High-temperature Mesophase Research Articles

On the Multi-Layered Adsorption of Alcanols in the Vicinity of Liquid–Vapor Transition at the Saturated Hydrocarbon–Water Interface

Structure of adsorption layer of long-chain monoatomic alcohols: 1-dodecanol and 1-tetracosanol at the interfaces n-hexane – water and n-hexadecane – water in the vicinity of “liquid – vapor” thermotropic phase transition is investigated by the method of X-ray reflectometry at synchrotron source. Model-independent structural data obtained on the adsorption layers under investigation deviate considerably from the structural parameters which have been proposed previously within a model-based representation and discussed in previous publications on said systems. It is shown that in the low-temperature mesophase the adsorption film consists of a Gibbs monolayer, a transitional liquid region with thickness of two to three monolayers ~50 Å and an extended (wide up to 200 Å) layer of micelles. Presence of a plane of the closest approach of micellar layer to the adsorption film at the interface is established. Transition to the high-temperature mesophase is followed by liquefying and partial evaporation of alcanol film along with observed depletion of micellar layer down to its complete disappearance.

Dendrimeric triphenylbenzenes: helical versus zig-zag arrangement in columnar mesophases

ABSTRACTA small series of dendrimers consisting of a star-shaped triphenylbenzene core unit and bulky lateral groups was synthesised and their mesomorphic properties were investigated via differential scanning calorimetry, polarising optical microscopy and small- and wide-angle X-ray scattering (WAXS). The dendrimers showed two enantiotropic hexagonal columnar mesophases. The combination of temperature-dependent and fibre-extruded WAXS revealed a helical arrangement in the low-temperature and a zig-zag orientation in the high-temperature mesophase. Furthermore, at the mesophase-to-mesophase transition, the intracolumnar reorganisation and the intercolumnar lattice extension occurred at different time scales, with the latter process being the slower one.

New chiral discotics with helical organization of the mesophase—liquid crystalline derivatives of dibenzotetraaza[14]annulene

Two optically pure derivatives of dibenzotetraaza[14]annulene, bearing four (S)- or (R)- 3,7-dimethyloctoxy peripheral chiral tails, respectively, and two hydroxybenzoyl meso substituents were synthesized using a convergent multi-step route. The structure of the products was determined by 1H and 13C NMR spectroscopy, ESI-MS, and elemental analysis. The mesomorphic behavior of the two chiral compounds, deciphered by differential scanning calorimetry (DSC), small-angle X-ray diffractometry (SA-XRD), and polarizing optical microscopy (POM) investigations, revealed the induction of two lamello-columnar phases, i.e., columnar stacks confined in smectic layers, whose columns may be arranged either according to the pg rectangular planar symmetry (for the low-temperature phase) or without registry between vicinal layers (for the high-temperature mesophase). Evidence of a helical organization of the molecules in the mesophases was obtained using a combination of electronic circular dichroism (ECD) and SA-XRD results, Molecular Dynamics simulations and quantum-chemical calculations.

Methylene-linked liquid crystal dimers and the twist-bend nematic phase

The transitional properties of three methylene-linked liquid crystal dimers are reported, namely, 1,5-bis(4-cyanoanilinebenzylidene-4′-yl)pentane (CN-5-CN), 1,5-bis(4-methoxyanilinebenzylidene-4′-yl)pentane (1O-5-O1), and 1,5-bis(4-ethoxyanilinebenzylidene-4′-yl)pentane (2O-5-O2). Each dimer exhibits two monotropic mesophases. The higher temperature mesophase is a normal nematic phase while the lower temperature phase is assigned as a twist-bend nematic phase. The assignment of the twist-bend nematic phase was based on the strong similarities in the optical textures observed to those reported recently for a structurally similar dimer. The complete miscibility of the mesophases exhibited by CN-5-CN and 1O-5-O1 has been established. The analogous hexamethylene-linked dimers exhibit only the normal nematic phase as do the corresponding ether-linked dimers. A review of the literature reveals another five methylene-linked odd-membered dimers that exhibit a nematic–nematic transition and, in each, the lower temperature nematic phase exhibits similar properties to those reported for the twist-bend nematic phase. The formation of this new nematic phase has been attributed to a negative bend elastic constant which results from the bent geometry of methylene-linked odd-membered dimers.

Peculiarities of the crystal structure of modified banana-shaped mesogen

The structure and phase behavior of an achiral banana-shaped compound—bis-{3,4,5-tri[4-(4-n-nonyloxybenzoyloxy)] benzoylamino}-1,3-phenylene (I)—have been investigated. This compound exhibits an enantiotropic high-temperature chiral mesophase; upon cooling it successively passes to the crystalline (Cr) phases: Crα (281.0–176.0°C), Crβ (175.0–72.0°C), and Crγ (71.0–40.0°C) (their temperature ranges of existence are indicated). The surface topology and results of linear measurements along the cross sections of certain portions of thin films of compound I in the Cr γ phase clearly prove the effect of the preserved chirality of the previous liquid-crystal phase in the crystalline state.

Mesomorphic properties of the neat enantiomers of a chiral pyramidic liquid crystal

The crown form of nona-octanoyloxy tribenzocyclononatriene (C8) with C(3) symmetry was prepared and separated into its enantiomers (C8A and C8B) by HPLC. Like the racemate, the neat enantiomers are also mesogenic, exhibiting two mesophases, M and Col(h). Differential scanning calorimetry, X-ray diffraction, carbon-13 NMR and UV circular dichroism are used to study the structural and dynamic properties of these chiral mesophases. The high-temperature mesophase (Col(h)) was identified by X-ray diffraction as columnar hexagonal; this phase is thermally unstable, undergoing racemization on heating before its clearing point is reached. The low-temperature mesophase (M) is most likely also columnar, but its symmetry could not be determined. In comparison with the racemate, the chiral mesophases are less well ordered, more tightly packed and they have a lower tendency to crystallize. On the other hand they are considerably more mobile, as reflected in their NMR spectra. No helical structure has been identified in the X-ray diffraction of the mesophases.

Liquid Crystalline Properties and Molecular Packing of Semifluorinated n-Alkanes F(CF2)10(CH2)mH

The phase transitions for a homologous series of semifluorinated n-alkanes, F(CF2)10(CH2)mH, with 2 ≤ m ≤ 19 (F10Hm), have been characterized using a polarizing microscope, DSC, X-ray diffraction, and dilatometry. Thermal analysis and optical textures indicate the presence of the smectic−smectic phase transition when the number of methylene units ranges from 6 to 12. The phase-transition temperature depends on the length (m) of the hydrocarbon segment. The density decreases with increases in the carbon number of the hydrogen segment. The data for both the thermal properties and the density suggest that the smectic−smectic phase transition is significantly first-order, which is primarily caused by a change in the molecular packing for the molecular rearrangement, while a transition for the clearing point is related to disordering of the entire molecule. The volume jump shows that F10Hm have the same molecular packing in the high-temperature mesophase and that their magnitude is greater than that of the conventional liquid crystalline compounds having an aromatic−mesogenic core. The effects of the fluorinated segment for the phase transitions are also discussed herein.

Synthesis and Characterization of Unsymmetrically β-Substituted Porphyrin Liquid Crystals: Influence of the Chemical Structure on the Mesophase Ordering

The present study addresses the synthesis and characterization of a series of columnar liquid crystal compounds based on the natural porphyrin hemin. These compounds, which have different alkyl side chains (Alk = butyl, octyl, dodecyl, and hexadecyl) and either free base (M = 2H) or metalated (M = Zn, Cu) porphyrin cores, display hexagonal columnar mesophases over wide temperature ranges that cover room temperature. In the octyl, dodecyl, and hexadecyl free base compounds, the formation of a low-temperature mesophase (Col h l ) on cooling from a high-temperature mesophase (Col h 2 ) is marked by a slow transition. Furthermore, the X-ray diffraction pattern of the Col h 1 mesophase indicates that intracolumnar noncovalent associations of three porphyrin molecules might be formed. As suggested by computer molecular modeling combined with the experimental data, the mobility of the propionic ester chains attached to the porphyrin core and the influence of metalation on porphyrin-porphyrin interactions may account for the mesophase structures encountered in the reported compounds.

Observation of Two-Step Thermochromism in Poly(3-docosylthiophene): DSC and Reflection Spectroscopy

Poly(3-docosylthiophene) was found to have a two-step thermochromic process for samples that are rapidly cooled from the melt. The melt-quenched samples are a red-orange color that changes first to purple and then to yellow upon heating. In contrast, slowly cooled materials are purple and have a single thermochromic transition to yellow, typical of most poly(3-alkylthiophene)s. This phase behavior is found in both regioregular and regioirregular samples. The thermal behavior of the polymers was studied using differential scanning calorimetry and visible reflection spectroscopy. The DSC thermograms of slowly cooled samples show two endotherms upon heating and three exotherms upon cooling. In contrast, rapidly cooled samples exhibit three endotherms upon heating and three exotherms upon cooling. Reflection spectra show color changes that correlate with the DSC features. A model is proposed to explain the data that involves trapping of a high-temperature mesophase under the rapid cooling conditions.

New mesomorphic organocyclosiloxanes II. Thermal behaviour and mesophase structure of organocyclohexasiloxanes†

Investigations of thermotropic phase transitions performed on organocyclosiloxanes [PhSi(O)OSiR]6, where R is Me3, Me2(CH2Cl) or Me2(CH≃CH2), have revealed that all these hexamers are mesomorphic compounds. The hexamers exhibit uncommon polymesomorphic behaviour forming two quite different mesomorphic structures. The molecular arrangement in the low temperature (LT) modification is characterized by two-dimensional (2D) long-range order with hexagonal packing. The X-ray diffraction pattern and peculiarities of molecular packing in the crystal lead us to suggest that the LT-mesophase is columnar, presumably of the Colhd type. The LT-mesophase is formed by dimeric moieties, which associate with each other in column-like substructures, the ring planes not orthogonal to the stack axis. The high temperature (HT) mesophase is a plastic crystal (3D-order), where molecules take up positions in a face-centred cubic lattice. This is a very uncommon example of thermal behaviour for plastic crystals that provides a unique opportunity to bridge the gap between plastic crystalline and liquid crystalline mesomorphic behaviour. The thermal and structural properties of the mesophases depend upon the type of side groups of the hexamers. The size of the ring also affects the phase behaviour and the mesomorphic structure. This conclusion is consistent with data obtained by us earlier for cyclotetrasiloxanes.

Application of Complementary Experimental Techniques to Characterization of the Phase Behavior of [C16mim][PF6] and [C14mim][PF6

A range of analytical techniques (DSC, conductivity measurement, Raman spectroscopy, small- and wide-angle X-ray diffraction (S-WAXS), quasi-elastic neutron scattering (QENS), and single-crystal X-ray diffraction) are applied to the characterization of the phase behavior of the low-melting-point liquid crystalline salts 1-hexadecyl-3-methylimidazolium hexafluorophosphate ([C16mim][PF6]) and 1-methyl-3-tetradecylimidazolium hexafluorophosphate [C14mim][PF6]. This is the first time that QENS has been applied to the structural analysis of this type of ionic liquid crystal. For the first time in this class of salts, a low-temperature phase transition is identified, which is assigned to a crystal−crystal transition. Conductivity and QENS data for [C16mim][PF6] suggest that the higher-temperature crystalline phase (CII) has greatly increased freedom in its long alkyl chain and anion than the lower-temperature crystalline phase (CI). This conclusion is supported by single-crystal X-ray diffraction results for [C14mim][PF6]. In both crystalline phases, as well as in the higher-temperature mesophase, the structure maintains a monodispersed layer structure with interdigitated alkyl chains. The structure of the mesophase is confirmed as smectic A by the S-WAXS and Raman spectroscopy results. Detailed analysis suggests that in this phase the alkyl chains undergo complete conformational melting.

Phase behavior of α, ω-dihexyl- α-quaterthiophene and ordering on a textured substrate

Ordering of α, ω-dihexyl- α-quaterthiophene (DH α4T) thin films on substrates including friction-transferred polytetrafluoroethylene (PTFE) is explored. DH α4T exhibits a high-temperature mesophase from 81 to 181 °C. Textural analysis reveals this phase to be a tilted, highly ordered smectic phase. Upon cooling from the isotropic melt, the smectic phase grows layer by layer on many surfaces. Smectic thin films grown on friction-transferred PTFE exhibit a high degree of alignment with edge dislocations being the primary defect. Upon crystallization from the mesophase, the material retains for the most part the order of the mesophase. Growth of the smectic phase on directional surfaces such as friction-transferred PTFE appears to be a route to control of surface ordering of DH α4T.

A novel class of banana-shaped azo compounds exhibiting antiferroelectric switching behaviour

The first banana-shaped liquid crystal materials with photo-active azo linkages are reported. The higher temperature mesophase X1 (believed to be B6) above the B1 phase exhibited by these azo compounds shows antiferroelectric switching characteristics. The observation of banana phases in azo compounds assumes significance from the point of view of photochromic studies.

X-ray diffraction and UV-Vis absorption studies of poly(di-n-hexylsilane) and poly(di-n-octylsilane) after quenching

Poly(di-n-hexylsilane) (PdHexSi) and poly(di-n-octylsilane) (PdOctSi) polymers have been studied using X-ray diffraction and UV-Vis absorption on warming after thermal quenching. These σ-conjugated conducting polymers exhibited qualitative similarities in their overall behavior. Quenching led to low temperature, ordered structures which retained the hexagonal interchain packing of the high temperature mesophase. This rapid cooling also suppressed the development of three-dimensional interchain ordering. In most other respects the behavior of PdHexSi was similar to that of slowly cooled samples. PdOctSi exhibited pronounced variations in both the X-ray diffraction and the UV absorption in comparison to its slowly cooled counterpart. These changes were found to be somewhat sample dependent. The diversity in PdOctSi behavior appears to stem from systematic changes in the side chain packing on passing from the n-hexyl to the n-octyl moiety.

Phase behavior of poly (pyromellitimide)s having flexible (n-alkyloxy)methyl side chains

Phase behavior of poly(pyromellitimide)s having (n-alkyloxy)methyl side chains (-CH 2 OC m H 2m+1 , m=4, 6, 8) has been studied by wide angle X-ray scattering, differential scanning calorimetry and 13 C solid-state NMR. While there are no observable transitions in tne polymers bearing short side chain(m=4, 6), there is one transition in the polymer bearing the longest side chain(m=8), which is assigned to mesophase-mesophase transition. All the samples show layered mesophase at room temperature, in which the side chains are amorphous but the main chains form two-dimensional crystals in each layer. In the polymer bearing the longest side chain(m=8), it shows another layered mesophase at high temperature. In the high temperature mesophase, the main chains do not form two-dimensional crystals in each layer; only the lateral packing of the main chains remains undisrupted.

An Efficient Synthesis, and Characterisation of Anthraquinone-based Discotic Liquid Crystals and Their X-Ray Diffraction Studies

We herein report an efficient synthesis of 1,2,3,5,6,7-hexaalkoxy-9,10-anthraquinones (also called rufigallol hexaethers) starting from gallic acid. These ethers are known to form columnar phases which are suitable for the study of charge transport, photoconductivity etc. Our X-ray diffraction studies show that the high-temperature mesophase is hexagonal columnar with correlation among the molecular cores along the column (Colh, also called Dh), and that, identified for the first time in these systems, there are one or two additional mesophases at low temperature depending on the system, some of them three-dimensionally ordered. Our results show that the mesophase (high-temperature) range does not dramatically decrease as the length of the alkyl chains at the 1,5 positions become longer than the rest. Since these molecules have absorption in the visible region they may also be the potential candidates for studying photovoltaic effects that are of interest in developing energy conversion applications.

A novel series of anthraquinone-based discotic liquid crystals with bulky substituents: synthesis and characterization

A novel series of anthraquinone-based discotic liquid crystals with bulky substituents, namely 1,5-dibenzyloxy-2,3,6,7-tetraalkyloxy-9,10-anthraquinones, has been synthesized starting from gallic acid. This is, to our knowledge, the first example of bulky substitution in a discotic C2-symmetric molecule forming columnar phases. Except for the lowest homologue, all members of this series are found to exhibit columnar mesophases; the low temperature mesophase appears to be three-dimensionally ordered whereas the high temperature mesophase is hexagonal columnar (Colh). We find that the introduction of benzyl substituents for alkyl chains (in the 1,5-positions) on the anthraquinone hexaalkoxylates stabilizes the three-dimensionally ordered phase, whereas it destabilizes the Colh phase, compared with the anthraquinone hexaalkoxylates. Interestingly, the three-dimensionally ordered phase extends down to -50 °C, making these new derivatives suitable for device applications.

Ferroelectric switching in a novel bent-shaped mesogen having two non-mesogenic units linked by an alkylene spacer

We report the synthesis, X-ray diffraction results and electro-optical switching measurements for a bent-shaped mesogen having two non-mesogenic units linked by an alkylene spacer. The novelty of the molecular structure lies in the carbonyl group of the ester linkage being directly attached to the spacer unit, unlike for banana-shaped molecules reported so far, in which it is one oxygen atom away from the spacer or the central aromatic unit. The compound shows two mesophases: the high temperature mesophase is a tilted smectic phase showing ferroelectric switching characteristics; the low temperature phase is more highly ordered with textural features similar to that of the B3 banana phase.

Photoconductivity of Discotic Liquid Crystals Derived from A Long-Chain Tetraphenylporphyrin

ABSTRACTPhotoconductivity of a mesogenic tetraphenylporphyrin (TPP) is shown in relation to the mesomorphic phase transition behavior. The TPP mesogen shows two types of mesophases having smectic-like order (lamellar mesophases : DL and DLC phases named in this work) and the photocurrent changes stepwise at the phase transitions with non-Arrhenius dependence whilst the darkcurrent tends to be Arrhenius. The hole mobility was measured for the random domain cell and was found to be faster than 10−4 cm2v−1sec−1 in the mesophases. The cell used for the photocurrent measurements was a sandwich-type one using two ITO glass slides as electrtodes and show rectification behavior for the crystal and the low-temperature mesophase (DLC) in contrast to non-rectification of photocurrent in the high-temperature mesophase (DL). These properties of charge carrier migration as an electronic process are profoundly related to molecular order as well as molecular motions in mesophase.

A kinetics study of crystallization from discotic mesophases

The polymorphism of hexa- n -octanoate of rufigallol (RHO) was used to investigate the crystallization kinetics when starting from two different discotic mesophases, namely, a hightemperature enantiotropic mesophase and a low-temperature monotropic mesophase. In all cases, an Avrami exponent of n 3 was obtained, suggesting two-dimensional sporadic nucleation and growth. Although the same overall mechanism was predicted to operate for all crystallization temperatures investigated, the Avrami constant b has a temperaturedependence which is different for different starting columnar phases, indicating a strong influence on the rates of nucleation and of growth of the different molecular arrangements of the starting mesophases. Differential scanning calorimetry was used for the studies, supplemented by direct observations through optical polarizing microscopy.