Chirality blue phase transitions

The analogue of the pressure-temperature plane in a liquid-gas system is the temperature-chirality plane for blue phase transitions. Along the lines of the classic liq-... 

X 10 N [26]. Chiral nenutic to blue phase transitions have been found for single components as well as for mixtures [36] und similar ccmditions. 

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. 

The complexes bearing one chiral substituent display a smectic A mesophase when the non-chiral chain is long, or an enantiotropic cholesteric and a monotropic SmA phase for shorter alkoxy chains. A TGBA phase is observed for the derivative which contains the chiral isocyanide combined with the diethyloxy, when the SmA to cholesteric transition is studied. The compound with two chiral ligands shows a monotropic chiral nematic transition. When this compound is cooled very slowly from the isotropic liquid it exhibits blue phases BP-III, BP-II, and BP-I. 

The phototuning of BPs can also be fabricated in a pure material system [147]. Das et al. reported a light-induced stable blue phase in photoresponsive diphen-ylbutadiene based mesogen 37. This compound was found to exhibit SmA and N during heating. When the temperature was kept at 118 °C, the photoisomerization induced an isothermal phase transition from SmA to N. Photoirradiation of the SmA film held at a higher temperature (124 °C) for 100 s resulted in transition to a phase with a characteristic classical BP texture showing in Fig. 5.30. The BP was thermodynamically stable and could be maintained at this state for several hours. The characteristic sharp reflection bands compared to the rather broad reflection bands observed for the chiral nematic phase confirmed the formation of BP. The photoinduced formation of the BP exhibited a reflection centered at 510 nm. Subsequent irradiation led to the blue shift to 480 nm in the reflection band. 

We estimate the chirality needed to allow the blue phase, and also the temperature range of the blue phase. When the temperature is close the isotropic transition temperature, the isotropic core does not cost much energy and can fill the void between the packed double-twist eylinders. The radius of the isotropic core is approximately the same as radius R of the double-twist cyhnder. From Figure 13.7, we know that qji 0.36n. From Equation (13.21) we have... 

In recent years a wide variety of chiral liquid crystalline phases has been discovered in certain chiral materials and these are mentioned above in the section on optical polarising microscopy. However, DSC has often not revealed the presence of such phases. In the case of the blue phases and the TGBA phase, the mesophase ranges are often too short to provide a distinct enthalpy peak. Of course, in some materials this situation is trae of the more usual liquid crystal phases. The transitions between the S( ferri phase and... 

Figure 7.1. Schematic picture of the temperature region near the nematic (N)-isotropic (ISO) phase transition. Top Nonchiral molecules have only nematic and isotropic phases. Bottom Chiral molecules have helical (H) and isotropic phases, and, depending on the chirality, up to three blue phases (BPI, BPII, and BPIII). BPI and BPII are cubic BPIII has the same symmetry as the ISO phase.

Figure 7.4. Transition temperatures T versus mole fraction chiral CE2 in a chiral-racemic mixture. The limited chirality range of BPII appears to be a universal feature of blue phases.

Figure 7.21. Generic phase diagram showing temperature T versus chirality q for blue phases. Universal features include the survival of BPI and BPIII, but not BPII, at high chirality and termination of the BPIII-isotropic transition at critical point c.p.

Further specific volume measurements with chiral mesogens, cholesteric, and blue phases have been made [26-28, 159-161]. The influence of chiral dopants on C/A and C/N phase transitions has been studied [37, 162]. Earlier studies, particularly on cholesteric mesophases and their mixtures, have been published, for example, for cholesteryl acetate [163], cholesteryl myristate [53], cholesteryl nonanoate [54], cholesteryl... 

We have already seen that the transitional behavior of dimers depends strongly on the length and parity of the spacer. Blatch et al. [33] studied two series of chiral dimers, one symmetric (5)2MB.OnO.(5)2MB, and the other nonsymmetric, CBO-nO.(5)2MB, (see Fig. 1) in order to establish whether the form chirality of the chiral phases would also depend critically on the parity of the spacer. For the CBO-nO.(5)2MB series with n=l and 9 a blue phase was observed but not for n = 6 and 8. This was rationalized in terms of the smaller pitch for the odd relative to the even mem-bered dimers which arises from the smaller twist elastic constant of odd dimers and is related to their lower orientational order. Surprisingly, the helical twisting powers of the dimers in a common monomeric nematic solvent appear to depend solely on the nature of the chiral group and not on its environment. Thus similar helical twisting powers are observed for both odd and even membered dimers. 

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