A nematic to nematic transformation exhibited by a rod-like liquid crystal

Abstract

A novel, highly polar rod-like liquid crystal was found to exhibit two distinct nematic mesophases (N and N X ).When studied by microscopy and X-ray scattering experiments, and under applied electric fields, the nematic phases are practically identical. However, calorimetry experiments refute the possibility of an intervening smectic mesophase, and the transformation between the nematic phases was associated with a weak thermal event. Analysis of measured dielectric data, along with molecular properties obtained from DFT calculations, applying the Maier-Meier relationship allowed for the degree of antiparallel pairing of dipoles in both nematic phases to be quantified. Based on the results, we conclude that the onset of the lower temperature phase is driven by the formation of antiparallel molecular associations.

Rational Design of Rod‐Like Liquid Crystals Exhibiting Two Nematic Phases

“…In both the higher‐ and lower‐temperature nematic phases, the observed scattering was broadly similar to that reported previously for 1 ; three diffuse peaks were seen at “small” angles parallel to the external aligning field (i.e., along the director, Figure c). The positions of each of the three peaks were determined by deconvolution (Figure ), the results are presented in Table .…”

“…The enthalpy associated with the N X –N transition for both 1 and 2 is vanishingly small (0.2 kJ mol −1 for both), and this results in the associated entropy of transition being extremely small for both materials (Δ S N x –N /R, 0.06 and 0.07 for 1 and 2 , respectively). As was noted by us previously, the small value of the enthalpy/entropy associated with the N X –N transition is consistent with a transition between two phases with the same macroscopic symmetry . A phase diagram was constructed between 1 and 2 as shown in Figure a.…”

“…It is interesting to note that the position of each individual peak is the same in both the N and N X phases; however, their relative size changes with the intensity of the peak at 6.3 Å increasing and that of the peak at 4.9 Å decreasing. If we assume that 4.9 and 6.3 Å are indeed the widths of an unpaired molecule and a paired species, respectively, then this suggests the degree of dimerisation is inversely proportional to temperature and therefore higher in the N X phase than in the nematic, a result consistent with measurement of the Kirkwood factor of compound 1 …”

“…Several other nematic or nematic‐like mesophases are known to exist (chiral nematic (N*), discotic nematic (N D ), re‐entrant nematic (N RE ), biaxial nematic (N B ), blue phases (BPI, BPII, BPIII)) or are either predicted to exist or have possibly been discovered (cubatic nematic (N cub ), splay‐bend nematic (N SB )). Recently, we have reported a polar liquid‐crystalline material ( 1 , Table ) that exhibited two distinct nematic mesophases, with a weak first‐order transition between the two phases . Other examples of nematic‐to‐nematic transitions have been observed in binary mixtures of re‐entrant materials, in frustrated chiral rod‐like systems and in main‐chain liquid crystal polymers .…”

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].…”