Nematic phase homologous series

Zugenmaier and Heiske [60] presented the crystal and molecular structures of the homologous series of 4 -(hydroxy-l-n-alkoxy)-4-cyanobi-phenyls (CBO(CH2)nOH) n = 4, 5, 7-11). The chemical structure of the compounds is shown in Fig. 1. All compounds of the series exhibit a nematic phase. The crystal and molecular data of the investigated compounds CBO(CH2)nOH and some derivatives are presented in Table 3. 

Mandal et al. [89-91] investigated the crystal structures of three members of the homologous series of 5-(4 -n-alkylcyclohexyl)-2-(4"-cyanophenyl)-pyrimi-dines. The crystal structures of the ethyl (ECCPP), pentyl (PCCPP), and heptyl (HCCPP) compounds were determined. The chemical structure of the compounds is presented in Fig. 15. The two lower homologues possess only a nematic phase, while the heptyl compound has a smectic phase in addition to a nematic phase. 

Several 4-(3-alkyl-2-isoxazolin-5-yl)phenol derivatives that possess liquid crystal properties have also been obtained (533-535). In particular, target compounds such as 463 (R = pentyl, nonyl) have been prepared by the reaction of 4-acetoxystyrene with the nitrile oxide derived from hexanal oxime, followed by alkaline hydrolysis of the acetate and esterification (535). A homologous series of 3-[4-alkyloxyphenyl]-5-[3,4-methylenedioxybenzyl]-2-isoxazolines, having chiral properties has been synthesized by the reaction of nitrile oxides, from the dehydrogenation of 4-alkyloxybenzaldoximes. These compounds exhibit cholesteric phase or chiral nematic phase (N ), smectic A (S4), and chiral smectic phases (Sc ), some at or just above room temperature (536). 

NMR spectra and Tj measurements at different temperatures. The local polymer chain motion varies over a frequency range of 104-106 Hz in the nematic phase. The activation energy of this motion is found to increase with decreasing number ( ) of methylene units in the spacer, and exhibits odd-even fluctuations. In a study of a homologous series of main-chain LC polyesters, 13C CP/MAS and variable-temperature experiments reveal a conformation-ally more homogeneous and a less dynamic nature for the even-chained than for the odd-chained polymer structures.300... 

Shirakawa, T, Hayakawa, T., and Tokuda, T, Pressure-volume-temperahire relations and isotropic-nematic phase transitions for a 4 -n-alkyl-4-cyanobiphenyl homologous series, J. Phys. Chem., 87,1406-1408 (1983). 

The narrower a range of the nematic phase in a homological series of different compounds (as an example see Fig. 6.1 la) the stronger are first order features of the N-SmA transition. In some sense, the SmA phase feels the proximity of the isotropic phase. In other words, we may say that, in the isotropic phase, there are traces of both nematic and smectic A short-range order. 

We mentioned already cholesteryl benzoate as one of the first discovered liquid crystals, although the chemical structure of cholesterol was unknown at the time. From this compound the name cholesteric phase was derived. p-Azoxyanisole (PAA), which belongs to the nOAOB homologous series (cf. Table I), was one of the first synthesized liquid crystals with known chemical structure. It is a typical representative of a rodlike compound. The first six members of this series form the nematic phase,... 

To date, four homologous series of carborane and one series of tricarbollide mesogens have been investigated. Series 38,40, 64 were investigated up to n = 10, series 66L to n = 18, and diesters 59D were prepared to n = 22. In all series of carborane derivatives no smectic phases were detected. For instance, diester 59D[22] exhibits only a nematic phase, while in the hydrocarbon analogs the smectic phases onset at n = 5 and completely replace the nematic phase at n = 12. 

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