Can even-membered liquid crystal dimers exhibit the twist-bend nematic phase? The preparation and properties of disulphide and thioether linked dimers

Figures & data

Figure 1. The molecular structures of (a) the CBnSSnCB and (b) the CBnSmCB dimers.

Scheme 1. The synthetic route used to prepare the CBnSSnCB series.

Scheme 2. The synthetic route used to prepare the CBnSmCB dimers.

Figure 2. (Colour online) The schlieren texture of the nematic phase shown by CB6SS6CB (T = 77°C).

Table 1. The temperatures and associated scaled entropy changes of the CBnSSnCB series. The data have been measured using DSC unless stated otherwise.

n	TCrI/°C	TNTBN/°C	TNI/°C	∆SCrI/R	∆SNI/R
2	138	–	107*	8.91	–
3	151	–	105	9.37	1.04
4	132	–	80*	12.5	–
5	142	–	97*	12.6	–
6	102	–	78	10.6	0.85
7	111	40*	95	16.8	–
8	76	–	69	17.4	1.39
9	94	–	84	17.7	2.04

*Measured using the polarised optical microscope.

Figure 3. (Colour online) The optical textures seen for isolated droplets of CB7SS7CB in the (a) nematic (T = 94°C), (b) twist-bend nematic (T = 40°C) and (c) crystal phases (T = 38°C).

Figure 4. Molecular structure of CB15CB.

Figure 5. (Colour online) (a) The schlieren texture of the N phase (T = 87°C) and (b) the brushstroke texture of the N_TB phase (T = 106°C) for the X_CB15CB = 0.6 mixture.

Figure 6. Binary phase diagram for mixtures of CB7SS7CB and CB15CB plotted as a function of the mole fraction CB15CB, X_CB15CB, in the mixture. The filled circles indicate nematic–isotropic transitions, open circles twist-bend nematic–nematic transitions and the broken line connects the melting points.

Figure 7. The dependence of the transition temperatures on the number of carbon atoms in each alkyl segment, n, for the CBnSSnCB series. The melting points are connected by the broken line and the values of T_NI by the solid line. The open circle denotes the N_TBN transition seen for CB7SS7CB.

Figure 8. (Colour online) Molecular structures of (a) CB6SS6CB and (b) CB7SS7CB as determined by geometry optimization (DFT/B3LYP/6-311G(d,p)). The views on the right are along the S-S bond.

Figure 9. (Colour online) The nematic schlieren texture exhibited by CB7S9CB (T = 95°C).

Figure 10. Binary phase diagram for mixtures of CB5S7CB and CB13CB plotted as a function of the mole fraction CB13CB, X_CB13CB, in the mixture. The filled circles indicate nematic–isotropic transitions, open circles twist-bend nematic–nematic transitions and the broken line connects the melting points.

Table 2. The transition temperatures and associated scaled entropy changes of the CBnSmCB series. The data have been measured using DSC. The transitional properties of the members of the CBnCB series having the same spacer length are also listed.

Dimer	TCr- /°C	∆SCrI/R	TNTBN/°C	TNI/°C	∆SNI/R	Ref
CB5S7CB	119	11.0	–	(107)	1.14
CB13CB	106	9.40	(105)	122	0.91	[63]
CB7S9CB	92	13.6	–	105	1.55
CB17CB	99	15.3	(97)	117	1.45	[61]

Figure 11. (Colour online) (a) Space filling model for CB7S9CB and (b) viewed along the spacer.

Figure 12. (Colour online) The electrostatic potential surface (top), and ball and stick model showing the molecular dipole moment (bottom) for the ground state all-trans conformation of CB5S7CB.

Paterson DA, Abberley JP, Harrison WT, et al. Cyanobiphenyl-based liquid crystal dimers and the twist-bend nematic phase. Liq Cryst. 2017;44:127–146. doi: 10.1080/02678292.2016.1274293

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Cruickshank E. [ PhD thesis]. University of Aberdeen; 2021.

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