Supramolecular liquid crystals displaying competitive hydrogen bonding: Small molecule mesogens assembled through different hydrogen bond assemblies

A series of supramolecular assemblies created through competitive hydrogen bonding of a n-alkoxy benzoic acid hydrogen bond donor and a mixture of hydrogen bond acceptors: a liquid crystal-forming rigid bi-pyridyl and a non-mesogenic tetrafunctional pyridyl species. The associative chain structures displayed mesogenic characteristics at loadings dependent on the concentration of the disruptive tetrafunctional agent- systems eliminated liquid crystallinity at 85% inclusion in heating transitions and 90% in cooling. The assembled structures displayed considerable mesophase stability. This was observed through long liquid crystalline lifetimes even at high compositions of 4PD, which generally dampened due to a generalized eutectic effect. It is believed that the freedom of mobility inherent in the C8 Acid allows for the reorganization of the hydrogen bonds in the liquid crystalline state providing for a very high loading of disruptor and long mesophase lifetimes.     

Analysis of optical and thermal properties of double hydrogen bonded liquid crystal binary mixtures

Eighteen binary mixtures are prepared from three thermotropic double hydrogen bonded liquid crystal complexes formed by Mesaconic Acid (ME) and various alkyloxy benzoic acids, namely ME + 5BAO, ME + 11BAO and ME + 12BAO where 5BAO, 11BAO and 12BAO represents pentyloxy benzoic acid, undecyloxy benzoic acid and dodecyloxy benzoic acid respectively. Two sets of binary mixtures (ME + 12BAO with ME + 5BAO and ME + 11BAO with ME + 5BAO) are formed by varying their molar ratio in steps of 0.1 to 0.9 which leads to the formation of eighteen binary mixture complexes. Existence of different energy bands in the binary mixtures is identified from the analysis of Fourier Transform Infrared Spectroscopy (FTIR). Phases such as nematic and smectic C are observed in all the binary mixtures which are confirmed through Polarizing Optical Microscopy (POM). Transition temperatures corresponding to the individual mesophases are further verified using Differential Scanning Calorimetry (DSC). Phase diagram of two sets of binary mixtures are constructed with the aid of POM and DSC data. Optical tilt angle measurement; a primary order parameter; is calculated in smectic C phase of the binary mixtures and the resultant data obtained is fitted to critical exponent value is in concurrence with the Mean Field theory predicted value. Thermal studies such as stability factor, order of phase transitions and specific heat capacity value of the mesophases are extracted from the DSC thermograms.     

Design,synthesis and characterization of hydrogen bonded thermotropic liquid crystals

A homologous series of seven linear hydrogen bonded thermotropic liquid crystals are isolated from p-n-octyloxy benzoic acid (8BAO) and p-n alkyl benzoic acids (nBA, where n varies from 2 to 8). In all the seven complexes, hydrogen bonding is a result of complementary proton-donor and electron-acceptor pairs derived from carboxylic acids of 8BAO and nBA. Spectroscopic characterization such as Fourier Transform Infrared spectroscopy (FTIR) and Nuclear Magnetic Resonance (¹HNMR, ¹³CNMR) are performed for all the seven complexes to confirm the intermolecular hydrogen bonding and the nature of chemical environment respectively. Seven mesogen are examined by Polarizing Optical Microscope (POM) for identification of the phase polymorphism through standard textures. Transition temperatures and corresponding enthalpy values of the individual mesogenic phases are experimentally obtained from the analysis of the respective Differential Scanning Calorimetry (DSC) thermograms. Phase diagram of this homologous series is constructed based on the data of transition temperatures, further qualitative analysis of the phase abundance is discussed. Thermal stability factor for each of the mesogenic phase has been determined. Order of phase transition is experimentally obtained by Navard and Cox technique. Specific heat for each phase transition of individual complexes is derived from the DSC data. Change of color of the texture with temperature in the same phase is referred as parachromatism and is observed in nematic phase of two mesogens.     

Influence of flexible chain,polar substitution and hydrogen bonding on phase stability in Shiff based (4)PyBD(4I)BrA-nOBA series of liquid crystals

Shiff based Linear Hydrogen Bonded Liquid Crystalline homologues, viz., (4)PyBD(4^I)BrA-nOBAs for n = 2,3,4,5,6,7,8,10,11 and 12 are synthesized. ¹H-NMR and ¹³C-NMR studies confirm the formation of HBLC and IR confirms the linear hydrogen bonding. Liquid crystal phase abundance, transition temperatures, enthalpy and order of transition are investigated by Polarized Optical Microscopy textural studies and Differential Scanning Calorimetry. Nematic, Smectic-A, -D, -C, B_hexatic, -B_Crystal, -F, -I and –G phases are identified. Large LC phase variances, viz., NAB_hexFG and NACFG are exhibited by intermediate chain length with n = 5 and 6. A-C transition exhibited unique II-order nature. II-order nature for G-Solid transition explained. Phase diagram drawn with flexible chain length infers INA, and ACF and CIF MultiCritical Points and Lifshitz behaviour. NA TriCiritical Point predicted in [3+7] binary. Influences of chain length, linear HB and end polar(-Br) substitution for LC phase stability are is discussed. Results of Phase stability are discussed in the wake of the body of the data and reports in other LCs. DSC enthalpy suggests for utility of intermediate and higher homologues for device savvy tilted Smectic LC phases.     

Design,synthesis and characterization of hydrogen bonded liquid crystals formed between methyl malonic acid and p-n-alkyloxy/alkyl benzoic acids

Two novel homologous series of hydrogen bonded liquid crystals (HBLC) comprising of 14 mesogens are formed between Methyl Malonic acid (MM) and p-n-alkyloxy/alkyl benzoic acids (nBAO/nBA). FTIR study confirms the formation of hydrogen bond. Phases exhibited by these mesogens are characterized by optical textural studies and Differential Scanning Calorimetry (DSC) studies. Phase diagram is constructed from these studies. Other thermal parameters such as order of the transition, thermal stability factor and specific heat are also elucidated. Optical tilt angle measurement and optical filtering action are also carried out. The influence of oxygen atom is also discussed.     

Synthesis of liquid crystals based on hydrogen-bonding of 4-(Octyloxy)benzoic acid with 4-alkylbenzoic acids

A molecular recognition process has been used to form new mesogenic molecular structures, where intermolecular hydrogen bonding occurs between 4-(octyloxy)benzoic acid (8BAO) and four 4-alkylbenzoic acids (nBAs, n = 2, 5, 6, 7). The synthesis of these complexes has been attained by resorting to mechanochemistry. The resulting materials have been characterized by polarizing optical thermal microscopy, differential scanning calorimetry, vibrational spectroscopy, X-ray powder diffraction, and ¹H NMR relaxometry. All the elements of the series show the formation of a mesophase. For one of the complexes, its electro-optical properties have also been assessed, resulting comparable to those of other widely used liquid crystals.     

Thermal and optical studies on induced smectic phases of inter molecular hydrogen bonded liquid crystals between decyloxy benzoic acid and citric acid

Hydrogen bonded liquid crystal complex (HBLC) is prepared from mesogenic 4-decyloxy benzoic acid (10OBA) and aliphatic nonmesogenic citric acid (CA). Liquid crystal (LC) phases are investigated by polarizing optical microscopy (POM), differential scanning calorimetry (DSC) studies. Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectroscopy (NMR) validate the intermolecular complementary, cyclic type of hydrogen bond (HB) and molecular environment in the designed HBLC complex. Mesomorphic phases like nematic, smectic X (Sm X) and smectic G (Sm G) are characterized by various textures and using different techniques such as POM, DSC and optical tilt angle measurements. Thermal span width and thermal stability factor for the observed phase is calculated. The complexes are prepared in different mole ratio and their corresponding influences on the phase transitions are discussed. Also it is observed that the HB units play a vital role in stabilizing the new Sm X phase. The variation in thermal stability of smectic phases due to the influence of aromatic cores and length of end chain in the different mole ratio of the present HBLC complexes are also discussed. The variation of tilt angle with respect to temperature in the smectic phase has been experimentally calculated and analyzed. The lowered melting and clearing transition temperatures and extended thermal span width in the Sm X phase are also reported.     

The role of hydrogen bonding in pseudo-macrocyclic ring formation in 2,6-dimethyl-1,3,5,7-cyclooctatetraene 1,3,5,7-tetracarboxylic acid monohydrate

2,6-Dimethyl-1,3,5,7-cyclooctatetraene-1,3,5,7-tetracarboxylic acid 3, C₁₄H₁₄O₉, was prepared by thermolysis of the corresponding semibullvalene and characterized by spectroscopic and single-crystal X-ray diffraction studies. The monohydrate of 3 crystallizes in the tetrahedral space group P-4n2, a = 11.0924(3) Å, c = 12.6799(5) Å, V = 1560.15(9) ų, Z = 4. The cyclooctatetraene ring adopts a tub-shaped conformation with a crystallograpically imposed twofold rotational symmetry, and is composed of localized C=C double bonds in the 1,3,5, and 7 positions with an average interatomic distance of 1.327 (5) Å and C–C single bonds with an average interatomic distance of 1.489(5) Å. The average C=C–C angle in the ring is 122.6(3)°. Each symmetry generated eight-membered ring contains four carboxyl groups, two of which are coplanar with the methyl groups across the C=C ring atoms. However, across the C–C bonds the carboxyl groups and the methyl groups show a torsion angle of 64.3(4)°. The presence of a water molecule in the crystal lattice generates a three-dimensional network of close hydrogen bondings between water and the carboxyl groups of multiple rings. This creates a network of orthogonal 10-membered rings between the 8-membered rings. Two given cyclooctatetraene rings are intermolecularly hydrogen bonded not only directly through their carboxyl groups but also via a bridging water molecule. This effect is rare in polycarboxylic acids and their monohydrates which bond only with water or among themselves.     

Structural characterization of oxidized lumiflavin hydrochloride hydrate: Comparison with the hydrogen bonding at the cofactor of flavoproteins

In order to examine the possibility of hydrogen bonding around the flavin ring, an X-ray diffraction study of the title compound was undertaken. Crystals of lumiflavin hydrochloride hydrate [2(C₁₃H₁₂N₄O₂) 4 HCl 7 H₂O] are triclinic, space group ,Z=2,a=11.064(1),b=17.903(1),c=9.891(1)Å, =111.9(6), -96.4(8), =91.6(7)°. We present the crystal structure of this compound and compare it with other flavin derivatives and with the structure of the active site of some flavoproteins.     

Intra- vs intermolecular hydrogen bonding. The crystal structure of 5-methyl-2-hydroxyacetophenone thiosemicarbazone monohydrate and salicylaldehyde-2-methylthiosemicarbazone monohydrate

The crystal structure of 5-methyl-acetophenonethiosemicarbazone monohydrate,A, and salicylaldehyde-2-methylthiosemicarbazone monohydrate,B, were determined using single crystal X-ray diffraction.A crystallizes in the monoclinic space groupC2/c, with lattice parametersa=14.161(2),b=15.753(1) ?,c=11.084(1) ?, β=112.59(1)° andZ=4, yielding a calculated density ofD _calc=1.352 mg/m³.B crystallizes in the triclinic space groupP1, witha=7.233(2) ?,b=7.371(2) ?,c=11.841(2) ?, α=82.77(2)°, β=78.33(2)°, γ=63.06(2)° andD _calc=1.371 mg/m³ forZ=2,. In bothA andB the immine nitrogen and the sulfur atom areanti with respect to N2-C8. WhileA presents the usual intramolecular six membered hydrogen bond ring,B has instead an intermolecular hydrogen bond between the hydroxy moiety of the salicyladehyde and a water molecule. AM1 calculations agree with the experimental conformations observed in both compounds. Contribution No. 1619 of the Instituto de Química, UNAM.     

Influence of lithium ions on the NLO properties of KDP single crystals

Potassium Dihydrogen Phosphate (KDP) and its isomorphous deuterated form are popular due to their applications in frequency converters and electro‐optic modulation. Different attempts have been made to dope KDP with inorganic additives, organic materials and amino acids. Since many of the metal ions possess more electro negativity which increases the non centrosymmetry, it is of interest to dope them in KDP. The influence of lithium ion (Li⁺) on NLO properties of KDP crystal has been studied in the present investigation. Single crystal of Lithium ion doped Potassium Dihydrogen Phosphate (KDP) was grown by slow evaporation technique. The enhancement in SHG efficiency after addition of ion Lithium (Li⁺) was observed by Kurtz Powder SHG test. It was found that the SHG efficiency of KDP after addition of Lithium ion is 1.33 times more than pure KDP. The crystal structure and cell parameters of grown crystal were determined by X‐ray diffraction. The Energy Dispersive X‐ray analysis (EDAX) gives the chemical composition of grown crystal. The functional groups were identified by FT‐IR spectral analysis. The presence of Lithium in the material of grown crystal was detected by Atomic absorption spectroscopy (AAS). The optical absorption and transmission studies were done by UV‐Visible spectral analysis. The grown crystal was subjected to thermogravimetric analysis (TGA) and Differential Scanning Calorimetry (DSC). (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of oxygen on electrical properties of CaF2 crystals

The O²⁻ ions appear as an unwanted impurity that contaminates the crystals of alkaline and rare earth fluorides during their preparation. The easy incorporation into the lattice and difficulties with their detection make the measurement of electrical properties problematical. In order to elucidate the influences of oxygen on CaF₂ crystals we have measured electrical conductivity. The crystals were prepared from a raw material of various moisture content, or they were doped by CaO, respectively. The measured courses exhibit equal activation energy E = 0.64 eV in the middle range. We thus conclude that synthetizing the crystal contaminated by moisture yields incorporation of O²⁻ ions only. We proved that even a two hours delay of melted raw material in gaseous products of Teflon pyrolysis does not suffice to remove oxygen. The method is effective only when applied after drying of raw material in vacuum.     

Influence of growth conditions on microstructure and properties of GaSe crystals

GaSe crystals have been grown from melt. There are several reasons why it is difficult to meet ideal demands for nonlinear optic material, GaSe single crystal. First, these crystals have a tendency towards lamination because of great difference in a and c crystal lattice parameters and very weak Vander der Waals forces in c direction. Next, there is a great difference in saturation vapor pressure of the components, which can cause nonstoichiometry of a melt-grown crystal composition. Another obstacle in the growth of perfect GaSe crystals is dendrite formation caused by instability of the growth front. To overcome this obstacle we used Bridgman technique and have found the temperature and pressure conditions, and growth velocity which provide growth of perfect bulk single crystals of about 100 mm in length and 20 mm in diameter. Sharp Laue patterns and a rocking curve confirm perfect structure of the grown crystals. Electron-probe X-ray microanalysis shows stoichiometric composition of GaSe crystals and X-ray phase analysis reveals presence of single-phased hexagonal structure.     

Influence of X-ray irradiation on the optical properties of chromium-doped KliSO4 crystals

The absorption and circular-dichroism spectra of chromium-activated KLiSO₄ crystals both nonirradiated and irradiated with an X-ray beam have been studied. It was established that in nonirradiated crystals chromium ions are mainly trivalent (Cr³⁺) and have octahedral coordination. In irradiated crystals, along with the centers provided by (Cr³⁺) ions, new centers are formed associated with (^Cr4+) and (^Cr5+) ions.     

Influence of dopants and preparation method on the thermoluminescence properties of corundum crystals

Thermoluminescence (TL) of several types of corundum crystals is described. Undoped crystals showed a weak TL at 15 and 110°C with emission at 330 and 495 nm. Good TL response at 275°C is shown by Cr doped (0.03 wt%) crystals grown by Verneuil method in the oxygen cone of the flame annealed in hydrogen at 1500–1800°C. The most intensive TL response is shown by crystals doped with Ti and Mo (∼ 10⁻³wt.%) grown by Czochralski method in reducing conditions. The former may be used to the detection of γ-rays dose ≥ 10⁻³ − 10⁻² Gy, the latter ≥ 10⁻⁷ − 10⁻⁶ Gy.     

Influence of the conditions of heat treatment on the mechanical properties of MgO crystals

The effect of cooling conditions after annealing at 1000°C on the mechanical properties of MgO single crystals is studied.     

N‐H⋅⋅⋅O,O‐H⋅⋅⋅O hydrogen bonded supramolecular formation in the cocrystal of salicylic acid with N‐containing bases

Cocrystals of salicylic acid (derived from reaction between aspirin and coformers) with 4,4′dipyridyl, nicotinamide, isonicotinamide, N,N′‐diacetylpiperazine and piperazine have been examined with intent to improve physicochemical properties of antipyretic agent. All of the resulting cocrystals, salicylic acid/4,4′dipyridyl (2:1), salicylic acid/nicotinamide (1:1), salicylic acid/isonicotinamide (1:1), salicylic acid/piperazine (1:0.5) and salicylic acid/ N,N′‐diacetylpiperazine (2:1) (derived from reaction between aspirin and piperazine) are obtained solution cooling/evaporation experiments. The structural analysis has shown that the well‐known COOH ⋅⋅⋅ N heterosynthon was considered the key element in the cocrystals design strategy. The carboxylic acid ⋅⋅⋅ pyridine hydrogen bond is an often used supramolecular synthon. The results from X‐ray Powder Diffraction, DSC, Raman and single‐crystal X‐ray analysis revealed the formation of cocrystal of salicylic acid with several coformers.     

Influence of the nonuniform distribution of electronic defects on the physical properties of garnet crystals

The optical and kinetic properties of Y₃Fe₅O₁₂ garnet single crystals have been investigated. A method for determining the conduction electron concentration in the samples using energy-dispersive X-ray fluorescence analysis has been proposed, which makes it possible to estimate the quality of obtained material based on a unified approach.     

Influence of diffraction in crystals on the coherence properties of X-ray free-electron laser pulses

The spatial and temporal evolution of the field of random X-ray femtosecond pulses and their coherent properties upon pulse propagation in free space and under dynamical diffraction in perfect crystals in the Bragg and Laue geometries has been analyzed on the basis of the formalism developed in statistical optics. Particular attention is paid to the influence of large pulse propagation distances, which are characteristic of lengthy channels of X-ray free-electron lasers.