An Elastic Hydrogen-Bonded Cross-Linked Organic Framework for

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An Elastic Hydrogen-Bonded Cross-Linked Organic Framework for Effective Iodine Capture in Water Yunxiao Lin,†,‡,∥ Xuanfeng Jiang,†,∥ Samuel T. Kim,† Sampath B. Alahakoon,§ Xisen Hou,† Zhiyun Zhang,† Christina M. Thompson,§ Ronald A. Smaldone,§ and Chenfeng Ke*,† †

Department of Chemistry, Dartmouth College, 6128 Burke Laboratory, 41 College Street, Hanover, New Hampshire 03755, United States ‡ Zhiyuan College, Shanghai Jiao Tong University, Shanghai 200240, P. R. China § Department of Chemistry and Biochemistry, University of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080, United States S Supporting Information *

ABSTRACT: A crystalline microporous hydrogenbonded cross-linked organic framework has been developed through covalent photo-cross-linking of molecular monomers that are assembled in a crystalline state. The elastic framework expands its void space to adsorb iodine rapidly with a high uptake capacity in an aqueous environment as well as recovering its crystalline form after the release of iodine.

C

ovalent organic frameworks (COFs),1 which are crystalline polymers possessing permanent pores, are prominent in gas storage/separation,2 catalysis,3 and energy-related applications.4 The crystalline nature of COFs with defined pore sizes allows precise structural design to selectively sequester environmental pollutants in our ecosystem, e.g., heavy-metal contaminants5 and radioactive wastes,6 which pose increasing threats to public health. Although some COFs have been reported to be stable in acidic or basic aqueous environments,3a,7 the chemical stability of COFs in general limits their practical application5 for adsorbing environmentally impactful ions/molecules, since COFs are synthesized via reversible reactions.8 Improved chemical stability is often associated with trading off the high crystallinity in COFs.1d Compared with COFs, highly crystalline porous molecular materials, such as hydrogen-bonded organic frameworks9 (HOFs) and porous organic molecules10 that rely on weak interactions to stabilize their frameworks, are often too labile for their wide adoption in environmental settings. Developing hydrogen-bonded cross-linked organic f rameworks (HCOFs), which involves chemically cross-linking molecular precursors that have been preorganized into a crystalline state by hydrogen-bonding interactions (Figure 1), will leverage advantages of both COFs and HOFs, thus (i) providing an indepth understanding of the three-dimensional (3D) frameworks, (ii) affording high chemical stability for selective adsorption of environmentally impactful guests, and most importantly (iii) bringing new features such as elasticity to the existing crystalline porous organic materials. In this approach, the molecular precursors first crystallize via multivalent hydrogen-bonding interactions, affording potentially porous © 2017 American Chemical Society

Figure 1. Illustration of the design of HCOFs through SCSC transformation. HCOFs possess guest-induced expandable voids because of the flexible cross-linkers.

molecular materials that are similar to HOFs.9 Solvent molecules in the crystal, however, do not have to be removed before the subsequent chemical cross-linking step, which will enable the utilization of a large library of such potentially porous molecular materials that otherwise lose their permanent porosity after solvent evacuation. Covalently cross-linking these well-organized molecules without interrupting their crystallinity will improve the chemical stability of the network, where successful examples have been demonstrated in related areas.11 As a result, HCOFs can deform through energy dissipation upon solvent or guest molecule “swelling” by breaking hydrogen bonds and subsequently restoring their original Received: March 30, 2017 Published: May 15, 2017 7172

DOI: 10.1021/jacs.7b03204 J. Am. Chem. Soc. 2017, 139, 7172−7175

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and recyclable properties of HCOF-1 and its potential for practical applications in the active enrichment and removal of radioactive iodine isotopes (129I and 131I) that are liberated during nuclear fuel treatment6 and nuclear accidents such as the Fukushima nuclear disaster.14 Our design (Figure 2a) started with the construction of molecular precursor 1, which was synthesized from tetraphenylethylene15 (TPE) in four steps via (1) nitration followed by (2) reduction,16 (3) reaction with cyanuric chloride, and (4) octasubstitution with propargylamine. Similar to melamine motifs in HOFs,9a the alkynylmelamine arms in 1 were designed as hydrogen-bonding motifs as well as reactive precursors for thiol−yne13 cross-linking. Slow vapor diffusion of diethyl ether into a dioxane solution of 1 produces 1crystal in space group C2/m (Figure 2b). There exist three types of channels, two featuring dimensions of ∼15.0 Å × 10.4 Å and ∼7.3 Å × 5.5 Å along the [100] direction (Figure 2d) and another with dimensions of ∼7.3 Å × 6.2 Å along the [101] direction (Figure S33). These micropores are interconnected, allowing cross-linkers to diffuse throughout the crystals by multiple pathways. The solvent-accessible void space of 1crystal is 50.9%. Actively removing dioxane from the crystal lattice results in a loss of crystallinity (Figure S37), suggesting that the solvent is critical in stabilizing the hydrogen-bonded framework. The alkynyl groups adopt several different conformations, and the distances between neighboring alkynyl groups are 6.0, 7.3, 8.1, 8.4, and 9.7 Å (Figure S34). This information suggests that ethanedithiol (HSCH2CH2SH, EDT) can be used as a potential cross-linker to form C−S bonds (∼1.8 Å) between two alkynyl groups that are ∼8 Å apart, since the distance between the sulfur atoms in EDT is 4.4 Å in its anti conformation. Single crystals of 1·2(dioxane) were soaked in neat EDT in the dark overnight and then photoirradiated under a mercury lamp or ambient light. After removal of unreacted EDT, the obtained HCOF-1 was activated using supercritical CO2. Since the thiol−yne reaction proceeds by a double radical hydrothiolation mechanism,13 it requires 8−16 equiv of EDT to consume all of the alkyne groups of 1, with 8 equiv of EDT resulting in a densely cross-linked polymer and 16 equiv affording a dendrimeric product, as confirmed in a solutionphase synthesis (Figure S11). No alkynyl −CC− vibrational band was observed in the Raman spectrum of HCOF-1 (Figure S12), and its solid-state 13C CPMAS NMR spectrum (Figure S7) suggests full alkyne-to-dithioether conversion. Elemental analysis of the cross-linked HCOF-1 reveals the chemical formula of H C OF-1 as [C 62 H 52 N 24 ]·[C 2 H 6 S 2 ] 8.0−8.1 · [CO2]2.5−3.0. Thermogravimetric analysis (TGA) recorded 37% mass loss attributed to [−SCH2CH2S−] between 250− 370 °C (Figure S18), suggesting complete removal of unreacted EDT and >98% thiol-to-thioether conversion. These results suggest that only a stoichiometric amount of EDT (8 equiv) was consumed during the photo-cross-linking. The obtained HCOF-1 crystals maintained their morphology (Figure 1c) as well as their transparency and fluorescence (Figure S15). The diffraction data (Figure S35) were not suitable to resolve the crystal structure of HCOF-1, which may be attributed to the generated chiral centers with random R and S configurations (Figure S46). The unit-cell parameters of HCOF-1 (a = 12.00 Å, b = 25.70 Å, c = 50.17 Å, β = 96°) determined by powder X-ray diffraction (PXRD) are similar to those of its molecular precursor 1 (a = 11.945(2) Å, b = 26.364(5) Å, c = 15.094 (3) Å, β = 98.517(3)°), with the c axis

conformation by reforming non-covalent interactions, thus giving rise to intrinsic elasticity12 at the molecular level. Challenges associated with the development of HCOFs are (1) maintaining their crystallinity when cross-linking the molecular building blocks and (2) preserving the porous structures that remain accessible to guest molecules. Pioneering works by Wuest11f and Schlüter11g,h have shown the successful preparation of cross-linked crystalline materials from molecular crystals through photoirradiated single-crystal-to-single-crystal (SCSC) transformations, where the utilization of mild photoreactions mitigates disruptions to the materials’ crystallinity, although the pore accessibility after photo-cross-linking remains unexplored. Inspired by these pioneering works, herein we report the design and synthesis of HCOF-1 (Figure 2a) through SCSC

Figure 2. (a) Synthesis of HCOF-1 from 1crystal. Inset: optical images of 1crystal and HCOF-1. (b) Crystal structure of 1 connected via intermolecular hydrogen bonds (dashed lines). (c) TEM image of HCOF-1 after sonication. (d, e) Packing diagrams of (d) 1 and (e) HCOF-1 (simulated structure) along the [100] direction with their pore surfaces highlighted in blue/gray. Dioxanes are excluded from the crystal lattice of 1.

transformation from molecular precursor 1 via photoirradiated thiol−yne reactions.13 HCOF-1 adsorbs I2 rapidly in an aqueous environment with high uptake capacity and efficiency, associated with an increase in the material’s density that simplifies its isolation. Interestingly, the adsorbed I2 can interrupt the crystallinity of HCOF-1, which expands its void space to accommodate more I2 beyond its theoretical capacity. The crystallinity of HCOF-1 can be recovered by release of the enriched I2 after solvent evacuation, demonstrating the elastic 7173

DOI: 10.1021/jacs.7b03204 J. Am. Chem. Soc. 2017, 139, 7172−7175

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Journal of the American Chemical Society and cell volume increased 3 times, indicating that the asymmetric unit of HCOF-1 expanded by a factor of 3 after thiol−yne cross-linking compared with the initial cell. Transmission electron microscopy (TEM) analysis of the crystals after sonication (Figure 2c) revealed exfoliated bundles of micrometer-sized fibrillar structures, suggesting that the periodic alignment of 1crystal was well-retained. Compared with the PXRD profile of 1crystal (Figure 3a), the crystalline 3D

Figure 4. (a) A saturated iodine aqueous solution (ca. 1.2 mM) upon addition of HCOF-1 (3.0 mg). (b) SEM image of I2⊂HCOF-1 and the EDS analysis profile. (c) Time-dependent UV/vis absorption spectra of I2 aqueous solution (1.14 mM) upon addition of HCOF-1 (3.0 mg). Inset: calculated I2 adsorption efficiencies at various times. (d) Timedependent UV/vis absorption of I2 desorption from I2⊂HCOF-1 (1.0 mg) in DMSO (10 mL).

Figure 3. PXRD profiles of (a) 1crystal, (b) HCOF-1 after CO2 activation, (c, d) I2⊂HCOF-1 with I2 loading ratios of (c) 0.2 g/g and (d) 2.1 g/g, and (e) recovered HCOF-1 sample after I2 desorption and evacuation of solvents.

morphology with some bending and twisting, and a large amount of iodine was detected by energy-dispersive spectroscopy (EDS). PXRD profiles (Figure 3c,d) of I2⊂HCOF-1 suggest that the framework was transformed from a crystalline form to an amorphous form, with no diffraction peak observed at 0.6−15° (Figure S38). These results suggest that the hydrogen-bonded superstructure of HCOF-1 was interrupted upon I2 adsorption, which may be attributed to the N−H···I hydrogen bonding17 and N···I and S···I halogen bonding18 interactions. Since the dithioether cross-linkers are flexible, HCOF-1 can expand its cavity to accommodate more I2 molecules. Indeed, the I2 uptake capacity measured in I2 vapor at 75 °C further increased to 2.9 ± 0.1 g/g because the energy required to interrupt the hydrogen-bonding network is reduced at an elevated temperature. This I2 uptake capacity of HCOF-1 is comparable to the highest sorption values of porous materials, including metal−organic frameworks (MOFs)19 and porous polymers.17a,20 More importantly, HCOF-1 was able to adsorb I2 directly from the aqueous environment at ambient temperature, which has not been demonstrated by MOFs or porous polymers before. This rapid I2 adsorption and high uptake capacity could be attributed to the hydrophobic nature of the pores that are surrounded by hydrogen-bonding motifs. Notably, the absorbed I2 molecules can be rapidly released from HCOF-1 in dimethyl sulfoxide (DMSO) within 30 min at >93% releasing efficiency with recovery of the crystal’s original color. In comparison, the release speed is considerably slower in MeOH (Figure S25), suggesting that the solvent molecules can compete with the hydrogen-bonding interactions between I2 and HCOF-1, therefore releasing the I2 in solution. After solvent evacuation, the superstructure of HCOF-1 recovered to its original state, as revealed by its PXRD profile (Figure 3e), which was nearly identical to that of HCOF-1. Our results demonstrate a rare example in which an elastic crystalline organic framework can expand and contract its cavity as a result of strong framework−substrate interactions. In summary, we have developed HCOF-1, a microporous cross-linked organic framework that possesses high crystallinity

structure is well-preserved in HCOF-1, with subtle shifting (0.25°) of the (020) peak at 6.99° to a higher 2θ value (Figure 3b), suggesting that the b axis is slightly reduced. The 3D structure of HCOF-1 was simulated on the basis of the singlecrystal X-ray data for its molecular precursor 1 by hypothesizing that the dithioethers are formed between the alkynyl groups that are 8.1 and 8.3 Å apart (Figures 2e, S45, and S46). The obtained HCOF-1 exhibits high chemical stability against a broad spectrum of organic solvents, acidic/basic aqueous solutions (pH = 0−14; Figure S39), and UV irradiation (7 days). The crystal samples of HCOF-1 float (Figure 4a) on the surface of the I2 aqueous solution initially due to its low density (ρ = 0.925 g/cm3). Within 30 min with some mechanical agitation, the crystals gradually turned darker and sank to bottom of the vial. Practically, this density increase of HCOF-1 upon I2 adsorption greatly simplifies the collection of I2⊂HCOF-1. A time-dependent UV/vis experiment (Figure 4c) showed that upon the addition of HCOF-1, the I2 concentration decreased from 288 to 18 ppm within 30 min and below 1 ppm after 24 h. HCOF-1 is also capable of adsorbing iodine in other solvents, with an uptake ability of H2O > MeOH ≈ cyclohexane (Figure S24). The I2 uptake capacity of HCOF-1 in aqueous solution was accurately measured as 2.1 ± 0.1 g of I2 per gram of HCOF-1 by immersing the crystal samples in a high-concentration aqueous solution of KI3, where the dynamic equilibrium of I3− ⇄ I2 + I− in KI3 solution allows dynamic release of I2, and this value was also confirmed by I2 titration, elemental analysis, and TGA experiments (see the Supporting Information). The I 2 adsorption capacity suggests a maximum of 16.5 I2 molecules per TPE moiety in HCOF-1, which exceeds its theoretical uptake capacity based on the void volume in the simulated structure of HCOF-1 (Figure S47). A scanning electron microscopy (SEM) image (Figure 4c) showed an iodine-enriched sample possessing crystal-like 7174

DOI: 10.1021/jacs.7b03204 J. Am. Chem. Soc. 2017, 139, 7172−7175

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and chemical stability, via single-crystal-to-single-crystal transformation from its molecular precursor. The synthesized HCOF-1 adsorbs I2 rapidly in the aqueous environment, associated with a significant density change as well as a crystalline-to-amorphous morphology transformation. The adsorbed I2 molecules interrupt the superstructure of HCOF1, which expands its framework to host more substrate molecules. Releasing the I2 from HCOF-1 restores its crystallinity. Our strategy enables the employment of a plethora of irreversible reactions and flexible linkers for the design of crystalline porous organic frameworks and their potential applications in pollutant sequestration. Furthermore, it demonstrates a general pathway for the development of elastic porous organic materials that selectively adsorb guest molecules and dynamically change their guest-accessible pore size.

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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/jacs.7b03204. Experimental details, iodine sorption studies, porosity measurements, and simulation details (PDF) Crystallographic data for 1 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Chenfeng Ke: 0000-0002-4689-8923 Author Contributions ∥

Y.L. and X.J. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Dartmouth College for startup funds. We gratefully acknowledge Prof. Richard Staples (Michigan State University) for X-ray data.

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REFERENCES

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DOI: 10.1021/jacs.7b03204 J. Am. Chem. Soc. 2017, 139, 7172−7175