Polarization-Modulated Bent-Core Liquid Crystal Thin Films without Layer Undulation

Area of Science:

• Soft matter physics
• Materials science
• Nanotechnology

Background:

• Spatial confinement influences molecular organization in soft matter.
• Bent-core smectic liquid crystals exhibit complex structures under confinement.
• Previous studies using freeze-fracture transmission electron microscopy (FFTEM) on nitro-substituted bent-core mesogens (n-OPIMB-NO₂) indicated undulated smectic layers with ~8 nm periodicity.

Purpose of the Study:

• To investigate the molecular organization of thin films of bent-core smectic liquid crystals using cryogenic transmission electron microscopy (cryo-TEM).
• To compare cryo-TEM findings with previous FFTEM results and understand discrepancies.
• To elucidate the effect of sample thickness on the observed molecular structures.

Main Methods:

• Cryogenic transmission electron microscopy (cryo-TEM) was employed to study approximately 100 nm thick films of 8-OPIMB-NO₂.
• Comparison with existing freeze-fracture transmission electron microscopy (FFTEM) data from thicker samples (5-10 μm).

Main Results:

• Cryo-TEM revealed only density modulation with a periodicity of 16.2 nm in the thin films.
• No smectic layer undulation was observed in the thin films, contrasting with FFTEM results.
• The discrepancy was attributed to differences in sample thickness and preparation methods (quenched thin films vs. fractured thick samples).

Conclusions:

• Sample thickness significantly influences the observed molecular organization in bent-core smectic liquid crystal films.
• Cryo-TEM provides insights into the bulk structure of thin films, while FFTEM visualizes fractured surfaces.
• Understanding these thickness-dependent effects is crucial for applications in organic electronics and photovoltaics.