Self-Recognizing π-π Stacking Interactions Designed for the Generation of Ultrastable Mesoporous Hydrogen-Bonded Organic Frameworks
Creating crystalline porous materials with large pores is typically challenging due to undesired interpenetration, staggered stacking, or weakened framework stability. Here, we report a pore size expansion strategy by self-recognizing π-π stacking interactions in a series of two-dimensional (2D) hydrogenbonded organic frameworks (HOFs), HOF-10x (x=0,1,2), self-assembled from pyrene-based tectons with systematic elongation of π-conjugated molecular arms. This strategy successfully avoids interpenetration or staggered stacking and expands the pore size of HOF materials to access mesoporous HOF-102, which features a surface area of ~ 2,500 m 2 /g and the largest pore volume (1.3 cm 3 /g) to date among all reported HOFs. More importantly, HOF-102 shows significantly enhanced thermal and chemical stability as evidenced by powder x-ray diffraction and N 2 isotherms after treatments in challenging conditions. Such stability enables the adsorption of dyes and cytochrome c from aqueous media by HOF-102 and affords a processible HOF-102/fiber composite for the efficient photochemical detoxification of a mustard gas simulant.
Introduction<!>Results and discussion<!>Thermal and chemical stability tests. To experimentally support the calculated π-π energies in these<!>Fabrication of HOF/fiber composites for photocatalysis. Inspired by the observed high stability of<!>Conclusions<!>Methods
<p>Recently, hydrogen-bonded organic frameworks (HOFs), [1][2][3][4] self-assembled from pre-designed molecular tectons using intermolecular H-bonding interactions, are rapidly expanding into a library of novel functional crystalline porous materials with diverse structures and applications including gas storage and separation, 5,6 chiral separation, 7 chemical sensing, 8 proton conduction, 9 and catalysis. 10,11 However, since the first report of HOF materials with permanent microporosity about a decade ago, 12 the development of HOFs has been hindered due to poor stability as a result of the weak H-bonding nature. Alternatively, various synthetic porous solids such as silicates, 13 carbons, 14 MOFs, 15 and COFs 16 have achieved a wide range of pore sizes from micro-to mesopores. Thus, the augmentation of pore size within HOFs to also access a stable HOF with permanent mesoporosity remains a long-standing challenge.</p><p>The isoreticular expansion strategy has been impactful for the construction of MOFs [17][18][19] and COFs 20,21 with large pores and high surface areas. However, direct extension of ligands to prepare mesoporous HOFs with three-dimensional (3D) structures has typically resulted in a high degree of framework interpenetration, [22][23][24] significantly reducing the open aperture size and limiting the inclusion of large guest molecules. Besides, large pore size and high stability are usually contradictory to each other and rarely combined into one single porous material. 25,26 Thus, development of an alternative pore size expansion strategy for the construction of stable mesoporous HOF materials is highly desirable. Among various approaches to stabilize HOFs, design of two-dimensional (2D) HOFs with intermolecular π-π interactions was found to be the most effective. 1 π-π stacking interactions are ubiquitous in both Nature, driving nucleobase stacking within DNA and RNA molecules, and in synthetic materials, prompting molecular recognition between various aromatics. 27 Though π-π stacking interactions are not desirable for high surface areas, 28 co-planar π-conjugated large aromatic tectons are ideal for stable HOFs due to their inert reactivity and high chemical resistance to solvent, acid and base. Such tectons usually form 2D layers by H-bonding while cooperative π-π stacking interactions exist between 2D layers, which has proven to be a reliable approach to stabilize HOF materials. 8,11,[29][30][31][32] For example, a series of HOFs with isostructural or quasi-isostructural honeycomb frameworks has been prepared by assembling tectons with C 3 -or C 6 -symmetries. 29,33 However, as the length of tectons increases, the π-π stacking between 2D layers tends to be in a staggered manner rather than eclipsed, which leads to a decrease in the pore size of these HOFs. In contrast to the eclipsed stacking in H-bonding self-assembly of tectons with one kind of fuse aromatic ring, the staggered stacking is typically observed when tectons with two or more same type of aromatic rings is used (Scheme 1A and 1B). To address this challenge, we propose to design tectons with two different kinds of connected bulky fused aromatic rings. First, different kinds of bulky fused aromatic rings favor to form self-recognizing π-π stacking, thus efficiently preventing staggered stacking (Scheme 1C). Second, this strategy can increase the pore size and π-π stacking interactions areas between 2D layers simultaneously, which we hoped would lead to very stable HOF materials.</p><p>To test our hypothesis, we designed and synthesized a series of HOFs, HOF-10x (x=0,1,2), using tecton molecules based on a pyrene core connected by four molecular arms terminated with carboxylate groups.</p><p>All three tectons form 2D H-bonded layer structures, which are further stabilized by the eclipsed π-π stacking between layers. More importantly, a systematic investigation of the crystal structure, permanent porosity, and chemical/thermal stability of three isostructural pyrene-based HOFs indicates that the expansion of tectons with self-recognizing large fused aromatic rings demonstrates to be an efficient pore expansion strategy without sacrificing the framework stability.</p><!><p>Synthesis and characterization of isostructural HOFs. Pyrene, a classic fused benzene ring molecule with planar conformation and large π-conjugated system, is chosen as the center of the tectons. Previously, 1,3,6,8-tetrakis (p-benzoic acid) pyrene (H 4 TBAPy), has been used to construct a stable HOF, known as PFC-1 11 (we name it as HOF-101 as this material was made prior to the publication of PFC-1).</p><p>To systematically study the influence of intermolecular π-π interactions on the stability of HOFs, two additional pyrene-based tectons analogues to H 4 TBAPy (Figure 1A,B,C), 1,3,6,8-tetracarboxy pyrene (H 4 TCPy) (Figure S1-S2) and 1,3,6,8-tetra(6-carboxynaphthalen-2-yl) pyrene (H 4 TNAPy) were synthesized to build an isostructural HOF family. Microcrystals of the HOF materials (HOF-100, HOF-101, and HOF-102) can be easily made via a fast precipitation method by introducing acetone into a concentrated N,N-dimethylformamide (DMF) solution of corresponding tectons (Figure S3-S5). However, the crystal structures of HOF-100 and HOF-102 were not fully solved by single-crystal X-ray diffraction measurements and only their unit cells were determined. With the experimental unit cell and a molecular modeling method, we simulated the crystal structure of HOF-100 and HOF-102 from powder x-ray diffractions (PXRD) (Table S1, Figure S6-S7). As expected, the resulting HOFs featured a H-bonding self-assembled 2D layer structure through carboxy dimer complementary H-bonds. See Figure 1. Each layer stacks with the neighboring one as AA mode along the (100) direction to form one-dimensional channels. The resulting channels in these HOFs show a systematic increase of pore size (diagonal di-mensions: 0.8 ×1.2 nm for HOF-100, 1.8 ×2.4 nm for HOF-101, and 2.5 ×3.0 nm for HOF-102). Further examination of the neighboring layers of these HOFs indicates strong π-π interactions (Figure 1D,E,F). The porosity of solvent free (activated) HOF-100, HOF-101, and HOF-102 was determined through N 2 physisorption measurements isotherms collected at 77 K (Figure 2A-C). The following trend of N 2 uptake capacity was observed: HOF-100 (235 cm 3 /g) < HOF-101 (605 cm 3 /g) < HOF-102 (815 cm 3 /g). Importantly, HOF-100 and HOF-101 (a small step of mesopore uptake was observed) showed typical type-I isotherms of microporous materials, while HOF-102 displayed a type-IV isotherms, indicative of mesoporous materials. The Brunauer-Emmett-Teller (BET) areas were calculated to be 900 m 2 g for HOF-100, 2,100 m 2 g for HOF-101, and 2,500 m 2 g for HOF-102. Additionally, the pore volume of HOF-102 (1.3 cm 3 /g), to the best of our knowledge, is the highest among HOF materials (Table S2). To accurately determine the pore size distribution of activated HOF-102, an Ar physisorption measurement was performed at 87 K and yielded a type-IV isotherm as expected. The pore size distribution obtained using an NLDFT model indicated a mesopore centered at 2.8 nm (Figure S8). To evaluate the π-π interaction energies of these HOFs, we performed DFT calculations with both dispersion and counterpoise corrections on the π-π stacked dimers (details in SI). The energy dramatically increased from -53.4 kJ/mol for HOF-100, -137 kJ/mol for HOF-101(PFC-1), to -208 kJ/mol for HOF-102 (Table S3). The values for HOF-101 and HOF-102 were higher than that of H-bonding energy (-99.4 kJ/mol), demonstrating that structural stability of these HOFs can be attributed in part to the π-π stacking interactions.</p><!><p>HOFs, we probed their thermal and chemical stabilities. First, we tested the thermal stability by activating the HOFs materials under 100 °C and 200 °C and then compared the N 2 isotherms (Figure 2A-C).</p><p>HOF-102 exhibited only a small decrease (13%) in N 2 uptake at the higher temperature, while the losses in HOF-101 (67%) and HOF-100 (84%) were larger. In addition, according to thermogravimetric analysis (TGA) under nitrogen (Figure 2D), the thermal decomposition temperatures of HOFs are in the order: 370 °C (HOF-102) > 310 °C (HOF-101) > 250 °C (HOF-100). In situ variable -temperature PXRD showed PXRD peak broadening and shifting for HOF-101 at 275 °C (Figure 2E), while the crystallinity of HOF-102 remained unchanged up to 320 °C (Figure 2F). To investigate the chemical stabilities of these HOFs, we exposed HOF-10x (x=0,1,2) to a series of challenging conditions, including different organic solvents (such as THF, DCM, EtOH, acetone), water vapor, boiling water, PBS buffer, pH=1/pH=12/pH=14 aqueous solution, concentrated HCl (12M), and H 2 SO 4 (18M) solution (Table 1). HOF-102 maintained its crystallinity after treatment under these conditions, as demonstrated by PXRD (Figure 3A), mass balance studies (Figure S9), and N 2 isotherms after treatments (Figure 3B).</p><p>In comparison, the chemical stabilities of HOF-10x (x=0,1,2) are in agreement with the π-π energy calculation trend. HOF-100 and HOF-101 both dissolve quickly in NaOH solution (pH =12) solution, and HOF-100 loses its crystallinity in the presence of boiling water and concentrated H 2 SO 4 (Figure S10-S11). To further compare the chemical stabilities of these HOFs with other porous materials, several highly robust, benchmark MOFs (HKUST-1 34 , UiO-66, 35 and ZIF-8 36 ) were also tested (Figure S12-S15). The MOFs did not retain their crystallinity under certain conditions (Table 1 and Figure S16-S18), most notably under PBS buffer, strong inorganic acid and pH=14 basic solution.</p><p>The surface wettability of the HOFs was characterized using contact-angle measurements (Figure S19).</p><p>The contact angles of water and decane on HOF-10x (x=0,1,2) were between 8° and 15°, indicating the amphiphilic nature of these materials. The exceptional stability of HOF-102 prompted us to explore the adsorption performance of water vapor and ammonia. The water vapor and ammonia isotherms of HOF-102 were measured at 298 K (Figure 3C and 3D). The PXRD measurements suggest that HOF-102 is still crystalline in the presence of water vapor and NH 3 (Figure S20). Therefore, the ultra-high chemical stability of HOF-102 against harsh aqueous conditions was not due to its hydrophobicity and can be attributed to the high π-π stacking interactions. The water vapor loading capacity of HOF-102 at 1 atm is 0.7g/g, and the ammonia uptake capacity is about 250 cm 3 /g at 1 atm, which are comparable with the water vapor and NH 3 sorption abilities of MOF materials. [37][38][39] Adsorption of dyes and biomolecules in water. To take advantage of this chemical stability, we explored the potential of HOF-102 as a sorbent in water media and tested its ability to adsorb methylene blue (MB). The uptake kinetics were measured at a 200 ppm initial concentration by measuring the change of UV-vis spectra at given time intervals (Figure S21-S22). Remarkably, MB was absorbed by HOF-102 quickly from the aqueous solution within a few minutes. The adsorbed MB was washed away by organic solvent such that HOF-102 was reused for at least 6 times without losing its adsorption capabilities (Figure 4A-B). To understand the adsorption of MB on HOF-102, isothermal titration calorimetry (ITC) experiments (Figure S23 and Table S4) were performed, and the calculated binding affinity (K a = 3.89 ± 0.22 × 10 4 M -1 ) for MB on HOF-102 is higher than the reported value using carbon nanotube as a sorbent. 40 Encouraged by the sorption performance of HOF-102 in water, cytochrome c (Cyt-c, 3.6×2.4×2.2 nm) 41 was selected to probe the capability of HOF-102 as an adsorbent for macromolecules. It was expected that Cyt-c could access the pores of HOF-102 because of the similar dimensions of the square aperture of HOF-102 (2.4 nm) with the short dimensions of Cyt-c. Activated HOF-102 was soaked in a buffer solution of Cyt-c, and the macromolecule's concentration was monitored by UV-vis spectroscopy in solution (Figure S24). The complete adsorption of Cyt-c occurred within four hours (Figure 4C). The color of HOF-102 turned from yellow to orange to form a Cyt-c@HOF-102 composite, and the sorption capacity was ~30 wt. %. Retained crystallinity was observed in the PXRD of Cyt-c@HOF-102 (Figure S25). N 2 isotherms (Figure 4D) and pore size distributions (Figure S26) indicated the occupation of Cyt-c within the mesopores of HOF-102 due to a significant decrease of pore volume.</p><!><p>HOF-102, we set out to fabricate HOF-102 on fibers to form flexible and wearable materials. An acetone suspension of HOF-102 nanocrystals was dropcast onto a cotton fiber surface followed by solvent evaporation at ambient conditions (Figure 5A and 5B). The uniform HOF-102 coating on fiber was observed through scanning electron microscopy (SEM) images (Figure 5C-D). The crystallinity and poros-ity of the HOF/fiber composite were examined by PXRD and N 2 isotherms (Figure S27-S28), which were in good agreement to the free powder samples of HOF-102. Previously, our group has demonstrated that the H 4 TBAPy based MOF NU-1000 42 can be used as a stable photocatalyst for the selective aerobic oxidation of the mustard gas simulant, 2-chloroethyl ethyl sulfide (CEES), to the nontoxic product 2-chloroethyl ethyl sulfoxide (CEESO) by LED (light emitting diode) irradiation of appropriate wavelength. As a proof of concept for the utility of the composite, we investigated the performance of HOF-102/fiber as a composite photocatalyst by utilizing the pyrene-based chromophores in the HOF structure. With 1 mol% catalyst used (based on pyrene), the HOF-102/fiber composite converted 48% of the starting material in 5 minutes and 100% in 30 minutes (Figure 5E), as monitored through gas chromatography-mass spectrometry (GC-MS). The selectivity of the catalytic reaction for the monooxygenated product (CEESO) over the overoxidized toxic 2-chloroethyl ethyl sulfone (CEESO 2 ) was confirmed via 1 H nuclear magnetic resonance (NMR) spectroscopy of the reaction mixture (Figure S29-S30). Furthermore, it was reused for at least three times without compromising catalytic activity (Figure 5F) due to bleaching of the chromophore, and the maintained crystallinity in the PXRD of HOF-102 after catalysis demonstrated the material's chemical stability under the reaction conditions.</p><!><p>A new pore-expansion strategy based on self-recognizing intermolecular π-π stacking interactions was developed to prepare a series of isostructural hydrogen-bonded organic framework materials, HOF-10x (x=0,1,2), with tunable pore size and enhanced stabilities. The mesoporous HOF-102 was stable in harsh conditions such as PBS buffer solution and concentrated acids/bases, indicating viability as an efficient and durable adsorbent material for Cyt c in aqueous solution. The developed strategy to access high stability in HOF-102 also demonstrated solution processability with the deposition of the material on fiber to obtain a photochemically active composite material. We are hopeful that this work will open an avenue for the rational design and synthesis of additional mesoporous HOF materials with impressive stabilities and solution processabilities.</p><!><p>Detailed procedures for the tecton synthesis are reported in the Supplementary Information.</p><p>Gener al pr ocedur e for HOF synthesis. HOF-10x (x=0,1,2) was synthesized by introducing acetone into linker solution in DMF. With HOF-102 as an example, H 4 TNAPy (200 mg, 0.225 mmol) was dissolved in 60 mL of DMF under heating at 120 °C for 1 h to get a clear yellow solution. After cooling down to room temperature, H 4 TNAPy solution was poured into 160 mL of acetone under stirring (400 rpm) within 2 min. The suspension was kept stirring for 12 h and isolated by centrifugation at 8000 rpm for 5 min. The obtained yellow powder was further washed with acetone (4 x 45 mL), and then dried at room temperature (yield 178 mg, 89%).</p><p>Gener al pr ocedur e for chemical stability test. 50 mg of HOF or MOF powder was immersed into 30 mL of HCl (12 M), H 2 SO 4 (18 M), water (pH 1, 12 and 14), PBS (pH 7.2), or boiling water for 24 h.</p><p>The samples were then isolated by centrifugation and washed by water (3 x 45 mL) and acetone (3 x 45 mL). Then, they were dried under vacuum at 80 °C for 12 hours before PXRD or N 2 adsorption test.</p><p>Gener al pr ocedur es for fabr ication of HOF/fiber composite. 70 mg HOF-102 was dispersed into 2 mL of acetone under ultrasonication for 2 min to get a uniform suspension. The 1 mL of suspension was drop cast onto cotton textile (400 mg, 5 cm x 5 cm) carefully, and the textile was dried in a hood overnight to evaporate the solvent. Then the drop casting procedure was repeated again on the dried textile piece to get a mass loading of about 15% on the fiber.</p>