Enantioselective noncovalent synthesis of hydrogen-bonded double-rosette assemblies

The noncovalent synthesis of enantiomerically pure hydrogen-bonded assemblies (M)- and (P)-1(3).(CA)(6) is described. These dynamic assemblies are of one single handedness (M or P), but do not contain any chiral components. They are prepared by using the "chiral memory" concept: the induction of supramolecular chirality is achieved through initial assembly with chiral barbiturates, which are subsequently replaced by achiral cyanurates. This exchange process occurs quantitatively and without loss of the M or P handedness of the assemblies. Racemization studies have been used to determine an activation energy for racemization of 105.9+/-6.4 kJ mol(-1) and a half-life time to racemization of 4.5 days in benzene at 18 degrees C. Kinetic studies have provided strong evidence that the rate-determining step in the racemization process is the dissociation of the first dimelamine component 1 from the assembly 1(3).(CA)(6). In addition to this, it was found that the expelled chiral barbiturate (RBAR or SBAR) acts as a catalyst in the racemization process. Blocking the dissociation process of dimelamines 1 from assembly 1(3).(CA)(6) by covalent capture through a ring-closing metathesis (RCM) reaction produces an increase of more than two orders of magnitude in the half-life time to racemization.     

Diastereoselective noncovalent synthesis of hydrogen-bonded double-rosette assemblies

Chiral centers present either in the dimelamine components of calix[4]arene 1 or in the cyanurate components CA quantitatively induce one handedness (P or M) in the corresponding hydrogen-bonded assemblies 1(3).(CA)(6) (de>98 %). The high degree of chiral induction results from the presence of six chiral centers in close proximity (C(alpha)) to the core of the assembly. A much lower level of chiral induction is observed for assemblies with chiral centers that are more remote (C(beta)). All diastereomerically pure assemblies 1(3).(CA)(6) exhibit very high CD activities (deltavarepsilon(max) approximately 100 L mol(-1) cm(-1)), in sharp contrast to the low CD activities (deltavarepsilon(max)相似文献   

Self-organization of gold-containing hydrogen-bonded rosette assemblies on graphite surface

The self-organization of supramolecular structures, in particular gold-containing hydrogen-bonded rosettes, on highly oriented pyrolytic graphite (HOPG) surfaces was investigated by tapping-mode atomic force microscopy (TM-AFM) and scanning tunneling microscopy (STM). TM-AFM and high-resolution STM results show that these hydrogen-bonded assemblies self-organize to form highly ordered domains on HOPG surfaces. We find that a subtle change in one of the building blocks induces two different orientations of the assembly with respect to the surface. These results provide information on the control over the construction of supramolecular nanoarchitectures in 2D with the potential for the manufacturing of functional materials based on structural manipulation of molecular components.     

Rational modulation of the periodicity in linear hydrogen-bonded assemblies of trimesic acid on surfaces

We demonstrate a surprising cooperative adsorption process at the liquid-solid interface, involving self-assembly in which a three-fold hydrogen-bonding unit (trimesic acid, TMA) is forced into a linear pattern by noncovalent interaction with an alcohol. Our work shows that the unexpected linear pattern formed by coadsorption of TMA and alcohols can be modulated in size by choosing alcohols with different chain lengths.     

A novel type of hydrogen-bonded assemblies based on the melamine.cyanuric acid motif

This paper reports the formation of novel hydrogen-bonded assemblies 1(3).CA obtained upon mixing cyanuric acid (CA) with melamine derivatives 1, in which two of the three possible H-bonding arrays have been blocked. The four components are held together by 9 hydrogen bonds and form a rigid planar structure in which a central CA (three ADA motifs: A = acceptor, D = donor) is hydrogen bonded to three peripheral melamine derivatives (DAD motif). Furthermore, the synthesis and assembly studies are described of hydrogen-bonded assemblies 2-4.CA, comprised of three melamine derivatives that are covalently connected, and CA. The overall thermodynamic stability of assemblies 2-4.CA is superior to 1(3).CA (I(Tm) = 9 vs 3.6). The presence of the 2.CA complex in chloroform was confirmed by (1)H NMR spectroscopy and MALDI-TOF mass spectrometry. Substitution of the trimelamines with chiral or fluorescent groups (R(3)) enabled the study of the assemblies by CD and fluorescence spectroscopy. Titration experiments revealed strongly enhanced stabilities even in the presence of polar solvents, such as THF and CH(3)OH. Depending on the polarity of the solvent, stacking between the planar assembly units was observed.     

Kinetic stabilities of double,tetra-, and hexarosette hydrogen-bonded assemblies

A study of the kinetic stabilities of hydrogen-bonded double, tetra-, and hexarosette assemblies, comprising 36, 72, and 108 hydrogen bonds, respectively, is described. The kinetic stabilities are measured using both chiral amplification and racemization experiments. The chiral amplification studies show that solvent polarity and temperature strongly affect the kinetic stabilities of these hydrogen-bonded assemblies. For example, the activation energy for the dissociation of a tetramelamine from a tetrarosette assembly, a process that involves the breakage of 24 hydrogen bonds, was determined at 98.7 +/- 16.6 kJ mol(-1) in chloroform and 172.8 +/- 11.3 kJ mol(-1) in benzene. Moreover, racemization studies with enantiomerically enriched assemblies reveal a strong dependence of the kinetic stability on the number and strength of the hydrogen bonds involved in assembly formation. The half-lives for double, tetra-, and hexarosette assemblies were found to be 8.4 min, 5.5 h, and 150 h in chloroform at 50 degrees C, respectively. For higher generations of these types of assemblies, the kinetic stabilities become so high that they can no longer measured in a direct manner.     

The influence of ethylene glycol chains on the thermodynamics of hydrogen-bonded supramolecular assemblies in apolar solvents

Substitution of hydrogen bond directed supramolecular assemblies with ethylene glycol chains leads to a reduction in the association constant in apolar solvents, where the reduction of the association constant is dependent on the length of the aliphatic spacer connecting the hydrogen bonds and the ethylene glycol chain.     

Synthesis of metal-hydrazone complexes and vapochromic behavior of their hydrogen-bonded proton-transfer assemblies

We synthesized and investigated a new series of metal-hydrazone complexes, including deprotonated [MX(mtbhp)] and protonated forms [MX(Hmtbhp)](ClO(4)) (M = Pd(2+), Pt(2+); X = Cl(-), Br(-); Hmtbhp = 2-(2-(2-(methylthio)benzylidene)hydrazinyl)pyridine) and hydrogen-bonded proton-transfer (HBPT) assemblies containing [PdBr(mtbhp)] and bromanilic acid (H(2)BA). The mtbhp hydrazone ligand acts as a tridentate SNN ligand and provides a high proton affinity. UV-vis spectroscopy revealed that these metal-hydrazone complexes follow a reversible protonation-deprotonation reaction ([MX(mtbhp)] + H(+) ? [MX(Hmtbhp)](+)), resulting in a remarkable color change from red to yellow. Reactions between proton acceptor [PdBr(mtbhp)] (A) and proton donor H(2)BA (D) afforded four types of HBPT assemblies with different D/A ratios: for D/A = 1:1, {[PdBr(Hmtbhp)](HBA)·Acetone} and {[PdBr(Hmtbhp)](HBA)·2(1,4-dioxane)}; for D/A = 1:2, [PdBr(Hmtbhp)](2)(BA); and for D/A = 3:2, {[PdBr(Hmtbhp)](2)(HBA)(2)(H(2)BA)·2Acetonitrile}. The proton donor gave at least one proton to the acceptor to form the hydrogen bonded A···D pair of [PdBr(Hmtbhp)](+)···HBA(-). The strength of the hydrogen bond in the pair depends on the kind of molecule bound to the free monoanionic bromanilate OH group. Low-temperature IR spectra (T < 150 K) showed that the hydrogen bond distance between [PdBr(Hmtbhp)](+) and bromanilate was short enough (ca. 2.58 ?) to induce proton migration in the [PdBr(Hmtbhp)](2)(BA) assembly in the solid state. The hydrogen bonds formed not only between [PdBr(Hmtbhp)](+) and HBA(-) but also between HBA(-) and neutral H(2)BA molecules in the {[PdBr(Hmtbhp)](2)(HBA)(2)(H(2)BA)·2Acetonitrile} assembly. The H(2)BA-based flexible hydrogen bond network and strong acidic host structure result in an interesting vapor adsorption ability and vapochromic behavior in this assembly because the vapor-induced rearrangement of the hydrogen bond network, accompanied by changes in π-π stacking interactions, provides a recognition ability of proton donating and accepting properties of the vapor molecule.     

Effect of the layer-by-layer (LbL) deposition method on the surface morphology and wetting behavior of hydrophobically modified PEO and PAA LbL films

We demonstrate that the surface morphology and surface-wetting behavior of layer-by-layer (LbL) films can be controlled using different deposition methods. Multilayer films based upon hydrogen-bonding interactions between hydrophobically modified poly(ethylene oxide) (HM-PEO) and poly(acrylic acid) (PAA) have been prepared using the dip- and spin-assisted LbL methods. A three-dimensional surface structure in the dip-assisted multilayer films appeared above a critical number of layer pairs owing to the formation of micelles of HM-PEO in its aqueous dipping solution. In the case of spin-assisted HM-PEO/PAA multilayer films, no such surface morphology development was observed, regardless of the layer pair number, owing to the limited rearrangement and aggregation of HM-PEO micelles during spin deposition. The contrasting surface morphologies of the dip- and spin-assisted LbL films have a remarkable effect on the wetting behavior of water droplets. The water contact angle of the dip-assisted HM-PEO/PAA LbL films reaches a maximum at an intermediate layer pair number, coinciding with the critical number of layer pairs for surface morphology development, and then decreases rapidly as the surface structure is evolved and amplified. In contrast, spin-assisted HM-PEO/PAA LbL films yield a nearly constant water contact angle due to the surface chemical composition and roughness that is uniform independent of layer pair number. We also demonstrate that the multilayer samples prepared using both the dip- and spin-assisted LbL methods were easily peeled away from any type of substrate to yield free-standing films; spin-assisted LbL films appeared transparent, while dip-assisted LbL films were translucent.     

Preparation and thermal properties of polyquinazolones: Effect of the chemical structure of bisbenzoxazinones on the thermal properties

Four kinds of polyquinazolones were synthesized from 4,4′-diaminodiphenylether and bisbenzoxazinones having different central aromatic nuclei (benzene, biphenyl, benzophenone, diphenylmethane). All the polymers were prepared by solution polycondensation at 190°C at a concentration of 15% solid in m-cresol, and were obtained with intrinsic viscosities in the range of 0.3–0.8 dL/g in 97–98% yield. Their thermal properties were studied by using dynamic thermogravimetry and isothermal weight loss. All the polymers were found to produce high char yields (55–75%) in nitrogen. Oxidative thermal stability of the polymers was dependent on the chemical structure of bisbenzoxazinone and was governed by the stability of the bonds in the quinazolone units. The following order of oxidative thermal stability of the quinazolone units was determined from this study: diphenylmethane < benzophenone < biphenyl < benzene.     

Resorcarenes in the boat conformation as building blocks for hydrogen-bonded assemblies including two ammonium cations

Crystal structures are reported for various co-crystals of rccc-resorcarenes with triethylammonium chloride. Usually, two molecules of a C2v-symmetric tetraester 2 in the boat conformation are linked through four hydrogen-bonded chloride anions to give dimeric assemblies. Two of the chloride anions may be replaced by four hydrogen-bonded ethanol molecules in an otherwise similar structure. These assemblies, which consist of six or eight components, posses voluminous, negatively charged chambers in which two triethylammionium cations, 3+, are included as guests by strong electrostatic and hydrogen-bonding interactions. The host-guest N-H...Cl hydrogen bonds were clearly detected at 173 K. These are the first examples of hydrogen-bonded, solid-state capsules trapping two ions of the same charge in close proximity. In the 1:2 complex with 3+ Cl-, the molecule of the parent resorcarene 1 also adopts a boat conformation whose cavity is considerably extended by four hydrogen-bonded chloride anions. The pocket formed in this way again includes two 3+ ions as a result of electrostatic and hydrogen bonding host-guest interactions. All these structures show that the boat conformers of resorcarenes can be used as a novel motif for the construction of hydrogen-bonded assemblies capable of molecular inclusion and encapsulation.     

Synthesis and characterization of hydrogen-bonded assemblies of W6S8L6 clusters

Ligand-exchange reactions involving octahedral W6S8 clusters and a family of pyridine-based ligands (isonicotinic acid, isonicotinamide, 4-hydroxypyridine, 4-aminopyridine, 4-pyridineacetamide) have been explored with the goal of preparing compounds that crystallize in hydrogen-bonded arrays. Two new compounds, W6S8(4-pyridineacetamide)6.DMF.4-pyridineacetamide (1) and W6S8(4-aminopyridine)6.4DMF (2), were isolated and characterized by single-crystal X-ray diffraction. Both compounds crystallize in the P2(1)/c space group with a = 16.461(1), b = 33.08(2), c = 13.165(10) A, beta = 103.270(15) degrees for 1 and a = 13.8988(5), b = 13.2791(5), c = 15.6293(6) A, beta = 108.5410(10) degrees for 2. Each compound was further characterized by 1H NMR spectroscopy, elemental (CHN) analysis, and thermogravimetric analysis. Examination of the structures shows that 1 forms a three-dimensional hydrogen-bonded network in which each 4-pyridineacetamide ligand interacts with ligands on neighboring clusters or with the free ligand of crystallization. This is the first hydrogen-bonded network formed from W6S8 clusters. In 2, the amino groups act as hydrogen-bond donors toward DMF molecules of crystallization, but an extended array is not formed. In addition, the binding strengths of these pyridine-based ligands to the W6S8 cluster were studied through quantitative 1H NMR studies of ligand-exchange reactions. A qualitative relationship was found between ligand binding strengths and Hammett substituent constants for this group of ligands.     

Amplification of chirality: the "sergeants and soldiers" principle applied to dynamic hydrogen-bonded assemblies

The amplification of supramolecular chirality has been studied in dynamic chiral hydrogen-bonded assemblies 1(3).(CA)(6) using "Sergeants and Soldiers" experiments. Previously, we have shown that chiral centers present in either the dimelamine component 1 or the cyanurate component CA quantitatively induce one handedness (M or P) in the assembly. This offers the possibility to study the amplification of chirality under two different kinetic regimes. When chiral dimelamines 1 are used, the exchange of chiral components and (M/P)-interconversion, i.e., interconversion between the (M)- and (P)-isomers of assembly 1(3).(CA)(6), take place via identical pathways (condition A). When chiral cyanurates CA are used, the exchange of chiral components occurs much faster than (M/P)-interconversion (condition B). Experimentally, a much stronger chiral amplification is observed under condition B. For example, the observed chiral amplification for a mixture of chiral and achiral components (40:60) is 46% under condition B and 32% under condition A. Kinetic models were developed to fit the experimental data and to simulate chiral amplification in dynamic systems in general. These simulations show that it is theoretically possible that the diastereomeric excess in a dynamic system is more than 99% with less than 1% chiral component present!     

Synthesis and energy-transfer properties of hydrogen-bonded oligofluorenes

A set of fluorene oligomers has been synthesized by stepwise palladium-catalyzed (Suzuki) couplings of fluorene monomers. Ureidopyrimidinones (UPy), functional groups that can dimerize via quadruple hydrogen bonds, were attached to both ends of the oligofluorenes. The resulting bis-UPy-terminated oligomers self-assemble into supramolecular chain polymers. For comparison, oligofluorenes of the same oligomer lengths but without terminal hydrogen-bonding groups were synthesized. Chains of hydrogen-bonded fluorenes can be simply endcapped by a variety of chain stoppers, molecules that have one UPy group. In this manner, we have endcapped the hydrogen-bonded fluorene chains with either oligo(p-phenylenevinylene) or perylene bisimide. Energy-transfer experiments in solution and the solid state demonstrate that oligofluorenes can donate energy to a variety of energy acceptors, but that this energy transfer occurs most effectively when the donor fluorene is hydrogen-bonded to the acceptor.     

Effect of magnetic nanoparticles on the thermal properties of some hydrogels

Magnetic hydrogels (ferrogels) based on poly(vinyl alcohol) (PVA) and poly(hydroxyethyl methacrylate) (PHEMA) with magnetite (Fe₃O₄) nanoparticles were prepared. PVA ferrogels were synthesised by submitting the aqueous solution of polymer and Fe₃O₄ to freezing-thawing (F-T) cycles yielding a physical gel. Different samples were prepared by varying (i) the concentration of PVA, (ii) the concentration of magnetite and (iii) the number of F-T cycles applied. PHEMA ferrogels were prepared by a crosslinking polymerization reaction in the presence of magnetite yielding chemical gels. Different samples were prepared by varying (i) the concentration of HEMA, (ii) the concentration of EGDMA and (iii) the concentration of magnetite nanoparticles. All ferrogel samples were first dried before been analysed in a thermogravimetric analyzer. The resulting thermograms showed that the concentration of magnetite nanoparticles does affect the thermal stability of either ferrogels system, a general improvement in comparison with PVA and PHEMA hydrogels, respectively, being observed. The apparent activation energy (E_a) of the thermal degradation for PVA ferrogels was evaluated and calculated applying the Flynn-Wall and the Kissinger methods. Values of apparent E_a increased with the content of Fe₃O₄ in the ferrogel sample.     

Effect of thermal treatment on the micromechanical properties of segmented polyurethanes

The effects of thermal treatments on segmented polyurethanes were studied by low frequency mechanical spectrometry. Complementary informations on the morphology of microphases and characteristic temperatures were given by other techniques such as DSC and SAXS. We have shown that low temperature treatments affected the morphology of the soft phase. High temperature treatments induced modification of dynamic modulus spectra attributed to the evolution of the microphase separation rate and the degree of crystallinity of some hard domains.     

Effect of glass cullet on thermal properties of kaolinite

Waste compromises environmental preservation as well human health in many countries. Recycling is an alternative that sometimes represents the only economical activity for a significant population in the big cities. Almost 3% of waste materials in Brazil are vitreous. Ceramic production adding waste glass is possible with advantages of costs reduction associated to decrease on firing temperatures and to the raw material itself. At present paper up to 80 mass% of waste glass was added to clay. The sintering temperature decreased linearly and the shrinkage increased with glass content, an effect more pronounced for high glass amount.     

Effect of thermal treatment on interfacial properties of beta-lactoglobulin

The changes in the secondary conformation and surface hydrophobicity of beta-lactoglobulin subjected to different thermal treatments were characterized at pH values of 7, 5.5 and 4 using circular dichroism (CD) and hydrophobic dye binding. Heating resulted in a decrease in alpha-helix content with a corresponding increase in random coil at all pH values, this change being more pronounced for small heating times. Heating also resulted in an increase in surface hydrophobicity as a result of partial denaturation, this increase being more pronounced at pH 4. Thermal treatment resulted in a shift of the spread monolayer isotherm at air-water interface to smaller area per molecule due to increased flexibility and more loop formation. Thermal treatment led to an increase in interfacial shear elasticity and viscosity of adsorbed beta-lactoglobulin layer at pH 5.5 and 7. Interfacial shear elasticity, shear viscosity, stability of beta-lactoglobulin stabilized emulsion and average coalescence time of a single droplet at a planar oil-water interface with adsorbed protein layer exhibited a maximum for protein subjected to 15 min heat treatment at pH 7. At pH 5.5, the interfacial shear rheological properties and average single drop coalescence time were maximum for 15 min heat treatment whereas emulsion stability was maximum for 5 min heat treatment. At pH 7, thermal treatment was found to enhance foam stability. Analysis of thin film drainage indicated that interfacial shear rheological properties do not influence thin film drainage.