EPR studies on molecular orientation in a surface-stabilized paramagnetic liquid crystal cell

Area of Science:

• Materials Science
• Spectroscopy
• Physical Chemistry

Background:

• Determining molecular orientation in liquid crystals (LCs) is crucial for understanding their properties.
• Electron Paramagnetic Resonance (EPR) spectroscopy offers a potential route for such investigations.

Purpose of the Study:

• To develop and validate a novel EPR spectroscopy method for precise molecular orientation determination in surface-stabilized LC cells.
• To model the orientation of a specific paramagnetic LC molecule ((2S,5S)-2,5-dimethyl-2-heptyloxyphenyl-5-[4-(4-octyloxybenzenecarbonyloxy)phenyl]pyrrolidine-1-oxy) in its racemic and enantiomerically enriched forms.

Main Methods:

• Utilized EPR spectroscopy on a paramagnetic LC molecule with a spin source fixed in its rigid core.
• Analyzed g-value profiles as a function of magnetic field angle in surface-stabilized LC cells (4 microm thickness).
• Developed orientation models for nematic (N), chiral nematic (N*), and crystalline phases, incorporating molecular rotation and helical superstructures.

Main Results:

• Successfully simulated observed g-value profiles using distinct orientation models for N, N*, and crystalline phases.
• Determined molecular g-values along the long (g(parallelM)) and short (g(perpendicularM)) axes in the N phase by fitting the temperature profile using the Haller equation.
• Validated the determined g-values against principal g-values of a similar nitroxide and MM3-optimized LC structure.

Conclusions:

• The developed EPR method provides accurate insights into molecular orientation in various LC phases.
• The study successfully characterized the distinct orientational behaviors of the paramagnetic LC molecule in different phases.
• The findings contribute to a deeper understanding of structure-property relationships in liquid crystals.