English

• Azobenzene compounds, making up most of the azo dyes, currently maintain their central role in the dye industry, constituting approximately 70% of the total dye production [1]. These dyes are extensively used in the textile, plastics, printing, pharmaceutical, food, and cosmetic industries [2,3]. However, the textile industry stands as the predominant consumer, accounting for 80% of the produced synthetic dyes and generating a staggering 70 billion tons of dye-containing wastewater each year [4]. A majority of these azo dyes are non-biodegradable, water-soluble, and biologically active, with some considered as carcinogenic and mutagenic [5-7]. This influx of synthetic dyes intensifies water contamination, particularly in freshwater resources, contributing to a significant environmental challenge. Given the escalating concerns related to water scarcity, wastewater containing dyes emerges as a fundamental environmental issue, posing a substantial impediment to sustainable development. In addition, there are increasing concerns regarding the potential harmful effects of the degradation products of azobenzene compounds. Consequently, there is an urgent need for effective strategies to address the selective encapsulation of azobenzene compounds in water.

  On the other hand, azobenzene compounds are a class of widely used stimuli-responsive molecules to construct smart materials [8]. These smart materials can exhibit changes in their structural, physical and chemical properties when exposed to specific internal or external stimuli, such as light, heat, pH and magnetism [9-12]. Azobenzene compounds as exquisite molecules have demonstrated notable success in diverse applications to orchestrate the photo-controlled properties of biomacromolecules [13,14], organic-inorganic hybrid materials [15-17] and contributing to the fabrication of supramolecular materials adept at converting light into mechanical energy [18]. Among them, azobenzene supramolecular systems based on host-guest interactions have been extensively utilized to construct light-responsive functional materials via noncovalent recognition and self-assembly [14,19-21]. (E)-Azobenzene exhibits the formation of inclusion complexes with several macrocyclic hosts, including α- or β-cyclodextrins [13,22-24], pillar[5]arenes-pillar[7]arenes [25], and cucurbit[7]urils-cucurbit[8]urils [26-28]. For example, a dendritic polyamidoamine-based supramolecular system, integrating pillar[5]arene and azobenzene motifs, has been developed for the targeted treatment of drug-resistant colon cancer [29]. The assembly of cucurbit[8]uril and azobenzene derivatives in aqueous solution generates a supramolecular organic framework capable of detecting nitrogenous reductase activity in Escherichia coli [30]. Additionally, Yang and co-workers have reported the construction of stimuli-responsive, biocompatible nano-valves, employing azobenzene-functionalized components and β-cyclodextrin-modified poly(glycidyl methacrylate) [31]. In contrast, the more polar and bent (Z)-azobenzene isomers demonstrates significantly lower binding affinity due to its inappropriate shape complementary and increased hydrophilicity [32]. This distinction allows light to serve as a mediator for the complexation and disassociation of complexes through the photoisomerization of azobenzene compounds [33]. Numerous innovative azobenzene supramolecular systems have been developed, exploiting the synthesis of macrocyclic azobenzene compounds and their host–guest chemistry for a broad spectrum of potential applications. Light-modulated azobenzene supramolecular systems based on host-guest interactions have been developed for photo-switchable adhesives [14], biological imaging [34], light-responsive gold nanoparticles [13], competitive binding [35], modification of surface [36], and drug delivery vehicles [21,37]. Novel macrocycles play a central role in supramolecular chemistry, distinguished by their unique geometries, preorganized cavities, and tunable binding affinities. These attributes render them highly effective for applications in molecular recognition, self-assembly, and stimuli-responsive systems [38-41]. Continued efforts in the design and synthesis of macrocyclic compounds are essential for broadening their applicability across diverse scientific and technological domains. Nevertheless, most of the host molecules for azobenzene compounds in photo-responsive systems are limited to cyclodextrins, pillar[n]arenes and cucurbit[n]urils. To construct versatile photo-switchable azobenzene supramolecular system, novel host molecules with robust affinity for azobenzene compounds in water are in high demand.

  In this work, BPy-Box·4Cl, consisting of two 3,3′-bipyridinium units bridged by two p-xylene units, showcases non-coplanar inclined bipyridyl units that form two trapezoidal binding pockets. It adopts a distorted quadrilateral cavity with dimensions of approximately ~7.08 × 8.50 Ų. Leveraging the highly electron-deficient tetracationic skeleton and chloride counterion characterized by high hydration enthalpy, this distinctive configuration is suitable for encapsulation of linear guest molecules in water through electrostatic interaction, sandwich, or T-shape π-stacking interaction [42]. We initially conducted an assessment of host-guest recognition in water using a range of representative linear guest molecules. These guests include molecules varying in charge states, with a particular focus on azobenzene compounds. As illustrated in Scheme 1, the tested guest molecules included cationic dyes (methylene blue), neutral compounds in trans double bond configuration, including (E)−4-(4-pyridylazo)pyridine, (E)-azobenzene, (E)-stilbene, methyl yellow (MY), (E)−4,4′-azodianiline, and anionic azobenzene compounds in trans double bond configuration (MO, ADASS, and congo red (CR)). Experiments of NMR titration and UV-visible absorption spectrophotometric titration were carried out to study the binding affinities of the host-guest systems. In NMR titration, the guest molecules at a concentration of 250 mmol/L or 500 mmol/L was incrementally added to a deuterated aqueous solution of BPy-Box·4Cl (1 mmol/L, 500 µL) [43]. Given that some guest molecules possess characteristic maximum absorption wavelength, the ultraviolet spectroscopy can demonstrate the significant change in absorbance corresponding to the maximum absorption wavelength of the guest molecules upon the addition of the host molecules. Job plot was utilized to determine the stoichiometry of guest molecules and BPy-Box·4Cl in the supramolecule system, and the results revealed a predominant formation of host-guest complexes with a stoichiometry of 1:1 (Figs. S19, S24, S28, S36, and S43 in Supporting information). Following the determination of various proportions in the host-guest spectrograms, the association constants could be calculated by monitoring the variation of chemical shifts and changes in the UV-visible absorption peak (Figs. S19-S44 in Supporting information).

  Scheme 1

  

  Scheme 1.  Chemical structures and binding constants of compounds studied in this work.

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  In the case of the representative cationic dye, BPy-Box·4Cl exhibited a binding constant of 150 L/mol with MB (Figs. S19 and S20 in Supporting information). This result is unexpected given the electrostatic repulsion anticipated between them. The weak binding is likely attributed to the partial inclusion of the 4-(N,N'-dimethylamino) moiety of MB [28]. A preliminary qualitative investigation indicated that BPy-Box·4Cl could not encapsulate some neutral molecules effectively, including (E)−4-(4-pyridylazo)pyridine, (E)-azobenzene, and (E)-stilbene with binding constants K_a < 1 L/mol (Figs. S21-S23 in Supporting information). Methyl yellow and (E)−4,4′-azodianiline, both featuring electron-donating substituents, showed slightly higher binding affinities with BPy-Box·4Cl compared to MB, with a binding constant of 536 and 329 L/mol, respectively (Figs. S24-S27 in Supporting information). The electrostatic attraction plays a crucial role as a driving force in host-guest complexation. Anionic azobenzene compounds as guest molecules were further investigated. To our delight, BPy-Box·4Cl also showed remarkable binding ability for recognizing monoanionic MO in water, fitting well into the 1:1 receptor-substrate binding model. BPy-Box·4Cl formed a complex with MO, displaying an association constant of approximately ~1438 L/mol (Fig. 1a, Figs. S28-S35 in Supporting information). This binding affinity is comparable to other azobenzene supramolecular systems, such as azobenzene-cyclodextrin (~2000 L/mol) [20] and azobenzene-pillar[n]arene (~2200 L/mol) [44] host-guest system. Subsequently, the dianionic ADASS was employed to investigate the electrostatic attraction-enhanced host-guest system. The binding constant between BPy-Box·4Cl and ADASS displayed a significant enhancement, increasing by an order of magnitude to 14,000 L/mol (Figs. 1c and d in Supporting information). This result was confirmed through UV-visible and NMR titration (Figs. 1b-d, Figs. S36-S42 in Supporting information). Moreover, a redshift of the UV maximum absorption peak with an increase in the proportion of BPy-Box·4Cl was observed. This phenomenon is attributed to enhanced planarity of MO or ADASS molecules due to the formation of host-guest complexes between BPy-Box and MO or ADASS through electrostatic interaction, π-π stacking and other noncovalent interactions. This structural change in the guest molecules alters their electronic environment, leading to a shift in their UV absorption spectra towards longer wavelengths (redshift) [45,46]. However, BPy-Box·4Cl could not form a host-guest complex with the dianionic CR. Despite the presence of two sulfonic acid anions facilitating electrostatic interactions, the structural steric bulkiness of Congo red prevented it from engaging in 1:1 host-guest complexation with BPy-Box·4Cl (Figs. S43 and S44 in Supporting information). These findings suggest that the combined effects of electrostatic attraction and shape complementarity can significantly enhance binding affinity.

  Figure 1

  

  Figure 1.  UV-visible spectra of (a) MO, (b) ADASS titrated with BPy-Box·4Cl. Inset: The photographs of guest and host with different mole fraction. (c) ¹H NMR titration of BPy-Box·4Cl (0.5 mmol/L, D₂O) binding with ADASS (50 mmol/L, D₂O). (d) Titration isotherm created by monitoring changes in the chemical shift for BPy-Box·4Cl caused by the addition of ADASS. Inset: Calculated changes of mole fractions for BPy-Box⁴⁺ (black trace) and ADASS⊂BPy-Box⁴⁺ (blue trace) over the substrate-receptor mole ratio.

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  Furthermore, the anti-interference encapsulation ability of different dye molecules by BPy-Box·4Cl in binary (Figs. 2a-d) and ternary systems (Figs. 2e-f) were investigated. The binding constants in the mixed solutions were diminished. However, there is still a significant ability of BPy-Box·4Cl to selectively encapsulate MO and ADASS in both binary and ternary systems. BPy-Box⁴⁺ showed strong binding affinity with ADASS in complicated conditions, with a binding constant of ~1785, 2525 L/mol in binary system and ~1441 L/mol in ternary system (Fig. 2). Moreover, the binding constant between BPy-Box·4Cl and ADASS in seawater could still reach ~10³ L/mol, in the presence of numerous coexisting ions, exhibiting good anti-interference encapsulation and detection ability in the natural seawater environment (Fig. 2g). The notable variations in binding affinity observed between BPy-Box·4Cl and representative common dyes such as MB, MY, MO, and CR bestow upon BPy-Box·4Cl an ideal host macrocyclic molecule for selective encapsulation of ADASS in real-world environment.

  Figure 2

  

  Figure 2.  Titration isotherm of the binary systems was generated by monitoring changes in absorbance of MO or ADASS caused by the addition of BPy-Box·4Cl (5 mmol/L, H₂O). (a) MO (50 µmol/L, H₂O) and MB (50 µmol/L, H₂O), (b) MO (50 µmol/L, H₂O) and CR (50 µmol/L, H₂O), (c) ADASS (50 µmol/L, H₂O) and MB (50 µmol/L, H₂O), (d) ADASS (50 µmol/L, H₂O) and CR (50 µmol/L, H₂O). Titration isotherm of the ternary systems was generated by monitoring changes in absorbance of MO or ADASS caused by the addition of BPy-Box·4Cl (5 mmol/L, H₂O). (e) MO (16.7 µmol/L, H₂O), MB (16.7 µmol/L, H₂O), and CR (16.7 µmol/L, H₂O), (f) ADASS (16.7 µmol/L, H₂O), MB (16.7 µmol/L, H₂O), and CR (16.7 µmol/L, H₂O). (g) Titration isotherm was generated by monitoring changes in absorbance of ADASS caused by the addition of BPy-Box·4Cl in seawater. Red lines are curve fitting using a 1:1 receptor-substrate binding model. Inset: UV-visible spectra of binary and ternary systems titrated with BPy-Box·4Cl. (h) Chemical structures of MO⊂BPy-Box⁴⁺ and ADASS⊂BPy-Box⁴⁺.

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  1D and 2D NMR experiments were conducted to investigate the interactions between the anionic azobenzene compounds and BPy-Box·4Cl in solution. Upon the addition of equimolar amounts of MO to BPy-Box·4Cl in DMSO-d₆ or D₂O, the ¹H NMR spectra indicated a noticeable change in the peak shape and chemical shift of host and guest proton (Figs. S32-S35 in Supporting information). Remarkably, different from the situation in DMSO‑d₆, the NMR proton signal of the mixture containing MO and BPy-Box·4Cl exhibited broadened peaks in D₂O (Fig. S35 in Supporting information). This provides evidence for the high probability of forming self-assembled aggregates or supramolecular polymers in water. The ¹H-¹H rotating-frame overhauser enhancement spectroscopy (ROESY) was further conducted to confirm the binding between MO and BPy-Box·4Cl, showing some intermolecular correlations across space between the host and guest molecules (Fig. S33 in Supporting information). Similarly, in the mixture containing ADASS and BPy-Box·4Cl in water, through-space intermolecular correlations were also observed in the ¹H-¹H ROESY NMR spectra, revealing the formation of host-guest complexation (Fig. S41 in Supporting information). Interestingly, broadened proton peak signals were not observed for ADASS⊂BPy-Box·4Cl complex in D₂O (Fig. S42 in Supporting information), suggesting the inability to form self-assembled aggregates or supramolecular polymers.

  The infrared spectra of the host, guest, and host-guest complexes were compared. A significant broadening of the infrared characteristic peak at 3360 cm^-1, which corresponds to the stretching vibration (ν_=CH) indicative of the formation and aggregation of the host-guest complexes (Fig. S6 in Supporting information). Fortunately, through the vapor diffusion of acetone into the aqueous solution containing BPy-Box·4Cl and MO or ADASS, the single crystal structures of MO⊂BPy-Box⁴⁺ and ADASS⊂BPy-Box⁴⁺ were successfully obtained. The driving force of the host-guest complexation was elucidated clearly by analyzing the single-crystal structure of the host-guest complex (Figs. 3a and b, Figs. S7 and S18, and Table S1 in Supporting information). BPy-Box⁴⁺ and MO molecule strongly interacted with each other via multiple interactions, including electrostatic attraction, CH···O (~3.461 Å), CH···π (~2.794–2.898 Å), anion···π (~2.230–2.715 Å), π···π (~3.104 Å, ~3.410 Å), and dipole-dipole interactions (~3.173 Å) (Figs. S7-S12 in Supporting information). This resulted in exceptionally stable self-assembled dimers (Fig. 3a), which is in well agreement with the packing pattern of two I₃^- encapsulated the BPy-Box⁴⁺ cavity [42]. Additionally, the pH-dependent UV-visible spectra of MO and MO⊂BPy-Box⁴⁺ were examined (Figs. S47-S50 in Supporting information), revealing no significant difference between them. Both under strong acid and base conditions, MO⊂BPy-Box⁴⁺ formed a supramolecular dimer, which indicated pH shows no significant effect on the recognition process (Figs. S45 and S46 in Supporting information). This observation further indicates the stability of MO⊂BPy-Box⁴⁺ to pH (Figs. S47 and S48 in Supporting information). As for ADASS⊂BPy-Box⁴⁺, similar intermolecular interactions and pH stability were observed (Figs. S13-S18, S49 and S50 in Supporting information). At pH < 5, ADASS is protonated and converts to its carboxylic acid form, resulting in weaker electrostatic interactions with BPy-Box⁴⁺ and minimal changes in chemical shifts. However, at pH > 5, the carboxylic acid group of ADASS is more inclined to interact with BPy-Box⁴⁺, facilitating stronger electrostatic interactions. This enhanced interaction leads to significant chemical shift changes, indicating effective host-guest recognition (Figs. S45 and S46 in Supporting information). However, owing to the planar structure of -COO^-, another ADASS molecule is capable of packing parallel to the encapsulated one outside the cavity, without observed formation of self-assembled dimers. This dense packing pattern results in a continuous layered structure in the single crystal of ADASS⊂BPy-Box⁴⁺, while the tetrahedron structure of -SO₃^- leads to an interrupted polygonal columnar structure in the single crystal of MO⊂BPy-Box⁴⁺ (Figs. 3c and d). Overall, the differing charge numbers and geometry of the two guest molecules result in two distinctly different stacking structures.

  Figure 3

  

  Figure 3.  Crystal structures of (a) MO⊂BPy-Box⁴⁺ and (b) ADASS⊂BPy-Box⁴⁺. The same interaction distances were omitted for clarity. (c) The packing mode of MO⊂BPy-Box⁴⁺. (d) The packing mode of ADASS⊂BPy-Box⁴⁺. Color codes: white C, light green C, grey H, red O, blue N, yellow S, and green Cl.

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  Moreover, computational modeling using B3LYP-D3 functionals by ORCA at the level of 6–31G(d) was carried out to understand the binding in MO⊂BPy-Box⁴⁺ and ADASS⊂BPy-Box⁴⁺ supramolecular systems, respectively. The independent gradient model based on the Hirshfeld division (IGMH) analysis intuitively reveals the noncovalent bonding interactions between the anionic azobenzene compounds and BPy-Box⁴⁺ (Figs. 4a and b). The favorable selective encapsulation of anionic azobenzene compounds is probably attributed to the sufficiently large space that allows for the establishment of stereoelectronic complementarity between the guest and BPy-Box⁴⁺. Within this space, a variety of interactions are found, including electrostatic attraction, CH···O, CH···π, anion···π [47-49], π···π, and other noncovalent interactions.

  Figure 4

  

  Figure 4.  Intermolecular binding isosurfaces of (a) MO⊂BPy-Box⁴⁺ and (b) ADASS⊂BPy-Box⁴⁺ complexes acquired from IGMH analysis (ρ = 0.002 au) top view and side view. Counterions are omitted for the sake of clarity.

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  Furthermore, the photoisomerization of azobenzene compounds provides superior performance as a fundamental building block in the creation of smart materials and photo-controlled extraction/release of the azo compounds. To validate the photo-switchable properties of the novel azobenzene supramolecular system with robust host-guest interactions, ADASS⊂BPy-Box⁴⁺ complex was exposed to UV (365 nm wavelength) and white light (400–780 nm wavelength) irradiation (Fig. 5). Both ADASS and the host-guest complex exhibited sensitivity to external light stimulation, while the BPy-Box⁴⁺ skeleton remained inert to external light stimulation (Figs. 5a and b). Rapid and reversible trans-to-cis photo-isomerization of ADASS was observed under UV and white light irradiation, as evidenced by the change of NMR chemical shift (Fig. S51 in Supporting information). This photo-isomerization was nearly complete within 3 min, paving the way for the development of fast and high-accuracy sensors and actuators (Fig. 5d).

  Figure 5

  

  Figure 5.  (a) Schematic diagram of ADASS and ADASS⊂BPy-Box⁴⁺ under UV (365 nm wavelength) and white light irradiation. UV-visible spectra of (b) BPy-Box⁴⁺ (0.05 mmol/L, H₂O), (c) ADASS⊂BPy-Box⁴⁺, and (d) ADASS (0.05 mmol/L, H₂O) after UV irradiation and (d) after white light irradiation.

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  The trans-to-cis photoisomerization results in the dissociation of the host-guest complex owing to the shape mismatch between (Z)-ADASS and BPy-Box⁴⁺ [50-53]. This dissociation under UV irradiation and recomplexation under white light irradiation were confirmed by NMR experiments (Fig. S51). The successful implementation of this photo-switchable host-guest system offers a promising platform for designing high-performance photo-promoted smart materials, such as photo-driven materials for extraction/release of the azo compounds.

  In conclusion, the tilted conformation endows electron-deficient BPy-Box·4Cl a suitable host molecule for encapsulating anionic linear azobenzene compounds. BPy-Box·4Cl shows a much stronger binding affinity with methyl orange than methylene blue, methyl yellow, and Congo red, which may be useful for selective dye encapsulation and detection in water. BPy-Box·4Cl binds with (E)-dianionic disodium 4,4′-azobisbenzoate with an association constant of 14,000 L/mol through electrostatic attraction-enhanced complexation, which is an order of magnitude higher than that in azobenzene-cyclodextrin and azobenzene-pillar[n]arene systems. The cationic cyclophane exhibited good selectivity and anti-interference properties for binding with anionic azo compounds in the binary, ternary, and natural seawater systems.

  The planar structure of -COO^- and the tetrahedron structure of -SO₃^- lead to two distinct packing patterns in the single crystal structures of MO⊂BPy-Box⁴⁺ and ADASS⊂BPy-Box⁴⁺ complexes. Computational modeling is further carried out for a more full understanding of the formation of the host-guest system, which vividly shows the main driving forces are electrostatic attractions, CH···O, CH···π, anion···π, and π···π, interactions. Moreover, the photo-isomerization of (E)-ADASS to (Z)-ADASS upon UV stimulation caused the reversible dissociation of the host-guest complex, due to inappropriate shape complementary between (Z)-ADASS and BPy-Box⁴⁺ cavity. The reversible photo-switchable ADASS-BPy-Box⁴⁺ supramolecular system was nearly accomplished within 3 min upon UV or white light irradiation. The high efficiency of the novel photo-controlled azobenzene-cyclophane supramolecular system provides a new tool for the encapsulation/release of azo compounds and the design of other small smart materials.

  Declaration of competing interest

  The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Yu Tan has patent pending to Yu Tan and Baoqi Wu. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Baoqi Wu: Writing – original draft, Formal analysis, Data curation. Rongzhi Tang: Writing – review & editing, Validation, Supervision, Software, Resources, Funding acquisition, Formal analysis. Zhi-Wei Li: Methodology, Formal analysis, Data curation. Feng Lin: Formal analysis, Data curation. Zongyu Sun: Formal analysis, Data curation. Huanyu Xia: Formal analysis, Data curation. Lin Jiang: Formal analysis, Data curation. Yu Tan: Writing – review & editing, Writing – original draft, Supervision, Resources, Funding acquisition, Conceptualization.

  Acknowledgments

  We gratefully acknowledge the support by the National Natural Science Foundation of China (No. 52473225) and the Guangdong Basic and Applied Basic Research Foundation (No. 2023A1515110262).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.110896.

  
  
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  Scheme 1  Chemical structures and binding constants of compounds studied in this work.

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  Figure 1  UV-visible spectra of (a) MO, (b) ADASS titrated with BPy-Box·4Cl. Inset: The photographs of guest and host with different mole fraction. (c) ¹H NMR titration of BPy-Box·4Cl (0.5 mmol/L, D₂O) binding with ADASS (50 mmol/L, D₂O). (d) Titration isotherm created by monitoring changes in the chemical shift for BPy-Box·4Cl caused by the addition of ADASS. Inset: Calculated changes of mole fractions for BPy-Box⁴⁺ (black trace) and ADASS⊂BPy-Box⁴⁺ (blue trace) over the substrate-receptor mole ratio.

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  Figure 2  Titration isotherm of the binary systems was generated by monitoring changes in absorbance of MO or ADASS caused by the addition of BPy-Box·4Cl (5 mmol/L, H₂O). (a) MO (50 µmol/L, H₂O) and MB (50 µmol/L, H₂O), (b) MO (50 µmol/L, H₂O) and CR (50 µmol/L, H₂O), (c) ADASS (50 µmol/L, H₂O) and MB (50 µmol/L, H₂O), (d) ADASS (50 µmol/L, H₂O) and CR (50 µmol/L, H₂O). Titration isotherm of the ternary systems was generated by monitoring changes in absorbance of MO or ADASS caused by the addition of BPy-Box·4Cl (5 mmol/L, H₂O). (e) MO (16.7 µmol/L, H₂O), MB (16.7 µmol/L, H₂O), and CR (16.7 µmol/L, H₂O), (f) ADASS (16.7 µmol/L, H₂O), MB (16.7 µmol/L, H₂O), and CR (16.7 µmol/L, H₂O). (g) Titration isotherm was generated by monitoring changes in absorbance of ADASS caused by the addition of BPy-Box·4Cl in seawater. Red lines are curve fitting using a 1:1 receptor-substrate binding model. Inset: UV-visible spectra of binary and ternary systems titrated with BPy-Box·4Cl. (h) Chemical structures of MO⊂BPy-Box⁴⁺ and ADASS⊂BPy-Box⁴⁺.

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  Figure 3  Crystal structures of (a) MO⊂BPy-Box⁴⁺ and (b) ADASS⊂BPy-Box⁴⁺. The same interaction distances were omitted for clarity. (c) The packing mode of MO⊂BPy-Box⁴⁺. (d) The packing mode of ADASS⊂BPy-Box⁴⁺. Color codes: white C, light green C, grey H, red O, blue N, yellow S, and green Cl.

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  Figure 4  Intermolecular binding isosurfaces of (a) MO⊂BPy-Box⁴⁺ and (b) ADASS⊂BPy-Box⁴⁺ complexes acquired from IGMH analysis (ρ = 0.002 au) top view and side view. Counterions are omitted for the sake of clarity.

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  Figure 5  (a) Schematic diagram of ADASS and ADASS⊂BPy-Box⁴⁺ under UV (365 nm wavelength) and white light irradiation. UV-visible spectra of (b) BPy-Box⁴⁺ (0.05 mmol/L, H₂O), (c) ADASS⊂BPy-Box⁴⁺, and (d) ADASS (0.05 mmol/L, H₂O) after UV irradiation and (d) after white light irradiation.

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