Two vinyl-terminated bent core-shaped liquid crystalline molecules that exhibit thermotropic antiferroelectric SmCPA phases have been covalently attached onto a hydrogen-terminated silicon(111) surface. The surface attachment was achieved via a mild procedure from a mesitylene solution, using visible light at room temperature. AFM measurements indicate that a smooth monolayer has been formed. The thickness of the monolayer was evaluated with ellipsometry and X-ray reflectivity. Although the molecules differ in length by four carbon atoms, the thickness of the resulting monolayers was the same. The measured thicknesses correspond quite well with the smectic layer thickness in the bulk liquid crystalline material, suggesting a similar self-organization within the monolayer. From attenuated total reflectance infrared (ATR-IR), which clearly shows the C−H and C═O vibrations, a tilt angle of the mesogens is deduced that also corresponds well with the tilt angle in the liquid crystalline state. X-ray photoelectron spectroscopy (XPS) measurements confirm the high quality of the monolayers, with only marginal silicon oxide formation. The elemental composition and amounts of different O and C atoms deduced from the high-resolution XPS correspond very well with the calculated compositions.
ABSTRACT The phase retardation of several banana type liquid crystals in their B1 and B2 phases is measured using a temperature controlled shear cell. In different phases the LC-cells are sheared to improve their order and the phase retardation before and after shearing is compared. It is shown that the phase retardation increases and the domain size enlarges as a result of shearing.
The synthesis and mesomorphic properties of 30 bent‐core compounds with a fluorine substituent in one of the outer rings are reported. The banana‐shaped compounds are all derived from resorcinol and contain esters as linking groups between the five aromatic rings. The different mesophases have been characterized by polarizing optical microscopy, differential scanning calorimetry, X‐ray diffraction studies and electro‐optical investigations. The compounds with the longer terminal chains exhibit an antiferroelectric SmCP phase. Upon introduction of a fluorine substituent the layer spacing increases, as compared with the corresponding unsubstituted compound. The introduction of one terminal vinyl group in the mono‐substituted bent‐core mesogens has no significant influence on the liquid crystalline properties and the switching remains antiferroelectric. Due to the introduction of the terminal double bond, these banana‐shaped compounds are suitable for the preparation of siloxane polymers or attachment to a hydrogen‐terminated silicon surface.
Two series of asymmetric banana-shaped compounds have been synthesized and studied. In the 1,3-phenylene bis[4-(4′-alkoxybenzoyloxy)]benzoate series the lack of symmetry was derived solely from the difference in length of the two terminal alkoxy chains. In the 3,4′-biphenylene bis[4-(4′-alkoxybenzoyloxy)]benzoate series the asymmetric nature originates from the 3,4′-substitution of the central biphenyl group and from the difference in length of the two terminal chains. All the melting points of the asymmetrical compounds in the series with the central phenyl unit are lower than those of the symmetrical compounds. The liquid crystalline B1 or B2 phase was retained in all cases. In the series with the central biphenyl unit the compounds with the shortest chain attached to the para-position of the central biphenyl unit have the lowest melting points. A significant lowering of the melting points in comparison with the symmetrically substituted compounds, however, could not be achieved. All the compounds of both series show a layer spacing which is comparable to those of the symmetrically substituted parent compounds. The observed switching behaviour of both the symmetric and asymmetric compounds with a B2 phase was antiferroelectric.
The synthesis and thermotropic properties of four homologous series of salicylaldimine-based dimer liquid crystals are reported. Two 4-(4-alkoxy-2-hydroxybenzylideneamino)benzoyloxy groups are connected to a central part consisting of a 1,3-phenylene, 1,5-pentylene, 2,2-dimethyl-1,5-pentylene or 3,3-dimethyl-1,5-pentylene unit. The terminal alkoxy chains have been varied from 4 to 16 carbon atoms in length. All the compounds exhibit liquid crystalline phases whose behaviour depends on the nature of the central part and the length of the alkoxy terminal chains. All compounds of the series with the central phenyl part exhibit enantiotropic B-phases, and the sequence B6–B1–B2 on increasing terminal chain length was observed. Replacement of the phenyl group with a pentyl central group partly suppresses the formation of B-phases. The longer homologues of this series show the B1 phase, while the shorter exhibit an intercalated SmCc mesophase. The introduction of methyl substituents to the pentyl spacer causes the melting points to fall dramatically and the formation of B-phases is totally suppressed. The compounds with the long tails show intercalated SmAc phases and those with short tails show intercalated SmCc phases.
Eight banana‐shaped side chain liquid crystalline oligomers and polymers have been synthesized by hydrosilylation of vinyl‐terminated bent‐core mesogens with trimethylsilyl‐terminated siloxanes. The synthesized oligomers and polymers, and their olefinic precursors, were investigated by polarizing optical microscopy (POM), differential scanning calorimetry, X‐ray diffraction (XRD), electro‐optical experiments and Maldi‐Tof. The short‐tailed olefins form a Colr mesophase, whereas those with longer chains exhibit the SmCPA mesophase. All the oligomers and polymers studied show liquid crystalline properties and do not crystallize upon cooling. Most oligomers with around four repeating siloxane units, show a lamellar (layer) structure and antiferroelectric switching properties, the SmCPA phase. XRD shows that the layer spacings are hardly influenced by the length of the terminal tails. The oligomer prepared from the smallest olefinic precursor, having the shortest alkyl tail, shows an XRD pattern reminiscent of a columnar phase, although POM displays domains of opposite chirality, and no switching behaviour could be detected. The polymers with around 35 repeating siloxane units are liquid crystalline, but due to their high viscosity a thorough characterization of the liquid crystalline phases was impossible.
This thesis describes the liquid crystalline properties of molecules with a bent shape. The objective of the research is to allow further insight in structure-property relationships for this class of liquid crystals. Specifically, we are interested in chemically stable compounds with switchable mesophases in order that they can potentially be used in display devices, as a possible alternative for chiral smectic-C compounds. One important objective is to obtain materials with the desired mesophases at temperatures as close to room temperature as possible. Another objective is to obtain materials that can be attached to polymer backbones or hydrogen-terminated silicon surfaces.In chapter 1 an overview of liquid crystals is given with a focus on bent-shaped (or banana-shaped) compounds. Bent-core mesogens often contain a 1,3-disubstituted phenyl or a 3,4'-disubstituted biphenyl group, which causes the bend in the molecules. These achiral bent molecules can form chiral phases, due to a combination of tilted molecules in layers, and a polar component perpendicular to the director of the bent molecules. There are four possibilities for the molecules to arrange themselves in the so-called SmCP phase; the polarization direction can be parallel or antiparallel in adjacent layers, giving rise to ferroelectricity (P F ) and antiferroelectricity (P A ), respectively. Secondly, the molecules can tilt in syn (C S ) or anti (C A ) fashion in successive layers. This results in two macroscopic chiral organizations (SmC S P F and SmC A P A ) and two racemic organizations (SmC A P F and SmC S P A ). In principle the antiferroelectric ground state is energetically preferred due to the escape from macroscopic polar order, and the possibility of out-of-plane interlayer fluctuations.In order to investigate whether banana-phases could also be obtained in dimeric molecules with an odd number of flexible units in the spacer, three series of compounds were described in chapter 2 . All three series show a pronounced odd-even effect in the isotropization temperatures with the parity of the spacer. Upon increasing the terminal tail length, the nematic phase was suppressed as was predicted by previous literature. Furthermore, the compounds with an odd number of flexible units between the mesogens did not show features of banana-phases, as was reported for some similar series. A reversal of the ester connecting groups between the aromatic rings resulted in a change from a monolayer smectic organization to (partly) intercalated smectic organizations.In chapter 3 , the liquid crystalline properties of four series of salicylaldimine-based dimers is studied. The bend in the molecules is obtained by connecting the mesogenic units via1,3-phenylene, 1,5-pentylene, 2,2-dimethyl-1,5-pentylene or 3,3-dimethyl-1,5-pentylenegroups. As observed in several other series a 1,3-disubstituted phenyl group promotes formation of B-phases. Upon increasing the terminal chain length the phase sequence B 6 -B 1 -B 2 (SmC int -Col r -SmCP) is observed. If the central phenyl group was replaced by a more flexible odd spacer, the switchable SmCP phase completely disappeared. Comparison of unsubstituted pentyl spacers with dimethyl substituted pentyl spacers, showed that the latter promoted intercalated smectic phases. Furthermore, these methyl substituents suppressed the melting temperatures.In chapters 4, 5 and 6 , five-ring banana-shaped molecules with esters as linking groups between the rings, and a central 1,3-substituted phenyl group, were studied. This parent-structure was modified in different manners and the influence of these modifications on the liquid crystalline properties was investigated.Bananas withtwo terminal alkoxy tails of different lengthwere compared to their symmetric analogues in chapter 4 . The non-symmetric compounds showed lower melting points while the influence on the isotropization temperatures was small. The switchable SmCP phase was retained. From comparing symmetrical and non-symmetrical bananas with an equal number of carbon atoms in their terminal chains, it was concluded that non-symmetry only slightly destabilizes the SmCP phase whereas the melting points are decreased considerably, thereby increasing the liquid crystalline range. Replacement of the central phenyl group with a 3,4'-disubstited biphenyl group, showed that for this series the melting points can not be lowered by introducing two terminal alkoxy tails of different length. The mesophase range for the biphenyl compounds is significantly larger than for the phenyl analogues, mainly due to higher isotropization temperatures.In chapter 5 , the liquid crystalline properties of compounds with one terminal vinyl group were studied. The unsaturated compounds showed the same mesophase and switching behavior as their saturated analogues described in chapter 4 . The SmCP phase was slightly destabilized however.The influence of a fluorine substituent at the ortho position with respect to the alkoxy group in one of the outer aromatic groups, on the liquid crystalline properties of banana-shaped mesogens, is described in chapter 6 . In contrast to the difluorine substituted bananas, which show ferroelectric switching behavior, the compounds with one fluorine substituent exhibited antiferroelectric switching. The SmCP phase was slightly stabilized if compared to the non-substituted analogues, since the isotropization temperatures increased upon introduction of one (and also a second) fluorine substituent.For all bananas described in chapters 4, 5 , and 6 , the liquid crystalline properties were only slightly changed when compared to the non-modified parent-structure. The switchable SmCP mesophase was in most cases retained, and the melting points could be lowered to about ~80°C by asymmetric tail lengths combined with a terminal vinyl group.Polymerization of banana-shaped compounds could in principle give a system showing (switchable) mesophases at even lower temperatures. Therefore, the mono-unsaturated bananas described in chapter 5 have been used to prepare siloxane polymers. The liquid crystalline properties of these polymers, one series with a relatively short and a second series with longer trimethylsilyl terminated polysiloxane backbones, are described in chapter 7 . Most of the "short" polymers exhibited the SmCP A phase, with the same switching properties as their olefinic precursors. Crystallization could not be observed, but upon cooling from the SmCP phase a transition to another unidentified mesophase was detected. The longer siloxanes also exhibited two mesophases. However, due to the high viscosity these compounds could not be characterized by polarization optical microscopy and switching experiments, but a lamellar ordering is likely for these materials.In chapter 8 a vinyl-terminated banana from chapter 5 and two fluorine substituted bananas from chapter 6 , were covalently attached to silicon surfaces via an extremely mild method. The two bananas with the fluorine substituent are isomers and only differ in the position of the substituent: close, or at a more distant position from the double bond. The presence and quality of these banana-shaped self-assembled monolayers was investigated, and indeed showed the presence of a monolayer with a thickness comparable to the d -spacing of the corresponding banana molecules in bulk. Moreover, the monolayers of the fluorine substituted bananas were investigated with angle-resolved X-ray photoelectron spectroscopy. These measurements showed that the fluorine substituents could be detected on positions in the monolayer with respect to the silicon bulk that were expected, based on the molecular structure of the molecules.
Abstract Three series of symmetrical dimer liquid crystals, i.e. bis(dodecyloxybenzoyloxybenzoyloxy)alkanes (BC12) and bis(dodecyloxy- and hexadecyloxybenzoyloxyphenyl) alkanedicarboxylates (QC12 and QC16) with a spacer ranging from two to eight methylene units, have been synthesized and their thermotropic properties were characterized. The isotropization temperatures show a pronounced odd - even effect as a function of the parity of the spacers. Most compounds exhibit a SmC mesophase. The QC12 and QC16 compounds, with an eight methylene spacer, possess an additional low temperature tilted SmI or SmF mesophase. Changing the orientation of the ester group as in the BC12 series results in a dramatic change of the thermotropic behavior. Keywords: Layer SpacingTwin Liquid CrystalX-ray
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