Chiral nematic phases

CLCs can be fabricated by two main methods. The first and simplest way is based on chiral mesogens, which can exhibit the cholesteric phase at a certain temperature range. The cholesteric phase produced by this method consists of pure materials and thus may exhibit advantages such as good uniformity, enhanced stability, and fast response time to stimuli. However, they are typically produced at 

---

Figure C2.2.2. Isotropic, nematic and chiral nematic phases. Here n denotes tire director. In tire chiral nematic phase, tire director undergoes a helical rotation, as schematically indicated by its reorientation around a cone.

Altliough in figure C2.2.2 they are sketched witli rodlike molecules, botli nematic and chiral nematic phases can also be fonned by discotic molecules. 

Chira.lNema.tlc, If the molecules of a Hquid crystal are opticaHy active (chiral), then the nematic phase is not formed. Instead of the director being locaHy constant as is the case for nematics, the director rotates in heHcal fashion throughout the sample. This chiral nematic phase is shown in Figure 7, where it can be seen that within any plane perpendicular to the heHcal axis the order is nematic-like. In other words, as in a nematic there is only orientational order in chiral nematic Hquid crystals, and no positional order. Keep in mind, however, that there are no planes of any sort in a chiral nematic Hquid crystal, since the director rotates continuously about the heHcal axis. The pitch of the helix formed by the director, ie, the distance it takes for the... 

Chiral nematic Hquid crystals are sometimes referred to as spontaneously twisted nematics, and hence a special case of the nematic phase. The essential requirement for the chiral nematic stmcture is a chiral center that acts to bias the director of the Hquid crystal with a spontaneous cumulative twist. An ordinary nematic Hquid crystal can be converted into a chiral nematic by adding an optically active compound (4). In many cases the inverse of the pitch is directiy proportional to the molar concentration of the optically active compound. Racemic mixtures (1 1 mixtures of both isomers) of optically active mesogens form nematic rather than chiral nematic phases. Because of their twist encumbrance, chiral nematic Hquid crystals generally are more viscous than nematics (6). 

Chiral Smectic. In much the same way as a chiral compound forms the chiral nematic phase instead of the nematic phase, a compound with a chiral center forms a chiral smectic C phase rather than a smectic C phase. In a chiral smectic CHquid crystal, the angle the director is tilted away from the normal to the layers is constant, but the direction of the tilt rotates around the layer normal in going from one layer to the next. This is shown in Figure 10. The distance over which the director rotates completely around the layer normal is called the pitch, and can be as small as 250 nm and as large as desired. If the molecule contains a permanent dipole moment transverse to the long molecular axis, then the chiral smectic phase is ferroelectric. Therefore a device utilizing this phase can be intrinsically bistable, paving the way for important appHcations. 

A similar effect occurs in highly chiral nematic Hquid crystals. In a narrow temperature range (seldom wider than 1°C) between the chiral nematic phase and the isotropic Hquid phase, up to three phases are stable in which a cubic lattice of defects (where the director is not defined) exist in a compHcated, orientationaHy ordered twisted stmcture (11). Again, the introduction of these defects allows the bulk of the Hquid crystal to adopt a chiral stmcture which is energetically more favorable than both the chiral nematic and isotropic phases. The distance between defects is hundreds of nanometers, so these phases reflect light just as crystals reflect x-rays. They are called the blue phases because the first phases of this type observed reflected light in the blue part of the spectmm. The arrangement of defects possesses body-centered cubic symmetry for one blue phase, simple cubic symmetry for another blue phase, and seems to be amorphous for a third blue phase. 

If the molecules are chiral or if a chiral dopant is added to a discotic Hquid crystal, a chiral nematic discotic phase can form. The director configuration ia this phase is just like the director configuration ia the chiral nematic phase formed by elongated molecules (12). Recendy, discotic blue phases have been observed. 

The positional order of the molecules within the smectic layers disappears when the smectic B phase is heated to the smectic A phase. Likewise, the one-dimensional positional order of the smectic M phase is lost in the transition to the nematic phase. AH of the transitions given in this example are reversible upon heating and cooling they are therefore enantiotropic. When a given Hquid crystal phase can only be obtained by changing the temperature in one direction (ie, the mesophase occurs below the soHd to isotropic Hquid transition due to supercooling), then it is monotropic. An example of this is the smectic A phase of cholesteryl nonanoate [1182-66-7] (4), which occurs only if the chiral nematic phase is cooled (21). The transitions are aH reversible as long as crystals of the soHd phase do not form. 

The separation of Hquid crystals as the concentration of ceUulose increases above a critical value (30%) is mosdy because of the higher combinatorial entropy of mixing of the conformationaHy extended ceUulosic chains in the ordered phase. The critical concentration depends on solvent and temperature, and has been estimated from the polymer chain conformation using lattice and virial theories of nematic ordering (102—107). The side-chain substituents govern solubiHty, and if sufficiently bulky and flexible can yield a thermotropic mesophase in an accessible temperature range. AcetoxypropylceUulose [96420-45-8], prepared by acetylating HPC, was the first reported thermotropic ceUulosic (108), and numerous other heavily substituted esters and ethers of hydroxyalkyl ceUuloses also form equUibrium chiral nematic phases, even at ambient temperatures. 

Other more exotic types of calamitic liquid crystal molecules include those having chiral components. This molecular modification leads to the formation of chiral nematic phases in which the director adopts a natural helical twist which may range from sub-micron to macroscopic length scales. Chirality coupled with smectic ordering may also lead to the formation of ferroelectric phases [20]. 

The prime requirement for the formation of a thermotropic liquid crystal is an anisotropy in the molecular shape. It is to be expected, therefore, that disc-like molecules as well as rod-like molecules should exhibit liquid crystal behaviour. Indeed this possibility was appreciated many years ago by Vorlander [56] although it was not until relatively recently that the first examples of discotic liquid crystals were reported by Chandrasekhar et al. [57]. It is now recognised that discotic molecules can form a variety of columnar mesophases as well as nematic and chiral nematic phases [58]. 

Here, ry is the separation between the molecules resolved along the helix axis and is the angle between an appropriate molecular axis in the two chiral molecules. For this system the C axis closest to the symmetry axes of the constituent Gay-Berne molecules is used. In the chiral nematic phase G2(r ) is periodic with a periodicity equal to half the pitch of the helix. For this system, like that with a point chiral centre, the pitch of the helix is approximately twice the dimensions of the simulation box. This clearly shows the influence of the periodic boundary conditions on the structure of the phase formed [74]. As we would expect simulations using the atropisomer with the opposite helicity simply reverses the sense of the helix. 

The simplest mesophase is the nematic phase. It is very fluid and involves highly disordered molecules having only short-range positional order, but with the molecules preferentially aligned on average in a particular direction (the director). If the constituent compound is racemic then it is possible to form a phase from the enantiomerically pure compound which is a chiral nematic phase. 

When the mesogenic compounds are chiral (or when chiral molecules are added as dopants) chiral mesophases can be produced, characterized by helical ordering of the constituent molecules in the mesophase. The chiral nematic phase is also called cholesteric, taken from its first observation in a cholesteryl derivative more than one century ago. These chiral structures have reduced symmetry, which can lead to a variety of interesting physical properties such as thermocromism, ferroelectricity, and so on. 

Several 4-(3-alkyl-2-isoxazolin-5-yl)phenol derivatives that possess liquid crystal properties have also been obtained (533-535). In particular, target compounds such as 463 (R = pentyl, nonyl) have been prepared by the reaction of 4-acetoxystyrene with the nitrile oxide derived from hexanal oxime, followed by alkaline hydrolysis of the acetate and esterification (535). A homologous series of 3-[4-alkyloxyphenyl]-5-[3,4-methylenedioxybenzyl]-2-isoxazolines, having chiral properties has been synthesized by the reaction of nitrile oxides, from the dehydrogenation of 4-alkyloxybenzaldoximes. These compounds exhibit cholesteric phase or chiral nematic phase (N ), smectic A (S4), and chiral smectic phases (Sc ), some at or just above room temperature (536). 

Note 3 With chiral nematic substances forming chiral nematic mesophases of short pitch (<700 nm), up to three blue phases occur in a narrow temperature range between the chiral nematic phase and the isotropic phase. 

Cholesteric - the liquid crystal phase formed by molecules with a chiral centre (also called the chiral nematic phase). 

Cr Cub, Cubv d E G HT Iso Isore l LamN LaniSm/col Lamsm/dis LC LT M N/N Rp Rh Rsi SmA Crystalline solid Spheroidic (micellar) cubic phase Bicontinuous cubic phase Layer periodicity Crystalline E phase Glassy state High temperature phase Isotropic liquid Re-entrant isotropic phase Molecular length Laminated nematic phase Correlated laminated smectic phase Non-correlated laminated smectic phase Liquid crystal/Liquid crystalline Low temperature phase Unknown mesophase Nematic phase/Chiral nematic Phase Perfluoroalkyl chain Alkyl chain Carbosilane chain Smectic A phase (nontilted smectic phase)... 

It is also possible to attach chiral peripheral groups to benzo[15]crown-5 as demonstrated by Shinkai [28]. In 8 (Scheme 4), a cholesteryl subunit was attached using an ester group as linker and a chiral nematic phase was observed. 

Subsequent studies on the chiral nematic phase of 8 and 10 showed the influence of complexed salts on the liquid crystalline properties. In the complexes (Scheme 5), the helical pitch was strongly affected by the complexed cation and the counterions [29]. 

Crown ethers of the type discussed in this section have been used as sensors, membranes, or materials for chromatography. Shinkai used cholesterol-substituted crown ether 10 as a sensor for chirality in chiral ammonium compounds (Scheme 16). It was found that the pitch of the cholesteric phase exhibited by 10 was changed upon addition of the chiral salt. As the wavelength of reflection for incident light depends on the pitch, a color change was observed that was visible to the naked eye [45, 46]. Such chirality sensing systems were known before but chromophores had to be bound to the crown ether in order to observe color changes [47]. This problem could be overcome by 10, which uses intrinsic properties of the chiral nematic phase. 

In most cases, the addition of a rod-shaped molecule to a crown ether leads to the formation of a nematic phase. In the case of addition of a chiral unit, such as cholesterol, chiral nematic phases can be observed. Some molecules can also show smectic phases when the attached rod shaped unit is long and inflexible. 

Li and co-workers investigated derivatives of 43c,d [62]. Instead of the terminal heptyl chains, chiral (S)-2-methylbutyl or dodecyl chains were used. For the methylbutane side chain, a chiral smectic and a chiral nematic phase were observed and the clearing points were higher than in 43c,d due to the shorter chain while they were lower for the longer dodecyl side chains. It was also observed that the trans-compounds possess higher clearing temperatures compared to their c/.v-derivatives. 

Crown ether binaphthyl derivatives 128-131 (Scheme 71) were synthesized and investigated by Akagi [139], Compounds 128-131 were used to induce chiral nematic phases (N ) in liquid crystals. It was found that the helical twisting power increased with decreasing ring size. Helical polyacetylenes were synthesized in the N phases. It was found that the interdistance between the fibril bundles of the helical polyacetylene was equal to the half-helical pitch of the N liquid... 

Fig. 8 (a) Schematic representation of the structure of the chiral nematic phase of DNA, showing continuously twisting nematic layers, giving rise to a p/2 periodicity easily observable in the side view on the left, (b) N droplets observed in polarized microscopy. The bright and dark stripes correspond to p/2 (size bar is 10 pm). Adapted with permission from [27]... 

Thisayukta J, Niwano H, Takezoe H, Watanahe J (2002) Enhancement of twisting power in the chiral nematic phase by introducing achiral banana-shaped molecules. J Am Chem Soc 124 3354-3358... 

Zero-Dimensional Nanoparticle Additives in the Chiral Nematic Phase... 

Our group pursued another approach of combining the properties of nanoparticles with chiral nematic liquid crystal phases. The idea was to decorate gold nanoparticles with chiral molecules known to be strong inducers of chiral nematic phases. To realize the idea, we prepared a series of alkylthiol-capped gold nanoparticles, either pure monolayer or mixed monolayer, with all or about every second of the alkylthiols end-functionalized with (5)-naproxen (e.g., 6 in Fig. 11) [349]. 

Finally, dispersions of MWCNT in chiral nematic liquid crystals were studied as well. These experiments suggested no change in the helical twisting characteristics of the chiral nematic phase. However, the MWCNTs were thought to disrupt the translational order in the SmA phase (decrease of the SmA-N phase transition) yet follow the twist of the nematic director in the chiral nematic phase [498]. 

Chiral monophosphorus ligands, in hydrogenation, 10, 16 Chiral nematic phase, calamitic mesogens, 12, 200 Chiral N,P ligands, in hydrogenation, 10, 17 Chiral phosphines... 

When the twisting power of the closed-ring isomer is larger than that of the openring isomer, it is expected that UV irradiation should induce the phase change from the nematic to chiral nematic phases.1481 A diarylethene 31a, with two diarylethene units in a chiral cyclohexane, was incorporated into K15 and the phase change concomitant with photoisomerization was measured. 

---