Alkyl cyanobiphenyls

Table 2. Selected rotational viscosities and Leslie viscosities ui of the liquid crystalline n-alkyl-cyanobiphenyl series (nCB, n 5-8) obtained by FC flow experiments at di rent temperatures 6kT=Tc T relative to the nematic-to-isotropic transition temperature Tq (clearing point). Within the large error estimations of 10% for yj and 2, 100% for (04 + 05), 300% for 03, and 400% for oi, the results are essentially consistent with data reported in the literature. ...

As mentioned above, the value of the molecular hyperpolarisability /3 is large for the well known alkyl-cyanobiphenyl (nCB) molecules, that possess strongly delocalized molecular electronic orbitals with oppositely attached electron-donor and electron-acceptor groups. Due to the rod-like shape of these molecules, the component of /3 associated with the perturbation of the electronic density along the long molecular axis 63 is usually much larger than the other components. In first approximation therefore only fSsss h s to be... 

All the experimental data for MBBA, alkyl cyanobiphenyls and alkyl cyanopyrlmidines (97-102) show that d K... 

The isotropic phases of the nCBs (alkyl-cyanobiphenyls) were also studied by Butler [19] over wide temperature ranges including both pretransitional and normal liquid regions of the fluid. No evidence of pretransitional effects was observed in the Brillouin shifts, even for 12CB which exhibits... 

Frost and Lalanne [77] have performed similar experiment in MBBA. The molecular theory of orientational fluctuations and Kerr effect can be found in the work of Flytzanis and Shen [76]. The optical Kerr effect in the alkoxyazoxybenzene homologous series is reported by Hanson et al. [78] and the same technique was used by Coles [79] in the studies of the pretransitional dynamics of alkyl cyanobiphenyl homologs. 

Alkyl cyanobiphenyls provide classical examples of interdigitated bilayer smectic A phases [20]. These materials are also used in applications of smectic liquid crystals (see Sec. 3 of this chapter) in thermally addressed storage devices. Figure 27 shows... 

AS/R, for the cyanobiphenyl dimers with the number of atoms, n, in the flexible spacer predicted with the continuous model for the torsional potential (—) and by the discrete model For comparison the corresponding predictions for the alkyl cyanobiphenyl monomers based on the continuous potential are also shown (—). 

Another way to consider these predictions for the continuous potential model is that in the limit of long spacers the relative orientations of the two mesogenic groups will be decoupled. In this limit it would then be expected that the dimers should behave like monomers. To see if this expectation is likely to be met the transitional entropy for an alkyl cyanobiphenyl was calculated using the continuous model with the same parameters as for the dimers. The results are shown in Fig. 27 and reveal that as the length of the terminal chain is increased so AS/R exhibits an overall increase with a small alternation superimposed on this. In addition, the transitional entropy for the monomers is comparable to that for the odd dimers, in keeping with the fact that the conformational distribution for the dimers does... 

Table 8-1. Latent Heats measured in pure 8CB by different techniques. A comparison of latent heats (error bars in parentheses) from Intensity fluctuation microscopy (IFM latent heats are estimated from the measured (/q = I.2 0.I) x 10 [66] and Landau parameters in [65]), Adiabatic Scanning Calorimetry (ASC errors from [69]) and Modulated differential scanning calorimetry (MDSC errors estimated from smaller error bar). Also listed is the molar fraction X of lOCB in 8CB-10CB (alkyl-cyanobiphenyl) mixtures at which the latent heat appears to vanish...

Figure Bl.19.8. (a) STM image (5.7 mu x 5.7 mu) of 10-alkylcyanobiphenyl on graphite (b) model showing the packing of the molecules. The shaded and unshaded segments represent the alkyl tails and the cyanobiphenyl head groups, respectively. (Taken from [38], figure 2.)...

Fig. 4a,b. The effect of substitution of an alkoxy tail for an alkyl tail in cyanobiphenyls a transition sequence is K kf N b transition sequence is K N /... 

The first report on the liquid crystalline properties of these compounds was published by Gray and Mosley [44] in 1976. The series of 4 -n-alkyl-4-cyanobiphenyls (CBn) have been widely studied by different methods due to their readily accessible nematic ranges around room temperature. The compounds have the phase sequences crystal-nematic-isotropic for CBS, CBIO, and monotropic nematic for CBS, CB4 crystal-smectic A-nematic-isotropic for CB9 crystal-smectic A-isotropic for CBll. The lower homologous CB2 is nonmesogenic. The general chemical structure of the compounds CBn is presented in Fig. 1. 

Note Examples are potassium salts of unbranched alkanoic acids, lecithin, certain polyisocyanates, cellulose derivatives with side-chains, such as (2-hydroxypropyl)cellulose, and cyanobiphenyl derivatives of alkyl(triethyl)ammonium bromide. 

Correlations between molecular structure and the LC state have been fruitfully employed to design 4-alkyl and 4-alkoxy-4 -cyanobiphenyls, which form the basis for electrooptic display devices. The reasoning behind the design of these nematogens is as follows. In a family of nematogens of the general formula... 

For device purposes, molecules used should be stable to heat and light. The LC phases have to be stable over a wide temperature range around room-temperature. This can be achieved by using mixtures of mesogens. A mixture of cyanobiphenyls, one with alkyl substituent (CjH,) and the other with alkoxy substituent (CgHi70), has proved to be most useful. 

Bromomethyl-2-cyanobiphenyl (4.6 g) was alkylated onto 2-n-butyl-4-chloro-5-(hydroxymethyl)-imidazole. For separation of the product was used a flash chromatography in 1 1 hexane/ethyl acetate over silica gel. The regioisomeric products yielded 2.53 g of the faster eluting isomer. Recrystallization from acetonitrile yielded 1.57 g of analytically pure 2-n-butyl-4-chloro-l-[2,-cyanobiphenyl-4-yl)methyl]-5-(hydroxymethyl)-imidazole, melting point 153.5 -155.5°C. 

Our group investigated the influence of the spacer length of 2,3-disubsti-tuted norbornene derivatives with cyanobiphenyl-mesogenic units [42] and with methoxybiphenyl-mesogenic units [43] on the mesophases. Exo,endo-his-[(4 -cyanobiphenyl-4-yl)oxy-n-alkyl]norborn-5-ene-2,3-dicarboxylates with different alkyl spacer lengths (IV-n, n=2-12) were synthesized in a straightforward manner. 

Fig. 33 Illustration of the two extremes, homogeneous and segregated, of the distribution of hydrogen-bonded side chains along the backbone in the case of a substoichiometric composition. System PAA(LC) investigated consists of poly(acrylic acid) and side chains containing cyanobiphenyl mesogens connected with an alkyl chain to imidazole-based hydrogen-bond acceptors (Scheme 10c). Reprinted with permission from [242]. 2006 American Chemical Society...

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