Mesophase cholesteric

Induction of chirality in No phases was also shown to be possible using charge transfer interactions, via jt-jt stacking. The binary mixture of mesogen 8, which is electron rich, with chiral electron-deficient molecule 9 was shown to induce a twist in the mesophase.15 Furthermore, nonmesogenic 8 gave rise to a cholesteric mesophase, denoted as being of the columnar type (N ), when present in a ternary system with TNF (trinitrofluorenone, an electron acceptor)... 

The study of the cholesteric mesophases obtained by doping thermotropic nematics with chiral, nonracemic compounds, has lead to relevant information about the stereochemistry of the dopants. Chiral interactions change the structure of the phase and therefore molecular chirality can be mapped onto an achiral (nematic) phase to yield a superstructural phase chirality. In 1984 Sol-ladie and Zimmermann published the first review summarizing the state of the art at that time.52 Later on, several review articles updated this subject.53-55... 

Discotic liquid crystals arise from disk-shaped molecules as nematic or cholesteric mesophases. Their structural characteristics are similar to the respective ealamitie mesophases, that is, the normals of the disks are oriented parallel. Instead of the smectic mesophases, diseotie columnar liquid crystals arise from eonnecting the disks to each other. The columns of the discotic columnar mesophase form a two-dimensional lattice whieh is in a hexagonal or rectangular modification. In addition, the columns may be tilted (Fig. 2f,g). 

Solvent viscosity vs, concentration plots for cellulose dissolved in TFA-CH2CI2 (70/30, v/v) do not exhibit a maximum (1I,S1) in contrast to the typicid behavior of polymer liquid crystal solutions. This same behavior is exhibited by other cellulose-solvent systems (52,fiQ). Conio et al. (59) si gest that due to the close proximity of the cholesteric mesophase to its solubility limit, it is only observed in a metastable condition. 

Note 5 The term chiral nematic mesophase or chiral nematic is preferred to cholesteric mesophase or cholesteric. 

Several cholesteric mesophases have been studied as media for the photodehydro-cyclization of (4+1). The results are collected in Table 6. 

The experiments of Table 6 indicate that a cholesteric mesophase with a right-handed helix gives an excess of (+)-[6]-helicene, which is known to have also a right-handed helix. The optical yield is small but significant. Nakazaki et al. 72) did not find induction in an isotropic mesophase (Table 6, c), while Hibert73) got still an enantiomeric excess in an isotropic and in several compensated nematic mesophases (Table 6, f, i, j, k, 1). Hibert explained these findings by discerning two effects ... 

The helix of the mesophase has probably a steric influence upon the conformational equilibrium of cis-syn (4+1). It seems unlikelv that this contribution is caused by CPL, generated in the cholesteric mesophase 75). 

Price and Wendorff31 > and Jabarin and Stein 32) analyzed the solidification of cholesteryl myristate. Under equilibrium conditions it changes at 357.2 K from the isotropic to the cholesteric mesophase and at 352.9 K to the smectic mesophase (see Sect. 5.1.1). At 346.8 K the smectic liquid crystal crystallized to the fully ordered crystal. Dilatometry resulted in Avrami exponents of 2, 2, and 4 for the respective transitions. The cholesteric liquid crystal has a second transition right after the relatively quick formation of a turbid homeotropic state from the isotropic melt. It aggregates without volume change to a spherulitic texture. This process was studied by microscopy32) between 343 and 355.2 K and revealed another nucleation controlled process with an Avrami exponent of 3. 

The cholesteric mesophase is a helically disturbed nematic phase. As in the nematic phase, the centers of gravity of the mesogenic molecules are statistically disordered, whereas the long molecular axes possess an orientational long range order with respect to the director. The director, however, is not constant in space, but continuously... 

The Principles of Formation and Some Properties of Smectic, Nematic and Cholesteric Mesophases of Liquid-Crystalline Polymers... 

Copolymerization of two mesogenic monomers is, at the present time, the only pathway to obtain polymers with cholesteric mesophase (see Part 4.4). On the other hand, only by copolymerizing smectogenic and nematogenic monomers and investigating the properties of copolymers in a broad interval of compositions, is it possible to establish the principles of formation of each type of mesophase. We demonstrate... 

The first success was achieved when optically active (chiral) monomeric units were combined with a nematic LC polymer 105,123,143,144). The approach was based on the idea that a cholesteric mesophase may actually be realized as a helical nematic structure. Then by chemical binding of chiral and mesogenic units into a chain, accomplished by copolymerization or copolycondensation (in case of linear polymers) of nematogenic and optically active compounds, it was found feasible to twist a nematic mesophase and obtain copolymers of cholesteric type (Table 13). 

Only one case was reported where a cholesteric mesophase was obtained by copoly-merizating of approximately equimolar amounts of cholesterol-containing monomers... 

The main feature identifying a cholesteric mesophase in polymers is the presence of optical texture with selective circularly-polarized light reflection. This indicates the formation of 1-helical cholesteric structure in LC copolymers. The X-ray patterns of actually all cholesteric copolymers described (with the exclusion of polymers 3.1 and 4.1, Table 13) correspond to those of nematic and cholesteric low-molecular liquid crystals, which is manifested in a single diffuse reflex at wide scattering angles. At the same time, for copolymers 3.1 and 4.1 (Table 13) small angle reflexes were observed 123), that are usually missing in low-molecular cholesterics. 

We have investigated various factors which contribute to solvent-induced partial resolution or race-mization of 1,1 -binaphthyl (BN). Only photochemical interconversions of BN conducted in cholesteric mesophases influenced the steady state concentration of atropisomers. Thermal equilibriun in cholesteric media or photochemical interconver-sions in chiral isotropic solvents did not alter appreciably the atropisomeric ratio of initially racemic BN. Solvent order accelerates the rate of BN thermal racemization. A discussion of the physical properties of the solvents and BN responsible for the observations is presented. 

In an attempt to discern the factor(s) most responsible for ordered solvent induced alterations of reaction rates and specificities, we have investigated the influence of cholesteric liquid-crystalline and other optically active media upon the induction or loss of optical activity in the atropisomers of 1,1 -blnaphthyl (BN, equation 1). We find that optical induction is negligible from thermal (ground-state) lsomerizations (usually <0.1%) but is larger for excited-state lsomerizations conducted in cholesteric mesophases (up to 1.1%). The factors responsible appear to be the geometry and polarizability of the 15N triplet state and rather specific solvent-solute interactions in ordered... 

Table I. Cholesteric mesophases and their transition temperatures.

Our spectroscopic studies of BN in mixture B and in hexane support our contention that ground state conformers are forced by cholesteric mesophases toward extremes of 0 (i.e., closer to 0° or 180° than in hexane solvent). As the two naphthyl groups become more coplanar, their u-overlap increases. Consequently, the 0-0 transitions in absorption (and excitation) occur at longer wavelengths (lower energies) (43). For the same reasons, the cholesteric solvent compresses excited singlets of BN, causing their fluorescence spectra to be red-shifted with respect to those in hexane. 

In conclusion, we believe that our ability to observe higher atropisoraeric excesses from Irradiations of BN in cholesteric mesophases than from thermal lsomerlzations can be traced to the larger interaction energies associated with the excited state species and its environment. The cumulative effect of these Interactions is manifested more specifically on a reactive solute when the solvent molecules are uniquely ordered than when they are isotropically dispersed. 

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