Helicoidal chirality

In coordination chemistry helicoidal chirality is frequently encountered. It is the chirality of OC-6 complexes possessing two cis geometry) or three bis(chelate) substituents, and also of helicates in supramolecular chemistry. 

Activation parameters. Symmetry. Chirality in molecules devoid of chiral centers. Helicoidal chirality. 

Mesophase with a helicoidal superstructure of the director, formed by chiral, calamitic or discotic molecules or by doping a uniaxial nematic host with chiral guest molecules in which the local director n precesses around a single axis. 

See Fig. 3 for an illustration of the helicoidal molecular distribution in a chiral nematic mesophase. 

Cholesteric mesophases form only from molecules having chiral centers. Since the helicoidal structures of cholesterics are supramolecular, the chiral centers in... 

It is possible to conceive of situations where the chemical linking of molecular components around a template is not as crucial as the formation of defined, non-covalent interactions during templating. This may be exemplified by the polymerisation of a nematic liquid crystalline crosslinker in the presence of a template, a non-polymerisable cholesteric mesogen [23]. The chiral dopant forces the crosslinker to form a cholesteric phase. After polymerisation of the crosslinker, the polymer still exhibits a helicoidal structure which is stable over a wider temperature range than the initial cholesteric phase. It is not reported in this work whether extraction of the chiral mesogen has been attempted or not. 

Liquid crystalline (LC) solutions of cellulose derivatives form chiral nematic (cholesteric) phases. Chiral nematic phases are formed when optically active molecules are incorporated into the nematic state. A fingerprint texture is generally observed under crossed polarizers for chiral nematic liquid crystals when the axis of the helicoidal structure is perpendicular to the incident light (Fig. 2). 

As seen in Fig. 10, the larger the pitch, the slower the rate the modulus evolution. The modulus of the right-handed mesophase (AEC-4) developed faster than that of the left-handed mesophase (AEC-1) with similar pitch. The difference in relaxation behavior of the AEC solutions may be because of the smaller driving force to reform helicoidal structures of chiral nematic phases from flow-induced nematic mesophases for the mesophases with larger pitch (nematic-like). 

Fig. 33. a Concentric cylindrical twist walls separating different chiral smectic regions. Inside each region the layer surface adopts a helicoidal shape, b Each cylindrical twist wall is comparable to that found in a conventional TGB phase, a helical shape being given to the screw dislocations constituting them (after Gilli and Kamaye [43])... 

Theoretical investigations by Brand [ 135] and Brand and Pleiner [136] predicted that a monodomain liquid-crystalline elastomer exhibiting a cholesteric or a chiral smectic C phase should display piezoelectric properties due to a modification of the pitch of the helix under strain. So, a piezoelectric voltage should be observed across the sample when a mechanical field is applied parallel to the helicoidal axis. In this description, the crosslinking density is supposed to be weak enough to allow the motion of the director, and deformations of the sample (compression, elongation, etc.) are assumed to be much smaller than those that should lead to a suppression of the helix. The possibility of a piezoelectric effect do not only concern cholesteric and chiral smectic C phases, but was also theoretically outlined for more exotic chiral layered systems such as chiral smectic A mesophases [137]. 

Cellulose and some derivatives form liquid crystals (LC) and represent excellent materials for basic studies of this subject. A variety of different structures are formed, thermotropic and lyotropic LC phases, which exhibit some unusual behavior. Since chirality expresses itself on the configuration level of molecules as well as on the conformation level of helical structures of chain molecules, both elements will influence the twisting of the self-assembled supermolecular helicoidal structure formed in a mesophase. These supermolecular structures of chiral materials exhibit special optical properties as iridescent colors, and... 

Lyotropic cellulosics mostly exhibit chiral nematic phases, although columnar phases have also been observed. The molecules in the thermotropic state also form chiral nematic order, but it is sometimes possible to align them in such a way that a helicoidal structure of a chiral nematic is excluded. Upon relaxation they show banded textures. Overviews on lyotropic LC cellulosics are... 

The optical properties of chiral nematic phases are closely related to their supermolecular Structures, as stated by the considerations of de Vries. In particular, the planar textures exhibit beautiful colors correlated to the pitch P of the helicoidal structures by Eq. (1), if the selective reflection wavelength lies in the visible range, and many examples are shown in Fig. 2. 

In addition to the alkoxymethyl derivatives, dihydroxosilicon(IV) phthalocyanines bearing alkoxy peripheral groups were prepared ((1) M = Si(OH)2, R = OC H2 +i, n = 2, 4, 6, 8, 10, 12, 14, 16, 18), and all displayed a Colh phase at low temperatures, with a decreasing mesophase stability due to the presence of axial hydroxyl groups.Similar silicon and silicon-free phthalocyanines substituted by chiral (S)-citronellol were shown to form a Colh and, at higher temperatures, a Col, with a helicoidal structure. 

A similar model can be considered even when a system of polymers forms microfibrils or filaments distributed with a constant interdistance. The separation of filaments can be controlled by the balance of several forces hydration, electrical doublelayer repulsion and van der Walls attraction. This conception derives from a paper by Elliot and Rome (1969) who interpreted in that way the interdistance control between muscle cell filaments. We do not know the exact shape of the isopotential surfaces corresponding to that system of forces in the vicinity of fibrils formed by a set of chiral pol5nners, but they must show in many cases a helicoidal symmetry. The complete system can closely resemble the model suggested by Rudall (1955). 

The phase angle therefore continuously increases or decreases. The structure given by Eq. (5.5) is a well-known helicoidally modulated structure. The tilt direction of chiral molecules organized in layers and tilted with the long axes away from the layer is almost always helicoidally modulated even... 

This interaction has already been discussed. Here we consider favourable structures for the two possible signs of the coefficient (Fig. 5.6, sixth row). The negative sign is the reason for lock-in, while the positive sign of bq is for interactions favoming perpendicular orientations in neighboming layers, which combine with chiral interactions and additionally stabilize helicoidal modulations in the SmC phases. 

In the most simple chiral polar tilted smectics, ferroelectric liquid crystals, the flexoelectric phenomenon influences the structure of the SmC phase only quantitatively. It affects the elastic and chiral couplings and consequently slightly changes the transition temperature to the tilted phase and the pitch of the helicoidal modulation. 

As stated in the introduction, the first LC thermosets were prepared in 1973 by Blumstein and Strzelecki et al. [6,53-56] via the thermal polyaddition of acrylate-terminated schiff-base mesogens within the mesophase. Strzelecki et al. also reported the first preparation of networks with a helicoidal structure. Copolymerization of the mixture of LC diacrylate, LC monoacrylates having a mesogen, and/or a chiral group within the cholesteric mesophase resulted in the LC networks with a characteristic cholesteric texture. 

Revol JF, Bradford H, Giasson J et al (1992) Helicoidal self-ordering of cellulose microfibrils in aqueous suspension. Int J Biol Macromol 14 170-172 Revol JF, Godbout L, Dong XM et al (1994) Chiral nematic suspensions of cellulose crystallites ... 

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