Clearing columnar discotics

Chains of 8-AOB can be both stiffened and/or projected in off-angle directions when unsaturated groups are introduced. For example, a double bond places a stiff, bent segment along an alkyl chain. The liquid crystalline phase of 8-AOB is lost when an E or Z configuration double bond is placed near a chain middle (between C4 and C5 8(3)-D-AOB), but the clearing temperature - 356.SK for the E isomer and 354 K for the Z isomer, compared to 356.5 K for the saturated (parent) molecule - is affected only slightly [108]. By contrast, placement of the double bond at the ends of the alkanoyloxy chains (8(6)-D-AOB), stabilizes the columnar discotic phase which persists from 306 to 343 K [108]. 

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]. 

In general, for side chain liquid-crystalline polymers, macroscopic molecular alignment is not easy and therefore clear evidence of electronic charge carrier transport was confirmed first in liquid crystals with low molecular weight. In the 1990s, fast electronic conduction was verified in discotic columnar phases of triphenylene derivatives [79,80] and hexabenzocoronene derivatives [81,82] as well as smectic phases of 2-phenylbenzothiazole [83, 84] and 2-phenylnaphthalene derivatives [85], as shown in Fig. 14. Carrier... 

The general, 8-diketonate template has an overall lath-like structure, and subtle structural modifications can lead to crossover between smectic and columnar mesomorphism, or to structural change between discotic to calamitic shape, simply by changing the nature of the substituent and R (Figure 33). This is an important aspect of this type of complex, which would not be appreciated fully and understood if discussed separately. For instance, symmetrical systems (R = R, D2h) can have either disk-like or X-shaped structures, whereas the unsymmetrical compounds (R R ) can possess either zigzag- or cross-shaped structures (Figure 33). For the latter, both cis (C2v) and trans (C2h) isomers are also possible, and it is reasonable to think that mixtures are always present and that transicis interconversion occurs at elevated temperatures. This study is clearly a textbook case in addition to being a perfect illustration of induction, modification, or suppression of mesomorphism on complexation. 

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). 

Three-ring mesogens with two longer lateral alkyl chains (e.g. 12) do not show mesophases. However, cross-shaped compounds (30) have nematic phases (uniaxial) with clearing points above 100°C [46, 59]. That is surprising because 1,2,4-tris(4-n-alkyl-oxybenzoyloxy)benzenes (19f) are not liquid crystalline, but l,3,5-tris(4-n-alkyloxy-phenyl)benzoates exhibit nematic discotic phases [83]. Apparently, such substituted benzene derivatives are located at the boundary between calamitic and discotic compounds. However, there is no example of this type that shows both columnar and nematic or smectic phases, as occurs in poly-catenar and double-swallow tailed compounds (see Sec. 5 and Chap. XII of this volume). 

All the complexes showed a discotic hexagonal disordered columnar mesophase (D jj). The mesophases were identified by X-ray diffraction. Platinum complexes exhibit a very clear thermochromism the color of the liquid crystalline discotic mesophase turns from green to red with increasing temperature from room temperature. 

---