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

1 Introduction

Cholesterol-based liquid crystal dimers have emerged as an important class of soft materials owing to their ability to combine the inherent molecular chirality of the cholesteryl unit with the structural versatility of aromatic mesogenic segments (Demus, 1989; Gottarelli et al., 2008; Yelamaggad et al., 2008; Sarkar et al., 2013; Berladean et al., 2025). These nonsymmetric dimers, typically consisting of a cholesterol moiety linked to an achiral mesogenic unit via a flexible spacer, have attracted considerable attention for their rich mesomorphic behaviour and their ability to stabilise a wide variety of conventional and unconventional liquid-crystalline phases (Yu et al., 2020; Okazaki et al., 2009; Rabari and Prajapati, 2024). The mesophase behaviour of such systems is governed by a delicate interplay between molecular chirality, spacer flexibility, molecular shape, and intermolecular interactions (Date et al., 1992; Creed et al., 1987). Consequently, subtle variations in molecular architecture often lead to profound changes in phase sequence, transition temperatures, and mesophase stability. Of particular interest are the frustrated mesophases exhibited by highly chiral cholesterol-based dimers, notably the blue phases (BPs) and twist grain boundary (TGB) phases. Blue phases arise from the competition between the tendency of chiral molecules to adopt a twisted arrangement and the requirement for efficient three-dimensional packing, resulting in complex cubic superstructures (Kitzerow and Bahr, 2001). These phases are typically observed over a narrow temperature range between the isotropic and chiral nematic phases and are characterised by selective reflection of visible light and exceptionally fast electro-optic response. Twist grain boundary phases represent another class of frustrated mesophases formed due to the incompatibility between helical chiral ordering and lamellar smectic organisation. Depending on the local smectic arrangement, TGB phases are classified as TGBA, TGBC, and TGBC*. Owing to their unique structural organisation and technological relevance, these frustrated phases continue to attract significant attention in photonics, electro-optics, and advanced display technologies (Kanakala and Yelamaggad, 2021; Li et al., 1997; Chaudhary and Ghildyal, 2024; Crooker, 1989).

Extensive investigations have demonstrated that the formation and stabilisation of such mesophases depend strongly on the nature of the aromatic mesogenic core attached to the cholesterol unit. Variations in molecular rigidity, conjugation length, polarity, and intermolecular interactions introduced by the aromatic segment can significantly alter the mesomorphic behaviour (Inui et al., 2025; Albuquerque et al., 2019). Consequently, a wide range of mesogenic cores, including biphenyl, azobenzene, chalcone, stilbene, tolane, diphenylacetylene, and Schiff base derivatives, have been incorporated into cholesterol-based dimers to tailor their liquid crystalline properties (Donaldson et al., 2010; Ajay Mallia and Tamaoki, 2004; Shanker and Yelamaggad, 2012; Nayak et al., 2019). Among these, Schiff base mesogens have received particular attention due to their facile synthesis, structural diversity, and excellent mesogenic properties. The azomethine (–CH = N–) linkage contributes significantly to molecular anisotropy and conjugation while simultaneously imparting favourable intermolecular interactions that promote mesophase formation (Dominiak et al., 2003; Soto Bustamante et al., 1996; Nafee et al., 2019; Alamro et al., 2021). Schiff base-containing liquid crystals are well known to exhibit nematic, smectic, chiral nematic, and frustrated mesophases depending on molecular architecture. In addition to their liquid crystalline behaviour, Schiff base materials have attracted interest for applications in molecular electronics, sensing, nonlinear optics, photonics, and stimuli-responsive materials (Soliman et al., 2025; Taha et al., 2024; Yadav et al., 2026).

Our group has previously demonstrated that cholesterol-based dimers incorporating Schiff base, diphenylacetylene, and biphenyl mesogenic cores can stabilise a variety of frustrated mesophases, including blue phases and twist grain boundary phases, often within complex polymorphic phase sequences (Yelamaggad et al., 2008; Kanakala and Yelamaggad, 2021; Veerabhadraswamy et al., 2019; Shanker and Yelamaggad, 2011; Shanker et al., 2019; Shanker et al., 2022). These studies revealed that the nature of the aromatic core, together with the length and parity of the flexible spacer, plays a decisive role in determining molecular chirality, mesophase periodicity, and phase stability. While these studies established important correlations between molecular architecture and mesophase behaviour, they primarily focused on identifying phase sequences arising from variations in mesogenic cores and spacer structures. A systematic investigation into the combined influence of terminal chiral substituents and spacer architecture on hierarchical chirality transfer and temperature-dependent supramolecular chirality has remained unexplored. The present work addresses this gap by employing two enantiomeric terminal chiral series together with systematic spacer variation to establish comprehensive structure–property relationships linking molecular design, mesophase organisation and chiroptical behaviour. In addition, temperature-dependent CD spectroscopy is employed to demonstrate thermally induced supramolecular chirality inversion through simultaneous inversion of both the chromophore absorption and selective reflection bands, providing new mechanistic insight beyond our previous studies.

2 Molecular design

Cholesterol-based nonsymmetric dimers incorporating Schiff-base mesogenic units exhibit exceptionally rich mesomorphic behaviour and have therefore attracted considerable research interest. Depending on their molecular architecture, these systems can stabilise a variety of conventional and frustrated liquid crystalline phases, including chiral nematic (N), smectic (SmA and SmC), blue phase (BP), and twist grain boundary (TGB and TGBC*) phases, as well as unusual phase sequences and re-entrant mesophases. In our previous studies, we have systematically investigated several cholesterol-based dimers containing Schiff base and related mesogenic cores to establish structure–property relationships governing their thermal and mesomorphic behaviour. These investigations revealed that even subtle modifications to the aromatic core or spacer architecture can significantly influence phase stability and mesophase organisation.

As part of these efforts, we also explored analogous systems containing o-hydroxy Schiff base (salicylaldimine) cores, which exhibited distinct thermal behaviour and highlighted the important role of the mesogenic unit in determining liquid-crystalline properties. Building on these observations, the present study focuses on the design and synthesis of new cholesterol-based dimers incorporating two-ring Schiff-base mesogenic cores connected via flexible spacers of varying length and parity. The Schiff base core was selected because of its proven ability to promote mesophase formation, stabilise frustrated liquid crystalline phases, and generate complex polymorphic behaviour in cholesterol-containing systems. To further investigate the influence of molecular chirality, the Schiff base mesogens were functionalized with chiral terminal groups derived from either citronellol or octan-2-ol enantiomers. In addition, flexible spacers of different length and parity were employed to examine the characteristic odd–even effect on mesomorphic behaviour. Through this molecular design, the present work aims to elucidate the combined influence of spacer architecture and terminal chirality on the thermal and liquid crystalline properties of cholesterol-based Schiff base dimers and to gain further insight into the structural factors governing the formation of complex chiral mesophases.

Chemical structure diagram displaying a molecule with two benzene rings linked by a nitrogen group, an ester linkage, and a bulky side chain. Text below details CSBC-(R/S)n and CSBO-(R/S)n series, specifying n values as three, four, five, and seven, R substituent as either (R)- or (S)-citronellyloxy or octan-2-yloxy, and X as hydrogen.

3 Synthesis and molecular structural characterisation

Cholesterol was treated with either 4-bromobutyroyl chloride, 5-bromopentanoyl chloride, 6-bromohexanoyl chloride, or 8-bromooctanoyl chloride in the presence of a mild base (pyridine) in THF to obtain cholesteryl ω-bromoalkanoates (1a-d). These products were reacted with 4-nitrophenol in the presence of a mild base to get cholesteryl ω-(4- nitrophenoxy)alkanoates (2a-d), which were subsequently subjected to catalytic hydrogenation (10% Pd-C; H₂ balloon, 1 atm.) to afford cholesteryl ω-(4-aminophenoxy) alkanoates (4a-d). 4-hydroxybenzaldehyde was O-alkylated with either (S)-(+)-citronellyl bromide or (R)-(−)-citronellyl bromide under the Williamson ether synthesis reaction conditions to obtain the corresponding 4-((3,7-dimethyloct-6-en-1-yl)oxy)benzaldehydes (3a and b). The other important chiral intermediate, such as 4-(octan-2-yloxy)benzaldehydes (3c and d), were synthesised by treating 4-hydroxybenzaldehyde with either (R)-(−)-2-octanol or (S)-(+)-2-octanol in the presence of diisopropyl azodicarboxylate (DIAD). The final Schiff bases (CSBC and CSBO) were synthesised by condensation of aldehydes 3a-d with the amines 4a-d in the presence of a mild acid, which acts as a catalyst. The molecular structures of the final compounds and their intermediates were characterised by spectroscopic methods; the target compounds were additionally characterised using microanalytical data (see Supplementary Material for details).

4 Results and discussion

The target compounds synthesised were investigated for their thermal behaviour using POM and DSC. Several representative mesophases have been examined by XRD measurements. The results of the microscopic and calorimetric studies reveal that all sixteen Schiff bases (CSBC and CSBO) exhibit a wide range of mesophases. For convenience, the evaluation results for the CSBC-(R/S)n and CSBO-(R/S)n dimers are presented below.

5 Thermal behaviour of CSBC-(R/S)n series

The liquid crystalline behaviour of the newly synthesised Schiff base dimers belonging to *CSBC-(R/S)n*** series has been presented in Table 1. All eight dimers (four pairs of enantiomers) exhibit enantiotropic mesomorphism, as determined by DSC and POM studies. The DSC traces recorded during the first heating-cooling cycles of these mesogens are shown in Supplementary Figure S1. The occurrence of enantiotropic mesophases, viz., N*, TGB, and SmA, was established by POM for the first two compounds, viz., *CSBC-(R)3 and CSBC-(S)3. On cooling the unaligned dimer under investigation from the isotropic liquid state, a pseudo-focal-conic texture appears (Figure 1a), which, upon shearing,* yields the characteristic Grandjean planar texture of the cholesteric (N*) phase. With further cooling of the unperturbed sample, the pseudo focal-conic texture of the N* phase transforms into another textural pattern comprising striking filaments (Figure 1b) that instantly change to yield regions of homeotropic (pseudo-isotropic/dark) domains and focal-conic textures resulting from the planar orientation of the dimers.

TABLE 1. Phase sequences, phase transition temperatures (^°C) ^a and associated enthalpies (kJ/mol) of transitions recorded for the Schiffs bases of *CSBC-(R/S)n*** series of dimers. Cr: Crystal; SmC: Chiral smectic C phase; SmA: Smectic A phase; TGB: twist grain boundary phase; N: Chiral nematic phase. I = Isotropic liquid state.

Peak temperatures in the DSC, traces obtained during the first heating-cooling cycles at 5

^°C/min rate.

The transition temperature are taken from the POM, study as these transitions are too week to be detected in the DSC, trances.

Mesophase freezes into a glassy state.

While the homeotropic regions imply an orthogonal alignment of the mesogens (director) with respect to the layer normal, the focal-conic fans are indicative of a layered structure. These observations clearly suggest the occurrence of the SmA phase through a transient TGB phase. The existence of the TGB phase was evidenced based on the observation of an attractive filamentary growth pattern, leading to long filaments, when the sample was heated slowly (0.5 °C/min) from the dark domains of the homeotropically aligned SmA phase (Figure 1c). The next pair of enantiomers, *CSBC-(R)4 and CSBC-(S)4*, display an interesting phase transitional behaviour. They show enantiotropic TGB and SmA phases meaning that the former phase appears adjacent to the isotropic liquid state of the samples. To visualize the optical textures stemming from both planar and homeotropic orientation of mesogens, the untreated (ordinary) glass slides were used. On cooling from the isotropic liquid state, the TGB phase appears in the form of droplets that coalesce to form a pattern comprising the Grandjean planar and distorted pseudo focal conic textures.

In some regions of the cell, especially at the edges and around the air packets, filaments were seen. Upon further cooling, the SmA phase appears with a texture comprising both homeotropic regions and a focal-conic fan pattern resulting from the planar orientation of the mesogens. When the samples were examined in slides favouring homogeneous orientation, the Grandjean planar texture of the TGB phase was seen. As expected, the filament texture of the TGB phase was seen when examined using slides treated for homeotropic orientation. Thus, the POM examination clearly reveals the presence of the TGB and SmA phases in these dimers. The subsequent enantiomeric pair, *CSBC-(R)5 and CSBC-(S)5, display enantiotropic SmA and N phases in addition to a metastable SmC* phase. The existence of SmA and N* phases was evidenced by the observation of their characteristic textural patterns under POM, as previously described for the mesogens. Interestingly, unlike the previous pair of enantiomeric dimers, these compounds do not exhibit the TGB phase in their phase sequence. The SmC* phase was identified based on the development of banded focal-conic texture when cooled from focal conic texture of the preceding SmA phase; indeed, in the dark (homeotropically aligned) regions of the SmA phase, a dull greyish pattern appears when transition to SmC* phase takes place (Figure 1d). Compounds *CSBC-(R)7 and CSBC-(S)7* exhibit only one mesophase, identified as the SmA phase. The observed evolution of mesophases with spacer length agrees well with previous reports on cholesterol-based liquid crystal dimers and related Schiff-base systems, in which shorter spacers promote frustrated mesophases such as TGB and, in some molecular architectures, blue phases because the increased conformational restriction enhances packing frustration and twist. The range of this frustrated TGB phase for different spacer length is shown in Supplementary Figure S3. As the spacer length increases, conformational flexibility improves molecular packing efficiency and favours the development of layered smectic phases. Similar odd-even spacer effects have also been reported, where parity influences molecular bend, layer organisation and phase stability (Veerabhadraswamy et al., 2019; Shanker and Yelamaggad, 2011). The present CSBC series follows these general trends, with short spacers stabilising TGB phases, intermediate spacers favouring SmA/SmC* polymorphism, and the longest spacers predominantly exhibiting the SmA phase.

It is well known that the SmA phase formed by the self-assembly of cholesterol-based dimers has several interesting structures (periodicities). Such structural differences are dependent on the relative lengths of the spacer and terminal chains and, to some degree, on the structure of the aromatic cores attached to cholesterol. Dimers with long spacers, for example, exhibit an intercalated smectic A (SmA_c) phase with d/L = 0.5 (where d is the layer spacing derived from XRD studies and L is the approximate all-trans molecular length of the dimer), while dimers with short spacers form monolayer SmA (d/L = 1). Where the lengths of the spacer and terminal tail are approximately identical, the incommensurate smectic A (SmA_ic) phase with opposing periodicities occurs. Several groups have recently reported the presence of an interdigitated SmA (SmAd) phase (d/L = 1.5–1.8) formed by bimolecular antiparallel dimers. An X-ray diffraction study was carried out for the SmA phase of dimer *CSBC-(R)3* as a representative case. As discussed above, the SmA phase of the chosen dimer should have a monolayer structure, as the terminal tail is much longer than the spacer. 1D intensity profiles recorded as a function of e scattering angle 2θ for the cholesteric phase at 170 °C and 165 °C as well as for the TGBC* phase at 120 °C and 60 °C, are shown in Figure 2. The small- and wide-angle peak positions, spacings d corresponding to the low-angle reflections, and d/L ratio have been collected in Table 2; here, L is the all-trans molecular length (Å) of the dimer *CSBC-(R)3*.

TABLE 2. XRD and related data of the N* and SmA phases formed by the dimer *CSB-(R)3: The spacings (d) corresponding to the low-angle reflections, the wide-angle peak positions, the molecular length (L*) in the most extended form with an all-trans conformation of the alkoxy chains and d/L ratio.

The XRD traces of the N* phase recorded at two different temperatures, which appear identical, possess a broad reflection at the wide-angle region (2θ ≈ 20°) in addition to a broad, less intense peak in the low-angle region (0 < 2θ < 5°). While the diffuse peak in the small-angle region evidences liquid-like ordering, the less intense peak in the low-angle region indicates short-range order in the mesophase.

The diffractograms of the SmA phase recorded at two different temperatures show a low-angle reflection and a diffuse peak at high angles, centred at 5.1 Å, indicative of a liquid-like order within the layer. As shown in Table 2.12, the layer spacing d closely matches the theoretically predicted all-trans molecular length (L) of the dimer, and accordingly, the d ₁/L ratio is nearly one, which is characteristic of a monolayered SmA phase.

6 Thermal behaviour of CSBO-(R/S)n series

The phase sequences and transition temperatures of the cholesterol-based dimers belonging to the CSBO series, as adjudged from microscopic and calorimetric investigations, are presented in Table 3. Supplementary Figure S2 depicts the DSC traces obtained during the first heating-cooling cycles for the eight dimers synthesised. It is clear from accumulated data that all the dimeric compounds synthesised are mesomorphic. In general, they exhibit enantiotropic N* and SmA phases, along with either SmC* or the transient (short-lived) TGB phases.

TABLE 3. Phase sequences, phase transition temperatures (^°C)^, and associated enthalpies (kJ/mol) of transitions recorded for the dimers belonging to series CSBO-(R/S)n compounds. Cr: Crystal; SmC: Chiral smectic C phase; SmA: Smectic A phase; TGB: twist grain boundary phase; N: Chiral nematic phase.

To begin with, the dimers CSBO-(R)3 and CSBO-(S)3, held between the normal glass slides, were examined for their phase trinational behaviour under POM. On cooling the thin film of the isotropic phase of any one of these samples, it displayed the characteristic pseudo-focal-conic fan textural pattern of the N* phase (Figure 3a), where an arbitrary ordering of the helix persists. The expected oily streaks of the Grandjean planar texture were observed when the aforesaid pseudo-focal-conic texture was perturbed by shearing the sample. In this planar texture, the helix axis of the phase stays normal to the substrate’s surface (along the viewing path). Upon further cooling the unperturbed sample, a phase transition to the TGB phase takes place; this phase, which stays over a very short thermal range, showed the characteristic filament texture (Figure 3b). Upon cooling the compound further, tiny batonnets grow which coalesce to yield a focal conic fan pattern along with homeotropic (pseudo isotropic/dark) texture (Figure 3c). This mesophase is assigned as a SmA phase based on the fact that the existence of striking focal conic texture suggests a layered structure, whereas the homeotropic domains specifies an orthogonal organisation of the director with respect to the layer planes. The enantiomers *CSBO-(R)4 and CSBO-(S)4 exhibit thermodynamically stable SmA and N* phases; their presence was confirmed by textural observations. An identical phase-transitional behaviour was observed for the dimers CSBO-(R)7 and CSBO-(S)7. Compounds CSBO-(R)5 and CSBO-(S)5 (Figure 3d) exhibited enantiotropic N* and SmA phases, along with a short-lived TGB phase; the presence of these mesophases was evidenced based on the optical textural investigation as described for the dimers CSBO-(R)3 and CSBO-(S)3.*

The phase evolution observed in the CSBO series is also consistent with earlier investigations of highly chiral cholesterol-based dimers. Short spacers favour transient TGB phases because the increased molecular rigidity and chiral frustration hinder the formation of stable lamellar packing, whereas increasing spacer length progressively stabilises conventional smectic ordering (Supplementary Figure S3). Unlike systems possessing stronger twisting power that stabilise blue phases, the present molecular architecture preferentially relieves frustration through TGB formation, highlighting the important role of the Schiff-base core and terminal substituent in determining the nature of the frustrated mesophase.

As a representative case, the mesophases of dimer *CSBO-(R)5* were probed by XRD. The diffractograms obtained for the different mesophases are shown in Figure 4. The data deduced from these diffractograms are presented in Table 4. As shown in Figure 4a, the N* phase exhibits only a broad peak in the wide-angle region. The layered smectic phases, such as SmA and SmC* (Figures 4b,c), possess two sharp reflections at the low-angle region and a diffused scattering maximum at the wide-angle area. Needless to say, the diffused peak is typical of the liquid-like ordering within the smectic planes. The first sharp reflection with layer thickness d1 and a small peak (d2) in the small diffraction angle region suggests a layered structure.

TABLE 4. XRD and related data of the N, SmA and SmC phase of dimer *CSBO-(R)5*: The spacings (d) corresponding to the low-angle reflections, the wide-angle peak positions, the molecular length (L) in the most extended form with an all-trans conformation of the alkoxy chains and d/L ratio.

The second, less intense peak (d ₂) must stem from a portion of the mesogen. As shown in Table 4, the layer spacing d _*1* values closely match the theoretically predicted all-trans molecular length (L) of the dimer, and thus, the d ₁/L ratio is nearly one, which is typical of a monolayered SmA phase. The d _*1* values of the SmC* phase are lower than the molecular length (L) of the dimer measured using all-trans conformation, where the methylene entities are fully stretched. That is, the d/L ratios are less than one, suggesting a tilted monolayer arrangement of LC molecules within the smectic layers. Thus, the XRD data together with the textural pattern confirm the presence of the SmC* phase. The calculated tilt angles were found to be in the range of 31–34°.

7 Chiroptical behaviour

The CD technique, in which circular dichroism is measured as a function of temperature, is being used as a handy tool to examine the behaviour of chiral mesophases, such as the N* and SmC* phases. The CD study helps determine the chiral system’s handedness by examining how circularly polarised light (CPL) interacts with the N* phase formed by the two enantiomers of the chiral compounds. In other words, the N* phase exhibited CD activity at a given wavelength when incident light gets resolved into two components, i.e., right and left CPL, and based on the helical sense of the phase formed by a particular enantiomer, the CPL of that particular handedness gets reflected totally, whereas its counterpart gets transmitted. The temperature-dependent circular dichroism spectra of representative enantiomeric dimers are presented in Figures 5a–d, while the corresponding peak positions and ellipticity values are summarised in Supplementary Table S1. A schematic representation of the transfer of molecular chirality into supramolecular helicity, along with the proposed mechanism of thermally induced helicity inversion, is shown in Figure 6.

Since liquid-crystalline phases are optically anisotropic, circular dichroism (CD) measurements may, in principle, be influenced by extrinsic optical effects such as linear dichroism (LD), linear birefringence (LB), reflection and light scattering, leading to apparent CD signals. Therefore, appropriate control experiments were performed to ensure that the observed temperature-dependent CD spectra originate from intrinsic molecular and supramolecular chirality. Representative samples were examined using the LD measurement mode of the spectropolarimeter, and no measurable LD signal was detected over the investigated spectral range (Figures 5e–h). In addition, the samples were rotated about the incident light axis in 90° intervals and the CD spectra were recorded after each rotation. No detectable changes in spectral profile, intensity or sign were observed throughout the rotation experiment. According to the established treatment of photoselectively induced linear artefacts, contributions arising from LD and LB exhibit a characteristic angular dependence, whereas genuine circular dichroism remains invariant with rotation about the optical axis. The observed rotational invariance therefore demonstrates that the measured CD spectra are free from significant contributions originating from linear optical artefacts. Further evidence supporting the intrinsic origin of the measured CD signals is provided by the excellent mirror-image relationship observed for the corresponding R- and S-enantiomeric pairs together with the complete disappearance of the CD signal in the isotropic phase (Figures 5e–h). Reflection and light-scattering artefacts are not expected to reproduce such highly reproducible phase-dependent and enantiomer-dependent behaviour. Moreover, the simultaneous and reversible inversion of both the selective reflection band and the chromophore absorption band during the N–SmC phase transition is characteristic of a genuine thermally driven reorganisation of the supramolecular helical structure. Collectively, these observations confirm that the measured CD spectra arise from intrinsic chiroptical responses and provide a reliable basis for the mechanistic interpretation of hierarchical chirality transfer and thermally induced supramolecular chirality inversion presented below.

In the chiral nematic (N) phase, the CD spectra display two characteristic bands arising from distinct levels of chiral organisation. The intense long-wavelength band centred around 550 nm originates from selective Bragg reflection by the periodic cholesteric helix and therefore probes the long-range supramolecular helical architecture. The second band, located near 350 nm, arises from the π→π transition of the Schiff-base chromophore. Unlike the reflection band, this Cotton effect originates from excitonic coupling between neighbouring chromophores and therefore reflects the local chiral arrangement of the mesogenic units.

The simultaneous presence of these two bands demonstrates that molecular chirality is transferred hierarchically from the stereogenic centres to both the local chromophore organisation and the macroscopic cholesteric helix. Comparison of the corresponding R- and S-enantiomeric pairs reveals nearly perfect mirror-image CD spectra throughout the investigated temperature range. Both the selective reflection band and the chromophore absorption band undergo complete sign inversion while retaining essentially identical spectral profiles and intensities. Such mirror-image behaviour confirms efficient transfer of molecular stereochemistry into supramolecular organisation without detectable racemisation or preferential aggregation. Although the absolute molecular configuration determines the handedness of the assembled helix, it has little influence on the thermodynamic stability of the mesophases, as reflected by the nearly identical phase sequences and transition temperatures of each enantiomeric pair.

The most significant observation of the present work is the simultaneous inversion of both CD bands during the N–SmC phase transition. Upon cooling into the SmC* phase, the signs of both the selective reflection band and the chromophore absorption band reverse completely. Since the molecular stereochemistry remains unchanged throughout the thermal cycle, the observed inversion cannot originate from any change in molecular chirality. Instead, it directly demonstrates a reversal of the supramolecular helical sense accompanying the phase transition. The simultaneous switching of both the long-range reflection band and the local chromophore absorption band indicates that the structural reorganisation occurs cooperatively over multiple hierarchical length scales, extending from local chromophore packing to the macroscopic helical superstructure.

This behaviour is illustrated schematically in Figure 6. In the N* phase, molecular chirality is expressed through continuous rotation of the director, resulting in the characteristic cholesteric helix. During cooling into the SmC* phase, positional layering develops together with molecular tilt. The azimuthal rotation of the tilt direction from one smectic layer to the next generates a different chiral superstructure, in which the preferred sense of helical rotation becomes opposite to that stabilised in the cholesteric phase. Consequently, although the molecular configuration (R or S) remains fixed, the supramolecular helix undergoes complete inversion of handedness. Figure 6, therefore, illustrates that the chirality switch originates from the reorganisation of molecular packing rather than from an alteration of molecular stereochemistry, providing a visual interpretation of the experimentally observed CD inversion.

The inversion of both CD bands further suggests that helicity reversal is not confined solely to the macroscopic cholesteric pitch but also involves reorganisation of the local excitonically coupled chromophore network. Such cooperative switching indicates strong coupling between orientational order, positional layering and chiral intermolecular interactions. The transition, therefore, represents a hierarchical inversion of chirality in which structural changes at the molecular packing level propagate throughout the entire supramolecular assembly.

A similar thermally induced inversion of supramolecular helicity has previously been observed by our group in ferroelectric liquid-crystalline systems, indicating that helicity inversion is associated with a competition between orientational twist and layered chiral ordering. However, to the best of our knowledge, systematic experimental observation of the simultaneous inversion of both the selective reflection and the chromophore CD bands across the N–SmC transition in cholesterol-based Schiff-base liquid crystal dimers has rarely been reported. This observation, therefore, represents one of the most significant findings of the present work and highlights the ability of subtle molecular design to control not only mesophase formation but also the direction of supramolecular helicity.

Collectively, the combined CD measurements and the mechanistic model presented in Figure 6 establish that spacer architecture regulates molecular packing, whereas terminal chirality determines the direction of helicity transfer. More importantly, the N–SmC transition acts as a thermally driven switch that reverses the handedness of the entire supramolecular assembly while preserving molecular stereochemistry. Such reversible switching of supramolecular chirality offers an attractive molecular design strategy for responsive chiroptical soft materials, circularly polarised photonic systems and temperature-controlled optical switching devices.

8 Conclusion

A new family of cholesterol-based nonsymmetric Schiff-base liquid crystal dimers incorporating systematically varied spacer lengths and terminal chiral substituents has been successfully designed, synthesised and comprehensively characterised. All sixteen compounds exhibit rich enantiotropic mesomorphism comprising combinations of N, SmA, SmC and TGB phases, demonstrating that subtle modifications of molecular architecture profoundly influence self-assembly and phase evolution in highly chiral liquid crystal dimers. Systematic variation of spacer length revealed clear structure–property relationships governing molecular packing and mesophase stability. Short spacers enhance conformational constraints and packing frustration, thereby favouring the formation of TGB phases, whereas increasing spacer length progressively stabilises layered smectic organisation. Intermediate spacer lengths provide the optimum balance between molecular rigidity and flexibility required for the formation of tilted SmC* phases, while longer spacers predominantly favour orthogonal SmA organisation. Variable-temperature X-ray diffraction further confirms monolayer SmA packing and tilted SmC* structures, providing direct structural evidence for the proposed packing models.

The most significant outcome of the present work is the identification of a thermally induced supramolecular chirality inversion (TISCI) accompanying the N* to SmC* phase transition. Circular dichroism measurements demonstrate that the corresponding R- and S-enantiomeric pairs exhibit mirror-image spectra, confirming faithful transfer of molecular stereochemistry into supramolecular helicity. More importantly, simultaneous inversion of both the selective reflection and chromophore absorption CD bands establishes that the phase transition is accompanied by complete reversal of the supramolecular helical sense while the molecular stereochemistry remains unchanged. This observation demonstrates that TISCI arises from the cooperative reorganisation of molecular packing and hierarchical chiral self-assembly rather than from any alteration of molecular chirality, thereby providing direct experimental evidence for the reversible switching of supramolecular helicity in cholesterol-based liquid crystal dimers.

Overall, this work demonstrates that spacer architecture governs mesophase evolution, conformational flexibility regulates molecular packing, terminal chirality determines supramolecular handedness, and TISCI provides a reversible mechanism for switching supramolecular helicity through thermally driven self-assembly. Beyond advancing the fundamental understanding of hierarchical chirality transfer in soft condensed matter, these findings provide practical molecular design principles for the development of next-generation chiral liquid crystal materials with precisely tunable mesophases and switchable chiroptical responses. Such materials hold considerable promise for emerging applications in reconfigurable photonic devices, circularly polarised light technologies, optical information storage, smart sensors, and advanced electro-optical systems, where dynamic control of supramolecular chirality is highly desirable.

Generative AI statement

The author(s) declared that generative AI was used in the creation of this manuscript. Generative AI was used solely to improve the language and presentation of the manuscript and to assist in preparing the graphical schematic. All experimental work, data generation, scientific interpretation, analysis, conclusions, and final content were conceived, performed, verified, and approved by the authors.

Any alternative text (alt text) provided alongside figures in this article has been generated by Frontiers with the support of artificial intelligence and reasonable efforts have been made to ensure accuracy, including review by the authors wherever possible. If you identify any issues, please contact us.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/frsfm.2026.1928826/full#supplementary-material

Supplementary material Supplementary material at the source

Footnotes

Edited by: Viorel Cîrcu, University of Bucharest, Romania

Reviewed by: Ludovico Valli, University of Salento, Italy

Shanker Govindaswamy, Bangalore University, India