Second chiral nematics

The introduction of a second chiral atom in the system leads to a reduction in the mesogenic properties and only a monotropic chiral nematic transition is observed for compound 23. However, when this compound is cooled down from the isotropic liquid state at a cooling rate of 0.5 °Cmin , very unusual blue phases BP-III, BL-II and BP-I are observed in the range 103-88 °C. Blue phases usually require pitch values below 500 nm. Hence the pitch value of the cholesteric phase for 23 must be very short, suggesting that the packing of two chiral carbons forces a faster helical shift for successive molecules packed along the perpendicular to the director. 

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. 

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]. 

The second family of materials, 10, were shown to exhibit chiral nematic to TGBC phases sequences in addition to the more conventional chiral nematic - TGBA" - TGBG sequence. For the R = hexyl, R = decyl and R = hexyl, R = dodecyl homologues TGBG phases with ranges of up to 35 °G were obtained. 

In cholesteric (or chiral nematic) liquid crystals the situation is very close to usual nematics. However, due to the chirality of the molecules, the lowest state of elastic energy in cholesterics does no longer correspond to the uniform director orientation, but to the twisted one with a pitch Pq = 27r/yo where yo is the wave vector of cholesteric. Thus for cholesterics the second term in expression (2.24) must be rewritten as... 

An explanation of the nomenclature should be made here. First, chiral nematic molecules need not come from cholesteryl derivatives, so we use the term chiral nematic instead of cholesteric when referring to liquid crystal materials. The chiral nematic/cholesteric phase itself we will call helical. Second, blue phases got their name from their blue appearance in early investigations. Blue phases are not always blue, however we now know that they may reflect light of other colors, including near infrared. Finally, BPIII was known as the fog phase or the gray fog phase in early publications. Although these terms are descriptive of this phase s appearance, BPIII seems to have survived. 

As the temperature is decreased the chiral nematic structure transforms to a higher order phase. The phase may go through a first order phase transition and crystallize in which case the optical properties are of little interest herein. It may transform to a glass, in which case the optical properties, such as birefringence, pitch, etc., are frozen and may be used in static, or time and environment-independent devices or applications (as discussed in Sec. 2.5 of this Chapter), or it may go through a second order or second order plus a weak first order phase transition to a higher order liquid crystalline phase. Here, for simplicity, we are not considering the so-called re-entrant phases [ 14]... 

The best known of the chiral liquid crystal phases is the cholesteric phase or chiral nematic (N ). Here an asterisk is used to indicate a chiral phase. The cholesteric phase (Figure 2.10) was the first liquid crystal to be discovered by Reinitzer in 1888. Reinitzer observed pure cholesterol benzoate under the microscope and noticed two apparent melting points solid crystal to the phase that is now known as cholesteric and then a second melting point to the isotropic liquid phase. Cholesterol is a chiral molecule. 

The phase chirality is given by the molecular chirality and the suprastructural chirality of the orientational or positional order. Thus, a phase has to be specified by symbols for the molecular and the suprastructural chirality. Chiral nematics as given in FIGURE 1 are then described by R or S or P or M for the molecules (first letter) and by P M fin- the helicity of the phase (second letter) RP or SP. 

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