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
Nematic phases, applied in almost all commercial liquidcrystal (LC) displays, are LC phases with mesogens isotropically distributed and exhibiting orientational long-range order of one preferred molecular axis along a common direction defined by the so-called director. These are the phases with the lowest viscosity and besides the anisotropic properties, are most similar to isotropic liquids. Biaxial nematics should realise a long-range orientational order of all three molecular axes along three mutually perpendicular directors, but maintain the isotropic distribution of the molecular centres of gravity and their molecular mobility. Such phases are not only appealing from the viewpoint of basic research and theoretical modelling, but are also of technological interest to speed up the switching of LC displays. [1,2,3] After the prediction by Freiser, [4] Saupe and Yu were the first to discover such mesophases in a narrow region of the phase diagram of a lyotropic LC.[5] The much-pursued synthesis of molecules for board-shaped mesogens, though suitable for forming thermotropic biaxial nematic phases, did not, however, reveal a widely accepted biaxial mesophase. [6] Only recently, biaxial nematic phases of multipodes [7] and polymers [8] with side-on attached nematogens, the rotation of which about the long axis is hindered, could be confirmed. In the area of low-molecular-weight mesogens, theory predicted V-shaped mesogens [1,9] and mixtures of rod-and disc-shaped molecules [1,10] as potential candidates for biaxial nematic phases. Indeed, the biaxial nematic phase at high temperatures, above 100 8C, was found in a series of oxadiazole derivatives, [11] the results of which were most widely accepted and confirmed. [12] We were interested in parameters controlling biaxiality in such phases and focused therefore on shape-persistent molecules 1 with a well-defined bending angle (Scheme 1), [12][13][14] which is supposed to be a principal parameter in theoretical predictions. [1,9] They were designed to self-assemble exclusively in nematic phases owing to the substitution pattern of alkoxy chains.[13] However, only planar bending units with a dipole along the bisector revealed nematic phases.
A series of biaxial V-shaped, shape-persistent molecules has been synthesised by stepwise coupling of phenylene ethynylene arms to an oxadiazole bending unit. Studies of their thermotropic nematic phases point to phase biaxiality.
Deuterium and carbon-13 NMR spectroscopy were used to study both the high temperature uniaxial nematic and the low temperature biaxial nematic glass of a shape-persistent V-shaped mesogen. It was found that biaxial ordering determined in the domains of the latter has symmetry lower than D(2h) and is compatible with C(2h) symmetry or lower. In particular, elements of the ordering matrix including biaxial phase order parameters were determined from (2)H NMR at two temperatures, one just below the glass transition, and the other deep inside the biaxial glass, which allowed for the characterization of the dominant molecular motions at these temperatures. (13)C NMR magic angle spinning sideband patterns, collected both in the high temperature nematic phase and in the nematic glass, clearly show the difference between them in terms of the phase symmetry.
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