Molecularengineering of discotic nematic liquid crystals

Connecting two columnar phase forming discotic mesogens via a short rigid spacer leads to the formation of a π-conjugated discotic dimer snowing discotic nematic (N _D) phase. Attaching branched-alkyl chains directly to the core in hexaalkynylbenzene resulted in the stabilisation of N_D phase at ambient temperature. Pentalkynylbenzene derivatives possessing a combination of normal- and branched-alkoxy chains display a very broad N_D, phase which is stable well below and above the room temperature.     

Photophysical and electrochemical properties of monoporphyrin rare earth liquid crystalline materials

The synthesis and characterization of [5-(p-alkacyloxy ) phenyl-10,15,20-tri-phenyl] porphyrin (APTPP) and its lanthanide complexes(lanthanide ions: Ho(III), Dy(III), Er(III), Yb(III)) are reported. They form hexagonal columnar discotic columnar (Col_h) liquid crystals over an extended domain of temperature. Luminescence spectra of the compounds are discussed. Quantum yields of Q band are in the region 0.004570-0.05847. The electrochemical property is studied by cyclic voltammetry. The synthesized APTPP and its lanthanide complexes exhibit two one-electron reversible redox reactions and three redox reactions, respectively. The photovoltaic properties and charge-transfer process of the liquid crystalline compounds are investigated by surface photovoltage spectroscopy (SPS) and electric-field-induced surface photovoltaic spectroscopy (EFISPS) techniques, and the bands are analogous with the ultraviolet (Uv) -visible absorption spectra, which reveal that all compounds are P-type semiconductors. All of compounds are nonelectrolytes.     

Search for optical biaxiality in discotic liquid crystals

Using a high-finesses Fabry-Perot structure, we examine the behaviour of discotic liquid crystals for possible biaxiality. This is done by confining the discotic liquid crystals in the Fabry-Perot cavity and examining the mode structure in presence and absence of an applied electric field. It is concluded that at least in the discotic liquid crystal that we have examined, there is no evidence of biaxiality.     

Morphology of open films of discotic hexagonal columnar liquid crystals as probed by grazing incidence X-ray diffraction

The structure and the orientation of thermotropic hexagonal columnar liquid crystals are studied by grazing incidence X-ray diffraction (GIXD) for different discotic compounds in the geometry of open supported thin films. Whatever the film deposition mode (either spin-coating or vacuum evaporation) and the film thickness, a degenerate planar alignment with the liquid crystalline columns parallel to the substrate is found. However, if a specific thermal process is applied to the liquid crystal film, homeotropic anchoring (columns normal to the interface) can be stabilized in a metastable state.     

Rufigallol-based self-assembled supramolecular architectures

Discotic liquid crystals have been attracting growing interest not only because of the fundamental importance as model systems for the study of charge and energy transport but also due to their potential application in organic electronic devices. The 1,2,3,5,6,7-hexahydroxy-9,10-anthraquinone, commonly known as rufigallol, is one of the earliest systems reported to form columnar mesophases. Over the past 25 years, more than 100 discotic liquid crystals based on this core have been realized and studied for various physical properties. This review summarizes synthesis and thermal behaviour of all these materials. A brief summary of various physical studies on these materials has also been given.     

Recent results on some columnar paramagnetic metallomesogens

The liquid crystalline and selected physical properties of some vanadyl and manganese(III)-porphyrin-TCNE complexes are discussed. These materials form linear chains from a magnetic point of view and columnar discotic LC phases. Some of the manganese complexes exhibit a bulk magnetic phase below ∼20 K. The discussed materials are studied by means of DSC, X-ray powder diffraction, dielectric spectroscopy, reversal current characterization and magnetic susceptibility measurements. A broader view on some physical properties of columnar paramagnetic metallomesogens is given.     

Orientation of discotic and ferroelectric liquid crystals in a macroporous silicon matrix

Macroporous silicon with deep regular channels 3–4.5 μm in diameter was infiltrated with discotic and ferroelectric liquid crystals (LCs) at the temperature of the isotropic phase, and then, the system was slowly cooled to room temperature, with the liquid crystalline mesophase formed. The orientation of the LC molecules in the porous matrix was studied by FTIR spectroscopy. The alignment of LCs was ascertained by comparing the behavior of various vibrational bands of a liquid crystal introduced into the porous matrix with that for LC inside the bulk cells of planar and homeotropic alignment. The molecules of the discotic LC show a planar orientation of their column’s axis with respect to the surface of the macroporous silicon wafer; i.e., they are perpendicular to the channel axis. The long molecular axis of the ferroelectric LC is aligned with the pore walls, having homeotropic orientation with respect to the wafer surface. In a macroporous silicon matrix, both kinds of LCs show unexpected enhancement of the low-frequency vibrational bands. From Fizika Tverdogo Tela, Vol. 44, No. 6, 2002, pp. 1145–1150. Original English Text Copyright ? 2002 by Perova, Astrova, Tsvetkov, Tkachenko, Vij, Kumar. This article was submitted by the authors in English.     

Molecular organization of discotic mesogenic cis-dichlorobis(3,4,5-trialkoxyphenylisonitrile)platinum (II) complexes on Langmuir and Langmuir–Blodgett films

Langmuir and Langmuir–Blodgett films of a new class of square planar metallomesogens, the cis-dichlorobis(3,4,5-trialkoxyphenylisonitrile)platinum (II) complexes (PtC_n), have been prepared.

The structure and the stability of the floating monolayers of these discotic complexes have been investigated by means of compression isotherms, surface potential measurements and Brewster angle microscopy (BAM). A certain influence of the peripheral substituents on the properties of the Langmuir films are observed. It is shown that the hexoxy derivative (PtC₆), which shows mesogenic behavior at room temperature, has a more marked tendency to self-organization in ordered multilayers than the butoxy complex (PtC₄) which is not liquid crystalline at room temperature.

The structural properties of films, transferred at different pressures, have been studied by AFM and UV–VIS, FT-IR transmission and reflection–absorption infrared spectroscopy (RAIRS). The transfer of the monolayers onto solid substrates leads to Langmuir–Blodgett films in which face-on orientation is found for the molecule cores. The organization of the aliphatic chains depends on the pressure at which the monolayers are transferred.     

Luminescence properties and relaxation processes of strongly correlated organic radical TTTA crystals and molecules

Photoluminescence of strongly correlated organic radical 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) crystals was measured at room temperature in order to elucidate the relaxation process in the excited state, which is responsible for an initiation of the photoinduced magnetic phase transition in this material. The electronic structure and luminescence properties of TTTA molecule were also investigated to clarify the relaxation process. We found that a luminescence band lies at 1.8 eV in the high-temperature (HT) phase crystal having almost same characteristics as that in TTTA molecule, suggesting that the intramolecular lattice distortion plays an important role for the relaxation process. On the other hand, a broad luminescence band appears at 1.4 eV with a large Stokes shift in the low-temperature (LT) phase crystal. The large Stokes shift observed shows that rearrangement of the dimerized TTTA molecules toward the evenly spaced radicals takes place due to a large intermolecular lattice distortion. This molecular rearrangement in the excited state initiates the photoinduced magnetic phase transition from the LT to HT phases in this material.     

High pressure and low-temperature polymorphism in cyclopentanol

The polymorphism of cyclopentanol (C₅H₁₀O) has been investigated as a function of temperature at ambient pressure and as a function of hydrostatic pressures to 3.7?GPa at room temperature. Differential scanning calorimetry (DSC) and Raman spectra reveal that two plastic phases and two fully ordered crystalline phases are formed during cooling. High pressure Raman and infrared spectra show that cyclopentanol undergoes two-phase transformations. At around 0.6?GPa, the liquid cyclopentanol transforms to a solid plastic structure. On further compression to 1.9?GPa, one fully ordered crystalline phase is observed. Based on pieces of evidence such as peak splitting and emergence of new peaks, it can be concluded that the ordered crystalline structure has a lower symmetry. In addition, the decrease in the wavenumber of the O–H stretching modes at low temperature and high pressure suggests the ordered crystalline phases are characterized by the formation of hydrogen-bonded molecular chains.     

Detecting columnar deformations in a supermesogenic octapode by proton NMR relaxometry

We used proton ( ¹H nuclear magnetic relaxation (NMR) dispersions to study the molecular dynamics in the isotropic phase and mesophases (nematic and columnar hexagonal) of a supermesogenic octapode formed by laterally connecting calamitic mesogens to an inorganic silsesquioxane cube through flexible spacers. The dispersions of the spin-lattice relaxation time (T₁) are interpreted through relaxation mechanisms used for the study of molecular dynamics in low-molar-mass liquid crystals but adapted to the case of liquid crystalline supermolecules. At high frequencies (above 10MHz) the behaviour of the T₁ with the Larmor frequency is similar for all phases and is ascribed to local reorientations and/or rotations. At intermediate and low frequencies (below 10MHz) our results show notable differences in the T₁ behaviour with respect to the mesophases. The nematic (N) and isotropic (Iso) phases’ low-frequency results are similar and are interpreted for both phases in terms of order director fluctuations (ODF), revealing that even in the isotropic phase local nematic order is detected by proton NMR relaxometry. Local nematic order in the Iso phase is interpreted in terms of the presence of nematic cybotactic clusters induced by the interdigitation of mesogens that is promoted by the silsesquioxane octapode molecular structure. In the columnar hexagonal (Col _h phase, the T₁ dispersions show that elastic columnar deformations (ECD) dominate the nuclear magnetic relaxation below 10MHz. This result shows that the columnar packing of the octapode clearly restricts the collective fluctuations of the mesogenic units inspite of their local nematic order.     

Surface diffusion of matter in discotic liquid crystal threads

The first measurements of the surface diffusion coefficients in columnar discotic liquid crystal thread and film by an optical absorption technique are presented. A comparison of the results with the diffusion in the bulk specimen helps to introduce the number of diffusion sites in the surface and in the bulk. We propose then a structure of point defects in the mesophase. An estimation of jump frequency is deduced. The dependence of the surface diffusion versus the temperature seems in agreement with the Arrhenius law.     

Molecular dynamics simulations of a flexible liquid crystal

Molecular dynamics simulations have been performed for a liquid crystal composed of a Gay—Berne core site with two alkyl chains of different length (C₇ and C₃) at either end of the molecule. Calculations have been carried out for 512 molecules in the NVT ensemble for simulation times of up to 8.0ns at two distinct densities. The liquid crystal phases of the material have been fully characterized by measurements of orientational order parameters and radial distribution functions in each phase. Results are also presented for conformational distributions and effective torsional potentials of the system. We conclude that models of this nature represent a powerful approach to the study of flexibility in mesogenic systems and open up possibilities for predicting both the phase behaviour and bulk properties of liquid crystals based solely on a prior knowledge of intermolecular interactions.     

Dependence of intramolecular valence delocalization on crystal form in mixed-valence trinuclear iron phenylacetate [Fe 2 III FeIIO(PhCH2CO2)6(py)3]

A dependence of intramolecular valence delocalization on crystal form in mixed-valence trinuclear iron phenylacetate [Fe ₂ ^III Fe^IIO(PhCH₂CO₂)₆(py)₃] was found. This complex was obtained in two different crystal forms: a columnar and a needle crystal form. Mössbauer spectra of the columnar crystals show a temperature-dependent valence delocalization, while the needle crystals show a trapped mixed-valence state from a lower temperature up to room temperature.     

Correlation of the molecular structure of discotic nematic liquid crystals with their orientational order and specific features of the nematic-isotropic phase transition

The orientational order parameter S of molecules in high-temperature discotic nematic liquid-crystal phases of triphenylene derivatives is investigated as a function of the length of side flexible molecular chains at different temperatures. It is established that the orientational order parameters S in the range of the transition from the nematic phase to the isotropic liquid phase (the N _D -I transition) are smaller than those predicted from the molecular-statistical theory and computer simulation. It is shown that the N _D -I transition is close to both the isolated Landau point and the tricritical point (regardless of the chemical structure of the molecules and the anisotropy of dispersion intermolecular interactions). Consistent explanations are offered for a number of experimental findings, such as the anomalously small changes in the enthalpy and entropy upon the N _D -I transition (as compared to those revealed upon the N-I transition in calamitic nematic liquid crystals), the anomalously strong response of the isotropic phase of discotic nematic liquid crystals to external fields (thermodynamically conjugate to the order parameter S) and the long relaxation times of this response, and the formation of cybotactic discotic molecular clusters in the isotropic phase in the vicinity of the N _D -I transition.     

Charge conversion of uranium ions in CaF2

Single crystals of CaF₂ doped with uranium ions exhibiting a light red colour have been grown in an inert atmosphere. ESR spectra belonging to U³⁺ centers unstable at room temperature could be observed only after UV-irradiation at liquid nitrogen temperature. The formation and annealing of these centers as well as the optical absorption date indicate that a new charge conversion mechanism is effective in such crystals.     

Isotope effects in the Lyman and Werner systems of molecular hydrogen

A prototype model of amphiphilic discotic binary solutions is proposed, and its statistical mechanics investigated by Monte Carlo computer simulation procedures. Data are presented from a large number of simulations, over 50 separate systems in all, carried out at a single relatively low pressure but covering almost the entire temperature-composition range of phase space beyond the melting line, including successful simulations of equilibrated cluster distributions throughout the isotropic solution phase. These data are in agreement with the well known theory of linear aggregation, but a major surprise is found in that the associated equilibrium constant possesses a strong concentration dependence at low temperature. The global aggregation behaviour is described in detail and discussed in the context of the standard state that is usually invoked when applying linear aggregation theory to experimental systems. It is concluded that as the temperature decreases the ideal behaviour of these cluster distribution data shows more and more influence from the nearby infinite polydispersity limit. At sufficiently high surfactant concentration our model discotic solutions melt first into partially ordered states of aligned aggregates (columns of discs) surrounded by liquid solvent. Both a nematic phase and a hexagonally ordered columnar phase are observed. Simulation procedures are used to investigate the phase behaviour and especially the microscopic structure of these lyotropic liquid crystal systems. The thermodynamic behaviour of our prototype molecular model shows a qualitative resemblance to experimental results obtained from aqueous solutions of the discotic amphiphile known as TP6EO2M.     

[5‐(p‐alkoxy)phenyl‐10, 15, 20‐tri‐phenyl] porphyrin and their rare earth complex liquid crystalline

Three series of porphyrin liquid crystalline compounds, [5‐(p‐alkoxy)phenyl‐10, 15, 20‐tri‐phenyl] porphyrin and their rare earth complexes (Tb (III), Dy (III), Er (III), Yb (III)), with a hexagonal columnar discotic columnar(Col_h) phase have been synthesized. These compounds were characterized by elemental analysis, molar conductances, UV‐visible spectra, infrared spectra, luminescence spectra, and cyclic voltammetry. These compounds exhibit more than one mesophases, which transition points of temperature change from ?33.6 to 16.0 °C, and transition points of temperature for isotropic liquid also increase from 4.9 to 38.2 °C, with increasing chain length. Their surface photovoltage (SPV) response have also been investigated by the means of surface photovoltage spectroscopy (SPS) and field‐induced surface photovoltage spectroscopy (EFISPS). It was found that their SPV bands are analogous with the UV‐visible absorption spectra and derived from the same transition. Copyright © 2007 John Wiley & Sons, Ltd.     

The structure and energetics of defects in liquid crystals

The study of defects in ordered media has become a subject of considerable interest to condensed matter physicists in recent years. This article presents a systematic account of the structures, energies and interactions of defects in the nematic, smectic, cholesteric and discotic phases of liquid crystals. Relevant experimental observations are also described.     

A new approach to describing the statistical laws governing the formation of the contour of individual bands in the optical absorption spectra of liquid systems

A new approach to the problem of a quantitative study of the statistical laws governing the formation of the contour of individual bands in the optical absorption spectra of unordered molecular systems (liquids, solutions, liquid crystals, etc.) was developed. The approach is based on a joint consideration of two independent mechanisms of the inhomogeneous broadening of spectral bands; these mechanisms are related to fluctuations of the isotropic and anisotropic components of the total interaction potential of a molecule with the surrounding medium. A generalized expression for the total statistical contour was derived, which made it possible to describe quantitatively the Q-band contour in the vibrational-rotational IR absorption spectrum of a HCl solution in CCl₄ at room temperature.