M. B. Ros

Structural studies of mesogenic orthopalladated imine derivatives

Abstract Structural studies by 1H NMR of 12 orthopalladated complexes of the type: {Pd2(μ-Y)2(C10H21OC6H4C(X)NC6H4R)2} (XH, CH3; YOCOCH3, Cl, Br, SCN; ROC10H21, C10H21) are reported. Although in all cases only the trans isomers of these complexes have been observed, the presence of other types of isomerism has been detected. Thus, the thiocyanate complexes show isomers arising from the relative disposition of the asymmetrical bridging ligands, whereas the acetato-bridged complexes must be a mixture of the D and L enantiomers due to their rigid open-book shape. From the results of different spectroscopic studies (low temperature and selective decoupling) carried out on the ketone-derived imine complex {Pd2(μ-OCOCH3)2(C10H21OC6H4C(CH3)NC6H4C10H21)2} we suggest that a structural distortion due to steric hindrances exists for this compound.

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.

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.

Metallo-mesogens based on ortho-palladated polar imines: III. Influence of the polar group position

Abstract In order to study the influence of the molecular position of the polar group on the liquid crystal behaviour of palladium containing complexes, four series of complexes of the type Pd2(μ - Y)2{p-X-C6H3-CH = N-C6H4-OC8H17}2 and Pd2(μ - Y)2{p-C8H17O-C6H3-CH = N-C6H4-X}2; X:-H, -CH3,-OCH3, -Cl, -NO2, -CN and Y: -OOC.CH3, -Cl have been synthesized. A comparative study has revealed that the rigid central core is mainly responsible for the mesogenic behaviour of these complexes (acetato-bridged compounds do not form liquid crystals) and the polar group position only modifies this behaviour slightly. However, it has been observed that the nature of the anilinic ring substituent has a significant influence on the mesogenic properties of the chloro-bridged complexes.

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.

Ferroelectric liquid crystal behavior in platinum orthometallated complexes

Di-μ-chloro-bis[(η3-2-methylallyl)platinum] reacts with chiral imines, p-R1C6H4CHNC6H4OR2-p [HL*; with R1, R2, or both equal to either (R)- or (S)-1-methylheptyl], to yield the C,N-cyclometallated platinum(II) compounds [Pt2(μ-Cl)2(L*)2] (2). Treatment of 2 with Tl[(p-H21C10OC6H4CO)2CH] produces mononuclear orthoplatinated complexes (3) incorporating a β-diketonate. Both complexes 2 and 3 are mesomorphic materials that display smectic A (SmA) and chiral smectic C (SmC*) phases. Their mesogenic properties are compared with those of their palladium analogues, reported previously. The platinum complexes show somewhat more stable mesophases. A study of the ferroelectric properties of compounds 2 and 3 has been performed.

Fabrication and nonlinear optical properties of monodomain polymers derived from bent-core mesogens

Monodomain polymers derived from bent-core mesogens were obtained using the in situ photopolymerization technique under an in-plane electric field. The orientation of the samples was characterized using x-ray diffraction. It was found that the samples presented homeotropic alignment and ferroelectric ordering. Stable second-harmonic generation response was found at room temperature and under no field. The second-order susceptibility tensor was fully determined.

Determination of the helical pitch in the smectic C*α phase by means of gyrotropy measurements

The existence of a short pitch helical structure in the smectic C* α phase has been confirmed in two compounds. This conclusion has been reached by using optical activity measurements along the helix axis and along a direction perpendicular to it. No measurable gyrotropy is detected along the helix, in agreement with earlier results. However, a rather large optical activity appears in the perpendicular direction. This optical rotation is difficult to detect, because it coexists with birefringence, and their separation requires the use of non-standard polarimetric techniques. The optical activity data, together with additional birefringence measurements performed simultaneously, have allowed us to obtain the helical pitch and the molecular tilt of the materials. The pitch is in the range 80-40 nm in the compounds studied, and its variation with temperature shows some irreproducible features on heating. On cooling from the smectic A phase, however, the pitch behaviour is perfectly reproducible.

ortho-Palladated metallomesogens: the effects of structural modifications on their liquid crystalline behaviour

The influence of structural changes such as the length, nature or number of terminal chains and the nature of the intermetallic bridge, on the liquid crystalline properties of palladium complexes [Pd(µ-X)Lj]2 (X–= CH3COO–, Cl–, Br–) derived from imines is reported. Acetato bridges are unfavourable for liquid crystal behaviour, but both halogen bridges clearly promote mesophase formation; SA and SC where X is Cl, and SA and N where X is Br. As far as the terminal tails are concerned, three main conclusions can be drawn: (i) the longer the terminal chains, the lower the melting points and the more ordered the mesophases, (ii) alkoxy tails increase the melting points but stabilize more ordered phases in comparison with alkyl tails, and (iii) the presence of a terminal substituent in the freely rotating anilinic ring has a more favourable effect on mesophase formation than when it is in the fixed (ortho-palladated) aldehydic ring. X-Ray experiments in the mesophase have revealed that molecular interdigitation is only present for the complexes containing two tails.