J. Etxebarria

Bent-core liquid crystals in the route to functional materials

Several results on bent-core liquid crystals considered as new supramolecular functional materials with potential for applications are reviewed. Various topics are discussed where these materials present unique attributes with clear advantages over conventional mesogens. In particular the electrooptic responses of some mesophases are examined from the viewpoint of their possible use in electrooptic devices. Other issues are also treated such as nonlinear optical properties, flexoelectricity or the design of biaxial nematics from bent-core molecules. Future challenges and prospects for new possibilities with these materials are briefly outlined.

Bent-core liquid crystals in a route to efficient organic nonlinear optical materials

The design, synthesis and nonlinear optical (NLO) response of some bent compounds, at both the molecular and macroscopic level, are reported. Based on a bent-core NLO-phore with a μβ0 in the range of 390 × 10−48 esu (1907 nm), the second order susceptibility tensor for second harmonic generation is evaluated at excitation wavelengths of 1064 and 1600 nm in a mesogenic material with a SmCP mesophase. The NLO coefficients are in the range 10–100 pm V−1. Thus it is deduced that a molecular design leading to bent-core liquid crystals with large β values is a correct strategy to achieve NLO-materials for bulk second order effects.

Liquid crystalline and nonlinear optical properties of bent-shaped compounds derived from 3,4'-biphenylene

The synthesis of different bent-core molecules derived from 3,4′-biphenylene bearing azo, azoxy, imine or ester linkages in their lateral structures is reported. Structure–activity relationships for their liquid crystalline behaviour are discussed. SmCP, USmCP and Colob mesophases are found depending on the type and number of these connecting units. The sequence ester ≈ azoxy > imine > azo can be proposed for the mesophase range, with significant differences observed in terms of mesopahase stabilization. SHG studies on these compounds give nonlinear coefficients in the range of 1–8 pm V−1. The molecular origin for these values is analyzed semi-quantitatively. It was concluded that the SHG performance of bent-core mesogens in general can still be increased substantially. An approach to improve the properties of these materials is briefly outlined.

Laterally Azo-Bridged H-Shaped Ferroelectric Dimesogens for Second-Order Nonlinear Optics: Ferroelectricity and Second Harmonic Generation

Two classes of laterally azo-bridged H-shaped ferroelectric liquid crystals (FLCs), incorporating azobenzene and disperse red 1 (DR-1) chromophores along the FLC polar axes, were synthesized and characterized by polarized light microscopy, differential scanning calorimetry, 2D X-ray diffraction analysis, and electro-optical investigations. They represent the first H-shaped FLC materials exhibiting the ground-state, thermodynamically stable enantiotropic SmC* phase, i.e., ground-state ferroelectricity. Second harmonic generation measurements of one compound incorporating a DR-1 chromophore at the incident wavelength of 1064 nm give a nonlinear coefficient of d(22) = 17 pm/V, the largest nonlinear optics coefficient reported to date for calamitic FLCs. This value enables viable applications of FLCs in nonlinear optics.

Azo Isocyanide Gold(I) Liquid Crystals, Highly Birefringent and Photosensitive in the Mesophase

Isocyanide ligands bearing an azo group and one alkoxy chain OC(n)H(2n+1) have been synthesized. They are calamitic liquid crystals for n > 4 and display nematic (n = 8, 12) and SmA (n = 12) mesophases. Their gold(I) compounds [AuX(CNR)] (X = Cl, C(6)F(5); R = C(6)H(4)N horizontal lineNC(6)H(4)OC(n)H(2n+1), n = 4, 8, 12) have been obtained by displacement of a weakly coordinated ligand. The chloro gold(I) compounds exhibit nematic (n = 4) and SmA mesophases, and decompose at temperatures higher than 200 degrees C, before reaching the clearing point. The pentafluorophenyl gold(I) compounds show nematic and SmA (n = 12) mesophases. All the derivatives are photosensitive in solution because of trans to cis isomerization of the azo group under UV light, which reverts photochemically or thermally to the trans isomer. Irradiation in the mesophase also induces isomerization with consequent destabilization of the mesophase to an isotropic liquid; the mesophase is recovered as soon as illumination stops. These azo mesogens show high birefringence values, higher for the linear gold complexes than for the free azo ligand.

Second harmonic generation measurements in aligned samples of liquid crystals composed of bent-core molecules

The second-order susceptibility tensor of three liquid crystals composed of banana-shaped molecules has been fully determined. The measurements were carried out in single domain samples, aligned by means of an electric field parallel to the cell surface. Special care was taken with the appropriate determination of the index mismatch between the fundamental and second harmonic waves. This was achieved by using samples of different thickness. This parameter is very important for obtaining reliable values of the second order susceptibility tensor components, and is seldom directly measured. The suitability of the experimental procedure was checked by comparing the results obtained for the reference compound P-8-O-PIMB with previously reported measurements for this material. The dij coefficients obtained in this work are in the range 1–4 pm V−1 and are compared with other values obtained in previous reports.

Novel liquid-crystalline mesogens and main-chain chiral smectic thiol-ene polymers based on trifluoromethylphenyl moieties

The synthesis and characterization of a series of novel liquid-crystalline molecules containing a trifluoromethylphenyl core or a 2,3-bis(trifluoromethyl)phenyl core is described. All trifluoromethylphenyl-containing compounds (W530, W551, and W558) have mesomorphic properties while 2,3-bis(trifluoromethyl)phenyl-containing compounds (W555, W556, W557) are room-temperature crystals without mesophases. Thereafter, based on the chemical structure of W558, to pursue a mesogen with a large electroclinic effect in the SmA* phase for development into electromechanical actuators, a chiral smectic thiol-ene monomer (K0901) is synthesized and polymerized in organic solvents. The polymer is characterized by GPC, DSC, PLM, XRD and shows an expected I–SmA* –Glass phase sequence though the electroclinic effect is relatively small.

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.

Non-linear optical properties of metallorganic ferroelectric liquid crystals

Second harmonic generation (SHG) efficiencies d eff were determined for Pt-containing ferroelectric liquid crystals (FLCs). A relatively simple method is presented which allows for the simultaneous determination of the second order susceptibility tensor coefficients d ij together with the dispersion and birefringence at the fundamental frequency. The relation between the d ij coefficients is qualitatively interpreted in terms of the molecular shape and of a scheme proposed for the molecular orientation in the ferroelectric phase. Contrary to the usual situation with calamitics, d 23 is much higher than the rest of the coefficients. This peculiarity, which reflects the 3D character of the molecular response, is interesting since d 23 is directly involved in d eff for phase matched SHG.

Temperature dependence of the second-order susceptibility in calamitic ferroelectric liquid crystals

The dij coefficients of the second-order susceptibility tensor for second harmonic generation were detemined for a calamitic ferroelectric liquid crystal (FLC) specifically designed for applications in non-linear optics. The measurements were performed at different temperatures in the SmC* phase. In accordance with the design methodology of these materials, the coefficient along the polar axis d22 is the greatest, and depicts an expected behaviour in the whole range of the ferroelectric phase. On the other hand, the temperature dependence of d21, d23, and d25, is anomalous to some extent, including a sign inversion of d23 at a certain temperature. These results, which contrast with those reported recently for non-calamitic FLCs, were qualitatively interpreted in the light of different conformations which, by virtue of plausible distortions of the molecular core, could coexist in the sample.