Cybotaxis dominates the nematic phase of bent-core mesogens: a small-angle diffuse X-ray diffraction study

• Published in: Soft matter Vol. 7; no. 3; pp. 895 - 91
• Main Authors: Francescangeli, Oriano, Vita, Francesco, Ferrero, Claudio, Dingemans, Theo, Samulski, Edward T
• Format: Journal Article
• Language: English
• Published: 01.01.2011
• Subjects: Compatibility; Diffraction; Diffusion; Evolution; Fluctuation; Liquid crystals; Nematic; X-rays
• ISSN: 1744-683X; 1744-6848
• DOI: 10.1039/c0sm00745e

New temperature dependent X-ray diffraction (XRD) data on the bent-core mesogens based on the nonlinear 2,5-bis( p -hydroxyphenyl)-1,3,4-oxadiazole (ODBP) unit enable a consistent interpretation of the supramolecular structure in this class of liquid crystals. Strong evidence for cybotaxis in the high temperature phase explains the small-angle four-spot pattern and calls into question prior XRD interpretations. We find that the data can be satisfactorily explained by skewed cybotaxis, a stratified arrangement of tilted, bent-core mesogens (BCMs). We also observe the temperature-induced evolution of skewed cybotaxis to normal cybotaxis-strata wherein the long axes of the BCMs are normal to the layer fluctuations. Our XRD interpretation is compatible with the NMR data that exhibit biaxiality in the nematic phase of ODBP mesogens. In the nematic phase of the bent-core mesogen ODBP-Ph-C 7 H 15 derived from 2,5-bis( p -hydroxyphenyl)-1,3,4-oxadiazole, evanescent smectic fluctuations transition from skewed to normal cybotaxis on elevating the temperature. Concomitantly the tilt decreases as the transition is approached and the local biaxial supramolecular structure becomes uniaxial.