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

The synthesis and characterisation of two series of cyanobiphenyl-based liquid crystal dimers containing sulfur links between the spacer and mesogenic units, the 4MODIFIER LETTER PRIME-[1,omega-alkanediylbis(thio)]bis-[1,1MODIFIER LETTER PRIME-biphenyl]-4-carbonitriles (CBSnSCB), and 4MODIFIER LETTER PRIME-({omega-[(4MODIFIER LETTER PRIME-cyano[1,1MODIFIER LETTER PRIME-biphenyl]-4-yl)oxy]alkyl}thio)[1,1MODIFIER LETTER PRIME-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, 3(4)-{6-[(4MODIFIER LETTER PRIME-cyano[1,1MODIFIER LETTER PRIME-biphenyl]-4-yl)thio]-hexyl}[1(1),2(1):2(4),3(1)-terphenyl]-1(4)-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.