Cholesterol-Based Liquid Crystal Dimers Flip Their Handedness With Heat

Chirality, the property of handedness that distinguishes a left-handed object from its mirror image, is usually treated as a fixed molecular attribute: once a molecule is built with R or S stereochemistry, that configuration does not change unless chemical bonds are broken. A new study of cholesterol-based liquid crystal dimers, published in Frontiers in Soft Matter, now shows that the supramolecular helices these molecules assemble into can do something the molecules themselves cannot. By simply heating or cooling the material through a phase transition, the researchers observed the handedness of the entire helical superstructure reverse completely, even though the stereochemistry of every molecule remained untouched.

The work, led by Sachin Ashok Bhat and Channabasaveshwara V. Yelamaggad at the Centre for Nano and Soft Matter Sciences in Bengaluru, together with collaborators at BMS College of Engineering and the University of Mysore, describes a family of sixteen nonsymmetric liquid crystal dimers. Each dimer couples a rigid, chiral cholesteryl unit to a two-ring Schiff-base mesogenic core through a flexible alkylene spacer of systematically varied length. The two ends of the molecule play different roles: the cholesterol fragment carries the permanent molecular chirality, while the Schiff-base segment, with its azomethine linkage, provides the anisotropic shape and conjugation needed for liquid crystalline ordering. The chiral tail was drawn from one of two sources, either citronellol or octan-2-ol, giving two series of compounds, CSBC and CSBO, each containing matched R and S enantiomeric pairs.

All sixteen compounds proved to be enantiotropic liquid crystals, meaning they pass reversibly between crystalline, mesomorphic and isotropic states on heating and cooling. The phase behaviour, mapped by polarised optical microscopy and differential scanning calorimetry, was remarkably rich. Depending on spacer length and terminal substituent, the dimers formed chiral nematic phases, smectic A layers, tilted chiral smectic C phases, and, in several cases, the twist grain boundary phase, a frustrated mesophase that arises when the tendency of chiral molecules to twist conflicts with their drive to form flat, layered smectic order.

The spacer length emerged as the decisive variable in determining which phases appeared. Short spacers, such as the three-carbon and four-carbon linkers, enhanced conformational restriction and packing frustration, stabilising the twist grain boundary phase. In the CSBC series, for example, the three- and four-carbon dimers showed TGB phases adjacent to the isotropic liquid, while the five-carbon members favoured chiral nematic and smectic A phases with a metastable chiral smectic C phase on cooling. The seven-carbon dimers, by contrast, exhibited only the smectic A phase. This progression reflects a general principle in dimeric liquid crystals: as the spacer lengthens, molecular flexibility increases, packing becomes more efficient, and layered smectic organisation wins out over frustrated twist. The familiar odd-even effect, in which spacer parity modulates molecular bend and phase stability, was also evident across both series.

Variable-temperature X-ray diffraction provided the structural underpinning for these observations. For the representative dimer CSBC-(R)3, the smectic A phase showed a layer spacing of about 46 angstroms at 120 degrees Celsius, closely matching the calculated all-trans molecular length of 46.66 angstroms. The resulting d/L ratio of nearly one is the signature of a monolayer smectic A structure, in which each layer is one molecular length thick. For CSBO-(R)5, the smectic C phase told a different story: layer spacings fell well below the molecular length, with d/L ratios of 0.86 at 40 degrees and 0.83 at 30 degrees, corresponding to calculated tilt angles between roughly 31 and 34 degrees. These measurements confirmed that the molecules lean over as they enter the smectic C phase, a geometric change that turns out to be central to the chirality switching behaviour.

The most striking results came from temperature-dependent circular dichroism spectroscopy, a technique that measures how a material responds differently to left- and right-circularly polarised light. In the chiral nematic phase, the spectra displayed two distinct bands. A strong long-wavelength band centred near 550 nanometres arises from selective Bragg reflection by the periodic cholesteric helix, probing the long-range supramolecular architecture. A second band near 350 nanometres originates from the pi-to-pi-star transition of the Schiff-base chromophore and reflects the local chiral arrangement of neighbouring mesogenic units through excitonic coupling. The presence of both bands demonstrates that chirality is transferred hierarchically, from the stereogenic centres of the cholesterol unit to the local chromophore packing and onward to the macroscopic helix.

The corresponding R and S enantiomers produced nearly perfect mirror-image CD spectra across the entire temperature range, with both bands inverting sign while retaining identical profiles and intensities. This confirms that molecular stereochemistry is faithfully translated into supramolecular helicity without racemisation or preferential aggregation. Notably, the absolute configuration had little effect on thermodynamics: each enantiomeric pair showed essentially identical phase sequences and transition temperatures, indicating that handedness governs the direction of the helix but not the stability of the phases themselves.

The pivotal discovery was what happened at the transition from the chiral nematic phase to the chiral smectic C phase. On cooling, both the selective reflection band and the chromophore absorption band reversed sign simultaneously and completely. Because the molecular stereochemistry is fixed throughout the thermal cycle, this inversion cannot reflect any change in the molecules themselves. Instead, it demonstrates a genuine reversal of the supramolecular helical sense. The authors propose a mechanistic picture: in the chiral nematic phase, molecular chirality is expressed through continuous rotation of the director, producing the cholesteric helix. On entering the smectic C phase, positional layering develops together with molecular tilt, and the azimuthal rotation of the tilt direction from one layer to the next generates a different chiral superstructure whose preferred helical sense is opposite to that of the cholesteric phase. The reorganisation of molecular packing, not any alteration of molecular configuration, drives the switch.

The team took care to rule out optical artefacts. Because liquid crystalline phases are anisotropic, apparent CD signals can in principle arise from linear dichroism, linear birefringence, reflection or scattering. Control experiments detected no measurable linear dichroism over the investigated spectral range, and rotating the samples about the incident light axis in 90-degree intervals left the spectral profiles, intensities and signs unchanged. Genuine circular dichroism is rotationally invariant, whereas artefactual contributions from linear optical effects are not, so the observed rotational invariance, together with the mirror-image behaviour of the enantiomeric pairs and the disappearance of the CD signal in the isotropic phase, establishes that the signals are intrinsic. The simultaneous, reversible inversion of both bands across the phase transition is characteristic of a real thermally driven reorganisation of the helical structure.

The researchers note that a similar thermally induced helicity inversion had previously been observed by their group in ferroelectric liquid-crystalline systems, suggesting that helicity inversion reflects a general competition between orientational twist and layered chiral ordering. However, the systematic observation of simultaneous inversion of both the selective reflection and chromophore CD bands across the nematic-to-smectic-C transition in cholesterol-based Schiff-base dimers has rarely been reported. The finding establishes thermally induced supramolecular chirality inversion, or TISCI, as a direct experimental manifestation of chirality transfer across multiple structural length scales, from local chromophore packing to the macroscopic helical superstructure.

The practical implications extend well beyond fundamental soft matter physics. Because the handedness of the supramolecular assembly can be switched reversibly with temperature while the molecular chirality stays constant, these dimers offer a design strategy for responsive chiroptical materials, circularly polarised photonic systems, optical information storage, smart sensors and temperature-controlled optical switching devices. The structure-property relationships uncovered here, in which spacer architecture regulates packing, conformational flexibility tunes phase stability, and terminal chirality sets the direction of helicity transfer, provide molecular design principles for next-generation chiral liquid crystals whose mesophases and optical responses can be tuned with precision. In a field where controlling light with chiral soft matter is a growing pursuit, a material that flips its entire helical handedness on command is a compelling addition to the toolkit.

Subject of Research: Thermally induced supramolecular chirality inversion in cholesteryl Schiff-base liquid crystal dimers

Article Title: Hierarchical chirality transfer and thermally induced supramolecular chirality inversion in cholesteryl schiff-base liquid crystal dimers

Article References: Hierarchical chirality transfer and thermally induced supramolecular chirality inversion in cholesteryl schiff-base liquid crystal dimers. (n.d.). https://doi.org/10.3389/frsfm.2026.1928826

Image Credits: AI Generated

DOI: 10.3389/frsfm.2026.1928826

Keywords: liquid crystals, chirality, cholesterol, Schiff base, circular dichroism, supramolecular helicity, smectic phases, chiral nematic, twist grain boundary, phase transitions, chiroptical materials, self-assembly