Helical order

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. 

At lower temperatures, the standard 3D helical order with all chains having not only the same chirality but also the same phase can be established. The two magnetic phase transitions present very different features. In fact, the transition to the... 

Preferential Screw Sense Helical Ordering of Polysilanes... 

Substituent Effects and Cooperative Helical Order Effect... 

It is considered that, if ideal, optically active poly(alkyl(aryl)silane) homopolymer and copolymer systems could be obtained which had stiffer main-chain structures with longer persistence lengths, it should be possible to clarify the relationship between the gabs value and the chiral molar composition. The magnitude of the chirality of the polyisocyanates allowed precise correlations with the cooperativity models.18q In the theory of the cooperative helical order in polyisocyanates, the polymers are characterized by the chiral order parameter M, which is the fraction of the main chain twisting in one helical sense minus the fraction of the main chain twisting in the opposing sense. This order parameter is equal to the optical activity normalized by the value for an entirely one-handed helical polymer. The theory predicts... 

Figure 4.34 Degree of preferential helical order (M) and estimated as a function of chiral molar composition in (a) copolymers 44 and (b) copolymers 45.

The first observed example of helical order in a discotic mesophase was found in hexa-hexylthiotriphenylene (18).24-26 Molecule 18 has two mesophases, at high temperature a Colhd mesophase and at low temperature a Colho... 

Trzaska and co-workers showed a similar propeller mechanism for the formation of helical columns from disclike metallomesogens (29-31).34 These metallomesogens also have C3 symmetry and 30 and 31 are provided with chiral side chains. In the hexagonal columnar mesophase these chiral side chains induce a Cotton effect in the chromophore of the helically arranged core. Heating the mesophase to the isotropic liquid results in the disappearance of the Cotton effect because of the loss of helical order. This effect illustrates the need for the molecules to be positionally ordered in order for the side-chain chirality to be transferred to the supramolecular column. 

A helical arrangement within columns was also found for other metal 3-diketonate complexes provided with chiral side chains (32) by Serrano and co-workers.35,36 These compounds form rectangular columnar mesophases with helical order within the columns. A spin-coated sample of 32 showed a positive exciton-splitted signal in the CD spectra, which was interpreted as a left-handed (M) helix. Annealing of the film resulted in much higher optical activities and a shift of the absorption maxima. The observed optical changes clearly point to a chiral organization of the columns in the mesophase. 

In the compounds CsCuCl3 and CsCuBr3 the sequence of long axes of elongated octahedra form a helix along the c axis, so that the unit cell becomes tripled in this direction (Fig. la). With local coordinate axes as in Fig. 3, this sequence is described by the motif xy zx yz Similar but more complex helical ordering of distortions within one chain is realized in N(CH3)4CuCl3. Other compounds display the... 

We can see that only two types of ordering can become equilibrium configurations. Moreover, these are precisely the two low-temperature structural phases found in hexagonal perovskites (Fig. 1). The parameters A, and A2 are expected to have different signs, which is in agreement with their definition in equation (37). According to the theory, the helical ordering (Fig. la) is achieved at A2j A I < 1/2. 

---