Polyphilic molecules

Fig. 2a-f. Mesogenic molecules with differing degrees of polar and sterical asymmetry a symmetric molecule with rigid core and two hydrocarbon tails b terminally polar molecule (arrow indicates the permanent dipole) c swallow-tailed (hiforked) molecule d hanana shaped molecule e terminally fluorinated molecule f polyphilic molecule (hatched areas correspond to the fluorinated fragment)... 

Fig. 12a-c. Schematic representation of the tilted layer structures for the polyphilic molecules in a strongly fractured conformation a the random up-down configuration b polar packing of molecules within the layer c two-dimensional (modulated) polar structure (Blinov et al. [44])... 

Keywords Columnar mesophase Cubic mesophase Dendrimer Liquid crystal Metallomesogen Micro-segregation Organic semiconductor Perfluorinated molecule Polyphilic molecule Self-assembly... 

Fig. 47 (a) Examples of polyphilic molecules with star shaped molecular topologies and (b) their mesophase morphologies [295]. (c) Janus-type porphyrin 177 [277] and (d) modes of self assembly of the completely RF-substituted porphyrin 156 left, for structure see Fig. 42, Colortho G -29 °C Colortho199 °C Iso) and the partly fluorinated porphyrin 177 right, ColTeJp2mg, Cr -22 °C Co n,Jp2mg 163 °C Iso) Colortho = orthorhombic columnar 3D phases (b) Reproduced with permission [295], copyright 2008, The Royal Society of Chemistry (RSC) (d) reproduced with permission [277], copyright 2011, American Chemical Society (ACS)...

A new direction in LC research is based on polyphilic molecules with a T-shaped molecular topology [8, 310-313]. Usually, in these molecules a rigid rod-like core... 

Fig. 14 (a) Chemical structures of the polyphilic dispersion-promoter molecules, (b) Tailor-designed polyphilic molecules promoting CNT dispersion in the nematic host. Pyrene anchoring group (blue), mesogenic CB unit (dark red), flexible hydrocarbon or ethylene oxide spacer (green), and liquid crystal host (light red) [464]. (Reproduced by permission of The Royal Society of Chemistry)... 

There is considerable interest in the properties of new mesomorphic materials, which are composed of molecules with novel architectures. These include rod-coil molecules [1], polyphilic molecules [2, 3], block-copolymers [4] and dendritic molecules [5]. In many of these systems microphase separation can be used to build new materials containing structures that are ordered on the nanoscale. Examples include the formation of spheres or rods within a uniform matrix of dififerent chemical... 

In principle, there is no reason to exclude the polar ordering of dipoles in the smectic A phase (Fig. 2b) along the molecular axis. No examples of such phases have been observed for low-molar-mass liquid crystals or for LC polymers. Tour-nilhac et al. [101] suggested an approach to ferroelectricity in the phase of polyphilic molecules consisting of aromatic, aliphatic, and perfluorinated parts they realized their approach successfully for an achiral smectic C phase only. An attempt to extend that approach to polyphilic LC copolymers [102] indicated antiferroelec-tric behavior of the synthesized materials. 

We start with some elementary information about anisotropic intermolec-ular interactions in liquid crystals and molecular factors that influence the smectic behaviour. The various types of molecular models and commonly accepted concepts reproducing the smectic behaviour are evaluated. Then we discuss in more detail the breaking of head-to-tail inversion symmetry in smectic layers formed by polar and (or) sterically asymmetric molecules and formation of particular phases with one and two dimensional periodicity. We then proceed with the description of the structure and phase behaviour of terminally fluorinated and polyphilic mesogens and specific polar properties of the achiral chevron structures. Finally, different possibilities for bridging the gap between smectic and columnar phases are considered. 

Many of the mesogenic molecules are stericaUy asymmetric, which is determined by the fractures and bending of the molecular core as well as by the presence of the tail chains of different nature, including the branched, biforked or polyphilic moieties (Fig. 2c-f). In terms of the multipole model of molecular asymmetry introduced by Petrov and Derzhanski [34], we can speak about longitudinal or transverse steric dipoles or multipoles (Fig. 3). 

Ad layering (Fig. 16a). On the other hand, additional conformational entropy related to the sequenced hydrocarbon-perfluorinated chain prevents the crystallization of the aromatic parts of the molecules in the smectic layers as occurs for hydrocarbon analogues of the same tail length [4]. The stability of Cd phase in polyphilics is more delicate in nature and will be discussed below. 

Finally, we should mention that the asymmetry of molecular shape, polyphilic effects and conformational constraints are the dominant factors in the stabilization of polar ordering in achiral mesogens. The examples presented above are, therefore, highly significant. They show that many liquid crystalline structures are intrinsically polar and may be effectively stabilized by suitable design of the mesogenic molecules. 

Compounds like 88 (Fig. 25a), in which the Rp-segments are decoupled from the rod-like core by longer aliphatic spacers, can be regarded as polyphilic ABC molecules which are expected to lead to triply segregated smectic phases as shown in Fig. 19c. However, in compound 88, for example, the SmA-phase is composed of only two sets of distinct sublayers, though there are three mutually incompatible... 

Molecules composed of more than two distinct incompatible units can be regarded as polyphilic if polyphilicity is manifested by formation of a periodic LC structure where the distinct molecular segments are organized in at least three distinct nanocompartments. Some examples of polyphilic LCs were already included in the... 

X-Shaped Polyphiles Liquid Crystalline Honeycombs with Single Molecule Walls... 

Attaching only one lateral chain to the rod-like core (T-shaped amphiphiles 181-183) gave rise to LC honeycomb phases where the cross-section of the honeycomb walls contains two rods arranged side-by-side, i.e., the honeycombs have double walls (Fig. 61a) [8]. However, attaching two lateral chains to opposite sides of the aromatic core (X-shaped polyphiles like compound 187) generates polygonal honeycombs with walls that are only one molecule thick (Fig. 61b) [319]. As a consequence of the thinner walls, effectively more space is left available for the lateral chains inside the cells. Hence, honeycombs with smaller cells could be achieved by using two short lateral chains instead of only one chain with the same total volume [330]. 

Tschierske and coworkers reported on new complex liquid crystalline phases of polyphilic block molecules or their metal complex [91-93]. These triblock rod-coil molecules consist of a rod-like p-terphenylene unit, and two hydrophobic alkyl chains at both ends of the rod, and oligo(ethylene glycol) with a terminal carbohydrate unit at a lateral position of the rod [94]. Depending on the size of the hydrophilic and hydrophobic segments, a series of unusual liquid crystalline phases were detected (Fig. 18). When the carbohydrate unit is directly conjugated to the rod building block, a simple smectic (Sa) phase was observed. In this liquid crystalline phase, the molecules are... 

Most of the amphiphiles discussed hitherto have only two distinct parts in their molecules which are different in their polarity. Polyphilic compounds possess more than two of them (see Fig. 33) depicting the cal-amitic biphenyl derivative 50 with three different parts a polyfluorinated one, an alkoxy chain, and the aromatic (biphenyl) section. The mesomorphic properties of this triphilic examples are compiled in Table 15. 

The polyphilic quality leads to a further ordering of the molecules in their meso-phase Fig. 34 gives a simple schematic drawing of a directed lamellar phase. The molecules have a polar orientation within the layers and long range correlations between them. This special mesophase structure causes macroscopically polar properties. With regard to this kind of molecular arrangement some polyphilic compounds have been successfully studied in respect of ferroelectric properties [161-165]. 

Cheng, X.H., et al. 2003. Calamitic bolaamphiphiles v th (semi)perfluorinated lateral chains Polyphilic block molecules with new liquid crystalline phase structures. J Am Chem Soc 125 10977. 

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