Banana-calamitic dimers: unexpected mesophase behaviour by variation of the direction of ester linking groups in the bent-core unit

Abstract

A five-ring bent-core unit and a phenyl benzoate moiety are linked together by means of a flexible spacer. Starting from dimers having different length of the aliphatic parts, that is the terminal alkyloxy chains and the spacer, the direction of each of the four ester connecting groups in the bent-core unit is reversed step by step. In this way thermodynamically stable polar and non-polar mesophases in unusual sequences could be obtained. For instance, a SmC s P AF -SmC ś P AF transition and the existence of two nematic phases (N, N X ) have been observed. The nematic phases of the banana-calamitic dimers exhibit unusual properties comparable to those of the nematic phases reported for some monomeric bent-core mesogens. The electric field-induced enhancement of the SmCP-SmA transition temperature of one compound proves the existence of polar clusters already in the non-polar SmA phase.

Twist-bend heliconical chiral nematic liquid crystal phase of an achiral rigid bent-core mesogen

“…Recently, the twist-bend nematic (N TB ) phase, a fundamentally new type of nematic ground state of achiral molecules, exhibiting layer-free, helical liquid crystal ordering of nanoscale pitch, has been structurally identified and characterized [1,2]. The twist-bend nematic phase was initially suggested as a theoretical possibility by Meyer over 40 years ago [3] and discussed in 2000 by Dozov [4] and Memmer [5], who suggested that the tendency for local bend curvature in the director field of bent molecules (for example, Figure 1a [6,7] having nematic characteristics, spontaneous chirality [8,9], and smectic textural features [10,11] but no smectic-like lamellar x-ray diffraction peaks [12] have been interpreted in terms of the N TB structure [4]. The N TB phase is in general polar and structurally chiral despite being formed from achiral molecules.…”

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).…”

Banana-calamitic Dimers: Further Variations of the Bent-core Mesogenic Unit

“…To have an information on the mesophase behaviour of the corresponding alkyloxy substituted derivatives, reference compounds E (R = -OC 12 H 25 ) are additionally given in Table 1 and Fig. 1 [44]. The CN-substituted dimer 1a with a short odd-methylene spacer length (m = 3) exhibits two monotropic phases: a nematic and a smectic phase.…”

“…In the X-ray pattern of the lower temperature phase no evidence of a layer structure could be found, although the fan-like texture indicates the existence of a smectic-like phase. Thus the optical and X-ray measurements point to an Nx phase as found for instance for liquid crystalline bent dimers of cyanobiphenyl connected by flexible spacers which has recently been characterized as a nematic phase with a conical twist-bend helical arrangement of the molecules and which has been called nematic twist-bend (NTB) phase [44,[58][59][60][61][62][63][64].…”

“…Thus the following structural features have been varied to study their influence on the liquid crystalline behaviour: the length of the aliphatic wing groups and of the flexible spacer, the chemical constitution of the calamitic moiety, and the direction of the ester groups which connect the phenyl rings of the bent-core moiety [43,44]. Selected compounds, (especially GTP240 -3-E 2 ) exhibiting nematic phases served as materials for electro-optical studies.…”