Optical and photophysical properties of light-harvesting phenylacetylene monodendrons based on unsymmetrical branching

The optical and photophysical properties of phenylacetylene dendritic macromolecules based on unsymmetrical branching are investigated using steady-state and time-dependent spectroscopy. Monodendrons, up to the fourth generation, are characterized with and without a fluorescent perylene trap at the core. The higher generation monodendrons without the perylene trap exhibit high molar extinction coefficients (>10(5) M(-1) cm(-1)) and high fluorescence quantum yields (65-81%). When a perylene trap is placed at the core, then the monodendrons typically exhibit high energy transfer quantum yields (approximately 90%), as well as subpicosecond time scale excited-state dynamics, as evidenced by ultrafast pump-probe measurements. The photophysical properties of the unsymmetrical monodendrons are compared to those of phenylacetylene monodendrons with symmetrical branching, which have been described recently. The high fluorescence quantum yields and large energy transfer quantum efficiencies exhibited by the unsymmetrical monodendrons suggest they have potential for applications in molecular-based photonics devices.     

Synthesis and optical properties of conjugated dendrimers with unsymmetrical branching

An improved synthetic approach to conjugated monodendrons with unsymmetrical branching structures is reported. Dendrimers containing two or three such conjugated monodendrons are synthesized and their optical properties are studied. Such dendrimers exhibit broad absorptions and very high fluorescence quantum yields, making them promising candidates for applications in molecular-based photonics.     

Perfectly branched and hyperbranched poly(ether amide)s

Poly(ether amide) dendrimer segments based on 5-(2-aminoethoxy)-isophthalic acid have been synthesized using the convergent approach and mild reaction conditions. The monodendrons were combined to dendrimers via different core molecules and the resulting properties of the molecules were compared to those of the monodendrons. The melt polycondensation of the AB₂ monomers with analog structure resulted first in undefined polymer structures. However, by thermal polyaddition reaction of suitable functionalized oxazoline monomers, well defined hyperbranched poly(ether amide)s with phenol terminal groups could be synthesized.     

Synthesis and NaOTf mediated self-assembly of monodendritic crown ethers

The synthesis of ten benzyl ether based self-assembling monodendrons containing benzo[15]crown-5 at their focal point is presented. These dendritic building blocks self-assemble either directly or via complexation with NaOTf in two-dimensional smectic B, smectic A, and p6mm hexagonal columnar (Phi(h)) and three-dimensional Pm3n cubic lattices. Retrostructural analysis of these lattices and of the lattices generated from the same monodendrons containing various other functional groups at their focal point by X-ray diffraction experiments provided for the first time a correlation between the molecular structure and the shape of the monodendron, the shape of the supramolecular dendrimer and the symmetry of the lattice. It has been shown that complexation with NaOTf provides the following five different trends: a) stabilization of the three-dimensional Pm3n cubic lattice self-organized from spherical dendrimers that are self-assembled from conic monodendrons; b) stabilization of the two-dimensional S(A) phase generated from parallel-piped monodendrons; c) no effect on the stability of the two-dimensional S(B) phase generated from parallel-piped monodendrons; d) stabilization of the two-dimensional p6mm hexagonal columnar phase self-organized from cylindrical supramolecular dendrimers that are self-assembled from tapered monodendrons; and e) destabilization of the two-dimensional p6mm hexagonal columnar phase self-organized from cylindrical supramolecular dendrimers self-assembled from half-disc monodendrons. Mechanisms of NaOTf mediated self-assembly processes were suggested. These monodendritic crown ethers and their NaOTf complexes provide the largest diversity of liquid crystalline phases encountered so far in any library of supramolecular dendrimers.     

Supramolecular ordering of amide dendrons in lyotropic and thermotropic conditions

Self-assembled superstructures of amide dendrons, from first to third generation including monodendrons and covalently linked dimers, were extensively examined, and the supramolecular ordering processes in thermotropic and lyotropic conditions were compared. The superstructures as determined by X-ray diffraction and DSC revealed that the first and second generation dendrons showed nearly identical superstructures regardless of the assembly conditions. But, the third generation dendrons showed a more sensitive self-organizing behavior. The structure obtained from the gel state was lamellar with a more extended conformation, while the structure from the melt state revealed the columnar superstructures of contracted branches. The superstructure formed from the gel state also showed a structural change upon raising the temperature and assumed a structure similar to the thermotropically driven one, implying that the structure formed from the gel is thermodynamically unstable. The formation of lamellar- or cylinder-type superstructures from amide dendrons was primarily dependent on the shape of dendrons, which is associated with the branch size (generation) and the surrounding conditions.     

Synthesis and characterization of monodendrons based on 9-phenylcarbazole

A series of 9-phenylcarbazole ethynylene monodenrons have been prepared by palladium-catalyzed coupling reactions creating well-organized arrays of redox centers. The tert-butyl groups attached to the 3,6-positions of peripheral 9-phenylcarbazole monomers provide adequate solubility to a limited degree. Trimer and 7-mer monodendrons were prepared using a monomer with 3, 3-diethyltriazene at its focal point. To facilitate purification, the synthesis of 15-mer monodendron, however, required a monomer bearing a 3-hydroxy-3-methyl-but-1-ynyl group at its focal point as a masking group for the terminal acetylene functionality. Although the solubility was limited, high generation monodendrons were found to be readily soluble in carbon disulfide, a solvent of high polarizability. Spectroscopic studies showed that there is limited through-bond conjugation over the monodendrons, but fluorescence studies suggested the presence of long-range through-space interactions in the higher members of the series.     

Synthesis of light-harvesting dendrimers focally anchored with crown ethers or terpyridine ligands

Crown ethers and terpyridine ligands have been successfully attached to the focal point of light harvesting phenylacetylene monodendrons through Pd-catalyzed coupling reactions. The structures of these functional monodendrons were characterized by 1H and 13C NMR spectroscopy, mass spectrometry and elemental analysis. Such binding-ligand anchored dendrons exhibit broad absorption, large molar extenction coefficients and high fluoresence quantum yields. Coordination of crown ethers with alkali ions results in a significant increase in absorption strength in the UV range, but little alteration in either intensity or position of fluorescence emission. Coordination of terpyridine ligands with Ru2+, however, does efficiently quench the fluorescence from the dendrons,albeit only the smallest dendron exhibits efficient binding.     

Interplay between H‐Bonding and Alkyl‐Chain Ordering in Self‐Assembly of Monodendritic L‐Alanine Derivatives

This paper reports on the synthesis and self‐organizing properties of monodendrons consisting of L ‐alanine at the focal point and alkyl chains with different length at the periphery. The structures of thin films and monolayers are studied by temperature‐resolved grazing‐incidence X‐ray diffraction and scanning force microscopy. The interplay between H‐bonding and ordering of the alkyl chains results in a rich temperature‐dependent phase behavior. The monodendrons form H‐bonded stabilized clusters with the number of molecules depending on the length of the aliphatic chains and temperature. The clusters play the role of constitutive units in the subsequent self‐assembly. Short alkyl chains allow the material to form thermodynamically stable crystalline phases. The molecules with longer side groups exhibit additional transitions from the crystalline phase to thermotropic columnar hexagonal or columnar rectangular liquid‐crystalline phases. In monolayers deposited on highly ordered pyrolytic graphite, the materials show ordering similar to thin films. However, for the compound bearing hexadecyl chains the affinity of the alkyl groups to graphite dominates the self‐assembly and thereby allows epitaxial growth of a 2D lattice with flat‐on oriented molecules.     

Langmuir and grafted monolayers of photochromic amphiphilic monodendrons of low generations

Four generations of monodendrons with multiple dodecyl alkyl tails (AA-N, N representing number of alkyl tails from 1 to 8), an azobenzene spacer group, and a carboxylic acid polar head have been studied at the air-water and air-solid interface using AFM, GIXD, X-ray reflectivity, and UV-vis spectrometry. The one and two tail molecules formed orthorhombic lateral packing with long-range intramonolayer ordering. Good agreement between molecular models and thickness measurements indicated that the one and two tail molecules orient along the surface normal. The increase in the cross-sectional mismatch caused by the presence of the multiple chains for the higher generations disrupted the long-range ordering and forced the alkyl tails to adopt quasi-hexagonal structure. The higher generations (AA-4 and AA-8) formed a kinked structure with the alkyl tails oriented perpendicular to the surface with the azobenzene group tilted at a large degree toward the surface. The photoisomerization behavior in dilute solutions, at the air-water interface, and for grafted layers demonstrated that lower generation monodendrons maintained the photochromic behavior after chemical grafting to the silicon substrates, although the confinement of the molecules in monolayers significantly increased the reorganization time.     

Supramolecular self-organization of "Janus-like" diblock codendrimers: synthesis, thermal behavior, and phase structure modeling

We report on the design and synthesis of three series of segmented amphiphilic block codendrimers, and on their self-organizing behavior in liquid-crystalline mesophases. Connecting two prefunctionalized monodendrons, each differing in their chemical constitution and generation number, yielded these diblock supermolecules. One wedge of the codendrimer was made hydrophobic, and is based on a branched poly(benzyl ether) monodendron functionalized at the periphery by lipophilic aliphatic fragments (also known as Percec dendrons). The other segment was made hydrophilic by the grafting of hydroxyl-containing moieties onto the focal functions of the former dendrons. Both types of dendrons were prepared independently by convergent methods and then joined in the ultimate stage of the synthetic procedure by cross-coupling reactions. In this way, the proportion of the dendritic blocks was varied independently to allow control of the hydrophilic/hydrophobic balance (HHB), the hydrogen-bonding ability, and consequently the capacity to tune the mesomorphic properties of the resulting "superamphiphiles" was anticipated. Essentially all the dendritic compounds display a thermotropic mesomorphism directly at or near room temperature as determined by using X-ray diffraction, polarized optical microscopy, and differential scanning calorimetry. The nature and the supramolecular organization of the mesophases, namely columnar and cubic phases, are correlated to the size of the respective block monodendrons and the chemical structures of the dendromesogens. The molecular organization within the cubic phases can be geometrically described and well understood by the space-filling polyhedron model.     

Facile synthesis and optical properties of bithiophenesilane monodendrons and dendrimers

Three generations of bithiophenesilane monodendrons and dendrimers consisting of 3-45 2,2'-bithiene-5,5'-diyl units were synthesized by means of effective coupling reactions between the corresponding bithienyllithium derivatives and chlorosilanes. These compounds show efficient photoluminescence in the violet-blue region, the quantum yield of which is 5-15 times higher than that for the parent bithiophene or bithiophenesilanes.     

Synthesis and mesomorphic properties of cyanurates and isocyanurates. Branched mesogens as model crosslinks for liquid crystalline thermosets

A number of new s-triazines (cyanurates and isocyanurates) with diaromatic mesogenic branches have been synthesized and their thermal properties investigated. No liquid crystal phases were found in the series of isocyanurates, while the cyanurates form enantiotropic calamitic mesophases (nematic and in one case smectic). No discotic mesophases could be detected. The mesogenic power of the cyclotrimers is reversed with respect to that of the monomers from which they can be obtained (cyanates and isocyanates). Molecular calculations reveal that the cyanurates can adopt an extended rod-like conformation due to the flexibility introduced by the oxygen bridge between the central ring and the mesogenic branches. The direction of the ester group in the phenyl benzoate mesogenic branches has a tremendous influence on the mesomorphic properties, with the result that mesophases could only be observed if the benzoic acid part was attached to the triazine ring.     

Toward globular macromolecules with functionalized interiors: design and synthesis of dendrons with an interesting twist

[see reaction]. Design and synthesis of a novel class of monodendrons, in which the functional units can potentially be directed toward the concave interiors of dendrimers, are described. The key feature of the design is the placement of the amphiphilic and the AB2 functional groups in orthogonal planes.     

Synthesis, liquid-crystalline properties, and supramolecular nanostructures of dendronized poly(isocyanide)s and their precursors

A series of novel dendronized pi-conjugated poly(isocyanide)s were synthesized successfully by using a Pd-Pt mu-ethynediyl dinuclear complex ([ClPt{P(C2H5)3}2C[triple bond]CPt{P(C2H5)3}2Cl]) as the initiator. The polymerizations of the dendronized monomers follow first-order kinetics, indicating that living polymerization takes place. The obtained polymers exhibit narrow polydispersities in the range of 1.03-1.20. Thermal properties of the poly(isocyanide)s as well as their isocyanide monomers and precursors with formamido (HCONH-) moieties as apexes were investigated by using differential scanning calorimetry (DSC), polarized optical microscopy (POM) and wide-angle X-ray diffraction (WAXD). Both the peripheries and the apex groups of the dendrons affect the formation of supramolecular column and/or cubic phases of the precursors and monomers. The formamido precursor forms a liquid-crystalline phase due to intermolecular hydrogen bonding. The isocyanide monomer lacks this hydrogen-bonding ability and does not display an organized mesophase. All of the rigid poly(isocyanide)s with the monodendrons exhibit columnar liquid-crystalline phases. Interestingly, cylindrical structures of a poly(isocyanide) were directly visualized by using transmission electron microscopy (TEM).     

Self-assembly of facially amphiphilic dendrimers on surfaces

Facially amphiphilic dendrimers have been shown to provide significant difference in surface behavior due to subtle changes in structure. The monodendrons are capable of providing hydrophobic surfaces, while the didendrons provide superhydrophobic surfaces. This provides an example of how a molecular level change could result in significant changes in surface behavior. This difference is attributed to the conformational differences exhibited by these dendrimers on surfaces.     

Prediction of branch on branch and topological characteristics of low‐density polyethylene polymerization by a novel stochastic approach

A novel approach using Monte Carlo method applied to simulation of low‐density polyethylene (LDPE) polymerization in tubular reactor showing topological characteristics, and the comprehensive kinetic mechanism has been taken into consideration. The results show the precise details of the structure of a chain in the three levels of the backbone, the main branches, and branches on branch. The chain types include dead polymer, dead polymer with unsaturated end, and live polymer with primary radical, secondary radical, and tertiary radical. In this work, the branches on branch were identified in terms of number, length, and position of the branch. Sixty percent of branches on branch are 1 to 5 carbons long, and the longest branch on branch is about 50 carbons. Thus, this study provides a tool for more accurately mapping the polymer chains architecture, superior to determine the number, and position of long‐ and short‐chain branches in past researches. Finally, this approach will advance the prediction of microstructure‐related properties of polymer one step further.     

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.     

Properties of star-branched polymer brushes

A simple model of a polymer brush was constructed. The star polymers with three arms were terminally attached with one arm (the stem) to an impenetrable surface with the other two arms (branches) free. The excluded volume effect was included into the model as the only interaction. Therefore, the system was studied in good solvent conditions. The simulations were carried out by means of the dynamic Monte Carlo method using the local changes of chain conformations to sample efficiently the conformational space. The influence of both the number of chains (the grafting density) and the length of chains on the static properties of the polymer brush was studied. The internal and local structure of a formed polymer layer was determined. It was shown that the size of the stems increased rapidly with the increase of the grafting density, while the size of the branches diminished. The changes of the spatial orientations of the stems and the branches for different grafting densities were shown and discussed.     

Molecular recognition directed self-assembly of tubular supramolecular architectures from building blocks containing monodendrons as exo-receptors and crown- or pseudo-crown-ethers as endo-receptors

The principles of self-assembly of tapered exo-receptors constructed from monodendrons containing crown-ether, oligooxyethylenic groups and polymer backbones as endo-receptors into tubular supramolecular architectures are described with selected examples. In the case of polymers with flexible backbones it is suggested that a helical chain conformation is induced during the assembly of its own tapered side groups. This mechanism is supported by the enhanced self-assembling ability of tapered side groups attached to a backbone with rigid helical conformation.     

Synthesis,Photophysical, and Two‐Photon Absorption Properties of Elongated Phosphane Oxide and Sulfide Derivatives

A series of rod‐shaped and related three‐branched push–pull derivatives containing phosphane oxide or phosphane sulfide (PO or PS)—as an electron‐withdrawing group conjugated to electron‐donating groups, such as amino or ether groups, with a conjugated rod consisting of arylene–vinylene or arylene–ethynylene building blocks—were prepared. These compounds were efficiently synthesized by a Grignard reaction followed by Sonogashira coupling. Their photophysical properties including absorption, emission, time‐resolved fluorescence, and two‐photon absorption (TPA) were investigated with special attention to structure–property relationships. These fluorophores show high fluorescence quantum yields and solvent‐dependent experiments reveal that efficient intramolecular charge transfer occurs upon excitation, thereby leading to highly polar excited states, the polarity of which can be significantly enhanced by playing on the end groups and conjugated linker. Rod‐shaped and related three‐branched systems show similar fluorescence properties in agreement with excitation localization on one of the push–pull branches. By using stronger electron donors or replacing the arylene–ethynylene linkers with an arylene–vinylene one induces significant redshifts of both the low‐energy one‐photon absorption and TPA bands. Interestingly, a major enhancement in TPA responses is observed, whereas OPA intensities are only weakly affected. Similarly, phosphane oxide derivatives show similar OPA responses than the corresponding sulfides but their TPA responses are significantly larger. Finally, the electronic coupling between dipolar branches promoted by common PO or PS acceptor moieties induces either slight enhancement of the TPA responses or broadening of the TPA band in the near infrared (NIR) region. Such behavior markedly contrasts with triphenylamine‐core‐mediated coupling, which gives evidence for the different types of interactions between branches.