On the Hunt for the Holy Grail of Liquid Crystal Science

Abstract
The thermotropic biaxial nematic thermotropic liquid crystal is a highly sought material in which the molecular centers of gravity are distributed like in a liquid but all molecular axis are aligned along individual directions (directors) at long range. There have been two periods of intense research on the biaxial nematic phase since the first theoretical proposal of Freiser 1970 and its discovery in the area of lyotropic liquid crystals by Saupe 1980 between 1990 and 2000 and 2004 till 2010. Finally it was concluded that not any material exhibited real nematic phase biaxiality but formed biaxial nematic cluster phases.[1]

Our aim was to find the right shape with the highest molecular biaxiality. The molecules were tailored to be shape-persistent, thus the materials cannot change significantly their conformation and eventually their aspect ratio remains unchanged. These molecules were further engineered to show nematic phases and although a biaxial nematic phase has been discovered for bent nematogens with the tetrahedral angle (magic angle), these materials were found to be thermodynamically unstable (monotropic).[2] Recently, we highlighted a board-shaped lead structure, with a highly promising aspect ratio and broad enantiotropic nematic phase intervals.[3] This could be shown to generate very small biaxial aggregates. This presentation will highlight the molecular design of such nematogens and their phase behavior studied by polarized optical microscopy, differential scanning calorimetry, X-ray diffraction and Conoscopy.

[1] M. Lehmann, V. Goertz, in Handbook of Liquid Crystals, eds. J. W. Goodby, P. J. Collings, T. Kato, C. Tschierske, H. Gleeson and P. Raynes, Wiley-VCH, Weinheim, 2nd edn, 2014, vol. 3, ch. 13, pp. 335–373, “Design of Biaxial Nematic Mesogens”.
[2] J. Seltmann, K. Müller, S. Klein, M. Lehmann, Chem. Commun 2011, 47, 6680-6682.
[3] M. Lehmann, S. Maisch, N. Scheuring, J. Carvalho, C. Cruz, P. J. Sebastião, R. Y. Dong, Soft Matter 2019, 15, 8496-8511.