Disk-like molecule

Discotic LC are formed by disk-like molecules with aromatic cores and side chains that are either hydrophobic (i.e., thermotropic) or hydrophilic (i.e., lyotropic). The discotic nematic (No) phase behaves like a normal nematic phase formed by rod-like molecules, and the disk-like molecules are oriented with their short molecular axes parallel to the director but show no positional order. More ordered columnar phases are commonly formed by thermotropic discotics. The two-dimensional structure can pack the columns into a hexagonal or rectangular columnar phase, while within the columns, disks can be... 

Extensive studies have been conducted to investigate the formation of chiral columns or helical superstructures in chiral and nonchiral disk- [53], star- [54, 55], and board-shaped [56] molecules. However, spontaneous deracemization has never been unambiguously demonstrated in discotic columnar phases consisting of nonchiral or racemic molecules. We recently observed clear evidence showing chiral resolution in a disk-like molecules with a dibenzo[g,p]chrysene core [57]. 

Fig. 14 Disk-like molecules with a dibenzo[g,/>]chrysene core used for the deracemization experiment...

Nagayama H, Varshney SK, Goto M, Araoka F, Prasad V, Takezoe H (2010) Spontaneous deracemization of disk-like molecules in the columnar phase. Angew Chem hit Ed 49 445 148... 

In principle, all the six-membered ferric wheels [Fe6CI6(LM l7)6] (39) are isostructural and have idealized S g-molecular symmetry. However, there are fundamental differences concerning their crystal packing. For example, all the disk-like molecules of 39a are arranged in parallel and are piled in cylindrical columns, with all the iron centers superimposed. Each column is surrounded by six parallel columns, which are alternately dislocated by 1/3 c and 2/3 c against the central one (Fig. 13). 

To some extent, the design criteria for discotic mesogens are somewhat simpler than those for their calamitic counterparts. In many cases, it is possible to choose a favorite disk-like molecule, then add 6-8 peripheral alkyl chains to generate a mesomorphic (i.e., hquid-crystalline) material. Thus, phthalocyanines, triphenylenes, truxenes, and many other systems will generate mesogenic (i.e., liquid-crystal-like) materials with appropriate substitution these examples are shown in Fig. 24. Note that it is not necessary for the mesogen to have a planar core. 

In calamitic systems it is the long axis of the molecules that is correlated in the mesophases but in discotic systems it is the short axis and different types of organization are seen, although disk-like molecules also form a nematic phase in which a unique axis is orientationally correlated (Fig. 25). However, below the nematic phase is a series of columnar phases (Col) in which the disks are stacked up into columns, which are themselves arranged according to some symmetric pattern. Typical... 

A few words of clarification about the nomenclature are relevant here. Columnar phases have been known for many years they were evident, for example, in the work of Spegt and Skoulios (28) on metal soaps, although these are not classical disk-shaped molecules. In 1977, however, a hexasubstituted benzene derivative was reported (29), this derivative was the first example of a properly disk-like mesogen, and the term discotic was coined to describe the mesophases it formed. Thus, for example, the discotic hexagonal phase was labeled Dh- The introduction of this nomenclature has actually caused confusion as disk-like molecules are not alone in their capacity to form columnar phases (indeed, some... 

In contrast to calamitic mesogens, discotic liquid crystals are built from disk-like molecules that can arrange into different structures, such as the discotic nematic mesophase, the discotic columnar mesophase, or the discotic hexagonal meso-phase. 

Liquid crystals derive both their beauty and their usefulness from their ordered, yet liquid, state. At rest, they possess at least some orientational order at rest, but lack the full three-dimensional positional order of solid crystals. Orientational order implies that the molecules tend to point in the same direction, while positional order implies that the molecules centers of mass tend to lie on lattice points. Spherically symmetric molecules, for example, can have positional order, but no orientational order. Thus, when one heats a simple positionally ordered crystalline solid to its melting point, all of its positional order is destroyed in a single step, producing an isotropic liquid. Orientational order is, however, possible for rod-like or disk-like molecules. Crystalline solids made of such molecules can melt into isotropic liquids in multiple steps at intermediate stages of melting, there can be orientational order with no, or only partial, positional order. That is, the molecules might tend to be aligned in a common direction, but the centers of mass of the molecules take on random positions. Such intermediate phases are liquid crystals.------------------------... 

Scaled particle theory has not yet been discussed. Equation 13.4.39] is taken from Baumer and Findenegg but originally dates back to Helfand et al. The equation is rigorous for hard disk-like molecules it is combined with a mean field lateral Lennard-Jones pair interaction. In this equation their = na lA if a is the diameter of the disk, is the depth of the Lennard-Jones pair interactions (i.e. the minimum in fig. 1.4.1.a). In this case 6 = a F its maximum in a close-packed monolayer corresponds to (max) = 0.906. The accent emphasizes this different scaling. Baumer and Findenegg applied [3.4.39] to dilute monolayers of 1-chlorobutane, perfluorohexane and fluorobenzene, adsorbed on water from the gas phase. 

The pressure dependence of the Raman-active vibron modes (Figs. 11(a) and 11(b)) was studied on unloading at 300 K in new phases. 1-N2 exhibits typical behavior for such molecular crystals branching of vibrational modes and increasing of separation between them with pressure due to increasing intermolecular interactions. All of the vibrational modes originate from the same center, which is close to the frequency of the V2 disk-like molecules in E-N2. Thus, the structure of the i phase is characterized by the presence of just one type of site symmetry for the molecules and the large number of vibrational modes arises from a unit... 

Liquid crystalline phases can show not only long-range orientational order as nematic phases do but also long-range positional order. When this positional order is one-dimensional, the mesophase is called lamellar or smectic when it is two-dimensional, it is called columnar. The latter case is often found with thermotropic liquid-crystal disk-like molecules. Such molecules stack in columns that assemble on a 2-D lattice of hexagonal, rectangular, or oblique symmetry. The molecules in a given column only show 1-D liquid-Hke order and the uncorrelated columns are free to slide past each other, which ensures the mesophase fluidity [73]. 

As mentioned above, the liquid-crystal state exists between the solid and liquid states and is stabilized by the presence of intermolecular, anisotropic dispersion forces that result from the anisotropic nature of the molecules that form the phases. Thus, rod-like molecules are much longer than they are broad and, hence, possess one unique, long axis. Similarly, disk-like molecules are rather flat and, hence, possess one, unique short axis (Figure 2). 

Praefcke and co-workers reported a series of disk-shaped, dinuclear or//5o-palladated benzalimine complexes, 116, which were the first example of organometallic complexes showing the nematic phase of disk-like molecules,... 

Figure 4.5.4 Topology of cyclodextrin adducts, (a) A fitting cylinder is entrapped in an axial position, (b) a sphere may be sandwiched between the wide ends, and (c) a large disk-like molecule may act as cover.

Sorai M, Tsuji K, Suga H, Seki S (1980) Studies on disk-like molecules. I. Heat capacity of benzene-hexa-n-hexanoate. Mol. Cryst. Liq. Cryst. 59 33... 

Sorai M, Yoshioka H, Suga H (1982) Studies on mesogenic disk-like molecule. III. Heat capacity rf benzene-hexa-n-octanoate from 13 to 393 K. Mol. Cryst. Liq. Cryst. 84 39... 

Similarly to the molecular engineering of calamitic molecules to produce ferroelectric smectic C phases [129], disk-like molecules with chiral peripheral chains tilted with respect to the columnar axis were predicted to lead to ferroelectric columnar mesophases [130]. Indeed, as it is the case with all flat disk-shaped mesogenic molecules, the tilt is mainly associated with the flat rigid aromatic cores of the molecules, the side-chains being in a disordered state around the columnar core. Thus, the nearest part of the chains from the cores makes an angle with the plane of the tilted aromatic part of the molecules. If the chiral centre and the dipole moment are located close to the core, then each column possesses a non-zero time averaged dipole moment, and therefore a spontaneous polarization. For reasons of symmetry, this polarization must be, on average, perpendicular to both the columnar axis and to the tilt direction in other words, the polarization is parallel to the axis about which the disk-shaped molecules rotate when they tilt as shown in Fig. 29. 

I. Self-Diffusion, Viscosity and Density of Nearly Spherical and Disk Like Molecules in the Pure Liquid Phase... 

---