Banana-shaped Liquid Crystals
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.
| Main Author: | |
|---|---|
| Other Authors: | |
| Format: | Doctoral thesis biblioteca |
| Language: | English |
| Subjects: | crystals, liquid crystals, physicochemical properties, fysicochemische eigenschappen, kristallen, vloeibare kristallen, |
| Online Access: | https://research.wur.nl/en/publications/banana-shaped-liquid-crystals |
| Tags: |
Add Tag
No Tags, Be the first to tag this record!
|