Switchable Surface Hydrophobicity-Hydrophilicity of a Metal-Organic Framework

: Materials whose surface can be switched from high/superhydrophobicity to superhydrophilicity are useful for myriad applications. Herein we report a metal-organic framework (MOF) assembled from Zn II ions, 1,4-benzenedicarboxylate and a hydrophobic carborane-based linker, whose crystal-surface can be switched between hydrophobic and superhydrophilic. Switching is achieved through chemical treatment to remove some of the building blocks.

Smart solid surfaces that exhibit switchable wettabilitymainly between high or superhydrophobicity, and superhydrophilicityare actively being sought due to their diverse potential applications such as self-cleaning materials, microfluidics, tunable optical lenses, drug delivery vectors, and sensors. [1,2] In general, the wettability of a solid surface can be tuned by modifying its surface geometry and/or its chemical composition. For example, superhydrophobic surfaces can be fabricated by creating structures that mimic the rough surface of the lotus leaf. [3,4] However, when such lotus leaf-like surfaces are built from materials that response to specific stimuli, they can exhibit switchable wettability. For instance, a lotus leaf-like surface built from the photosensitive material TiO2 can exhibit light-dependent switching. [5] To date, the majority of materials that exhibit switchable wettabilityand therefore, that could be used for fabricating such smart surfacescomprise photo-sensitive inorganic oxides (e.g. TiO2 [4] and ZnO, [6] ); organic polymers (e.g. light-responsive azobenzene-containing polymers, [7] temperature-responsive poly-N-isopropylacrylamide, [8] and electrical-responsive polypyrrole [9] ); self-assembled monolayers (SAMs; e.g. electrical-responsive 16-mercaptohexanoic SAMs [10] and pH-responsive mixed SAMs [11] ); and counterion exchange-responsive ionic liquids. [12] Herein we report that metal-organic frameworks (MOFs) (also known as porous coordination polymers [PCPs]) can be a novel class of responsive materials that exhibit switching of their crystal surface, between hydrophobic to superhydrophilic. Researchers have recently begun to investigate hydrophobic and superhydrophobic MOFs to enhance their aqueous stability and enable new applications (e.g. self-cleaning). [13] Some advances have been made to this end. For instance, Kitagawa et al. reported a superhydrophobic MOF, which they obtained by controlling the corrugation of its crystal surface by using aromatic linkers. [14] However, the most common strategy to synthesize these hydrophobic MOFs is via introduction of hydrophobic molecules as linkers (e.g. trifluoromethyl groups, [15,16] carboranes, [17] and alkyl chains [18,19] ) or as guests. [20] Among these hydrophobic molecules, carboranes are an interesting class of exceptionally stable boron-rich clusters that possess material-favorable properties such as thermal and chemical stability. [21] Here, we have exploited this class of molecules to synthesize a responsive MOF that exhibits switching between hydrophobicity and superhydrophilicity. Specifically, we used an ortho-carborane functionalized with pyridylmethylalcohol groups at the C-positions [22] (hereafter, "oCB-L", Figure 1a) as a hydrophobic linker in combination with Zn-bdc (where bdc is 1,4-benzenedicarboxylate). This switching behaviour is achieved by alternatively exposing the MOF to NaOH/DMF solution and to slightly acidic aqueous solution.
Thermogravimetric analysis (TGA) of oCB-MOF-1 shows a continuous weight loss of 22.3% from 90 o C to 250 ºC, which we attributed to the loss of the four guest and two coordinated DMF molecules (calcd. 24.9%; Supporting Information, Figure S3). Above 350 o C, this framework decomposes over multiple steps. Elemental analysis, infrared (IR-ATR) and TGA measurements of a sample heated under vacuum at 85 ºC for 12 h confirmed the removal of the four guest DMF molecules (Supporting Information, Figure S4, Table S3). The IR spectrum confirmed the presence of DMF molecules in the activated oCB-MOF-1', evidenced by a strong peak at 1650 cm -1 . In the same direction, TGA analysis of oCB-MOF-1' revealed initial weight loss of 8.3%, from 120 o C to 250 ºC, corresponding to loss of the two coordinated DMF molecules (calcd. 10.0 %). Additionally, PXRD measurements showed that oCB-MOF-1' retains its original structure. Sorption measurements revealed that desolvated oCB-MOF-1' is nonporous to N2 at 77 K and 1 bar. However, it is porous to CO2 (69.4 cm 3 g -1 at 0.9 bar; BET surface area: 296 m 2 g -1 ) at 195 K and 0.8 bar, for which it showed reversible type-I isotherms (Supporting Information, Figure S5). We reasoned that this selectivity could be explained by the pore aperture (3.2 x 6.4 Å), which is large enough to be accessible for CO2 but not for N2 (kinetic diameters for CO2: 3.30 Å and N2: 3.7 Å). The isosteric heats of adsorption of CO2 (Qst) were derived from the Claussius-Clapeyron equation, using the adsorption branches of the isotherms measured at 288 K, 273 K and 258 K. The Qst of oCB-MOF-1´ was 29.2 kJmol -1 at low coverage, decreasing at a rate of up to 26.4 kJmol -1 at high coverage. Note here that when oCB-MOF-1 was heated at 200 ºC for 12 h, all six DMF molecules were removed (Supporting Information, Figure S4, Table S3). However, PXRD analysis showed a loss of crystallinity, and CO2 sorption measurements revealed a decrease of 35% of the BET area compared to that of oCB-MOF-1'.
Having determined that oCB-MOF-1 retains porosity, we then evaluated the influence of the carborane units on its hydrophobic properties. Thus, we performed water contact-angle measurements of crystalline powder of as-synthesized oCB-MOF-1 and desolvated oCB-MOF-1' packed on a glass surface. The contact angle (Ɵc) in each case was 140° and 138°, respectivelyvalues which are characteristic of a highly hydrophobic solid. We finally characterized the contact angle of oCB-MOF-1 shaped in the form of a disk pellet (diameter = 13 mm), which was fabricated by pressing a dry crystalline powder under a pressure of 10 tons for 5 min (Figure 2). This pellet, which showed a roughness factor r (defined as the ratio of the actual area of the rough surface to its projected area on the horizontal plane) of 1.7  0.1, was also found to be hydrophobic with a Ɵc of 108°. We also quantified the contact-angle hysteresis (CAH) that was found to be 32° (Supporting Information, Table S4). This relatively large CAH was attributed to the surface roughness of the disk pellet that can be the responsible for the contact line pinning. The hydrophobic character of oCB-MOF-1' is also revealed by water-vapor adsorption measurements. Similarly to other highly hydrophobic or superhydrophobic MOFs (e.g. ZIF-8 and perfluorinated MOFs), [25] oCB-MOF-1' can barely adsorb water: at 95% relative humidity, it exhibits a very low uptake of only 0.05 g H2O/g oCB-MOF-1' (Supporting Information, Figure  S6). This low water adsorption clearly confirms that the pore surface of oCB-MOF-1' is hydrophobic and as such, would prevent entry of water into the pores. Due to the high hydrophobicity of oCB-MOF-1, it is highly stable in water. For example, oCB-MOF-1 is stable when incubated in liquid water over a wide pH range (from 2 to 12; pH adjusted with HCl or NaOH) for at least 15 h at room temperature (Supporting Information, Figure S8). Indeed, the simulated (derived from the single crystal structure of oCB-MOF-1) and experimental (resulting from the powders incubated at different pHs) PXRD patterns suggest that minor structural transformations occur under these conditions, which are however reversible after immersing these powders in DMF for 2 h.
Interestingly, we observed that oCB-MOF-1, either handpacked or shaped in the form of a disk pellet, undergoes a switch from hydrophobic (Ɵc = 140° from the hand-packed; Ɵc = 108° from the pellet) to superhydrophilic (Ɵc = 0° in both cases) upon immersion in a solution of NaOH (10 eq.) in DMF for 1 h at room temperature. Moreover, this switch was reversible: initial hydrophobicity (Ɵc = 137° from the hand packed; Ɵc = 101° from the pellet) was fully recovered when hydrophilic oCB-MOF-1 was immersed in H2O (pH < 6.5) for 1 h at room temperature ( Figure 2). Note that the roughness factor r of the disk pellet as well as the CAH did not varied significantly during this switching process, evidencing that the chemical nature of oCB-MOF-1 is mainly responsible of this switching phenomena. The r value calculated from the hydrophilic disk pellet was 1.7  0.1, whereas that from the recovered hydrophobic pellet was 1.5  0.1. In this latter, the CAH was found to be 35º. Remarkably, we were able to demonstrate at least two complete cycles of switching without observing any significant loss in hydrophobicity (Figure 2d and Supporting Information, Figure S8).
We attributed the switching phenomenon to changes in the surface chemistry of the oCB-MOF-1 crystals. Thus, we hypothesized that under basic conditions, the hydrophobic oCB-L linkers are selectively deboronated to their corresponding nido species [26] and/or extracted [27] from the crystal surface. The consequence of these processes would be that the more hydrophilic nido species and/or Zn4(bdc)2 layers are mainly exposed to the crystal surface, making it hydrophilic. To verify these assumptions, we analyzed the supernatant from the NaOH/DMF treatment by 1 H-and 11 B-{ 1 H}-NMR spectroscopy, MS spectrometry and ICP-MS (Figures 2 and Supporting  Information, Figures S9a, S10a, S11a). Remarkably, 11 B-{ 1 H}and 1 H-NMR, and MS, indicated only the nido species resulting from the deboronation of the oCB-L linker (and not the closo form of the oCB-L and bdc linkers), whereas ICP measurements confirmed the absence of any Zn II ions. These observations are consistent with the fact that mainly oCB-L linkers from the crystal surface are selectively removed (either via deboronation and subsequent detachment, or vice versa), which exposes the more hydrophilic Zn4(bdc)2 layers on the crystal surface.
Here, we would like to highlight that we obtained identical results as above, when we performed the treatment with Et3N in DMF (85 ºC, 3 h), NaOH in MeOH (r.t., 1 h), and NaOH in CH3CN (r.t., 1 h) ( Supporting Information, Figures S9b, S10b,   Figure 2. a) Contact angle images of oCB-MOF-1 (left column, disk pellet; right column, hand-packed) before treatment (top), after immersion in a DMF solution containing 10 equivalents of NaOH (middle) and after immersion in H2O (bottom).; b) In the first treatment, only carborane nido species were detected in the 1 Hand 11 B-{ 1 H}-NMR spectra of the supernatant. Treatment with water leads to release of the bdc linkers and Zn II ions, as detected by 1 H-and 11 B-{ 1 H}-NMR and ICP, respectively; c) Photographs of the press disk pellet of oCB-MOF-1; d) Static contact angle values for two cycles of treatment (see text for details). S12-13). This deboronation reaction is well known to occur in carboranes by a wide variety of nucleophiles in various basic conditions. [21,26] However, when we performed the treatment using aqueous NaOH (r.t., 1 h), we did not observe any changes in the crystal hydrophobicity (Ɵc = 139° from hand-packed). We rationalized this result by considering the non-wettability of the highly hydrophobic crystal surface of as-synthesized oCB-MOF-1 to water (in sharp contrast to the organic solvents that we had tested: DMF, MeOH or CH3CN).
Contrariwise, we attributed the recovery of the initial hydrophobicity to the removal, mainly, of the hydrophilic Zn4(bdc)2 layers under slightly acidic aqueous conditions (which is probably due to the protonation of bdc linkers). This removal would again leave the hydrophobic oCB-L linkers exposed on the crystal surface. At this point, 1 H-and 11 B-{ 1 H}-NMR spectra of the supernatant showed the presence of the bdc linker but not of any carborane species (Supporting Information, Figures S9c, S10c, S11b). Consistently, ICP measurements confirmed the release of Zn II ions during the treatment (13.5 ppm), whereas the MS spectrum showed mainly the bdc linker, plus a weak peak corresponding to the nido oCB-L linker. An ideal mechanism for such switching behavior is shown in Figure 3.
To demonstrate that this switchable phenomenon is due chiefly to the crystal surface and not to any internal modifications, we ground a sample of superhydrophilic oCB-MOF-1 crystals to break them up and physically re-expose the hydrophobic oCB-L linkers on the crystal surface. As expected, we immediately observed a pronounced increase in the hydrophobicity of the ground crystals, which exhibited a Ɵc of 129° (from handpacked; Supporting Information, Figure S14). This result is consistent with the non-observation of any significant modifications in the crystallinity or in the CO2 sorption capabilities of oCB-MOF-1' throughout the switching process (Supporting Information, Figures S15-16). In fact, the only significant variation that we observed during this process was in the water sorption properties. Initially, the hydrophobic oCB-MOF-1' exhibited a type-II isotherm, but upon switching, the resultant hydrophilic oCB-MOF-1' showed a type-V isotherm, which corresponds to an increase in uptake at a relative pressure of 0.3 (Supporting Information, Figure S6). Although neither sample showed any significant water uptake, the uptake (0.09 gg -1 ) in the hydrophilic oCB-MOF-1' was twice that in the hydrophobic one (0.05 gg -1 ). A similar trend has been reported between ZIF-8 and its aldehyde-functionalized SIM-1 analog. [28] These differences at low pressure can be attributed to the presence of a higher number of polar groups on the crystal surface in the hydrophilic oCB-MOF-1. Note here that initial type-II isotherm with a maximum uptake of 0.05 gg -1 was recovered when switching again to the hydrophobic oCB-MOF-1 (Supporting Information, Figure S6).
In conclusion, we have reported a novel MOF, oCB-MOF-1, whose crystal-surface can be switched between hydrophobic and superhydrophilic via chemical treatment. It was assembled from Zn II ions, 1,4-benzenedicarboxylate and a hydrophobic carborane-based linker, which connects the constituent Zn4(bdc)2 2D layers to yield the 3D network of the final MOF. We found that the carborane moieties confer enhanced hydrophobicity to this MOF (Ɵc = 140°). Given the versatile compositions of MOFs, and the fact that they can be fabricated by design, we hope that our work here facilitates development of MOFs with reversible wettability properties triggered by stimuli such as light, temperature or pH. Such materials would ultimately prove utile for obtaining smart porous surfaces (e.g. membranes and coatings) that exhibit switchable wettability. COMMUNICATION A metal-organic framework, based on 2D layers of Zn4(bdc)2 connected through a pillaring hydrophobic carborane based linker, undergoes a switching from hydrophobic to superhydrophilic, and vice versa, upon chemical treatment.