competition experiments with guests bearing a carboxylic acid instead of a sulfonic acid in the binding motif have demonstrated that the sulfonic acid has superior binding strength. Also, the importance of a combination of noncovalent interactions has been shown via competition experiments with a guest lacking the uriedo moiety. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3792–3799, 2004     

尼罗红为探针对芳醚树枝形聚合物微环境的研究

本工作合成了一系列羧基为外围末端基团的芳醚树枝形聚合物Gn(n=1—4),利用吸收光谱,稳态和时间分辨荧光发射光谱研究了Gn对尼罗红分子的增溶作用和Gn微环境极性.研究结果表明,尼罗红在Gn中的溶解度随代数增加而增大,从G1到G4,尼罗红溶解度相比水中分别增加了3、35、47和215倍.Gn的微环境极性比水低,2-4代芳醚树枝形聚合物具有相似的微环境极性.     

新型线状-树枝状两亲嵌段共聚物的合成

本文设计合成了一系列由不同链长的聚丙烯酸(PAA)为亲水嵌段和不同代数聚苄醚树枝体(Dendr.PBE)为疏水嵌段的杂化共聚物(PAA-Dendr.PBE)。     

Poly(propylene imine) dendrimers as hydrogen donor in Type II photoinitiated free radical polymerization

Three generations of poly(propylene imine) dendrimers, namely (PPI-16, PPI-32 and PPI-64; 16, 32 and 64 for generations 3, 4 and 5, respectively) were used as hydrogen donors in photoinitiated free radical polymerization of methyl methacrylate by using one of the following photosensitizers; benzophenone and thioxanthone. The effect of generation number of the dendrimer on photoinitiation efficiency and molecular weight of the resulting polymers was investigated. Glass transition temperatures and particle size measurements of the resulting polymers indicated the presence of nearly stretched polymer chains around the dendrimers. The location of hydrogen donating sites was evaluated by photolysis studies in the absence of monomer by using a stable radical namely, 2,2,6,6-tetramethylpiperidine-N-oxyl free radical (TEMPO) and showed that hydrogen abstraction occurs from the inner tertiary amino groups. The TEMPO attached dendrimers were further used in the nitroxide mediated radical polymerization (NMP) of styrene to yield star polymers.     

Ionic azo-codendrimers: Influence of the acids contents in the liquid crystalline properties and the photoinduced optical anisotropy

A series of codendrimers has been prepared by the grafting of poly(propylene imine) PPI-(NH₂)₁₆ (G = 3) with two different carboxylic acids: the aromatic-based 5-[4′-(4-cyanophenylazo)phenyloxy]pentanoic acid (AZO) and the aliphatic 2,2-bis(undecanoyloxymethylene)propionic acid (BUPA) in different proportions. Their thermal properties, liquid crystalline behavior and photoinduced optical anisotropy have been investigated. The formation of the ionic compounds was confirmed by nuclear magnetic resonance (NMR) spectroscopy, and Fourier transform infrared (FT-IR) spectroscopy. The influence of the acid percentage in the liquid crystalline behavior was investigated by means of differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffractometry (XRD). All the resulting co-dendrimers present liquid crystalline properties displaying an evolution from nematic mesophase for the azo-richer dendrimers to smectic A mesophase for the BUPA-richer dendrimers. Only materials with low azobenzene contents up to four AZO units per dendrimer on average were processable by casting from dichloromethane solutions and resulted in transparent thin films appropriate for optical studies. The photoinduced dichroism reached was higher in the dendrimer with higher azo content (four units per dendrimer) being this material a good compromise between processability and optical response.     

水溶性树枝形聚合物微反应器笼效应研究

本工作以羧基为末端基团的芳醚树枝形聚合物G_n(n=1-4)和核心为十二烷基链的聚酰胺-胺树枝形聚合物PG_m(m=1-3)作为光化学微反应器,研究了1-苯基-3-对甲苯基丙酮在水溶液中的α-光解反应,并计算了笼效应.研究结果表明,芳醚树枝形聚合物G_n的笼效应随代数增加而增大,高代数树枝形聚合物表现出对底物分子更强的限制性作用,增加树枝形聚合物的浓度可以提高笼效应;聚酰胺-胺树枝形聚合物PG_m对CH₃-DBK的限制作用远低于芳醚树枝形聚合物G_n.     

Interaction of single-walled carbon nanotubes with poly(propyl ether imine) dendrimers

We study the complexation of nontoxic, native poly(propyl ether imine) dendrimers with single-walled carbon nanotubes (SWNTs). The interaction was monitored by measuring the quenching of inherent fluorescence of the dendrimer. The dendrimer-nanotube binding also resulted in the increased electrical resistance of the hole doped SWNT, due to charge-transfer interaction between dendrimer and nanotube. This charge-transfer interaction was further corroborated by observing a shift in frequency of the tangential Raman modes of SWNT. We also report the effect of acidic and neutral pH conditions on the binding affinities. Experimental studies were supplemented by all atom molecular dynamics simulations to provide a microscopic picture of the dendrimer-nanotube complex. The complexation was achieved through charge transfer and hydrophobic interactions, aided by multitude of oxygen, nitrogen, and n-propyl moieties of the dendrimer.     

Synthesis and catalytic activities of Pd-phosphine complexes modified poly(ether imine) dendrimers

In this paper, we report synthesis of new alkyldiphenyl phosphine ligand modified poly(ether imine) dendrimers up to the third generation. The phosphinated dendrimers were obtained by functional group transformations of the alcohols present at the periphery of the dendrimers to chloride, followed by phosphination using LiPPh₂. The modification at the peripheries of the dendrimers was performed successfully to obtain up to 16 alkyl diphenylphosphines in the case of a third generation dendrimer, in good yields for each individual step. After phosphination, dendritic ligands were complexed with Pd(COD)Cl₂ to give dendritic phosphine-Pd^II complexes. Both the ligands and the metal complexes were characterized by spectroscopic and spectrometric techniques including high-resolution mass spectral analysis for the lower generations. Evaluation of the catalytic efficacies of the dendrimer-Pd^II metal complexes in mediating a prototypical C-C bond forming reaction, namely the Heck reaction, was performed using various olefin substrates. While the substrate conversion lowered with catalyst in the order from monomer to third generation dendrimer, the second and third generation dendrimers themselves were found to exhibit significantly better catalytic activities than the monomer and the first generation dendrimer.     

Discotic liquid crystalline poly(propylene imine) dendrimers based on triphenylene

The design, synthesis, and mesomorphic properties of a new series of homodendrimers consisting of the commercially available poly(propylene imine) (PPI) dendrimers (G = 1-5), PPI-(NH(2))(n)() (n = 4, 8, 16, 32, 64), functionalized with a discotic triphenylene moiety are reported. The liquid crystalline behavior was investigated by means of differential scanning calorimetry (DSC), polarizing-light optical microscopy (POM), and X-ray diffractometry (XRD). All of the homodendrimers showed mesomorphic properties, with the second to fifth generations giving a hexagonal columnar mesophase (Col(h)) and the first generation a rectangular columnar mesophase (Col(r)). The X-ray study reveals that these mesophases show a highly ordered structure with segregation of triphenylenes and dendrimers into separate columns and a regular stacking distance inside the triphenylene columns. GPC analysis showed that the dendrimers had good monodispersity and MALDI-TOF studies of the first three generations gave good evidence that all of the terminal amino groups of the dendrimers were functionalized with a discotic unit.     

Unsurpassed cage effect for the photolysis of dibenzyl ketones in water-soluble dendrimers

Amphiphilic water-soluble poly(alkyl aryl ether) dendrimers Gn (n = 1-3) with charge-neutral tetraethylene glycol monomethyl ethers at their periphery were synthesized as microreactors to control the photochemical reactions of dibenzyl ketone derivatives in aqueous solutions. Photophysical studies demonstrated that Gn can encapsulate organic molecules and provide a hydrophobic microenvironment. The product distribution of photolysis of dibenzyl ketone derivatives can be successfully controlled by encapsulating the substrates within dendrimers, and an unsurpassed cage effect of 1.00 is reached in high generation dendrimers, revealing that a thick and compact "shell" was formed at the periphery of the dendrimers. The cage effect is also significantly influenced by the substituent at the para-position of the guest molecules. The higher generation dendrimers exhibit a better confined microenvironment and the aggregates possess more compact cavities to "lock" the guests than the corresponding unimolecular dendrimers. After photolysis, the separation of products can be easily achieved by extracting from the dendrimer solutions and the dendrimers are simply recovered and reused.     

金属离子对咔唑修饰的芳醚树枝形聚合物的荧光猝灭作用

合成了一系列外围带有咔唑, 核心带有或不带有氮杂冠醚结构的1-3代芳醚树枝形聚合物(CZ-Gn-CR, CZ-Gn-OH, n=1-3). 利用光物理方法研究了金属离子Li+、Na+、K+、Ca2+、Cu2+、Zn2+、Eu3+和Tb3+与树枝形聚合物之间的相互作用. 研究表明, Eu3+、Tb3+和Cu2+通过静态过程猝灭树枝形聚合物的荧光, 测定了Eu3+、Tb3+和Cu2+对CZ-Gn-CR、CZ-Gn-OH荧光的猝灭速率常数; 核心的冠醚结构可以增强树枝形聚合物与金属离子的相互作用. Li+、Na+、K+、Ca2+和Zn2+对CZ-Gn-CR和CZ-Gn-OH的紫外吸收光谱和荧光光谱均没有明显影响.     

Structure and conformation of DAB dendrimers in solution via multidimensional NMR techniques.

NOESY-HSQC 3D-NMR and NOESY 2D-NMR techniques have been used on a 750 MHz spectrometer to study the chain conformations of different generation DAB dendrimers (poly[propylene imine] dendrimers) in chloroform and benzene solutions. The high-field multidimensional NMR techniques provided the chemical shift dispersion needed to resolve all of the unique resonances in the dendrimers. By studying the NOE interactions among dendritic chain protons, information about through space interactions between protons on different parts of the dendrimer chain is obtained, which is directly related to the conformation of the dendrimer. These experiments also give further proof of the chemical shift assignments obtained from the HMQC-TOCSY 2D and 3D NMR experiments. The concentration effects on chemical shifts have also been observed, revealing information about the interactions between solvent and different parts of dendrimer molecules. These studies clearly show for DAB dendrimers, that folded chain conformations can occur in nonpolar solvents such as benzene and extended chain conformations are predominant in polar solvents such as chloroform.     

Liquid crystalline dendrimers based on cinnamates and coumarins

New poly(propylene imine) (PPI) dendrimers modified at the periphery with mesomorphic 4-cyanobiphenyl units and cinnamate- or coumarin-based photoactive moieties were synthesised and structurally characterised. The photoactive moieties were structurally designed to be liquid crystalline compatible and to reduce the tendency towards crystallisation of the final material. Thermal and liquid crystal properties were studied by POM, DSC, TGA and XRD. The results show that all dendrimers displayed a smectic A mesophase. Dendrimers with high transparency in the visible region but with different absorption spectra were obtained, rendering materials that are easy to monitor in light-irradiation experiments.     

Photoresponsive nanocarriers based on PAMAM dendrimers with a o‐nitrobenzyl shell

Gn (n = 3, 4, and 5) poly(amidoamine) (PAMAM) dendrimers were synthesized and peripherally modified with photocleavable o‐nitrobenzyl (NB) groups by reacting o‐nitrobenzaldehyde with the terminal amine groups of PAMAM dendrimers, followed by reducing the imine to amine groups with NaBH₄. The NB‐modified dendrimers, Gn‐NB (n = 3, 4, and 5), were characterized by nuclear magnetic resonance and fourier transform infrared spectroscopy. The results showed that the NB groups were successfully attached on the periphery of the dendrimers with near 100% grafting efficiency. Such a photosensitive NB shell could be cut off on irradiation with 365 nm ultraviolet (UV) light. The encapsulation and release of guest molecules, that is, salicylic acid (SA) and adriamycin (ADR), by Gn‐NB were explored. The encapsulation capability of these dendrimers was found to increase as the guest molecular size was decreased and have dependence on the generation of dendrimers as well. For both of SA and ADR, the average encapsulation numbers per dendrimer decreased in the order of G4‐NB > G5‐NB > G3‐NB, indicating that the fourth generation dendrimer was a better container for the guest molecules. The rate of SA release was found to be greater with UV irradiation than that without, suggesting that the NB‐shelled PAMMAM dendrimers could function as a molecular container/box with photoresponsive characteristics. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 551–557, 2010