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

• Main Authors: Rebecca Walker, Damian Pociecha, Alfonso Martinez-Felipe, John MD Storey, Ewa Gorecka, Corrie T Imrie
• Format: Article
• Language: English
• Published: MDPI AG 2020-03-01
• Series: Crystals
• Subjects: liquid crystals; twist-bend nematic phase; hydrogen-bonding; supramolecular liquid crystals; supramolecular dimers; supramolecular trimers; spontaneous symmetry breaking
• Online Access: https://www.mdpi.com/2073-4352/10/3/175

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&#8217;-cyanobiphenyl-4-yl) hexyloxy]-benzoic acid (CB6OBA) with a series of small stilbazole-based compounds 4-[(E)-2-(n-alkoxyphenyl)ethenyl]pyridines (<i>n</i>OS) of varying terminal chain length (<i>n</i>) to obtain the CB6OBA:<i>n</i>OS complexes. Complexes with <i>n</i> &#8804; 7 exhibit nematic and twist-bend nematic behaviour, followed on cooling by a smectic C phase for <i>n</i> = 4&#8722;7, and finally, a hexatic-type smectic X phase for <i>n</i> = 3&#8722;7. Mixtures with <i>n</i> = 8&#8722;10 exhibit a smectic A phase below the conventional nematic phase, and on further cooling, a biaxial smectic A_b 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&#8217;-methoxy[1,1&#8217;-biphenyl]-4-yl)hexyl]oxy}phenyl)ethenyl]pyridine (1OB6OS) or 4-[(E)-4&#8217;-(6-{4-[(E)-2-(pyridin-4-yl)ethenyl]phenoxy}hexyl)[1,1&#8217;-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.