1. Introduction

Chirality, a fundamental characteristic of natural and biological systems, is essential to the basic units of life. In complex biomacromolecule systems such as the double helix structure of DNA and the α-helices and β-fold structures in proteins, supramolecular helicity plays a crucial role in functional performance^[1]. In recent years, with the rapid development of applications of chiral polymers in chiral recognition^[2], chiral resolution^[3], asymmetric catalysis^[4] and circularly polarized luminescenc^[5], achieving supramolecular chirality control holds profound significance for their applied research^[6]. Gerenally, supramolecular chirality is closely related to non-covalent interactions, such as hydrogen bonding, π-π stacking, acid-base interactions, ionic bonds, or van der Waals forces^[7-9]. It can be modulated through the introduction of external stimuli (e.g., temperature^[10-12], solvents^[13-15], light irradiation^[16-18] and chiral additives^[19,20]) as well as by adjusting molecular internal structures (e.g., absolute configuration^[21-23], chemical modifications^[24-26] and the odd-even effect^[27-29]). However, regulating the supramolecular chirality of polymers by altering the external environment of the assemblies is often restricted by the precise control of medium conditions, which limited some potential applications of chiral polymers.

The odd-even effect subtly modulates the chiroptical properties of nanomaterials via slightly structural variations at the molecular level. Gray and McDonnell proposed the SED/SOL-rule during their investigation of non-steroidal cholesteric liquid crystals. According to this rule, for compounds of S absolute configuration, an even number (E) of carbon atoms separating the chiral chromophore center from the molecular core gives rise to a left-handed (D) cholesteric helix (SED), whereas an odd number (O) yields a right-handed (L) cholesteric helix (SOL). The opposite relationship holds for the R configuration, denoted as REL/ROD^[30]. Meanwhile, the SED/SOL-rules also describe the relationship between the site of stereogenic center, the absolute configuration and the helicity. Recently, Zhang et al. have demonstrated that by modulating the length of the spacer between the side-chain chiral center and the flexible azobenzene (Azo) unit in polymer assemblies, the circular dichroism (CD) properties exhibit a distinct odd-even effect, strictly following the established SED/SOL-rules^[31]. Subsequently, they further achieved an abnormal even-odd effect in the CD signal based on the previous results by significantly increasing the degree of polymerization (DP), thereby realizing the two-fold even-odd effect within this polymer system^[32]. Nevertheless, these strategies ignored the structure of the polymer’s own main-chain and its critical role in regulating interactions between side-chain modules.

In recent years, our group have succeeded in preparing various stereoregular polyesters through enantioselective copolymerization of epoxides and cyclic anhydrides^[33,34] In the present study, a series of enantiopure epoxides bearing varying spacer lengths between the chiral center and the azobenzene unit (Scheme 1, mR-Azo, m = 3, 4, 5, 6, 7, 8, and 9) were designed and synthesized. By employing different anhydrides in the regioselective ring-opening copolymerization (ROCOP) with these epoxides, various azopolyesters with main-chain chirality were synthesized, for investigating the subtle relationship between the polymer backbone and supramolecular chirality, with an expectation of control over chiral consistency or chirality inversion in the assemblies.

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Scheme 1. The two-fold odd-even effects of supramolecular chirality in polyester assemblies can be precisely controlled by adjusting the polymer structure and the spacer units between the main chain and side-chain azobenzene groups. ROCOP: regioselective ring-opening copolymerization; THF: tetrahydrofuran; EtOH: ethanol; Azo: azobenzene; PA: phthalic anhydride; SA: succinic anhydride; ODD: m = 3, 5, 7, and 9; EVEN: m = 4, 6, and 8.

2. Experimental Section

Taking the preparation of azopolyester (6R-Azo/PA) as an example: In a nitrogen-filled glovebox, 5.0 mg (S,S)-SalenCo(III)NO₃, 5.1 mg [PPN][NO₃] (PPN = bis(triphenylphosphine)iminium), and 62.8 mg phthalic anhydride (PA) were sequentially added into a 10 mL Schlenk flask, followed by the addition of 168.2 mg of synthesized epoxide monomer 6R-Azo and 1.15 g of 1,2-dimethoxyethane (DME). The reaction was then allowed to proceed at 30 °C. After a defined period, a small aliquot of the reaction mixture was withdrawn under a dry nitrogen atmosphere for ¹H NMR analysis to determine the conversion of the epoxide monomer, at which point the reaction was quenched. To the reaction mixture were added an appropriate amount of CH₂Cl₂ and 2 mL of 2.0 mol/L HCl in ethyl acetate, and the resulting mixture was stirred. A 100 mL round-bottom flask was charged with a large volume of methanol, and the reaction solution was added to precipitate the polymer. The polymer was then redissolved in CH₂Cl₂ and reprecipitated with methanol at least three times, yielding a yellow powdery polymer. Finally, the 6R-Azo/PA copolymer was dried in a vacuum oven at 50 °C to remove residual solvent.

3. Results and Discussion

Various enantiopure epoxides bearing variable spacer lengths (m) between the chiral stereocenter and the Azo chromophore were synthesized and characterized by ¹H NMR and chiral high-performance liquid chromatography (HPLC) (Scheme S1 and Figure S1). Subsequently, these epoxides were subjected to the alternating ring-opening copolymerization with cyclic anhydrides (Table S1), affording a series of main-chain chiral azopolyesters with similar degrees of polymerization. These polymers have the distinct main-chain backbones and variable flexible spacer lengths between the chiral center and the side azobenzene moiety. All azopolyesters were characterized by gel permeation chromatography (GPC), ¹H NMR, and HPLC, confirming their structures and high stereoregularity (Figures S2, S3). The aggregation of two or more azobenzene chromophores in a confined space induces supramolecular interactions, giving rise to an exciton-coupled CD effect. Usually, the azopolyesters were first dissolved in a good solvent, tetrahydrofuran (THF), followed by the addition of a poor solvent, ethanol (EtOH), to induce gradual polymer assemblies. The effects of subtle variations in the polymer main-chain structure and the spacer length between the chiral stereocenter and the azobenzene unit on the CD spectra were clearly evident (Figure S4). Among these, the CD spectra of the assemblies formed from the flexible main-chain azopolyester (mR-Azo/SA) (Figure 1a) displayed a positive bisignate Cotton effect when m+1 = even (m = 3, 5, 7, and 9; pink background), whereas a negative bisignate Cotton effect was observed when m+1 = odd (m = 4, 6, and 8; blue background). These results are fully consistent with the conventional SED/SOL-rule, where an even number (E) of atoms separating the chiral center from the rigid core in an R configuration leads to the formation of P-helical (D) aggregates, and the same applies to ROD. Interestingly, when a rigid benzene ring structure was introduced into the azopolyester main-chain (mR-Azo/PA), all assemblies exhibited a negative bisignate Cotton effect (Figure 1b). Specifically, the CD signals of the assemblies with m+1 = odd remained unchanged (blue background), demonstrating chiroptical consistency relative to the flexible main-chain azopolyester. In contrast, when m+1 = even (yellow background), the CD signal of the assemblies was inverted, revealing a chiroptical inversion that deviates from the SED/SOL-rule. Following the nomenclature established above, the 3R/5R/7R/9R-Azo/PA is designated as RED. The CD spectral intensity is presented in Figure 1c. Furthermore, assemblies were obtained by varying the THF/EtOH ratio in the mixed solvent(Figure 1d), and the resulting helical patterns in THF/EtOH mixtures at different volume ratios followed the same trend as described above. (Figure 1e, Figures S5, S6).

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Figure 1. The maximum CD spectra of (a) mR-Azo/SA and (b) mR-Azo/PA (m = 3~9) in mixed solvents with different volume ratios of THF/EtOH; (c) Trend of the maximum CD spectra of azopolyesters; (d) The formation process of solvent self-assembly of polyesters with different structures; (e) Scheme illustrating the odd-even effects of azopolyester assemblies with varying structures. CD: circular dichroism; THF: tetrahydrofuran; EtOH: ethanol; ROCOP: regioselective ring-opening copolymerization; ODD: m = 3, 5, 7, and 9; EVEN: m = 4, 6, and 8.

This chiroptical inversion—from SED/SOL-rule to anti-SED/SOL-rule—is attributed to the rigidity of the polymer main-chain backbone (Table S2, Figure S8, S9). The helical orientations of several polymer assemblies with analogous structures support this conclusion (Figure S10). For polymers with a flexible main chain (mR-Azo/GA and mR-Azo/CHA), the assemblies exhibited alternating positive and negative bisignate Cotton effects, indicating that the helical orientation varied with the number of methylene units. In contrast, for polymers with a rigid main-chain (mR-Azo/MA and mR-Azo/CPrA), the assemblies consistently displayed the negative bisignate Cotton effect, demonstrating that the helical orientation remained unchanged regardless of the number of methylene units. Previous studies have shown that azobenzene units bearing odd- or even-numbered spacers exhibit different inclinations relative to the main chain normal, giving rise to distinct helical tendencies in the resulting assemblies^[32]. This variation in tendency enables the chirality of the assemblies to alternate with the parity of the flexible spacer. Consequently, when both the polymer main- and side-chains are flexible, the helical tendency is pronounced, leading to a chiroptical inversion in the polymer assemblies that strictly follows the SED/SOL-rule. In contrast, when the polymer main-chain transitions from flexible to rigid, the planar rigid structure locks the main-chain conformation. Under such conditions, the azobenzene units are constrained to arrange around the polymer helix during assembly, resulting in rigid main-chain polymers displaying behavior determined solely by the chiral center, which deviates from the SED/SOL-rule. Interestingly, the CD signals of azopolyester assemblies with rigid main-chain exhibited a pronounced odd-even effect in intensity, wherein assemblies bearing odd-numbered spacers displayed substantially stronger signals than their even-numbered counterparts. This behavior is attributed to the opposite helical tendency that arises when azobenzene units with odd-numbered spacers adopt an orientation opposite to that of the chiral center, thereby preserving higher asymmetry within the assemblies and yielding stronger CD signals.

A change in the aggregation mode frequently leads to chiroptical inversion of the supramolecular chirality^[35]. Therefore, the molecular packing modes of the two polyesters were analyzed using ultraviolet-visible (UV-vis) spectra. Typically, the main absorption peak at 357 nm in the UV-vis spectra is attributed to the π-π* electron transition of isolated trans-azobenzene groups. However, when the azobenzene polymer is in an aggregated state, the formation of H-aggregates and J-aggregates results in a blue shift (340 nm) and a red shift (377 nm), respectively. When the azobenzene groups adopt a strictly face-to-face π-π stacking arrangement, the corresponding main absorption peak appears at approximately 320 nm in the UV-vis spectra. Unfortunately, the packing mode did not follow an alternating odd-even pattern. Nevertheless, the molecular packing modes of the two polyesters followed the same trend: under conditions of lower flexibility (m < 6), the assemblies exhibited predominantly H-aggregation. When the flexibility increased (m ≥ 6), the assemblies adopted a packing mode dominated by π-π stacking (Figure 2).

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Figure 2. The maximum UV-vis spectra of (a) mR-Azo/SA and (b) mR-Azo/PA (m = 3~9) in mixed solvents with different volume ratios of THF/EtOH; (c) Schematic illustration of supramolecular packing of polymers in the aggregated state. UV-vis: ultraviolet-visible; THF: tetrahydrofuran; EtOH: ethanol.

The odd-even effect trends observed in the two azopolyester architectures were corroborated by differential scanning calorimetry (DSC), polarized optical microscopy (POM), wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS). In this study, the liquid crystalline properties of the azopolyesters also exhibited a pronounced odd-even effect as the flexible spacer varies. The DSC results are presented in Figure 3a. For the flexible main-chain azopolyester (mR-Azo/SA), the liquid crystal–isotropic phase transition temperature (T_LC-iso) exhibited a pronounced odd-even effect, decreasing progressively for odd-numbered flexible spacers and increasing progressively for even-numbered ones (Figure 3b). The POM images of the azopolyesters all showed clear birefringence (Figure 3b); however, well-defined textures were difficult to discern. Further analysis of the WAXD and SAXS data (Figure 3c and Figure 3d) revealed that, with increasing flexible spacer length, distinct diffraction peaks appeared in the WAXD patterns, indicating the formation of a liquid crystalline phase. In the SAXS patterns, scattering peaks were observed with a scattering vector ratio of 1:2, which is characteristic of a smectic phase. As described by Inukai et al., the odd-even effect of molecular shape can influence the helical twisting direction of the liquid crystal in the smectic phase, thus adhering to the SED/SOL-rule^[36,37].

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Figure 3. (a) The DSC curve of mR-Azo/SA; (b) T_LC-iso of mR-Azo/SA; (c) WAXD pattern of mR-Azo/SA, all data were obtained at room temperature; (d) SAXS pattern of mR-Azo/SA, all data were obtained at room temperature. DSC: differential scanning calorimetry; WAXD: wide-angle X-ray diffraction; SAXS: small-angle X-ray scattering.

For the azopolyester with a rigid main-chain, a distinct liquid crystal–isotropic phase transition was also observed on the DSC curve, confirming its liquid crystalline order (Figure 4a). The T_LC-iso values exhibited an odd-even effect in which no significant variation was observed for odd-numbered spacers, whereas even-numbered spacers showed a progressive increase (Figure 4b). This odd-even effect was also reflected in the corresponding enthalpy changes, with values for odd-numbered spacers consistently exceeding those for even-numbered ones. Additionally, the rigid main-chain azopolyester exhibited a glass transition temperature, which also displayed an alternating odd-even effect (Figure S11). POM observations revealed that azopolyesters bearing odd-numbered flexible spacers exhibited clear birefringence (Figure 4b), though well-defined textures were difficult to discern, whereas those with even-numbered spacers showed no apparent birefringence. This suggests that azopolyesters with odd-numbered flexible spacers possess superior liquid crystalline order compared to their even-numbered counterparts. The WAXD patterns of the polymers exhibited weak diffraction peaks, whereas the SAXS patterns showed no discernible diffraction features (Figure 4c and Figure 4d). Accordingly, the supramolecular chirality of the assemblies in this case arises from the chiral nematic liquid crystal induced by the main-chain chiral center. The helical orientation is governed solely by the configuration of this chiral center, independent of the SED/SOL-rule. In summary, when the polymer main chain transitions from flexible to rigid, the accompanying phase transition from a smectic to a chiral nematic phase leads to a switch in the supramolecular chirality of the assemblies from following the SED/SOL-rule to violating it.

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Figure 4. (a) The DSC curve of mR-Azo/PA; (b) T_LC-iso of mR-Azo/PA; (c) WAXD pattern of mR-Azo/PA, all data were obtained at room temperature; (d) SAXS pattern of mR-Azo/PA, all data were obtained at room temperature. DSC: differential scanning calorimetry; WAXD: wide-angle X-ray diffraction; SAXS: small-angle X-ray scattering.

4. Conclusion

In summary, a series of stereoregular azopolyesters with main-chain chirality and side azobenzene group through the regioselctive ring-opening copolymerization of cyclic anhydrides and enantiopure epoxides with different spacer lengths (m) between the chiral stereocenter and the Azo chromophore. The supramolecular chirality of the assemblies of the azopolyesters with flexible main-chain strictly followed the conventional SED/SOL-rule, whereas that with rigid main-chain followed the anti-SED/SOL-rule, manifesting a double odd-even effect. Moreover, the packing mode of the assemblies was strictly dependent on the spacer length between the Azo group and the chiral stereocenter. The longer spacers favor π-π stacking. Additionally, the liquid crystalline property also influences the supramolecular chirality. The stereoregular azopolyesters from mR-Azo/SA formed a smectic liquid crystal and followed the SED/SOL-rule, whereas that from mR-Azo/PA formed a chiral nematic liquid crystal and deviated from it. These findings have advanced our understanding of the relationship between polymer main-chain backbone structure and the supramolecular chirality, providing insights into the precise control of the helicity of polymer assemblies.

Supplementary materials

The supplementary material for this article is available at: Supplementary materials.

Acknowledgements

We used DeepSeek to improve the readability and grammatical accuracy of the manuscript. No AI tools were used for data generation, analysis, or interpretation of results. The authors take full responsibility for the integrity, originality, and accuracy of the work.

Authors contribution

Jing JL: Investigation, visualization, writing-original draft.

Fu YS: Conceptualization, supervision, writing-review & editing.

Zhao R: Writing-review & editing.

Liu Y: Conceptualization, funding acquisition, writing-review & editing.

Lu XB: Conceptualization, supervision, funding acquisition, writing-review & editing.

Conflicts of interes

The authors declare no conflicts of interest.

Ethical approval

Not applicable.

Consent to participate

Not applicable.

Consent for publication

Not applicable.

Availability of data and materials

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Funding

This work was financially supported by the National Natural Science Foundation of China (Grant No. 92356305).

Copyright

© The Author(s) 2026.