Reversible Luminescence Vapochromism and Crystal-to-Amorphous

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Reversible Luminescence Vapochromism and Crystal-to-Amorphousto-Crystal Transformations of Pseudopolymorphic Cu(I) Coordination Polymers Gihaeng Kang,† Youngeun Jeon,† Kang Yeol Lee,‡ Jineun Kim,*,† and Tae Ho Kim*,† †

Department of Chemistry and Research Institute of Natural Science, Gyeongsang National University, 501 Jinju-Daero, Jinju 52828, Republic of Korea ‡ School of Mechanical Engineering, Korea University, 145 Anam-ro, Seongbuk-Gu, Seoul 02841, Republic of Korea S Supporting Information *

ABSTRACT: Four solvent-responsive one-dimensional copper(I) coordination polymers (CPs), namely, {[Cu4I4L(MeCN)2]·CH2Cl2}n (1), {[Cu4I4L(MeCN)2]·CHCl3}n (2), {[Cu4I4L(MeCN)2]·0.5p-xylene}n (3), and [Cu4I4L(MeCN)2]n (4), were prepared by reaction of CuI with N,N′bis[2-(cyclohexylthio)ethyl]pyromellitic diimide (L) via self-assembly under varying solvent conditions. CPs 1−4, which are pseudopolymorphic supramolecular isomers derived from solvent molecules, are composed of Cu4I4 cubane clusters. The ligands in CPs 1−3 adopted a synconformation, whereas in CP 4 they were observed in the anticonformation. This occurred via syn to anti transitions upon heating, followed by exposure to MeCN vapor. In addition, a reversible anti to syn transition was achieved by agitating in mixed organic solvents. It was shown that ligand transition from the syn- to the anti-conformation occurred through crystal-to-amorphous-to-crystal transformations. Furthermore, CPs 1−3 exhibited reversible solvent exchange and crystal transformation by exposure to vapors from volatile organic compounds.

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rhomboid dimers and stair-step polymers. Although acetonitrile and dichloromethane were used as solvents and structural transformations were induced by controlling the ligand/metal ratio, some reversible crystal transformations were also observed.15 We later reported the preparation of four CPs containing Cu2I2 clusters by reaction of CuI with a ligand bearing both pyrazine and sulfur moieties. In addition, we reported a controlled reversible crystal transformation that was derived by either heat or from the exchange of solvent molecules at specific temperatures. Acetonitrile played a key role in solvent exchange by opening the space inside the CP framework, and although reversible transformation was seen, crystal transformation based on solvent vapor was not observed.16 We therefore chose to investigate the possibility of reversible crystal transformations induced by volatile organic compound (VOC) vapors by employing ligand conformation change and crystal-to-amorphous-to-crystal (CAC) transformations, according to stepwise stimulation by heat and solvent vapors.18−21 A dithioether ligand, N,N′-bis[2-(cyclohexylthio)ethyl] pyromellitic diimide (L), bearing an electron-deficient pyromel-

oordination polymers (CPs) have recently received a great amount of attention, with studies focusing on the control of geometric structures of compounds formed by coordination between well-designed ligands and metal cations. Furthermore, the production of compounds exhibiting useful functionalities has also been investigated. CPs have the potential for use in various fields, such as catalysis,1−3 ion exchange,4,5 selective adsorption,6−8 hydrogen storage,9,10 sensors,11,12 and magnetism-based applications.13,14 Moreover, a number of studies are currently underway regarding the crystal transformation of CPs by control of anionic, solvent, ligand, and metal cationic molar ratios. Predicting such structural changes is not easy; hence, many chemists have invested effort in identifying and controlling the influences of the factors that induce structural changes. Our aim was to focus on reversible/nonreversible structural transformations in copper(I) CPs by variation of temperature and solvent. Furthermore, we wished to investigate their emission changes under the above conditions. We expect that a number of structural transformations will be related to conformational changes of the sulfur-containing organic ligands and the labile Cu−S coordination bond.15−17 We investigated the preparation of four CPs by reaction of CuI with a dithioether ligand bearing a pyromellitic diimide spacer. We found that by varying the ligand to metal ratio, complexes with different structures were formed, including © XXXX American Chemical Society

Received: August 20, 2015 Revised: September 21, 2015

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DOI: 10.1021/acs.cgd.5b01199 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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litic diimide spacer was synthesized by reacting pyromellitic dianhydride with 2-(cyclohexylthio)ethylamine according to a previous literature method.15 Plate-shaped single crystals suitable for single crystal X-ray diffraction (SCXRD) analysis were obtained by recrystallization in CH2Cl2 (Scheme S1, Supporting Information). The four luminescent copper(I) CPs, namely, {[Cu4I4L(MeCN)2]·CH2Cl2}n (1), {[Cu4I4L(MeCN)2]·CHCl3}n (2), {[Cu4I4L(MeCN)2]·0.5pxylene}n (3), and [Cu4I4L(MeCN)2]n (4), were synthesized under controlled solvent conditions with a 4:1 CuI to L molar ratio (Scheme 1). Crystalline CPs 1−3 were obtained within 12 Scheme 1. Preparation and Crystal Transformation of CPs 1−4

Figure 1. Intermolecular interactions in CPs: (a) 1, (b) 2, (c) 3, and (d) 4. The I−···π and S···π interactions and the C−H···O and C−H··· Cl hydrogen bonds are represented by dashed yellow lines. Hydrogen atoms without intermolecular interactions are omitted for clarity. pXylene molecules are omitted for clarity in CP 3.

interactions. CPs 1 and 2 crystallized as orthorhombic Pbca space groups exhibiting crystallographically imposed inversion centers. In these crystals, the ligands were situated in the center of symmetry. CPs 3 and 4 crystallized as monoclinic P21/n space groups exhibiting crystallographically imposed inversion centers. The 1D chain structures of CPs 1−4 were composed of a single cubane Cu4I4 cluster along with two acetonitrile molecules and one ligand molecule. In CPs 1−4, two copper ions were coordinated by three iodide ions and one sulfur atom, whereas the other two copper ions had a distorted tetrahedral structure, coordinated by three iodide ions and the nitrogen atom of an acetonitrile molecule, which exhibits stretching vibrations at 2273 and 2304 cm−1 (Figure S7 in the SI). The Cu−Cu (2.6045(6)−2.8797(13) Å), Cu−S (2.2937(15)− 2.307(2) Å), and Cu−N (1.963(3)−1.989(2) Å) distances were within the ranges of other previously reported Cu(I)sulfur complexes.15 The 1D chain structure of CP 1 is stacked in the [100] direction with dichloromethane molecules trapped between the chains. Since the Cu4I4 clusters sit on both faces of the pyromellitic diimide plane, the crystal structure shows a symmetrical intermolecular interaction consisting of two C− H···O hydrogen bonds (C15···O2, 3.189(8) Å, H15C···O2, 2.48 Å), two I−···π interactions (I2···C9, 3.620(5) Å), and two S···π interactions (S1···C9, 3.331(6) Å). In addition, the stabilization of the structure is increased by the presence of two C−H···Cl hydrogen bonds (C16···Cl1, 3.16(3) Å, H16A···Cl1, 2.19 Å) between the dichloromethane molecules, and one C− H···Cl hydrogen bond (C2···Cl2, 3.70(2) Å, H2B···Cl2, 2.74 Å) between the ligand L and a dichloromethane molecule (Figure 1a). CP 1 therefore forms a 2D net structure that is parallel to the (110) plane (Figure S3 in the SI). For CP 2, the 1D chains are lined up along the [100] direction, and chloroform molecules fill the spaces between the chains. Similar to CP 1, CP 2 possesses two C−H···O hydrogen bonds (C15···O1, 3.184(5) Å, H15B···O1, 2.53 Å), two I−···π interactions (I2···C12, 3.607(3) Å), and two S···π interactions (S1···C12, 3.278(3) Å) (Figure 1b). Furthermore, an additional

h using DCM/MeCN (3:1), CHCl3/MeCN (3:1), and pxylene/MeCN (1:1), respectively, while CP 4 was obtained as a minor product without guest molecules, and CP 1 was the major product on using DCM/MeCN (3:1) at room temperature for over one month. This suggests that CPs 1−3 are kinetically stable, while CP 4 is thermodynamically stable. Crystals suitable for SCXRD were obtained by carefully applying a solution of CuI to the top of a solution of L, and subsequent diffusion of the two reactants. Characterization of CPs 1−4 was carried out by elemental analysis (EA), thermogravimetric analysis (TGA), differential thermal analysis (DTA), photoluminescence (PL), SCXRD, powder X-ray diffraction (PXRD), and Fourier transform infrared spectroscopy (FT-IR) (see SI for further information). As the chemical formulas for CPs 1−4 either do not contain solvents, or bear different solvents, these CPs exhibited solventinduced supramolecular isomerism. In addition, reversible structural transformations between CPs 1−4 were also observed. CPs 1−3 showed the potential to undergo vaporinduced solvent exchange, accompanied by small structural changes and vapochromism. These reversible processes are believed to originate from MeCN coordinating to Cu4I4 clusters, and opening the space inside the molecular skeleton to allow facile solvent exchange. Moreover, the crystal transformations from CPs 1−3 to CP 4 proceeded via a stepwise method using MeCN vapor, following heating to 120 °C. This resulted in both a change in ligand conformation and a CAC transformation. The structures of pseudopolymorphic isomers CPs 1−4 were determined to have one-dimensional (1D) chain structures via SCXRD (Tables S1−S3 and Figures S1−S6 in the SI). Figure 1 shows the structures of the CPs including intermolecular B

DOI: 10.1021/acs.cgd.5b01199 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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C−H···O hydrogen bond (C16···O2, 3.397(10) Å, H16···O2, 2.43 Å) exists between the ligand L and a chloroform molecule, which contributes to structure stabilization. This enables a twodimensional (2D) net structure to be formed parallel to the (110) plane (Figure S4 in the SI). In the structure of CP 3, the 1D chains are stacked along the [001] direction, while p-xylene molecules fill the space between the chains. Compared to CPs 1−2, CP 3 possesses a skeletal structure that is slightly slanted due to the size of the guest molecules. It also possesses two C−H···O hydrogen bonds (C28···O1, 3.203(11) Å, H28C···O1, 2.53 Å, C30···O4, 3.209(11) Å, H30A···O4, 2.48 Å), two I−···π interactions (I1···C11, 3.602(7) Å, I4···C15, 3.666(6) Å), and one S···π interaction (S2···C12, 3.424(8) Å) (Figure 1c). However, unlike CPs 1−2, CP 3 does not show direct interactions between its skeletal chain structure and the p-xylene guest molecule. As a result, the crystal structure forms a 2D net structure parallel to the [101] surface through the five interactions (Figure S5 in the SI). CP 4, which contains no guest molecule, is lined up in the [301̅] direction. As CP 4 only exhibits one I−···π interaction between the pyromellitic diimide plane and the Cu4I4 cluster (I4···C3, 3.619(2) Å) (Figure 1d), the structure is stabilized only by this interaction, and a 2D net structure parallel to the [103] plane is formed (Figure S6 in the SI). Moreover, the ligands of CPs 1−3 adopt a syn-conformation with respect to arrangement of the sulfur atoms in the same direction as the pyromellitic diimide plane, while CP 4 adopts an anticonformation (Figure 1 and Figure S8 in the SI). Figure 2a shows the photoluminescence spectra of CPs 1−4 at an excitation wavelength of 400 nm. CPs 1−4 showed luminescence maxima at 604, 606, 588, and 583 nm, respectively, exhibiting the typical orange luminescence spectrum of the cubane tetramer. Such luminescence likely results from the mixing of X-to-ligand charge-transfer (XLCT) and metal-to-ligand charge transfer (MLCT), wherein the former charges are transferred from electron-rich iodide ions I− (X, halogen) to relatively electron-poor ligands, and in the latter charges are transferred from the metal cluster to the ligand.22 In CPs 1 and 2, two I−···π and two S···π interactions cause withdrawal of electron density from the Cu4I4 cluster, and thus luminescence peaks were observed at longer wavelengths than for CPs 3 and 4. In addition, CP 3 has one less S···π interaction than CPs 1 and 2, inducing less electron density withdrawal, and thus its luminescence peak appeared at a shorter wavelength than those of CPs 1 and 2. Furthermore, as CP 4 contains a C−H···I hydrogen bond and does not interact with electron-deficient pyromellitic diimide spacers, its luminescence peak was observed at the shortest wavelength of all four CPs. As shown in Figure 2b, CPs 1−3 showed reversible solvent exchange according to the specific ratios of VOC vapors, and as a result, transformation between the crystals occurred more readily.16,23 We investigated whether powdered specimens of CPs 1−3 exhibited vapochromism when coated on a glass slide (left to right) and exposed to a mixed solvent vapor with a specific ratio at room temperature (Figure S9 in the SI).24,25 When the prepared specimens were exposed to the vapor of a dichloromethane/acetonitrile 5:1 mixture, CPs 2 and 3 exhibited comparable luminescence with CP 1, thus being in accord with solvent exchange using dichloromethane and structural transformation from CPs 2 and 3 to CP 1. When the vapor from a chloroform/acetonitrile 5:1 mixture was used, solvent exchange using chloroform and

Figure 2. (a) Solid-state photoluminescence spectra of CPs 1−4. (b) Photographic images of CPs 1−3 exposed to different solvent vapors.

crystal transformation to CP 2 occurred. Luminescence vapochromism was also observed. Similarly, solvent exchange by p-xylene and structural transformation to CP 3 occurred through the use of the vapor from a p-xylene/acetonitrile 5:1 mixture, and transformation was verified via PXRD and PL spectroscopy (Figures S14 and S15 in the SI). These solvent exchanges and crystal transformations are believed to originate from the coordination capability of acetonitrile on copper(I) ions disturbing the crystalline structure, and consequently, opening the space inside the complex framework to allow solvent exchange. Model studies have previously confirmed intracrystalline solvent-molecule exchange using solvent mixtures containing acetonitrile.16 However, in the present study, we report reversible solvent-molecule exchange and structural transformation between CPs 1−3 using solvent vapors, and thus these materials have potential for use in VOC sensors. TGA/DTA measurements were also taken for CPs 1−4 to examine their thermal stability (Figure S16 in the SI), with all TGA curves displaying a similar weight loss pattern. Both the solvent (i.e., the guest molecule) and the coordinated MeCN molecules were eliminated between 80 and 120 °C, while the ligand decomposed between 300 and 450 °C. The DTA curves of CPs 1−4 showed two heat absorption peaks between 100 and 250 °C. As such, heat absorption tends to arise from the elimination of guest molecules and coordinated MeCN molecules, the DTA data suggest that structural transformation occurred in conjunction with weight loss. Thus, as shown in Figure 3, the crystal transformation pathway was verified by heating CPs 1−3 at 120 °C for 30 min, followed by exposure to MeCN vapor for 1 h. Initially, heating at 120 °C resulted in C

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stability, and XRD patterns of each CP were examined. CPs 1− 3 exhibited reversible solvent exchange and crystal transformation in the presence of VOC vapors. Moreover, transformation from CPs 1−3 to CP 4 was accompanied by ligand transition from the syn- to the anti-conformation with concurrent CAC transformation. When the resulting CP 4 products were stirred in mixed organic solvents of specific ratios, CPs 1−3 were regenerated, demonstrating that fully reversible structural transformations can occur in CPs 1−4. Thus, we could conclude that CPs 1−4 have the potential for use as VOC sensors at room temperature. Future studies will be conducted on structural transformations that add ligands and guest molecules capable of imposing changes in noncrystalline coordination polymers.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01199. CCDC nos. 1417587−1417591 (L and CP 1−CP 4) contain the supplementary crystallographic data for this paper. These data can be obtained from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif Details of experimental procedures for the preparation of CPs; optical, X-ray diffraction, and thermogravimetric analyses (PDF) Crystallographic information files (CIF)

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Figure 3. (a) Photographic images under UV light (on/off). (b) PXRD patterns representing the CAC transformations.

AUTHOR INFORMATION

Corresponding Authors

simultaneous detachment of the guest molecules (i.e., dichloromethane, chloroform, and p-xylene) and coordinated acetonitrile molecules. This led to loss of the luminescent characteristics, but formation of the noncrystalline CPs 1′−3′, as evidenced by broad PXRD patterns (Figure 3b). Disappearance of the acetonitrile peaks in the FT-IR spectra upon heating confirmed once again that coordinated acetonitrile had been removed (Figure S7 in the SI). Surprisingly, when acetonitrile vapor was applied to CPs 1′−3′, acetonitrile became coordinated with the metal center once again, leading to crystal transformation to CP 4 and reappearance of the luminescence characteristics. Observation of this process by SEM showed that sections of the thin plate-like crystals started to lose their crystalline shape and began to crack when solvents escaped from the structure upon heating to 120 °C. SEM images also showed that the application of acetonitrile vapor to these cracked structures resulted in the formation of hard block-type crystals (Figure S17 in the SI). Consequently, structural transformation of CPs 1−3 to CP 4 by CAC transformation took place using a combination of heat and acetonitrile vapor, and was accompanied by simultaneous conformation changes of the ligands from syn to anti. Furthermore, when CP 4 was stirred in mixtures of DCM/MeCN (5:1), chloroform/MeCN (5:1), and p-xylene/MeCN (5:1) without dissolution, CPs 1−3 were regenerated (Figure S18 in the SI), demonstrating the reversibility of the structural transformations in combination with regulation of ligand conformation. In conclusion, we successfully synthesized L containing a pyromellitic diimide spacer group and subsequently prepared four 1D CPs (1−4) by reaction of L with CuI under a range of solvent conditions. SCXRD was used to confirm the structures of the crystals. In addition, the emission spectra, thermal

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (Nos. 2014R1A1A4A01009105 and 2015R1D1A4A01020317).

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