The Columnar Phases

Berardi et al. [66] have also investigated the influence of central dipoles in discotic molecules. This system was studied using canonical Monte Carlo simulations at constant density over a range of temperatures for a system of 1000 molecules. Just as in discotic systems with no dipolar interaction, isotropic, nematic and columnar phases are observed, although at the low density studied the columnar phase has cavities within the structure. This effect was discovered in an earlier constant density investigation of the phase behaviour of discotic Gay-Berne molecules and is due to the signiflcant difference between the natural densities of the columnar and nematic phases... 

The discotic mesophases are classified in two types columnar, and nematic discotic. The structure of the nematic discotic mesophase (Np, Figure 8.3, left) is similar to that of rod-like molecules, but constituted by disk-like units. In columnar mesophases, the molecules are stacked in a columnar disposition and, depending on the type of columnar arrangement, several columnar mesophases are known. The most common lattices of the columnar phases are nematic discotic (No), columnar nematic (Ncoi), columnar hexagonal (Coin), and columnar rectangular (Col,) mesophases. 

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. 

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. 

Investigation of symmetric and non-symmetric triphenylenes combining three fluorinated and three non-fhiorinated chains (compounds 149 and 150 in Fig. 40) has shown that these compounds can possess a higher stability of the columnar phases compared to those with all chains semiperfluorinated (compound 148), whereas the distribution of the fluorinated chains (symmetric vs. non-symmetric) does not play a significant role [272],... 

Copper complex 114c also shows columnar hexagonal phases [125]. Copper leads to an increase in the columnar phase range. The melting point is lowered to 94 °C while the decomposition does not occur before 345 °C. 

Fig. 9 Textures of the columnar phases of compound 34b. Pseudo-focal conic texture at 92 °C of the Colhd phase (bottom) and at 74 °C in the body-centred orthorhombic columnar phase (top). In order to increase the dynamic range of the photographs various photos with different exposure times have been superimposed. The black Maltese cross is perfectly dark in the centre for the Colhd- In the low temperature phase undulation of the columns produces birefringence in the dark branches...

The columnar phases of oligobenzoate Hekates are most probably formed from -shaped conformers which pack disordered in hexagonal columnar phases and for symmetric mesogens they can tilt and aggregate in highly ordered helical arrangements to form the 3D columnar structures (see Fig. 10a, b). Indications... 

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... 

Compounds 1-3 exhibit mechano- and/or thermochromic photoluminescent properties accompanied by cubic-columnar LC phase transitions. Table 1 shows the LC properties of compounds 1-3. Compounds 1 and 3 show cubic phases and columnar phases on rapid cooling and slow cooling, respectively. A columnar phase is not observed for compound 2 on cooling. The differential scanning calorimetry (DSC) measurements of compounds 1-3 show that the cubic phases of compounds 1-3 are metastable phases, while the columnar phases are stable. The X-ray diffraction patterns indicate that compounds 2 and 3 form Pmhn cubic phases and PH a rectangular columnar phases. 

Fig. 3 UV-Vis absorption (solid line) and photoluminescence (dashed line) spectra of (a) I and (b) 3. Top in chloroform solutions middle, in the cubic phases bottom in the columnar phases...

Hence, columnar (discotic) liquid crystals with an aromatic central core are of significant interest for application in OLEDs as an HTL, if the columnar phase can be aligned with the columns orthogonal to the substrate surface, i.e. the flow of holes from the anode to the ETL and/or emission layer is... 

Note that, for some time, the columnar phase of these polycatenar mesogens was given the symbol 0. However, following the arguments presented above, the use of Col is more appropriate and is now vddely accepted. 

Now let us turn our attention to the related palladium and platinum complexes (59). The first point to note is that for the hexacatenar complexes derived from 3,4,5-trialkoxystilbazoles, the mesomorphism is dominated by the columnar phase, although very recent X-ray studies, yet to be published (60), show that there are in fact two columnar phases in the phase diagram. 

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