A Chameleon Chiral Polar Liquid Crystal:  Rod-Shaped When Nematic, Bent-Shaped When Smectic

Abstract

The first antiferroelectric liquid crystal (AFLC) exhibiting a (chiral) nematic phase, a combination which has long been the goal of synthetic chemists working with polar liquid crystals but which at the same time represents a fundamental contradiction in terms of translational order, was recently reported by Nishiyama and co-workers. We have investigated this chiral twin dimer by optic, electrooptic, and dielectric methods and conclude that it is not an ordinary AFLC material, but one where the peculiar properties of bent-core smectics are combined with those of ordinary rod-shaped liquid crystals. The compound exhibits a new type of nematic−smectic phase transition, connected with a change of molecule conformation from rod- to bent-shaped. This also has an important impact on the chiral interactions in the system. Toward the high-temperature end of the smectic phase, the energy balance between bent conformation smectic and straight conformation nematic can be shifted by an electric field such that the transition to the nematic phase with stretched-out molecules can be field-induced.

Current Topics in Smectic Liquid Crystal Research

“…[103][104][105] Lately, the orthoconic effect has also been found in several other materials (some of them are depicted in Table 3), quite different from the initially used mixture (which will be discussed in more detail below): in bent-core mesogens, [106,107] in a bistereogenic AFLC (M7BBM7) structurally related to MHTAC, [90] in a "twin AFLC mesogen" compound (BMHBOP-8), [88] and recently also in a siloxane bimesogen (Br11-3-11Br). [94] Siloxane bimesogens are actually some of the best-studied, high-tilt AFLCs, which have been investigated repeatedly since 1997.…”

“…However, it has turned out to be extremely difficult to produce materials that form both a nematic and an anticlinic smectic phase (not counting the class of bent-core molecules, which are outside the scope of this Review); the rare reports where this combination has been claimed [87,110] are inconclusive and/or not quite accurate. [88,111] Most AFLCs form the SmA* phase directly from the isotropic phase. Although not as easy as with a nematic phase, a decent alignment can often also be achieved in this case.…”

“…Considering that the optical tilt angle q increases discretely from zero to % 25-308 (the exact value differs somewhat between different batches of the mixture [86] ) at the SmA*-SmC* transition, 7 it is amazing that the corresponding layer shrinkage is only about 1 , which corre- Figure 22. A demonstration of the orthoconic effect using the "twin AFLC mesogen" compound BMHBOP-8 [87,88] (see Table 3) in a cell of 0.8 mm thickness. Only by special electric-field treatment could we fulfill all the practical requirements for observing the orthoconic effect (horizontal tilt plane and complete helix suppression) in this compound.…”

“…An even more adverse type of phase transition was found in the "twin"-molecule compound BMHBOP-8, [87] where an extremely short-pitch cholesteric phase was followed by a smectic phase through a transition in which the molecular shape changed, thus resulting in a complete loss of alignment. [88] An improved alignment could be achieved with both these types of bimesogenic compound by electric-field treatment and/or slow cooling into the smectic phase [88,93] (see the BMHBOP-8 example in Figure 22), but such alignment methods are not realistic in commercial display production. Some orthoconic AFLCs belonging to the class of bent-core mesogens have been identified, [106,107] but they do not align well and the mesophase temperature ranges are also generally far too high.…”

Distinctive dielectric properties of nematic liquid crystal dimers

“…Calamitic, discotic or bent-core mesogenic groups have been used among others as 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 F o r P e e r R e v i e w O n l y building blocks for dimeric systems, connected by longitudinally or laterally by flexible linking groups of different types, length or parity, and revealing a highly interesting hierarchy of mesomorphic behaviour [6][7][8][9][10][11][12][13]. However, in the recent years the simplest calamitic-calamitic dimers have attracted considerable attention due to the discovery of a novel nematic phase, originally for cyanobiphenyl based odd dimers with methylene links ( , -bis(4,4'-cyanobiphenyl)-alkanes CBnCB).…”