Polyphilicity

Keywords Smectic liquid crystals, Molecular conformations, polar and steric frustrations, Polyphilic and perfluorinated mesogens. X-ray diffraction... 

Polyphilic Liquid Crystals - Structure and Phase Behaviour. . . 223... 

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. 

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)... 

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). 

Fig. 9. Molecular model of polyphilic (four-block) mesogen...

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])... 

Fig. 13. X-ray intensity profile with diffi action vector along qx (in the plane of layers) in the smectic X phase for the mixture of polyphilic compounds M70. The solid line through the data points is the smn of three Lorentzian peaks which are shown hy broken curves (Blinov et al. [44])...

Fig. 14. Temperature dependences of the layer spacing in the smectic Ad and Cd phases for terminally polar polyphilic compounds F4H11OCB (squares), FsHnOCB (circles), FgHioOCB (triangles) and FioHnOCB (diamonds). The arrows indicate the smectic Ad - smectic Cd phase transition points (Ostrovskii et al. [45])...

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. 

Fig. 16a-c. Structure of the smectic Aj and Ca layers for the polyphilic compound FsHnOCB a smectic Aj layers b steric mismatch in the limit case of hilayer A2 layering c smectic Ca layers... 

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. 

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

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... 

Fig. 46 Examples of polyphilic rod-like compounds and possible modes of self-assembly of compound 174 (a) segregated non-centrosymmetric layer which would lead to longitudinal ferroelectricity if adjacent layers would organize with the same direction and (b) non-segregated centrosymmetric layer [297]...

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. 50 Molecular structure of the star-polyphile 180 and schematic picture of the molecular arrangement in the (12.6.4) tiling of the lyotropic Colhex phase [306]...

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