From molecular biaxiality of real board-shaped mesogens to phase biaxiality? On the hunt for the holy grail of liquid crystal science

Abstract

In the search of the predicted biaxial nematic phase, a series of shape-persistent board-shaped mesogens with maximum molecular biaxiality and a dipole along the minor molecular axis were designed to form nematic (N) mesophases. One compound exhibits a wide nematic temperature range, which can be supercooled to room temperature. A comprehensive variable temperature X-ray study on aligned samples reveals patterns being dominated by the form factor of very small aggregates, from which the aspect ratio of the lead compound with length (L) : breadth (B) : width (W) of 10.73 : 3.16 : 1.23 could be obtained. The ratio is close to the predicted optimum molecular biaxiality by Straley's hard particle model. Hence variable temperature proton relaxation studies on this mesogen were carried out over a wide frequency range. The global fit of the frequency dispersions at five temperatures with a motional model requires in addition to the usual rotation/reorientation contribution, two independent director fluctuations contributions: one for the conventional nematic order director (n) fluctuations and the other for the minor director (m) fluctuations (normal to n). The correlation length of the minor directors determined by NMR could extend to 5–8 molecules in the W direction, but only to the nearest neighbour in the B direction, as found by X-ray diffraction. Both X-ray and NMR studies indicate that these new types of lead structure are extremely promising to find the long sought-after biaxial N mesophase.