Insights into the cybotactic nematic phase of bent-core molecules

Abstract

We present X-ray diffraction evidence for clusters exhibiting a skewed cybotactic supramolecular structure in the nematic phase of the butyloxy-phenyl diester, ODBP-Ph-OC₄, a bent core mesogenic diester of the nonlinear diol 2,5-bis-(p-hydroxyphenyl)-1,3,4-oxadiazole (ODBP). The temperature dependence of the off-meridional four-spot pattern shows that the tilt angle in the cybotactic clusters changes with temperature. The correlation lengths exhibit no evidence of critical behavior but there is no underlying smectic C phase in ODBP-Ph-OC₄. The tilted cybotactic clusters persist throughout the 85 °C wide nematic phase, and even remain evident in the supercooled nematic. This inherently biaxial supramolecular structure—clusters of stratified and tilted mesogens—embedded in an otherwise translationally disordered nematic host medium may account for the NMR biaxiality observed in nematic phases of ODBP mesogens.