Disc-like molecules

In summary, a vast number of materials has been used to fonn LB films. However, in the majority of cases an effort to characterize the film stmcture or even to show that a regular layer stmcture has been achieved is lacking. Work on the stmcture of films of disc-like molecules such as porjDhyrins and phthalocyanines is especially limited. Some references can be found in [29]. 

The prime requirement for the formation of a thermotropic liquid crystal is an anisotropy in the molecular shape. It is to be expected, therefore, that disc-like molecules as well as rod-like molecules should exhibit liquid crystal behaviour. Indeed this possibility was appreciated many years ago by Vorlander [56] although it was not until relatively recently that the first examples of discotic liquid crystals were reported by Chandrasekhar et al. [57]. It is now recognised that discotic molecules can form a variety of columnar mesophases as well as nematic and chiral nematic phases [58]. 

Fig. 10a,b. The distance dependence of the Gay-Berne potential for disc-like molecules... 

Since the discovery of discotic liquid crystals [121], the mesophases formed by rod-like and disc-like molecules have been considered as belonging to different liquid crystalline classes. Indeed, the conventional rod-like and disc-... 

Note 2 The adjective diseotic is also employed to describe the nematic mesophases formed by diseotic mesogens. The mesophases formed by a columnar stacking of disc-like molecules are described as columnar mesophases. 

Note 2 The symmetry and structure of a nematic mesophase formed from disc-like molecules is identical to that formed from rod-like molecules. It is recommended therefore, that the subscript D is removed from the symbol No , often used to denote a nematic formed from disc-like molecules. 

Note 6 Sanidic structures are analogous to the columnar mesophases formed by disc-like molecules. 

In the last few years disc-like molecules have been shown to form liquid crystals (Chandrasekhar, 1994). Typical of them are hexasubstituted esters of benzene (I) and certain porphyrin esters (II) (see below). In the liquid crystalline state, the disc-like molecules are stacked aperiodically in columns (liquid-like), the different columns packing in a two-dimensional array (crystal-like). The phases have translational periodicity in two dimensions but liquid-like disorder in the third. In addition to the columnar phase(D), the disc-like molecules also exhibit a nematic phase (Nj,). A transition between D and phases has been reported. 

A group of antibiotics isolated from soil fungi, the enniatins,77 also show ionophoric activity. Enniatin B (16), a cyclohexadepsipeptide, has a greater selectivity for Na+ over K+. With the latter ion stoichiometries other than 1 1 are found in solution. These are attributed to stacking interactions between the adjacent disc-like molecules as observed in the solid state.76 Interestingly, T1 ,... 

Sorai, M. and Suga, H. Studies on mesogenic disc-like molecules. II. Heat capacity of benzene-hexa-n-heptanoate from 13 to 393 K. Mol. Cryst. Liq. Cryst. 73, 47 (1981)... 

In discotic phases the orientation of the molecules is perpendicular to the molecular plane. Here, the columns can be arranged in a nematic or columnar manner. In the nematic phase the molecules possess a centre of gravity randomly ordered, but with the short molecular axis of each molecule more or less parallel. In the columnar phase, beside the preferable orientation of the short molecular axes, the disc-like molecules are ordered forming columns. Depending on the correlation strength between he columns these phases can be subdivided into ordered or disordered. A third possibility is to have a thermodynamically preferable position of the columns in the mesophase, like in a hexagonal cell. Additionally, a tilt of the columns is also possible. 

Fig. 1 Organization of rod-like molecules (top) and disc-like molecules (bottom) in LC phases (for clarity the alkyl chains are not shown in the models of the phase structures). Abbreviations Iso isotropic liquid state N nematic LC phase SmA smectic A phase, SmC smectic C phase (tilted), Col columnar phase [8]...

Fig. 39 Examples of triphenylene based disc-like molecules incorporating fluorinated segments in their peripheral chains (77° C) [267, 268]...

Disc-like molecules with a pentakis(phenylethynyl)phenoxy core commonly form nematic phases which can be strongly stabilized by the presence of only one... 

Fig. 41 Structure of a disc-like molecule with one RF-chain and (b) the organization in the...

Nagayama H, Varshney SK, Goto M, Araoka F, Ishikawa K (2010) Spontaneous deracemization of disc-like molecules in the columnar phase. Angew Chem Int Ed 48 445 148... 

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... 

Fig. 15 Self-assembly of an intramolecularly hydrogen bonded disc-like molecule [121]...

FIG. 6.13 Schematic representation of some characteristic liquid crystal phase structures. Nematic, smectic and cholesteric phases formed from rod-like molecules columnar phases formed from disc-like molecules (from Jansen, 1996). 

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... 

Thermotropic liquid crystals can then be furflier subdivided into high molecular mass, main and side-chain polymers [10] and low molecular mass, the latter class of compounds being one of the areas of this review. The phases exhibited by the low molecular mass molecules are then properly described with reference to the symmetry and/or supramolecular geometry of the phases, which are briefly introduced here and are discussed in more detail further below. Thus, the most disordered mesophase is the nematic (N), which is found for calamitic molecules (N), discoidal molecules (Nq) and columnar aggregates (Nc), among others. The more ordered lamellar or smectic phases (S) [11, 12] are commonly shown by calamitic molecules, and there exists a variety of such phases distinguished by a subscripted letter (e. g. Sa, Sb)- Columnar phases (often, if incorrectly, referred to as discotic phases) may be formed from stacks of disc-like molecules, or from... 

As discussed earlier, structural anisotropy is paramount in importance when considering the intermolecular interactions necessary to promote mesomorphism. The structural anisotropy is related to the molecular shape and we shall now endeavour to investigate its effect upon the mesomorphic properties of several complexes. In several instances we have discussed N, Sa and Sc phases arising from calamitic molecules, and in later sections we shall discuss disc-like molecules exhibiting columnar mesophases. Unfortunately such rigid definitions of molecular... 

Phthalocyanines represent a class of inherently disc-like molecules and consequently were among some of the first examples of metallomesogens exhibiting columnar mesophases, being first reported by Simon and co-workers in 1982 [122]. Thus, they described an octasubstituted phthalocyanine copper(Il) complex (Figure 65) which exhibited a mesophase between 53°C and the start of decomposition at about 300°C X-ray diffraction studies showed lines with reciprocal spacing of 1 3 v 4 7,... 

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