Nematic twist-bend phase with nanoscale modulation of molecular orientation

Abstract: A state of matter in which molecules show a long-range orientational order and no positional order is called a nematic liquid crystal. The best known and most widely used (for example, in modern displays) is the uniaxial nematic, with the rod-like molecules aligned along a single axis, called the director. When the molecules are chiral, the director twists in space, drawing a right-angle helicoid and remaining perpendicular to the helix axis; the structure is called a chiral nematic. In this work, using transmission electron and optical microscopy, we experimentally demonstrate a new nematic order, formed by achiral molecules, in which the director follows an oblique helicoid, maintaining a constant oblique angle with the helix axis and experiencing twist and bend. The oblique helicoids have a nanoscale pitch. The new twist-bend nematic represents a structural link between the uniaxial nematic (no tilt) and a chiral nematic (helicoids with right-angle tilt).

• The paper demonstrates that the twist-bend nematic phase exhibits an oblique helicoidal structure with a nanoscale pitch (~8-9 nm) bridging nematic and chiral nematic phases.
• The study employs advanced TEM, XRD, and FFTEM techniques to validate the unique structural, elastic, and optical properties of dimeric liquid crystals.
• The findings offer theoretical insights into molecular interactions, paving the way for precise control in display technologies and novel liquid crystal applications.

Analysis of the Nematic Twist-Bend Phase in Liquid Crystals

The discussed paper presents a comprehensive investigation into a novel nematic twist-bend (N$_{tb}$) phase in liquid crystals, where achiral molecules exhibit nanoscale modulation of molecular orientation. This research explores the distinct characteristics of liquid crystals that form an oblique helicoidal structure without the need for molecular chirality, as previously predicted by Meyer and supported by various theoretical and computational models.

Structural Properties and Experimental Verification

The authors deploy a sophisticated array of experimental techniques including transmission electron microscopy (TEM) and X-ray diffraction (XRD) to evidence the N$_{tb}$ phase in dimeric liquid crystal materials. Their Fourier-transform infrared electron microscopy (FFTEM) observations convey a period of nanoscale pitch (~8-9 nm), significantly shorter than typically seen in chiral nematic (N*) phases. These structural insights indicate that the oblique helicoidal configuration provides a structural bridge between the uniaxial nematic (N) and chiral nematic (N*) phases.

Elastic and Optical Characteristics

Elastic properties, such as the bend elastic constant (K$_3$), deviate from typical nematic behavior as they exhibit low values near the N to N$_{tb}$ phase transition, an anomalous feature confirmed across experiments with materials M1 and M2. This finding suggests a distinctive molecular mechanism that facilitates the N$_{tb}$ phase, likely involving bent-core or "banana-shaped" molecules.

The study also highlights the optical behavior of the N$_{tb}$ phase, where birefringence is observed to decrease in the phase due to the conical helix configuration. The optical and electro-optic responses differ significantly between the N and N$_{tb}$ phases, as demonstrated by the striking differences in the Frederiks transition—wherein the reorientation of the optic axis alteration is much more constrained in the N$_{tb}$ phase due to its nanoscale periodicity.

Theoretical Implications and Future Prospects

Theoretical models suggest that the unique twist-bend structure of N$_{tb}$ could bridge a better understanding of the molecular interactions leading to short-pitch helicoidal phases without relying on molecular chirality. The outcomes pose implications for flexoelectricity in liquid crystals and potential new applications in display technologies, where molecular orientation and elastic properties can be precisely manipulated at the nanoscale.

The discovery and subsequent characterization of the N$_{tb}$ phase guide future studies focusing on the hydrodynamical properties of these materials, their response to external fields, and the role of domain boundaries and defects in real-world applications. This research offers avenues for exploring novel liquid crystalline phases stabilized by dimeric and bent-core molecules, potentially leading to new materials with customized optical and mechanical properties.

Overall, the findings signify an important step in the study of liquid crystal phases, underscoring a complex interplay of molecular characteristics that result in nanoscale chiral order—an area open to abundant further research and technological exploitation.