Crystal-smectic G transformation investigated by temperature-dependent Raman study.

Area of Science:

• Materials Science
• Condensed Matter Physics
• Spectroscopy

Background:

• Liquid crystals exhibit diverse mesophases, including the crystal and Smectic G (SmG) phases.
• Understanding phase transitions is crucial for developing advanced materials with tailored properties.

Purpose of the Study:

• To investigate the crystal-SmG phase transition dynamics in terephthalidine-bis-butylaniline (TBBA) and terephthalidine-bis-heptylaniline (TB7A).
• To elucidate the molecular mechanisms underlying the observed phase transition using temperature-dependent Raman spectroscopy and DFT calculations.

Main Methods:

• Temperature-dependent Raman spectroscopy was performed on TBBA and TB7A from 10 K to 406 K.
• Analysis focused on peak position and linewidth variations of specific Raman bands (1120-1240 cm⁻¹ and 1500-1700 cm⁻¹).
• Density Functional Theory (DFT) calculations (B3LPY/6-31G(d)) were used to determine optimized geometry and vibrational wavenumbers.

Main Results:

• Raman spectral changes clearly indicated a crystal-SmG transition at approximately 334 K for both compounds.
• Increased vibration of the long alkyl tail and rotation around the molecular axis were identified as key drivers of the transition.
• DFT calculations provided optimized molecular geometry and vibrational modes corresponding to observed Raman bands.

Conclusions:

• The study successfully characterized the crystal-SmG transition in TBBA and TB7A.
• Molecular dynamics, specifically alkyl chain vibrations and rotations, are confirmed as the primary mechanisms for this transition.
• Raman spectroscopy and DFT are effective tools for studying phase transitions in liquid crystalline materials.