Structure-Property Correlations in Cyanobiphenyl-Based Dimer-Like Mesogens.

Area of Science:

• Materials Science
• Organic Chemistry
• Physical Chemistry

Background:

• Investigates dimer-like liquid crystal (LC) compounds, extending previous research on re-entrant nematic (Nre) phase dependence on molecular structure.
• Focuses on compounds linking a cyanobiphenyl core with a salicylaldimine segment via an oxy(oligomethylene)oxy spacer.

Purpose of the Study:

• To comprehensively investigate the effects of spacer length/parity and terminal chain length on the thermal behavior and phase transitions of dimer-like LC compounds.
• To explore fundamental relationships between molecular structural features and thermal properties, particularly the odd-even effect.

Main Methods:

• Synthesis of five series of dimer-like compounds with varying spacer lengths (hexyl to decyl) and terminal chain lengths (pentyl to decyl).
• Characterization using optical and calorimetric studies to determine phase behavior and transition temperatures.
• X-ray diffraction analysis to confirm phase structures, specifically the partial bilayer order of the SmA (SmAd) phase.

Main Results:

• A pronounced odd-even effect was observed in thermal properties, with even-member compounds showing higher clearing temperatures and transition enthalpies.
• The odd-even effect attenuates with increasing spacer length.
• Compounds with even-parity spacers exhibited stable nematic (N) and/or smectic A (SmA) phases, while odd-spaced compounds predominantly showed a monotropic N phase; re-entrant nematic (Nre) phases were not observed in the newly synthesized compounds.

Conclusions:

• Dimer-like compounds exhibit thermal behavior analogous to both LC monomers and dimers, serving as bridging structures.
• The study validates the significant influence of molecular structure, particularly spacer parity and length, on liquid crystalline phase behavior.
• The precise origin of re-entrant behavior in specific compounds remains difficult to ascertain due to minimal structural differences.