Novel cholesteric glassy liquid crystals of monosubstituted ferrocenes: synthesis and selective reflection properties of a dimesogen, and crystal structure of a monomesogen

Abstract

Several monomesogens of ferrocene derivatives bearing the N‐benzoyl‐N′‐arylthiourea (BATU) bidentate ligand have been synthesized. A non‐symmetric dimesogen in which two structurally different mesogenic groups, namely the BATU and cholesteryl moieties, inter‐linked by a flexible spacer has also been synthesized. The achiral monomesogens exhibit enantiotropic smectic C and nematic phases, while the chiral monomesogens show exclusively a cholesteric phase with selective reflection in the infrared region. The dimesogen, on the other hand, exhibits an enantiotropic cholesteric phase with selective reflection in the visible region with iridescent colours. This represents the first ferrocene based organometallic liquid crystalline compound exhibiting this unique property. The cholesteric phases formed transform to glassy liquid crystals which can be fixed on sudden cooling to 0°C. The Grandjean texture observed in the cholesteric temperature range was retained in the glassy state depicting helical orientation in the glassy solid. Furthermore, X‐ray crystal structure determination of one of the compounds (Fc‐BATU‐12) provided evidence implying the involvement of structure‐determining intra‐ and inter‐molecular interactions in all the liquid crystalline compounds involving BATU units. The crystallographic data confirms that these systems fulfil the two prerequisites for calamitic (rod‐like) mesomorphism, namely critical length/depth ratio (>5) and an extended linear shape.

Acknowledgement

We thank Dr Ameerunisha Begum for drawing the schemes and charts, and Dr Andreas Hoischen, Department of Physical Chemistry for measuring the reflectance spectra.