Macroscopic properties of the nematic phase of boomerang-shaped "C-7": Evidence of biaxiality

Abstract

We report the first evidence of macroscopic biaxiality of the nematic phase of the boomerang-shaped mesogen ODBP-Ph-C-7 (C-7). Studying the macroscopic alignment and the dielectric and the optical properties of the nematic phase of C-7, we found effects that could be attributed only to the existence of three different dielectric constants and three different directors, which lay in the very definition of biaxiality. In contrast to the case of a uniaxial nematic liquid crystal, alignment of C-7 cannot be labeled as homeotropic or planar: if one director is homeotropic then the other two are planar. We show that cells with different homeotropic directors and, consequently, different pairs of planar directors, to a certain extent are, equivalent to planar cells with different dielectric anisotropies. This difference, which is an unambiguous sign of biaxiality, is detected experimentally and turns out to be extremely sharp. The dielectric and the optical properties of C-7 are found to undergo dramatic changes within the temperature range of the nematic phase. These data are summarized in a tentative phase diagram of C-7, which shows that while the medium is elastically biaxial, its dielectric and optical properties can be uniaxial or even isotropic.