Light-Emitting Chiral Nematic Dimers with Anomalous Odd-Even Effect

Area of Science:

• Materials Science
• Organic Chemistry
• Physical Chemistry

Background:

• Chiral nematic (N*) liquid crystals exhibit unique optical properties.
• Dimers with varying spacer lengths and terminal groups are synthesized to study structure-property relationships.
• Understanding mesomorphic behavior is crucial for developing advanced materials.

Purpose of the Study:

• To investigate the thermal and photophysical properties of novel photoluminescent, chiral nematic liquid crystalline dimers.
• To explore the influence of molecular structure, specifically spacer parity and terminal tail length, on liquid crystal behavior.
• To elucidate the anomalous odd-even effect observed in the formation of blue phases.

Main Methods:

• Synthesis of cholesterol and Schiff base/salicylaldimine core-based dimers.
• Thermal analysis (e.g., DSC, polarized optical microscopy) to identify liquid crystal phases.
• Spectroscopic techniques (e.g., UV-Vis, fluorescence, CD spectroscopy) to probe photophysical properties.
• Investigation of structure-property relationships by varying molecular architecture.

Main Results:

• All synthesized dimers exhibited the chiral nematic (N*) liquid crystalline phase.
• An unusual odd-even effect was observed: blue phases (BPs) were present in dimers with even-membered spacers but absent in odd-membered counterparts.
• Photoluminescence was confirmed in dilute solutions and condensed states (solid, N* phase, isotropic liquid).
• Circular dichroism (CD) spectra indicated significant chiral correlations and a right-handed helical twist in the N* phase.

Conclusions:

• The synthesized dimers display anomalous mesomorphic behavior, particularly concerning blue phase formation.
• The observed odd-even effect is attributed to the overall molecular shape influenced by terminal tail orientation.
• These photoluminescent liquid crystals possess tunable optical properties, with potential applications in advanced display and photonic technologies.