Roof-shaped Nematogens – A New Approach towards the Thermotropic Biaxial Nematic Phase

• Applicant: Professor Dr. Matthias Lehmann
• Subject Area: Preparatory and Physical Chemistry of Polymers ; Organic Molecular Chemistry - Synthesis and Characterisation
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 449759211

This project is aiming towards a new approach to realize the biaxial nematic phase in the area of low molar mass molecules. The nematic phase is a liquid crystal phase, i.e. a phase with anisotropic properties and liquid like molecular mobility, as known from LC-Displays. In the biaxial nematic phase the molecules have no positional order (they are liquid-like), but all molecular axis possess a long range orientational order – in the conventional nematic phase only the long axis is oriented owing to the molecular mobility. The aim is therefore to stop the rotation by a convenient molecular architecture. This phase is long sought because of its multiple theoretical predictions and promising applications, but never completely confirmed by the scientific community.The project is based on a recently discovered lead structure with a 1,3,5,8-tetrasubstituted anthraquinone core of a polar roof-shaped structure to which shape-persistent oligo(phenyleneethenylene)arms were attached. This molecular architecture is very close to the molecular optimum biaxiality, which is proposed to generate the biaxial nematic phase. However, the formation of a monodomain necessary for the unequivocal confirmation of phase biaxiality and the stable phase at ambient temperature could not be achieved. Therefore, the aim is to synthesize a series of roof-shaped nematogens based on the lead structure with lateral and terminal chains of different nature and lengths to optimize the molecular biaxiality and the lateral interactions along the minor axis. The latter is detrimental to guarantee the orientational order of all three molecular axes. Additionally, we synthesize derivatives, which can interact by H-bonding with substrate surfaces, such as glass. This will enable us to align the materials in monodomains, which are necessary to confirm the phase biaxiality by the applied techniques. The materials properties will be studied by polarized optical microscopy (orthoscopy, conoscopy), differential scanning calorimetry and X-ray scattering and molecular models will be setup.The knowledge of the structure-property relationship for a small series of target molecules will be used for the optimization of the molecular system. Eventually, mixtures of selected nematogens are studied in order to achieve the room temperature biaxial nematic phase. The optimized nematic molecules and their mixtures will be studied in liquid crystal cells, i.e. their alignment, polarity and switching behavior. With this research in molecular engineering, we expect to approach a real biaxial nematic phase of low molar mass molecules, which is of great basic scientific importance and can be applied in future new liquid-crystalline switching devices.

• DFG Programme: Research Grants