M. R. De La Fuente

Dielectric relaxation in liquid crystalline dimers

Dielectric measurements are reported for liquid crystalline dimers consisting of two mesogenic groups joined through ether links by a flexible alkyl chain. Results are presented for the odd-symmetric dimer, α,ω-bis[(4-cyanobiphenyl)-4′-yloxy]undecane (BCB.O11), and the odd-asymmetric dimer, α-[(4-cyanobiphenyl)-4′-yloxy]-ω-(4-decylanilinebenzylidene-4′-oxy) nonane (CB.O9O.10). The real and imaginary parts of the electric permittivities of aligned samples were measured as functions of frequency over the range 102–109 Hz at temperatures throughout the nematic phase. Measurements were fitted as a function of frequency to the Havriliak–Negami function, and yielded relaxation times and dielectric strengths for the relaxations at each of the temperatures studied. The static dielectric anisotropy for both materials was positive. For BCB.O11, the parallel and perpendicular components of the permittivity exhibited one low frequency and one high frequency relaxation, while the parallel component of the permittivity...

Simulation of the First Hydration Shell of Nucleosides D4T and Thymidine: Structures Obtained Using MP2 and DFT Methods

A comparative theoretical analysis on the effect of the solvent on the molecular structure and energetics of the most stable conformers of the nucleoside analogue D4T (stavudine) and of the natural nucleoside thymidine (Thy) was carried out. Solvent effects were considered using the Tomasis polarized continuum model (PCM) and including a variable number (1-13) of explicit water molecules surrounding the nucleoside in order to simulate the first hydration shell. More than 200 cluster structures with water were analyzed. B3LYP and MP2 quantum chemical methods were used. The CP-corrected interaction energies for D4T and water molecules were computed. For cases where literature data are available, the computed values were in good agreement with previous experimental and theoretical studies. In the isolated state, conformer I (anti-gg-gg) appears the most stable for D4T molecule and conformer II (anti-gg-gt) for Thy molecule. In D4T with eight water molecules, conformer II changes to conformer I. Thus, conformer I seems preferred when water molecules are situated in the first hydration shell. However, in hydrated thymidine conformer Ia (anti-gg-tg) is the more stable one, and the first hydration shell is more extended than in D4T molecule. The effect of the hydration on the total atomic charges and intermolecular distances were also discussed. Several general conclusions on hydrogen bonds network and involved interaction energies were underlined.

Structure–activity studies of ferroelectric and antiferroelectric imine ligands and their square-planar complexes

The synthesis and characterization of the mesomorphic and dielectric properties of a series of new imine-based liquid crystalline compounds that exhibit tilted SmC* phases is reported. The presence of an imino linkage within the mesogenic nucleus of these compounds is significant due to the ability of salicylaldimines to coordinate to metals. Structure–activity studies have also been carried out by varying the structural elements in the ligands. The structural variations include changing the length of the chiral chain and incorporating fluorocarbon segments in the achiral terminal chain.

Dielectric studies on orientationally disordered phases of neopentylglycol ((CH3)2C(CH2OH)2) and tris(hydroxymethyl aminomethane) ((NH2)C(CH2OH)3)

The dielectric spectra up to frequencies of 1 GHz of the substances 2,2-dimethyl-1,3-propanediol (or neopentylglycol, NPG) and 2-amino-2-(hydroxymethyl)-1,3-propanediol (or tris(hydroxymethyl)aminomethane, TRIS) have been determined in the orientationally disordered FCC and BCC phases respectively, in the ranges 305 - 353 K for NPG and 343 - 375 K for TRIS (in this case, in the supercooled BCC orientationally disordered phase). The dielectric relaxation of both substances has been studied by considering modified Debye theories as well as the Jonscher equation. Significant deviations from the single Debye relaxation were found in both cases. The temperature dependence of the relaxation time, assuming Arrh?nius or Eyring activation models, allows one to evaluate of the activation enthalpies and entropies. Large values were obtained for both parameters, showing that there are strong intermolecular interactions via hydrogen bonds, as had been proved previously for solid phases of alicyclic and branched (tetrahedral molecules) alcohols.

Cyanostilbene bent-core molecules: a route to functional materials

The synthesis and characterization of novel compounds, that incorporate the high current interest functional cyanostilbene unit, are reported. Four examples, derived from 3,4′-biphenylene, have a 2D molecular-shape that promotes a variety of bent-core liquid crystalline phases (columnar, polar smectic C, and dark-conglomerate phases). The mesomorphism has been characterized by optical microscopy, differential scanning calorimetry, X-ray diffraction, and by detailed electrooptic and dielectric studies. The liquid crystalline properties can be modulated depending on the position of the cyano group on the CC bond (α- or β-isomers) and the number of cyanostilbene units in the molecule (one or two). The polar smectic C mesophases have an antiferroelectric ground state. Furthermore, the multiresponsive nature of the cyanostilbene structure has led to very attractive and diverse photoactivity, including luminescence and second harmonic generation. The results obtained broaden the possibilities for the use of bent-core liquid crystals to design novel multiresponsive soft materials.

Dielectric and optical studies near the chiral smectic C-smectic A transition of (S)-2-hydroxy-4-decyloxybenzylidene-4′-amino-2p″-methylbutylcinnamate

Abstract The complex dielectric permittivity of HDOBAMBC has been measured in the vicinity of the chiral smectic C-smectic A transition as a function of temperature and frequency. The Goldstone mode contribution has been clearly identified and its relaxation frequency has been determined over the whole S*C range. High accuracy tilt angle measurements have also been performed by using a new method which allows its determination in a direct and virtually continuous way. These experimental results together with additional measurements of the polarization and helical pitch have been compared with those of DOBAMBC and analized in the framework of the generalized Landau theory. As for DOBAMBC a reentrant behaviour S*C-SC-S*C has been observed under an electric field near the S*C-SA transition.

Study of the critical behavior and scaling relationships at the N-to-I phase transition in hexyloxycyanobiphenyl.

An exhaustive analysis of the critical behavior of the nematic to isotropic (N-to-I) phase transition on the liquid crystal hexyloxycyanobiphenyl (6OCB) has been performed. To do so, the accurate evolution of various physical magnitudes (static dielectric permittivity data together with specific heat and volumetric determinations) around the N-to-I transition has been required. The specific heat data with the isobaric thermal expansion coefficient and the derivative of the static dielectric permittivity with temperature have been proven to be related to each other by a scaling relationship. However, some discrepancies have been observed for the dielectric data in relation to such a scaling relationship and the critical behavior of the N-to-I phase transition. All information has been used to get some insight on the strength of the first-order N-to-I phase transition of the 6OCB in relation to the other counterparts in the nOCB series of compounds.

Molecular and collective modes in ferroelectric liquid crystals studied by dielectric spectroscopy

The complex dielectric permittivity has been measured for a ferroelectric liquid crystal in the range 102-109Hz. Six different relaxations have been obtained and characterized: soft mode (SmA* and SmC* phases), Goldstone mode (SmC* phase), rotation around molecular long axis, rotation around molecular short axis, ferroelectric domain mode (SmC* phase) and an internal motion associated with a polar group. Strengths and frequencies of these modes have been obtained for the different phases for different bias fields. Using these results together with spontaneous polarization and molecular tilt measurements we have also obtained the rotational viscosities associated with the soft mode and the Goldstone mode. We explain the results in the light of the so-called Landau extended model, concluding that the biquadratic coupling between polarization and tilt is quite important with regard to the bilinear coupling. This fact has been used to explain the noticeable increase of the activation energy of the frequency o...

Low and high frequency dielectric spectroscopy on a liquid crystal with the phase sequence N∗-SA-S∗c

Abstract Broadband dielectric measurements on a multicomponent ferroelectric liquid crystal mixture have been performed. The alignment was homeotropic and the cell and sample holder were the same in the whole frequency range. Two relaxation processes have been observed in all liquid crystal phases with shapes given by the Havriliak-Negami and inverted Havriliak-Negami functions. Strengths and frequencies of both modes have been obtained for the different phases. The values of the latter and their activation energies allowed us to assign the low and high frequency mechanism to the molecular reorientation around the transversal axis and around the longitudinal axis, respectively. The behaviour of this high frequency mode does not show any jump in the SA-S∗c phase transition, neither in the amplitude nor in the frequency, indicating that the appearance of the spontaneous polarization in the S∗c phase is not the consequence of the freezing of this mode.

Simultaneous determination of fundamental parameters for ferroelectric liquid crystals

Abstract An analysis of an experimental technique which allows for a simultaneous determination of several parameters with special interest for ferroelectric liquid crystals is presented. These parameters are the spontaneous polarization, the rotational viscosity and the switching time. In addition, the susceptibility associated with the soft mode free of contributions related to the helicoidal structure can also be obtained. The experimental results for these parameters for the ferroelectric liquid crystal HDOBACEEC are reported. A comparison between the switching time values deduced from the rotational viscosity and those obtained by optical measurements is performed.