Liquid Crystalline Switches

A detailed understanding of the influence of the structure, charge, and counterion on the thermal and mesomorphic properties requires the preparation and further study of ferrocenium materials. The above-reported results prove that liquid-crystalline switches should, in principle, be available by the incorporation of ferrocene/ferrocenium into mesomorphic materials. 

Irie and coworkers utilized this aryl framework in a wide variety of molecular switches, ranging from photo-electrochemical switches to liquid-crystalline switches. One outstanding example is the reversible switching of surface morphology in single crystals of 5 (/ i = Ph, i 2 = (Fig. 3). 

Mesomorphic dendrimers containing electroactive units have potential for construction of dendrimer based molecular switches. Deschenaux et al. reported [154] the synthesis and liquid-crystalline properties of a novel dendrimer containing six mesomorphic ferrocene units. Apart from exhibiting a broad enantiotropic smectic A phase as determined by polarized optical microscopy, DSC, and XRD studies, thermogravimetry revealed the excellent thermal stability of the macromolecule. 

Thermostats, mercury-switch, 16 42 Thermosyphon, 13 225, 226 Thermosyphon reboiler, 11 774 Thermotropic LCPs, 13 381-382 Thermotropic liquid crystalline compounds, 15 87-89t... 

A conglomerate in real liquid crystalline phases was first observed in the smectic phase of a rod-shaped mesogen with two stereogenic centers in its tail [42], We used a racemic mixture which was supposed not to electrically switch. Evidence for conglomerate formation was provided by clear electro-optic switching and texture observation under a polarizing microscope domains with stripes, which themselves display fine stripes. These stripes are tilted in two different directions with respect to the primary stripes. This is a still very rare example now that fluid soft matter is known to resolve spontaneously into a three-dimensional conglomerate. 

More modern, much faster responding liquid crystalline displays use ferroelectric liquid crystals such as p decyloxybenzylidine-//-amine-2-methylbulylcinnamale, which can be switched between two stable states by application of an external field. Such properties may also be useful in optical data storage. 

Switching of Liquid Crystalline Properties through Fulgide Photochromism I 117... 

Like in other chiroptical switches (Section 5.3.1), solvent polarity was found to play an important role. Diastereoselective cyclization was observed in THF and toluene, but not in nonpolar solvents such as n-hexane. Upon photoexcitation, diarylethenes 24 (Scheme 11) can adopt a planar and a twisted conformation, and photocyclization only proceeds through the planar conformation. In the case of chiral diarylethene 27a, there are two diastereomeric planar conformations leading to the diastereomers of the cyclic product 27b. The stereoselectivity in the photocyclization process is enhanced because of a decrease in the excited state energy of the unreactive twisted form, providing a relaxation pathway for the less favorable planar diastereoisomer in more polar solvents. Chiral photochromic diarylethenes are among the most prominent photoswitches known today, featuring nondestructive read-out, excellent reversibility, and the potential for construction of switchable molecular wires and modulation of liquid crystalline phases (see Section 5.5.3).[40,411... 

Scheme 26 Switching between three different liquid crystalline states after irradiation at one wavelength. Nematic liquid crystal 41 and dopant 8 were used.

The tetralins (181-183) collated in Table 3.23 are red liquid crystals with a transition moment orthogonal to the molecular long axis, see Figure 3.13. Since they are liquid crystalline themselves they are very soluble in nematic host mixtures. Such mixtures exhibit high negative dichroic ratios and can be used in White and Taylor GH-LCDs with positive contrast. Furthermore, mixtures containing an additional anthraquinone dye of positive dichroic ratio can be used to switch from one colour to another under the action of an electric field. 

ICP thin films were used as driving electrodes for polymer-Dispersed Liquid-Crystals (PDLC) display devices. Liquid-crystalline-based display devices, which are commonly made of a liquid-crystal compound sandwiched between two substrates coated with a conducting layer of indium tin oxide (ITO), whose substitution with ICP electrodes could improve the optical and mechanical properties of the display devices. On the way to all-organic displays, PDLC sandwiched between two plastic substrates coated with ICP layers are promising devices for paper-like displays for electronic books which require flexibility, lightness, and low-power consumption. The electro-optical characteristics (transmission properties, drive voltages and switching times) of the PDLC devices depend on the nature of the ICP substrate used [13]. 

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