Stable Ferroelectric Liquid Crystals Derived from Salicylaldimine-Core

Cholesteric mesophase based 1D photonic materials from self-assembly of liquid crystalline block and random terpolymers containing chromonic molecules

We describe the influence of competing self-organizing phenomena on the formation of cholesteric mesophase in liquid crystalline brush block terpolymers (LCBBTs) and liquid crystalline random brush terpolymers (LCRBTs) containing chromonic molecules. A library of LCBBTs and LCRBTs are synthesized using ring-opening metathesis polymerization (ROMP) of norbornene side-chain functionalized monomers comprising cholesteryl mesogen (NBCh9), chromonic xanthenone (NBXan), and poly(ethylene glycol) (NBMPEG). Compression molded films of LCRBTs containing chromonic molecules display multilevel hierarchical structure in which cholesteric mesophase co-exists with π-π stacking of the chromonic mesophase along with PEG microphase segregated domains. This is unexpected as conventional LCBCPs and LCBBCs that lack chromonic molecules do not form cholesteric mesophases. The presence of π-π interactions modifies the interface at the IMDS so that both chromonic and cholesteric mesophases coexist leading to the manifestation of cholesteric phase for the first time within block architecture and is very reminiscent of previously published LCRBCs without chromonic molecules. The key to the observed hierarchical assembly in these LCBBTs containing chromonic molecules lies in the interplay of LC order, chromonic π-π stacking, PEG side chain microphase segregation, and their supramolecular cooperative motion. This unique "single component" polymer scaffold transforms our capacity to attain nanoscale hierarchies and optical properties from block architecture similar to nanoscale mesophases resulting in random architecture.

Highly frustrated liquid crystal phases in optically active dimers: synthesis and rich phase transitional behavior

New cholesterol-based Schiff base dimers exhibit a rich mesomorphism.

Ferroelectric Liquid Crystals: Synthesis and Thermal Behavior of Optically Active, Three-Ring Schiff Bases and Salicylaldimines

The chiral ferroelectric smectic C (SmC*) phase, characterized by a helical superstructure, has been well exploited in developing high-resolution microdisplays that have been effectively employed in the fabrication of a wide varieties of portable devices. Although, an overwhelming number of optically active (chiral) liquid crystals (LCs) exhibiting a SmC* phase have been designed and synthesized, the search for new systems continues so as to realize mesogens capable of meeting technical necessities and specifications for their end-use. In continuation of our research work in this direction, herein we report the design, synthesis, and thermal behavior of twenty new optically active, three-ring calamitic LCs belonging to four series. The first two series comprise five pairs of enantiomeric Schiff bases whereas the other two series are composed of five pairs of enantiomeric salicylaldimines. In each pair of optical isomers, the configuration of a chiral center in one stereoisomer is opposite to that of the analogous center in the other isomer as they are derived from (3 S)-3,7-dimethyloctyloxy and (3 R)-3,7-dimethyloctyloxy tails. To probe the structure-property correlations in each series, the length of the n-alkoxy tail situated at the other end of the mesogens has been varied from n-octyloxy to n-dodecyloxy. The measurement of optical activity of these chiral mesogens was carried out by recording their specific rotations. As expected, enantiomers rotate plane polarized light in the opposite direction but by the same magnitude. The thermal behavior of the compounds was established by using a combination of optical polarizing microscopy, differential scanning calorimetry, and powder X-ray diffraction. These complementary techniques demonstrate the existence of the expected, thermodynamically stable, chiral smectic C (SmC*) LC phase besides blue phase I/II (BPI or BPII) and chiral nematic (N*) phase. However, as noted in our previous analogous study, the vast majority of the Schiff bases show an additional metastable, unfamiliar smectic (SmX) phase just below the SmC* phase. Notably, the SmC* phase persists over the temperature range ≈80-115 °C. Two mesogens chosen each from Schiff bases and salicylaldimines were investigated for their electrical switching behavior. The study reveals the ferroelectric switching characteristics of the SmC* phase featuring the spontaneous polarization (P_S ) in the range 69-96 nC cm^-2 . The helical twist sense of the SmC* phase as well as the N* phase formed by a pair of enantiomeric Schiff bases and salicylaldimines has been established with the help of circular dichroism (CD) spectroscopic technique. As expected, the SmC* and the N* phase of a pair of enantiomers showed mirror image CD signals. Most importantly, the reversal of helical handedness from left to right and vice versa has been evidenced during the N* to SmC* phase transition, implying that the screw sense of the helical array of the N* phase and the SmC* phase of an enantiomer is opposite.

Effect of the Functional Groups of Racemic Rodlike Schiff Base Mesogens on the Stabilization of Blue Phase in Binary Mixture Systems

Four series of rodlike racemic Schiff base mesogens possessing different alkyl chains and two types of linkages, ester and alkynyl linkages, were synthesized and applied to induce cubic blue phases (BPs) in simple binary mixture systems. The mesophases of these Schiff base mesogens were confirmed by variable-temperature X-ray diffraction and the characteristic texture from polarized optical microscopy (POM). In general, when chiral additive S-(+)-2-octyl 4-(4-hexyloxybenzoyloxy)benzoate (S811; 20-40 wt %) is added into the rodlike racemic salicylaldimine-based mesogens, the cubic BPs could be observed and its temperature range is larger than 20 K. The widest temperature range of the cubic BP (35 K) can be observed in the blending mixture composed of rodlike racemic salicylaldimine-based mesogen OH-TI_n possessing alkynyl linkage and 35-40 wt % S811. However, Schiff base mesogens possessing alkynyl linkage show a direct isotropic to chiral nematic transition when equal amount of chiral dopant is added. Notably, the termination temperature of BPs is very close to room temperature (ca. 35 °C) after 40.0 wt % S811 is added into the salicylaldimine-based mesogens possessing terminal alkyl chains and ester linkage. Interestingly, wide BPs (>30 K) can also be induced by adding chiral additive 1,4:3,6-dianhydro-2,5-bis[4-(n-hexyl-1-oxy)benzoic acid]sorbitol (ISO(6OBA)₂) with a high helical twisting power into the racemic Schiff base mesogen possessing ester linkage. Cubic BPI and BPII can be confirmed by reflectance spectra and POM. The results of reflectance spectra indicate that the binary mixture composed of salicylaldimine-based mesogens and S811 easily exhibits a supercooling effect and induces BPI. However, only BPII can be observed in all binary mixtures containing Schiff base mesogens. On the basis of our experimental results and molecular modeling, we suppose that the values of biaxiality, polarizability, and the dipole moment of molecular geometry are the main factors that affect BP stabilization.