From antiferroelectricity to ferroelectricity in smectic mesophases formed by bent-core molecules

This contribution gives an overview of ferroelectric switching liquid crystalline phases formed by bent-core molecules. First a description of some general principles behind the mesophase formation within bent-core systems will be given, followed by a short review of the mesophase structures formed by such molecules. Then, different classes of ferroelectric switching bent-core mesogens will be described. This type of switching behaviour has been reported for several subtypes of polar smectic phases (B2, B5, B7 and SmCG) and recently for columnar mesophases. In this discussion particular attention will be made to polyphilic bent-core molecules, composed of three incompatible units, a bent aromatic core, alkyl chains and an oligosiloxane unit. The importance of the decoupling of the layers into microsegregated sublayers for the ferroelectric organisation is discussed. Many of the ferroelectric switching mesophases show dark textures with distinct regions of opposite chirality in their ground states. It is discussed that this might be due to a helical superstructure formed as a result of an escape from macroscopic polar order. Hence, the materials themselves are not ferroelectric in the ground state, but upon alignment within an electric field in the measuring cells the ferroelectric states are stabilised by surface interactions, leading to a ferroelectric switching system. The designing principle was extended to mesogenic dimers with bent-core structural units. For these compounds, depending on the number of dimethylsiloxane units in the spacer either ferroelectric or antiferroelectric switching was observed, whereby the effect of parity is reversed to that observed for conventional calamitic dimesogens. Finally, a carbosilane-based first generation dendrimer is reported. It shows a ferroelectric switching phase, for which a non-correlated organisation of tilted polar smectic layers is proposed (SmCPR).