Doping chiral nematic

Abstract It is well known that spontaneous deracemization or spontaneous chiral resolution occasionally occurs when racemic molecules are crystallized. However, it is not easy to believe such phenomenon will occur when forming liquid crystal phases. Spontaneous chiral domain formation is introduced, when molecules form particular liquid crystal phases. Such molecules possess no chiral carbon but may have axial chirality. However, the potential barrier between two chiral states is low enough to allow mutual transformation even at room temperature. Therefore the systems are essentially not racemic but nonchiral or achiral. First, enhanced chirality by doping chiral nematic liquid crystals with nonchiral molecules is described. Emphasis is made on ester molecules for their anomalous behavior. Second, spontaneous chiral resolution is discussed. Three examples with rod-, bent-, and diskshaped molecules are shown to give such phenomena. Particular attention will be paid to controlling enantiomeric excess (ee). Actually, almost 100% ee was obtained by applying some external chiral stimuli. This is very noteworthy in the sense that we can create chiral molecules (chiral field) without using any chiral species. 

Nakata M, Takanishi Y, Watanabe J, Takezoe H (2003) Blue phases induced by doping chiral nematic liquid crystals with non-chiral molecules. Phys Rev E 68 041710-1-6... 

Several exciting phenomena described for non-chiral nematic systems were also reported for nanoparticle-doped chiral nematic liquid crystals. We mentioned the work of Kobayashi et al., who, most notably, demonstrated a frequency modulation twisted nematic (FM-TN) mode and fast switching characteristics using metal nanoparticles as dopants [301-307, 313, 314],... 

Mixtures of a nematic liquid crystal (LC or LC ) with small quantities of gold nanoparticles coated with alkylthiolates (<5 wt%) including an alkylthiolate functionalized with a chiral group have been studied (Figure 8.29) [72]. All mixtures show nematic mesophases with transition temperatures and phase stability very similar to those oftheliquid crystal precursors LC or LC. The introduction ofachiral center into the mixtures (mixtures of Au ) produce chiral nematic mesophases. A similar result is obtained in mixtures of Au and LC doped with the chiral dopant (s)-Naproxen. 

The study of the cholesteric mesophases obtained by doping thermotropic nematics with chiral, nonracemic compounds, has lead to relevant information about the stereochemistry of the dopants. Chiral interactions change the structure of the phase and therefore molecular chirality can be mapped onto an achiral (nematic) phase to yield a superstructural phase chirality. In 1984 Sol-ladie and Zimmermann published the first review summarizing the state of the art at that time.52 Later on, several review articles updated this subject.53-55... 

M Voigt, M Chambers, and M Grell, Circularly polarized emission from a narrow bandwidth dye doped into a chiral nematic liquid crystal, Liq. Cryst., 29 653-656, 2002. 

Fig. 10 (a, b) Schematic mechanism demonstrated for a reflective color M-paper with magnetically controllable characteristics, (c, d) The intensity of magnetic field dependence on the reflection spectra of chiral nematic mixtures doped with magnetite nanoparticles that are surface modified with oleic acid and a chiral pyridine-based dopant, as well as photographs of both formulations before and after a magnetic field of 1,000 GS was applied (see photograph insets above) [364], (Copyright 2010, Taylor Francis)... 

Very large values of gCPPL may be more easily obtained via doping of LCs, e.g. the near maximum value of gCPPL = 1.8 for 0.2% achiral ter(fluorene) doped in chiral nematic LC film (35-pm thick) at the wavelength range of selective reflection for the LC [135],... 

Similar results were reported for the combination of chiral azobenzenes doped in nematic liquid crystals [59,60]. 

The viscosity of a typical cholesteric made by doping a nematic with a modest amount of chiral nematic is much lower (around 1 P or so) than that of the typical pure eholesteric. Perhaps this is because the pitch of the doped nematic is higher than that of the typical pure cholesteric, or because the twist elastic constant of the doped nematic is much lower. 

Besides degree of substitution, nature of substituents, solvent, concentration, and temperature, other factors that change the polymer-solvent interactions can affect the pitch of lyotropic cellulosic mesophases. Doping inorganic salts " or small chiral molecules into the lyotropic mesophase changes the polymer-solvent interactions. As the results, the pitch of the chiral nematic mesophase changes accordingly. 

In this liquid crystal phase, the molecules have non-symmetrical carbon atoms and thus lose mirror symmetry. Otherwise optically active molecules are doped into host nematogenic molecules to induce the chiral liquid crystals. The liquid crystals consisting of such molecules show a helical structure. The most important chiral liquid crystal is the cholesteric liquid crystals. As discussed in Section 1.2, the cholesteric liquid crystal was the first discovered liquid crystal and is an important member of the liquid crystal family. In some of the literature, it is denoted as the N phase, the chiral nematic liquid crystal. As a convention, the asterisk is used in the nomenclature of liquid crystals to mean the chiral phase. Cholesteric liquid crystals have beautiful and interesting optical properties, e.g., the selective reflection of circularly polarized light, significant optical rotation, circular dichroism, etc. 

MBBA AI-(4-methoxybenzylidene)-4-n-butylaniline = —1.8 x lO pm ). These values are close to those obtained with organic compounds of the chiral binaphthyl type. The observations show the formation of chiral nematic phases N under the influence of the doping agent, with values of pitch (p) close to the visible region of the spectrum. The sense of the induced helix is negative (M). In the case of MBBA, the CD spectrum shows a powerful induced negative effect in the region of the imine chromophores of the MBBA receptor. 

Yamaguchi T, Inagawa T, Nakazumi H, Irie S, Irie M. 2001. Photoinduced pitch changes in chiral nematic liquid crystals formed by doping with chiral diarylethene. J Mater Chem 11(10) 2453 2458. 

Chiral nematic mesoporous films of Eu " doped Zr02 have been produced via a hard-templating approach using nanoctystalline cellulose-templated silica (Fig. 11). It was found that these chiral nematic nanostructures are capable of modulating the spontaneous emission of the Eu ions. The emission lines of the Eu " at 596 nm, 613 and 625 nm were significantly suppressed, and an increase in the luminescence lifetime is observed. It was suggested that these new chiral luminescent nanomaterials could find potential applications in sensing and new optical nanodevices. 

Li et al. reported two light-driven chiral molecular switches 36 and 37 with tetrahedral and axial chirality [116]. When chiral switch 36 was doped in nematic LC host E31 at 15 wt% concentration, phototuning the reflection color over the entire visible region was observed. An amazing feature of this photoresponsive CLC system is the quick relaxation. After 1 min of exposure to bright white light, it returned to... 

The effects of bistability and hysteresis in supertwisted nematic layers were first investigated in [122]. To obtain twist angles larger than 90 , nematics were doped with a small amount of an optically active material. Thus a cholesteric (or chiral nematic) with a large pitch P was created, so that the pitch value had to adjust the boundary conditions for the directors on the substrates. The corresponding texture was first discovered by Grandjean and is discussed in Chapter 6. In 1984 the display based on the Supertwist Birefringent Effect (SBE) was proposed [123]. 

Figure 34. Magnetic field dependence of the relative pitch p (H)/p (0) in a chiral nematic. These data come from [137] and correspond to PAA doped with choles-teryl acetate.

In the White-Taylor device the chiral nematic (p A) is doped with an anisotropic dichroic dye. With homeotropic boundary conditions and low voltages, the focal-conic texture becomes axially aligned in the plane of the device. The dye spirals with the director and the random directions of the helix axis in the plane of the device ensure that unpolarized light is absorbed uniformly in this state. Application of a high field (see... 

By using the chiral electron acceptor (-)-TAPA, even the cholesteric versions of the nematic discotic phase Ng or of the nematic columnar one Ncoi can be obtained, for example, by doping a nematic discotic pentayne ether with up to 30 mol% (-)-TA-PA or by inducing the N oi phase in a ternary mixture composed of pentayne ether (4) (n = 16) with TNF and (-)-TAPA the latter electron acceptor does not itself lead to the induction of a columnar mesophase with such pentayne donors [26, 59 d]. 

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