Superstructure, helical

The diaminopyridodipyrimidinedione 73 has been synthesised as a cytosine-like Tecton designed for self assembly into a helical superstructure. Self recognition occurs in the solid state <96MI02 96CA(125)221771 >. 

Chirality (or a lack of mirror symmetry) plays an important role in the LC field. Molecular chirality, due to one or more chiral carbon site(s), can lead to a reduction in the phase symmetry, and yield a large variety of novel mesophases that possess unique structures and optical properties. One important consequence of chirality is polar order when molecules contain lateral electric dipoles. Electric polarization is obtained in tilted smectic phases. The reduced symmetry in the phase yields an in-layer polarization and the tilt sense of each layer can change synclinically (chiral SmC ) or anticlinically (SmC)) to form a helical superstructure perpendicular to the layer planes. Hence helical distributions of the molecules in the superstructure can result in a ferro- (SmC ), antiferro- (SmC)), and ferri-electric phases. Other chiral subphases (e.g., Q) can also exist. In the SmC) phase, the directions of the tilt alternate from one layer to the next, and the in-plane spontaneous polarization reverses by 180° between two neighbouring layers. The structures of the C a and C phases are less certain. The ferrielectric C shows two interdigitated helices as in the SmC) phase, but here the molecules are rotated by an angle different from 180° w.r.t. the helix axis between two neighbouring layers. 

Fig. 16 TEM pictures of helical superstructures from PS-PIC copolymers in water. The chemical structure and composition of the corresponding copolymers are indicated below the pictures. Reprinted with permission from [238], Copyright (2001) American Chemical Society...

Nolte and coworkers reported on the formation of micelles with a helical superstructure from AB amphiphilic diblock copolymers prepared from an amine-end-capped polystyrene that was used as the initiator for the polymerization of various dipeptide-derived isocyanides (Fig. 16). 

Comehssen JJLM, Fischer M, Sommerdijk NAJM, Nolle RJM. Helical superstructures from charged poly(styrene)-poly(isocyanodipeptide) block copolymers. Science 1998 280 1427-1430. 

Fig. 41. Schematic representation of the columnar triple-helical superstructure derived from the X-ray data for (LP2, LU2) each spot represents a PU or UP base pair spots of the same type belong to the same supramolecular strand the dimensions are compatible with an arrangement of the PTP and UTU components along the strands indicated (see also text) the aliphatic chains stick out of the cylinder, more or less perpendicularly to its axis a single helical strand and the full triple helix are respectively represented at the bottom and at the top of the column [9.152].

Besides molecular shapes and amphiphilicity, chirality also has a large influence on LC self assembly, leading to series of LC phases with helical superstructures, reduced symmetry, and chirality induced frustration [43-46]. 

A cholesteric, or chiral nematic (N ) phase. This is a positionally disordered fluid in which the constituent molecules align on average their axes along a common direction called the nematic director. Being the DNA helices chiral, the orientational order develops an additional macro-helical superstructure with the twist axis perpendicular to the local director. The phase thus consists of local nematic layers continuously twisted with respect to each other, with periodicity p/2 (where p is the cholesteric pitch see Fig. 8a) [27,28]. For 150-bp helices, the N phase appears at a concentration around 150 mg/mL in 100 mM monovalent salt conditions. This LC phase is easily observed in polarized optical microscopy. Since the N pitch extends to tens of micrometers (that is, across... 

Extensive studies have been conducted to investigate the formation of chiral columns or helical superstructures in chiral and nonchiral disk- [53], star- [54, 55], and board-shaped [56] molecules. However, spontaneous deracemization has never been unambiguously demonstrated in discotic columnar phases consisting of nonchiral or racemic molecules. We recently observed clear evidence showing chiral resolution in a disk-like molecules with a dibenzo[g,p]chrysene core [57]. 

Pelzl G, Diele S, Jakli A, LischkaCh, Wirth I, Weissflog W (1999) Helical superstructures in a novel smectic mesophase formed by achiral banana-shaped molecules. Liq Cryst 26 135-139... 

However, the particular synthetic requirements in the preparation of conjugated polymers have thus far severely limited the number of similarly hierarchically structured examples. Pu et al. reported different types of conjugated polymers with fixed main-chain chirality containing binaphthyl units in their backbone which exhibited, for example, nonlinear optical activity or were used as enantioselective fluorescent sensors [42—46]. Some chirally substituted poly(thiophene)s were observed to form helical superstructures in solution [47-51], Okamoto and coworkers reported excess helicity in nonchiral, functional poly(phenyl acetylenejs upon supramolecular interactions with chiral additives, and they were able to induce a switch between unordered forms as well as helical forms with opposite helical senses [37, 52, 53]. 

Under the influence of polycinchonine acrylate (1, 1 mol%) the 5-configured ester (3) was formed in 35 % ee at - 78 C in toluene. Interestingly, if instead of the polymer the monomeric derivative cinchonine propionate (2) is used under the same reaction conditions the product with inverted absolute configuration [(f -3), 4.4 % ee] is formed This result, which was reported in the original article without any comment, can be interpreted as follows the polymer exhibits a chiral (probably helical) superstructure, which dominates the chiral induction in the reaction [4]. 

Fig. 10 Self assembly of poly (acetylenes) bearing pendant L-valine moieties into helical superstructures. Reprinted with permission from [71]...

Figure 37 Electron micrographs of self-assembled fibres, (c) Electron micrograph showing the dense network of long helical fibres in a steroid/cyclohexane organogel, (d) Preparation of single fibres, demonstrating their helical superstructure [425]. (e) Cyclic oligopeptides, selfassembling into nanofibres [435,436], Reproduced with permission of The Royal Society of Chemistry and The American Chemical Society. Parts (c) and (d) 1986 American Chemical Society...

In a further step, Yam and coworkers constructed helical superstructures emplo5nng chiral alkynylplatiniun(II)-terpyridyl complexes, obtaining metallogels that show helical fibrous nanostructures. The chiral supramolecular structiue and the spectroscopic properties depend on the extent of aggregation through Pt- Pt and n-n interactions, which can be influenced by varying the nature of counteranions, as revealed by UV/vis, circular dichroism, and luminescence studies (221). 

The chiral side chain polymers derived from asymmetric esters of terephthalic acid and hydroquinone can form (in a broad temperature range, including ambient temperature) an unusual mesophase (the isotropic smectic phase, IsoSm ) characterized by high transparency and optical isotropy within the visible wavelength range, combined with a hidden layered smectic ordering and some elements of helical superstructure at shorter dimensions of 10 to 250 nm. The short-pitch TGB A model seems to be the most adequate for the mesophase structure. 

First X-ray measurements show that the helical superstructure of the cholesteric and chiral smectic C phase can be untwisted by stretching the elastomer (5). High strains of 300% are necessary for this purpose (compared to 20% for the achiral elastomers). Nevertheless these results show that the chiral lc elastomers have the potential to act as mechano-optical couplers (cholesteric phase) or as piezo-elements (chiral smectic C phase) (5), because the mechanically induced change of the helical superstructure has to change the optical transmission or reflection properties or the spontaneous polarization. Both effects however have not yet been measured directly. 

Figure 20. Transmission electron micrographs of the (a) right-handed and (b) left-handed helical superstructures obtained from assembled polystyrene—polyisocyanopeptide block copolymers 99 and 102, respectively.

Although the precise mechanism of formation of these superstructures is still unclear, a hierarchical organization is evident, since transfer of chirality takes place from the amino acid in the monomer to the secondary helical structure of the polyisocyanide blocks and, ultimately upon assembly, to the helical superstructure. 

---