Ferroelectric LC

Azomethine derived ferroelectric liquid crystals As DOBAMBC, many ferroelectric LC s were prepared utilizing amyl alcohol as the chiral source. The reason for the small spontaneous polarization of DOBAMBC is the separation between the C=0 dipole moment and the chiral carbon. This effect can be explained by the intramolecular rotation or vibration of the carbonyl dipole moment relative to the chiral carbon, because they are not adjacent. There are some rules linking the molecular structure and the direction of the spontaneous polarization (minus or plus). In order to reduce the separation between the carbonyl dipole moment and the chiral carbon, ferroelectric LC s were synthesized utilizing a secondary alcohol as the chiral source. Ferroelectric LC s with large spontaneous polarizations have large dipole moments at the chiral centre. Ferroelectric LC s with halogen or nitrile units connected directly to the chiral carbon were synthesized from chiral lactic acids or amino acids. 

To obtain fast LC photoresponse, a new guest/host system was developed, in which ferroelectric LCs (FLCs) were used as a host LC. FLCs exhibit spontaneous polarization (Ps) and show microsecond responses to change in applied electric field (flip of polarization) in a surface-stabilized state.1261 If a flip of polarization of FLC molecules in the surface-stabilized state can be induced by light in the presence of an applied electric field, photoresponse in the microsecond time region might be achievable. 

Liquid crystals are interesting supramolecular systems which can show second harmonic generation when they are aligned appropriately. Ferroelectric LCs [250] as well as bent-core molecules have been used to this purpose, and show reasonable second harmonic generation [251]. These materials combine non-linear optical effects with simple processing procedures on account of their liquid crystalline flow characteristics and the possibility of organising them with electric and magnetic fields. 

Photochemical control of properties of SmC LC phase was achieved by doping azobenzene A-4 possessing a chiral carbon atom to a ferroelectric LC A-5 [61]. When the SmC LC is in the surface stabilized state, the bulk dipole moment can be flipped by an external electric field. As the hysteresis curve for the Z form is narrower than that of E form, irradiation of UV light to cause E-... 

Figure 5 Polarization flip of ferroelectric LC (A-5) containing A-4 by photochq EIZ isomerization of A-4.

A chiral photochromic thioindigo T-l was reported by Lemieux et al. When it was doped in racemic LC T-2, it successfully altered the spontaneous polarization of the induced ferroelectric LC (Sc ) by photochemical E-Z isomerization [91,92]. 

Figure 7. ADMET-active monomer for the synthesis of main-chain ferroelectric LC oligomers n = 2—4.

Since the early days of ferroelectric low molar mass LC research, the exploration of the NLO properties of these substances has been a topic of interest. For a recent review on SHG in ferroelectric LCs, see [215]. Unfortunately, the SHG efficiency of most ferroelectric LCs investigated so far [216-219] is orders of magnitude below that of state-of-the-art inorganic crystals, such as lithium niobate. The main reason for the low electronic NLO activity is that the ferroelectric LC compounds and mixtures used in these investigations were optimized for display applications rather than for NLO performance. Quite recently, some research groups started the development of ferroelectric LC compounds specifically devoted to NLO applications, and indeed improved the SHG efficiency by orders of magnitude [220-224]. 

Quite recently, Schmitt et al. [223] presented highly efficient NLO ferroelectric LC materials (48) whose SHG properties were investigated in 13 pm thin, homeo-tropically aligned layers. 

In order to achieve amorphous polar solids, some research groups have explored ferroelectric SCLCPs, and have shown that the basic rules governing the relationships between molecular structure and macroscopic ferroelectric LC properties are the same [225-228]. The main difference between low molar mass and polymer ferroelectric LCs, however, is the existence of stable glassy phases in most of the latter. 

A.4.2. Photochemical Flip of Polarization and Alignment Change in Ferroelectric LCs. Photochromic reactions can also strongly influence the... 

Much attention has been paid to molecular design of effective guest molecules to induce the photochemical flip of polarization in ferroelectric LC systems. An azobenzene derivative with a chiral cyclic carbonate group (13) was designed to induce a large value of polarization and examined as a chiral dopant to induce a... 

Figure 3.19. Photochemical flip of polarization in ferroelectric LCs. Source Sasaki et al., 1994.

With a different ferroelectric LC-polymer it was even possible to obtain very homogenous FS-films of thicknesses of only 6 to 16 layers as determined by both optical and X-ray reflectometry (5polymeric films contained a small fraction of polymerizable mesogenic side-groups, it was even possible to photocrosslink these systems. It should be mentioned that in this first case the crosslinking caused some changes in film morphology, but the resulting films are very stable and do not even break when small holes are formed by mechanical stress (JS),... 

By combining regiospecific synthesis with alternating hydrophilic and hydrophobic side chain moieties, Bjornholm, McCullough, and coworkers [285,286] formed regioregular amphiphilic PATs that can spontaneously self-assemble at hydrophilKc or hydrophobic interfaces to form oriented monolayers or bilayers. Finally, there are also efforts to induce chiral liquid crystalline phases. Goto et al. [287] have recently reported on a family of ferroelectric LC substituents. These materials also exhibit a rich array of... 

The main objective of this chapter is to review and outline the research studies and perspectives on liquid crystalline elastomers and LC anisotropic networks, with emphasis on recent interesting innovations on network-stabilized ferroelectric LC (FLC) gels, discotic columnar networks, and self-assembly hydrogen-bonded LC network. We will also present hybrid networks based on ladderlike polysiloxanes that have been developed in our group as advanced functional film materials. 

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