Porous Interdigitation Molecular Cage from Tetraphenylethylene

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Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Porous Interdigitation Molecular Cage from Tetraphenylethylene Trimeric Macrocycles That Showed Highly Selective Adsorption of CO2 and TNT Vapor in Air Jia-Bin Xiong,† Jin-Hua Wang,† Bao Li,*,† Chun Zhang,*,‡ Bien Tan,*,† and Yan-Song Zheng*,† †

Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China ‡ College of Life Science and Technology, Huazhong University of Science and Technology, Wuhan 430074, China S Supporting Information *

ABSTRACT: A new [3 + 3] trimeric macrocycle, based on tetraphenylethylene having an aggregation-induced emission effect, could form a interdigitation molecular cage with a big void by host−guest interactions. The cage could accommodate two TNT molecules and detect TNT at the 1.7 fg level per mL of air. Moreover, the cage could give permanent pores that had a BET surface area of 347 m2/g and could adsorb 7.8 wt % CO2 with high CO2/N2 selectivity up to 32 (273 K, 1 bar).

improved selectivity and fluorescence quantum yields.20 Here, we report that two new TPE trimeric macrocycle molecules can interdigitate each other to form a host−guest molecular cage bearing a vast void, which can accommodate two 2,4,6trinitrotoluene (TNT) explosive molecules and selectively adsorb carbon dioxide. TPE trimeric macrocycle 5 was synthesized according to the route shown in Scheme 1. The starting material di(pmethylphenyl)diphenylethylene 1 was nitrated with nitric acid to afford di(p-methylphenyl)di(p-nitrophenyl)ethylene 2 in 86% yield. Then under reduction with hydrazine hydrate in the

M

olecular cages bearing an internal void are attracting increasing interest due to enormous application potential in gas storage,1 separation,2 reactive species stabilization,3 reaction vessel,4 catalysis,5 sensor,6 etc. To date, a wide variety of molecular cages, including covalent bonded cages,7 reversible covalent bonded cages,8 coordination bonded cages,9 hydrogen bonded cages,10 halogen-bonded cages,11 ion bonded cages,12 etc., have been reported. Among these molecular cages, those self-assembled from macrocyclic compounds are of special interest to us, because they not only can form giant cages with a very vast internal void,13 which will have the potential to mimic biological cages composed of protein and DNA,14 but also easily produce self-assembled molecular cages or capsules just by a combination of two macrocyclic components through noncovalent bonds. 10−12 It is well-known that macrocyclic compounds easily form host−guest complexes by the insertion of guest molecules into the cavity of the macrocycle or mutual inclusion,15 which have afforded a wide variety of host−guest supramolecular gels or polymers.16 However, except for vesicles formed by host−guest interactions,15,17 cages based on host− guest interaction are very rare. One of these rare examples is the calix[4]arene trimeric cage formed by a cyclic, mutually included arrangement of three calixarene molecules, which can be exploited to retain freons and methane.18 Recently, aggregation-induced emission (AIE) phenomena are attracting extreme interest due to their enormous application potential in bioimaging, chemo/biosensors, optoelectronic materials, stimuli-responsive systems, etc.19 Among the AIE molecules, tetraphenylethylene (TPE) and its derivatives are the most studied ones with AIE effect due to their easy preparation and reliable AIE characteristics. To increase the sensitivity and fluorescence intensity of TPE receptors, cyclizations between TPE molecules or within one TPE molecule have been carried out.20 These resultant TPE macrocyclic compounds truly exhibit © XXXX American Chemical Society

Scheme 1. Synthetic Route of TPE Trimeric Macrocyclic Compound 5

Received: November 10, 2017

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DOI: 10.1021/acs.orglett.7b03483 Org. Lett. XXXX, XXX, XXX−XXX

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TPE unit), the dimeric cage looked like a fork with four arms consisting of TPE units (Figure S15D), which could pack into the rectangular channel with a 12.2 Å length and 7.72 Å width. This served as a reminder of Atwood’s13a and Ward’s10a giant hydrogen bonded cages, which have packed interstices between them. But because of the four arms, as connecting nodes, the interdigitation cages could pack into more regular and more spacious crystal channels (Figure S15E). When TNT was added, the interdigitation cage could accommodate two TNT molecules in its void to give a single crystal of 1:1 5-TNT complex in DMSO (CCDC 1525305, Figure 2A). Unexpectedly, the electron-deficient TNT was nearer to the electron-deficient pyridyl ring in an almost parallel position rather than to the electron-rich aniline ring of the TPE unit (Figure 2B). In addition to slipped π−π stacking (3.51 Å) between TNT and the pyridyl rings with a very small overlapping area, there were strong n−π (n denotes lone pair electron) interactions (3.10 and 3.14 Å) between the nitro oxygen of the TNT molecule and the pyridyl ring. According to conventional speculation, due to strong electron-deficient effect, TNT easily interacts with electron-rich aromatic compounds by D−A (donor−acceptor) π−π interactions. However, this speculation is never confirmed by the single crystal structure of a complex of TNT with an aromatic host receptor.21 To the best of our knowledge, this is the first crystal structure of TNT-binding mode in a receptor, which discloses that n−π interactions between the TNT nitro groups and aromatic rings plays a key role in TNT association. This finding provides a new idea for designing a fluorescent or chromophoric sensor for explosives such as nitroaromatics, nitramines, nitroalkanes, and nitrate esters by exploiting their nitro groups as binding sites. Interestingly, after encapsulation of TNT molecules, the cage void became bigger. First, two folded pyridyl rings were staggered more than that without TNT. The distance between two folded pyridyl rings increased to 7.20 Å from 7.07 Å. Second, the included pyridyl rings inserted more deeply into the cavity of the macrocycle, with decreasing distance between the included pyridyl ring of one macrocycle and the normal pyridyl rings of another macrocycle. As a result, four arms of the cage with TNT were stretched more uniformly than ones without TNT. Therefore, the cages packed more densely and gave round channels (Figure 2D). Noticeably, the round channels had a diameter of 13.5 Å, possessing a bigger space than that without TNT. This is a rare example for guest molecules to expand the void and interstice.22 The cage can offer permanent pores after guest and solvent molecules are removed. The crystals of 5-TNT were immersed in methanol four times and then in ethyl ether two times. By this

presence of Pd/C, 2 was transferred into di(p-methylphenyl)di(p-aminophenyl)ethylene 3 in 88% yield. With this important intermediate in hand, macrocycle 5 composed of three TPE units and three pyridinediformyl units was obtained in 18% yield by condensation of it with 2,6-pyridinediformyl chloride 4. As expected, the TPE trimeric macrocycle exhibited AIE effect (Figure S14). After a hot solution of 5 in DMSO was allowed to stand at about 30 °C, colorless rectangular single crystals suitable for Xray diffraction were obtained. The crystal structure (CCDC 1525304) confirms that the macrocycle is composed of three TPE units and three pyridyl rings (Figure 1). Due to strong

Figure 1. (A) Crystal structure of 5 with cofacially folded TPE and pyridinediformyl units. (B) Interdigitation molecular cage formed by mutual insertion of macrocycles 5 with the folded pyridinediformyl groups. The numbers in structure denote distance in Å.

intramolecular hydrogen bonds between formamide hydrogen and pyridyl nitrogen, the formamide groups were in the same plane with the pyridyl ring. However, the formamide groups were not coplanar with the TPE unit due to lack of hydrogen bonds. The rigid structure of 2,6-pyridinediformyl and TPE units led to the folded conformation of 5 in which one 2,6-pyridinediformyl unit and one TPE unit that were not neighboring each other were cofacially folded with a dihedral angle of 12.4° between them. The resultant conformation afforded a rectangular cavity 14.3 Å long and 12.0 Å wide (Figure 1A). Interestingly, through ArH−π hydrogen bonds (2.78 and 2.82 Å) between the folded TPE and pyridyl units and CH3−π (3.13 Å) interactions between normal TPE cores, one macrocyclic molecule could insert into the cavity of another by the folded pyridinediformyl unit and form an interdigitation dimer. Because the cavity of the macrocycle was giant, the free space remained up to 7.07 × 14.3 × 9.85 Å3 in the interdigitation dimer after the cavity was partially occupied by the pyridinediformyl unit. This interdigitation dimer furnished a real host−guest supramolecule cage (Figure 1B). Viewed from another direction (perpendicular to the folded pyridyl ring or

Figure 2. (A) Crystal structure of two TNT molecules accommodated in interdigitation cage of 5. (B) TNT binding mode in the cage. (C) Crystal structure of cage-TNT complex viewed along direction perpendicular to the folded pyridyl ring. (D) Crystal channels connected by the interdigitation cages. The numbers in structure denote distance in angstrom; for clarity, the solvent molecules and hydrogen atoms were omitted. In A and C, two TNT molecules are shown in spacefill style. B

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gases, the CO2/N2 selectivity was high, up to 32 (Figure S17). To give insight into the mechanism of CO2 storage, isosteric heat of adsorption was also calculated from the CO2 isotherms measured at 273 and 298 K. At onset of adsorption, the adsorption heat of CO2 is 36.4 kJ mol−1 (Figure S18). This value indicated strong physisorption aroused by weak noncovalent bonds such as n−π, quadrupole−dipole, and hydrogen bonding interactions between CO2 and the interdigitation cage, which would be helpful for facile release of the stored CO2. The suspension of 5 in H2O/THF emitted strong fluorescence while the solution of it in THF had no emission, demonstrating that 5 is a typical AIE compound. After TNT solution in ethanol was added, the fluorescence of the suspension could be gradually quenched (Figure S19). When 10 equiv TNT were added, about half of the fluorescence was quenched. Interestingly, in the presence of 1% (weight, related to water; the same below) NaNO3, the fluorescence could be attenuated up to 5-fold by 10 equiv of TNT. As a result, the intensity ratio I0/I had a linear rise with TNT concentration. According to the Stern−Volmer equation, the linear quenching constants of 5 by TNT were determined to be 1.6 × 106 M −1 (Figures S20−S21). Accordingly, in light of the three times standard deviation concept, the detection limit for TNT solution was 3.1 × 10−8 M (Figure S22). Moreover, when the air saturated with TNT vapor was gradually bubbled into the suspension by a syringe, the fluorescence could be gradually quenched. However, the highest quenching efficiency was only about 10% even more volumes of TNT saturating air was bubbled into the suspension (Figure S23). Just like that using TNT solution, in the presence of 1% inorganic salt, including KCl, NaBr, Na3PO4, NaCl, NaF, Na2SO4, and NaNO3, the quenching efficiency by TNT vapor was raised significantly (Figure S24). Among them, NaNO3 showed the most enhancing effect and gave a quenching efficiency up to 68%. Even the diluted TNT vapor with air could obviously quench the fluorescence of the suspension (Figure 4).

treatment, TNT and DMSO were completely removed and the crystal solid became a powdered one. However, by powder XRD measurement, the solvent-treated crystals showed a good crystal structure with a very similar pattern to that of the as-prepared crystals (Figure S16). Gas adsorption and desorption experiments of the solvent-treated crystals (previously desolvated at 120 °C for 10 h under vacuum) exhibited a BET surface area of 347 m2/g (Langmuir surface area of 446 m2/g) and the total volume of 0.37 cm3 g−1. A steep nitrogen gas uptake at low relative pressure in the nitrogen adsorption isotherms (Figure 3A) suggested an abundant micropore structure. But the

Figure 3. (A) Nitrogen sorption isotherms of the cage at 77 K. (B) Pore size distributions of the cage calculated using the DFT method. The peak at 126.5 Å was proposed to be an artifact by a reviewer, but we are unable to explain its origin. (C) CO2 and N2 adsorption and desorption isotherms by the cage. Empty symbols denote gas adsorption and filled symbols denote desorption. STP = standard temperature and pressure.

Figure 4. After 200 mL of TNT-containing air was bubbled, change in fluorescent spectra of the suspension of 5 in 95:5 H2O/THF in the presence of 1% NaNO3 with dilution of TNT-saturating air by fresh air. [5] = 1.0 × 10−7 M.

hysteresis of the desorption probably resulted from reversible structural change in the crystal upon adsorption of nitrogen.23 The pore size distribution calculated by the DFT method (Figure 3B) was 5.9, 8.0, 12.7, and 17.2 Å. This datum confirmed that the void of the cage (5.9 and 8.0 Å) was retained and the partial channel structure (12.7 and 17.2 Å with lower volume) did not collapse, which was in accordance with the powder XRD test. Through n−π, quadrupole−dipole and hydrogen bonding interactions, the cage should adsorb CO2. As shown in Figure 3C, the cage could take up 7.8 wt % CO2 at 273 K and 1 bar, approximately equal to six CO2 molecules adsorbed by one cage, indicating that one pyridyl ring bound one CO2 molecule. By comparing the initial slopes of adsorption isotherms of these two

Probably the added salt could facilitate absorption of TNT into the pores of the cage by increasing the polarity of the aqueous suspension. With dilution of TNT-saturating air by fresh air, the quenching efficiency was gradually decreased. After the TNTsaturating air was diluted 1.0 × 104 times, the quenching efficiency still reached 4.0%. This result demonstrated that N2, O2, CO2, H2O, etc. in air all did not interfere with the detection of TNT and the cage exhibited very high selectivity for adsorption of TNT vapor. At this dilution point, the detection limit was measured to be 0.18 ppt (0.18 volumes of TNT vapor in 1012 volumes of air) or 1.7 fg of TNT per mL of air (Figures S25− C

DOI: 10.1021/acs.orglett.7b03483 Org. Lett. XXXX, XXX, XXX−XXX

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S26). This sensitivity for detection of TNT vapor was among the highest ones (femtogram sensitivity to TNT per mL of air21) and comparable to that of trained canine. In conclusion, a TPE macrocyclic compound composed of three pyridinediformyl and three TPE units was synthesized and could form an interdigitation molecular cage. Due to the giant cavity of the macrocycle, the interdigitation cage formed by the macrocycle still had a big void, which could accommodate two TNT molecules. Even after the TNT and solvent molecules were removed, the void of the cage was still retained and could give permanent pores. These pores could be exploited in both the selective absorption of CO2 and supra-sensitive sensor for TNT vapor. Other application properties, such as separation of molecule isomers by this interdigitation cage, are under investigation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b03483. Synthetic and experimental details, more spectra (PDF) Accession Codes

CCDC 1525304−1525305 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Bao Li: 0000-0003-1154-6423 Yan-Song Zheng: 0000-0002-1807-4580 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank National Natural Science Foundation of China (21072067 and 21673089) and the Fundamental Research Funds for the Central Universities (HUST: 2015ZDTD055) for financial support, and thank the Analytical and Testing Centre at Huazhong University of Science and Technology for measurement.

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REFERENCES

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DOI: 10.1021/acs.orglett.7b03483 Org. Lett. XXXX, XXX, XXX−XXX