Optically active bimesogens incorporating branched central spacers

Area of Science:

• Materials Science
• Organic Chemistry
• Physical Chemistry

Background:

• Liquid crystalline materials, including dimers, bimesogens, and oligomers, are extensively studied.
• The influence of the central spacer group on the properties of these mesogenic systems has received less attention.
• Chirality within the molecular structure can significantly impact liquid crystal phase behavior.

Purpose of the Study:

• To investigate the role of the central spacer in phenyl 4-cyanobenzoate bimesogens.
• To synthesize and characterize a chiral bimesogen (compound 1) with a methyl group at the 2-position of the central spacer.
• To evaluate the helical twisting power and phase behavior of compound 1 and its analogues.

Main Methods:

• Synthesis of a chiral bimesogen incorporating a substituted phenyl benzoate core.
• Measurement of helical twisting power (HTP) in nematic liquid crystal hosts (5CB and E7).
• Differential scanning calorimetry (DSC) and polarized optical microscopy (POM) for phase characterization.

Main Results:

• Compound 1, a chiral bimesogen, demonstrated significant helical twisting power (0.36 and 0.35 μm⁻¹ wt%⁻¹ in 5CB and E7, respectively).
• Compound 1 exhibited a monotropic chiral nematic phase but did not display a twist-bend modulated phase.
• Analogues with larger mesogenic units showed varied liquid crystal phases, including chiral nematic, twist-bend (TB), smectic C (SmC), and smectic B (SmB) phases.
• Analogues with smaller aspect ratios were non-mesogenic.

Conclusions:

• The central spacer's structure, particularly its chirality, plays a crucial role in dictating the liquid crystalline properties of bimesogens.
• Chirality in the spacer can induce helical structures and chiral nematic phases.
• The aspect ratio and nature of the mesogenic units significantly influence the type and stability of observed liquid crystal phases, including TB, SmC, and SmB phases.