Enantiotropic Nematics From Cross‐Like 1,2,4,5‐Tetrakis(4′‐alkyl‐4‐ethynylbiphenyl)benzenes and Their Biaxiality Studies

Abstract

The theoretically predicted optimum length/breadth/width ratio for maximizing shape biaxiality was investigated experimentally by the facile and successful synthesis of cross-shaped compound 3, which showed enantiomeric nematic phase behavior. This cross-like core structure could alternatively be viewed as two fused V-shaped mesogens, which have recently immerged as a new direction in biaxial nematic research, at the bending tips that can act as a new structure for biaxial investigations. Whilst the thermal analysis data of compound 3 did not meet the expected theoretical values for biaxial nematics, surface-induced biaxiality was evidenced by optical studies. Cluster-size analysis within the nematic phase of compound 3 revealed the formation of meta-cybotactic nematics, which approached the cluster sizes of cybotactic nematics. The split small-angle 2D X-ray diffraction patterns of magnetic-field-aligned samples indicated that the nematic phase was composed of small smectic C-like clusters with the tilting of molecules within the clusters. The wide-temperature-range enantiomeric nematic phase of cross-like compound 3 enabled the molecular skeleton to serve as an alternative skeleton to bent-rod mesogens, which exhibited nematic phases with the potential competition of transitions to higher-order liquid-crystalline phases and crystallization, for future biaxial investigations.

Influence of Mesogenic Properties of Cruciform-Shaped Liquid Crystals by Incorporating Side-Arms with a Laterally-Substituted-Fluorine

“…General Procedure: Compounds 3a, 4a and 5a were prepared according to the literature procedure [23,26].…”

“…The mesophase stability, as well as the small nematic temperature range hampered further biaxial investigations. Recently, based on the structure shown in Figure 1, enantiotropic nematic behaviors were reported by replacing the peripheral phenyls with biphenyls to stabilize the nematic phase [23]. Nevertheless, compound 1's low clearing temperature can be of advantage in practical applications.…”

Extended conjugated mesogens: synthesis and mesomorphic properties of H-shaped mesogens based on 3,3ʹ,5,5ʹ-tetrasubstituted 2,2ʹ-bithiophene with oligo(1,4-phenyleneethynylene) arms

“…These entropy changes are significantly smaller than standard values [33,34] and almost the same as those of hockey-stick compounds (0.1-0.3), [12] V-shaped mesogens (0.2-0.5) [12] or 1,2,4,5-tetrakis(4ʹ-alkyl-biphenyl-4-ethynyl)benzenes (0.2 and 0.3). [15] The small entropy values can be attributed to the shape of the molecule. The biaxial nature of the molecule reduces the orientational order in the nematic phase and hence the entropy change at the N-I transition.…”

“…[15] The longer alkyl chains of the X-shaped mesogens do not stabilise the crystalline phase but assist the formation of the mesophase. The X-shaped core, based on 1,2,4,5-tetrasubstituted benzene, has a high symmetry, whereas the 4(PE) 2 2T core, containing 2,2ʹ-bithiophene, has a skewed H-shape.…”

“…rod-like, [6][7][8][9][10] V-shaped, [11][12][13] star-shaped [14,15] and discotic [16][17][18][19][20] mesogens). The mesogens generally consist of a conjugated core and aliphatic chains.…”