4-Sulfocalix[4]arene/Cucurbit[7]uril-Based Supramolecular Assemblies through the Outer Surface Interactions of Cucurbit[n]uril.

Upon mixing of aqueous solutions of the freely soluble building blocks cucurbit[7]uril (Q[7]) and 4-sulfocalix[4]arene (SC[4]A), white microcrystals instantly separate in near-quantitative yield. The driving force for this assembly is suggested to be the outer-surface interaction of the Q[n]. Dynamic light scattering, scanning electron microscopy, and NMR (diffusion-ordered NMR spectroscopy) analyses have confirmed the supramolecular aggregation of Q[7] and SC[4]A. Titration 1H NMR spectroscopy and isothermal titration calorimetry have shown that the interaction ratio of Q[7] and SC[4]A is close to 3:1. Moreover, the Q[7]/SC[4]A-based supramolecular assembly can accommodate molecules of some volatile compounds or luminescent dyes. Thus, this work offers a simple and highly efficient means of preparing adsorbent or solid fluorescent materials.

Introduction

Generally, supramolecular assembly involves the formation of complexes of molecules held together through noncovalent interactions,[1−4] including hydrogen bonding,[5−8] van der Waals forces,[9,10] metal coordination,[11−14] π–π interactions,[15−18] and so on. It is the core of supramolecular chemistry and has brought together supramolecular chemistry and material science,[19,20] polymer science,[21−23] biomedical science,[24−26] and catalysis.[27−29] Therefore, supramolecular assembly is regarded not only as a forefront of contemporary science,[30,31] but also as the future of chemistry.[31,32]

Cucurbit[n]urils (Q[n]s)[33−37] have a long history, stretching back to 1905 when Behrend and co-workers reported a reaction between glycoluril and formaldehyde,[33] although it took 76 years before Mock and co-workers structurally assigned the first member of the Q[n] family, the cucurbituril Q[6].[34] Q[n]s are characterized by a rigid hydrophobic cavity and two polar portals rimmed with carbonyl groups and have proven to be ideal building blocks for the construction of various supramolecular assemblies, not only through host–guest interactions,[38−43] but also metal ion coordination.[44−47] From our own research and that of others, it is clear that the electrostatically positive outer surface of Q[n] can serve as a driving force in the formation of various novel Q[n]-based supramolecular architectures and materials,[47−52] most notably in the presence of various inorganic anions, such as polychloride d-transition metal anions [Md-blockCl],[53,54] polyoxometallate anions,[49] and aromatic organic anions.[55,6]uril-Based Supramolecular Assemblies: Possible Application in Radioactive Cesium Cation Capture. J. Am. Chem. Soc.. 2014 ">56] We ascribe such constructions to the so-called outer-surface interactions of Q[n], including C–H···π interactions between methine groups of glycoluril moieties of Q[n] and aromatic rings, π–π interactions between carbonyl groups of glycoluril moieties of Q[n] and aromatic rings, hydrogen bonding between methine groups of glycoluril moieties of Q[n] and protruded oxygen atoms of polyoxometallate anions or inorganic cluster anions, but mostly ion–dipole interactions between the electrostatically positive outer surface of Q[n]s and inorganic or organic anions.[57,6]uril-based multifunctional supramolecular assemblies: synthesis, removal of Ba(II) and fluorescence sensing of Fe(III). Dalton Trans.. 2018 ">58]

Like cucurbit[n]urls, calix[n]arenes also have a long history, stretching back to 1872 when Baeyer found that many products could be obtained from reactions between phenols and aldehydes.[59,60] After more than seven decades, Zinke and Ziegler assigned some of these products,[61,62] which were termed calixarenes by Gutsche in 1978.[63] Generally, calix[n]arenes are also characterized by a hydrophobic cavity constructed from n-phenyl rings bridged by n-methylene groups. There is at least one reactive site on each phenyl ring, allowing for easy functionalization of these calix[n]arenes.[64−66] As third-generation hosts, calix[n]arenes, like cucurbit[n]urils, find various applications in drug delivery,[67,68] sensing,[69,70] separation and recognition,[71,72] catalysis,[73,74] and so on. 4-Sulfocalix[n]arenes[75] (hereinafter abbreviated as SC[n]As) contain not only aromatic rings but also sulfonate groups, which can interact with the outer surface of Q[n]s. Thus, SC[n]As could be ideal building blocks for the construction of Q[n]/SC[n]A-based supramolecular assemblies. Such supramolecular assemblies may show not only the respective intrinsic properties of the Q[n] and SC[n]A but also new properties due to their novel forms.

Referring to the first work on the supramolecular assemblies of Q[6] and SC[4 or 6]As by Lin and co-workers,[76] water-soluble Q[7] (Figure a) and SC[4]A (Figure b) were selected as building blocks. White microcrystals precipitated almost instantaneously in near-quantitative yield (>90%) upon simple mixing of solutions of Q[7] and SC[4]A in neutral water. Dynamic light scattering (DLS), diffusion-ordered NMR spectroscopy (DOSY), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) further confirmed the supramolecular aggregation of Q[7] and SC[4]A. 1H NMR spectroscopy and isothermal titration calorimetry (ITC) showed the interaction ratio of Q[7] and SC[4]A to be close to 3:1. It was anticipated that such a supramolecular assembly would have space to accommodate molecules of some volatile compounds or fluorescent dyes, such that it might be used as an adsorbent material or a solid fluorescent material.

Figure 1

Crystal structures of (a) Q[7] and (b) SC[4]A.

Results and Discussion

Interaction of Q[7] with SC[4]A

Both Q[7] and SC[4]A are water soluble. However, mixing their aqueous solutions almost instantaneously produces a white precipitate in near-quantitative yield (>90%). This phenomenon suggests a strong interaction between these components, which we interpret as an outer-surface interaction of the Q[n], including the π–π interaction of the carbonyl bonds of Q[7] with the aromatic rings of SC[4]A and ion–dipole interaction between the electropositive outer surface of Q[7] and the electronegative sulfonyl groups of SC[4]A.[57] To probe the interaction of Q[7] and SC[4]A in more detail, we first used a titration 1H NMR technique to determine their interaction in neutral D2O. The experimental results showed that SC[4]A gradually disappeared from the solution with increasing number of equivalents of Q[7] and the ratio of precipitated Q[7] to SC[4]A was close to 3:1 (Figure S1 and Table S1 in the Supporting Information). The assembled supramolecular precipitate can dissolve in acidic solution. Figure shows the 1H NMR spectrum of the Q[7]/SC[4]A-based precipitate redissolved in 4 M DCl/D2O solution, along with the spectra of pure Q[7] and SC[4]A in the same acidic medium. Compared to those of the pure molecules, the signals of both Q[7] and SC[4]A in the supramolecular assembly showed some changes in chemical shift. For the Q[7] molecule, all proton resonances experienced an upfield shift (Δδ ≈ 0.02 ppm), suggesting that these molecules reside in a shielding area. For the SC[4]A molecule, all proton resonances experienced a downfield shift (Δδ ≈ 0.05 ppm), suggesting that these molecules reside in a deshielding area, that is, at the portals of the Q[7] molecules.

Figure 2

1H NMR spectra of (a) pure SC[4]A, (b) pure Q[7], and (c) Q[7]/SC[4]A-based supramolecular assembly, each in 4 M DCl/D2O solution.

Diffusion-ordered NMR spectroscopy (DOSY) is commonly employed to confirm an aggregation system in solution, since the results can directly correlate the 1H NMR signals between two interacting components with the corresponding diffusion coefficient.[77−80] In the present case, the DOSY spectrum of a sample with a 3:1 ratio of Q[7] and SC[4]A in D2O was recorded. The average diffusion coefficient for the Q[7] component was evaluated as 6.10 × 1010 m2 s–1, as compared to 4.20 × 10–10 m2 s–1 for the SC[4]A component. On mixing the two components in a 3:1 ratio in D2O, the average diffusion coefficient decreased to 3.20 × 10–10 m2 s–1. This obvious decrease in the diffusion coefficient implies that the two components become aggregated (Figure S2 and Table S2 in the Supporting Information).

Isothermal titration calorimetry (ITC) measurements can provide quantitative physical and chemical data for an interaction system. Thus, a solution of SC[4]A was incrementally injected into a solution of Q[7] (0.10 mM) at 25 °C to record the exothermic binding isotherm (Figure S3 in the Supporting Information). The interaction molar ratio determined was 0.36, close to the 3:1 ratio of Q[7] to SC[4]A, and the interaction association constant (Ka) of the two components was evaluated as (2.13 ± 0.23) × 105. Moreover, the relatively large negative enthalpy reveals that the assembly process is typically driven by a favorable enthalpy change (Table S3 in the Supporting Information).

Dynamic light scattering (DLS) can also provide aggregation information on an interaction system. Figure shows DLS data for Q[7]/SC[4]A obtained by simply mixing solutions of Q[7] and SC[4]A in a molar ratio of 3:1. A single hydrodynamic diameter distribution centered at around 375 nm can be seen, attributable to the formation of aggregated assemblies, as compared to distributions centered at 4.4 nm for Q[7] and 4.1 nm for SC[4]A.

Figure 3

DLS profiles of Q[7], SC[4]A, and Q[7]/SC[4]A-based supramolecular assembly in neutral water at 25 °C.

Thus far, we have described the supramolecular assembly of Q[7] and SC[4]A in aqueous solution, which instantaneously yields less water-soluble microcrystals with a certain interaction ratio (3:1 Q[7] to SC[4]A). Although we have tried various methods, we have not yet succeeded in obtaining suitable single crystals of the Q[7]/SC[4]A-based supramolecular assembly. A powder X-ray diffraction (PXRD) pattern of the Q[7]/SC[4]A-based assembly suggested that it was partially crystallized (Figure S4 in the Supporting Information). An SEM image provided direct visual information for the formation of the Q[7]/SC[4]A-based supramolecular assembly, with particle diameters in the range 0.2–0.9 μm (Figure a). Figure b shows a TEM image of the highly dispersed Q[7]/SC[4]A-based aggregate in acetonitrile; the sizes of the Q[7]/SC[4]A-based aggregates are in the range of 4–13 nm. Close inspection reveals that each aggregate in the image is actually a nanosized crystal (Figure c). The numerous channels between these Q[7]/SC[4]A-based aggregates or nanocrystals could make this supramolecular assembly potentially useful for the absorption of volatile compounds or fluorophore molecules.

Figure 4

(a) SEM image; (b) TEM image of the Q[7]/SC[4]A-based supramolecular assembly; inset, an enhanced TEM image of nanosized crystal in the Q[7]/SC[4]A-based supramolecular assembly.

The thermal stability of the Q[7]/SC[4]A-based supramolecular assembly was assessed by thermogravimetric analysis in nitrogen (Figure S5 in the Supporting Information). It was found that the assembly underwent weight loss in two stages. The first stage was assigned to the evaporation of intramolecular and intermolecular water, with a maximum at 112 °C, as compared to 149 °C for Q[7] and 122 °C for SC[4]A. The peak temperature of decomposition and carbonization was 413 °C, higher than those of 388 °C for Q[7] and 320 °C for SC[4]A.

Adsorption Behaviors of the Q[7]/SC[4]A-Based Supramolecular Assembly

In spite of the absence of a single-crystal X-ray structure, the basic interaction between Q[7] and SC[4]A was characterized using various techniques. The water-insoluble microcrystals could be redissolved in HCl, and the interaction ratio of Q[7] and SC[4]A was established as 3:1. Most importantly, microscopy revealed extensive channels between the microcrystals and the empty cavities of Q[7], and SC[4]A themselves could make the Q[7]/SC[4]A-based supramolecular assembly a promising adsorbent material.

Previous work has revealed that various Q[n]-based supramolecular assemblies exhibit selective adsorption of different volatile compounds. For example, a Q[5]/K+/p-hydroxybenzoic acid ternary supramolecular assembly exhibits highly selective methanol adsorption,[81] and two different hemimethyl-substituted Q[6]-based supramolecular assemblies show strong preferences for methanol adsorption and diethyl ether adsorption, respectively.[82] In the present case, various channels can be observed in the Q[7]/SC[4]A-based supramolecular assembly and the Brunauer–Emmett–Teller surface area of the activated assembly, as calculated from the isotherm, is 3.1 m2 g–1 (Figure S7 and Table S4 in the Supporting Information). Further experimental results showed that the Q[7]/SC[4]A-based supramolecular assembly exhibits different adsorption properties for selected volatile compounds (gram per gram). The respective capacities are 0.14 g for methanol, 0.09 g for ethanol, 0.06 g for acetone, 0.10 g for acetonitrile, 0.21 g for dichloromethane, 0.23 g for trichloromethane, 0.22 g for tetrachloromethane, 0.12 g for diethyl ether, and 0.10 g for n-hexane (Figure and Table S4). Thus, it exhibits a somewhat higher selectivity for polychloromethanes. Compared to the related capacities of Q[7] and SC[4]A alone, there are no significant differences among these three hosts, except that Q[7] shows a higher capacity for methanol (0.42 g).

Figure 5

Adsorption profiles of volatile materials on Q[7]/SC[4]A-based supramolecular assembly: (⬢) methanol, (⬟) ethanol, (●) acetone, (▼) acetonitrile, (▲) diethyl ether, (◆) dichloromethane, (▶) trichloromethane, (◀) tetrachloromethane, and (■) n-hexane.

Considering not only the various channels between the microcrystals but also the probable empty cavities of Q[7] and SC[4]A in the Q[7]/SC[4]A-based supramolecular assembly, we anticipated that such an assembly would also have space to accommodate molecules of various fluorescent compounds and thereby yield solid fluorescent materials. Referring to the method used in ref (83), a mixed solvent (water/acetonitrile, 1:9, v/v) was eventually selected for all experiments in this work. Some typical fluorophore guests, such as G1, G2, G3, and G4, were selected for adsorption on the Q[7]/SC[4]A-based supramolecular assembly (Figure a). Experimental results indicated that the supramolecular assembly adsorption-based luminescent materials exhibited fluorescence emissions in the solid state. For example, loading the Q[7]/SC[4]A-based supramolecular assembly with G4 (G4@Q[7]/SC[4]A) gave a powder that appeared light-orange under daylight, with a bright-yellow fluorescence emission with maximum intensity at 579 nm, compared to the red G4 under daylight, and very weak dark-red fluorescence emission with maximum intensity at 637 nm under 365 nm UV light (Figure b,c). In comparison, loading pure Q[7] or pure SC[4] with G4 gave powders that appeared light-yellow or pale-white, respectively, under daylight, with a weak light-blue (λexc = 561 nm) or weak light-purple (λexc = 558 nm) fluorescence emission under 365 nm UV light (Figure b,c). Significant changes were seen for G4, not only in color (from red, 637 nm, to bright-yellow, 579 nm) but also in the strength of the fluorescence emission (over 74 times under the same experimental condition). The results of other adsorption experiments are presented in Figures S8–S10 in the Supporting Information. It is interesting that the interactions of different dyes with the Q[7]/SC[4]A-based supramolecular assembly gave rise to different fluorescence emission phenomena. For example, the assembly adsorbs G1 and exhibits an enhanced blue fluorescence emission; there are no obvious differences from G1 itself or the G1/Q[7] or G1/SC[4]A systems (Figure S8 in the Supporting Information). Similar phenomena can be observed for G2 and the G2/Q[7]/SC[4]A, G2/Q[7], and G2/SC[4]A systems (Figure S9 in the Supporting Information). The fluorescence emissions of the G3/Q[7]/SC[4]A, G3/Q[7], and G3/SC[4]A systems show obvious changes compared to that of G3. The wavelengths of maximum intensity are 487 nm (cyan) for G3/Q[7]/SC[4]A, 488 nm (cyan) for G3/Q[7], and 507 nm (green) for G3/SC[4]A, as compared to 457 nm (blue) for G3 (Figure S10 in the Supporting Information). We also investigated the interactions of these dyes with Q[7] or SC[4]A alone (Figure S11 in the Supporting Information). 1H NMR titration experiments revealed that Q[7] or SC[4]A can include all of these dye guests, except that interaction of G1 with SC[4]A resulted in the formation of a precipitate, suggesting that the Q[7]/SC[4]A-based supramolecular assembly could not only include dye molecule guests in the cavities of Q[7] or SC[4]A units but also in its channels.

Figure 6

(a) Structures of the selected dyes G1, G2, G3, and G4 and (b) the comparison of the colors of G4, G4 + Q[7], G4 + SC[4]A, G4 + Q[7]/SC[4]A, under daylight and UV light (365 nm); (c) fluorescence spectra of solid G4 (λexc = 637 nm); G4 + Q[7] (λexc = 561 nm), G4 + SC[4]A (λexc = 558 nm), and G4 + Q[7]/SC[4]A (λexc = 579 nm).

Further adsorption capacity experiments were performed by immersing the Q[7]/SC[4]A-based supramolecular assembly (10 mg) in a solution (0.5 mL) containing a dye (2 × 10–3 M) (Figure S12 in the Supporting Information). The loading capacities for the Q[7]/SC[4]A-based supramolecular assembly (mol g–1) were evaluated as 5.26 × 10–5 for G1, 4.74 × 10–5 for G2, 4.10 × 10–5 for G3, and 9.20 × 10–5 for G4.

Conclusions

In summary, by combining water-soluble Q[7] and SC[4]A as building blocks, a simple method for synthesizing a Q[7]/SC[4]A-based supramolecular assembly with numerous channels has been demonstrated. The driving force for construction of this supramolecular assembly is suggested to be the outer-surface interaction of Q[7]. Titration 1H NMR spectroscopy and isothermal titration calorimetry (ITC) have indicated that the interaction ratio of Q[7] and SC[4]A is close to 3:1. Moreover, the Q[7]/SC[4]A-based supramolecular assembly can not only accommodate some volatile compounds, in particular, polychloromethanes, but can also selectively adsorb certain organic dyes to yield multiemitting, including white-light-emitting, solid fluorescent materials. Thus, the present work not only further enriches the palette of supramolecular assemblies based on the outer-surface interaction of cucurbit[n]urils, but also combines two major branches of macrocyclic chemistry, cucurbit[n]uril chemistry, and calixarene chemistry, in one system. More importantly, such study might open up new Q[n]-based supramolecular chemistry in the fields of porous materials and light emitters. More extensive investigations on the Q[n]/C[n]A-based supramolecular assemblies and their functions are currently underway.

Experimental Section

Materials

SC[4]A was purchased from TCI. Q[7] was prepared in our laboratory according to a literature method.[35,36] Other chemicals were of analytical grade and were obtained from Aladdin and used as received without further purification.

Measurements

All 1H NMR spectra, including those for titration experiments and DOSY spectra, were recorded at 20 °C from solutions in D2O on a Varian Inova-400 spectrometer. TEM images were obtained with an FEI Tecnai G2 F20 field-emission transmission electron microscope. SEM images were obtained with a Zeiss∑IGMA+X-Max20 field-emission scanning electron microscope (for SEM imaging, Au (1–2 nm) was sputtered onto the grids to prevent charging effects and to improve the image clarity).

DLS Measurements

DLS data were obtained on a Brookhaven BI-APDX apparatus at 25 °C. Sample solutions for DLS measurements were prepared by filtration through a 450 nm Millipore filter into a clean scintillation vial. DLS measurements were first performed on separate aqueous solutions of either Q[7] and SC[4]A. These solutions were then mixed and further DLS measurements were carried out in the same way. All of the DLS measurements were performed at a scattering angle of 90°.

Isothermal Titration Calorimetry (ITC) Experiments

ITC measurements were performed on a Nano ITC instrument (TA). All solutions were prepared in purified water and degassed prior to titration experiments. Thirty consecutive 8 μL aliquots of a 10 mM solution of SC[4]A were injected into the microcalorimetric reaction cell containing 1 mL of 1 mM Q[7] solution at 25 °C. A correction for heat of dilution was applied by injecting the SC[4]A solution into deionized water and subtracting these data from those of the Q[7] – SC[4]A titration. Computer simulations (curve fitting) were performed using the Nano ITC analyze software.

Preparation of Q[7]/SC[4]A-Based Supramolecular Assembly

SC[4]A (0.04 g, 0.05 mmol) and Q[7] (0.2 g, 0.15 mmol) were separately dissolved in water. The Q[7] solution was then added dropwise to the SC[4]A solution with stirring, whereupon a white flocculent precipitate was immediately formed almost quantitatively. This precipitate was collected by filtration; yield: 0.205 g (85%).

Adsorption Studies on the Q[7]/SC[4]A-Based Supramolecular Assembly with Selected Volatile Compounds

The requisite amount of solid assembly (0.5–1.0 g) contained in a tared open glass phial was placed in a sealable glass vessel, which was then evacuated with the aid of a vacuum pump. Pumping was continued until the sample achieved constant weight. A second open container containing a few milliliter of a volatile liquid, either methanol, ethanol, n-hexane, diethyl ether, acetonitrile, dichloromethane, trichloromethane, tetrachloromethane, or acetone was then added, and the vessel was resealed. The weight change of the sample was then determined at 0.25–1 h intervals over 24 h to obtain the vapor absorption profile.

Preparation of Q[7]/SC[4]A/Dye Solid-State Fluorescent Materials

Portions (10.0 mg) of Q[7]/SC[4]A were added to aliquots (0.5 mL) of 0.01 M solutions of dyes in acetonitrile. The mixtures were stirred for 1 min. Powder samples with different colored fluorescences were recovered by filtration: cyan with pyrenemethanamine hydrochloride (G1), blue with umbelliferone (G2), cyan with levofloxacin hemihydrate (G3), and yellow with 2-[4-(dimethylamino)styryl]-1-methylpyridinium iodide (G4).