Calix[4]arene-based Porous Organic Nanosheets - ACS Applied

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Functional Nanostructured Materials (including low-D carbon)

Calix[4]arene-based Porous Organic Nanosheets Dinesh Shetty, Tina Skorjanc, Jesus Raya, Sudhir Kumar Sharma, Ilma Jahovic, Kyriaki Polychronopoulou, Zouhair Asfari, Dong Suk Han, Sajeewa Dewage, John-Carl Olsen, Ramesh Jagannathan, Serdal Kirmizialtin, and Ali Trabolsi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03800 • Publication Date (Web): 24 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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ACS Applied Materials & Interfaces

Calix[4]arene-based Porous Organic Nanosheets Dinesh Shetty,† Tina Skorjanc,† Jesus Raya,φ Sudhir Kumar Sharma,┴ Ilma Jahovic,† Kyriaki Polychronopoulou,‡ Zouhair Asfari,ǁ Dong Suk Han,╨ Sajeewa Dewage,† John-Carl Olsen,§ Ramesh Jagannathan,┴ Serdal Kirmizialtin*,† & Ali Trabolsi*,† †

New York University Abu Dhabi (NYUAD), Saadiyat Island, UAE. Engineering Division, New York University Abu Dhabi, Saadiyat Island, UAE.

┴ φ

CNRS/Université de Strasbourg 1, rue Blaise Pascal, Strasbourg – France 67000.

‡

Mechanical Engineering Department, Khalifa University of Science and Technology, P.O. Box 127788, Abu Dhabi, UAE. ǁ Laboratoire de Chimie Analytique et Sciences Séparatives, Institut Pluridisciplinaire Hubert Curien, 67087 Strasbourg Cedex, France. ╨

Chemical Engineering Program, Texas A&M University at Qatar Education City, Doha, Qatar

§

Department of Chemistry, University of Rochester, RC Box 270216, Rochester, NY 14607-0216, USA.

ABSTRACT: Calixarenes are a common motif in supramolecular chemistry but have rarely been incorporated in structurally well-defined covalent 2D materials. Such a task is challenging, especially without a template, because of the non-planar nature and conformational flexibility of the calixarene ring. Here, we report the first-of-kind solvothermal synthesis of a calix[4]arene-based 2D polymer (CX4-NS), that is porous, covalent, and isolated as few-layer thick (3.52 nm) nanosheets. Experimental and theoretical characterization of the nanosheets are presented. AFM and TEM results are consistent with the calculated lowest energy state of the polymer. In the lowest energy state, parallel layers are tightly packed, and the calixarenes adopt the 1,2-alternate conformation, which gives rise to a two-dimensional pattern and a rhombic unit cell. We tested the material's ability to adsorb I2 vapor and observed a maximum capacity of 114 wt%. Molecular simulations extended to model I2 capture showed excellent agreement with experiments. Furthermore, the material was easily regenerated by mild ethanol washings and could be reused with minimal loss of efficiency.

Keywords: calix[4]are; organic nanosheets; iodine adsorption; molecular dynamics; porous materials

INTRODUCTION The discovery of isolated graphene1-4 in 2004 triggered a broad research effort toward the development of robust twodimensional functional polymers and their production on the bulk scale. Work in many groups has led to the synthesis of promising materials including metal dichalcogenides,5,6 nanographenes,7-9 extended polycyclic aromatic hydrocarbons,9,10 metal organic framework (MOF) nanosheets11 and covalent organic framework (COF) nanosheets12-17 that, owing to modular syntheses and easy functionalization, have emerged as versatile platforms for catalysis, sensing, and drug delivery.14,18-21 COF nanosheets (aka CONs) have, in particular, attracted recent interest. CONs are typically synthesized by dynamic covalent chemistry and isolated via top-down fabrication techniques such as self-22 or solvent-assisted12 ex-foliation and mechanical23,24 or chemical25 delamination. Unfortunately, one common drawback of the dynamic and reversible nature of their constituent bonds is lack of stability in strongly acidic or strongly basic solution. Also, CON monomers have generally

been limited to conformationally rigid, planar molecules that form polymer layers of single-atom thickness. Incorporation of 3D monomeric units bearing functional groups would provide more opportunity for post-assembly modification. We envisioned that reliance on irreversible covalent bonds to form a polymeric network and the introduction of macrocycles as monomers26,27 would generate a highly stable, easily modified, and potentially multifunctional 2D material. Calix[4]arene is a well-known macrocycle28-30 with guest recognition properties. It has a polar rim, a non-polar rim and a hydrophobic cavity. The rims can be selectively functionalized to provide analyte-selectivity or to facilitate polymerization.31-33 We anticipated that installing calixarenes within an organized 2D polymeric framework would result in a material that retained the macrocycle's recognition properties. Though the nonplanarity and conformational flexibility of calixarene renders its incorporation into 2D polymers a challenge, a 2D-metal organic co-ordination network based on a functionalized calix[4]arene has recently been described.34 Here, we report the template-free

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solvothermal synthesis of a calix[4]arene containing 2D polymer (CX4-NS). The polymer is porous, entirely covalent, and isolated by exfoliation as 3.52 nm-thick nanosheets composed of nine 2D polymeric layers. Incorporation of the calixarenes is intended to allow for post-assembly modification and the creation of functional surfaces for host– guest chemistry. We show that the π-electron-rich nanosheets adsorb vapor- and solution-phase iodine35,36 and use computational modeling, including molecular dynamics and Monte Carlo simulation, to gain insight into the atomic level details of nanosheet formation and I2 uptake. RESULTS AND DISCUSSION The synthesis of CX4-NS was accomplished by solvothermal Sonogashira-Hagihara cross-coupling reaction between 5,11,17,23-tetrabromocalix[4]arene-25,26,27,28-tetrol and 4,4'-diethynyl-1,1'-biphenyl in anhydrous 1,4-dioxane at 120 o C (Figure 1) for 72 h. These higher temperature, solvothermal conditions resulted in a completely different morphology than the amorphous covalent polycalix[4]arene networks we recently isolated from cross-couplings run at 65 °C in THF.3638 It is important to note that, the classical synthetic approach with the same linker resulted mostly in crosslinked 3Dpolymers. The product of the high-temperature reaction was centrifuged and washed multiple times with different solvents including THF, water, acetone, CHCl3, DMF and DMA. In contrast to the reversible covalent bonds that are commonly used to form 2D COFs, the C-C coupling reaction used here results in static covalent bonds. Furthermore, the bulky, yet flexible calixarenes that become incorporated within the 2D network give rise to weaker interlayer attractions that facilitate layer separation. To exfoliate the bulk material, we tested different solvents and found that ethanol allows for the isolation of few-layered nanosheets.

Figure 1. Synthetic route to porous calix[4]arene nanosheets (CX4-NS) by Sonogashira–Hagihara cross-coupling under solvothermal conditions.

As in case of our previous reports on cross-linked 3Dpolymers, both FTIR and NMR analysis confirms the succussful coupling of monomers to form the network structure. FTIR spectrum of CX4-NS showed the absence of alkyne -C-H absorption bands near 3275 cm−1 (which are present in the spectrum of monomer 2) and the presence of a peak near 2240 cm−1 and a broad peak near 3370 cm−1, which correspond to alkyne -C≡C- stretching and calix[4]arene –O-H stretching, respectively (Figure S4). Also, we observed a broad peak correspond to the methylene carbons (−CH2−) of the calix[4]arenes and acetylene resonances (-C≡C-) of the linker in the solid-state cross-polarization magic angle spinning (CP/MAS) 13C NMR spectrum (Figure S5). Raman

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spectra (Figure 2d) of exfoliated nanosheets show a G band near 1585 cm−1 that can be assigned to the vibration of sp2hybridized carbon. Another D band at 1415 cm−1 can be attributed to defects in the network structure.39,40 The powder X-ray diffraction (PXRD) pattern generated by CX4-NS (Figure S6) shows a broad signal typical of the amorphology of polymers synthesized by C-C coupling reactions.37,41 The broad diffraction peak located near 2θ = 22° suggests the existence of π-π interactions and is consistent with a theoretically predicted interlayer distance of ~0.39 nm. Thermogravimetric analysis (TGA) of the polymer indicates excellent thermal stability over 600 oC (Figure S7). The porosity of the bulk nanosheets was characterized by an N2 sorption isotherm, which shows an H4 type hysteresis loop corresponding to desorption and indicates a Brunauer−Emmett−Teller (BET) surface area of 468 m2g−1 (Figure 2a). Pore size distribution calculated using NLDFT reveals mainly the presence of mesopores with an average pore diameter of 89.7 Å and a cumulative pore volume of 0.52 cm3 g−1 (Figure S8). As compared to the bulk material, ethanol exfoliated nanosheets exhibited (Figure 2a) decreased BET surface area (225 m2g−1) and cumulative pore volume (0.09 cm3g−1), suggesting disrupted layer stacking. Interestingly, the diameter of the smallest pores indicated by the N2 isotherm (~ 2.5 nm) is in exact agreement with that predicted by theoretical calculations. We also measured CO2 and CH4 adsorption isotherms on activated bulk samples at both 273 K and 298 K. For both gases and at both temperatures, the isotherms displayed rapid rises at low relative gas pressures (P/P0 < 0.02), followed by slower increases (Figure S9 and S10). The bulk morphology of CX4-NS was verified by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM indicates the agglomeration of irregularly shaped nanosheets, whereas high-resolution TEM reveals thin sheet-like structures (Figures 2b, 2c and S11). The lamellar features of the material was further confirmed by atomic force microscopy (AFM), which demonstrates sheetlike morphology of 3.52 nm thickness and a rather broad lateral size distribution of 1 to 5 µm (Figures 2e and 2f). To further investigate nanosheet formation, we conducted computational modeling by applying and extending methods that have been used to study monomeric and dimeric calixarene derivatives.42-46 Density functional theory (DFT) was used to identify the minimum energy structures of the calixarene-containing nanosheet by varying the unit cell parameters (see SI for details). In the lowest energy structure, the calixarenes adopt a 1,2-alternate conformation that results in a two-dimensional pattern that has a rhomboid unit cell. The structure has a pore diameter of about 23.5 Å and a density of 0.69 g/cm3. The distances between the intermolecular phenoxy groups of calixerene rings are found to be around 1.94 Å providing the stability to this motif through hydrogen bonding network (Figure 3). Additional stability is achieved by π-π interactions between the well-aligned phenyl rings of the linker moieties that are brought together with a distance of 3.9 Å. Henceforth, we refer to this structure as CX4-NS-diamond. Sheet formation is also viable with the calixarenes in other conformational states (A, B, C, and D in Table 1). Figures S12-13 shows how the monomeric unit modulates the corresponding macrostructure. Overall our material is amoprpous in nature and possibly having number of defects sites because of the irreversible nature of the covalent bonds,

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ACS Applied Materials & Interfaces therefore, we beleive the observed structure in our theoritical calculation is one of the possible network scenarios. Considering the flexibility of calix[4]arene, one would expect a diverse set of linker conformations to be thermally accessible within the polymer. We used molecular dynamics (MD) simulations, that relied on the Dreiding/X6 forcefield,47 to explore the influence of conformational flexibility on the

stability of the low energy states and to study the possibility of interconversion between nanosheet structures. We calculated the energies of the lowest energy structures computed by the classical forcefield with DFT, and were encouraged to find that both methods produced the same relative order of corresponding energies (Table 1). This agreement gave us the

Figure 2. Polymer characterization. (a) N2 uptake isotherms at 77 K for bulk powder (black curve, adsorption; red curve, desorption) and as-exfoliated CX4-NS nanosheets (green curve, adsorption; blue curve, desorption); calculated surface areas = 468 and 225 m2g−1, for powder and nanosheets, respectively; (b and c) High-resolution TEM images of CX4-NS, with scale bars of 100 and 20 nm, respectively, that show nanosheet morphology. (d) Raman spectra of exfoliated CX4-NS; G band appeared near 1585 cm−1and D band at 1415 cm−1. (e) AFM image of CX4-NS with (f) the height profile measured around the red ellipse in e.

confidence to use MD simulations that can also take into account the entropic effects of the flexible calixarenes and therefore provide estimates of the relative free energies of the nanosheet formations. The simulations involved in sampling conformational space of microstructures at the basin of each local energy state detailed in SI. Table 1 summarizes thermodynamic parameters obtained for each state explored. Similar to the minimum energy results, free energy estimates also favored the CX4-NS-diamond motif, by 26.55 kcal/mol over the second lowest energy state. The large free energy differences between the CX4-NS-diamond motif and alternative structures suggests that, at room temperature, the predominantly observed nanosheet structure is in fact CX4NS-diamond. Its stability was found to be due primarily to the enthalpy (H = 196.08 kcal/mol), with the entropic contribution being relatively minor (TS = 1.9 kcal/mol) allowing us to suggest an explanation for the mechanism of stabilization by looking at the energy terms (Table 1). Simultions suggest that the stabilization of CX4-NS-diamond is governed by covalent geometry constraints and dispersion forces resulted in

lowering Ebonding and Evdw more effectively than other conformational states. The computational modeling results are in good agreement with AFM measurements (Figure 2e and 2f). To relate the simulations to experiment, we defined and determined the average height of the MD-modelled layers, . Because the microscopy experiments were not of sufficient resolution to distinguish individual layers, a direct measurement of layer heights was not possible. Nevertheless, the height profiles (h) from AFM were divided by ∆z from simulation to estimate the number of layers, n = h/∆z, observed in AFM. We argue that if n is close to an integer, then simulations and experiment have likely the same nanosheet structure. Remarkably, for CX4NS-diamond, we found n = 3.52 nm / 0.39 nm = 9.0, which, to two significant figures being an integer, suggests that the computational model is in accord with the AFM data. None of the computed thicknesses of the other nanosheet structures described in the SI give rise to a whole number in this calculation, which further supports CX4-NS-diamond as


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being, among several possible sheet formations, the one actually isolated. Radioactive iodine is a volatile solid that, when accidently released as a gas in nuclear power plant explosions, poses a serious threat to the public and the environment.48,49 Thus, having access to materials that effectively adsorb and sequester iodine is desirable. Because of its π-electronrichness and large surface area, we anticipated that CX4-NS would exhibit high affinity for iodine. Iodine vapor capture experiments were conducted under typical nuclear fuel reprocessing conditions (75 oC and ambient pressure).50,51 In a typical measurement, 20 mg of CX4-NS powder was loaded in a pre-weighed glass vial, placed in a sealed vessel filled with nonradioactive iodine, and heated. Gravimetric measurements were made at intervals until the adsorption equilibrium had been reached (Figure 4a). The time required for maximum uptake was approximately 8 h, and the amount of I2 adsorbed was 114 wt%, performance comparable to that of most reported nanosheet materials.52,53 Intense absorption peaks near 616 and 629 eV in the XPS spectrum of the iodineloaded sample are attributable to the 3d electrons of I2 (Figure 4b). Deconvoluted XPS spectra indicate that iodine adsorbed on the nanosheets remains as either I3− or neutral I2 (Figure S14). TGA analysis (Figure S15) of an I2-loaded sample revealed significant mass loss (12%), indicative of I2 release, in the range of 100 to 200 oC.

Figure 3. Tube representations of the thermodynamically favored CX4-NS structure. (a) Lowest energy 1,2-alternate conformation of calix[4]arene monomer unit. View of the nanosheet forming super-structure in the (b) xy and (c) xz planes. (d) Close-up view showing the interlayer π-π stacking and interlayer hydrogen bonds.

We found that sorption of I2 by CX4-NS is reversible. Captured I2 could be easily removed by immersing the I2loaded samples in ethanol at room temperature for 12 h, during which time the colorless solutions gradually became dark brown (Figure 4d), indicating I2 release. After regeneration, the polymers were dried and could be re-used with negligible decrease in adsorption capacity (Figure 4c). We also found that CX4-NS was capable of removing iodine from cyclohexane solution (Figure S16), though the removal

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rate was much slower (50% removal after 12 h) than for adsorption of iodine vapor. Iodine adsorption primarily occurs because of favorable non-bonded interactions between iodine and the extensive πsurface of the nanosheet.36 To elucidate the atomic details of these interactions we employed grand canonical Monte Carlo (GCMC) simulations using a configurational-biased algorithm54-56 where the chemical potential µ, volume V and temperature T were kept constant while allowing I2 exchanges between vapor and bound phases. We studied I2 capture capacity, which provides a direct comparison to experimental results, as well as we monitor I2 distribution in CX4-NS in atomic detail. To mimic the experimental conditions for I2 capture, we first ensured that the structural preference of the nanosheets does not change at 75 ￮C. Simulations at elevated temperatures indicated that CX4-NS-diamond is still the most stable form (Table 1). Based on the CX4-NS-diamond geometry GCMC simulation result was in close accord with the experimentally determined I2 uptake capacity: simulations predicted 120 wt% I2 uptake per repeat unit, whereas the measured value was 114 wt%. To understand the distribution of I2 molecules within the polymer matrix, we computed 3D density profile of the I2 distribution within the polymer from equilibrium simulations. Figure 5 shows that I2 vapor localizes around the π-electron rich outer surface of the calixarene phenyl rings and even more so around the triple bonds of the linkers (Figure 1). The affinity of I2 around the triple bonds can be estimated from the relative density using, ∆Gbind = ̶ kBT ln(ρ/ρb), where kBT is the thermal energy and ρ/ρb is the relative density with respect to bulk I2 vapor, giving rise to ∆Gbind ~ ̶ 3.7 kcal/mol at 75 ￮C.

Figure 4. Iodine adsorption experiments. (a) Iodine vapor uptake over time and maximum I2 vapor adsorption capacity (inset; both are in mg g−1) by CX4-NS (~114 wt%); (b) XPS spectra of I2loaded CX4-NS; region of interest is iodine potential. (c) Recycling efficiency of CX4-NS in I2 vapor adsorption; efficiency decreased by ~10% after 3 cycles. (d) Regeneration of CX4-NS in EtOH after I2 vapor adsorption.


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ACS Applied Materials & Interfaces Table 1. Thermodynamic parameters obtained from computational modeling of CX4-NS with four different conformations given in Figures S12 and S13. Energy (kcal.mol-1)

A

B

C

D

DFT Energy(hartree)

-409.69

-408.82

-407.03

-407.10

MM Energy

-11.89

27.39

64.99

189.40

Enthalpy (H)

169.53

196.08

236.40

221.70

Free energy (G ) at

167.63

194.31

234.55

220.94

EvdW

-9.54

-8.21

11.14

31.12

ECoulomb

-10.64

-11.51

-7.94

-10.06

Ebonding

97.24

122.95

136.47

108.49

G 75 ￮C

196.84

226.77

251.91

250.14

25

CONCLUSION In summary, we report a porous covalent calix[4]arenecontaining nanosheet material, CX4-NS, that features (i) high stability due to the non-reversibility of its constituent covalent bonds, (ii) an organized arrangement of embedded calix[4]arene host molecules, (iii) phenoxy groups that allow

￮C

Figure 5. I2 distributions in the CX4-NS polymer matrix from Grand Canonical Monte Carlo simulations. (a) A snapshot from simulations. Yellow ball-and-stick figures represent instantaneous I2 vapor molecules. (b) top and (c) side views of the construct. The 3D density map of I2 is shown in surface representation (see SI for details). The gray isosurface corresponds to iodine density of ρ > 20ρb, where ρb is the bulk density of I2 vapor, transparent yellow represents ρ > 100ρb, and solid yellow represents ρ > 200ρb. Images were created by VMD program.

for post-synthetic functionalization, and (iv) two different pore types: one due to the cavities of the macrocycle, and the other resulting from the networks 2D connectivity. We studied the nanosheet structures by experimental and theoretical means and charcterized the structure, CX4-NS-diamond, of its individual layers. We also measured the material's iodine uptake capacity (114 wt% after 8 h) and found it to be comparable to that of similar 2D covalent organic polymers. We anticipate that the material's incorporation of calix[4]arene and the possibity of easy functionalization will facilitate its use in the areas of sensing and catalysis.

EXPERIMENTAL SECTION Synthesis of CX4-NS. The polymers was synthesized under solvothermal conditions in a Schlenk tube. Compounds 1 (0.5 g) and 2 (0.275 g, 2 eq.) were placed into a Schlenk tube and dissolved in 1,4-dioxane (50 mL). Purged the mixture with argon for 15 min. PdCl2(PPh3)2 (0.144 g, 0.3 eq.), CuI (0.065 g, 0.5 eq.), and diisopropylamine (0.5 mL) were introduced to the reaction mixture under argon. The contents were degassed by three freezepump-thaw cycles. The tube was then placed in heating oven at 120 ˚C for 72 h. The reaction mixture was centrifuged and the solid was collected and washed with THF, water, acetone, CHCl3,


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DMF and DMA to remove impurities. Finally, the washed polymer was dried in a vacuum oven at 50 ˚C for 12 hours. For exfoliation, dried powder was dispersed in ethanol and sonicated for 2 h. Iodine capture and recycling measurement. Samples were prepared as follows: 25 mg of polymeric powder was placed in glass petri which was placed in a sealed glass chamber containing solid iodine flakes which were placed below the top level of the petri dish. The sample was heated with the iodine at 348 K and ambient pressure. After adsorption of the iodine vapor over (for 024 h), the I2-loaded powder was cooled to room temperature and weighed. Iodine uptake capacity was calculated as follows: Cu = (W2 − W1)/W1 × 100 wt %, with Cu being the iodine uptake capacity and W1 and W2 being mass of the polymer before and after adsorption, respectively. Recycling of the polymer was performed by washing the I2-loaded polymer repeatedly with 10 mL aliquots of EtOH. Iodine release was monitored by UVvisible analysis of the EtOH solution. After complete release of I2, the recovered polymer was dried and used again for iodine adsorption.

ASSOCIATED CONTENT Detailed experimental procedures, full characterization of structural data, and details of theoretical study associated with the SI.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by New York University Abu Dhabi. We thank NYUAD for their generous support for the research program. We also thank the Core Technology Platforms at NYUAD and Miss Khulood Alawadi for the 3D drawings assistance. This research was carried out on the High Performance Computing resources at New York University Abu Dhabi and AD181 faculty research grant to SK.

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Calix[4]arene-based Porous Organic Nanosheets - ACS Applied

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