Roof-shaped nematogens with optimummolecular biaxiality -A new avenue towards the real thermotropic biaxial nematic phase ?

Recently roof-shaped nematogens 1 with an anthraquinone core were identified to approach anoptimum biaxial aspect ratio.[1,2] While biaxial aggregates of hundreds of nematogens have beendemonstrated by NMR relaxometry, the macroscopically aligned biaxial nematic could not beenconfirmed to date. This is a common challenge encountered already in two intense research periods,during which several claims about the discovery of the thermotropic biaxial nematic phase of a lowmolar mass material have been opposed.[3]

The present contribution highlights the study of a number of roof-shaped molecules such as 1 and 2 with a slightly distinct substitution pattern. Combined comprehensive X-ray scattering and solid-state NMR investigations reveal that only one of these nematic materials (1) shows a pronounced local interdigitation of the lateral chain along the minor director. This is realised by the long lateral chains pointing towards the anthraquinone core.

The self-assembly mechanism enhances the local biaxial order in the nematic phase and impacts on the XRS order parameters of biaxial nematogens 1 and 2. The local freezing of the rotation about the molecular long axis is a prerequisite to find the delicate thermotropic biaxial nematic phase. Fréedericksz transitions of the nematic phases are studied in the presence of electric and magnetic fields to probe phase biaxiality.