Sequences in dendrons and dendrimers

Sequential incorporation of a variety of functional groups forms the basis for specificity in biomacromolecules. Introduction of such diversity and sequencing ability in artificial macromolecules is fundamentally interesting. In this paper, three different synthetic approaches have been used to build dendrons and dendrimers in which all the monomer units are different from each other. The synthetic strategies described in this paper involve the use of (i) an ABB(p) monomer, (ii) an ABB' monomer, and (iii) an ABB(m) monomer. The complementarity and the versatility of these synthetic approaches should render them useful for a variety of applications.     

Snowflake-like dendrimers via site-selective synthesis of dendrons

[structure: see text] Snowflake-shaped dendrimers were prepared via site-selective synthesis of dendrons, where an attachment of encapsulating dendritic branches and an extension of phenylacetylenic units were alternatively manipulated on the structure of AB(2) (diethyltriazeno for A and bromo for B) substituted diphenylacetylene using a combination of Suzuki and Sonogashira cross-coupling reactions.     

A family of fluorenyl dendrons for porphyrin dendrimers synthesis

A series of dendrons bearing various number of fluorenyl donor groups have been synthesized. First, the reference compound 2-(bromomethyl)-9H-fluorene (8) with one fluorenyl unit, then dendron 10, with two fluorenyl arms, and finally new generation dendrons, 11 and 12, bearing four peripheral fluorenyl arms were synthesized and characterized. A series of different generations of porphyrin dendrimers, obtained from these dendrons are also presented. Preliminary results on higher generation dendrimers are reported as well. Under mild basic conditions, surprisingly, a new compound 1 incorporating a fluorenyl unit in the cycle and three pendant fluorenyl arms was obtained by an intramolecular reaction of brominated tetrapod dendron 12.     

Modular peripheral functionalization of thiophene dendrons and dendrimers

A series of thiophene dendrons and dendrimers with peripheral functional groups were designed and synthesized. Two methodologies using thiophene dendrons and dendrons as synthetic building blocks, namely, (1) periphery functionalization; (2) a combination of focal and periphery functionalization have been demonstrated.     

Synthesis and properties of dumbbell-shaped dendrimers containing 9-phenylcarbazole dendrons

We report the synthesis and structural characterization of two dumbbell-shaped dendrimers incorporating 9-phenylcarbazole units as dendrons, as well as their thermal, morphological, photophysical, and electrochemical properties.     

Synthesis and characterization of dendrons and dendrimers skeleton-constructed with azobenzene moiety

A series of dendrons and dendrimers skeleton-constructed with azobenzene moiety based upon 4-carboxy-4′-(1,2-propanediolether)-azobenzene as an AB₂ monomer, via a convergent approach, proceeding in a repeated stepwise growth manner starting from 4-carboxy-4′-(n-butylether)-azobenzene as a peripheral monomer, were synthesized, and characterized by NMR, FTIR, and MALDI-TOF-MS analysis. Their regular molecular architecture and thus monodispersed molecular weights were confirmed by GPC. The UV-Vis absorbance and ¹H NMR spectrum study indicated that the azobenzene moieties in CHCl₃ solution took fully trans-cis isomerization under UV irradiation, and reversely isomerization back to the trans by visible light irradiation or by heat.     

Synthesis of carbosilane dendrons and dendrimers derived from 1,3,5-trihydroxybenzene

Several carbosilane wedges of generations 1-3 have been synthesized, following the divergent method, containing at the focal point a C-Br bond and as peripheral functional groups SiMeCl₂, SiMe(C₃H₅)₂, SiMe₂Cl, SiMe₂H, and ester units SiMe₂{(C₃H₆)N(C₂H₄CO₂Me)₂}. The dendrons functionalized with SiMe(C₃H₅)₂ and SiMe₂{(C₃H₆)N(C₂H₄CO₂Me)₂} groups were used to synthesize spherical dendrimers derived from 1,3,5-(HO)₃C₆H₃, leaving the outer groups unchanged. The allyl dendrimers thus obtained were used as precursors to prepare new dendrimers functionalized with SiMeCl₂, SiMe₂Cl, SiMe₂H, amine units SiMe₂{(C₃H₆)NH₂} and also ester units SiMe₂{(C₃H₆)N(C₂H₄CO₂Me)₂}.     

Supramolecular structural diversity among first-generation hybrid dendrimers and twin dendrons

Hybrid dendrimers, obtained by complete monofunctionalization of the peripheral amines of a "zero-generation" polyethyleneimine dendrimer, provide structurally diverse lamellar, columnar, and cubic self-organized lattices that are less readily available from other modified dendritic structures. The reaction of tris(2-aminoethyl)amine (TREN) with 4-dodecyloxybenzimidazolide provides only the corresponding zero-generation TREN dendrimer. From the mixture of tri- and disubstituted TREN derivatives obtained from first-generation self-assembling dendritic imidazolides, the hybrid dendrimer and a twin dendron could be separated, purified, and characterized. The hybrid dendrimers display smectic, columnar hexagonal (Phi(h)), and cubic (Pm_3n) lattices. The TREN twin dendrons, on which only two peripheral amines have been acylated, exhibit centered-rectangular columnar (Phi(r-c)), Phi(h), and Pm_3n lattices. The existence of a thermoreversible Phi(h)-to-Pm_3n phase transition in the first-generation hybrid dendrimers and twin dendrons is exploited to elucidate an epitaxial relationship between the two mesophases. We postulate a mechanism by which the transition proceeds. The thermoreversible Phi(h)-to-Pm_3n phase change is accompanied by optical property changes that are suitable for rudimentary signaling or logic functions. This structural diversity reflects the quasiequivalence of flat-taper and conical self-assembling dendrons and the ability of flexible dendrimers to accommodate concomitant conformational and shape changes.     

Thermally responsive dendrons and dendrimers based on reversible furan-maleimide Diels-Alder adducts

[reaction: see text]. Benzyl aryl ether dendrons and dendrimers containing thermally reversible furan-maleimide Diels-Alder adducts were prepared up to the third generation. The covalent cleavage and reassembly of the dendrons and dendrimers were evaluated by 1H NMR.     

Star-shaped mesogens of triazine-based dendrons and dendrimers as unconventional columnar liquid crystals

Dendrons Gn-Cl and Gn-NH (n = 2-4) and novel dendrimers Gn-N approximately N-Gn (n = 2-4) based on triazine and piperazine units were efficiently prepared in good yields without employing the protection and deprotection processes and are fully characterized by 1H NMR and 13C NMR spectroscopies, elemental analysis, and mass spectroscopy. These compounds are transparent and possess good thermal stability. G4-Cl shows a monotropic columnar phase in a narrow range with a coexisting crystalline phase. Dendron G4-NH shows a rectangular column-phase, and dendrimer G4-N approximately N-G4 exhibits a monotropic hexagonal columnar phase. These identifications were supported by the polarizing optical scope and powder XRD studies.     

Convergent synthesis of PAMAM dendrimers using click chemistry of azide-functionalized PAMAM dendrons

Azide-functionalized PAMAM dendrons containing an azidopropylamine focal point were synthesized by the divergent method and applied for the construction of symmetric PAMAM-like dendrimers containing 1,2,3-triazole rings as connectors via stitching with two different multi-terminal alkynes. The stitching method was based on the click chemistry protocol, i.e., the copper-catalyzed cycloaddition reaction between an alkyne and an azide.     

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.     

Synthesis of azide-functionalized PAMAM dendrons at the focal point and their application for synthesis of PAMAM-like dendrimers

For the first time, the divergent synthesis of azide-functionalized PAMAM dendrons using azidopropyl amine as an azide focal point and convergent synthesis of symmetric PAMAM-like dendrimers containing 1,2,3-triazole rings as connectors via click chemistry with a dialkyne core is described.     

Highly congested liquid crystal structures: dendrimers, dendrons, dendronized and hyperbranched polymers

In this tutorial review we describe some studies concerning liquid crystal dendritic polymers. We have chosen to present several representative examples that illustrate the diverse kinds of LC dendritic structures, namely: dendrimers, dendrons, dendronized polymers and hyperbranched polymers. We review their synthesis, mesogenic properties and the way that they are arranged to form supramolecular liquid crystal assemblies.     

Polyamido amine dendrimers functionalized with poly(phenylenevinylene) dendrons at their periphery

A series of polyamido amine (PAMAM) dendrimers (Generations 2, 3, 4, and 6) fully functionalized at their periphery with first‐ and second‐generation poly (phenylenevinylene) (PPV) dendrons have been efficiently prepared. MALDI‐TOF mass spectrometry proved to be particularly useful for the characterization of the new hybrid dendrimers as well as for the estimation of the average number of PPV dendrons attached to the surface. The optical absorption and emission properties of these systems were studied. The materials display extremely high molar extinction coefficients and emit blue light with only slightly lower fluorescence quantum yields than the corresponding free dendrons. Self‐quenching interactions between PPV units were not observed in THF. However, the luminescence properties underwent a dramatic change when toluene was used as the solvent. The lower polarity of toluene caused shrinkage of the PAMAM structure and brought the PPV chromophores closer together, leading to self‐quenching interactions and excimer formation. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6409–6419, 2009     

Synthesis of polyester dendrimers and dendrons starting from Michael reaction of acrylates with 3-hydroxyacetophenone

A new AB₂ type building block for synthesis of dendritic compounds is made starting from Michael addition of 2 equiv of 3-hydroxyacetophenone to acrylate, followed by decarboxylation and deprotection of either carboxyl or hydroxyl protecting groups. [G1]-[G4] dendrons and [G2] dendrimer were synthesized by the reiterative [hydrogenolysis then DCC/DPTS coupling] sequence using a combination of convergent and divergent growth methods.     

Synthesis and structure of intramolecularly hydrogen bonded dendrons

[reaction: see text] A convenient synthesis of monodendrons whose conformation is restricted through the intervention of intramolecular hydrogen bonding and repulsive electrostatic interactions is described. X-ray crystal structure analysis of the second generation dendron shows the presence of a propeller-type secondary structure and indicates that the dendrons have assembled into a symmetrically interdigitated dimer in the solid state. 1H NMR and IR spectral data are in agreement with the presence of intramolecular hydrogen bonding between the amides and the pyridine N throughout the dendron structure in solution.     

Efficient synthesis of poly(phenylazomethine) dendrons allowing access to higher generation dendrimers

We have developed a novel synthetic method of phenylazomethine dendrons that uses 4,4'-methylenedianiline instead of 4,4'-diaminobenzophenone to synthesize the precursor of the phenylazomethine dendron and then oxidized the precursor to the next-generation dendron. For this method, the productivity of the dendrons has been significantly increased. Furthermore, as the synthesis of high-generation dendrons becomes easier, synthesis of DPA G5 was achieved. [structure--see text]