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.
Highlights
The experimental identification of a new nematic phase in 2011 is undoubtedly one of the most significant developments of the past decade in liquid crystal science [1]
In the twist-bend nematic, NTB, phase, the molecules are arranged in a heliconical superstructure of nanoscale periodicity while retaining a random distribution of their centres of mass [4]. This chiral superstructure is formed despite the constituent molecules being chemically achiral, and the NTB phase represents the first example of spontaneous chiral symmetry breaking in a fluid system
The H-bonded acceptor, CB6OBA, is the bent-shaped template and a twist-bend nematogen, and this is complexed with small rod-like stilbazole-based molecules of varying alkoxy chain length n, the 4-[(E)-2-(n-alkoxyphenyl)ethenyl]pyridines, nOS
Summary
The experimental identification of a new nematic phase in 2011 is undoubtedly one of the most significant developments of the past decade in liquid crystal science [1]. The H-bonded acceptor, CB6OBA, is the bent-shaped template and a twist-bend nematogen, and this is complexed with small rod-like stilbazole-based molecules of varying alkoxy chain length n, the 4-[(E)-2-(n-alkoxyphenyl)ethenyl]pyridines, nOS. In this case, the hydrogen bond donor is not liquid crystalline. Increasing the length of the terminal conventional liquid crystal dimers [36], and that the hydrogen bond between pyridyl-based chain attached to the hydrogen bond donor may reveal heliconical smectic phases as well as the NTB fragments and benzoic acids is strong enough to promote liquid crystalline complexes. Phase formation in non-symmetric hydrogen-bonded trimer complexes made up of unlike hydrogen bond donor and acceptor molecules: CB6OBA:1OB6OS and CB6OBA:CB6OS (Figure 2)
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