Chiral spacers

The above selectivities are affected by changes in the molar ratio between the guests and between guest and host in the solution mixture. They show, however, consistent trends. Qualitatively similar results of guest selectivity were obtained with N,N -bis(tri-p-tolylmethyl)urea and a number of related hosts47. Additional experiments using ureas substituted with chiral spacers are readily envisioned. 

Fig. 19. A series of 2nd-generation dendrimers with chiral spacers derived from (R,R)- or (S,S)-tartaric acid. Note that the (R,R)-threitol-acetonide building blocks in 49 - 51 are derived from (S,S)-tartaric acid, and vice versa [78-80]...

Guldi DM, Giacalone E, de la Torre G et al (2005) Topological effects of a rigid chiral spacer on the electronic interactions in donor-acceptor ensembles. Chem Eur J 11 7199-7210... 

Also, the structure of 63 is better described as zwitterionic with a C-N bond length of only 1.651 A <2001J(P2)133> in the case of methoxy instead of NMe2, the MeO- C=S separation is longer (2.550 A) and the two substituents behave as normal /rm-substituents. Furthermore, the 2,6,9-trioxo-bicyclo[3,3,l]nona-3,7-diene moiety 64 has been structurally characterized as part of Pd(ii) complexes <2003JOM(676)93> and as a novel chiral spacer unit in macrocyclic polyethers <2002SMC383, 2004T2857>. 

Cj Dendrimers with chiral spacers or chiral branching units (see Section 4.2.4)... 

Chiral dendrimer scaffold, made up of chiral branching building blocks or chiral spacers... 

Chow and Mak came to a similar conclusion on investigating the chiroptical properties of dendrimers containing enantiomerically pure threitol building blocks obtained from tartaric acid as spacers between the achiral phloroglucin branching units (see Fig. 4.73) [27]. They found that the chiral spacers in the dendrimer scaffold do not influence one another and contribute additively to the overall rotation. Moreover, they also observed that on introduction of both enantiomers one (R,R)-threitol unit precisely compensated the rotational contribution of one (S,S)-threitol unit, provided that the enantiomeric building blocks were located at equivalent positions within the dendrimer scaffold. However, CD-spectroscopic data revealed that the contribution of the exterior threitol units to the total rotation must be slightly different from that of the interior units. 

Fig. 4. 73 Targeted incorporation of the two enantiomers (RR, SS) of a chiral spacer (threitol) compensates the rotation in these dendrimers (according to Chow and Mak stereocentres are marked with a red asterisk)...

Chien and Cada [42] have prepared optically active and photoactive SCLC copolymers, 15, with the 4-alkoxyphenyl-4 -alkoxycinnamate chromophore, with the intention of creating LC polysiloxane networks that could be used to prepare macroscopically oriented organic ferroelectric polymers for electro-optical devices. Optical activity was introduced into the polymer by the use of a chiral spacer. Those copolymers which were mesogenic exhibited properties characteristic of a Sc. phase. UV-irradiation of thin films of the polymers in their mesomorphic states at 90°C, led to a loss of the IR absorption at 1635 cm-1 that is due to the cinnamate double bond, and to cross-linking. Long-term irradiation led to... 

It is also known that in side-chain LC polymers the copolymerization of optically active monomers with mesogenic monomers, in the same manner as the mixing of optically active compounds with nematic low molecular weight compounds, can induce the formation of a cholesteric mesophase. Therefore, it is expected that inclusion of chiral spacers in main chain liquid crystal polymers, which would be nematic... 

Blumstein and coworkers studied the cholesteric behavior of polyesters with azoxybenzene mesogenic units and the same chiral spacer, (+) 3-methyladipic acid. They could clearly observe oily streak textures, which are typical of low molecular weight cholesterics, for the following homopolymer and copolymers ... 

Cholesteric oily streak textures could again be clearly observed for these LC polymers containing chiral spacers. In general, the pitch of the helical packing increased in a regular manner not only with temperature, as judged by an iridescent color, but also with the amount of achiral component in the copolymers. 

To investigate the effect of chirality in dendrimers, a number of groups have incorporated chiral spacers into the poly(aryl ether) repeat unit. Chow et al.96-100 modified their dihydroxyphenoxypropanol repeat unit 27 to include the acetonide protected diol spacers (2R,3R) or (25,35)-threitol, 28 and 29. McGrath and co-workers incorporated similar chiral protected diols 30-33 into their studies,101-105 whereas Seebach and co-workers106-109 investigated the di- and tribranched chiral monomers 34-36. 

Diels-Alder Merrifield with aliphatic chiral spacer TidslOiPr) [24 [... 

An interesting example of the effect of introducing chirality into the bridging unit has been given by Suzuki et al. [8]. The ligand 3 contains two pyridine binding units separated by a chiral spacer. If (RJl)-3 is reacted with Ag(I), crystals of an extended... 

Cholesteric PEIs may also be obtained by extension with chiral spacers - a methyl substituent placed on the alkyl chain of the aforementioned thiophenol on the 3 carbon [20]. Photo-reactive cholesteric PEIs with this type of structure have subsequently been reported by the same author [23] formed from the condensation of 1,4-phenylene di-acrylic acid and N-(4-carboxyphenyl) trimellitimide with a chiral spacer prepared from (S)-3-bromo-2-methylpropanol and 4-mercaptophenol. The resulting polymer had enantriotropic cholesteric melt with grandjean texture upon slight shearing (6). 

PEIs derived from AT-(4-carboxyphenyl) trimellitimide and aliphatic spacers (11) are not thermotropic, irrespective of whether the spacer is chiral or not. If the spacers are semi-aliphatic and contain a benzene ring, then thermotropic PEIs may be formed with both a smectic and nematic phase. If chiral spacers are then used a chiral smectic A or C phase may additionally be obtained. Such phases may be ferroelectric, which is extremely rare for main-chain polymers [26]. Examples of chiral and non-chiral spacers used in the copolymers (12) are ... 

D-3-methyl-glyoxal is a common chiral spacer. In addition, D-butane-1, 3-diphenol, L-propane-1, 2-diphenol, and their dimers and trimers are used as the chiral spacers in cholesteric liquid crystalline polymers as well. 

The interest in chiral dimers was stimulated to a large extent by the expectation that placing the chiral centre in the spacer, at least for even dimers, should increase its orientational order compared to that for a chiral centre located in a terminal chain and this in turn should result in an enhancement of the form chirality of the phase. This suggestion has still to be extensively investigated but the limited data available indicates that dimers having chiral spacers actually exhibit ferroelectric smectic C phases with low values of spontaneous polarisation [140]. 

The influence on the mesomorphic properties of polymer structural parameters, for example, the length and nature of the polymer, the substituent s structure, the structural isomerism enantiomeric excess, e. g., of the chiral spacer segment, has been elucidated for the polymer systems (6) [42-46] based on the 4,4 -terephthaloyldioxydi-benzoyl mesogen. It is important to recog-... 

Scherow.sky, G., Muller, U., Springer, J., Trapp, W., Levelut, A. M., and Davidson, R, Liquid-crystalline side chain polymers containing a chiral spacer unit exhibiting chiral smectic phases, Liq. Cryst, 5, 1297-1306 (1989). 

Aromatic imide groups are known to be nearly planar, rigid, polar, and thermostable. However, aromatic imide structures are also known to be non-mesogen in nature. Poly(ester-amide) (PEI) derived from N- (4 -carboxyphenyl) trimellitimide and aliphatic spacers are not thermotropic, whether the spacer used is chiral or not. Semi-aliphatic spacers are observed to exhibit both a smectic and a nematic LC phase in the resultant thermotropic PEIs. The semi-aliphatic chiral spacers exhibit both chiral smectic phase (A or C ) and cholesteric phase. Such a chiral smectic LC-phase, which may be ferroelectric in nature, is extremely rare for LC-main-chain polymers [27]. It is a particular advantage of polar imide mesogens to favor the formation of layer-structures when combined with non-polar species [28]. 

Type C2b has two (or more) mesogenic groups that want to be parallel to the director, but they are twisted by a chiral spacer. These compounds can cause very strong chiral effects, but the molecules are often not mesogenic themselves. If the mesogenic axes are almost parallel, then helical inversions can be induced by small conformational changes. 

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