Synthesis, Adsorption, and Recognition Properties of a Solid Symmetric Tetramethylcucurbit[6]uril-Based Porous Supramolecular Framework

In this work, we reported a porous supramolecular framework (A) constructed of a symmetric tetramethylcucurbit[6]uril (TMeQ [6]) in aqueous HCl solutions; the driving force was the outer surface interaction of cucurbit[n]urils, as well as hydrogen bonding between latticed water molecules and portal carbonyl oxygens of TMeQ[6]. Adsorption experimental results revealed that the porous supramolecular framework can absorb certain fluorophore guests (FGs) to form luminescent assemblies (FG@As) by fluorescence enhancement or colour change, and some of them can respond to certain volatile organic compounds. ,us, the TMeQ[6]-based supramolecular framework could be used as a sensor for certain gas or volatile compounds.


Introduction
Symmetric tetramethylcucurbit [6]uril (TMeQ [6]) is the earliest reported partial alkyl-substituted cucurbit[n]uril in our laboratory [1]. TMeQ [6] was condensed by using the diether of dimethyl-glycoluril (1) and the dimer of glycoluril (2) in aqueous HCl solutions. e free TMeQ [6] is ellipsoid ( Figure 1) due to the tension between the two substituted methyl groups on the dimethyl-glycoluril and has well water solubility. erefore, TMeQ [6] was used to replace the water insoluble Q [6] when we investigated Q [6]-based host-guest interaction and coordination with metal cations in neutral aqueous solutions. For example, the structure feature of TMeQ [6] was confirmed using the host-guest interaction of TMeQ [6] with 2,2′-bipyridine in neutral aqueous solution when it was first isolated [1]. en, a series of host-guest interaction studies on TMeQ [6] were reported [2][3][4][5][6]; for example, the interaction of TMeQ [6] with three hydrochloride salts of phenylephrine isomers was investigated using a competitive interaction method which could explain how subtle differences in the structure of the title isomers lead to a significant difference in the stability of the corresponding host-guest inclusion complexes with the TMeQ [6] [2]; using TMeQ [6], binding interactions with a series of alkyldiammonium ions in aqueous solution were investigated in detail [3]; moreover, investigation of the host-guest interaction between TMeQ [6] and 2-(4-methoxyphenyl)-1H-imidazo [4,5-f] [1,10]phenanthroline hydrochloride salt (guest 1) revealed that TMeQ [6] can trigger fluorescence emission of this guest 1 by TMeQ [6], which has been developed to achieve a supramolecular probe capable of recognizing La 3+ and Ce 3+ through a dramatic decrease in fluorescence intensity [4]. Also the coordination and interaction of TMeQ [6] with a series of metal ions, such as alkali (A + ), alkaline earth (AE 2+ ), and lanthanide metal ions (Ln 3+ ), showed a difference from those of water insoluble Q [6] [5,6]. In particular, such differences in the interaction between TMeQ [6] and Ln 3+ could potentially be used for isolating heavier Ln 3+ from their lighter counterparts in neutral solution, and lighter lanthanide cations from their heavier counterparts in acidic solution.
Besides the Q[n]-based host-guest chemistry [7][8][9][10][11][12] and coordination chemistry [13][14][15][16], the positive electropotential outer surface of cucurbit[n]uril has attracted great attention, and Q[n]-based outer surface interaction chemistry was proposed in recent years [17]. In this emerging chemistry,  [18][19][20][21], exhibiting special adsorption for CH 2 �CH 19 2 and CO 2 [20] or showing anisotropic proton conductivity [21]. Nowadays, we know that the outer surface interaction of cucurbit[n]urils (Q[n]s), which is attributed to the positive electropotential outer surface of Q[n]s results in the formation of these novel Q[n]-based porous supramolecular frameworks [17]. Our group and others also demonstrated a series of Q[n]-based porous supramolecular frameworks in the absence and presence of different structure directing agents in different mediums [14,[22][23][24][25][26][27][28][29][30][31]. For example, allapally and Tian demonstrated a crystalline form of Q [6] via the outer surface interaction of cucurbit[n]uril, which exhibited high thermal stability, permanent porosity, and the highest specific surface area of any known solid-state forms of Q[6] [22]. When Lu and coworkers investigated the drug delivery function of Q [7], they obtained a Q[7]-based supramolecular framework through the outer surface interaction of Q[n]s [23]. Moreover, Q [6] can arrange into a supramolecular polycatenane stabilised by the outer surface interaction of cucurbit [6]urils [24]. Cao and coworkers demonstrated a series of supramolecular compounds constructed from decamethylcucurbit [5]uril (Me 10 Q [5]) and different HPAs or POMs; they found that the Me 10 Q[5]-HPA or POMs-based supramolecular framework exhibited reversible photochromic properties as well as excellent photocatalytic activity toward the degradation of methyl orange and rhodamine B under visible light irradiation [14,25,26]. Recently, we demonstrated a series of simple Q [10]-based supramolecular frameworks in the absence and presence of structure directing agents in aqueous HCl or HNO 3 solutions [27][28][29][30], which not only exhibited sequence selectivity isolation of specific metal cations, thus leading to these architectures being used as metal-selective materials [28], but also exhibited novel adsorption capacities for certain dyes and became novel luminescent materials [29], which could have special selective response to certain volatile compounds [30]. More recently, Fedin and coworkers demonstrated a series of novel supramolecular frameworks based on Q [6] and mono-/polynuclear bismuth(III)-and mercury(II)-ions. is work further extended the research on the construction of Q[n]-based supramolecular frameworks assembled through the inorganic anion as the structure directing agents [31].
us, we focused on the simple Q[n]-based porous supramolecular frameworks, their adsorption properties for dyes, and their response to various volatile organic compounds. In the present work, the symmetric tetramethylcucurbit [6]uril (TMeQ [6]) was selected as building block, and a facile TMeQ[6]-based porous supramolecular framework was prepared from aqueous HCl (1-3 M) solutions (A). is supramolecular framework can absorb various fluorophore guests (FGs), resulting in the formation of luminescent assemblies (FG@As) by fluorescence enhancement or colour change. Moreover, most FG@ A systems exhibited response to certain volatile organic compounds (VOCs) and could be used as solid sensors.

Experimental Section
2.1. Materials. TMeQ [6] was synthesised by literature method. Other chemicals (analytical grade) were obtained from Aladdin and used without further purification. technique and fitted with a nitrogen cold stream (−30°C). e diffraction data were integrated by using the SAINT program, which was also used for the intensity corrections for the Lorentz and polarization effects, and semiempirical absorption corrections were applied using SADABS program. e structure was solved by direct methods, and all of the nonhydrogen atoms were refined anisotropically by the full-matrix least-squares method on F [2] using the SHELXS-97 and SHELXL-97 program packages, respectively [32,33]. Carbon-bound hydrogen atoms were introduced at calculated positions and were treated as riding atoms with an isotropic displacement parameter equal to 1.2 times that of the parent atom. Twenty-four water molecules in A were omitted using the SQUEEZE option of the PLATON program. Analytical expressions for neutral-atom scattering factors were employed, and anomalous dispersion corrections were incorporated. e crystallographic data of A have been deposited at the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-1873879, containing the supplementary crystallographic data for this paper, which can be obtained free of charge from the Cambridge Crystallographic Data centre via ww.ccdc.cam.ac.uk/data_request/cif.

Preparation of Solid-State FG@A Fluorescent Materials.
TMeQ [6]-based A 1.0 g was added to 4.0 mL 5 × 10 −3 MFG acetonitrile solution with stirring for 1 h and filtered, and solid materials with different colour fluorescence were collected.

Adsorption and Response Investigations of FG@A for
VOCs.
e activated FG@A (0.5-1.0 g) was added to a tared open glass phial, and then the phial was put into a vacuum desiccator and evacuated using a vacuum pump until the sample achieved a constant weight. A few mL of the VOC (methanol, ethanol, ethyl ether, acetonitrile, acetone, dichloromethane, trichloromethane, tetrachloromethane, and so on) was then added to another open glass phial, put into the same vacuum desiccator, and resealed. e weight change was measured and corresponding solid fluorescence spectra were determined at ∼1-60 min intervals over several hours to obtain the vapour adsorption profile.

Measurement of Fluorescence Spectra of Solid FG@A
Assemblies. Fluorescence spectra of solid FG@A assemblies before and after adsorbing VOCs were recorded at 25°C using a Varian Cary Eclipse spectrofluorometer (Varian, Inc., Palo Alto, CA, USA), respectively.

Crystal Structures of Compounds A.
Various TMeQ[6]based supramolecular assemblies or frameworks were formed from TMeQ [6] molecules in the presence of inorganic anions or aromatic compounds through the outer surface interaction of cucurbit[n]urils in the different mediums [5,6]. Herein, a simple TMeQ[6]-based supramolecular framework was generated from aqueous HCl (1∼3 M) solutions (A). Above or below this acidity range, not only is the yield of TMeQ [6] crystals low, but also it takes too long. In aqueous HCl (2 M) solution, TMeQ [6] crystals occur in 3 h, left overnight, and the yield reaches up to about 80%. In this TMeQ [6]-based supramolecular assembly, each TMeQ [6] molecule (the central TMeQ [6]) interacts with four neighbouring TMeQ [6] molecules through the so-called outer surface interaction of cucurbit[n]uril (Figure 2(a)) [17]. e portal carbonyls of the central TMeQ [6] are always close to the outer surface without substituted methyl groups of two neighbouring TMeQ [6] molecules; in turn, portal carbonyls of another two neighbouring TMeQ [6] molecules are always close to the outer surface without substituted methyl groups of the central TMeQ [6] molecule, owing to the positive electropotential outer surface of cucurbit[n]urils. us, further extension of TMeQ [6] molecules results in the formation of a TMeQ [6]-based 2D supramolecular framework, in which every four TMeQ [6] molecules can arrange into two different square pores with sections of ∼25 and ∼62Å 2 , respectively ( Figure 2(b)). A close inspection revealed that the detailed interactions mainly include dipole interaction between portal carbonyl oxygens of a TMeQ [6] molecule and positive electropotential methane (O4-H35, O3-H34, O2-H34; O8-H15, O9-H15, O10-H14), methylene (O4-H37B, O2-H31A; O8-H17A, O10-H11A, O11-H12A), and even methyl groups (O4-H39C; O8-H19C) on the outer surface of a neighbouring TMeQ [6] molecule (Figures 2(c) and 2(d)), in addition to the hydrogen bonding between the latticed water molecules and portal carbonyl oxygens of neighbouring TMeQ [6] molecules in A (omitted for clarity). e distances between portal carbonyl oxygens of TMeQ [6] molecules and methine, and methylene and methyl protons from neighbouring TMeQ [6] molecules are in the range 2.343∼2.717Å. Figure 2(e) shows the TMeQ[6]-based supramolecular framework stacked by the TMeQ[6]-based 2D supramolecular frameworks with ∼3Å gaps; Figure S1 shows the TMeQ[6]-based supramolecular framework with numerous channels with sections of about 90Å 2 in compound A. All channels in A are filled with large amount of water molecules (the latticed water molecules), which link TMeQ [6] molecules through hydrogen bonding with portal carbonyl oxygens of TMeQ [6]

molecules in A.
Single crystal X-ray diffraction analysis reveals that TMeQ[6]based porous supramolecular assembly has the characteristics of frameworks and belongs to a typical supramolecular organic framework (SOF). erefore, the TMeQ[6]-based supramolecular framework could find applications in catalysis, sensors, adsorption, ion exchange processes, and so on. Herein, we first considered the adsorption properties in the present work.
Powder X-ray diffraction spectrum of the bulk of A essentially consists of that of simulation of pure crystalline phases ( Figure S2 in the ESI). Analysis of the thermal stabilities of A includes differential scanning calorimetry (DSC) and thermogravimetry (TG) ( Figure S3 in the ESI). For the air-dried A, the DSC with TG curves suggested that water molecules of crystallization were first eliminated with weight losses of ∼28% (calcd. 25%) when the temperature was increased from ambient to about 200°C. Further weight loss was observed starting from about 300°C, and sharp weight Journal of Chemistry losses were incurred at about 400°C, suggesting the decomposition of TMeQ [6]. [6]-Based Porous Supramolecular Framework. Quite a few experimental results have proven that Q[n]-based porous supramolecular frameworks can take up various dyes and fused ring compounds and form solid fluorescent assemblies [29,30]. In the present case, different pores, gaps, spaces, and channels can be observed in the TMeQ   immersing A in an acetonitrile solution containing FG6 yielded luminescent material FG6@A that exhibited a ∼2.5fold increase in fluorescence enhancement compared with the free FG6 (Figure 3(a)). Meanwhile, loading A with FG15 yielded luminescent material FG15@A that exhibited not only a ∼2.5-fold increase in fluorescence enhancement but also a fluorescence colour change from green to blue compared with the free FG15 (Figure 3(b)). e amount of FG6, FG15, or other adsorbates extracted by TMeQ[6]based assembly A from the acetonitrile solution could be measured using 1 H NMR to compare the integrity before and after extracting the adsorbates ( Figure S5 and Table S2). In general, loading porous framework A with a fluorescent guest could change conformation or aggregation state of the fluorescent guest and result in its fluorescence enhancement or quenching. Recent studies have shown that Q[n]-based supramolecular assemblies or frameworks can adsorb a variety of VOCs, whereas the adsorbed VOC could influence the florescence properties of the resulting FG@A luminescent materials [29,30]. us, the adsorption of 12 common VOCs was tested: dichloromethane, trichloromethane, tetrachloromethane, methanol, ethanol, acetone, acetonitrile, ethyl ether, benzene, toluene, formaldehyde, and tetrahydrofuran. e results revealed that the FG@A systems exhibited different responses to the selected VOCs, via fluorescence quenching or enhancement ( Figures S5-S17). For example, FG10@A exhibited little or no response to most VOCs, except alcohols, acetone, and acetonitrile, as demonstrated by significant fluorescence enhancement (Figure 4(a)). FG2@A and FG11@A exhibited similar responses to these via a significant fluorescence enhancement. Meanwhile, most FG@A systems exhibited little or no responses to all selected VOCs ( Figures S5, S7-S9, S15, and S16). Because the free FG4 and FG7 and the corresponding FG4@A and FG7@A systems all showed weak fluorescence, the adsorption properties of the selected VOCs were not investigated in the present work. It is interesting that FG15@ A exhibited little or no response to most VOCs, whereas loading FG15@A with trichloromethane resulted in obvious fluorescence enhancement, suggesting that FG15@A possessed high selectivity for trichloromethane (Figure 4(b)). More detailed investigations on these systems were performed by assessing fluorescence changes via real-time monitoring of the uptake of the selected VOCs. For example, titration fluorescence spectra of the loading of FG10@A with ethanol revealed a rapid increase in fluorescence intensity with increasing the adsorption time or the amount of absorbed formaldehyde (Figure 5(a)). Figure 5(b) shows that the fluorescence intensity of solid FG10@A at 392 nm increased with increasing the adsorption time; it took about 20 min to reach the max value (the equilibration time). Figure 5(c) shows the adsorption profile, and the adsorption or saturation adsorption of FG10@A for ethanol increased by 4% (wt%) within about 6 h. e equilibration time (20 min) was obviously much shorter than the saturation adsorption time (6 h). e response of a FG/A luminescent material to VOC was dependent on the properties of FG and VOC, and the detection limit (DL) of the FG/A luminescent material for VOCs varied with the different luminescent materials and the different VOCs. Based on the intensity/ time curve data ( Figure 5(b)) and the adsorption/time data (Figure 5(c)) used in the interpolation method, ΔI vs. the amount of ethanol adsorbed on solid FG10@A was plotted ( Figure 5(d)), and DL was calculated by multiplying the standard derivation of ten measurements in the absence of ethanol by three, and dividing by the slope of the linear calibration curve over a low concentration range. e DL of solid FG10@A for ethanol was ∼7.23 × 10 −7 mol·gram −1 . Titration experiments of the loading of FG10@A with acetone were also performed (Figures S18). e DL of solid FG10@A with acetone was 3.71 × 10 −7 mol·gram −1 . e titration fluorescence spectra of the loading of solid FG15@A with trichloromethane also showed a rapid fluorescence  In the present work, there are some other FG@TMeQ [6]-based luminescent assemblies which exhibit distinct selective response to certain volatile compounds, including the following: FG2@A to ethanol and dichloromethane; FG11@A to ethanol, dichloromethane, and benzene. e detailed adsorption properties of these systems have also been examined, and the detailed experimental results are shown in Figures S19-S23. e DLs of solid FG2@A for ethanol and dichloromethane were determined to be 9.59 × 10 −8 and 3.71 × 10 −7 mol·gram −1 , respectively, and the DLs of solid FG11@A for ethanol, dichloromethane, and benzene were determined to be 6.36 × 10 −8 , 3.87 × 10 −7 , and 4.39 × 10 −7 mol·gram −1 , respectively. In general, a change in fluorescence emission of an FG molecule could be influenced by changes in molecular configuration or aggregation state; loading of the FG into a porous assembly such as the Q [n]-based porous supramolecular assemblies could alter its configuration or aggregation state, resulting in fluorescence enhancement or quenching of the FG. us, we loaded the simple TMeQ[6]-based porous supramolecular assemblies with common fluorescent guests to yield novel luminescent materials. Moreover, most of the resulting solid FG@A systems responded to common VOCs, suggesting that the adsorbed VOCs influence the molecular structure configuration and aggregation state or charge distribution of the loaded FG molecule through intermolecular interactions between the loaded FG, the adsorbed VOC, and TMeQ [6]  molecules in the framework. Generally, when the adsorbed VOCs were removed, the fluorescence of solid FG@A was recovered. Analysis of the lifetime of the adsorption capacity or fluorescence emission revealed that some FG@A-VOC systems exhibited reversible adsorption and were relatively stable ( Figures S24 and S25). Previous studies have proven that the adsorption of neat FG for VOC was unable to induce an obvious fluorescence change in the selected FGs. Similar phenomena can be observed in the present work ( Figure S26).

Conclusion
In summary, we prepared a porous supramolecular framework (A) by simple crystallization TMeQ [6] in aqueous HCl solutions (1-3 M). Crystal structure analysis showed that the framework contained two different square holes in each TMeQ [6]-based layer and numerous gaps between the layers. e driving force was derived from the outer surface interaction of cucurbit[n]urils, as well as hydrogen bonding networks among the latticed water molecules with electrostatic potential negative portal carbonyl groups of TMeQ [6]. e TMeQ [6]-based porous supramolecular framework (A) exhibited special adsorption for certain fluorophore guests to form solid fluorescent materials (FG/As) with certain fluorescence emission, which can further adsorb different common volatile organic compounds (VOCs), accompanied by fluorescence response (enhancement or quenching). us, preparing porous Q[n]based supramolecular frameworks, loading TMeQ[6]-based porous supramolecular assemblies with FGs yielded solid fluorescent sensors, and detecting FG/As response to certain VOCs could launch a new field of Q[n]-based supramolecular chemistry. More extensive investigations of these Q [n]-based supramolecular assemblies and their functions are currently underway.
Data Availability e PXRD, NMR, and fluorescence spectra and adsorption data used to support the findings of this study are included within the supplementary information file.   Table S2: normalised adsorption data of A for five dyes, respectively (mol/g).