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Angle-controlled synthesis and redox properties of a tetraphenylethene-based hexacationic triangular macrocycle†

Yang Yu‡ ^a, Xiaowen Song‡^a, Yawen Li‡^b, Pingxia Wang^a, Lin Cheng^a, Ying Yang*^a, Gang He*^b and Liping Cao*^a
aCollege of Chemistry and Materials Science, Northwest University, Xi’an 710069, P. R. China. E-mail: yingyang@nwu.edu.cn; chcaoliping@nwu.edu.cn
^bFrontier Institute of Science and Technology, Xi'an Jiaotong University, Xi'an, Shaanxi 710054, P. R. China. E-mail: ganghe@mail.xjtu.edu.cn

First published on 25th July 2025

The varying sizes and angles of precursor building blocks have inspired the design and synthesis of a wide range of macrocycles. One of the most successful examples is cyclobis(paraquat-p-phenylene) cyclophane, known as the “blue box,” first reported by Stoddart et al.²⁰ However, a significant limitation of this method is its inability to synthesize macrocycles with directly connected aromatic units, as S_N2 reactions do not occur between pyridine and aromatic halides. To overcome this challenge, the Zincke reaction offers a potential approach for synthesizing pyridinium-containing macrocycles, where the cationic nitrogen is functionalized with aryl groups.^21,22 Several examples of macrocycles synthesized using this approach have already been reported.²³ The viologen fragment formed in the Zincke reaction exhibits favourable redox properties, undergoing two reversible single-electron reductions. These reductions can be triggered either by chemical reagents or via external stimuli such as applied voltage or light. Thus, viologen derivatives have found broad applications in electrochromic and photochromic devices.^24–28

Angle-controlled synthesis is a general and high-yielding synthetic strategy that requires the complementary molecular modules to have a robust structure with predefined bite angles²⁹ and appropriate stoichiometric ratios,^30–32 in order to effectively obtain a wide variety of 2D and 3D macrocycles with predefined shapes and symmetry.³³ Herein, we report a tetraphenylethene (TPE)-based hexacationic triangular macrocycle using the aforementioned strategy, which is prepared by reacting a 180° linear Zincke salt compound and aromatic amines with a 60° angle in a 1:1 stoichiometric ratio (Scheme 1 and Fig. S1–S3, ESI†). Two TPE-based aromatic amine precursors (S1 and S2) were employed as angle-controlling building blocks,^24,25 and a linear bipyridinium derivative (S3) was chosen as the linker.²³ Further reactions between TPE precursors and S3 led to the synthesis of a hexacationic triangular macrocycle (T⁶⁺·6PF₆⁻) and a dumbbell-shaped reference (D²⁺·2PF₆⁻).³⁴ Non-luminous T⁶⁺·6PF₆⁻ exhibited bright reddish-brown color upon the addition of various poor solvents, indicating its aggregation-induced emission (AIE) properties. Moreover, T⁶⁺·6PF₆⁻ and D²⁺·2PF₆⁻ possess favorable redox properties. The first and second reduction potentials of viologen fragments from D²⁺ to T⁶⁺ exhibit increasingly negative values, while their LUMO energy levels gradually increase, indicating a gradual weakening of the electron-accepting ability and unfavorable conditions for the corresponding reduction reactions. The transformation from T⁶⁺ to the radical T³⁽⁺˙⁾ and subsequently to neutral T can be achieved under redox conditions.

	Scheme 1 Synthetic routes to T6+·6PF6− and D2+·2PF6.

Single crystals of T⁶⁺·6PF₆⁻ were obtained by slow vapor diffusion of isopropyl ether into an acetonitrile solution of T⁶⁺·6PF₆⁻ at room temperature (Table S1, ESI†). The crystal structure (Fig. 1a) revealed that three 4,4′-bipyridine-1,1′-diium units and three TPE units formed a triangular cavity (17.317 Å) within an individual molecule of T⁶⁺. Given the right-handed (P-) and left-handed (M-) rotations of TPE units, two conformational isomers, PPM-T⁶⁺ and MMP-T⁶⁺, were identified in the lattice of T⁶⁺. The difference in the rotational conformation of the TPE units between PPM-T⁶⁺ and MMP-T⁶⁺ resulted in the formation of a mesomeric state for the whole system. In the unit cell, both wave-like and serrated stacking structures were formed via multiple C–H⋯π interactions (d = 2.83–2.84 Å) between TPE units (Fig. 1b and c). Interestingly, these C–H⋯π interactions induced the alignment and parallel arrangement of T⁶⁺ molecules into 1D nanotube-like assemblies. Adjacent 1D double nanotubes, oriented at opposite angles, then formed 2D nanotube layers in an alternating zigzag pattern (Fig. 1d). Ultimately, these 2D nanotube layers stacked in parallel, revealing a 3D nanotubular framework (Fig. S4, ESI†).

	Fig. 1 X-ray crystal structures of T6+·6PF6−: (a) and (b) top view, (c) side view of stacking networks along the c-axis, and (d) side view of a nanochannel formed from nanotubes along the b-axis.

Photophysical experiments showed that T⁶⁺·6PF₆⁻ exhibits different trends in absorption and emission intensities under the regulation of the solvent effect. The UV-vis spectra of T⁶⁺·6PF₆⁻ revealed two maximum absorption peaks corresponding to bipyridine and TPE in different solvents including acetone, 1,4-dioxane, DMF, EA, MeOH, H₂O, MeCN, THF, CHCl₃, CH₂Cl₂, and DMSO (Fig. S5, ESI†). The luminescence data indicated that T⁶⁺·6PF₆⁻ did not exhibit significant luminescence behaviour in a good solvent system. Under different solvent systems, the emission of T⁶⁺·6PF₆⁻ exhibited a bright reddish-brown color, precisely centered at 555 nm, while maintaining consistent and stable optical properties (Fig. S6, ESI†). Among them, adding 90% CH₂Cl₂ (poor solvent) to the MeCN (good solvent) solution of T⁶⁺·6PF₆⁻ could significantly enhance fluorescence (Fig. S7, ESI†), indicating that T⁶⁺·6PF₆⁻ with TPE components displayed AIE properties.

To investigate the redox characteristics of T⁶⁺, the electrochemical properties were examined via cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Compared to the reduction peaks of the reported dumbbell-shaped D²⁺ (−0.55 and −0.90 V) (Fig. S8 and S9, ESI†), T⁶⁺ exhibited two reversible reduction peaks (−0.56 and −0.93 V), suggesting that T⁶⁺ can accept six electrons in a 2e⁻ sequence, resulting in three distinct redox states. These findings indicate that T⁶⁺ has favourable redox properties. Furthermore, from the electrochemical data, the LUMO energy level of T⁶⁺ was calculated to be −4.24 eV (Fig. S14 and Table S2, ESI†). In comparison, the LUMO energy level of D²⁺ is −4.25 eV. Notably, the first and second reduction potentials of the viologen fragments in T⁶⁺ are increasingly negative relative to D²⁺, while their LUMO energy levels gradually increase, reflecting a weakening of the electron-accepting ability and less favourable conditions for reduction reactions.

The local electrostatic potential values for T⁶⁺/D²⁺ were mapped on the isosurfaces of their electron density. As expected, the two reversible single-electron reductions of T⁶⁺ were achieved through chemical reduction (Fig. 2a), which was proved by the observation of a decrease in electrostatic potential near the viologen units, indicating that these units are the primary sites for reduction (Fig. 2b and c). Radical T³⁽⁺˙⁾ was gradually generated by adding zinc powder to a DMF solution of T⁶⁺ under a nitrogen atmosphere, accompanied by a transition in the solution color to dark green. Two characteristic absorption peaks of the radical, at 660 nm and 720 nm, were observed in the UV-visible spectrum (Fig. 2d). The radical nature of T³⁽⁺˙⁾ was further confirmed by electron paramagnetic resonance (EPR) spectroscopy (Fig. 2e).

	Fig. 2 (a) A schematic representation of the two-step reduction of T6+. Electrostatic potential surfaces of (b) T6+ and (c) D2+. (d) UV/vis spectra of T6+, T3(+˙), and neutral T in DMF (c = 10 μM). (e) EPR spectra of T6+, T3(+˙) and T. (f) UV/vis spectra of T3(+˙) generated from the chemical reduction of a DMF solution of T6+ upon the addition of 1 eq. and 6 eq. of CoCp2. (g) UV/vis spectrum of radical T3(+˙) oxidized in air. The inset photographs of T6+, T3(+˙) and neutral T in DMF under daylight.

Subsequently, radical T³⁽⁺˙⁾ was further reduced to neutral T by adding sodium, and the system color gradually changed to reddish brown. The absorption peaks at 660 nm and 720 nm disappeared from the UV-Vis absorption spectrum, and the radical cation signals in the electron paramagnetic resonance (EPR) spectrum vanished as well (Fig. 2e), returning to the pre-reduction state. Notably, CoCp₂ was also found to induce the same reduction. Upon adding increasing equivalents of CoCp₂ (1.0 eq. to 6.0 eq.) to the DMF solution of T⁶⁺, significant changes were observed in the UV-Vis spectrum. The radical absorption peaks gradually increased to their maximum intensity at 3.0 eq. CoCp₂, indicating the generation of radical T³⁽⁺˙⁾. As the CoCp₂ concentration increased from 4.0 eq. to 6.0 eq., the radical absorption peaks began to decrease and eventually disappeared, signalling the further reduction of radical T³⁽⁺˙⁾ to neutral T (Fig. 2f and Fig. S13, ESI†).

When the radical T³⁽⁺˙⁾ was exposed to air without nitrogen protection, UV absorption spectra at 660 nm and 720 nm were recorded over time. Exposure to air resulted in gradual oxidation of T³⁽⁺˙⁾ to the cationic T⁶⁺, demonstrating its good reversible redox properties (Fig. 2g). A similar phenomenon was observed when D²⁺ underwent chemical redox reactions to generate radical D⁺˙ and neutral D (Fig. S10–S12, ESI†). Notably, the reversible redox transitions of T⁶⁺/D²⁺ could be observed not only via chemical reduction but also through electrochemical reduction. To explore their potential in electrochromic applications, we fabricated proof-of-concept electrochromic devices (ECDs) using DMF solutions of T⁶⁺/D²⁺ as the active materials. The ECDs based on T⁶⁺/D²⁺ exhibited green and dark green colors when external voltages of 1.5 V and 2.5 V were applied, respectively (Fig. 3a and Fig. S14a, ESI†). This color change indicated the accumulation of radicals (T³⁽⁺˙⁾/D⁺˙) upon voltage application. Upon increasing the voltage further to 2.5 V and 4 V, the radical cations (T³⁽⁺˙⁾/D⁺˙) gradually converted to their neutral forms (T/D), resulting in distinct color transitions from red to red-brown.

	Fig. 3 (a) A solution-based ECD with T6+ as an electrochromic material. Spectroelectrochemistry of T6+ (c = 1 × 10−3 M) for (b) the first reduction and (c) the second reduction. (d) Two reduction voltage values of T6+/D2+. (e) The optical memory of T3(+˙)/D+˙.

Furthermore, the spectroelectrochemistry of ECDs with T⁶⁺ as the active material was investigated. The spectroelectrochemical behaviour of T⁶⁺ showed two strong absorption peaks at 650 nm and 717 nm (Fig. 3b) when a potential of 2.5 V was applied, with a noticeable increase in absorption in the visible region due to the formation of the radical state T³⁽⁺˙⁾. As the applied potential was increased to 4 V, T³⁽⁺˙⁾ was gradually converted to neutral T, accompanied by a decrease in absorption in the visible region (Fig. 3c). A similar two-step reduction process was observed in ECDs based on D²⁺ when external voltages were applied. Compared to T⁶⁺, the ECDs based on D²⁺ showed a lower reduction voltage (Fig. 3d and Fig. S14b, ESI†), indicating that D²⁺ is more readily reduced (electron-receptive). Notably, compared to D²⁺-based ECDs (Fig. S14b and c, ESI†), the T⁶⁺-based ECDs demonstrated three times higher radical stability after 40 seconds of voltage cutoff. As a result, the color of T⁶⁺-based ECDs remained stable for a longer period after the power supply was disconnected (Fig. 3e), indicating greater radical stability and the requirement of fewer cycles for regeneration.

In summary, a hexacationic triangular macrocycle, T⁶⁺·6PF₆⁻, composed of TPE derivatives as apexes, was successfully synthesized in a one-pot manner via the Zincke reaction. T⁶⁺·6PF₆⁻ underwent three reversible redox states via chemical reduction and electrochemical reduction, displaying good redox properties, and upon the cyclization of T⁶⁺·6PF₆⁻, two-step reduction potentials became distinctly negative with increasing LUMO energy levels. The abovementioned results guided us in building an electrochromic component using T⁶⁺·6PF₆⁻. This work inspires the construction of shape-persistent macrocycles via angle-controlled synthesis strategies, which further provides a theoretical basis for the chemical reduction and electrochromism of macrocycles.

This work was supported by the National Natural Science Foundation of China (No. 22301241 and 22371229) and the Shaanxi Fundamental Science Research Project for Chemistry and Biology (22JHQ073).

Conflicts of interest

Data availability

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