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: Bhat, S. A., P. N., R., Veerabhadraiah, S. R., & Yelamaggad, C. V. (2026). Hierarchical chirality transfer and thermally induced supramolecular chirality inversion in cholesteryl schiff-base liquid crystal dimers. Frontiers in Soft Matter, 6, Article 1928826. 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

Grain boundaries are the two-dimensional interfaces where crystals of different orientations meet inside a metal. Although they occupy only a tiny fraction of a material’s volume, they dominate its mechanical behavior, acting as barriers to dislocation motion, sources of new dislocations, and sites of damage initiation. Metallurgists traditionally distinguish between symmetrical tilt grain boundaries, in which the misorientation axis lies within the boundary plane, and twist grain boundaries, in which the rotation axis is perpendicular to the interface. The two geometries produce very different atomic structures, and the new study set out to determine how those differences translate into strength during compressive deformation of a magnesium alloy containing 2 atomic percent yttrium, a common strengthening addition.

Using molecular dynamics simulations, the researchers built atomic models of both boundary types and subjected them to uniaxial compression, tracking every dislocation nucleation event, every cross-slip, and every stress spike in real time. The simulations relied on a modified embedded-atom method interatomic potential specifically developed for magnesium-yttrium alloys, ensuring that the interatomic forces captured the chemistry of the solute atoms realistically. The results were unambiguous: the compressive strength of models containing twist grain boundaries was significantly higher than that of models with symmetrical tilt boundaries, with flow stresses approximately 15 to 25 percent higher across the strain range of 5 to 20 percent.

The origin of this enhancement, the team found, is an intrinsic three-dimensional network of screw dislocations embedded within the twist boundary itself. Unlike the more planar structure of a tilt boundary, a twist boundary accommodates its misorientation through a grid-like array of screw dislocations that intersect one another at regular nodes. Under load, these nodes act as dynamic sites for dislocation pinning, cross-slip, and multiplication. In effect, the twist boundary arrives pre-equipped with a dense population of dislocation sources and obstacles, forcing the material to expend far more energy to sustain plastic flow. The tilt boundary, by contrast, offers fewer such anchoring points, allowing dislocations to sweep through more freely and limiting the stress the material can carry.

To quantify why particular slip systems activate at each boundary, the researchers performed a Schmid factor analysis, the classic criterion that predicts which crystallographic slip system will yield first under a given stress state. In the symmetrical tilt boundary model oriented at 45 degrees, the basal slip system, which involves dislocations with an a-type Burgers vector gliding on the close-packed basal plane, received a Schmid factor of only 0.11, while the pyramidal slip system, involving the harder c plus a dislocations, received a much more favorable value of 0.38. This quantitative contrast neatly explains the observed dominance of pyramidal slip at that boundary: the macroscopic resolved shear stress simply favors the pyramidal system, despite its intrinsically higher resistance.

The twist boundary told a subtler and more surprising story. In the twist boundary model with a 10-degree misorientation, the basal a Schmid factor of 0.38 would ordinarily predict easy basal slip, since basal dislocations are the most mobile carriers of plasticity in hexagonal metals like magnesium. Yet the simulations showed pyramidal c plus a slip activating locally anyway. The resolution lies in the local stress field of the dislocation network. At the nodes where screw dislocations intersect, stress concentrations reach approximately 1.5 times the macroscopic applied stress, and these amplified local stresses are sufficient to overcome the high critical resolved shear stress that normally keeps pyramidal systems dormant. In other words, the boundary’s internal architecture can override the textbook Schmid criterion, activating slip systems that the macroscopic stress state alone would never select.

This interplay between global geometry and local structure leads the authors to a unifying conclusion: slip system activation in these alloys is governed by a competition between the macroscopic Schmid factor and the intrinsic dislocation source character of the grain boundary. Neither factor alone is sufficient to predict deformation behavior. A boundary may be favorably oriented for basal slip, yet its embedded dislocation network can hijack the process and trigger harder pyramidal slip locally, with consequences for hardening, ductility, and damage tolerance. Conversely, a boundary with an unfavorable Schmid factor for pyramidal slip may still activate it if the resolved stress is large enough, as seen in the tilt boundary case.

The significance of these findings extends well beyond the simulation cell. Pyramidal c plus a dislocations are widely regarded as the key to improving the ductility of magnesium, because they provide the additional independent slip systems needed for arbitrary shape change at room temperature. If twist boundaries can be deliberately introduced, or their density engineered through thermomechanical processing, alloy designers could gain a new lever for promoting c plus a activity without resorting to exotic alloying additions. The work thus provides atomic-scale guidance for grain boundary engineering in magnesium alloys, a strategy in which the character and population of interfaces, rather than grain size alone, become the primary design variables.

For a metal that is the lightest structural metal in common use, roughly 75 percent less dense than steel and 33 percent less dense than aluminum, every increment in strength and ductility carries outsized weight in the race to electrify transport and cut emissions. Magnesium alloys already serve in steering wheels, seat frames, and laptop casings, and expanded use in vehicle bodies and aerospace structures depends on overcoming their deformation limitations. By revealing that the three-dimensional screw dislocation network of a twist boundary can raise flow stress by up to a quarter while simultaneously activating the very slip systems that confer ductility, this study reframes grain boundaries from passive obstacles into active, engineerable components of the deformation machine. As simulation tools like LAMMPS and visualization platforms such as OVITO continue to mature, atomic-scale studies of this kind are poised to guide the next generation of magnesium alloys, one grain boundary at a time.

Subject of Research: Molecular dynamics simulation of deformation mechanisms at tilt and twist grain boundaries in Mg-2Y magnesium alloys.

Article Title: Atomic simulation study of deformation mechanisms of symmetrical tilt and twist grain boundaries in magnesium alloys

Article References: Mei, W., Yang, L., Zhang, N., Yu, Y., Zhang, W., He, P., & Xiao, D. (2026). Atomic simulation study of deformation mechanisms of symmetrical tilt and twist grain boundaries in magnesium alloys. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13751-3

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13751-3

Keywords: magnesium alloys, grain boundaries, molecular dynamics, twist grain boundary, tilt grain boundary, screw dislocation network, Schmid factor, pyramidal slip, basal slip, grain boundary engineering, Mg-2Y alloy, compressive strength