Discotic liquid crystalline phase

The interest in the structures of simple R2Si(OH)2 compounds lies in the fact that one of them, Bu 2Si(OH)2, forms a discotic liquid crystalline phase (308,309). Despite many attempts, it has not proved possible to obtain crystals of Bu 2Si(OH)2 suitable for a crystallographic study, the material obtained from various solvents usually being of a fine fibrous nature. The discotic phase of Bu 2Si(OH)2 has been proposed (309) to be due to the formation of dimeric disks of molecules which remain on breaking the interdimer hydrogen bonds in a structure of type 65 at the transition between crystal and mesophase. As has been described, structure type 65 is found for several diols similar to Bu 2Si(OH)2, and it is thus quite likely that Bu 2Si(OH)2 does indeed have the proposed structure. 

Just as chiral induction can be realised in discotic liquid crystals, it can also be realised in assemblies of disc-like molecules or disc-like aggregates. As far as molecules are concerned, C3-symmetrical trisamides (Fig. 15), which actually exhibit discotic liquid crystalline phases, also form chiral columnar stacks through it-it interactions when dissolved in apolar solvents, which are depicted schematically in Fig. 15 [121]. An achiral compound of this type (15) exhibits no optical activity in dodecane, but when the compound is dissolved in the chiral CR)-(-)-2,6-dimelhyloctanc significant Cotton effects (only slightly less intense than those observed in a chiral derivative) are detected. The chiral disc-like trisamide 16 can also be used as a dopant at concentrations as low as 2.5% to induce supramolecular chirality in the stacks of achiral compound. In this case, the presence of the additional hydrogen... 

Just as chiral induction can be realized in discotic liquid crystals, so it can in assemblies of disc-like molecules or disc-like aggregates. As far as molecules are concerned, C3-symmetrical fm-amides (Fig. 6), which exhibit discotic liquid-crystalline phases, also form chiral columnar stacks through n-n interactions when dissolved in apolar solvents,which are depicted schematically in... 

Koerner et al. reported that cyanate ester monomer Cl and C3 could be oriented and polymerized in an a-c electric field [39] and that the orientation of the molecules was controllable by changing the frequency of the field. The cyanate ester LC monomer could self-organize in such a dynamic environment. This orientation-on-demand technique will provide a new method to control and modify high performance materials. It is claimed that triazine systems based on 2,4,6-tris[4-aminophenyl)-alkoxybenzylidene]-l,3,5-triazine form a discotic liquid crystalline phase [50]. This report made the investigation of such networks of even more interest. 

FIGURE 3.1 Schematic representations of commonly observed discotic liquid crystalline phases. Discotic nematic (a) columnar nematic (b) columnar hexagonal (c) columnar rectangular (d). 

Nematic liquid crystalline phases can be thought of as smectic phases absent the layering effect (Figure 13.7 A). There is clearly still a preferred orientation, a director, but that is the only type of order. Another type of phase termed the columnar or discotic liquid crystalline phase can form when the mesogen is more disk-shaped rather than cigar-shaped (Figure 13.7 A). 

It is not possible to predict from the related crystal structure alone whether the compound will melt to a liquid crystalline phase or not, because the anisotropic molecules (calamitic and discotic ones) form in favourable anisotropic packing. As a rule long shaped rod-like molecules quite often possess a layered arrangement in the solid state regardless of whether the compound is mesogenic or not. 

The directors (long molecular axes) of the constituent molecules in nematic phases are parallel to one another on average. This is the only order present in nematic liquid crystals, which are the most fluid type of liquid-crystalline phase. Molecules that form cholesteric phases must be optically active or contain an optically active dopant. As the phase name implies, the constituent molecules are frequently steroids and most commonly are cholesteric esters or halides. A conceptual model of the cholesteric phase includes layers of molecules in nematic-like positions, each layer being twisted slightly with respect to the ones above and below it. When the phase consists only of optically active molecules, the angle of twist between layers is typically less than one degree. Several subclasses of discotic phases exist. In all, the molecular planes of the constituent molecules are parallel. However, the discs can pack in nematic-like arrangements (ND) or in columns that are internally ordered (D ) or disordered (Dd) and may be stacked vertically,... 

Stilbenoid dendrimers are able to undergo aggregation. Depending upon the generation number, some of the pure substances form liquid-crystalline phases (Dha discotic hexagonal disordered phase Dra discotic rectangular disordered phase Dob discotic distorted phase). Differential scanning calorimetry (DSC) revealed phase transitions between 99°C and 0°C. 

Recent work focuses on non-classical mesogenes which are built up by self-assembly. One example is a family of polymers containing disk-like groups which form no liquid crystalline phase, but ean act as an electron acceptor or donor. Charge transfer complexation with a complementary low molecular mass compound induces nematic or columnar discotic liquid crystalline order [153,154]. Figure 13 demonstrates this with the example of a polyester, bearing electron-rich tetra(alkoxy)tri-phenylene-units in the main chain, mixed with the electron deficient aromatic 2,4,7-trinitro-9-fluorenone (TNF). While the pure polymer shows a non-ordered isotropic melt, a columnar phase appears on addition of TNF. 

Liquid-crystalline phases are characterized to some degree by the shape of the molecules and by their packing arrangements and ordering in the mesomorphic state. Typically, molecules can have cither disc- or rod-like shapes and can form discotic or calamitic mesophases, respectively. Ferrocene liquid crystal systems that have so far been synthesized tend to have molecular structures that are lath- or rod-like in shape, and consequently the phases observed are calamitic. However, this does not preclude the possibility that a polysubstituted ferrocene could be prepared where the molecular shape is disc-like, thereby holding out the prospect of possibly producing discotic/columnar phases. 

In the simplest liquid-crystalline phase, namely the uniaxial nematic, there is at rest a special direction designated by a unit vector n called the director (see Fig. 10-2). In the plane transverse to the director, the fluid is isotropic. The most common nematics are composed of oblong molecules that tend to point in a common direction, which defines the director orientation. Oblate, or disc-like, molecules can also form uniaxial nematics for these discotic nematics, the director is defined by the average orientation of the short axis of the molecule. Lath-like molecules or micelles (shaped like rectangular slabs), in which all three dimensions of the molecule are significantly different from each other, can form biaxial nematics (Praefcke et al. 1991 Chandrasekhar 1992 Fialtkowski 1997). A biaxial... 

Fig. 16. H- H DQ spinning sideband patterns of discotic HBC-C 2 in the solid state (top) and in the columnar liquid crystalline phase (bottom), where the discs rotate around the column axis as indicated. For details, see ref. 38.

Pyridone forms a hydrogen-bonded dimer with a closed structure [136]. This dimerization has been used for the design of crystalline assemblies such as three-dimensional networks [136b]. This hydrogen bonding has been used for the formation of discotic liquid-crystalline assemblies [137]. Disk-like complex 58 with dodecyl chains displays a hexagonal columnar phase (Dh) between 88 and 108 °C. Columnar mesophases are also exhibited by benzamide dimers [138]. 

The need to add either salt or a long-chain alcohol in order to prepare nematic phases from classical surfactants (e.g., SDS-decanol-water [154] decylammonium chloride-ammonium chloride-water [237]) may be understood in terms of this model. These additives shift the liquid crystalline phases away from their optimum concentration ranges and leading to a buildup of frustration. This frustration is resolved by the introduction of local structural fluctuations that destabilize the mesophases and lead to finite micelles of the appropriate sizes [177]. For instance, decanol limits the domains of existence of the mesophases for the system SDS-decanol-water [238]. The addition of NH4CI to the decylammonium chloride-water system suppresses the hexagonal phase and enhances the stability of the discotic phase [239]. 

Dan et al. reported the fabrication of discotic nematic liquid crystalline phase by giant GO flakes with very high aspect ratio (10" ) in water [120]. It was observed that higher aspect ratio led to a dramatic change in mesomorphic behavior and the phase transition took place at concentrations with one order of magnitude lower than usually observed. The GO flakes were prepared by an improved synthetic method which furnished more regular structures and better intact basal plane... 

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