Asymmetric liquid-crystal dimer

Note 4 A liquid-crystal dimer with different mesogenic groups linked by a spacer is known as an asymmetric liquid-crystal dimer. 

Asymmetric dimeric liquid crystals with charge-transfer groups constitute good models for the mesomorphic self-organization of the related polymers discussed above. Their mesophase structures have been studied in some detail [34]. In this context, the formation of intercalated smectic phases of the A, C, and I type is especially noteworthy [34c]. However, the nature of the specific interactions between the unlike mesogenic groups, as well as the conformation of the spacer, are still to be explained [34d]. 

There are a number of possible explanations for the formation of more than one photodimer. First, due care is not always taken to ensure that the solid sample that is irradiated is crystallographically pure. Indeed, it is not at all simple to establish that all the crystals of the sample that will be exposed to light are of the same structure as the single crystal that was used for analysis of structure. A further possible cause is that there are two or more symmetry-independent molecules in the asymmetric unit then each will have a different environment and can, in principle, have contacts with neighbors that are suited to formation of different, topochemical, photodimers. This is illustrated by 61, which contrasts with monomers 62 to 65, which pack with only one molecule per asymmetric unit. Similarly, in monomers containing more than one olefinic bond there may be two or more intermolecular contacts that can lead to different, topochemical, dimers. Finally, any disorder in the crystal, for example due to defective structure or molecular-orientational disorder, can lead to formation of nontopochemical products in addition to the topochemical ones formed in the ordered phase. This would be true, too, in those cases where there is reaction in the liquid phase formed, for example, by local melting. 

Variation of proton disorder with temperature in carboxyl dimers has been observed by Kanters, Roelofsen and Kroon [88] in the crystal structure of monofluoromalonic acid, using X-ray diffraction. At liquid nitrogen temperature the carboxyl group is ordered, but at room temperature the protons are disordered. Direct evidence for dynamic double proton exchange in carboxyl dimers was demonstrated by solid state NMR studies in />-toluic acid [89] and benzoic acid [90]. For the / -toluic acid dimer, Ernst and coworkers deduced an asymmetric double minimum potential 19 with an energy difference AG = 0.24kcalmol and an activation energy = 1.14 kcal mol between the two tautomers. At room temperature... 

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