Chain dynamics in a chiral C phase by deuteron spin relaxation study

Area of Science:

• Materials Science
• Physical Chemistry
• Condensed Matter Physics

Background:

• Chiral liquid crystals exhibit complex molecular dynamics crucial for their electro-optic properties.
• Understanding molecular reorientations and conformational transitions is key to designing advanced liquid crystal displays.
• Deuterium NMR spectroscopy is a powerful tool for probing molecular motion in ordered systems.

Purpose of the Study:

• To investigate molecular reorientations and internal conformational transitions in an aligned chiral liquid crystal (10B1M7).
• To assess the applicability of existing motional models to both uniaxial (smectic A) and tilted (smectic C*) liquid crystal phases.
• To determine the influence of the tilt angle in the smectic C* phase on molecular dynamics.

Main Methods:

• Deuterium spin-lattice relaxation measurements were performed on aligned 10B1M7 liquid crystal samples.
• Analysis focused on the smectic A (SmA) and chiral smectic C* (SmC*) phases.
• A motional model was applied and evaluated for its adequacy across different liquid crystal phases.

Main Results:

• The motional model, typically used for uniaxial phases, proved adequate for the tilted SmC* phase.
• Deuterium NMR spectra in the SmC* phase did not resolve rotations around the pitch axis, suggesting unobservable phase biaxiality.
• Relaxation rates in the SmC* phase correlated with the molecular director's tilt angle relative to the layer normal.
• Molecular tumbling motion exhibited a higher activation energy when transitioning from the SmA to the SmC* phase.

Conclusions:

• The study validates a unified motional model for both SmA and SmC* phases of chiral liquid crystals.
• Molecular tilt in the SmC* phase significantly influences relaxation rates, despite unobservable phase biaxiality.
• A notable increase in activation energy for tumbling motion occurs upon entering the SmC* phase from the SmA phase.