Multiple nematic phases observed in chiral mesogenic dimers

Abstract

A sequence of seven nematic phases has been observed in chiral mesogenic dimers, having odd number of carbon atoms in the spacer, thus a bent shape. The highest temperature phase is a chiral nematic (cholesteric) phase on heating or blue phases on cooling. The lowest temperature nematic phase expels the chiral twist and exhibits spontaneous bent-splay modulation. Fig. 1 General molecular structure of studied dimers.

Miscibility studies of two twist-bend nematic liquid crystal dimers with different average molecular curvatures. A comparison between experimental data and predictions of a Landau mean-field theory for the NTB–N phase transition

“…Cestari et al 6 have examined an odd liquid crystal dimer and observed a nematic-nematic transition but, as the result of an extensive investigation based on numerous techniques, were able to identify the low-temperature nematic as we shall see shortly. A slightly different dimer again with an odd spacer but having one chiral mesogenic group has been reported by Zep et al,7 surprisingly this system is found to form seven nematic phases over a narrow temperature range.…”

Conformational landscapes of bimesogenic compounds and their implications for the formation of modulated nematic phases

“…First predicted by Dozov [13], this liquid-crystalline state of matter has a locally helical structure with a pitch measured to be on the order of 10 nm [4,14], with this phase displaying a number of unusual and unique optical textures [15]. When the twist-bend phase is chiral, other 'nematiclike' mesophases have been reported whose structure is as yet unknown [5,16], whereas a nematic-to-nematic transition has also been recently reported for a polar rod-like compound [17]. A number of reviews concerning the twist-bend phase have been published recently [18][19][20].…”

“…The discovery of the twist-bend phase (N TB ) has given fresh impetus to the study of dimeric liquid crystals [1][2][3][4][5][6][7][8][9][10][11][12][13]. First predicted by Dozov [13], this liquid-crystalline state of matter has a locally helical structure with a pitch measured to be on the order of 10 nm [4,14], with this phase displaying a number of unusual and unique optical textures [15].…”

Molecular dynamics of a binary mixture of twist-bend nematic liquid crystal dimers studied by dielectric spectroscopy

“…In 31 this phase, formed by achiral molecules, the director exhibits 32 periodic twist and bend deformations forming a conical helix 33 having doubly degenerate domains of opposite handedness. 34 Although initially described for dimers of the CBnCB family, 35 the discovery of a low temperature nematic phase boosted 36 research in the area, and a number of materials combining 37 different mesogenic units linked by chains have been identified 38 which exhibit the N TB phase [16][17][18] and its chiral counterpart 39 [19,20]. It is now recognized that molecular curvature plays 40 a crucial role in stabilizing the twist-bend nematic phase.…”