ACCELERATED APPROACH TO THE SYNTHESIS OF METHOXYCARBONYL-TERMINATED POLYETHER DENDRIMERS

Dendrimers are macromolecules that radiate out from a central core, with the number of branch points on a given arm increasing exponentially from the core to the periphery.¹ Because of their novel properties, dendrimers have found many uses, including as unimolecular micelles, novel amphiphiles, and complexation agents.² These and other applications will benefit from more efficient methods of dendrimer preparation. The synthesis of dendrimer can be divided into both the divergent approach developed by Tomalia³ and Newkome,⁴ and the convergent method of Frechet.⁵ The convergent method was adopted in the synthesis of our target methoxycarbonyl-terminated polyether dendrimers. This method has the advantage over the divergent one in that the number of reaction in each step is restricted to a fixed small number irrespective of dendrimer generation, therefore completion of the reaction in each step can be controlled and monitored and large excess of reagent is not required even in the preparation of higher generation dendrimers.     

Pyrene as chromophore and electrophore: encapsulation in a rigid polyphenylene shell

Starting from the fourfold ethynyl-substituted chromophore 1,3,6,8-tetraethynylpyrene as core, a series of polyphenylene dendrimers was prepared in high yield by combining divergent and convergent growth methods. The fluorescence quantum yields (Q(f)>0.92) of the encapsulated pyrene chromophore were independent of the size of the polyphenylene shell. Fluorescence quenching studies and temperature-dependent fluorescence spectroscopy were performed to investigate the site isolation of the core. They indicate that a second-generation dendrimer layer is needed to efficiently shield the encapsulated pyrene and prevent aggregate formation. Alkali-metal reduction of the encapsulated pyrene core was carried out to afford the corresponding pyrene radical anions, for which hampered electron transfer to the core was observed with increasing dendrimer generation, which is further proof of the site isolation due to the polyphenylene shell. To improve film formation and solubility of the material, solubilizing alkyl chains were introduced on the periphery of the spherical particles. Furthermore, highly transparent films obtained by a simple drop-casting method showed blue emission mainly from the unaggregated species. The materials presented herein combine high quantum efficiency, good solubility, and improved film-forming properties, which make them possible candidates for several applications in electronic devices.     

Synthesis,characterization, and application of melamine‐based dendrimers supported on silica gel

Dendrimers based on melamine have been covalently attached to silica gel with convergent and divergent approaches. These approaches yield different products. Each step of the synthesis can be monitored with Fourier transform infrared spectroscopy, thermal gravimetric analysis, and X‐ray photoelectron spectroscopy. The organic molecules can be isolated by chemical etching the organosilica material with HF and then analyzed by mass spectrometry. The convergent approach, which relies on the solution‐phase synthesis of the dendrimer before its attachment onto silica gel, produces materials that are ～20% w/w organic regardless of the generation of the dendrimer used in the preparation of these composites and without apparent defects in the dendrimer structure. Materials prepared with the divergent approach show increasing percentages of organic material with each generation, but the dendrimers show structural defects including incomplete branching. In a survey of sequestration potential, materials obtained with the divergent approach removed more atrazine from solution than the more homogeneous materials obtained from the convergent approach. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 168–177, 2005     

Sugars within a hydrophobic scaffold: glycodendrimers from polyphenylenes

A new glycodendrimer type has been introduced that is designed on the basis of shape-persistent polyphenylene dendrimers. The sugar installation occurs not only on the dendrimer surface but also within the hydrophobic internal scaffold. The synthesis has been accomplished via both convergent and divergent routes by employing the Schmidt glycosylation and the Diels-Alder reaction. This new glycodendrimer has been found to exhibit water-solubility, while conserving hydrophobicity of the interior environment despite the incorporation of sugars.     

Organometallic dendrimers based on (tetraphenylcyclobutadiene)cyclopentadienylcobalt modules

The synthesis and characterization of novel organometallic polyphenylene dendrimers containing 24 or 44 phenyl rings and one cyclobutadiene(cyclopentadienyl)cobalt unit is reported. The dendrimers are made by the convergent CpCo(CO)(2)-mediated dimerization of di- or tetraethynyltolanes followed by a divergent core extension utilizing tetraphenylcyclopentadienone. The obtained dendrimers are air and water stable, soluble materials that show interesting differences in their hydrodynamic properties as evidenced by gel permeation chromatography. Scanning pulse voltammetry in solution shows that the dendrimers are oxidized at potentials ranging from 0.8 to 0.83 V. The more sterically encumbered the dendrimer, the higher its oxidation potential, that is, the more difficult oxidation is.     

Interactions in noncovalent PAMAM/TMPyP systems studied by fluorescence spectroscopy

Steady-state absorption and emission spectroscopy and time-resolved fluorescence measurements were employed in the study of meso-tetrakis(4-N-methylpyridinium)porphine (TMPyP) interactions with half-generation carboxyl-terminated poly(amidoamine) (PAMAM) dendrimers in water. TMPyP experiences a less polar environment and a strong fluorescence quenching effect upon dendrimer association. The tertiary amine functional groups in PAMAM dendrimers are likely to be responsible for the fluorescence quenching of TMPyP through an electron-transfer mechanism. The Stern-Volmer plots achieve a plateau at high dendrimer concentrations that was attributed to full porphyrin-dendrimer association, and an average fluorescence quantum yield of 15-20% relative to aqueous TMPyP was estimated. The association constant for the 1:1 complex with generation 2.5 at dendrimer-porphyrin ratio D/P = 1 is 5.75 x 10(7) M(-1), indicating a strong binding affinity. The dissociation of the complex with increasing ionic strength reinforces the role of electrostatic forces in porphyrin-dendrimer association. Comparison of Stern-Volmer plots obtained from quantum yields or lifetimes showed the importance of a static effect in these systems. The fluorescence decays of the porphyrin-dendrimer complex were fitted with a dispersed kinetics model. At intermediate dendrimer-porphyrin ratios (D/P approximately 1), diffusional quenching processes between free porphyrin and dendrimer were modeled with the Sano-Tachiya pair survival probability equation. Transient diffusional effects were dismissed as a possible explanation for the static effect detected.     

Dendrimer-functionalized single-wall carbon nanotubes: synthesis, characterization, and photoinduced electron transfer

We describe the synthesis and characterization of a series of single-wall carbon nanotubes (SWNTs) functionalized with polyamidoamine dendrimers. Importantly, the dendrimers are linked directly to the SWNT surface using a divergent methodology. This approach allows the number of functional groups on the nanotubes to be increased without provoking significant damage to the conjugated pi-system of the SWNTs. Several tetraphenylporphyrin moieties can be linked to the periphery of the dendrimers, and the photophysical properties of the resulting nanoconjugates have been investigated with a series of steady-state and time-resolved spectroscopy. The fluorescence kinetics provide evidence for two transient decays, one very short-lived (i.e., 0.04 +/- 0.01 ns) and one long-lived (i.e., 8.6 +/- 1.2 ns). A possible explanation is that some porphyrin units do not interact with the nanotubes, thus exhibiting a fluorescence lifetime similar to that of the free porphyrin. Complementary transient absorption measurements not only corroborate the fast decay of the photoexcited tetraphenylporphyrin but also confirm that intraconjugate charge separation evolves from the excited porphyrin to the SWNTs.     

Polyphenylene dendrimers with different fluorescent chromophores asymmetrically distributed at the periphery

A new synthetic approach leading to asymmetrically substituted polyphenylene dendrimers is presented. Following this method, polyphenylene dendrimers decorated with an increasing number of chromophores at the periphery have been obtained up to the second generation. Especially the synthesis of a polyphenylene dendrimer bearing three donor chromophores and one acceptor chromophore has been realized. Intramolecular energy transfer within this molecule is demonstrated by applying absorption and fluorescence measurements.     

Efficient synthesis of immolative carbamate dendrimer with olefinic periphery

An efficient synthetic strategy for immolative carbamate dendrons and dendrimers is described that requires no protection/deprotection in the convergent growth step. 1,3-Diamino-2-propanol was used as AB₂ building block and 4-nitrophenyl chloroformate as carbamate forming reagent. The method was demonstrated with a G3-dendron. A combination of convergent and divergent growth method was used to couple G2-dendrons to a G2-core or G3-dendrons to a tetrahedral G1-core with amine functional groups to form a spherical carbamate dendrimer (G4) possessing an olefinic periphery.     

Dendrimers with Oligospiroketal (OSK) Building Blocks: Synthesis and Properties

The development of novel dendrimers containing oligospiroketal (OSK) rods as building blocks is described. The linkage between the core unit (CU), branching units (BU), and OSK rods relies on the CuAAC reaction between terminal alkynes and azides. Two different strategies of dendrimer synthesis were investigated and it was found that the convergent approach is clearly superior to the divergent one. SAXS measurements and MD simulations indicate that the obtained dendrimer features a globular structure with very low density. Obviously, the OSK rods stabilize a rather loose mass‐fractal structure.     

Functionally layered dendrimers: a new building block and its application to the synthesis of multichromophoric light-harvesting systems

[reaction: see text] A divergent synthesis of internally functionalized dendrimers based on a modular functional monomer has been developed. This strategy was applied to the construction of a light-harvesting dendrimer containing one set of naphthopyranone dyes located at the interior and another set of coumarin chromophores located in the adjacent outer layer surrounding a porphyrin acceptor. Quantitative energy transfer from both donor pigments is observed, giving rise to exclusive emission from the porphyrin core over all excitation wavelengths.     

Multiple functionalization of benzophenones inside polyphenylene dendrimers--toward entrapped ions and radicals

Polyphenylene dendrimers possessing a defined number of keto groups in the dendritic scaffold have been synthesized by using a benzophenone-functionalized tetraphenylcyclopentadienone branching unit. A postsynthetic functionalization of the polyphenylene backbone was achieved by reacting the entrapped keto groups with organolithium reagents yielding monodisperse alcohol products. To investigate the accessibility and reactivity of the embedded groups, many functions of different size and nature, for example, the chromophore pyrene, were introduced. Moreover, suitable precursors for the synthesis of dendrimer entrapped species, trityl cations, trityl radicals, and ketyl radical anions, were obtained. To gain insight into the structure of these newly functionalized dendrimers, UV/vis, EPR, and NMR measurements have been performed. They showed a delocalization of the charge/spin into the polyphenylene dendritic arms leading to a stabilization of the ions/radicals. Remarkably, for the ketyl radicals, EPR measurements indicated the occurrence of intermolecular metal-bridged biradicals. They suggest the existence of a dendritic radical network of the dendrimers themselves.     

Investigation of ground- and excited-state photophysical properties of 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphyrin with ruthenium outlying complexes

The present work employs a set of complementary techniques to investigate the influence of outlying Ru(II) groups on the ground- and excited-state photophysical properties of free-base tetrapyridyl porphyrin (H(2)TPyP). Single pulse and pulse train Z-scan techniques used in association with laser flash photolysis, absorbance and fluorescence spectroscopy, and fluorescence decay measurements, allowed us to conclude that the presence of outlying Ru(II) groups causes significant changes on both electronic structure and vibrational properties of porphyrin. Such modifications take place mainly due to the activation of nonradiative decay channels responsible for the emission quenching, as well as by favoring some vibrational modes in the light absorption process. It is also observed that, differently from what happens when the Ru(II) is placed at the center of the macrocycle, the peripheral groups cause an increase of the intersystem crossing processes, probably due to the structural distortion of the ring that implies a worse spin-orbit coupling, responsible for the intersystem crossing mechanism.     

End-group dynamics of fluorescently labeled dendrimers

The molecular dynamics of dendrimer branches were studied by observing the excimer formation of pyrenyl end-groups. By a selective synthesis only one pyrene moiety per dendron was attached, resulting in a dendrimer with 3 fluorophors per molecule. The first 3 generations of the dendrimer were synthesized by the convergent-growth approach. By time resolved measurements of the eximer and monomer fluorescence an increased mobility of the end-groups was observed.     

Controlled MegaDalton assembly with locally stiff but globally flexible polyphenylene dendrimers

The divergent polyphenylene dendrimer synthesis of the largest chemically monodisperse molecules to date, up to 28 nm at 271.6 kDa for the sixth generation, is presented. Monodispersity, conformational flexibility, and an assembly behavior reminiscent of multimeric proteins for the locally stiff, macroporous dendrimers were evaluated with a combination of molecular and polymer characterization tools, namely size exclusion chromatography, atomic force microscopy, ultrahigh-mass MALDI-TOF mass spectrometry, and dynamic light scattering. Remarkably, the high-precision MegaDalton assembly of shape-adaptable dendrimers occurs in the absence of electrostatic or hydrogen-bonding interactions and is the product of Lilliputian solvophobic interactions, mediated by the dendrimer arm size, shape, and stiffness. This covalent/noncovalent approach offers a general molecular shaping motif that is completely different than what has been previously accessible with conventional self-assembly.     

Polycationic (mixed) core-shell dendrimers for binding and delivery of inorganic/organic substrates

The convergent synthesis of a series of polycationic aryl ether dendrimers has been accomplished by a convenient procedure involving quantitative quaternarization of aryl(poly)amine core molecules. The series has been expanded to the preparation of the first polycationic, mixed core-shell dendrimer. All these dendrimers consist of an apolar core with a peripheral ionic layer which is surrounded by a less polar layer of dendritic wedges. These cationic, macromolecular species have been investigated for their ability to form assemblies with (anionic) guest molecules. The results obtained from UV/Vis and NMR spectroscopies, and MALDI-TOF-MS demonstrate that all the cationic sites throughout the dendrimer core are involved in ion pair formation with anionic guests giving predefined guest/host ratios up to 24. The large NMR spectroscopic shifts of resonances correlated with the groupings located in the core of the dendrimers, together with the relaxation time data indicate that the anionic guests are associated with the cationic core of these dendrimers. The X-ray molecular structure of the octacationic, tetra-arylsilane model derivative [Si(C6H3[CH2NMe3](2)-3,5)4]8+. 8I- shows that the iodide counterions are primarily located near the polycationic sphere. The new polycationic dendrimers have been investigated for their catalytic phase-transfer behavior and substrate delivery over a nanofiltration membrane.     

Pi-conjugated dendrimers based on bis(enediynyl)benzene units

We have synthesized a new family of pi-conjugated dendrimers that are based on bis(enediynyl)benzene units by using both divergent and convergent approaches. The compounds at all three generations have strong bluish-green fluorescence, especially the third-generation dendrimer, which has the highest extinction coefficient and quantum efficiency in this series. [structure: see text]     

Multichromophoric polyphenylene dendrimers: toward brilliant light emitters with an increased number of fluorophores

Two routes for the introduction of highly fluorescent peryleneimide chromophores into the scaffolding of polyphenylene dendrimers via iterative Diels-Alder cycloadditions are presented. The key intermediates for the divergent dendrimer buildup were two cyclopentadienone branching units carrying two peryleneimides and two masked terminal alkynes. The difference between the two reagents is the mode of incorporation of the chromophores. In the first case, the chromophores were attached to the alpha-position of the tetraphenylcyclopentadienones. In the second case, peryleneimides are used as a "spacer" in the beta-position of the cyclopentadienones giving rise to dendrimers with extended molecular diameters (up to 12 nm) and 24 chromophores within their scaffold. Absorption and emission characteristics of the new multichromophoric nanoparticles were investigated and compared to those of the parent dyes. Additionally, an asymmetrically substituted first-generation dendrimer with six perylene diimide chromophores and one ester functionality is reported. The ester serves as a potential anchor group, and this nanoemitter paves the way to a multichromophoric fluorescence label. All dendrimers have good solubility in common organic solvents, high fluorescence quantum yields, and defined distances between the chromophores, making them attractive candidates for single-molecule spectroscopy.     

Phosphorus dendrimers: from synthesis to applications

Various methods of synthesis of phosphorus dendrimers are presented, in particular a fast method which allows creating each generation in only one step, while multiplying by 5 the number of end-groups. These methods of synthesis can easily be modified to place selectively on the surface, inside, within the branches or at the core, functional groups chosen according to the properties or the applications which they can bring. In many cases, the size of the dendrimer (its generation) has an influence on these properties. Various applications in the fields of catalysis, materials, and biology are presented. To cite this article: Anne-Marie Caminade et al., C. R. Chimie 6 (2003).     

Optically active BINOL core-based phenyleneethynylene dendrimers for the enantioselective fluorescent recognition of amino alcohols

A dramatic enhancement in fluorescence intensity from 1,1'-bi-2-naphthol (BINOL) to dendritic phenyleneethynylenes containing the BINOL core was observed. The strong fluorescence of the dendrimers allows a very small amount of the chiral materials to be used for sensing. The light harvesting antennas of the dendrimer funnel energy to the center BINOL unit, whose hydroxyl groups upon interaction with a quencher molecule lead to fluorescence quenching. This mechanism makes the dendrimers have much more sensitive fluorescence responses than corresponding small molecule sensors. The fluorescence of these dendrimers can be enantioselectively quenched by chiral amino alcohols. It is observed that the fluorescence lifetime of the generation two dendrimer does not change in the presence of various concentrations of 2-amino-3-phenyl-1-propanol. This demonstrates that the fluorescence quenching is entirely due to static quenching. Thus, formation of nonfluorescent ground-state hydrogen-bond complexes between the dendrimers and amino alcohols is proposed to account for the fluorescent quenching. A linear relationship has been established between the Stern-V?lmer constant of the generation two dendrimer and the enantiomeric composition of 2-amino-3-phenyl-1-propanol. Such enantioselective fluorescent sensors may allow a rapid determination of the enantiomeric composition of chiral molecules and are potentially useful in the combinatorial search of asymmetric catalysts and reagents.