Molecular ordering in a biaxial smectic-A phase studied by scanning transmission X-ray microscopy (STXM)

Results of STXM investigations of a binary mixture (1–TNF = 2 : 1; SmA_b 140 M 180 Iso) known to form a SmA_b phase [T. Hegmann, J. Kain, S. Diele, G. Pelzl and C. Tschierske, Angew. Chem. Int. Ed., 2001, 40, 887] are presented. Near edge X-ray absorption fine spectra (NEXAFS) of the 1–TNF board-like aggregates, in particular the intensity of the low energy peaks associated with aromatic ring π* orbitals (284.5–286.5 eV), show that the molecular plane of these aggregates is very sensitive to the relative orientation of electric field vector E of linearly polarized light, which is used to determine the molecular orientation in the LC phase. The observed strong in-plane dichroic signal suggests the predominant orientation of the 1–TNF aggregates to be along the smectic layer normal as well as long-range ordering of the in-plane molecular orientation (biaxiality). Orientational maps derived from series of measurements at different sample rotation angles around the specimen normal clearly show a Schlieren-type texture, and permit a detailed examination of exclusive ±½ disclination theoretically predicted for the SmA_b phase.