Short-pitch chiral nematics

The ability of the chiral 2-methylbutyl moiety to generate very short pitch chiral nematic phases was by now widely recognised. It was not therefore surprising that a number of research groups should attempt... 

There is a further geometry of practical interest for light incident on chiral nematic films, related to the pitch of the helix in which we consider light propagating in a direction normal to the helix axis, i.e., as in the fingerprint texture, but with a pitchp less than A. In this short pitch chiral nematic case, the chiral optical tensor is averaged in space and the macroscopic optic axis is col-linear with the helix axis [27]. The macro-... 

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. 

Thermotropic cholesterics have several practical applications, some of which are very widespread. Most of the liquid crystal displays produced use either the twisted nematic (see Figure 7.3) or the supertwisted nematic electrooptical effects.6 The liquid crystal materials used in these cells contain a chiral component (effectively a cholesteric phase) which determines the twisting direction. Cholesteric LCs can also be used for storage displays utilizing the dynamic scattering mode.7 Short-pitch cholesterics with temperature-dependent selective reflection in the visible region show different colors at different temperatures and are used for popular digital thermometers.8... 

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. 

Blue phases are liquid crystal phases that appear between chiral nematic phases with relatively short helical pitches and isotropic phases. Generally, blue phases have the following characteristics ... 

The pitch of the helix depends on concentration c of a dopant for small c Po ac and a is called helical twisting power of the dopant [15]. However, with increasing c the dependence becomes nonlinear and the heUx handedness can even change sign (the case of cholesteryl chloride dopant in p-butoxybenzyli-dene-p -butylaniline, BBBA, see Fig. 4.24). The same chiral, locally nematic phase with a short pitch in the range of 0.1-1 pm is traditionally called cholesteric phase because, at first, it has been found in cholesteryl esters. Such short-pitch phases manifest some properties of layered (smectic) phases. 

The pitch of a chiral nematic phase is the distance along the hehx over which the director rotates by 360°. It should be noted, however, that the stmcture repeats itself every half pitch due to the equivalency of and -ft Interesting optical effects occur when the wavelength of light in the liquid crystal is equal to the pitch. These will be described at length in Chapter 11. The pitch of a chiral nematic phase can be as short as 100 nm. Mixing the two optical isomers in various proportions allows the pitch to be increased from the pilch of either of the two pitre optical isomers. A racemic mixture (equal parts of each optical isomer) possesses an infinite pitch and is therefore nematic. Finally, the chiral nematic phase is often ealled the cholesteric phase, since many of the first compounds that possessed this phase were derivatives of cholesterol. 

There are also electro-optic effects using either a different geometry of surface stabilization or a completely different mechanism In the twisted ferroelectric smectic-C cell [54] the moleeules form in the zero field state a quarter helix which is removed when a dc field of either polarity is applied the optical effect is achieved in the same way as in a twisted nematic cell. Compounds with a short chiral smectic-C pitch in a thick cell are used for the distorted helix ferroelectric (DHF) device [55] this effect uses the optical difference between the zero-field state eharacterized by a fully developed short-pitch helix, and structures with a distorted or almost unwound helix in the presence of an applied field optically addressed spatial light modulators can take advantage of the DHF effect [56]. Further applications of ferroelectric liquid crystals are switchable diffraction gratings [57]. 

When a chiral material is added to a nematic liquid crystal at low concentrations, the pitch p appears to vary linearly with the concentration. A mixture of two compounds of opposite chirality can produce a nematic phase at a certain composition. At this compensation point, the pitch becomes infinite. Unwinding of helical structures can be achieved by external fields. Finally, it is mentioned that, for chiral nematics with relatively short pitch, there exist several intermediate phases known as the blue phases between the isotropic and the chiral nematic phases. These blue phases are... 

These peaks are normally centred close to Q = 2ti//o and Q 2%Iwq respectively, where Iq and Wq are the molecular length and width (typically, Iq 2.5 nm Wo=0.5 nm). They arise from the average end-to-end and side-to-side separations of the close-packed molecules. The peaks are diffuse because these positional correlations only extend over short distances, typically, a few molecular diameters. The widths of the diffuse maxima are inversely proportional to these correlation lengths, In fact, such correlations usually also exist in the isotropic phase, so it may be difficult to distinguish the powder patterns of the nematic from that of the isotropic phase. Note that the chiral nematic (cholesteric), where the pitch is typically 0,1 pm, will also give a very similar diffraction pattern. 

Blue phases appear in a small temperature range between the chiral nematic phase with a sufficiently short helical pitch length and the isotropic phase. The blue phase has very peculiar characteristics [13-15] ... 

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