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Struktur und Dynamik der nematischen Phasen aus bent-core Mesogenen mit starken smektischen Fluktuationen
Deutsche Forschungsgemeinschaft (DFG) ;
Nematic phases formed by bent-core mesogens have recently become a very active research topic. They exhibit remarkable structural, electro-optical and dielectric properties, which distinguish them from rod-shaped mesogens. Extensive theoretical studies about the role of molecular shape on phase behaviour indicate the existence of a whole class of phases without positional order distinguished by different symmetries. Such phases include biaxial and polar nematics, and tetrahedratic and three-atic phases, which can have several order parameters and display new types of behaviour in electric, flow- and temperature-gradient fields. One of the most exciting achievements in research on bent-core nematics has been the discovery of smectic fluctuations,which are responsible for apparent biaxial behaviour, and giant flexoelectric response. This is a new level of complexity in mesophase structures with only orientational order, and is of fundamental interest for basic science, as it has many possibilities or technological applications. In the proposed research, we offer an extensive investigation of the structure and dynamics of several classes of bent-core nematic compounds exhibiting clustering. The novelty of this proposal lies in the unexplored electro-optics and non-linear optics of bent-core nematic phases and largely unknown structural and dynamic properties (elastic, flexoelectric, etc.). X-ray, dielectric spectroscopy and generation of second harmonic will provide us with full characterisation of the nematic phases and the extent of smectic fluctuations. Detailed experimental studies of the Fréedericksz transition, the behaviour of inversion walls, flexoelectric effects, and the Cotton-Mouton effect are anticipated to provide insight into the elastic and polar properties for different types nematic phases. Extensive studies of those phenomena can greatly contribute to our understanding of the physics for this novel class of liquid crystal materials. Another unique feature of this proposal is a combination of these physical investigations with synthetic work focusing on the investigation of the effects of varying the molecular structure on the structure and properties of the nematic phases, allowing for a correlation of the physical properties with the molecular structure and the perspective to arrive at new biaxial and polar nematic phases.

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