Supramolecular main-chain liquid crystalline polymers and networks with competitive hydrogen bonding: networks and linear polymers created from tris- and bis-functionalised pyridyl networking agents

A series of supramolecular polymers and networks with variable liquid crystalline characteristics have been created. These species are formed through the benzoic acid/pyridine hydrogen bonding of a flexible bis-acid and a mixture of a rigid bis-pyridyl and non-mesogenic tris- and bis-pyridyl molecules. The tris networked systems displayed liquid crystalline characteristics up to and including 31.0% netpoint inclusion. Above this concentration, only crystalline and melting behaviours were observed. The bis-containing polymer system displayed liquid crystalline characteristics up to and including 22.5% inclusion. The phenomenon observed in the tris system would seem to be linked to a statistical correlation of hydrogen bond acceptors and donors. The elimination of liquid crystallinity at lower concentrations of the bis-pyridyl dopant could be attributed to the lower melting nature of that species. Smectic phases were found in both series of complexes in loadings up to 10% of the non-liquid crystalline component. There was also no observed phase segregation of the species after multiple heat/cool cycles and extended periods of time in the isotropic state. This would indicate that the thermodynamically more stable mesogenic phase cannot out-compete the non-liquid crystalline network.     

Polarization charge densities provide a predictive quantification of hydrogen bond energies

A systematic density functional theory based study of hydrogen bond energies of 2465 single hydrogen bonds has been performed. In order to be closer to liquid phase conditions, different from the usual reference state of individual donor and acceptor molecules in vacuum, the reference state of donors and acceptors embedded in a perfect conductor as simulated by the COSMO solvation model has been used for the calculation of the hydrogen bond energies. The relationship between vacuum and conductor reference hydrogen bond energies is investigated and interpreted in the light of different physical contributions, such as electrostatic energy and dispersion. A very good correlation of the DFT/COSMO hydrogen bond energies with conductor polarization charge densities of separated donor and acceptor atoms was found. This provides a method to predict hydrogen bond strength in solution with a root mean square error of 0.36 kcal mol(-1) relative to the quantum chemical dimer calculations. The observed correlation is broadly applicable and allows for a predictive quantification of hydrogen bonding, which can be of great value in many areas of computational, medicinal and physical chemistry.     

Study of the hydrogen bonding of 1,4-bis[(3,4,5-trihexyloxyphenyl)hydrazide]phenylene in crystalline and liquid crystalline phases using infrared,Raman, and two-dimensional correlation spectroscopy

The vibrational bands of a dihydrazide derivative, 1,4-bis[(3,4,5-trihexyloxyphenyl)hydrazide]phenylene (TC6), observed in the Raman and infrared spectra were assigned. The intermolecular hydrogen bonding vibrational bands due to CO and NH groups in the low-frequency Raman spectra were observed at 111 and 94 cm⁻¹ in the crystalline and liquid crystalline (LC) phases, respectively. The sequential order of changes in the hydrogen bonding and alkyl chains was opposite in the crystalline and LC phases. The modifications in the hydrogen bonding occurred prior to conformational changes in the hydrocarbon chains in the crystalline phase; however, a reverse trend was observed in the LC phase. Simultaneously, the two-dimensional (2D) IR and Raman correlation spectroscopic analysis showed that the amide I band of TC6 in the LC phase comprised at least five distinct bands. In addition, the hetero 2D correlation between the NH and CO groups confirmed that no free NH and CO groups existed in the LC phase.     

Supramolecular main-chain liquid crystalline polymers and networks with competitive hydrogen bonding: a study of flexible bis-acids and a mixture of rigid and flexible polypyridyls

A series of novel liquid crystalline supramolecular networks formed through hydrogen bonds have been synthesised. These networks are based on flexible bi-benzoic acid and two types of pyridyl species – a small rigid bipyridyl capable of forming liquid crystalline phases and polyfunctional non-mesogenic pyridyl species that can compete for available hydrogen bond donor molecules. It was found that the networks display monotropic nematic character at high concentrations of disrupting net points – up to 50% for tetrapyridyls, 40% for tripyridyls and 35% for bipyridyls. Weak smectic phases were observed in all systems in loadings up to 10% of each disrupting unit. It is believed that the reversibility of the hydrogen bond allows for the formation of the ordered mesophases in the presence of the non-liquid crystalline groups.     

含氟液晶研究进展

本文简述了含氟液晶的研究进展。根据小分子含氟液晶中氟原子或含氟基团的位置不同,将其分为3类：末端是氟原子或含氟基团的液晶、苯环上氢原子被氟原子取代的液晶、中心桥键上的氢原子被氟原子取代的液晶。根据小分子含氟液晶特点,归纳了氟原子或含氟基团对液晶分子物理性质的影响。同时对高分子含氟液晶的研究进展也做了介绍。     

Intermolecular force constants of hcn in the condensed phase

A simple theory of linear lattice is applied to the hydrogen bonded linear chain system of HCN to calculate the intermolecular force constants at different temperatures in the condensed phase. The strong CN bond is assumed to remain unperturbed in the hydrogen bond formation. The sharp change in intermolecular force constant while passing from the crystalline to the liquid phase is interpreted as a characteristic of this phase transition (fusion).     

Laterally attached liquid crystalline polymers as stationary phases in reversed-phase high-performance liquid chromatography. V. Study of retention mechanism using linear solvation energy relationships

A linear solvation energy relationship model was used to characterize the retention behavior of a stationary phase based upon a nematic side-on liquid crystalline polymer (SOLCP) in reversed-phase liquid chromatography. The set of solutes was constituted of a high variety of compounds whose molecular sizes were considerably smaller than the mesogenic unit size. The results showed good statistical fits for these retention data in 65:35, 75:25 and 85:15 (v/v) methanol-water mobile phases. Both the cavity term and excess molar refraction are the most important favorable retention-governing parameters, whereas the solute hydrogen bond acceptor basicity is the most unfavorable retention parameter. Hydrophobicity and pi-pi interactions decrease strongly when the percentage of methanol increases, leading to an important retention decrease despite the fact that the hydrogen bond interaction weakens as the organic solvent is added. The shape recognition ability of this side-on liquid crystalline stationary phase on polycyclic aromatic hydrocarbon solutes is partly explained by the solutes' high polarizability due to the presence of pi-electrons. However, the solute polarizability is not sufficient and a stationary phase's "structure effect" must to be taken into account for the shape discrimination observed. The strong interaction between liquid crystal molecules caused likely a adsorption retention mechanism rather than a partition mechanism.     

Self-assembly of main chain liquid crystalline polymers via heteromeric hydrogen bonding

Complexation between pyridines and carboxylic acids is driven by hydrogen bonding. This simple, single hydrogen bond is shown to be capable of serving concomitantly as both the agent of liquid crystallinity and as the coupling bond generating an extended linear chain structure. Three such complexes made from an aromatic diacid and three structurally different bis pyridyls were prepared. In each case the association complex self-assembles into an organized liquid crystalline phase. Discussion of this complexation as a step-growth polymerization process is presented along with an examination of the suitability of various methods for characterizing these materials.     

Dissecting Noncovalent Interactions in Carboxyl-Functionalized Ionic Liquids Exhibiting Double and Single Hydrogens Bonds Between Ions of Like Charge

We show that the carboxyl-functionalized ionic liquid 1-(carboxymethyl)pyridinium bis(trifluoromethylsulfonyl)imide [HOOC-CH₂-py][NTf₂] exhibits three types of hydrogen bonding: the expected single hydrogen bonds between cation and anion, and, surprisingly, single and double hydrogen bonds between the cations, despite the repulsive Coulomb forces between the ions of like charge. Combining X-ray crystallography, differential scanning calorimetry, IR spectroscopy, thermodynamic methods and DFT calculations allows the analysis and characterization of all types of hydrogen bonding present in the solid, liquid and gaseous states of the ionic liquid (IL). We find doubly hydrogen bonded cationic dimers (c⁺=c⁺) in the crystalline phase. With increasing temperature, this binding motif opens in the liquid and is replaced by (c⁺−c⁺−a⁻ species, with a remaining single cationic hydrogen bond and an additional hydrogen bond between cation and anion. We provide clear evidence that the IL evaporates as hydrogen-bonded ion pairs (c⁺−a⁻) into the gas phase. The measured transition enthalpies allow the noncovalent interactions to be dissected and the hydrogen bond strength between ions of like charge to be determined.     

Type V deep eutectic solvents: Design and applications

Type V deep eutectic solvents are a novel class of sustainable solvents. They are prepared by physically mixing solid, non-ionic components and are characterized by strong negative deviations from thermodynamic ideality. This work provides guidelines for the rational design of these solvents and reviews some of their recent applications. Emphasis is given on the choice of hydrogen bond donors and acceptors to achieve the necessary liquid phase non-ideality, namely on the use of pairs of molecules with high polarity asymmetry, and stresses the importance of assessing their solid–liquid phase diagrams. Polymorphism and cocrystal formation are also briefly addressed, together with predictive methodologies that have been developed to estimate their properties.     

一种通过分子间氢键自组装的侧链液晶聚合物

一种通过分子间氢键自组装的侧链液晶聚合物邢佩祥，赵英英，汤心颐，姜炳政（吉林大学化学系，长春，１３００２３）（中国科学院长春应用化学研究所高分子物理开放实验室）关键词氢键，侧链液晶聚合物，近晶相，自组装自Ｒｉｎｇｓｄｏｒｆ［１］等提出在介晶基团与主链...     

Hydration of mineral surfaces probed at the molecular level

By employing the nonlinear optical, interface selective experiment of sum frequency spectroscopy together with independent ab initio and density functional theory calculations, we determine the functional species of a corundum (001) surface: doubly coordinated OH groups which differ in their bond tilt angles. The interaction of the functional species with the adjacent water molecules is also observed. In a large pH range around the point of zero charge, the interaction is not controlled electrostatically but by hydrogen bonding. The functional species' tilt angles are crucial parameters, determining whether the species act as hydrogen bond donors or acceptors.     

'Self-organization' processes in proton and deuteron mixtures in open-chain hydrogen bond systems: polarization IR spectra of 4-mercaptopyridine crystals.

This paper is devoted to IR spectroscopic studies in polarized light of 4-mercaptopyridine (4-MPD) hydrogen-bonded crystals. These studies were preceded by determination of the 4-mercaptopyridine X-ray structure. Polarization spectra of 4-mercaptopyridine crystals were measured in the frequency ranges of V(N-H) and V(N-D) bands at room temperature, and also at the temperature of liquid nitrogen, for the two different crystalline faces: 'bc' and 'ac'. When investigating 'residual' v(N-H) band shapes for crystals that were diluted by deuterium, strong dichroic effects in the spectra were still observed, providing evidence for the existence in the hydrogen bond chains of domains containing exclusively protons or deuterons. This phenomenon proves the existence of a new kind of H/D isotopic effects in open chain hydrogen bond systems, namely 'self-organization' effects. Such effects, however, were not observed for other open chain hydrogen bond systems, e.g. alcohol crystals. Solid-state spectra of 4-mercaptopyridine were quantitatively interpreted, along with the strong polarization and the isotopic effects, when based on the 'strong-coupling' theory for linearly arranged hydrogen bond dimers.     

Effect of the site of hydrogen-bonding on the liquid crystalline behaviour of supramolecular complexes

Two series of hydrogen-bonded side-chain liquid crystal polymers have been prepared by mixing components containing carboxyl acid and pyridyl-based fragments. We have focused our attention on the effect that the position of the hydrogen bond donor or acceptor site attached to the side-chain backbone has on the hydrogen-bonding interactions and liquid crystalline phase transitions of the system. The liquid crystalline behaviour of the complexes is studied using Fourier transform infrared spectroscopy, differential scanning calorimetry, polarising optical microscopy and X-ray diffraction. The results indicate that the phase transition temperatures of the complexes are influenced by the site of hydrogen-bonding.     

Raman study of a liquid crystalline gel consisting of photochromic 4‐methyl‐N‐(4‐n‐heptyloxysalicylidene)aniline and a gelling agent (R,R′)‐1,2‐bis(dodecanoylamino)cyclohexane

A liquid crystalline physical gel has been prepared from the mixture of a nematic liquid crystal and a low molecular mass gelling agent containing a hydrogen‐bonding moiety. The newly synthesized liquid crystalline compound exhibited photochromism in the crystalline solid phase. Although photochromism was not observed in the nematic gel state of the mixture, the lifetime of photochromism in the solid phase became longer, compared with that of a single liquid crystalline compound. Some Raman bands of the mixture showed a marked change in both intensity and frequency through the phase transitions. These bands have been assigned to the vibrational modes related to the core part of molecule.     

Supramolecular liquid‐crystalline polyurethane

A novel supramolecular liquid‐crystalline polyurethane was prepared by mixing the polyurethane containing a pendant pyridyl unit as H‐bond acceptor and 4‐dodecyloxybenzoic acid as H‐bond donor. Intermolecular hydrogen bond formation was confirmed by Fourier transform infrared spectroscopy. The liquid‐crystalline behaviour of the complex formed was established by differential scanning calorimetry, polarizing optical microscopy and X‐ray diffractometry. The complex exhibited highly ordered smectic and nematic phases. The smectic‐nematic and nematic‐isotropic transitions were observed at 89°C and 120°C, respectively.     

Thermodynamic anomalies in a lattice model of water

We investigate a lattice-fluid model of water, defined on a three-dimensional body centered cubic lattice. Model molecules possess a tetrahedral symmetry, with four equivalent bonding arms, aiming to mimic the formation of hydrogen bonds. The model is similar to the one proposed by Roberts and Debenedetti [J. Chem. Phys. 105, 658 (1996)], simplified in that no distinction between bond "donors" and "acceptors" is imposed. Bond formation depends both on orientation and local density. In the ground state, we show that two different ordered (ice) phases are allowed. At finite temperature, we analyze homogeneous phases only, working out phase diagram, response functions, the temperature of maximum density locus, and the Kauzmann line. We make use of a generalized first-order approximation on a tetrahedral cluster. In the liquid phase, the model exhibits several anomalous properties observed in real water. In the low temperature region (supercooled liquid), there are evidences of a second critical point and, for some range of parameter values, this scenario is compatible with the existence of a reentrant spinodal.     

Polymorphs and Polymorphic Cocrystals of Temozolomide

Crystal polymorphism in the antitumor drug temozolomide (TMZ), cocrystals of TMZ with 4,4′‐bipyridine‐N,N′‐dioxide (BPNO), and solid‐state stability were studied. Apart from a known X‐ray crystal structure of TMZ (form 1), two new crystalline modifications, forms 2 and 3, were obtained during attempted cocrystallization with carbamazepine and 3‐hydroxypyridine‐N‐oxide. Conformers A and B of the drug molecule are stabilized by intramolecular amide N? H???N_imidazole and N? H???N_tetrazine interactions. The stable conformer A is present in forms 1 and 2, whereas both conformers crystallized in form 3. Preparation of polymorphic cocrystals I and II (TMZ?BPNO 1:0.5 and 2:1) were optimized by using solution crystallization and grinding methods. The metastable nature of polymorph 2 and cocrystal II is ascribed to unused hydrogen‐bond donors/acceptors in the crystal structure. The intramolecularly bonded amide N–H donor in the less stable structure makes additional intermolecular bonds with the tetrazine C?O group and the imidazole N atom in stable polymorph 1 and cocrystal I, respectively. All available hydrogen‐bond donors and acceptors are used to make intermolecular hydrogen bonds in the stable crystalline form. Synthon polymorphism and crystal stability are discussed in terms of hydrogen‐bond reorganization.     

An Experimental Exploration of C−H⋅⋅⋅X Hydrogen Bond in [CHCl3−X(CH3)2] Complexes (X=O,S, and Se)

Among the conglomeration of hydrogen bond donors, the C−H group is prevalent in chemistry and biology. In the present work, CHCl₃ has been selected as the hydrogen bond donor and are X(CH₃)₂ are the hydrogen bond acceptors. Formation of C−H⋅⋅⋅X hydrogen bond under the matrix isolation condition is confirmed by the observation of red-shift in the C−H stretching frequency of CHCl₃ and comparison with the simulated spectra. Stabilisation energy of all the three complexes is almost equal although the observed red-shift for the C−H⋅⋅⋅O complex is less compared to the C−H⋅⋅⋅S/Se complexes. The nature and origin of the hydrogen bond have been delineated using Natural Bond Orbital, Atoms in Molecules, Non-Covalent Interaction analyses, and Energy Decomposition Analysis. Charge transfer is found to be proportional to the observed red-shift. This work provides the first impression of C−H⋅⋅⋅Se hydrogen bond and its comparison with C−H⋅⋅⋅O/S hydrogen bond interaction under experimental condition.     

Achromatic polarization switch using a film-compensated twisted nematic liquid crystal cell

Two liquid crystal materials, 4,4'-bis(ω-hydroxyalkoxy)-α-methylstilbenes, with butyloxy and octyloxy terminal chains and denoted HAMS-4 and HAMS-8, respectively, were synthesized. The corresponding compounds, 4,4'-bis(ω-alkoxy)-α-methylstilbenes (AMS-n) (n = 2 to 7, 9, 10 and 11), which do not hydrogen bond, were also synthesized and used to study the influence of hydrogen bonding on phase behaviour. DSC, polarizing optical microscopy and miscibility studies were used for determination of the structure and property relationships of the various liquid crystalline phases exhibited by the two homologues and the AMS-n homologues. The FTIR spectra were measured at various temperatures on going from the crystalline phase to the isotropic phase through the intermediate phases. The characteristics of the OH stretch band, the CH₂ stretching, bending and rocking progression modes, as well as the ring C C stretch and the ring skeletal vibrations were studied to elucidate the modes and geometry of the hydrogen bonding of the terminal OH group, the conformation of the terminal chains and the intermolecular interaction between the aromatic mesogenic cores in the various phases.