Cyanobiphenyls dimers

Fig. 8. The dependence of the nematic-isotropic transition temperature, T i, on the total number of atoms, n, in the spacer for the cyanobiphenyl dimers linked via ether (O) [18], alkyl ( ) [16] and carbonate groups ( ) [16, 53]...

The results of these calculations for the N-I transition temperature based on the continuous torsional model are shown in Fig. 26 for the methylene linked cyanobiphenyl dimers with the spacer containing from 3 to 18 atoms. For ease of comparison, the transition temperatures have been scaled with the value for the sixth member of the homologous series. The results reveal that the alternation in is rapidly attenuated with increasing spacer length and that for spacers containing more than about 11 atoms, the alternation in Tn-i is essentially... 

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 (—). 

The dimer model proposed by Wilson [83] was found not to exhibit a nematic phase but smectic A and B phases and so, although semi-realistic features are included, the model is not able to reproduce the behaviour of real dimers such as the a,co-bis(4 -cyanobiphenyl-4-yloxy) [87], which exhibit nematic phases, although liquid crystal dimers exhibiting smectic phases are also known [88]. In these simulations, the GB-GB interaction appears to be too strong and so the system forms a smectic phase in which the GB and LJ units... 

The most extensively studied series of liquid crystal dimers are the a,m-bis(4 -cyanobiphenyl-4-yloxy)alkanes [ 18-28],... 

The most extensively characterised examples of non-symmetric dimers are the a-(4-cyanobiphenyl-4 -yloxy)-co-(4-n-alkylanilinebenzylidene-4 -oxy)al-kanes. 

FIGURE 7.6 Effect of the linking group on the odd-even character of the PM-type spacer. The transition entropy, 45ni/R, oscillates with the numher of methylene units (n) of the spacer. The upper three curves represent the dimer, and the lower two are for the monomer LCs. The filled symbols indicate the carbonate-type dimer, a ,ester-type dimer, a,< -bis[(4,4 -cyanobiphenyl)carbonyloxy]alkane (CB-n triangles), and the ether-type monomer LC, 4 -n-aUcoxy-4-cyanobiphenyl (nOCB squares). 

Optical and NMR studies [6,7] indicate, that molecules of cyanobiphenyl liquid crystals (n-CB, n=5-12) penetrate between the DMOAP alkyl chains and orient parallel to them. The experiments show that a t5rpical trilayer interfacial structure is formed, where the first liquid crystal monolayer is quite tilted, very polar and oriented almost parallel to the surfcice. As the cyanobiphenyl molectdes have a strong tendency to form dimers, the molecules in the second bilayer orient almost perpendicularly to the surface and represent an anisotropic base for the orientation of the rest of the liquid crystal. The trilayer is stabilized by the alkyl chains of DMOAP. The schematic picture of this interface is presented in Fig. 3.3. 

Secondly, it follows from Eqs (1.31) and (1.32) that the longitudinal molecular dipole d provides a much smaller contribution to the flexocoef-ficients than the transverse dipole d , since A/k 10. Thus we conclude that the dipole flexoeffect is expected to be important only for molecules with large transverse dipoles. Note that the significant dipole flexoeffect has indeed been determined for nematics composed of molecules with large transverse dipoles. For cyanobiphenyl liquid crystals Marcerou and Frost did not find any dipolar flexoelectric effect, which may be determined not only by the tendency to form dimers with antiparallel dipoles but also by a relatively small contribution from transverse molecular dipoles to the flexoelectric coefficients. 

Flexocoefficients have also been obtained from the threshold voltages and wave numbers of electric field-induced flexoelectric instabilities (which are discussed in more detaii in Chapter 4 by Buka et al. ) for ClPbislOBB and for the molecule 4-((3-(4-(4-(decyloxy) benzoyloxy)benzoyloxy) phenylimino) methyl)-3-hydroxyphenyl 4-(6-(4 -cyanobiphenyl-4-yloxy)hexyloxy) benzoate (BCCB), which is a dimer composed of a calamitic and a BC molecule. In both cases the flexocoefficients were found to have the same order as that of calamities. 

The NMR technique has provided useful information regarding the orientational characteristics of nematic liquid crystals [96]. In many examples, the order parameters of the mesogenic core comprising a linear array of aromatic nuclei have been accurately determined from the observed dipolar (Dhd) and quadrupolar (Av) splittings by using deuterium-substituted samples. An attempt was made to elucidate the nematic conformation of the polymethylene-type spacer involved in dimer compounds, o, ct)-bis(4-cyanobiphenyl-4 -yloxy)alkanes (CBA- , with = 9,10), by the combined use of NMR and RIS analysis. The chemical structures and the phase transition data obtained by differential scanning calorimetry (DSC) are shown in... 

Figure 8 Self-assranbly for cw-cyanobiphenyl-substituted porphyrins on Cu(lll). (a)-(f) Detailed STM images with corresponding molecnlar models for diffeent oUgomaic structnres. (a) Dimer (5 x 5nm ), (b) trimo- (6.3 x 6.3 nm ), (c) type I tetramo- (6.4 x 6.4nm ), (d) type II tetramer (6.2 x 6.2nm ), (e) pentama- (7.6 x 7.6nm ), (f) hexama- (8.2 x 8.2nm ). (Reproduced from Ref. 48. Wiley-VCH, 2009.)...

Figure 40 (a) The minimized structure of 4-pentyloxy-4 -cyanobiphenyl and its dimerization in the gas phase at absolute zero, (b) The... 

The molecules of n-cyanobiphenyl liquid crystals, schematized on Fig. 5.9, are known to be relatively stable. They form dimers in bulk. ... 

So far, such measurements have only been performed on cyanobiphenyl molecules ( -CB and n-OCB). These molecules exhibit polar ordering (with the cyano groups pointing towards the surface) on hydrophilic substrates such as water, glass, and certain polymer films [49-51], but not on hydrophobic layers [52]. On the latter, the molecules orient with their aliphatic chains in contact with the substrate, and arrange following their natural tendency to form apolar dimers (two molecules oriented head to head). In all cases where a surface dipolar ordering exists, it is lost after the first molecular layer [49]. 

The two most extensively studied series of liquid crystal dimers are the a,characteristic behavior of dimers. The acronyms used to refer to these series are BCBOn and m.0n0.m, respectively, in which n refers to the number of methylene groups in the flexible spacer and m that in the terminal alkyl chains. The particular interest in these series arises, in part, because they may be considered to be the dimeric analogues of the 4- -alkyloxy-4 -cyanobiphenyls (nOCB) and the N-(4-n-alkyloxybenzylidene)-4 -n-alkylanilines, respectively, which are probably the most widely studied series of conventional low molar mass liquid crystals. In addition, members of the m.OnO.m series were the first dimers for which extensive smectic polymorphism was observed. 

Figure 2 shows the dependence of the nematic-isotropic temperatures, NI on the number of methylene groups, n, for the BCBO series. A very large odd-even effect is apparent in which the even members of the series exhibit the higher values although this alternation is attenuated on increasing n [19]. For comparative purposes the clearing temperatures for the analogous conventional or monomeric low molar mass series, the 4-ra-alkyloxy-4 -cyanobiphenyls, are also shown in Fig. 2. The clearing temperatures of the monomeric compounds are considerably lower than those of the dimers...

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... 

In addition Pd(II) complexes of related ligands PCH and CCH were made. All complexes with PCH ligands show monotropic nematic phase while with the CCH ligands, CCH-3 (n = 3) is not liquid crystal, CCH-5 ( = 5) is monotropic nematic and CCH-7 ( = 7) is enantiotropic nematic. Orientational order of the dimer [PtCl2(5-CB)2] formed by linking of the two cyanobiphenyl via platinum dichloride bridge, dissolved in a nematic solvent was studied using deuterium NMR spectroscopy [13]. 

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