Encapsulation of Various Guests by an Anionic In-Metal–Organic

Article pubs.acs.org/crystal

Encapsulation of Various Guests by an Anionic In-Metal−Organic Framework Containing Tritopic BTB Ligand: Crystal Structure of Reichardt’s Dye Captured in an In-Metal−Organic Framework Eun-Young Cho,† Ja-Min Gu,† In-Hwan Choi,† Wan-Seok Kim,† Yong-Kyung Hwang,† Seong Huh,*,† Sung-Jin Kim,‡ and Youngmee Kim*,‡ †

Department of Chemistry and Protein Research Center for Bio-Industry, Hankuk University of Foreign Studies, Yongin 449-791, Korea ‡ Department of Chemistry and Nano Science and Institute of Nano-Bio Technology, Ewha Womans University, Seoul 120-750, Korea S Supporting Information *

ABSTRACT: The reaction between 1,3,5-benzenetribenzoic acid (H3BTB) and In(NO3)3 hydrate in diethylformamide yielded a new InIII-metal-organic framework, [(CH3CH2)2NH2]3[In3(BTB)4]·10DEF·14H2O (I). The countercation and solvent-free doubly interpenetrated I potentially contains 71.0% of solvent accessible void. Although the framework of I was not stable enough to maintain its original structure when the solvent molecules were removed, the as-prepared I was found to be a very good sorbent for acridine orange hydrochloride, a large Reichardt’s dye, and hydrophobic iodine molecule in solution. The as-prepared I exhibited increased uptake amount in the order of Reichardt’s dye > acridine orange hydrochloride > iodine. The largest uptake of the bulky Reichardt’s dye by I could be attributed to the optimized structural fitting of Reichardt’s dye into the large threedimensional void space of I. The structure of Reichardt’s dye-encapsulated I_RD was unambiguously revealed by X-ray crystallography for the first time.

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INTRODUCTION Metal−organic frameworks (MOFs) are very promising crystalline porous inorganic materials that can be employed for several applications such as hydrogen storage,1−3 carbon dioxide capture/separation,4−6 proton conduction,7−10 drug delivery,11−13 heterogeneous catalysis,14−16 and selective encapsulation of guest molecules.17−22 Among these potential applications, selective encapsulation of diverse guest species in confined spaces of MOFs is of great importance for designing new functional MOFs because selective encapsulation of guest molecules or ions in solution by robust porous frameworks can be utilized for a wide range of new applications. Several research groups developed new functional MOF systems through the encapsulation of various guest molecules inside MOF channels. MOF-76 having microporous one-dimensional (1D) channels was utilized for both the detection and selective adsorption of UVI ions.17 Redox-active ferrocene (Fc) was efficiently incorporated into MOF-5 through vapor phase transfer, and the resulting Fc@MOF-5 was structurally characterized.18 Biologically active caffeine (Caf) was successfully loaded into ZIF-8, and the resulting Caf@ZIF-8 exhibited a release of caffeine in solution.19 Metalloporphyrin captured HKUST-1 was revealed as the first example of heme biomimetic catalytic system.20 Paramagnetic benzodithiazolyl © 2014 American Chemical Society

and methylbenzodithiazolyl radicals were also selectively encapsulated into the MIL-53(Al) framework through gas phase diffusion.21 Cobalt(II) phthalocyanine (Co-Pc) was selfassembled in a confined space of bio-MOF-1 with an anionic framework.22 All these recent examples clearly suggest that the encapsulation of a variety of functional guest molecules inside MOF channels would provide new opportunities for advanced applications. Meanwhile, one of the most attractive advantages of InIIIbased MOFs compared with other MOFs may be their anionic frameworks.23 In-MOFs without multinuclear secondary building units (SBUs) contain an anionic mononuclear In(O2C)4− node having tetrahedral geometry.24 Despite the lack of multinuclear SBUs, the eight-coordinate tetrahedral In(O2C)4− node often leads to topologically interesting high surface MOFs due to the expanded dimension of a single InIIIbased node.25,26 Additionally, other auxiliary bridging linkers are not needed for the formation of a three-dimensional (3D) framework except for the single type of ditopic or polytopic carboxylate-based bridging ligand. A rich chemistry of these InReceived: April 25, 2014 Revised: August 26, 2014 Published: September 2, 2014 5026

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Table 1. Crystallographic Data for I, I_AO and I_RD empirical formula formula weight temperature (K) wavelength (Å) space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z density (calc) (Mg/m3) absorption coeff (mm−1) crystal size (mm) reflections collected independent reflections data/restraints/parameters goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff peak and hole (e·Å−3)

I

I_AO

I_RD

C72H36In2O16 1386.65 296(2) 0.71073 R3̅ 44.2269(19) 44.2269(19) 42.519(2) 90.00 90.00 120.00 72026(6) 18 0.575 0.316 0.43 × 0.40 × 0.21 426710 39806 [R(int) = 0.0764] 39806/120/733 0.869 R1 = 0.0608 wR2 = 0.1598 R1 = 0.2312 wR2 = 0.2055 0.774 and −0.304

C72.67H40In2O21.83 1492.02 100(2) 0.71073 R3̅ 44.1252(8) 44.1252(8) 42.2961(9) 90.00 90.00 120.00 71319(9) 18 0.625 0.324 0.45 × 0.35 × 0.28 440467 39539 [R(int) = 0.1039] 39539/22/739 1.074 R1 = 0.1153, wR2 = 0.3365 R1 = 0.2075 wR2 = 0.3758 1.928 and −0.603

C185H75In4NO34 3314.74 295(2) 0.71073 R3̅ 45.597(6) 45.597(6) 40.718(8) 90.00 90.00 120.00 73314(21) 9 0.676 0.317 0.20 × 0.20 × 0.12 154276 38108 [R(int) = 0.1972] 38108/48/815 0.671 R1 = 0.0925 wR2 = 0.2502 R1 = 0.3237 wR2 = 0.3075 0.900 and −0.465

tetrahedral In(O2C)4− nodes are very similar among these MOFs. The anionic frameworks of In-MOFs contain countercations near the InIII-nodes, and therefore they can be utilized as good cation exchangers.24 Depending on the type of cations, InMOFs exhibited distinct gas sorption behaviors. In addition, the anionic frameworks of In-MOFs would be suitable for encapsulation of various guest molecules within the frameworks because of their high surface areas and pore volumes. However, their guest encapsulation in solution rarely has been studied compared to gas sorption investigations of these In-MOFs.26,34 In this study, therefore, we report on a new 3D In-MOF containing a tritopic BTB3− bridging ligand and its ability to encapsulate three guest molecules carefully chosen based on their chemical structures, dimensions, and charges.

MOFs is being developed recently. For example, we reported robust In-MOFs containing only 2,6-naphthalenedicarboxylate (2,6-NDC), [(CH3CH2)2NH2][In(2,6-NDC)2]·2H2O·DEF,25 or 4,4′-biphenyldicarboxylate (4,4′-BPDC), [(CH3)2NH2][In(4,4′-BPDC)2]·4DMF·2H2O, and [(CH3CH2)2NH2][In(4,4′BPDC)2]·2DEF·H2O,26 together with their framework topology, catenation isomerism, and gas sorption properties. Although both 2,6-NDC and 4,4′-BPDC are very common ditopic carboxylate bridging ligands, the resultant In-MOFs are very intriguing in their topologies and properties. Other InMOFs containing ditopic 2,5-thiophenedicarboxylate,27 1,4naphthalenedicarboxylate,28 1,4-benzenedicarboxylate,29 2amino terephthalate,30 D-(+)-camphoric acid,31 and 5-aminoisophthalate32 ligands have been recently reported. Bu and coworkers also utilized tritopic 1,3,5-benzenetricarboxylate (1,3,5BTC) as a sole bridging ligand to form a series of (cation)3[In3(1,3,5-BTC)4].33 In this system, seven new InMOFs with varying countercations were efficiently prepared by simply changing the type of countercations. Qian and coworkers also prepared In-MOF, [(CH3)2NH2]3[In3(BTB)4]· 12DMF·22H2O named as ZJU-28, using tritopic 1,3,5benzenetribenzoic acid (H3BTB) as a sole bridging linker.34 The ZJU-28 exhibited second-order nonlinear optical activities induced by a series of four encapsulated cationic pyridinium hemicyanine dipolar chromophores with a systematically varied alkyl chain length. Thus, ZJU-28 is ideal for accommodating large cationic guest species. Up to now, only five similar InMOFs containing a single tetratopic carboxylate ligand 1,2,4,5benzeneteracarboxylate, 35 5-(3,5-dicarboxybenzyloxy)isophthalate,36 5-(bis(4-carboxybenzyl)amino)isophthalate,37 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin, 3 8 and 5,10,15,20-tetrakis(4-carboxybiphenyl)porphyrin38 have been independently reported. The coordination environments of

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EXPERIMENTAL SECTION

Materials. 1,3,5-Benzenetribenzoic acid (H3BTB) was prepared according to the literature.39 In(NO3)3 hydrate, acridine orange hydrochloride, Reichardt’s dye, and iodine were purchased from Sigma-Aldrich and used as received. Diethylformamide was purchased from TCI and used as received. Other reagent grade solvents were used as received. Instrumentation. NMR spectra were recorded on a Bruker Ascend 400 (400.13 MHz for 1H) spectrometer. 1H chemical shifts were referenced to the proton resonance signal resulting from protic residue in the deuterated solvent. Thermogravimetric analysis (TGA) was carried out on a TGA Q5000 (TA Instruments) under a nitrogen atmosphere. Fourier transform infrared (FT-IR) spectra in attenuated total reflection (ATR) mode were obtained on a Jasco FT/IR-4100 spectrometer. Elemental analysis was performed at Organic Chemistry Research Center, Sogang University (Seoul, Korea) by using EA1112 (CE Instruments, Italy). Powder X-ray diffraction (XRD) spectra were obtained with a Bruker D8 Focus diffractometer (40 kV, 30 mA, step size = 0.02°). UV−vis spectra were collected on a Scinco S-3100 spectrophotometer. Diffuse reflectance UV−vis spectra were measured 5027

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Figure 1. 3D structure of [In3(BTB)4]3− along the c-axis (a) and along the a-axis (b). Solvent molecules and hydrogen atoms are removed for clarity. The Connolly surface of solvent-free I viewed down the c-axis, probed with a probe radius of 1.4 Å (c). Encapsulation of I2 by I. The as-prepared I (10 mg) was added into 1.0 mM cyclohexane solution of iodine (10 mL) and gently stirred in the dark at room temperature. Aliquots of the solution (1.0 mL) were periodically taken and diluted with fresh cyclohexane (1.0 mL) for quantification by UV−vis spectroscopy. The measured molar extinction coefficient of 1.0 mM I2 in cyclohexane was 824.7 M−1 cm−1 at λmax = 522.3 nm (Figure S1). X-ray Crystallography. The X-ray diffraction data for I and I_AO were collected on a Bruker APEX-II diffractometer equipped with a monochromator in a Mo Kα (λ = 0.71073 Å) incident beam. The Xray data for I_RD and I_I2 were collected on a Bruker SMART APEX diffractometer. A crystal of I was mounted on a glass fiber and collected data at room temperature, and a crystal of I_AO was mounted on a silicone loop and collected data at 100 K. Crystals of I_RD and I_I2 were inserted into a Lindemann capillary with mother liquor. The CCD data were integrated and scaled using the BrukerSAINT software package, and the structure was solved and refined using SHEXTL V6.12.40 All hydrogen atoms were placed in the calculated positions. SQUEEZE/PLATON was used in structural refinement in X-ray experiment. The crystallographic data for I, I_AO, and I_RD are listed in Table 1. The selected bond distances are listed in Table S1, Supporting Information. Structural information was deposited at the Cambridge Crystallographic Data Centre (CCDC reference number are 976960 for I, 989914 for I_AO, and 988834 for I_RD).

with Jasco V-550 spectrophotometer. Optical microscopic images were collected on a Nikon Eclipse LV100POL microscope equipped with a DS-Fi1 CCD camera. The cryogenic volumetric N2 adsorption− desorption analysis was performed on a Belsorp-miniII at 77 K (BEL Japan). The as-prepared [(CH3CH2)2NH2]3[In3(C27H15O6)4]·10(C5H11NO)·14(H2O) (I) was dried at 333 K under high vacuum for 15 h. Low pressure volumetric CO2 adsorption measurements were performed on a Belsorp-miniII at 196 K (2-propanol/dry ice bath). Preparation of [(CH3CH2)2NH2]3[In3(C27H15O6)4]·10(C5H11NO)· 14(H2O) (I). In(NO3)3 hydrate (0.030 g, 0.1 mmol based on an anhydrous form) and H3BTB (0.044 g, 0.1 mmol) were dissolved in 10 mL of DEF. The reaction mixture was sealed in a Teflon-lined high pressure bomb and stored at 120 °C for 72 h. The colorless crystals were retrieved by filtration, washed with DEF, and air-dried (0.058 g, 49%). Anal. Calcd for C170H234In3N13O48 (F. wt. 3572.19): C, 57.16; H, 6.60; N, 5.10. Found: C, 56.52; H, 6.42; N, 5.24. Encapsulation of Acridine Orange Hydrochloride (AO) by I. The as-prepared I (10 mg) was added into 1.0 mM ethanol solution of acridine orange hydrochloride (10 mL) and gently stirred in the dark at room temperature. Aliquots of the solution (0.3 mL) were periodically taken and diluted with fresh ethanol (2.7 mL) to give 10fold diluted solution. From this diluted solution, a 0.3 mL solution was diluted again with fresh ethanol (2.7 mL) for quantification by UV−vis spectroscopy. The measured molar extinction coefficient of 1.0 mM AO in ethanol was 58500 M−1 cm−1 at λmax = 491.5 nm (Figure S1, Supporting Information). Encapsulation of Reichardt’s Dye (RD) by I. The as-prepared I (10 mg) was added into 1.0 mM dichloromethane solution of Reichardt’s dye (10 mL) and gently stirred in the dark at room temperature. Aliquots of the solution (0.2 mL) were periodically taken and diluted with fresh dichloromethane (1.8 mL) for quantification by UV−vis spectroscopy. The measured molar extinction coefficient of 1.0 mM RD in dichloromethane was 5108 M−1 cm−1 at λmax = 690.4 nm (Figure S1). The crystallographic data of I_RD was obtained using the crystal retrieved from the mixture after 72 h.

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RESULTS AND DISCUSSION Thermal reaction of H3BTB and In(NO3)3 hydrate in N,Ndiethylformamide (DEF) at 120 °C afforded the colorless block type crystals formulated as [(CH3CH2)2NH2]3[In3(BTB)4]· 10DEF·14H2O (I) (Figure S2, Supporting Information). I crystallizes in the R3̅ space group. As depicted in Figure 1, BTB3− bridges InIII ions in a chelating mode to form a 3D framework. The asymmetric unit contains two InIII ions and two and 2/3 BTB 3− ligands (Figure 2). I contains 5028

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temperature since the data with the reduced cell dimensions at low temperature did not give reliable structure solution. As the measurement temperature was lowered below room temperature, the captured solvent molecules presumably contracted, influencing the unit cell dimensions and volumes. For example, the unit cell dimensions and volume observed at 300 K, a = b = 44.330(13) Å, c = 42.403(17) Å, V = 72266.3(44.8) Å3, decreased at 170 K, a = 43.244(33) Å, b = 43.431(25) Å, c = 41.617(45) Å, V = 67958.04(93.98) Å3 (Table 2). The PXRD pattern of the bulk I indicated a good agreement with the simulated pattern based on X-ray crystallography (Figure 3). Except for the lowest diffraction plane (1 0 1), all

Figure 2. Asymmetric unit of [In3(BTB)4]3−. Displacement ellipsoids are shown at the 30% probability level. All hydrogen atoms were omitted for clarity. Symmetry operations: (i) 0.66667 − x + y, 0.33333 − x, −0.66667 + z, (ii) 0.33333 − y, −0.33333 + x − y, −0.33333 + z, (iii) 1 − x + y, 1 − x, z, (iv) 0.66667 − y, 0.33333 + x − y, 0.33333 + z, (v) 0.33333 − y, −0.33333 + x − y, 0.66667 + z, and (vi) −x + y, −x, z.

[(CH3CH2)2NH2]+ cations to compensate for the charge of the anionic framework constructed by four carboxylate ligands bonded to an InIII ion. The diethylammonium cations, DEF, and water solvent molecules were not refined because they were highly disordered, and the elemental analysis provided the total formula of I. An InIII ion is eight-coordinated, and the geometry of an InIII node is pseudotetrahedral constructed by four BTB3− ligands. BTB3− ligands having a 3-fold rotation axis reside alternatively along the c-axis. The framework is a crystallographically independent 2-fold interpenetrated 3D network (Figure S3, Supporting Information). The structure indicated a new 8,8-connected 2-nodal net with a Schläfli symbol of {35·413·510}{36·412·510} assuming two InIII ions act as two nodes without any simplification based on TOPOS analysis.41 The class is IIa with Zt = 1 and Zn = 2. Although the network of I can be considered as the (3,4)-connected net, assembled by 3-connecting trigonal BTB3− node and 4connecting pseudotetrahedral In(COO)4− node, the net differs from the typical cubic ctn net or bor net found in cubic-C3N4 and boracite (Mg3B7O13Cl), respectively,42 thereby it is a new net (Figure S4, Supporting Information). This phenomenon can be attributable to the deviation from the ideal tetrahedral structure for the pseudotetrahedral In(COO)4− node. The cation and solvent-free I indicated 71.0% of the void volume based on PLATON analysis.43 The real potential void volume should be slightly smaller than the calculated value due to the presence of countercations. The unit cell dimension of I was gradually reduced depending on the temperature as shown in Table 2. The structure determination was completed at room

Figure 3. PXRD patterns of the simulated and as-prepared sample.

other main diffraction planes including (21̅ 0), (2 01)̅ , (0 0 3) or (32̅ 1̅), (21̅ 3̅), (54̅ 0), and (64̅ 1) were observed at the calculated positions. The (1 0 1) peak was overlapped with the direct X-ray beam in our measurement. The thermogravimetric analysis (TGA) profile of the as-prepared I indicated a 35.4 wt % loss at 215 °C (Figure S5, Supporting Information). In spite of the lack of precise location information from the X-ray diffraction study, the TGA data suggested complete loss of 10 DEF and 14 water molecules in the as-prepared I. Above 215 °C, the second weight loss occurred continuously up to 400 °C, and the complete decomposition of three diethylammonium countercations might occur during this period. After their decomposition, charge mismatching might be balanced by protons generated from the diethylammonium countercations. The similarly formulated ZJU-28 was reported by Qian and co-workers in 2012.34 They used InCl3 precursor in a complex mixed solvent system of N,N-dimethylformamide (DMF)/1,4dioxane/H2O at 130 °C, while we used In(NO3)3 hydrate in a single solvent of DEF. Their colorless needle-like crystal was formulated as [(CH3)2NH2]3[In3(BTB)4]·12DMF·22H2O. Both frameworks contain the same ratio of InIII ions to BTB3− ligands, but the structure of each framework is very

Table 2. Cell Dimension Variation of [(CH3CH2)2NH2]3[In3(BTB)4]·10(DEF)·14(H2O) (I) Depending on Temperature

a b c α β γ V

300 K

290 K

270 K

250 K

230 K

210 K

190 K

170 K

44.330(13) 44.381(14) 42.403(17) 90.00 90.00 120.00 72266.3(44.8)

44.042(16) 44.210(14) 42.245(22) 90.00 90.00 120.00 71155.1(48.4)

44.023(22) 43.889(15) 42.042(25) 90.00 90.00 120.00 70599.9(54.7)

43.853(20) 43.769(16) 41.890(25) 90.00 90.00 120.00 69892.8(60.9)

43.748(20) 43.695(18) 41.785(29) 90.00 90.00 120.00 69362.5(70.5)

43.473(22) 43.747(27) 41.966(36) 90.00 90.00 120.00 69247.2(78.8)

43.532(29) 43.593(23) 41.744(35) 90.00 90.00 120.00 68832.6(77.8)

43.244(33) 43.431(25) 41.617(45) 90.00 90.00 120.00 67958.04(93.98)

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Figure 4. Chemical structures of acridine orange (AO) and Reichardt’s dye (RD) (a) and time-dependent uptake of AO, RD, and I2 by as-prepared I (b). Color-changed crystal images of as-prepared I, I_AO, I_RD, and I_ I2 (c).

different. Their MOF crystallized in the hexagonal P6̅2c space group (No. 190), while our MOF I did crystallize in the trigonal R3̅ space group (No. 148). Both MOFs have large void volumes that were occupied by highly disordered water molecules and cations. The Connolly surface of cation and solvent-free I is depicted in Figure 1c. Very large threedimensionally interconnected void volume is clearly discernible. Motivated by the large potential void volume of solvent-free I, gas sorption abilities were investigated by using N2 and CO2 gases at 77 and 196 K (Figure S6, Supporting Information). Unfortunately, the activated I after CHCl3-exchange exhibited negligibly small uptake of both N2 and CO2. These results imply that the framework of I may not be stable when the solvent molecules are removed during activation process. The PXRD pattern of the activated sample indicated most diffraction peaks found in the as-prepared I disappeared (Figure S7, Supporting Information). Despite the nonporosity for gaseous molecules of the activated I, three representative colored guest molecules were selected based on their molecular dimensions and charges in order to study the encapsulation abilities of the as-prepared I under similar experimental conditions. The large void volume is big enough to capture big dye molecules such as AO and RD since the distances of the center to center on each BTB3− are 15.486(1) Å and 27.034(1) Å alternatively (Figure 1b), and the size of the free RD molecule44 is 15.587 × 12.000 Å2 (Figure S8, Supporting Information). The same concentration of solutions (1 mM) for acridine orange hydrochloride (AO, orange color) in ethanol, Reichardt’s dye (RD, light green color) in methylene chloride, and iodine (I2, purple color) in cyclohexane were used for the guest encapsulation experiments. Since the as-prepared I contains both countercations and solvent molecules in its void space, the guest encapsulation experiments would provide the characteristics of replacement of these ions and molecules with various guests.

The chemical structures of AO and RD are indicated in Figure 4a. Because the positions of both AO and iodine molecules were not precisely located using X-ray diffraction, we estimated the stoichiometry of the encapsulated guest molecules by using the data obtained from the encapsulation experiments by UV−vis spectroscopic analyses. The uptake amounts for these guest molecules were found to be 1.7 mmol for AO, 2.8 mmol for RD, and 1.1 mmol for iodine per mmol of the as-prepared I. Because we used the HCl salt form of AO, it is a positively charged guest molecule. In contrast, RD is neutral but its physical dimension is much larger than that of AO. It took 75 h for the completion of the RD encapsulation, while it only took 25 h before encapsulation of AO stopped. The encapsulation of small, positively charged AO was much faster than the large, neutral RD. As the framework of I is negatively charged, the encapsulation of positively charged AO could be more favorable due to electrostatic attraction. This electrostatic attraction could affect the cell dimensions, and the smaller cell dimensions were observed as shown in Table 1. In fact, the encapsulation of AO ended in a very short period of time (∼25 h) as shown in Figure 2b. Therefore, the encapsulated AO partially replaced countercations in the framework. However, the initial rates of encapsulation of both dye molecules were found to be virtually identical as shown in Figure 4b. Furthermore, the final uptake amount for RD indicated a 1.6fold increase compared with AO. The rapid and high uptake of RD could be attributable to a stronger interaction between the framework of I and RD. This result clearly suggests that both the surface charge of the framework as well as the pore structure and surface functionality may play key roles in the encapsulation of specific guest molecules by MOF. The detailed interactions between RD and the anionic framework will be discussed later. Additionally, iodine encapsulation was tested. Iodine molecule is a volatile solid, and its capture inside MOFs is an important area of current research.45−49 The as-prepared I 5030

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could encapsulate hydrophobic, neutral iodine as shown in Figure 4b; however, the final captured amount is smaller than RD. Furthermore, the encapsulation kinetics is much slower than both AO and RD. This unexpected slow kinetics can be accounted for by the fact that hydrophobic iodine molecules do not possess strong interaction with the anionic framework of I compared with AO and RD. Color-changed crystal images depicted in Figure 4c clearly indicate the efficient encapsulation of guest molecules (see Figure S9, Supporting Information for digital photo images of crystals). The original colorless I changed its color into the corresponding guests’ colors (I_AO, orange; I_I2, purple; and I_RD, light green). In order to clarify the interactions between I and the aforementioned guest molecules, we attempted to analyze the single crystal structures of guest-loaded I. Data of I_AO were collected at 100 K, and I_RD was inserted into a Lindemann capillary with the mother liquor for X-ray data collection at room temperature (Table 1 and Figure 4c). I_I2 was also put into a capillary with the mother liquor for data collection (Figure 2c), and it showed very similar unit cell dimensions as the as-prepared I with R3̅ space group, a = b = 44.391(6) Å, c = 42.209(8) Å, and V = 72034(30) Å3. In the cases of AO and iodine, however, we could not get complete information on the guest molecules captured in I; we were able to observe the framework structures of I_AO and I_I2 as given in Figure S10, Supporting Information. We also found partially occupied solvent molecules in the I_AO structure, and those are 1/2 ethanol and eight and 1/2 water solvent molecules per [In3(BTB)4]3−. The as-solved I_AO indicated 60.8% of the void volume based on PLATON analysis. From the comparison with the original void volume, the encapsulated positively charged AO partially replaced the original ammonium cations of I. The framework structures of the guest-loaded I are basically identical to the as-prepared I. Even the sample used for iodine capture in cyclohexane for a prolonged period of time (8 days), maintained the original framework structure. Thus, we conclude the framework is rigid during the encapsulation of these guests in solution. In contrast, the crystal structure of I_RD was completely solved as depicted in Figure 5. The intact framework structure of I was unequivocally confirmed. The captured RD molecules were precisely located inside framework of I. To the best of our knowledge, this is the first example of the crystal structure of RD captured in a MOF. Despite several previous reports on the encapsulation of RD by large pore MOFs, the crystal structure of RD captured in the framework has not previously been reported.50−53 The unit cell dimensions of I_RD are deviated +1.37 Å on the a- and b-axes and −1.801 Å on the c-axis from those of the original I upon encapsulation of RD and water molecules (Table 1). We found partially occupied RD and water molecules, and those are 3/4 RD and 3/4 water molecules per [In3(BTB)4]3− in the I_RD structure. There is a 3-fold rotation axis at the middle of the BTB3− ligand, and partially occupied RD molecules were encapsulated in the framework through electrostatic interactions with C3 symmetry (Figure 5). Partially occupied water oxygen atoms were also encapsulated inside pores through hydrogen bonds with C3 symmetry (Figure S11, Supporting Information). The cation and solvent-free I_RD indicated 52.7% of the void volume based on PLATON analysis. This indicates that RD molecules were encapsulated by 25.8% of the original void volume (71.0%) of the cation and solvent-free I, and implies that the original solvent molecules (10 DEF and 14 H2O) were partially

Figure 5. Crystal structures of I_RD. (a) Encapsulated RD molecules and water molecules are represented in a CPK model, and the doubly interpenetrated frameworks are shown in a stick model with different colors: black and gray. Three partially occupied RD molecules occupy alternately in the frameworks with a C3 symmetry represented by different colors: three are blue and the other three are green. Partially occupied water oxygen atoms are red. (b) Expanded view of the partially occupied RD molecules represented in green and blue balland-stick models. Oxygen atoms are red. All hydrogen atoms were omitted for clarity.

replaced by RD under our experimental conditions. Since RD molecules were encapsulated in the framework through electrostatic interactions with C3 symmetry, the middle phenyl ring of RD resides parallel to the ab plane (Figure 5). That might be the reason for positive expansion along the [100] and [010] directions and negative expansion along the [001] direction in cell data. On the basis of six torsion angles, the conformation of a RD is more twisted (Figure 6 and Table S2, Supporting Information) than that of a free RD,44 and the main aromatic rings deviate significantly from a linear arrangement. This twisted conformation is possibly due to the perfect fitting in the pores as well as electrostatic interactions between RD molecules and anionic framework. Therefore, the interaction between RD, containing a cationic nitrogen atom, and the anionic portion of the framework of I is likely to be much enhanced because of efficient electrostatic interaction between RD and anionic frameworks (Figure 7). Diffuse reflectance UV−vis spectra for free RD and I_RD shown in Figure 8 clearly reveal that the captured RD in the framework of I exhibits less efficient charge transfer due to the structural distortion. As a result, the original black color of RD in the solid state changed into light-green in I_RD. 5031

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Crystal Growth & Design

Article

In summary, we prepared a new phase of In-BTB MOF and successfully characterized its crystal structure. Despite its 2-fold catenation, the as-prepared In-BTB MOF I exhibited a very large void volume for solvate molecules and countercations due to the large dimension of the BTB3− ligand. Although the solvent-free I did not exhibit meaningful adsorption of gaseous N2 and CO2, the framework structure of as-prepared I in solution was found to be stable enough for a prolonged period of time for guest encapsulation. Three representative guest molecules were tested for encapsulation by as-prepared I. Both AO and RD exhibited very rapid encapsulation kinetics relative to iodine, and the final uptake amount of the large RD is greater than the small AO. The remarkable encapsulation efficiency of the large, neutral RD can be attributed to the perfect fitting of the RD into the pore structure as well as electrostatic interactions between the dye and the anionic frameworks of I. These results clearly suggest the careful design of pore structure and functionality of MOFs would benefit for biomimetic absorption of target molecules.

Figure 6. Structures of the RD in the framework of I_RD (a) and a free RD (b).

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ASSOCIATED CONTENT

S Supporting Information *

Table of the selected bond distances for I, I_AO, and I_RD. The torsion angles for RD in the framework of I and free RD. Calibration curves for the quantification of AO, RD, and I2. Crystal shape image of I. Crystal structure of the 2-fold interpenetrated 3D frameworks of I. Pictorial representation of the (3,4)-connected net of I. TG profile of I. The adsorption− desorption isotherms for N2 and CO2. PXRD patterns of the asprepared I and the dried sample. The estimated physical dimensions of a free RD. The optical micrographs of asprepared I, I_AO, I_RD, and I_I2. Framework structures of I_AO and I_I2 shown along the c-axis. Framework structures for I_AO and I_I2 indicating partially-occupied water molecules encapsulated in the pores. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.H.). *E-mail: [email protected] (Y.K.).

Figure 7. Electrostatic interactions between a partially occupied RD (green bonds), containing a cationic nitrogen atom (blue), and anionic part of the framework (black bonds), and hydrogen bonds between carboxylate oxygen atoms (red) and a partially occupied water oxygen atom (red). All hydrogen atoms are omitted for clarity.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (20110008018, 2013R1A1A2006914), the Gyeonggi Regional Research Center (GRRC) Program of Gyeonggi Province (GRRC-HUFS-2014-B04).

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REFERENCES

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Figure 8. Diffuse reflectance UV−vis spectra for free RD and I_RD. Digital micrographs of the samples are shown together.

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