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

• Published in: Chemistry : a European journal 2012-07, Vol.18 (31), p.9543-9551
• Main Authors: Chen, Hsiu-Hui, Lin, Hsing-An, Lai, Yin-Hui, Lin, Shu-Yu, Chiang, Chien-Hung, Hsu, Hsiu-Fu, Shih, Tzenge-Lien, Lee, Jey-Jau, Lai, Chien-Chen, Kuo, Ting-Shen
• Format: Article
• Language: English
• Subjects: arenes; Bending; Chemistry; Clusters; Formations; liquid crystals; mesogens; Nematic; nematic phase; Optimization; Phase transformations; Two dimensional; X-ray diffraction; X-rays
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201103453

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. Take up your cross: An enantiotropic nematic phase with a wide temperature range has been achieved by using cross‐like mesogens. Biaxiality investigations by using conoscopic and 2D XRD studies imply the formation of small smectic C‐like meta‐cybotactic clusters in the nematic arrangement.