Macrocyclic liquid crystals

The same type of columnar structure is also proposed for other polycatenar materials, such as tetrone derivatives [51], phasmidic macrocyclic liquid crystals [52], silver (I) complexes [53], or 2,2 -bipyridine derivatives [54]. 

Since the discovery of crown ethers, cryptands, and other macrocyclic ligands by Cram, Lehn, and Pedersen, who were awarded the 1987 Nobel Prize in chemistry for their development and use of molecules with structure-specific interactions of high selectivity [1], a completely new research field was opened supramolecular chemistry [2-4-]. Since then, this research field has been extended in many fields such as molecular recognition, organic sensing, and liquid crystals. 

Hirose studied liquid crystal 105 consisting of a central dibenzo[18]crown-6 macrocycle and Percec-type side group dendrons (Scheme 53) [116]. 

Macrocyclization under high dilution conditions has been used to prepare different liquid crystals composed of 2-phenylpyrimidine or 5-phenylpyrimidine connected by PEG units <03JOC597>. Treatment of polyoxypropylenediamine (Jeffamine) with 1,4-benzoquinones afforded low but variable yields of the 2,5-bridged quinoid(aza)crown ethers not utilizing high dilution conditions <03TL5531>. 

In summary, the turnstile represents a new example of rotation in a molecular system. The fact that phenylacetylene macrocycles form discotic liquid crystals suggests the possibility that turnstiles might function as discotic ferroelectric liquid crystals [34]. A dipole appropriately incorporated on the spindle might allow for its own rotation to be controlled rapidly and reversibly by an external field. 

Applications of this method include among others the preparation of unsym-metrical diaryldiynes [27] and of l,4-bis[2-(4 ,4"-dialkoxyphenyl)ethenyl]benzenes for polymer [28] and liquid-crystal synthesis [29]. Such one-pot acetylene double couplings are also possible between (substituted) 6-bromoazalenes and substituted bromobenzenes [30], and even macrocyclic polyenepolyynes have been made in this way ([31, 32], Scheme 3). 

Phthalocyanines (Pc) constitute an important class of compounds for the formation of liquid crystals. Elements from groups Ia to Vb of the periodic table can coordinate to the four nitrogen atoms of the phthalocyanine macrocycle, leading to more than 70 types of metallic complex. Small divalent ions are accommodated in the centre of the ring forming a planar, tetracoordinated complex, while heavier ions are situated out of the plane (Figure 6.28). 

From the standpoint of molecular complexity, the most intriguing applications of thiophene and its derivatives focus either on the occurrence of the thiophene moiety in naturally occurring molecules or in pharmacological agents. Transformation of thiophene into other moieties also produces some remarkable molecules. From a commercial view, the unique electronic and physical properties of thiophene make it ideal for applications in polymers, liquid crystals and dyes. Examples of thiophene-containing cyclophanes and other macrocycles as well as organometallic derivatives complete this chapter. 

A recent observation has been that not all porphyrin LC phases are discotic. Thus 5,15-meso-substituted porphyrins have been prepared which show a variety of smectic phases that are not discotic. The porphyrin macrocycle could be acting to impose biaxial symmetry within the meso-phase, which makes the new porphyrins of great interest as new nematic liquid-crystal materials. 

Due to the disc-like geometry of the porphyrin macrocycle, discotic mesophases have generally been the most common for porphyrin liquid-crystalline materials. The first porphyrin liquid crystals featuring rod-like calamitic mesophases were described by Bruce and coworkers using substituted 5,15-di(phenyl) zinc porphyrin complexes as the basic molecular building block.The first materials... 

More importantly, upon addition of a silver salt, these helical tubules were segmented into slices of tubules while maintaining helical order in these discrete nanostructures. The preservation of the shape-persistent hexameric macrocycles in this transition is responsible for the retention of supramolecular chirality. Transition from chiral tubules into discrete nanostructures with maintaining the supramolecular chirality is reported for thermotropic liquid crystals of dendritic molecules [82-84], There is, however, no study about such a transition for weU-defined nanoscale synthetic... 

Liquid crystals formed by functionalized cyclo[8]pyrrole have been employed as sensors for nitroaromatic explosive compounds such as TNT (trinilrotoluene) (Figure 6). The macrocycles stack with the aromatic compounds forming an ordered mesophase that can be detected by microscopy. Sessler and coworkers have used anion-induced synthesis to produce a new series of 2,6-diamidopyridine-bipyrrole... 

Discotic liquid crystals of phenylacetylene macrocycles 29 were first reported by Zhang and Moore in 1994 [136]. Their synthesis involves the stepwise build-up of phenylacetylene hexamers [137] followed by a macrocyclization step [138]. The key reactions of the synthesis (Scheme 33) are the palladium-catalysed coupling of aryl iodides and acetylenes, and the selective deprotection steps allowing the ends of the growing oligomer to be distinguished. The synthesis shown is just one of the many possible sequences that can be used to prepare the macrocycles. The mesophase behaviour of the discotic derivatives prepared to date is shown in Table 35 [136]. In some ways... 

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