Experimental and Computational Study of a Liquid Crystalline Dimesogen Exhibiting Nematic, Twist-Bend Nematic, Intercalated Smectic, and Soft Crystalline Mesophases

Liquid crystalline dimers and dimesogens have attracted significant attention due to their tendency to exhibit twist-bend modulated nematic (NTB) phases. While the features that give rise to NTB phase formation are now somewhat understood, a comparable structure–property relationship governing the formation of layered (smectic) phases from the NTB phase is absent. In this present work, we find that by selecting mesogenic units with differing polarities and aspect ratios and selecting an appropriately bent central spacer we obtain a material that exhibits both NTB and intercalated smectic phases. The higher temperature smectic phase is assigned as SmCA based on its optical textures and X-ray scattering patterns. A detailed study of the lower temperature smectic ‘’X’’ phase by optical microscopy and SAXS/WAXS demonstrates this phase to be smectic, with an in-plane orthorhombic or monoclinic packing and long (>100 nm) out of plane correlation lengths. This phase, which has been observed in a handful of materials to date, is a soft-crystal phase with an anticlinic layer organisation. We suggest that mismatching the polarities, conjugation and aspect ratios of mesogenic units is a useful method for generating smectic forming dimesogens.


General Techniques
Miscellaneous solvents were purchased from Fisher Scientific dried by sequential percolation through columns of activated alumina and copper Q5 catalyst prior to use.
Unless otherwise noted, chemical reagents were purchased from commercial suppliers and used without further purification. The intermediate i4 was prepared as described previously. 1 Reactions were monitored by thin layer chromatography (TLC) using an appropriate solvent system. Silica coated aluminium TLC plates used were purchased from Merck (Kieselgel 60 F-254) and visualised using UV light at wavelengths of both 254 nm and 365 nm. Column chromatography was performed using flash grade silica from Fluorochem (40 -63μm particle size). Yields refer to chromatographically (HPLC) and spectroscopically ( 1 H NMR, 13 C 2 NMR and 19 F NMR) homogenous material.

Nuclear Magnetic Resonance
NMR spectra were recorded on a JEOL ECS spectrometer operating at 400 MHz ( 1 H), 100.5 MHz ( 13 C 2 ) and 376.4 MHz ( 19 F NMR) as solutions in deuterated chloroform. Spectra were referenced to the residual protic solvent for 1 H (7.26 ppm), 13 C 2 to the resonance of CDCl3 (77.16 ppm) and 19 F were unreferenced.

Mass Spectrometry
Mass spectra were recorded on a Bruker compact time of flight mass spectrometer with both ESI and APCI sources, and we extend our gratitude to Mr. Karl Heaton of the University of York for obtaining MS data.

High Performance Liquid Chromatography
High-performance liquid chromatography was performed on a Shimadzu Prominence

Small Angle X-ray Scattering
Small angle X-ray scattering was performed using a Bruker D8 Discover using copper Kα radiation (λ = 0.154056 nm) from a 1 μS microfocus source. Samples to be studied by SAXS Raw 2D SAXS patterns were processed by subtracting the 2D pattern of an air filled glass capillary; the background subtracted data was separately radially and azimuthally averaged to give scattering intensity as a function of Q and χ respectively. The low angle portion of radially averaged data was fitted with a Voigt function to yield the peak position(s) (i.e. layer spacing). Azimuthally averaged data was used for calculation of the orientational order parameters according to 3 4 . Data was scaled and exported as .tiff images for presentation in the manuscript.
All data processing was performed using in house developed Matlab scripts and functions which are available from the corresponding author upon reasonable request.

Electronic Structure Calculations
Computational chemistry was performed in Gaussian G09 rev D01 5 on either the ARC3 machine at the University of Leeds, the YARCC or Viking machines at the University of York. Post optimisation, a frequency calculation was used to confirm the absence of imaginary frequencies and so confirm the optimised geometries were true minima.

Synthetic Scheme
Scheme 1 3.   The alignment of CB8OFFFT during SAXS/WAXS studies was sufficient in the nematic and NTB phases to permit measurement of the orientational order parameters. We used the Kratky method as outlined elsewhere. 3,4 The alignment of the sample deteriorates following the SmCA-NTB transition and so order parameters were not calculated below a reduced temperature of 0.876 (93.5 °C). Both 〈 2 〉 and 〈 4 〉 increase as CB8OFFFT is cooled through its nematic range, whereas 〈 6 〉 is effectively zero. On entering the NTB phase (T/TN-Iso ~ 0.88, T = 95.3 °C) all three order parameters decrease, mirroring the behaviour of other systems studied by this method. 6

Fig SI-3:
Plot of the orientational order parameters of CB8OFFFT as a function of reduced temperature.