Sulfur-linked cyanobiphenyl-based liquid crystal dimers and the twist-bend nematic phase

ABSTRACT

The synthesis and characterisation of two series of cyanobiphenyl-based liquid crystal dimers containing sulfur links between the spacer and mesogenic units, the 4ʹ-[1,ω-alkanediylbis(thio)]bis-[1,1ʹ-biphenyl]-4-carbonitriles (CBSnSCB), and 4ʹ-({ω-[(4ʹ-cyano[1,1ʹ-biphenyl]-4-yl)oxy]alkyl}thio)[1,1ʹ-biphenyl]-4-carbonitriles (CBSnOCB) are described. The odd members of both series show twist-bend nematic and nematic phases, whereas the even members exhibit only the nematic phase. An analogous cyanoterphenyl-based dimer, 3⁴-{6-[(4ʹ-cyano[1,1ʹ-biphenyl]-4-yl)thio]-hexyl}[1¹,2¹:2⁴,3¹-terphenyl]-1⁴-carbonitrile (CT6SCB), is also reported and shows enantiotropic N_TB and N phases. The transitional properties of these dimers are discussed in terms of molecular curvature, flexibility and biaxiality. The same molecular factors also influence the birefringence of nematic phases. Resonant X-ray scattering studies of the twist-bend nematic phase at both the carbon and sulfur absorption edges were performed, which allowed for the determination of critical behaviour of the helical pitch at the transition to the nematic phase, the behaviour was found to be independent of molecular structure. It was also observed that despite the different molecular bending angle and flexibility, in all compounds the helical pitch length far from the N-N_TB transition corresponds to 4 longitudinal molecular distances.

Graphical Abstract

KEYWORDS:

• Twist-bend nematic phase
• liquid crystal dimers
• birefringence
• sulfur linked
• cyanobiphenyl
• resonant X-ray scattering

Acknowledgments

EG and DP acknowledge the support of the National Science Centre (Poland): (Grant Number 2016/22/A/ST5/00319). We acknowledge the use of Beamline 5.3.1 and 11.0.1.2 of the Advanced Light Source supported by the Director of the Office of Science, Office of Basic Energy Science, of The U.S. Department of Energy under contract no. DE-AC02-05CH11231. We thank Alex Liebman-Pelaez for designing the heating stage used for TReXS measurement.

Disclosure statement

No potential conflict of interest was reported by the authors.

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Funding

This work was supported by the Director of the Office of Science, Office of Basic Energy Science, The U.S. Department of Energy [DE-AC02-05CH11231]; National Science Centre (Poland) [2016/22/A/ST5/00319].