A new azobenzene‐group‐containing monomer and several respective functional side‐chain polymers grafted on a methylhydrosiloxane backbone (with two different degrees of polymerization; with and ...without the addition of a photoreactive benzophenone derivative) are designed, synthesized, and characterized. The resulting materials clearly show self‐assembly behavior and possess a nematic liquid‐crystal phase over a broad temperature range, which extends below 0 °C. The optical properties of these new photochromic liquid‐crystalline materials are determined from the absorbance spectra of oriented samples and by photoinduced birefringence studies. The results indicate a considerable dichroism of the side‐chain liquid‐crystalline polymers (SCLCPs), and hence demonstrate their potential applicability for optical storage.
A new azobenzene‐group‐containing monomer and respective functional side‐chain polymers grafted on a methylhydrosiloxane backbone are designed, synthesized, and characterized. The resulting polymers possess a nematic liquid‐crystal phase over a broad temperature range, which extends below 0 °C. The side‐chain liquid‐crystalline polymers exhibit considerable dichroism, and hence demonstrate their potential applicability for optical storage.
The voltage dependence of the effective birefringence with corresponding textures for a 1:1 mixture of bent‐core and rod‐shaped molecules measured in a 5 µm film with planar alignment coating is ...measured (see figure). The data are fitted assuming rotation of the second director around the main director as illustrated in the lower part. Rectangles indicate the tilt‐plane of the bent‐core and rod‐shape molecules.
Perfectly alternating copolymers can be synthesised by reacting diacrylates with terminal dienes using alternating diene metathesis polycondensation (ALTMET). This novel method is very versatile and ...allows preparation of diverse structures of polymers, including main‐chain liquid crystalline polymers.
Main‐chain liquid crystalline polymer obtained by ALTMET.
New banana‐shaped monomers 1a–1f, based on the 1,3‐phenylene bis{4‐4′‐(alkenyloxy)benzoyloxy}benzoate bent structure were prepared. The central 1,3‐phenylene ring was either unsubstituted (1a, 1b) or ...had a 2‐methyl substituent (1c, 1d) or a 4‐chloro substituent (1e, 1f), respectively. The occurrence of mesophases greatly depended on the chemical structure of the banana‐shaped molecules. Whilst 1,3‐phenylene bis{4‐4′‐(10‐undecenyloxy)benzoyloxy}benzoate (1b, acronym Pbis11BB) formed a monotropic B2 phase, the 4‐chloro derivatives showed conventional nematic and SmC phases. In contrast, the 2‐methyl derivatives did not form any kind of mesophases. MOPAC/AM1 semi‐empirical calculations allowed evaluation of the value and orientation of the electric dipole vectors and of the twist angles of the central phenyl ring with respect to the outer rings. Electro‐optical investigations showed that the B2 phase of 1b was antiferroelectric with a 45° tilt angle between 91–75 °C (P = 320 nC · cm−2) and a switching time in the range of 50 μs in a E = 15 V · μm−1 field.
Synthetic route for the preparation of monomers 1a–1f.
ClPbis11BB and Pbis11BB, two banana-shaped mesogens differing by a chlorine substituent on the central phenyl ring, show a nematic and a B2 phase, respectively. To obtain information on the ...structural features responsible for their different mesomorphic behavior, a study of the preferred conformations of these mesogens has been performed by NMR spectroscopy in two nematic media (Phase IV and ZLI1167), which should mimic the environment of the molecules in their own mesophases, avoiding problems of sample alignment by a magnetic field. To this aim, 2H NMR experiments have been performed on selectively deuterated isotopomers of ClPbis11BB and Pbis11BB and of two parent molecules, ClPbisB and PbisB, assumed as models in previous theoretical and experimental conformational studies. We found that only a limited number of conformations is compatible with experimental data, often very different from those inferred from theoretical calculations in vacuo, indicating a strong influence of the liquid crystalline environment on molecular conformation. No significant differences between chlorinated and non-chlorinated molecules were found, this suggesting that chlorine does not change the molecular conformational equilibrium, as previously proposed.
A series of liquid crystalline compounds based on 2,7‐bis(biphenyl)fluorenes was synthesised by a Suzuki cross‐coupling reaction and subsequent etherification with different bromoalkenes. By simple ...variations of the chain lengths the temperature range in which the nematic mesophase is present could be tuned. The described approach offers the possibility of covalent incorporation of the mesogens into polymers via olefin metathesis techniques, resulting in enlarged stability of the mesophase. Monomers and polymers could be easily aligned on rubbed polyimide substrates as revealed by polarised absorption spectroscopy.
Two families of ester‐type banana monomers are presented, 1,3‐phenylene bis{4‐4′‐(10‐undecenyloxy)benzoyloxybenzoate}s and 1,3‐phenylene bis{4′‐(10‐undecenyloxy)‐1,1′‐biphenyl‐4‐carboxylate}s, in ...which the nature of the substituents on the central phenyl ring and the side arms was varied. The mesophase behavior of the monomers, including B2 and B7 phases, was correlated with their chemical structure and was compared with that of analogous azomethine‐type banana mesogens. It is also shown that the banana monomers can be incorporated into new architectures of liquid crystal polymers.
Accelerating progress in the discovery of new bent-core liquid crystal (LC) materials with enhanced features relies on the understanding of structure-property relationships that underline the ...formation of LC phases. The aim of this study was to develop a model for the prediction of LC behaviour of five-ring bent-core systems using a QSPR approach that combines dimension reduction techniques (e.g. genetic algorithms etc.) for the selection of molecular descriptors and decision trees, multivariate adaptive regression splines (MARS) and artificial neural networks (ANN) as classification methods. A total of 27 models based on separate pools of calculated molecular descriptors (2D; 2D and 3D) and published experimental outcomes were evaluated. Overall, the results suggest that the acquired ANN LC classifiers are usable for the prediction of LC behaviour. The best of these models showed high accuracy and precision (91% and 97%). Since the best classifier is able to successfully capture trends in a homologous series, it can be used not only to screen new bent-core structures for potential LCs, but also for the estimation of influence of structural modifications on LC phase formation, as well as for the evaluation of LC phase stability.
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