Shape-persistent V-shaped mesogens—formation of nematic phases with biaxial order

Abstract

A homologous series of shape-persistent V-shaped molecules has been designed to form the biaxial nematic phase. Phenyleneethynylene moieties are attached to a bent fluorenone unit to create an apex angle of about 90u, which is determined from the single crystal structure. Two mesogens, one symmetric and another unsymmetric, have been synthesized by attaching a cyano group to one or both of the peripheral phenyl units, respectively. These groups introduce local dipoles essential for the formation of the nematic phases. The tendency to form a crystalline phase is reduced by laterally substituted hexyloxy chains which allow the nematic phase to be supercooled to a glassy state. Two of the three fluorenone derivatives exhibit a transition from the uniaxial nematic to the biaxial nematic phase. This transition has an undetectably small transition enthalpy, but the X-ray diffraction, polarizing optical microscopy, and conoscopy reveal the presence of the biaxial order in the low temperature nematic phase. Results and discussion SynthesisAn optimised strategy, shown in Scheme 1, was applied for the synthesis of unsymmetric phenyleneethynylene oligomers. The key precursor in the synthesis scheme is the 4-bromo-2,5dihydroxyiodobenzene 3. 13 After etherification with hexylbromide, unsymmetric compound 5 can be synthesised using a

Computer simulations of biaxial nematics

“…Kumar and coworkers have reported another class of rigid bent-core mesogens with aperture angle of 90 degrees [46] which appear to also form N b phases.…”

“…A large apex angle generally favours the N u with respect to layered organisations, while small values have given interlocked dimers (even though Ref. [46] reports N b mesogens with 90 degrees aperture). Certain intermediate apex angles destroy the N u [131,134,135].…”

Optical Simulation of Viewing Angle Property of Biaxial Nematic Bent-Core Liquid Crystal

“…Although there have been many efforts to develop the Nb-material which operates at room temperature, few materials with a wide nematic phase around room temperature has been reported [22]. With this reason, the electrooptical properties of the BLC molecules for the display applications have been focused in the smectic or isotropic phases [23][24][25].…”

Biaxial nematics: symmetries, order domains and field-induced phase transitions†

“…Biaxial order observations by IR spectroscopy on the same compounds (10), using surface-aligned samples, showed a nematic-nematic phase transition which, however, did not appear in microcalorimetry measurements on un-aligned samples (39). The XRD studies on both types of thermotropic biaxial systems show typical cybotactic cluster difractograms (22,23,(24)(25)40) analogous to those observed in some conventional uniaxial nematics and therefore do not exclusively imply a biaxial monodomain sample. The electric field response observed by XRD in surface aligned samples of bent-core nematics was interpreted as switching of the tranverse axis (22) of the biaxial sample.…”

“…Within this model, a number of recent experimental observations on thermotropic nematic phase biaxiality can be rationalized and interpreted consistently. These include observations of the intriguing electro-optic behaviour of bent-core nematics (14,41,(45)(46), biaxiality measurements by 2 H-NMR in bent-core and in calamitic-tetramer nematics, local biaxial structures revealed in XRD of a variety of bent-core and laterally substituted calamitic nematics (22)(23)(24)(25)40) and phase biaxiality measurements by IR sperctroscopy in calamitic tetramers (10) in relation to the results of calorimetry studies (39) on the same compounds. Furthermore, in the context of the cluster model, the above observations can be naturally related to direct observations of domain structures in bent-core nematics (18) and to the results of previous studies which invoked the presence of local structures in order to interpret the behavior of bent-core uniaxial nematics (47).…”