Optically biaxial interdigitated smectic A phase: liquid crystalline dimeric bidentate ligands and their metal complexes

Abstract

A detailed study into the synthesis and characterization of achiral nonsymmetric dimeric bidentate ligands formed by combining salicylaldimine and cyanobiphenyl rodlike anisometric segments through a flexible spacer (varying in its length and parity) and their Cu(II) and Pd(II) complexes is reported. The spacer parity greatly influences the phase behavior of ligands; notably, the dimers with odd-parity spacer stabilize nematic, uniaxial smectic A and biaxial smetic A phases, while the even-members display nematic and / or smectic phase/s. Unlike ligands, both the metal complexes solely show nematic behavior. An odd-even effect, as noted for ligands, was observed for the nematic-isotropic transition temperatures and the associated entropy values of copper complexes, while such behavior was not found in the case of palladium complexes. Chart 1 Molecular structure of the most extensively studied nonsymmetric dimers (CBOn.Om series; see ref. 2 and 4).

Optically active, mesogenic lanthanide complexes: design, synthesis and characterisation

“…Thus, the prominent odd-even effect observed for the free ligands is reflected in the thermal behaviour of the complexes also; Figure 6 illustrates such an effect observed for the gadolinium(III) complexes, in which the even-members II-3, II-5 and II-7 exhibit the higher transition temperatures with the indication that the effect tends to attenuate on increasing the number of carbon atoms in the spacer (m = n+1). In fact this behaviour is in agreement with a recent observation that the copper complexes exhibit an odd-even effect for the nematic-isotropic transition temperatures (20). Furthermore, the lanthanum complex with a triflate counter anion (IV-5) shows a lower transition temperature when compared with nitrate complexes, which is in accordance with the results of an earlier report (23).…”

“…The loss of polymesomorphism in this ligand and reduction in the transition temperature can be interpreted in terms of loss of anisotropy owing to the odd number of carbon atoms leading to a bent molecular conformation. Thus, as observed commonly (15,20), the transition temperatures, especially the clearing points of the ligands, depend critically on the parity of the spacer, in which the odd-member exhibits the lower value. It must be mentioned here that the phase transitions of all these ligands are highly reproducible for any number of heating and cooling cycles.…”

Stereochemical Rules Govern the Soft Self‐Assembly of Achiral Compounds: Understanding the Heliconical Liquid‐Crystalline Phases of Bent‐Core Mesogens

“…Optical investigation of homeotropically aligned samples (layer planes organized parallel to the substrate surfaces) between crossed polarizers show that at this phase transition the isotropic appearance of the homeotropic SmA phase is replaced by a gray schlieren texture as typical for the transition to a biaxial smectic phase (Figure a–d). The developing periodic stripe pattern (yellow arrows in Figure d) is typical for biaxial SmA b phases as well as for anticlinic tilted SmC a phases . The presence of 4‐brush disclinations (red arrow) between the stripes is in favour for an anticlinic SmC a structure of this biaxial smectic phase; in a non‐tilted biaxial SmA b phase only two‐brush disclinations would be expected .…”

Optically Biaxial, Re‐entrant and Frustrated Mesophases in Chiral, Non‐symmetric Liquid Crystal Dimers and Binary Mixtures

“…Furtherc ooling of the sample caused at ransition to another mesophase sharplya t5 3.6 8Cw ith an otable change in the optical textural pattern;a ss hown in Figure 7f,t he homeotropic (pseudo-isotropic) region and homogeneous (focal-conic) pattern of the preceding SmA phase transformt oalowb irefringence schlieren texture and as tripped focal-conic texture,r espectively.The schlieren texture in the homeotropic orientation is suggestive of the director field in the plane of the cells. These opticalt extural features are typical of an orthogonal biaxial SmA (SmA b )p hase [53,54,58,[73][74][75][76][77][78][79][80][81][82][83][84] which possessesa na dditional director (m)i nt he plane of the layers (see al ater section for details). For the sake of confirmation, the sample was examined in cells treated for homeotropic and homogeneous anchoring.…”

“…[69,70] In this phase, the mesogens within the layer are oriented along the layer normal( director n)a nd freely rotate about their long molecular axis. On the contrary,o ne of the most sought after phases in the SmA LCs is the SmA b phase [54,58,[73][74][75][76] as it is supposed to be av ery promising and viable mediumf or future advanced technologies. [84] In this phase, the constituent molecules are oriented along the layer normal( director n), but have an additional director (m)i nt he plane of the layers due to the restricted rotationo fm esogens aroundt heir long molecular axis.…”