4-n-Alkoxy-4 -cyanobiphenyls

Table 2. Crystal and molecular data of mesogenic 4 -n-alkoxy-4-cyanobiphenyls (CBOn)... 

Table 6. Thermotropic behavior of 4-n-alkoxy-4 -cyanobiphenyls [228], poly n-[(4 -(4"-cyanophenyl)phen-oxy)alkyl]vinyl etherjs (DP =17-32, pdi = 1.09-1.21) [122-127,212,213], n-[(4 -(4"-cyanophenyl)phen-oxy)alkyl]vinyl ethers [122-127] and a-ethoxy-t0-(4-n-alkoxy-4 -cyanobiphenyl)s [122-127].

The more obvious choice for a model compound corresponds to exactly one repeat unit of the polymer. The a-ethoxy-m-(4-n-alkoxy-4 -cyanobiphenyl)s exhibit nematic mesophases at n=4-9 and SmA mesophases at n = 8-ll, and therefore match the thermotropic behavior of the polymer better than the vinyl ether monomer. However the SmA mesophase is enantiotropic only at n = 11, and the nematic mesophase is monotropic at all of these spacer lengths. Compounds which take into account only the mesogen and spacer are also good, if not better, models of the polymers. In contrast to the ethyl ethers, all of the SmA and most of the nematic mesophases are enantiotropic, which means that the melting temperature mimics the relative temperature of the glass transition of the polymer backbone better. However, the nematic mesophase still appears at n = 6 -11, and the SmA mesophase doen t appear at n = 5-7, 10, 11. 

Poly[4-[acryloyloxy-n-alkoxy-4 -cyanobiphenyl-co-4-[acryloyloxy-n-alkoxy-4 -methoxy azobenzenel... 

For a mesophase to obtain, it is generally necessary that the terminal groups of the molecules contain permanent dipoles. For example, the N-I transition of 4-octyloxybenzoic acid is some 40° higher than that of 4-nonylbenzoic acid despite the essentially equivalent size of the two molecules. Similarly, the 4-alkoxy-4 -cyanobiphenyls form substantially more stable mesophases than 4-alkyl-4 -cyanobiphenyls. Polarity of the termini, however, frequently gives rise to very strong intermolecular attractions. In such cases, the melting points of the compound may be raised so high that the mesophase cannot survive. 

Let us take first the relatively simple example of 4-pentyloxy-4 -cyanobiphenyl (50CB). It has a strong dipole parallel to the long axis of the molecule, associated with the donor alkoxy group and the acceptor nitrile. Therefore, the individual molecules will be polarized and in the bulk phase they will be polarizable. The material will be prone to nanosegregation as the aromatic units will associate more strongly through n-n interactions than the s-s interactions of the chains. Parallel correlations of the molecules will be preferred, but to minimize the dipolar interactions, the molecules will be required to pack in a head-to-tail... 

Fig. 3 Effect of the linking group on the odd-even character of the polymethylene-type spacer [ 14]. The latent entropy, A5ni/R, oscillates with the number of methylene units ( ) of the spacer. The three upper curves represent the dimer, and the two lower curves are for the monomer LCs. Filled symbols indicate the carbonate-type dimer, a,o)-bis[(4,4 -cyanobiphenyl) oxycarbonyloxy]alkane (CBC-n filled circles) and the monomer LC, 4 -n-alkoxycarbonyloxy-4-cyanobiphenyl (/lOCCB filled squares). The open symbols indicate the ether-type dimer, a,co-bis[(4,4 -cyanobiphenyl)oxy] alkane (CBA-n open circles), the ester-type dimer, a,o)-bis[(4,4 -cyanobiphenyl)carbonyloxy] alkane (CB-n open triangles), and the ether-type monomer LC, 4 - -alkoxy-4-cyanobiphenyl (nOCB open squares)...

The alkyl and alkoxy cyanobiphenyls (n-CB and n-OCB) derivatives synthesized by Gray and coworkers and the bicyclohexyl derivatives (PCH- and CCH-n), 4, bind rapidly to a variety of metals. The Sheffield Liquid Crystal group [12] used these compounds for the synthesis of metallomesogens with a linear geometry M = Pd, Pt, Rh. 

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