Remarkable stabilisation of the intercalated smectic phases of nonsymmetric dimers by tert-butyl groups

• Published in: Liquid crystals Vol. 49; no. 7-9; pp. 969 - 981
• Main Authors: Walker, Rebecca, Pociecha, Damian, Faidutti, Camilla, Perkovic, Eva, Storey, John M. D., Gorecka, Ewa, Imrie, Corrie T.
• Format: Journal Article
• Language: English
• Published: Abingdon Taylor & Francis 15.07.2022; Taylor & Francis Ltd
• Subjects: Alkanes; branched chain; Chain branching; Dimers; Homology; Intercalation; Melting points; smectic phases; Substitutes; Transition temperatures; twist-bend
• ISSN: 0267-8292; 1366-5855
• DOI: 10.1080/02678292.2022.2055797

The synthesis and characterisation of two groups of nonsymmetric dimers, the 1-(4-cyanobiphenyl-4'−yloxy)-ω-(4-butyl, 4-(1-methylpropyl) or 4-t-butylanilinebenzylidene-4'−oxy)alkanes, and the 1-(4-cyanobiphenyl-4'−yl)-ω-(4-butyl, 4-(1-methylpropyl) or 4-t-butylanilinebenzylidene-4'−oxy)alkanes, are reported. The length and parity of the flexible spacer is varied. The tert-butyl homologues show higher melting points than the corresponding sec-butyl or n-butyl substituted dimers, suggesting that chain branching improves packing efficiency within the crystalline structure. The branched chain homologues have a stronger tendency to exhibit smectic phases than the n-butyl-substituted dimers, and for longer spacers are exclusively smectic. A comparison of the nematic-isotropic transition temperatures (T NI ) for dimers containing the different terminal chains is possible for one set of materials, and reveals a large reduction in T NI on passing from the n-butyl to sec-butyl-substituted, but a much smaller decrease on changing sec-butyl for tert-butyl. A different trend is observed for the smectic A-isotropic transition temperatures for which the tert-butyl substituted dimers show a higher value than the corresponding sec-butyl homologue, and only marginally lower than that of the n-butyl-substituted dimer. This surprising behaviour is interpreted in terms of the ability of the tert-butyl group to pack more efficiently into the intercalated smectic A phase as the spacer length increases.