Twist-Bend Nematogenic Supramolecular Dimers and Trimers Formed by Hydrogen Bonding

Abstract

A selection of novel non-symmetric supramolecular liquid crystal dimers and trimers formed by hydrogen-bonding have been prepared and their phase behaviour characterised by polarised optical microscopy, X-ray diffraction and temperature-dependent Fourier-transform infrared spectroscopy. We mix the bent twist-bend nematogen 4-[6-(4’-cyanobiphenyl-4-yl) hexyloxy]-benzoic acid (CB6OBA) with a series of small stilbazole-based compounds 4-[(E)-2-(n-alkoxyphenyl)ethenyl]pyridines (nOS) of varying terminal chain length (n) to obtain the CB6OBA:nOS complexes. Complexes with n ≤ 7 exhibit nematic and twist-bend nematic behaviour, followed on cooling by a smectic C phase for n = 4–7, and finally, a hexatic-type smectic X phase for n = 3–7. Mixtures with n = 8–10 exhibit a smectic A phase below the conventional nematic phase, and on further cooling, a biaxial smectic Ab phase and the same hexatic-type SmX phase. Supramolecular trimers, CB6OBA:CB6OS and CB6OBA:1OB6OS, formed between CB6OBA and dimeric stilbazoles [(E)-2-(4-{[6-(4’-methoxy[1,1’-biphenyl]-4-yl)hexyl]oxy}phenyl)ethenyl]pyridine (1OB6OS) or 4-[(E)-4’-(6-{4-[(E)-2-(pyridin-4-yl)ethenyl]phenoxy}hexyl)[1,1’-biphenyl]-4- carbonitrile (CB6OS), exhibit nematic and twist-bend nematic phases, and are the first hydrogen-bonded trimers consisting of unlike donor and acceptor fragments to do so.

Distinct twist-bend nematic phase behaviors associated with the ester-linkage direction of thioether-linked liquid crystal dimers

“…35 To date, a number of bent LC dimers composed of two mesogenic structures connected with an odd-numbered oligomethylene spacer have been shown to exhibit the N TB phase. In addition, several LC oligomers (e.g., trimers, 53,[58][59][60][61][62][63] tetramers, 58,59,64 a hexamer, 65 and other types of oligomers [66][67][68][69] ), polymers, 70 and bent-core molecules 71,72 have also been identified as twist-bend nematogens. It should be noted here that the bent molecular geometry is key to designing twist-bend nematogens.…”

Twist‐Bend Nematic Glasses: The Synthesis and Characterisation of Pyrene‐based Nonsymmetric Dimers

“…In consequence, the vast majority of twist-bend nematogens are odd-membered liquid crystal dimers (see, for example [8][9][10][11][12][13] ), although the N TB phase has also been reported for trimers, [14][15][16] higher oligomers, [17][18][19] bent-core materials [20,21] and hydrogen-bonded supramolecular systems. [22][23][24][25] Dozov also predicted the existence of heliconical smectic phases consisting of achiral molecules [2] and these have recently been discovered. [26][27][28] Low molar mass glassy liquid crystals have been known for many years, but typically these exhibit very low glass transition temperatures, well below room temperature, and to be observed often require high quenching rates.…”

“…Structure‐property relationships for the N TB phase are still being established and understood, but one seemingly essential prerequisite for the formation of the N TB phase is a pronounced molecular curvature. In consequence, the vast majority of twist‐bend nematogens are odd‐membered liquid crystal dimers (see, for example [8–13] ), although the N TB phase has also been reported for trimers, [14–16] higher oligomers, [17–19] bent‐core materials [20,21] and hydrogen‐bonded supramolecular systems [22–25] . Dozov also predicted the existence of heliconical smectic phases consisting of achiral molecules [2] and these have recently been discovered [26–28] …”

Chair- and V-Shaped of H-bonded Supramolecular Complexes of Azophenyl Nicotinate Derivatives; Mesomorphic and DFT Molecular Geometry Aspects

“…The spectra were recorded for H-bonded supramolecular complexes I/An as well as their individual components, Figure 1. The stretching vibration of the C=O was reported [62][63][64] to be at 1681 cm −1 . The formation of the H-bond between the nitrogen atom of base and the COOH group of acid replaces the dimeric H-bonds of the 4-alkoxybenzoic acid.…”