Matrix-molecule induced chiral enhancement effect of binary supramolecular liquid crystals

Abstract

Chiral enhancement effects associated with supramolecular liquid crystalline structures are studied, using a complex of achiral molecules (4,4′-bipyridine, 4Bpy) and chiral cholesteric liquid crystalline molecules (3-cholesteryloxycarbonylpropanoic acid, C4) as the model system. Non-mesogenic achiral molecule 4Bpy is used as the matrix element. The chiral enhancement of the supramolecular liquid crystalline structures can be revealed by circular dichroism (CD) and helical twisting power measurements. It is interesting that CD spectra of the complex exhibited an appreciably enhanced chiral property in comparison with that of the pure chiral cholesteric C4 molecules at room temperature. The helical pitch measurements by the Grandjean–Cano method also confirm the chiral enhancement effect. In addition, polarizing optical microscopy (POM) measurements indicate that the addition of matrix molecules leads to the explicit expression of the chiral liquid crystalline texture, i.e. twisted grain boundary (TGBA*) phase, at the liquid crystal temperature. Differential scanning calorimetry (DSC) and variable-temperature X-ray diffraction measurements further confirm the existence of the TGBA* phase. The binary supramolecular assembly structures are investigated by scanning tunneling microscope (STM) and the formation of hydrogen bonds between the chiral mesogens and the achiral matrix molecules can be directly observed. The variable-temperature Fourier transform infrared (FTIR) results further demonstrate that the hydrogen bonds persist until 168 °C. These results indicate that the introduction of a bifunctional aromatic base such as bipyridine could lead to self-assembled supramolecular architectures with discernible enhancements of chiral properties.