Shape-persistent linear, kinked, and cyclic oligo(phenylene-ethynylene-butadiynylene)s: self-assembled monolayers

Shape-persistent rigid phenylene-ethynylene-butadiynylenes form lamellar self-assembled monolayers (SAMs) at the HOPG/TCB interface, which were studied by scanning tunneling microscopy (STM) with submolecular resolution. Substitution of the terminating acetylene functions with polar cyanopropyldimethylsilyl groups leads to 2D phase separation and defined rod-rod interactions, which determine the packing distances between the rigid rods. The results stimulated the connection of rigid rods via septiarylene clamp units. They covalently link two rigid rod units and define the intramolecular rod-rod distance that matches the alkoxy substituent chain lengths. The systems can be described as half-ring structures of two rigid rods connected via a rotatable joint unit. These acetylene-terminated half-ring structures were also oligomerized under Cu and Pd catalysis to yield defined acyclic and cyclic oligomers. Detailed STM studies decoded the molecular origin of the surface patterning of such systems. The dodecyloxy side chains are adsorbed along the HOPG main axes and, together with the alkoxy backbone angle, determine the adsorption direction of the adlayers.     

Synthesis and characterisation of tetra-tetrazole macrocycles

The syntheses of tetra-tetrazole macrocycles, containing two bis-tetrazole units linked by a variety of alkyl-chain lengths from four to eight carbons, are described. The crystal structures of three of these derivatives are reported, and the molecular conformation in the solid state is compared to that of the previously reported tetra-tetrazole macrocycle and to other bis- and tris(tetrazole)benzene structures. The macrocycle conformation is influenced by the length of the alkyl-chain linker, the relative orientation of the tetrazole rings on the benzene ring and by intermolecular interactions. In the macrocycles based on 1,2-bis(tetrazole)benzene, the adjacent tetrazole rings on the benzene ring are prevented from becoming co-planar on intramolecular (steric) grounds. In the 1,3- and 1,4-bis(tetrazole)benzene derivatives, there is no such impediment, and a co-planar arrangement is observed where intra- and/or intermolecular stacking interactions exist. Deviations from co-planarity are associated with optimisation of intermolecular interactions between the tetrazole rings and adjacent alkyl chains. In the macrocycle based on 1,4-bis(tetrazole)benzene with four-carbon linkers, an intramolecular stacking interaction exists, which precludes the presence of any cavity. In the macrocycle based on 1,3-bis(tetrazole)benzene with six-carbon linkers, a cavity of 10.8×9.4 Å is observed for each molecule in the solid state, although the packing of adjacent molecules is such that there are no extended channels running through the crystal.     

The effect of fluorination: a crystallographic and computational study of mesogenic alkyl 4-[2-(perfluorooctyl)ethoxy]benzoates

The series of alkyl 4-[2-(perfluorooctyl)ethoxy]benzoates (F8-n) shows a systematic change of crystal structures depending on the length of the alkyl chain: separate packing of perfluorooctyl (Rf) and alkyl (Rh) chains from each other for shorter (n=2) and longer (n=11) members, alternate packing of Rf and Rh chains for middle (n=6,7) members, and an intermediate type of packing for n=4. Semiempirical MO calculations show slightly repulsive interactions between the Rf chains, and attractive ones between Rf and Rh chains and between Rh and the core of a molecular pair. It is concluded that fluorination determines the molecular shape of the crystal structures by making the chain rigid. It is confirmed that the interactions between Rf chains are small compared with those between other moieties and that they are forced to aggregate owing to the exclusion from other moieties. Thus, the effect is dependent on the geometries and intermolecular interactions of the other moieties.     

Thermal behavior and intermolecular interactions in blends of poly(3‐hydroxybutyrate) and maleated poly(3‐hydroxybutyrate) with chitosan

The thermal behavior and intermolecular interactions of blends of poly(3‐hydroxybutyrate) (PHB) and maleated PHB with chitosan were studied with differential scanning calorimetry, Fourier transform infrared (FTIR), wide‐angle X‐ray diffraction (WAXD), and X‐ray photoelectron spectroscopy (XPS). The differences in the two blend systems with respect to their thermal behavior and intermolecular interactions were investigated. The melting temperatures, melting enthalpies, and crystallinities of the two blend systems gradually decreased as the chitosan content in the blends increased. Compared with that of the PHB component with the same composition, the crystallization of the maleated PHB component was more intensively suppressed by the chitosan component in the blends because of the rigid chitosan molecular chains and the intermolecular hydrogen bonds between the components. FTIR, WAXD, and XPS showed that the intermolecular hydrogen bonds in the blends were caused by the carbonyls of PHB or maleated PHB and chitosan aminos, and their existence depended on the compositions of the blends. The introduction of maleic anhydride groups onto PHB chains promoted intermolecular interactions between the maleated PHB and chitosan components. In addition, the intermolecular interactions disturbed the original crystal structures of the PHB, maleated PHB, and chitosan components; this was further proven by WAXD results. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 35–47, 2005     

Synthesis and ordering of two-dimensional polymers

The work to be discussed in this lecture departs from the usual focus of polymer science on linear chains and explores the “bulk” synthesis and properties of polymer molecules that can be considered shape-persistent molecular objects. Shapes that are particularly interesting are those not common in the conformational space of linear chains, for example, two-dimensional polymers shaped as plates or discs and macromolecules shaped as cylinders or parallelepipeds. The lecture focuses on a molecular object which is a rigid and internally anisotropic two-dimensional polymer with planar dimensions greater than its thickness. The shape-granting skeleton of this two-dimensional polymer is built by covalent bonds. We have so far developed three different strategies for their bulk synthesis, all involving systems in which reactive oligomers organize spontaneously into the necessary planar assemblies to form the object. In one strategy molecular recognition events such as homochiral interactions play a key role in the formation of the two-dimensional flat polymers /1/. A different methodology relies on nanophase separation in rodcoil block molecules in which a rigid segment is covalently bonded to a flexible one sharing the same backbone. The third strategy involves the folding of oligomers into hairpin structures which self assemble into two-dimensional liquids. In these two last strategies the layered rodcoils or hairpins react to form the covalent backbones necessary to grant shape to the object. A computer simulation relevant to the experimental system suggests that large two-dimensional polymers can be formed by extremely short backbones. The lecture will also describe examples of unique properties in advanced materials that could emerge from these rigid two-dimensional objects. These examples include. materials with self-organized surfaces of high chemical definition and temporal stability, self assembling membranes, molecular reinforcement, and films with remarkably stable electrical or optical properties.     

Can a Proton be Encapsulated in Tetraamido/Diamino Quaternized Macrocycles in Aqueous Solution and Electric Field?

The proton‐binding behavior of solvated tetraamido/diamino quaternized macrocyclic compounds with rigid phenyl and flexible phenyl bridges in the absence or presence of an external electric field is investigated by molecular dynamics simulation. The proton can be held through H‐bonding interactions with the two carbonyl oxygen atoms in macrocycles containing rigid (phenyl) and flexible (propyl) bridges. The solute–solvent H‐bonding interactions cause the macrocyclic backbones to twist to different extents, depending on the different bridges. The macrocycle with the rigid phenyl linkages folds into a cuplike shape due to π–π interaction, while the propyl analogue still maintains the ellipsoidal ringlike shape with just a slight distortion. The potential energy required for proton transfer is larger in the phenyl‐containing macrocycle than in the compound with propyl units. When an external electric field with a strength of 2.5 V nm^?1 is exerted along the carbonyl oxygen atoms, a difference in proton encircling is exhibited for macrocycles with rigid and flexible bridges. In contrast to encapsulation of a proton in the propyl analogue, the intermolecular solute–solvent H‐bonding and intramolecular π–π stacking between the two rigid phenyl spacers leads to loss of the proton from the highly distorted cuplike macrocycle with phenyl bridges. The competition between intra‐ and intermolecular interactions governs the behavior of proton encircling in macrocycles.     

Introduction of Ferrocene as a Facilitator for the Construction of Supramolecular Polymers

Proper monomer design is the key to enhancing the strength of noncovalent interactions between the molecules toward the efficient formation of supramolecular polymers (SPs). We have designed and synthesized 1,n′-disubstituted ferrocene-azobenzene-long alkyl chains, Fc(CONH-Azo-TDP)₂, to afford SPs with a high probability. The design exploits the ‘‘molecular ball-bearing’’ property of the ferrocene core, which allows two azobenzene arms to rotate in the planes of cyclopentadienyl rings, generating the most suitable molecular conformation required for SP formation. This ferrocene monomer formed a supergel consisting of SPs supported by strong intermolecular (H-bonding and π-π stacking) interactions and higher enthalpy gain than the reference molecules, where the central ferrocene core was replaced by flexible aliphatic as well as rigid benzene linkers. The molecular conformation involved in SPs, the strength of noncovalent interactions, and the process of supramolecular polymerization were investigated through NMR, UV-Vis, XRD and TEM studies. The results demonstrate that ferrocene may act as a good modulator for constructing efficient SPs.     

Self‐Assembly of a Tripod Aromatic Rod into Stacked Planar Networks

Threefold symmetric rigid‐core molecules with an internally grafted poly(ethylene oxide) (PEO) chain were synthesized, and their self‐assembled structures were characterized using differential scanning calorimetry, TEM, and 1D and 2D X‐ray scatterings in the solid state. The tripod compounds based on short PEO chains (n=8, 13, 17, 21), self‐assemble into 2D channel‐like network structures, whereas the compound with the longest PEO chain (n=34) forms a lamellar liquid crystalline phase. The interiors of the channel structures are filled with flexible PEO chains along the double‐walled aromatic circumference. In these channel‐like networks, three aromatic rods connected in the meta‐position to each other are superimposed in parallel to other adjacent molecules to form the double‐walled aromatic frameworks stacked perpendicular to the resulting channels. These are novel examples of supramolecular channel‐like structures developed using amphiphilic diblock molecules based on a threefold symmetric rigid scaffold.     

Liquid crystalline homopolyesters having main chain calamitic mesogens and one or two side chain azobenzene moieties in the repeat units

Combined semi-rigid homopolyesters, containing both main chain calamitic mesogens and one or two side chain azobenzene units separated by aliphatic (hexamethylene, octamethylene and decamethylene) chains in the polymer repeat units, were prepared and their liquid crystalline properties characterized. Polyesters having two side chain azobenzene units and a main chain biphenyl moiety showed a higher ordered smectic B or smectic F phase, whereas the other polymers containing a main chain 2,5-diphenyl-1,3,4-thiadiazole unit and one or two side chain methoxyazobenzene units formed a smectic C phase despite the presence of different mesogens in the main and side chains. This is probably due to the compact molecular chain-packing and intra- and intermolecular interactions between the polymer backbones and the two azobenzene units.     

Crystal structure of poly(ethylene-oxide) supramolecular assemblies

The formation of poly(ethylene oxide) (PEO) supramolecular complexes is discussed in terms of intermolecular interactions and molecular packing. On the basis of the different known crystal structures, several mechanisms are proposed. First, the PEO complexes can be formed by an Intercalation or Inclusion process, guest molecules diffusing into the PEO unit cell. On the other hand, molecular complexes based on hydrogen bonding cannot be obtained by such a way, their formation requires the complete removal of the initial PEO structure either by melting or dissolution. Finally the relations between the crystal lamellar morphology, the host-guest interactions and the PEO chain mobility are discussed.     

Two dimensional chiral networks emerging from the aryl-F...H hydrogen-bond-driven self-assembly of partially fluorinated rigid molecular structures

Self-assembly of the partially fluorinated rigid molecules physisorbed at solution/graphite interface has been investigated by scanning tunneling microscopy. Upon adsorption, both the branched star-shaped compound 1 and the angulate rod 2 compromising diacetylene and acetylene interlinked benzene and pentafluorobezene formed two-dimensional chiral porous networks. The spontaneous formation of these architectures is likely attributed to the two effects: the compensation of the dipole moments of the branches and the formation of Ar-H...F hydrogen bonds. These results demonstrate that the immobilization of molecules at the liquid/solid interface can be driven by these weak intermolecular interactions instead of van der Waals interactions between alkyl chains and substrate.     

Supramolecular Aggregates of Dendritic Cyclophanes (Dendrophanes) Threaded on Molecular Rods with Steroid Termini

Multinanometer-sized assemblies with molecular weights exceeding 14 000 are obtained by the threading of two dendritic cyclophanes (dendrophanes) onto molecular rods in which two testosterone termini are attached by rigid spacers to a central phenyl ring bearing two quaternary ammonium side chains. The formation of these structurally defined aggregates, in which the dendrophanes preferentially encapsulate the steroid termini (see picture), is driven by a combination of apolar interactions, hydrophobic desolvation, and ion pairing, and depends strongly on the length of the spacer.     

L-苯丙氨酸衍生物与脂肪胺构筑双组分超分子凝胶

设计合成了系列单链L-苯丙氨酸衍生物,该系列衍生物单组分没有胶凝性能.选择脂肪胺作为配对物,与L-苯丙氨酸衍生物组成双组分体系后能够胶凝许多有机溶剂形成凝胶.流变学测试显示该凝胶体系弹性模量(G′)比粘性模量(G′′)约高一个数量级,有着很好的机械性能,并且呈现出典型的类固体的流变学行为.傅里叶变换红外(FT-IR)光谱、核磁共振(NMR)谱、小角X射线衍射(SAXS)和扫描电镜(SEM)结果表明,凝胶中胶凝剂分子形成纤维状或片层状的聚集体,羧基(―COOH)和氨基(―NH2)的酸碱作用、酰胺基团间(―CONH―)的氢键作用以及分子间范德华作用力是形成该凝胶的主要驱动力.凝胶中胶凝剂分子自组装形成具有周期性的层状有序结构,层状结构进一步组装形成纤维状聚集体,最终形成三维网状结构阻碍溶剂流动形成凝胶.     

Self-assembled monolayers of novel surface-bound dendrons: peripheral structure determines surface organization

The synthetic and functional versatility of dendrimers and their well-defined shapes make them attractive molecules for surface modification. We synthesized six structurally very similar surface-bound dendrons and used them as building blocks for the preparation of self-assembled monolayers (SAMs) on a gold surface. We studied the effects of the surface-bound dendron's main structure, peripheral substituents, and the coadsorption process on its self-assembling behavior. Using scanning tunneling microscopy (STM), we observed nanostripes for SAMs of the surface-bound dendron consisting of symmetrical benzene rings. When we changed the symmetrical dendron's structure slightly, by increasing or decreasing the numbers of benzene rings at one wedge, we found no ordered structures were formed by the asymmetrical dendrons. We also introduced two kinds of substituents, heptane chains and oligo(ethylene oxide) chains, to the symmetrical dendron's periphery. Heptane chains appear to enhance the interaction between symmetrical backbones, leading to the formation of stripes, while oligo(ethylene oxide) chains appear to weaken the interaction between symmetrical backbones, resulting in a homogeneous structure. Dendrons with both heptane and oligo(ethylene oxide) chains exhibit nanophase separation in a confined state, leading to the formation of a honeycomb structure. Electrochemical studies provide additional evidence for understanding the resulting surface organizations: surface-bound dendrons with symmetrical structures form denser monolayers than their asymmetrical analogues; SAMs comprising peripherally substituted dendrons exhibit blocking effects proportionate to their hydrophilic fraction.     

Side-chain effect on the structural evolution and properties of poly(9,9-dihexylfluorene-alt-2,5-dialkoxybenzene) copolymers

The structural evolution and properties of poly(9,9-dihexylfluorene-alt-2,5-dialkoxybenzene) with different lengths of alkoxy side chains on phenylene have been systematically investigated by means of thermogravimetric analysis (TGA), X-ray diffraction (XRD), differential scanning calorimetry (DSC), polarizing light microscopy (PLM), atomic force microscopy (AFM), and cyclic voltammetry (CV) techniques. The polymer self-organizes into a lamellar structure consisting of both two- and one-layer packing, and the two-layer packing style is the dominant structure. In addition, the two-layer and one-layer packing structures also accompany the presence of planar stacking and/or crystalline and noncrystalline structures, thus maintaining the stability of the packing. PF6OC6 shows three ordered phases (two crystalline phases and one nematic phase) during the heating process. With further increase of the length of alkoxy side chains, only two ordered phases (one crystalline phase and one nematic phase) are observed and the polymers show a melting-recrystallization phenomenon, which is steadily inhibited as the length of the alkoxy side chains increases. The optical and electrochemical properties of the polymers do not exhibit noticeable dependence on the length of the alkoxy side chains. However, the thermal stability, the vibronic structures, and the full width at half-maximum (fwhm) in photoluminescence spectra of the films gradually decrease, and the oxidation onset potentials and the corresponding HOMO energy levels slightly increase with increasing length of alkoxy side chains on phenylene. These results indicate that the length variation of alkoxy side chains does not change the electronic structure of the polymer backbones, but remarkably affects the microphase separation between the flexible side chains and the conjugated backbones.     

Ordered arrays of organometallic iridium complexes with long alkyl chains on graphite

Iridium(III) fac-tris(2-phenylpyridine) fac-[Ir(ppy)3] complexes equipped with long alkyl chains were prepared to examine their capability to form organized arrays on the surface of highly oriented pyrolytic graphite (HOPG). The molecules form lamellar arrays at the 1-phenyloctane/HOPG interface. From the analysis of the STM images, it was concluded that the molecules align with alkyl chains being interdigitated. Similar lamellar arrays were also obtained at the air/HOPG interface upon drop-casting of toluene solutions. The lamellar structure at the molecular level leads to rectangular two-dimensional crystalline domains a few hundred nanometers long (nanoslips). Infrared external reflection spectroscopy suggested that the adsorbed alkyl chains adopt the trans-zigzag conformation in the nanoslip, although the orientations of the zigzag plane of the alkyl groups are mixed. Cyclic voltammetry indicates fast electron transfer between the adsorbed molecules and the substrate and significant intermolecular electronic interactions. It was found that annealing at high temperatures is an effective method to prepare ordered assemblies more than a few micrometer scale (microslips). The orientations of the nanoslips prepared from the racemic mixture exhibited an apparent 12-fold symmetry, while its optically active enantiomer resulted in more irregular domains with a six-fold symmetry, implying an important role of chirality on packing at the molecular level and on the orientation of the domains at larger scales. When drop-cast from more concentrated solutions than a few hundreds of micromolar, multilayers were obtained, in which the alkyl chains in the molecules are more or less perpendicular to the surface. This structure can be transformed into the nanoslips upon standing.     

含酰胺结构超低膨胀聚酰亚胺薄膜的热膨胀行为

采用联苯二酐与3种含酰胺结构二胺制备了具有不同取代基团的聚酰胺-酰亚胺薄膜,考察了酰胺结构对薄膜力学、耐热及尺寸稳定性的影响,研究了聚集态结构与薄膜热膨胀行为的关系和规律.该系列薄膜具有超高强度和优异的耐热性能,拉伸强度高达280. 5 MPa,玻璃化转变温度在389～409℃,并在30～300℃温度范围内表现出超低负膨胀,热膨胀系数(CTE,ppm/℃,即106cm·cm-1·℃-1)在-3. 05～-1. 74ppm/℃之间.聚集态分析结果表明,酰胺结构使分子链间形成了强氢键相互作用,分子链在薄膜面内方向高度有序取向,并在膜厚方向堆积更为紧密,使薄膜表现出热收缩现象.通过不同体积大小的取代基团进一步调控分子链间相互作用及排列堆积,可实现薄膜在高温下近乎零尺寸形变,为设计制备超低膨胀聚合物基板材料提供了新思路.     

Aggregation feature of fluorine-substituted benzene rings and intermolecular C-H.F interaction: crystal structure analyses of mono- and trifluoro-L-phenylalanines

X-Ray crystal structures of four different fluorine-substituted phenylalanines (two mono- and two tri-substitutions) were analyzed to investigate the effect of fluorine atom on the association pattern of benzene rings. Although respective structures showed similar molecular packing in such a way that the layers of hydrophobic benzene rings and hydrophilic amino/carboxyl groups were alternately running along a crystallographic axis, the association patterns of benzene rings were different depending on the substitution position and number of fluorine atoms. The general features could be that the partially displaced face-to-face interactions are increased with increase in the number of fluorine atoms, whereas the edge-to-face interactions are decreased. The C-H bond next to a fluorine-substituted carbon atom could serve as a donor of an intermolecular C-H.F hydrogen bond.     

Tetrahedral zinc complexes with liquid crystalline and luminescent properties: interplay between nonconventional molecular shapes and supramolecular mesomorphic order

Novel metallomesogens with luminescent properties and liquid crystalline behavior at room temperature have been achieved by the preparation of zinc complexes with polycatenar pyrazole and bis(pyrazolyl)methane ligands. Their molecular structures do not have a conventional shape in that they are far from the typical rod-like and flat disc-like geometries of common liquid crystals. They consist of a nonplanar nucleus due to the methylene spacer and/or the coordination to the tetrahedral center, as confirmed by single crystal analysis of the cores. The different numbers and positions of side chains in the pyrazole ligand enabled us to access lamellar and columnar mesophases and, of particular interest, to obtain columnar arrangements at room temperature. Supramolecular models for the organization of the molecules in the mesophases are proposed on the basis of the small-angle XRD diffractograms. The zinc complexes display luminescence in the near UV-blue region with large Stokes shifts. An interplay between non-conventional molecular shapes (due to the tetrahedral core) and the supramolecular mesomorphic order (due to the ligand design) led to materials that interestingly embody two rather opposite properties, a columnar self-organizational ability and luminescence with weak intermolecular interactions.     

Nanostructures of polyelectrolyte gel–surfactant complexes

Small‐angle X‐ray scattering was used to investigate the nanostructures of complexes formed by slightly crosslinked anionic copolymer gels of poly(sodium methacrylate‐co‐N‐isopropylacrylamide) [P(MAA/NIPAM)] with cetyltrimethylammonium bromide (CTAB), and didodecyldimethylammonium bromide (DDAB), respectively, at room temperature (∼ 23°C). Several highly ordered supramolecular structures were observed in the polyelectrolyte gel–surfactant complexes. In P(MAA/NIPAM)–CTA systems, in sequence with decreasing charge density of the P(MAA/NIPAM) copolymer chains, structures of the Pm3n space group cubic, face‐centered cubic close packing of spheres, and hexagonal close packing of spheres were determined at a charge content of ≥ 75, 67, and 50%, respectively. The spheres and rods in these structures were the spherical and cylindrical micelles formed by the self‐assembly of CTA cations with their paraffin chains inside. Both the aggregation number and the size of the micelles decreased with a decreasing charge density of the copolymer chains. In the P(MAA/NIPAM)–DDA systems, the bilayer lamellar structures formed at charge contents ≥ 75% transferred to bicontinuous cubic structures of the Ia3d space group at charge contents of 50–67%. The rods in the Ia3d cubic structures were formed by the self‐assembly of double‐tailed DDA cations with polar moieties inside. The formation of these highly ordered structures were driven by both electrostatic and hydrophobic interactions of the charged copolymer chains/surfactants and the surfactants/surfactants inside the charged gels. The structures became less ordered by further decreasing the charge content of the P(MAA/NIPAM) chains. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2165–2172, 1999