Metallacyclodimeric Ensembles Consisting of Zinc(II) - American

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Host−Guest Chemistry of 1D Suprachannels and Dihalomethane Molecules: Metallacyclodimeric Ensembles Consisting of Zinc(II)-2,7bis(nicotinoyloxy)naphthalene Complexes Minwoo Park, Hyeun Kim, Haeri Lee, Tae Hwan Noh, and Ok-Sang Jung* Department of Chemistry, Pusan National University, Pusan 609-735, Korea S Supporting Information *

ABSTRACT: Self-assembly of ZnX2 (X = Cl, Br, and I) with 2,7-bis(nicotinoyloxy)naphthalene (L) as a hemicircular bidentate ligand containing a chromophore moiety yields a systematic metallacyclodimeric unit, [ZnX2(L)]2. These basic skeletons constitute, via interdigitated π···π interactions, a unique columnar ensemble forming a suprachannel. This can then be employed as an unusual “diiodomethane within the suprachannel” host−guest system, CH2I2@[ZnX2(L)]2. Specifically, the suprachannel significantly stabilizes the CH2I2 molecules in the order [ZnI2(L)]2 > [ZnBr2(L)]2 > [ZnCl2(L)]2. This suprachannel has significant halogen effects on the photoluminescence (PL), thermal properties, and host−guest inclusion.

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INTRODUCTION Arrays of molecular units, especially after the emergence of additional functionalities, have beckoned crystal engineers for the past decade.1−11 Indeed, the field of supramolecular materials, manipulation of molecular ensembles is both an attractive topic and, not coincidentally, an enticing challenge.8−11 A rigorous knowledge and understanding of the driving forces behind those ensembles is prerequisite to the design and construction of molecular arrays. Syntheses of metallacyclic molecules and their arrays are particularly coveted goals,12−19 in the fields of molecular adsorption, recognition, ion exchange, confinement catalysis, and luminescent chemosensing, since their functions are comparable with organic crown-ether, cyclodextrins, calixarenes, and cucurbiturils.20−26 Some hemicircular bidentate N-donors provide, via the introduction of appropriate metal ions, wider opportunities for task-specific metallacycles as receptors.12,13 Thus, on the basis of the geometry of the metal ions, the binding sites of the donating ligands, the reactivity, the length, and the charge of the spacers, various metallacycles for host−guest recognition systems have been designed and synthesized. Specifically, zinc(II) complexes of functional N-donor ligands have been extensively examined for metallo-enzymes, zinc finger proteins, transmetalation, recognition, photoluminescence (PL), and catalysts.27−33 As regards systematic research on the construction and recognition of “specific guests within a zinc(II) metallacyclic suprachannel containing chromophore moiety”, simple recognition systems have been reported, though the field remains relatively unexplored. In this context, two crucial aims of the present study were to explore the behavior of the included unstable haloalkanes within the open cavity of columnar suprachannels consisting of zinc(II) metallacyclodimers and to investigate the metallacyclodimeric system’s significant halogen effects. Herein, we report the construction of task-specific © 2014 American Chemical Society

zinc(II) metallacyclodimers as functional halomethane containers and evaluate their PL and thermal properties. The metallacyclic system consisting of the three ZnX2 (X− = Cl−, Br−, and I−) was designed as selective receptors for dichloromethane, dibromomethane, and diiodomethane, respectively.12 This suprachannel system is the effective stabilizer for diiodomethane, which is known to decompose upon exposure to light and release iodine.34 With its high density, diiodomethane is useful as a reagent for the determination of mineraland other solid-sample densities, as an optical contact liquid, and as carbene.35

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EXPERIMENTAL SECTION

Materials and Measurements. All chemicals, including zinc(II) chloride, zinc(II) bromide, zinc(II) iodide, 2,7-dihydroxynaphthalene, nicotinoyl chloride hydrochloride, and triethylamine, were purchased from Aldrich and used without further purification. Elemental microanalyses (C, H, N) were performed on crystalline samples at the KBSI Pusan Center using a Vario-EL III. Thermal analyses were undertaken under a nitrogen atmosphere at a scan rate of 10 °C/min using a Labsys TGA-DSC 1600. The infrared spectra of samples prepared as KBr pellets were obtained on a Nicolet 380 FT-IR spectrophotometer. 1H (300 MHz) and 13C (75 MHz) NMR spectra were recorded on a Varian Mercury Plus 300 with calibration against the solvent signal (CDCl3: δ = 7.26 ppm for 1H NMR; δ = 77.16 ppm for 13C NMR). Scanning electron microscopy (SEM) images were obtained on a Tescan VEGA 3. Powder X-ray diffraction data were recorded on a Rigaku RINT/DMAX-2500 diffractometer at 40 kV and 126 mA for Cu Kα. Excitation and emission spectra were acquired on a FluoroMate FS-2 spectrofluorometer. 2,7-Bis(nicotinoyloxy)naphthalene (L). Triethylamine (22 mmol, 2.67 g) was added to a stirred mixture of 2,7-dihydroxynaphthalene (5 mmol, 0.80 g) and nicotinoyl chloride hydrochloride (12 mmol, 2.14 g) in chloroform (120 mL) at 60 °C. The reaction mixture Received: April 21, 2014 Published: August 7, 2014 4461

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Table 1. Crystallographic Data for [ZnCl2(L)]2·CH2Cl2·2CH3CN, [ZnBr2(L)]2·CHCl3, and [ZnI2(L)]2·CH2Cl2 formula Mw cryst system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z dcalcd (g cm−3) μ (mm−1) GOF on F2 R1 [I > 2σ(I)]a wR2 (all data)b a

[ZnCl2(L)]2·CH2Cl2·2CH3CN

[ZnBr2(L)]2·CHCl3

[ZnI2(L)]2·CH2Cl2

C49H36Cl6N6O8Zn2 1180.28 triclinic P1̅ 7.617(2) 9.243(2) 19.802(5) 101.72(2) 98.53(2) 94.42(2) 1341.7(5) 1 1.461 1.248 1.065 0.1227 0.3670

C45H29Br4Cl3N4O8Zn2 1310.45 triclinic P1̅ 8.3082(2) 9.5031(2) 15.9217(3) 84.604 (1) 89.685(1) 84.055(1) 1244.76(5) 1 1.748 4.389 1.066 0.0539 0.1796

C45H30Cl2I4N4O8Zn2 1463.97 triclinic P1̅ 9.760(1) 17.0271(2) 17.4437(2) 115.223(1) 96.034(1) 94.901(1) 2580.54(5) 2 1.884 3.478 1.056 0.0453 0.1371

R1 = ∑∥Fo| − |Fc∥/∑|Fo|. bwR2 = (∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2])1/2.

was refluxed for 24 h. The mixture was then filtered, and the filtrate was washed with 0.5 N NaOH aqueous solution several times. The chloroform layer was dried over anhydrous magnesium sulfate and filtered. Evaporation of the chloroform yielded a yellow product, which, subsequently, was recrystallized from a solvent pair of chloroform and diethyl ether, thus affording pure 2,7-bis(nicotinoyloxy)naphthalene (L) as a yellow crystalline solid in a 78% yield. mp = 180 °C. Anal. Calcd for C22H14N2O4: C, 71.35; H, 3.81; N, 7.56%. Found: C, 71.30; H, 3.84; N, 7.47%. IR (KBr, cm−1): 1739 (s), 1591 (m), 1284 (s), 1238 (s), 1151 (m), 1085 (m), 1020 (m), 906 (m), 730 (m). 1H NMR (CDCl3, 300 MHz, δ): 9.34 (s, 2H), 8.94 (d, J = 4.8 Hz, 2H), 8.54 (d, J = 8.4 Hz, 2H), 8.13 (d, J = 9.6 Hz, 2H), 7.93 (s, 2H), 7.69 (dd, J = 8.4 Hz, J = 4.8 Hz, 2H), 7.57 (d, J = 9.6 Hz, 2H). 13C NMR (CDCl3, 75 MHz, δ): 163.82, 154.40, 150.65, 148.82, 137.62, 133.84, 129.62, 125.11, 124.18, 121.71, 118.70. [ZnCl2(L)]2·CH2Cl2·2CH3CN. An acetonitrile solution (5 mL) of ZnCl2 (13 mg, 0.1 mmol) was carefully layered onto a dichloromethane solution (5 mL) of L (37 mg, 0.1 mmol). Colorless crystals of [ZnCl2(L)]2·CH2Cl2·2CH3CN formed at the interface in 2 days in a 75% yield (45 mg). mp = 212 °C. Anal. Calcd for C49H36Cl6N6O8Zn2: C, 49.86; H, 3.07; N, 7.12%. Found: C, 49.70; H, 3.01; N,7.03%. IR (KBr, cm−1): 3568 (br), 3464 (br), 1745 (s), 1608 (m), 1514 (w), 1434 (m), 1286 (s), 1240 (s), 1197 (s), 1178 (m), 1143 (s), 1091 (s), 1051 (m), 898 (s), 734 (m), 690 (m). Φ = 0.080. [ZnBr2(L)]2·CHCl3. A methanol solution (5 mL) of ZnBr2 (22 mg, 0.1 mmol) was carefully layered onto a chloroform solution (5 mL) of L (37 mg, 0.1 mmol). Colorless crystals of [ZnBr2(L)]2·CHCl3 formed at the interface in 2 days in an 84% yield (55 mg). mp = 241 °C. Anal. Calcd for C45H29Br4Cl3N4O8Zn2: C, 41.24; H, 2.23; N, 4.28%. Found: C, 40.90; H, 2.20; N, 4.34%. IR (KBr, cm−1): 3525 (br), 1747 (m), 1608 (w), 1434 (w), 1284 (s), 1238 (s), 1199 (m), 1176 (w), 1145 (s), 1083 (m), 1051 (m), 904 (w), 732 (m), 692 (w). Φ = 0.042. [ZnBr2(L)]2·2.5CH2Br2. [ZnBr2(L)]2·2.5CH2Br2 was prepared in the same manner as the above [ZnBr2(L)]2·CHCl3, except that dibromomethane was employed instead of chloroform. mp = 221 °C. Anal. Calcd C46H32Br8N4O8Zn2 ([ZnBr2(L)]2·2CH2Br2): C, 35.90; H, 2.10; N, 3.64%. Found: C, 36.98; H, 2.13; N, 3.61%. IR (KBr, cm−1): 3523 (br), 1748 (m), 1608 (w), 1434 (w), 1283 (s), 1237 (s), 1198 (m), 1176 (w), 1144 (s), 1083 (m), 1052 (m), 904 (w), 732 (m), 692 (w). Φ = 0.039. [ZnI2(L)]2·CH2Cl2. An ethanol solution (5 mL) of ZnI2 (31 mg, 0.1 mmol) was carefully layered onto a dichloromethane solution (5 mL) of L (37 mg, 0.1 mmol). Colorless crystals of [ZnI2(L)]2·CH2Cl2 formed at the interface in 2 days in a 67% yield (49 mg). mp = 247 °C. Anal. Calcd for C45H30Cl2I4N4O8Zn2: C, 36.92; H, 2.07; N, 3.83%. Found: C, 37.10; H, 1.99; N, 3.81%. IR (KBr, cm−1): 3446 (br), 1749

(s), 1608 (m), 1432 (w), 1282 (s), 1240 (m), 1199 (m), 1176 (w), 1145 (m), 1085 (m), 1049 (m), 904 (w), 730 (m), 692 (w). Φ = 0.015. Quantum Yield. The PL quantum yield was calculated, on the basis of the absorption and fluorescence spectra measured in the solvent, as

Φsample = Φst

2 Su λst nDu 2 Sst λ u nDst

(1)

where Φst is the emission quantum yield of the standard (i.e., Firpic, Φ = 0.4236), λst and λu represent the absorbance of the standard and sample at the excited wavelength, respectively, Sst and Su are the integrated emission band areas of the standard and sample, respectively, nDst and nDu are the solvent refractive indices of the standard and sample, respectively, and u and s represent each zinc(II) compound and the standard, respectively. Crystal Structure Determination. X-ray data were collected on a Bruker SMART automatic diffractometer equipped with graphitemonochromated Mo Kα radiation (λ = 0.71073 Å) and a CCD detector at −25 °C. Thirty-six frames of two-dimensional diffraction images were collected and processed to obtain the cell parameters and orientation matrix. The data were corrected for Lorentz and polarization effects. The absorption effects were corrected using the multiscan method (SADABS).37 The structures were solved using the direct method (SHELXS 97) and refined by full-matrix least-squares techniques (SHELXL 97).38 The non-hydrogen atoms were refined anisotropically, and the hydrogen atoms were placed in calculated positions and refined only for the isotropic thermal factors. The crystal parameters along with the procedural information on the data collection and structure refinement are listed in Table 1.

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RESULTS AND DISCUSSION Synthesis. As shown in Scheme 1, the zinc(II) metallacyclodimeric species [ZnX2(L)]2 (X = Cl, Br, I; L = 2,7bis(nicotinoyloxy)naphthalene) were synthesized by selfassembly of ZnX2 with L, yielding colorless single crystals suitable for X-ray crystallographic analysis. IR and elemental analyses confirmed the formation of the proposed complexes. The reactions were performed originally in the 1:1 mole ratio of Zn(II):L, but the products were not significantly affected by either the mole ratio or the solvents. The reactions afforded cyclodimeric products instead of coordination polymeric species, irrespective of solvents, concentrations, and anions, 4462

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local geometry around the zinc(II) ion approximates to a typical tetrahedral arrangement, with two nitrogen donors from two ligands and two halides. L connects two zinc(II) ions to form a molecular circle, with a Zn···Zn separation of 15.506(4) Å for [ZnCl 2 (L)] 2 ·CH 2 Cl 2 ·2CH 3 CN, 15.647(1) Å for [ZnBr2(L)]2·CHCl3, and 15.541(1) and 16.929(1) Å for [ZnI2(L)]2·CH2Cl2. Figure 1 shows the molecular circles to

Scheme 1. Schematic Diagram for the Present Complexes

which indicated that the metallacyclodimeric species were thermodynamically stable. In this light, the discrete cyclodimeric zinc(II) complexes’ formation might be attributable to the intrinsic properties of the stable hemicircle-type conformation of L. The products were dissociated in N,Ndimethylformamide and dimethyl sulfoxide but were insoluble in acetone, acetonitrile, chloroform, dichloromethane, and hexane. Crystal Structures. The relevant bond lengths and angles are listed in Table 2. X-ray crystallographic measurements showed all of the skeletons except the halide anions to be very similar (Figure S1, Supporting Information). The single crystals of [ZnI2(L)]2·CH2Cl2, in contrast to those of [ZnCl2(L)]2· CH2Cl2·2CH3CN and [ZnBr2(L)]2·CHCl3, consisted of two slightly different cyclodimers. All of the skeletal structures afforded a 32-membered centrosymmetric cyclodimer. The

Figure 1. Top (left) and side (right) views of [ZnBr2(L)]2·CHCl3 showing yellow-highlighted suprachannels.

be stacked in an eclipsed mode along the a-axis (interdigitated π···π distances = 3.44(8) Å for [ZnCl2(L)]2·CH2Cl2·2CH3CN; 3.44(5) Å for [ZnBr2(L)]2·CHCl3; 3.35(1), 3.72(3) Å for [ZnI2(L)]2·CH2Cl2), thereby effecting the formation of 1D suprachannels of 3.5 × 6.9, 4.3 × 4.9, and 3.2 × 3.6 Å2

Table 2. Selected Bond Distances (Å) and Angles (deg) for [ZnCl2(L)]2·CH2Cl2·2CH3CN, [ZnBr2(L)]2·CHCl3, and [ZnI2(L)]2· CH2Cl2 [ZnCl2(L)]2·CH2Cl2·2CH3CN

[ZnBr2(L)]2·CHCl3

Zn(1)−N(1) Zn(1)−N(2)a Zn(1)−Cl(1) Zn(1)−Cl(2) N(1)−Zn(1)−N(2)a N(1)−Zn(1)−Cl(1) N(1)−Zn(1)−Cl(2) N(2)a−Zn(1)−Cl(1) N(2)a−Zn(1)−Cl(2) Cl(1)−Zn(1)−Cl(2)

2.08(1) 2.104(8) 2.213(4) 2.205(3) 104.4(4) 110.2(3) 105.9(3) 103.4(3) 104.8(3) 126.1(1)

Zn(1)−N(1) Zn(1)−N(2)c Zn(1)−I(1) Zn(1)−I(2) N(1)−Zn(1)−N(2)c N(1)−Zn(1)−I(1) N(1)−Zn(1)−I(2) N(2)c−Zn(1)−I(1) N(2)c−Zn(1)−I(2) I(1)−Zn(1)−I(2)

2.071(5) 2.065(5) 2.5362(7) 2.5501(7) 101.1(2) 108.0(1) 105.9(1) 110.7(1) 107.3(1) 121.80(3)

Zn(1)−N(1) Zn(1)−N(2)b Zn(1)−Br(1) Zn(1)−Br(2) N(1)−Zn(1)−N(2)b N(1)−Zn(1)−Br(1) N(1)−Zn(1)−Br(2) N(2)b−Zn(1)−Br(1) N(2)b−Zn(1)−Br(2) Br(1)−Zn(1)−Br(2)

2.060(5) 2.062(5) 2.3402(9) 2.3516(9) 98.1(2) 110.1(2) 108.0(1) 105.9(2) 105.1 (2) 125.89(4)

Zn(2)−N(3) Zn(2)−N(4)d Zn(2)−I(3) Zn(2)−I(4) N(3)−Zn(2)−N(4)d N(3)−Zn(2)−I(3) N(3)−Zn(2)−I(4) N(4)d−Zn(2)−I(3) N(4)d−Zn(2)−I(4) I(3)−Zn(2)−I(4)

2.067(4) 2.077(5) 2.5403(7) 2.5516(7) 95.0(2) 107.7(1) 111.1(1) 111.6(1) 105.3(1) 122.66(3)

[ZnI2(L)]2·CH2Cl2

a

1 − x, 1 − y, 1 − z. b1 − x, 1 − y, 2 − z. c2 − x, 3 − y, 2 − z. d2 − x, 3 − y, 1 − z. 4463

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dimensions, respectively (Figure S2, Supporting Information). Figure 2 shows these metallacyclodimers forming, via the

Figure 2. Top (top) and side (bottom) views of [ZnBr2(L)]2·CHCl3 showing interdigitated π···π interactions.

interdigitated π···π interactions, an ensemble constituting a unique columnar stacking structure in an eclipsed fashion. In the case of [ZnI2(L)]2·CH2Cl2, two different metallacyclodimers were stacked alternately along the a-axis (Figure S2). The most interesting feature, illustrated in Figure 3, is that all of the halomethane molecules exist within the suprachannels of the 1D ensembles, suggesting that the suprachannels are suitable for halomethane molecules. In the case of [ZnCl2(L)]2· CH2Cl2·2CH3CN, the acetonitrile solvate molecules stay in the outside vacancy of the 1D suprachannels (Figure S3, Supporting Information). Recognition of Dihalomethane Molecules. In order to determine the distinguishability in recognition among the three metallacyclodimers, a mixture of CH2Cl2, CH2Br2, and CH2I2 was employed in a series of substrate molecules. All of the solvate molecules of the present zinc(II) compounds could be removed at 50 °C in vacuum, as confirmed by reference to the IR, elemental analysis, and 1H NMR results. When the evacuated [ZnCl2(L)]2, [ZnBr2(L)]2, and [ZnI2(L)]2 species were immersed in CH2Cl2, CH2Br2, and CH2I2, respectively, the individual samples could absorb the solvent molecules, thus forming [ZnCl 2 (L)]2 ·CH 2Cl 2, [ZnBr2 (L)] 2·CH 2Br2, and [ZnI2(L)]2·CH2I2. Furthermore, each evacuated solid sample ([ZnX2(L)]2) was immersed in a mixture of CH2Cl2, CH2Br2, and CH2I2 (v/v/v = 1:1:1) for 1 day. The 1H NMR spectrum of each immersed sample was measured in Me2SO-d6 in order to confirm the incorporation of halomethane molecules, even though each compound was dissociated in the solution. [ZnCl2(L)]2 and [ZnBr2(L)]2 incorporated a mixture of CH2Cl2, CH2Br2, and CH2I2 in 7:2:1 and 1:1:2 ratios, respectively, while [ZnI2(L)]2 absorbed CH2I2 molecules

Figure 3. Illustration showing halomethane solvate molecules within [ZnCl2(L)]2·CH2Cl2·2CH3CN, [ZnBr2(L)]2·CHCl3, and [ZnI2(L)]2· CH2Cl2 suprachannels (top to bottom).

exclusively, as depicted in Figure S4 (Supporting Information). [ZnI2(L)]2, in other words, proved very useful in discriminating CH2I2. The effective inclusion of CH2I2 in [ZnI2(L)]2 species was further confirmed by UV−vis measurements. According to a control experiment, the pristine CH2I2 did not show the absorption band. After 1 day, however, CH2I2 was easily decomposed under light, exhibiting the absorption maximum at 495 nm. The respective [ZnCl2(L)]2, [ZnBr2(L)]2, and [ZnI2(L)]2 (100 mg) were immersed into mixed solvents of CH2I2 (8 μL) and CH2Cl2 (1.0 mL). The mixtures were left for 1 day in the dark. Then, the resulting mixtures were exposed to light for 1 day in order to decompose the remaining CH2I2. According to UV−vis spectroscopic measurements of the filtrate, the band intensity at λmax = 495 nm was in the order [ZnCl2(L)]2 > [ZnBr2(L)]2 > [ZnI2(L)]2, indicating that the absorption ability of CH2I2 molecules is in the order [ZnCl2(L)]2 < [ZnBr2(L)]2 < [ZnI2(L)]2 (Figure 4). This established, significantly, that the [ZnI2(L)]2 ensemble suprachannel enclathrates only CH2I2 molecules, even in mixed solvents. The inclusion of CH2I2, which is known to be unstable,34 means that the present suprachannel materials can be used as its stabilizer. We attempted to self-assemble in CH2Br2 and/or CH2I2 to obtain single crystal CH2Br2@ [ZnX2(L)]2 and CH2I2@[ZnX2(L)]2. The reaction of ZnBr2 with L in a mixed solvent of methanol and CH2Br2 produced the single crystals consisting of [ZnBr2(L)]2·2.5CH2Br2. As 4464

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[ZnBr2(L)]2·CHCl3, and at 517 nm (λex = 390 nm) for [ZnI2(L)]2·CH2Cl2. The PL bands of [ZnCl2(L)]2·CH2Cl2· 2CH3CN and [ZnBr2(L)]2·CHCl3 were blue-shifted relative to the corresponding ligand L (λem = 513 nm; λex = 370 nm), while [ZnI2(L)]2·CH2Cl2 was red-shifted. The ligand’s intrinsic band seemed to play a pivotal role in the appearance of the complexes’ PL bands. These results suggest that the PL bands can be ascribed to the LMCT.40,41 The bathochromic shifts of [ZnCl2(L)]2·CH2Cl2·2CH3CN and [ZnBr2(L)]2·CHCl3 relative to L could be explained by the electronegativity of chloride and bromide and the coordination environments of the zinc(II) ions.42−44 The present PL intensities, correspondingly, were in the order [ZnCl2(L)]2·CH2Cl2·2CH3CN > [ZnBr2(L)]2·CHCl3 > [ZnI2(L)]2·CH2Cl2. It is meaningful, additionally, to point out that [ZnCl2(L)]2·CH2Cl2·2CH3CN showed a stronger band than either [ZnBr2(L)]2·CHCl3 or [ZnI2(L)]2·CH2Cl2 at room temperature. Such a difference could be attributed to the significant increase of the Zn−Cl bond rigidity and the reduction in the loss of energy through nonradiative decay. The quantum efficiency of [ZnCl2(L)]2·CH2Cl2·2CH3CN (Φ = 0.08) was much lower than that of the known iridium(III) complex, Firpic (Φ = 0.42).36 Thermal Properties and Morphology. The thermogravimetric analysis results for [ZnCl2(L)]2·CH2Cl2·2CH3CN, [ZnBr2(L)]2·CHCl3, and [ZnI2(L)]2·CH2Cl2 are plotted in Figure S9 (Supporting Information). As is apparent, the skeletal structures were thermally stable up to 326, 326, and 336 °C, respectively, and collapsed in the 326−600 °C range. All of the compounds showed melting points (212, 241, and 247 °C, respectively) before decomposition. The solvate molecules began to evaporate at 25−80 °C. The skeletal structures’ hightemperature collapse seemed attributable to the stable, interdigitated π···π interaction-based arrangement. All of the compounds, moreover, were stable over a significantly wide temperature range after melting. In order to confirm the morphological change during calcination, [ZnCl2(L)]2·CH2Cl2· 2CH3CN, [ZnBr2(L)]2·CHCl3, and [ZnI2(L)]2·CH2Cl2 were calcined for 1, 2, and 4 h at 600 °C, and structurally determined with reference to SEM measurements. It was found that all of the compounds ultimately had changed to zinc(II) oxide crystals but that their morphologies differed slightly according to their respective halides, as shown in Figure 6 and Figure S7 (Supporting Information). The zinc(II) oxide crystal formation was further supported by EDX and powder XRD diffraction data. The final morphologies were characteristic flower-shaped ones. The specific process for [ZnCl2(L)]2·CH2Cl2·2CH3CN was as follows: after 1 h calcination at 600 °C, the crystals exhibited a molten-like state; after 2 h, they changed to flowershaped particles of 5 μm in size; and finally, after 4 h, they grew into the blunt-petaline morphology. The flowers’ unit-crystal size was about 1 × 3 μm2. The process of the formation of the flower shape was disclosed in time-course images as follows: melting → evaporation of organic portions → formation of unit crystals → orientation of unit crystals via intercrystal interaction → flower-shaped particles. The formation time of the flowershaped morphology differs slightly according to the halogen anions, as shown in SEM images of [ZnBr2(L)]2·CHCl3 and [ZnI2(L)]2·CH2Cl2 calcined for 2 h at 600 °C (Figure S10, Supporting Information). These results established that this method is an efficient means of exercising, via structural difference and calcination time, morphological control of zinc(II) oxide crystals. Zinc(II) oxide has been employed as an important additive to various products, including plastics,

Figure 4. UV−vis spectra of remaining solution after immersion of the evacuated [ZnCl2(L)]2 (green), [ZnBr2(L)]2 (red), and [ZnI2(L)]2 (blue) into CH2I2 in CH2Cl2 at room temperature. Black line represents the mixed CH2I2/CH2Cl2 solvents without [ZnX2(L)]2 species.

depicted in Figure S5 (Supporting Information), the solvate dibromomethane molecules exist within both the 1D suprachannels comprising the metallacyclodimeric species [ZnBr2(L)]2 and the outside vacancy of the 1D suprachannels. The reactions of ZnX2 with L in other dihalomethane solvent systems yielded the thin-plate-shaped crystalline products, indicating that the cyclodimeric skeletal structures were not affected by the solvents. The inclusion of solvate dihalomethane molecules in the crystals was confirmed via 1H NMR spectra (Figures S6−S8, Supporting Information). In fact, the total solvent-accessible volumes for the desolvated skeletons after removal of the solvate molecules are estimated to be 28.7, 18.5, and 16.3% for [ZnCl2(L)]2, [ZnBr2(L)]2, and [ZnI2(L)]2, respectively, calculated by PLATON.39 Notably, in this regard, [ZnI2(L)]2 predominantly adsorbs CH2I2 molecules, which is consistent with the “like attracts likes” model. During adsorption/desorption, the IR peaks remain unchanged except for the CH2I2 bands, suggesting that the suprachannel is retained thereafter. Thus, this suprachannel system can be applied as a tailored storage or stabilizer for unstable diiodomethane. But what is the critical driving force behind the diiodomethane container? We attributed the formation of each structure to the felicitous “size influence” and the “weak non-covalent halogen interaction” aspects. Photoluminescence. The photoluminescence (PL) spectra of the three zinc(II) crystals, together with those of L, were measured in the solid state at room temperature (Figure 5). The emission band was observed at 497 nm (λex = 370 nm) for [ZnCl2(L)]2·CH2Cl2·2CH3CN, at 507 nm (λex = 380 nm) for

Figure 5. Solid-state PL spectra of L (black), [ZnCl2(L)]2·CH2Cl2· 2CH3CN (green), [ZnBr2(L)]2·CHCl3 (red), and [ZnI2(L)]2·CH2Cl2 (blue) at room temperature. Inset: photograph of [ZnCl2(L)]2· CH2Cl2·2CH3CN under UV-lamp irradiation at 365 nm. 4465

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1:1:1), 1H NMR spectra and optical microscope images of crystalline [ZnX2(L)]2 products obtained in mixed solvents of CH2Br2 and/or CH2I2, the TGA/DSC curves of the present zinc(II) compounds, and SEM images of the calcined morphologies of [ZnBr2(L)]2·CHCl3 and [ZnI2(L)]2·CH2Cl2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +83-52-516-7421. Tel: +82-51-510-2591. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean Government [MEST] (2013R1A2A2A07067841).

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Figure 6. SEM images of morphology of thermal residue of [ZnCl2(L)]2·CH2Cl2·2CH3CN calcined for 1 (a), 2 (b), and 4 h (c) at 600 °C. Bar = 5 μm. Inset: powder XRD patterns for ZnO as thermal residue (white) and reference patterns (blue) from ICDD database (PDF no. 36-1451).

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ceramics, glass, cement, lubricants, paints, ointments, sealants, pigments, foods (zinc-nutrient sources), ferrites, fire retardants, and first-aid bandages. In materials science, the different morphologies of zinc(II) oxide crystals serve specific functional materials such as wide-band-gap semiconductors of excellent transparency, high electron mobility, and significant luminescence.45

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CONCLUSION Self-assembly of ZnX2 with 2,7-bis(nicotinoyloxy)naphthalene (L) as a hemicircular chromophore bidentate ligand yields cyclodimeric [ZnX2(L)]2 zinc(II) complexes as useful receptors. These form, via interdigitated π···π interactions, an ensemble constituting a suprachannel of unique columnar stacking structure that can stabilize diiodomethane. This system can function as an unprecedented stabilizing host via the appropriate host−guest recognition. Furthermore, it has significant halogen effects on photoluminescence (PL), thermal properties, and stabilization. This system, effectualized by interdigitated π···π interactions, appears to represent an important conceptual advance in suprachannel development. Calcination of its materials results in the formation of flowershaped zinc(II) oxide morphologies. Upcoming experiments will provide more detailed information on the complexes’ enormous recognition and photoluminescence potentials.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data in CIF format, crystallographic parameters of [ZnBr2(L)]2·2.5CH2Br2, ORTEP drawings of the present zinc(II) compounds, illustrations showing the suprachannel dimensions, packing diagrams of [ZnCl2(L)]2·CH2Cl2· 2CH3CN showing the encapsulation of CH2Cl2 and CH3CN molecules, the 1H NMR spectrum of each sample (i.e., [ZnCl2(L)]2, [ZnBr2(L)]2, and [ZnI2(L)]2) immersed for 1 day in a mixed solvent of CH2Cl2, CH2Br2, and CH2I2 (v/v/v = 4466

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Crystal Growth & Design

Article

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