A convenient one-pot synthesis, and characterisation of the ω-bromo-1-(4-cyanobiphenyl-4’-yl) alkanes (CBnBr)

A convenient synthetic route based on a sodium-mediated aromatic cross-coupling reaction is described for the multi-gram preparation of the ω-bromo-1-(4-cyanobiphenyl-4’−yl) alkanes (CBnBr, n = 2–10). These materials are not only key intermediates in the synthesis of oligomers and polymers but also exhibit fascinating liquid crystal behaviour in their own right. Nematic behaviour is observed for n⩾5, and the nematic-isotropic transition temperature, TNI, increases in essentially a linear manner on n. The properties of the ω-bromo-1-(4-cyanobiphenyl-4’−yloxy) alkanes (CBOnBr, n = 2–9) are also reported, and nematic behaviour is seen for n⩾3. The values of TNI show a weak odd-even effect on n in which the odd members show the higher values. The sense of this alternation is opposite to that seen for the 4-alkyloxy-4’-cyanobiphenyls, and this is attributed to the steric bulk of the bromine atom. The absence of smectic behaviour for both the CBnBr and CBOnBr series is attributed largely to electrostatic interactions that would arise from the concentration of the bromine atoms at the layer interfaces in an interdigitated smectic phase. A comparison of a range of cyanobiphenyl-based materials containing a chain with a terminal polar or polarisable group suggests that their phase behaviour is governed largely by their average molecular shapes

Molecular curvature and the twist-bend liquid crystal phases: the effect of the spacer

The synthesis and characterisation of ten members of the 4[{[4({5[4((4-cyanophenyl)phenyl]oxy)pentoxy}yl)phenyl]methylidene}amino] phenyl 4-alkyloxybenzoates is presented. These dimers are referred to using the acronym CBO5OIBeOm in which m refers to the number of carbon atoms in the terminal chain and varies from 1 to 10. The CBO5OIBeOm series exhibits nematic (N), twist-bend nematic (NTB), and smectic A (SmA) phases depending on the length of the terminal chain. Although the NTB phase is observed, these dimers do not exhibit the twist-bend smectic C (SmCTB) phases. By comparison, the structurally similar CB6OIBeOm series does exhibit the SmCTB phases, and this strongly suggests that the formation of the SmCTB phase is more sensitive to molecular curvature than that of the NTB phase

Liquid crystal dimers and the twist‐bend phases: non‐symmetric dimers consisting of mesogenic units of differing lengths.

The syntheses and characterisation of the 4‑[{[4‑({n‑[4‑(4 cyanophenyl)phenyl]‑n‑yl}oxy)phenyl]-methylidene}amino]phenyl-4-alkoxybenzoates (CBnOIBeOm) are reported with n= 8 and 10 and m= 1-10. The two series display fascinating liquid crystal polymorphism. All twenty reported homologues display enantiotropic nematic (N) phase at high temperature. When the length of the spacer (n) is greater than that of the terminal chain (m), the twist-bend nematic (NTB) phase is observed at temperatures below the N phase. As the length of the terminal chain is increased and extends beyond the length of the spacer up to three smectic phases are observed on cooling the N phase. One of these smectic phases has been assigned as the rare twist-bend smectic C subphase, the SmCTB‑α phase. In all the smectic phases, a monolayer packing arrangement is seen, and this is attributed to the anti-parallel associations of the like mesogenic units

A twist-bend nematic phase driven by hydrogen bonding

The liquid crystalline phase behavior of 4-[6-(4'-cyanobiphenyl-4-yl)hexyloxy]benzoic acid (CB6OBA) and 4-[5-(4'-cyanobiphenyl-4-yloxy)pentyloxy]benzoic acid (CBO5OBA) is described. Both acids show an enantiotropic nematic phase attributed to the formation of supramolecular complexes by hydrogen bonding between the benzoic acid units. In addition, CB6OBA provides the first example of hydrogen bonding driving the formation of the twist-bend nematic phase. The observation of the twist-bend nematic phase for CB6OBA, but not CBO5OBA, is attributed to the more bent molecular shape of the complexes formed by the former, reinforcing the view that shape is a key factor in stabilizing this new phase. Temperature-dependent FTIR spectroscopy reveals differences in hydrogen bonding between the two nematic phases shown by CB6OBA which suggest that the open hydrogen-bonded complexes may play an important role in stabilizing the helical arrangement found in the twist-bend nematic phase

New insights into the liquid crystal behaviour of hydrogen-bonded mixtures provided by temperature-dependent FTIR spectroscopy

The phase behaviour of equimolar mixtures of 6-(4'-cyanobiphenyl-4-yl)hexyloxybenzoic acid (CB6OBA) with either 1-(4-butylazobenzene-4'-oxy)-5-(4-oxypyridine)pentane (BuABO5OPyr) or 1-(4-butylazobenzene-4'-oxy)-6-(4-oxypyridine)hexane (BuABO6OPyr) is reported. CB6OBA shows a monotropic twist-bend nematic phase and an enantiotropic nematic phase whereas the two pyridyl-based components do not exhibit liquid crystallinity. Both equimolar mixtures (CB6OBA/BuABOnOPyr) show enantiotropic nematic phases. The nematic-isotropic transition temperature and associated entropy change are higher for the CB6OBA/BuABO6OPyr mixture than for the CB6OBA/BuABO5OPyr mixture. This may be accounted for in terms of the average shapes of the hydrogen-bonded 1:1 complexes formed between the two differing components in the mixtures. However, Fourier transform infrared spectroscopy reveals that this complex is not formed quantitatively, but instead a complex mixture exists over the complete temperature range studied, involving the 1:1 complex, both cyclic and open acid dimers, free acid and hence, free BuABOnOPyr molecules