Viologen-Based Photochromic Coordination Polymers for Inkless and

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Viologen-Based Photochromic Coordination Polymers for Inkless and Erasable Prints Wei-Qiu Kan,† Shi-Zheng Wen,*,‡ Yuan-Chun He,*,§ and Chao-Yue Xu† †

Jiangsu Province Key Laboratory for Chemistry of Low-Dimensional Materials, School of Chemistry and Chemical Engineering, Huaiyin Normal University, Huaian 223300, P. R. China ‡ Jiangsu Province Key Laboratory of Modern Measurement Technology and Intelligent Systems, School of Physics and Electronic Electrical Engineering, Huaiyin Normal University, Huaian 223300, P. R. China § College of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, P. R. China S Supporting Information *

ABSTRACT: Four coordination polymers, namely, [Zn(HL1)(L2)0.5]·H2O (1), [Cd(HL1)(L2)0.5]·H2O (2), [Zn(L1)(L3)0.5]· H2O (3), and [Cd(L1)(L3)0.5] (4) (H3L1 = (3,5-dicarboxyl-phenyl)-(4-(2′-carboxyl-phenyl)-benzyl)ether, H2L2Cl2 = 1,1′bis(4-carboxy-benzyl)-4,4′-bipyridinium dichloride, and L3Cl2 = 1,1′-dimethyl-4,4′-bipyridylium dichloride), have been synthesized hydrothermally. The structures of compounds 1−4 have been determined by single-crystal X-ray diffraction analyses, and further characterized by elemental analyses, infrared (IR) spectra, powder X-ray diffraction (PXRD) analyses, and thermogravimetric analyses. Compounds 1 and 2 display three-dimensional 2-fold interpenetrating frameworks, whereas compounds 3 and 4 exhibit two-dimensional layer structures. These compounds display photochromic behaviors from pale yellow to green under UV light, visible light, or sunlight. The photochromic mechanisms of these compounds have been studied by IR spectra, PXRD analyses, UV−vis absorption spectra, electron paramagnetic resonance spectra, density functional theory calculations, and X-ray photoelectron spectroscopy. The capabilities of compounds 1 and 2 as inkless and erasable printing media have also been tested. Moreover, the photomodulated fluorescence of these compounds has also been investigated.

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reusing printing media, which can decrease the paper wastes.13 In this context, photochromic materials are good candidates for inkless and erasable printing media because of their reversible color changes. However, many reported photochromic materials return to their initial colors in a few minutes. The short lifetimes of the photogenerated radicals limit these photochromic materials to be used for inkless and erasable printing, because the printed contents would disappear in the background in a few minutes. Therefore, the good photochromic materials used as inkless and erasable printing media should have good ability to retain the photogenerated color to make the printing contents readable in enough time. In addition, the photochromic materials should easily return to their initial colors in the dark rather than by heating, so that the printing contents can easily be erased. In this work, four photochromic metal−organic coordination polymers have been constructed from 3,5-(dicarboxyl-phenyl)(4-(2′-carboxyl-phenyl)-benzyl)ether (H3L1; Scheme 1) and two different viologens, 1,1′-bis(4-carboxybenzyl)-4,4′-bipyridinium dichloride (H2L2Cl2; Scheme 1) and 1,1′-dimethyl-4,4′-

INTRODUCTION Photochromic chemical species display reversible color changes by light irradiation.1,2 These materials have attracted intense attention due to their convenient visual monitoring and applications in the fields such as optical switches, solar energy conversion, data storage, photomasks, and so on.3−12 Typical photochromic organic molecules include diarylethene, spiropyran, azobenzene, Schiff base, viologen, and furylfulgide.13,14 Compared with the pure organic molecules, the photochromic properties of metal−organic coordination polymers can be adjusted by the combination of different metal ions and photochromic organic ligands.2,15,16 Viologens are 1,1′disubstituted 4,4′-bipyridinium derivatives, and they are generally used as electron acceptors to build photochromic metal−organic coordination polymers because the substituent groups on the two nitrogen atoms of viologen can be adjusted easily.14,17−22 On the other hand, the excess usage of printed matter has caused serious environmental problems such as deforestation and chemical pollution in recent years.13,23 There are two main solutions to solve these problems. One is using inkless and erasable printing, which can decrease the cost and environmental pollution resulting from the use of ink. The second is © XXXX American Chemical Society

Received: August 27, 2017

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DOI: 10.1021/acs.inorgchem.7b02206 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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Scheme 1. Structures of Organic Ligands Used in This Work

Article

EXPERIMENTAL SECTION

Materials and Methods. H3L1 was prepared according to the procedures reported.24 Other reagents were obtained commercially and used as purchased. A PerkinElmer 2400 LS elemental analyzer was used to collect element analyses data. The IR spectra data were collected on a Nicolet AVATAR360 spectrometer in the range of 4000−400 cm−1. TGA was performed on a NETZSCHSTA 449F3 thermal analyzer from room temperature to 800 °C under nitrogen. PXRD spectra data of the samples were collected on an ARL X’TRA diffractometer. The UV−vis absorption spectra were measured on a UV 1800 spectrophotometer. The EPR spectra were recorded with a Bruker ESP 300E electron paramagnetic resonance spectrometer. The XPS spectra were measured on a ThermoFisher ESCALAB250 X-ray photoelectron spectrometer (powered at 150 W) using Al Kα radiation (λ = 8.357 Å). The solid-state emission/excitation spectra were recorded on a PerkinElmer FLS-920 spectrometer at room temperature. DFT Calculations. The Vienna Ab initio Simulation Package VASP25 with plane waves as the basis set was used to analyze the electron-density distributions of the four compounds. The unit cells of the four compounds were chosen to carry out the calculations using the projector augmented wave method and the generalized gradient approximation expressed by the Perdew−Wang functional.26,27 The plane-wave cutoff energy of all the compounds was set as 450 eV. The Monkhorst−Pack scheme with the 4 × 5 × 7 (compounds 1, 2, and 4) and 5 × 5 × 5 (compound 3) were employed for Brillouin-zone integration in the ground state calculations. Caution! Viologen and cadmium are all poisonous for human health and experiments should only be handled with proper protocols to avoid accidents. Synthesis of [Zn(HL1)(L2)0.5]·H2O (1). H3L1 (39.2 mg, 0.1 mmol), Zn(OAc)2·2H2O (43.9 mg, 0.2 mmol), H2L2·Cl2 (49.7 mg, 0.10 mmol), and 15 mL of H2O were sealed in a 20 mL Teflon reactor and heated at 130 °C for 3 days. After cooling to room temperature, yellow crystals of compound 1 were obtained in a 61% yield. Anal. Calcd for C35H26ZnNO10 (Mr = 685.94): C, 61.28; H, 3.82; N, 2.04. Found: C, 61.40; H, 3.89; N, 2.13%. IR (cm−1): 3601 (w), 3484 (w), 3120 (m), 3052 (m), 2587 (w), 1698 (s), 1659 (s), 1629 (s), 1564 (s), 1442 (s), 1387 (s), 1254 (m), 1131 (m), 1092 (m), 1038 (m), 966 (w), 762 (s), 710 (m), 485 (w). Synthesis of [Cd(HL1)(L2)0.5]·H2O (2). The synthesis of compound 2 was similar to that of compound 1, except that Cd(OAc)2·2H2O (53.3 mg, 0.2 mmol) was used instead of Zn(OAc)2·

bipyridylium dichloride (L3Cl2; Scheme 1) under hydrothermal conditions. It should be pointed out that coordination polymers based on H3L1 have been first reported by Ma et al. in 2013.24 These compounds display diverse highly connected structures with excellent photoelectronic properties.24 Therefore, in this work, H3L1 has been chosen as an organic ligand to construct coordination polymers. The structures of compounds 1−4 have been fully characterized by single-crystal X-ray diffraction analyses, elemental analyses, infrared (IR) spectra, thermogravimetric analyses (TGA), and powder X-ray diffraction (PXRD) analyses. The photochromic mechanisms of these compounds have been investigated by IR spectra, PXRD analyses, UV−vis absorption spectra, electron paramagnetic resonance (EPR) spectra, density functional theory (DFT) calculations, and X-ray photoelectron spectroscopy (XPS). Moreover, the capabilities of compounds 1 and 2 as inkless and erasable printing media and the photomodulated fluorescence of compounds 1−4 have also been investigated.

Table 1. Crystal Data and Structure Refinement Parameters for Compounds 1−4 formula Fw crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z Dc/g cm−3 F(000) R(int) GOF on F2 R1 [I > 2σ(I)]a wR2 (all data)a a

1

2

3

4

C35H26ZnNO10 685.94 monoclinic P21/n 11.4783(11) 16.8081(16) 16.6435(16) 90 109.935(2) 90 3018.6(5) 4 1.509 1412 0.0625 1.032 0.0414 0.1059

C35H26CdNO10 732.97 monoclinic P21/n 11.3243(9) 17.1057(14) 16.8708(14) 90 109.3260(10) 90 3083.9(4) 4 1.579 1484 0.0447 1.028 0.0333 0.0808

C28H22ZnNO8 565.84 triclinic P1̅ 9.5469(12) 11.0049(14) 11.9188(15) 72.625(2) 83.538(2) 80.733(2) 1176.7(3) 2 1.597 582 0.0383 1.038 0.0399 0.0915

C28H20CdNO7 594.85 monoclinic P21/n 10.8853(9) 10.1209(9) 21.3570(18) 90 101.102(2) 90 2308.8(3) 4 1.711 1196 0.0500 1.021 0.0295 0.0697

R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = |∑w(|Fo|2 − |Fc|2)|/∑|w(Fo2)2|1/2. B


Article

Inorganic Chemistry 2H2O. Yellow crystals of compound 2 were obtained in a 38% yield. Anal. Calcd for C35H26CdNO10 (Mr = 732.97): C, 57.30; H, 3.55; N, 1.91. Found: C, 57.21; H, 3.59; N, 1.98%. IR (cm−1): 3731 (w), 3450 (w), 3050 (m), 2605 (w), 1720 (s), 1634 (s), 1551 (s), 1447 (s), 1382 (s), 1265 (s), 1170 (w), 1126 (m), 1048 (m), 961 (w), 853 (w), 762 (s), 719 (m), 481 (w). Synthesis of [Zn(L1)(L3)0.5]·H2O (3). The synthesis of compound 3 was similar to that of compound 1, except that L3·Cl2 (25.7 mg, 0.1 mmol) was used instead of H2L2·Cl2. Yellow crystals of compound 3 were obtained in a 42% yield. Anal. Calcd for C28H22ZnNO8 (Mr = 565.84): C, 59.43; H, 3.92; N, 2.48. Found: C, 59.29; H, 3.85; N, 2.40%. IR (cm−1): 3731 (w), 3445 (m), 3045 (m), 2890 (w), 2855 (w), 1646 (s), 1572 (s), 1451 (s), 1404 (s), 1347 (s), 1265 (s), 1195 (m), 1126 (s), 1052 (s), 953 (w), 827 (w), 784 (s), 714 (s), 666 (w), 523 (w), 446 (w). Synthesis of [Cd(L1)(L3)0.5] (4). The preparation of compound 4 was similar to that of compound 3, except that Cd(OAc)2·2H2O (53.3 mg, 0.2 mmol) was used instead of Zn(OAc)2·2H2O. Yellow crystals of compound 4 were obtained in a 55% yield. Anal. Calcd for C28H20CdNO7 (Mr = 594.85): C, 56.53; H, 3.39; N, 2.35. Found: C, 56.39; H, 3.49; N, 2.42%. IR (cm−1): 3731 (w), 3423 (m), 3051 (m), 2856 (w), 1625 (s), 1560 (s), 1451 (s), 1378 (s), 1260 (s), 1183 (m), 1122 (s), 1048 (s), 885 (w), 771 (s), 723 (s), 663 (w), 472 (w). X-ray Crystallography. The single-crystal X-ray diffraction data of compounds 1−4 were collected on a Bruker SMART APEX II diffractometer equipped with graphite monochromated Mo Kα radiation at 293 K. All the structures were solved by Direct Method of SHELXS-9728 and refined by full-matrix least-squares techniques using the SHELXL-9729 program. All the non-hydrogen atoms of compounds 1−4 were refined with anisotropic thermal parameters. All the hydrogen atoms on carbon atoms were generated geometrically. The hydrogen atoms of carboxylate groups for compounds 1 and 2, and the hydrogen atoms of water molecules for compounds 1−3 were located from difference Fourier maps. The detailed crystallographic data and structural refinement parameters of compounds 1−4 are summarized in Table 1. Selective bond lengths and angles of compounds 1−4 are listed in Tables S1−S4.

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RESULTS AND DISCUSSION Structures of Compounds [Zn(HL1)(L2)0.5]·H2O (1) and [Cd(HL1)(L2)0.5]·H2O (2). Compounds 1 and 2 have similar structures; therefore, only the structure of compound 1 is described in detail. The asymmetric unit of compound 1 contains one Zn(II) ion, one HL1 anion, half a L2 ligand, and one lattice water molecule. As shown in Figure 1a, Zn1 is fivecoordinated in a square-pyramidal coordination geometry, furnished by five oxygen atoms from three different HL1 anions and two different L2 ligands. The coordination geometry was determined by calculating the structural index parameter τ according to the literature.30 For a perfect tetragonal geometry, τ is equal to 0, while, for a perfect trigonal-bipyramid, it becomes 1. In the literature, the geometric parameter was defined as τ = (β − α)/60°, where β and α were the two largest donor−metal−donor angles. In compound 1, the two largest O−Zn−O angles are 156.04° and 155.05° (O9#2−Zn1−O8 and O4#1−Zn1−O3#3), respectively. Therefore, the coordination geometry of Zn1 in compound 1 (τ = 0.0165) is close to the perfect square-pyramid. The Zn−O bond lengths are in the normal range of 1.963(2)−2.066(2) Å as other Zn(II)containing coordination polymers.31−33 Two symmetry-related Zn1 ions are connected by two carboxylate groups to generate a binuclear unit with the Zn···Zn distance of 3.086 Å. Each HL1 anion links two binuclear units through the two carboxylate groups of the isophthalate to form a layer (Figure 1b). The carboxylic group of the biphenyl fragment remains uncoordinated. The layers are further bridged by the L2 ligands to

Figure 1. (a) ORTEP view of compound 1 giving the coordination environment of Zn(II) ion with hydrogen atoms and lattice water molecule omitted for clarity (30% probability displacement ellipsoids). Symmetry codes: #1 −x + 3/2, y − 1/2, −z + 1/2; #2 −x + 2, −y, −z + 1; #3 x + 1/2, −y + 1/2, z + 1/2; #4 −x + 1, −y, −z + 2. (b) View of the 2D layer generated by the Zn(II) ions and HL1 anions. (c) 3D framework of compound 1. (d) View of the 2-fold interpenetrating net with the Schläfli symbol of 412·63. C


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Inorganic Chemistry generate a 3D framework (Figure 1c). In addition, there are intramolecular O−H···O hydrogen-bonding interactions (Table S1), which further stabilize the framework of compound 1. Topologically, if the binuclear units are considered as 6connected nodes, and the HL1 anions and L2 ligands are viewed as linkers, the complicated 3D framework of 1 can be simplified as a 6-connected framework with a Schläfli symbol of 412·63. Two identical frameworks interpenetrate each other, giving a 2-fold interpenetrating framework (Figure 1d). Structure of Compound [Zn(L1)(L3)0.5]·H2O (3). The asymmetric unit of compound 3 contains one Zn(II) ion, one L1 anion, half a L3 cation, and one lattice water molecule. As shown in Figure 2a, Zn1 is five-coordinated in a squarepyramidal coordination geometry with a τ value of 0.0207,30 completed by five oxygen atoms from five different L1 anions. The Zn−O bond lengths are in the normal range of 1.9474(16)−2.0977(17) Å as other Zn(II)-containing coordination polymers.31−33 Two symmetry-related Zn1 ions are held together by four carboxylate groups to generate a binuclear unit with the Zn···Zn distance of 3.141 Å. Each L1 anion links three binuclear units to form a layer (Figure 2b). From the topological point of view, the L1 anions can be considered as 3-connected nodes, and the binuclear units can be simplified as 6-connected nodes. Therefore, the 2D layer can be simplified as a (3,6)-connected net with a Schläfli symbol of (43)(46·66·83) (Figure 2c). The L3 cations locate between the layers through the face-to-face π−π interactions between the phenyl rings (C3 to C8) of the L1 anions and the pyridinium rings (N1 and C23 to C27) of the L3 cations (with the centroid−centroid distances of 3.68 Å; Figure S1), forming a 3D supramolecular architecture (Figure 2d). Structure of Compound [Cd(L1)(L3)0.5] (4). Compound 4 has the same topological structure with compound 3, but the crystal cell parameters and some structural details are different from 3. The asymmetric unit of 4 consists of one Cd(II) ion, one L1 anion, and half a L3 cation. As shown in Figure 3a, Cd1 is six-coordinated in an octahedral coordination environment, defined by six oxygen atoms from five different L1 anions. The Cd−O bond lengths are in the normal range of 2.255(2)− 2.427(3) Å as other Cd(II)-containing coordination polymers.34−36 Two symmetry-related Cd1 ions are connected by four carboxylate groups to generate a binuclear unit with the Cd···Cd distance of 3.443 Å. Each L1 anion links three binuclear units to form a layer (Figure 3b). It should be noted that, in compound 3, all the binuclear units have the same direction in one layer, but in compound 4, the adjacent lines of binuclear units arrange in two different directions A and B in one layer (Figure 3b). From the topological point of view, the L1 anions can be considered as 3-connected nodes, and the binuclear units can be simplified as 6-connected nodes. Therefore, the 2D layer can be simplified as a (3,6)-connected net with a Schläfli symbol of (43)(46·66·83) (Figure 3c). The L3 cations locate between the layers through the face-to-face π−π interactions between the phenyl rings (C1 to C6) of the L1 anions and the pyridinium rings (N1 and C23 to C27) of the L3 cations (with the centroid−centroid distances of 4.04 Å; Figure S2), generating a 3D supramolecular architecture (Figure 3d). Although the L3 ligands in both 3 and 4 are nearly parallel to the layers formed by the metal ions and the L1 ligands, the directions of the L3 ligands in 4 are different from those in compound 3. In 3, all the L3 ligands lean to the left, whereas those in 4 lean to the left and right alternatively between the adjacent lines (Figure S3).

Figure 2. (a) ORTEP view of compound 3 showing the coordination environment of Zn(II) ion with hydrogen atoms and lattice water molecule omitted for clarity (30% probability displacement ellipsoids). Symmetry codes: #1 x − 1, y, z; #2 x − 1, y + 1, z − 1; #3 −x, −y + 1, −z + 1; #4 −x, −y + 2, −z; #8 −x, −y + 2, −z + 1. (b) View of the 2D layer formed by the Zn(II) ions and L1 anions in compound 3. (c) View of the (3,6)-connected net with a Schläfli symbol of (43)(46·66·83). (d) View of the 3D supramolecular architecture of 3. D


Article

Inorganic Chemistry

Photochromic Properties. Viologen-based compounds are generally photochromic due to the photoinduced electron transfer from the electron donors to the viologen ligands.14,17 Therefore, in this work, the photochromic behaviors of compounds 1−4 have been investigated. Compounds 1−4 display photochromic phenomena, and they are very photosensitive. They exhibit rapid and eye-detectable color changes in air at room temperature upon continuous irradiation with UV light (365 nm), visible light, or sunlight. It should be pointed out that, although many photochromic coordination polymers have been reported, the ones responding to sunlight are relatively rare. The utilization of sunlight for activation can facilitate the practical application of photochromic materials because the sunlight is widely available. Upon irradiation by a 250 W Xe lamp, the pale yellow samples of compounds 1−3 can quickly turn to pale green in 20 s for 1 and 3, and 40 s for 2, and the colors gradually turn to dark green with increasing of the irradiation time. For compound 4, the change in color is not obvious compared with compounds 1−3. However, there still exists an eye-detectable color change from pale yellow to pale green upon irradiation by visible light for 1 min (Figure 4). When irradiated by UV light or sunlight, the times for coloration are 20 s/30 s for 1, 20 s/40 s for 2, 20 s/30 s for 3, and 50 s/1 min for 4. The color changes for compounds 1−4 are reversible. For compounds 1 and 2, the color fading of the green samples can be achieved by leaving the samples in air in the dark for 7 days and 3 days, respectively. It is noteworthy that the colored samples of 3 and 4 are very stable in air and do not completely return to their original colors in the dark under ambient atmosphere after 4 months. The results indicate that there exist ultra-long-lived charge-separated states in the colored samples of 3 and 4, which is vital for transforming solar energy into chemical energy.37,38 However, the decoloration of the green samples of 1−4 can be achieved easily by heating the samples at 100 °C (1 and 2) or 130 °C (3 and 4) for 5 min. After heating, compounds 1−3 lost their crystallinities, but the heated samples can be further used for the reversible photochromic processes. In the following coloration and color fading processes, the color changes of the heated samples were not obvious compared with the unheated ones. Also, along with the increase of the cycles, the coloration and decoloration times of the samples increased. The coloration and color fading processes of compounds 1−4 can be repeated for at least 10 cycles through the irradiation− heating processes. Through the irradiation−dark processes, the reversible photochromic processes of 1 and 2 can be repeated for 10 cycles for 1 and 5 cycles for 2, while the reversible cycles of compounds 3 and 4 are difficult to judge because of the long decoloration times of them in the dark. The coloration times responding to different light sources, decoloration times, and reversible cycles of compounds 1−4 have been listed in Table S5 in the Supporting Information. IR spectra, PXRD, and EPR measurements were carried out to investigate the mechanisms of the photochromic behaviors for compounds 1−4. The IR spectra and PXRD patterns of compounds 1−4 before and after irradiation are identical, ruling out the possible structural changes and decompositions (Figures S4 and S5). As shown in Figure 4, the original yellow samples of compounds 1−4 are EPR silent, whereas the green ones are EPR active, giving viologen radical signals at g = 2.0032, 2.0035, 2.0034, and 2.0035, respectively. These values are close to those found in viologen complexes.39−42 Therefore, the photochromic mechanism of compounds 1−4 is the

Figure 3. (a) ORTEP view of compound 4 showing the coordination environment of Cd(II) ion with hydrogen atoms omitted for clarity (30% probability displacement ellipsoids). Symmetry codes: #1 x, y − 1, z; #2 x − 1/2, −y + 1/2, z + 1/2; #3 −x + 1/2, y − 1/2, −z + 1/2; #4 −x, −y + 1, −z + 1; #6 −x + 1, −y + 1, −z + 1. (b) View of the 2D layer formed by the Cd(II) ions and L1 anions in compound 4. (c) View of the (3,6)-connected net with a Schläfli symbol of (43)(46·66· 83). (d) View of the 3D supramolecular architecture of 4. E


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Inorganic Chemistry

Figure 4. EPR spectra of compounds 1−4 before and after irradiation by visible light. Inset: photographic images of compounds 1−4 showing the photochromic effects.

compounds 2 and 4, there is only one peak for the N atom, lying at about 405.02 eV for 2 and 404.85 eV for 4. After coloration, a new peak with lower binding energy at 401.99 eV for 2 appeared. For 4, after coloration, two new peaks with lower binding energies at 399.35 and 401.97 eV emerged. These results indicate that the N toms received electrons.42,46 The generation of viologen radicals in compounds 1−4 was further proved by UV/vis absorption spectra. After irradiation by visible light, new absorption peaks at about 630 nm appeared and became intense with extending of the irradiation time for compounds 1−4 (Figure S11). This characteristic absorption band is similar to those of the N,N′-disubstituted4,4′-bipyridinium radicals generated through photoinduced electron transfer.44,48,49 Kinetic studies based on the absorbance at 630 nm indicate that the photochromic processes of 1−4 cannot be fitted to lines because of the low R2 values (smaller than 0.9). Each conversion process can be separated into three stages. At the first stage, the rate increases fast in response to visible light. At the second stage, the rate constant decreases comparing with the first stage. At the third stage, the rate constant increases again (Figure S12). The variation in the conversion rate of the three stages may be caused by the fact that the photochemical reactions take place on the surfaces first, and then in the inner parts at the second stage. At the third stage, the inner parts are exposed to the light again manually. Effects of the Structures on the Photochromic Properties. Results from documents indicate that the photochromic behaviors of viologen-based compounds are mainly determined by the following three factors: (1) short distance between the pyridinium nitrogen atom and the donor atom with an approximately 90° angle between the donor-N+ line and the pyridinium plane; (2) the presence of O−H···N, C−H···O, or C−H···X (X = Cl, Br···) hydrogen bonds between

formation of viologen radicals caused by the photoinduced electron transfer.43−45 The mechanisms of the photochromic behaviors of 1−4 have been further confirmed by the DFT calculations. The DFT calculation results indicate that the HOMOs of these compounds are mainly dominated by the carboxylate groups of the HL1 and L1 anions, and the LUMOs mainly reside on the pyridinium rings of the viologen ligands (Figure S6). This implies that the carboxylate groups of the anions and the pyridinium rings of the viologen cations tend to donate and accept electrons, respectively. Therefore, the photochromic behaviors of 1−4 are caused by the photoinduced electron transfer from the carboxylate groups of the anions to the pyridinium rings of the viologen cations and the formation of viologen radicals. XPS measurements were also carried out to get better insight into the electron transfer mechanisms of these compounds. After coloration, the core-level spectra of Zn 2p/Cd 3d and C 1s of 1−4 had no clear changes, whereas those of O 1s and N 1s varied clearly (Figures S7−S10). There are two peaks for the O atoms of 1−4, lying at about 531.21 and 532.80 eV for 1, 531.05 and 532.78 eV for 2, 531.06 and 532.73 eV for 3, and 530.80 and 532.65 eV for 4, respectively. After coloration, the positions of these peaks remained almost unchanged, but a new peak with higher binding energy comparing with the first peak emerged at about 531.82 eV for 1, 531.72 eV for 2, 531.38 eV for 3, and 531.48 eV for 4, suggesting that the O atoms lost electrons.17,46,47 There are two peaks for the N atoms of 1 and 3, lying at about 402.02 and 405.34 eV for 1, and 401.99 and 405.00 eV for 3, respectively. After coloration, the peaks at 402.02 eV for 1 and 401.99 eV for 3 remained almost unchanged, but the peaks with higher binding energies at 405.34 eV for 1 and 405.00 eV for 3 disappeared. For F


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Inorganic Chemistry

Figure 5. Photographs of the compound 1-coated paper after leaving in the dark for different times: (a) 0 days, (b) 2 days, (c) 4 days, (d) 6 days. Photographs of the compound 2-coated paper after leaving in the dark for different times: (e) 0 days, (f) 1 day, (g) 2 days, (h) 3 days.

Figure 6. Time-dependent photoluminescent emission spectra of 1−4 upon visible light irradiation (λex = 420 nm) in air at room temperature.

the pyridinium ring and the donor; (3) the presence of π−π or C−H···π interactions between the pyridinium ring and the neighboring aromatic ring.14,40,43,44,48,50,51 In this work, for compounds 1 and 2, the nearest distances between the carboxylate oxygen atoms and the pyridinium nitrogen atoms are 3.731 and 3.761 Å, respectively (Figure S13a,b). These values satisfy the requirement for electron transfer (3.4−4.0 Å).43,44 However, the angles between the donor-N+ line and the pyridinium plane are 119.08° and 108.26°, respectively. These angles are out of the range of 70− 100°, which are unfavorable for the electron transfer.43 Therefore, there may be other pathways of electron transfer in compounds 1 and 2. By carefully inspecting the structures,

we found that there are C−H···O hydrogen bonds between the pyridinium carbon atoms and the donor oxygen atoms (Figure S13a,b), which may be the possible pathways of electron transfer.14,51 Moreover, the uncoordinated carboxyl oxygen and the 3D interpenetrated frameworks of 1 and 2 are also thought to favor the donor−acceptor interactions.52 In compounds 3 and 4, the nearest distances between the carboxylate oxygen atoms and the pyridinium nitrogen atoms are 3.640 and 3.554 Å, respectively (Figure S13c,d). The angles between the donorN+ lines and the pyridinium planes are 95.75° and 89.53°, respectively. These distances and angles are favorable for the electron transfer from the carboxylate oxygen atoms to the pyridinium rings.43,44 In addition, the π−π interactions between G


Article

Inorganic Chemistry

emission intensities of compounds 1−4 decreased gradually with the extension of irradiation time. After irradiation for 20 min, the intensities of 1−4 reached 28.6, 37.9, 26.1, and 23.6% of the original intensities. Such photomodulated luminescence has potential application in the photoswitching, which has also been observed in other photochromic coordination polymers.3,52,55 Thermal and Humidity Stability. Humidity stabilities of compounds 1 and 2 have been investigated by measuring the PXRD patterns of the samples after immersion in the water for 15 days. The samples after immersion retained their crystallinities, and the PXRD patterns were identical to those of the as-synthesized samples (Figure S5). The results indicate that compounds 1 and 2 have excellent humidity stabilities. The thermal stabilities of compounds 1−4 have been studied by TGA and variable-temperature PXRD. TGA data were collected in the temperature range of 30−800 °C under a N2 atmosphere. The TGA curves of compounds 1−4 are shown in Figure S15. Compound 1 shows a weight loss of 2.38% from 50 to 135 °C, which is attributed to the loss of one lattice water molecule (calcd 2.62%). The second gradual weight loss begins at 263 °C, and does not stop before 800 °C. Compound 2 undergoes dehydration from 46 to 121 °C (obsd 2.00%, calcd 2.46%). There is no further weight loss from 121 to 281 °C. After 281 °C, the organic components start to decompose. For compound 3, the weight loss of 3.39% from 80 to 165 °C is consistent with the release of one lattice water molecule (calcd 3.18%). The organic compositions start to decompose at 256 °C and do not stop before 800 °C. The anhydrous compound 4 is thermally stable up to around 343 °C, and the weight losses occur in a consecutive step and do not end before 800 °C. Variable-temperature PXRD measurements imply that the PXRD patterns of compounds 1 and 2 at 100 °C and that of compound 3 at 130 °C were not identical to those of the corresponding as-synthesized samples (Figure S5). On the basis of the fact that compounds 1−3 can proceed reversible photochromic processes after heating at 100 °C (compounds 1 and 2) or 130 °C (compound 3), we inferred that, after heating at 100 or 130 °C, compounds 1−3 lost their lattice water molecules, but the frameworks of them did not collapse. Then, the frameworks which have lost lattice water molecules kept stable at least up to 262 °C for 1, 281 °C for 2, and 255 °C for 3, which have been confirmed by the PXRD patterns (Figure S5). The PXRD pattern of compound 4 at 343 °C was identical to that of the as-synthesized sample (Figure S5), indicating that the anhydrous compound 4 is stable up to 343 °C.

the pyridinium rings and the neighboring aromatic rings (Figures S1 and S2) and the C−H···O hydrogen bonds between the pyridinium carbon atoms and the donor oxygen atoms (Figure S13c,d) also facilitate the photoinduced electron transfer from the carboxylate oxygen atoms to the pyridinium rings.14,50,51 Inkless and Erasable Printing. Since the colored samples of 1 and 2 can easily return to their initial colors in the dark, whereas the colored samples of 3 and 4 can return to their initial colors only by heating, only compounds 1 and 2 were used to test the abilities for inkless and erasable printing. The experiments were carried out according to the procedures reported in the literature.13 A finely ground sample of the compound was immersed in a certain amount of ethyl alcohol. After ultrasonic treatment for 1 h, a suspension of the compound formed. The suspension was then dropwise and evenly added to the surface of a filter paper. After the paper was dried, the coating was adhered to the filter paper. The surface of the coated paper was then covered with a stencil which has a flower-shaped blank. After exposure to the visible light for about 3 min, the stencil was removed from the surface of the coated paper. A green flower printed on the yellow background of the compound was obtained (Figure 5a,e). It can be seen from the figures that the color contrast between the flowers and the backgrounds was well enough for visual reading. The printed flower kept legible in the dark at ambient atmosphere for 4 days for the 1-coated paper and 1 day for the 2-coated paper (Figure 5). These times are long enough for temporary uses. The printed contents finally vanished in the backgrounds after 7 days for the 1-coated paper and 3 days for the 2-coated paper. The papers returned to blank and can be reused for the next round of printing. The printing and erasing process can be repeated for at least 10 cycles for the 1-coated paper and 5 cycles for the 2-coated paper without obvious loss in contrast with the background. Since the radicals of electrontransfer photochromic compounds are relatively unstable in the atmosphere of oxygen gas, the erasing of the printed paper can be accelerated by flushing oxygen gas on the printed paper for reuse.13,16 Therefore, oxygen gas flushing experiments were carried out to investigate the effect of the oxygen gas on the erasing process. The printed papers were placed in a dark box with a hole. Then, the oxygen gas was inlet continuously from the hole with a small flow rate. The printed flowers vanished in the background for 3 days for the 1-coated paper and 6 h for the 2-coated paper. The results indicate that the erasing processes can be accelerated clearly by flushing oxygen gas. Photomodulated Luminescence. As shown in Figure 6, the main emission peaks of the as-synthesized compounds 1−4 are located at 530, 547, 558, and 553 nm when excited at 420 nm. The free H3L1 ligand shows a main emission peak at 408 nm (λex = 335 nm). The excitation and emission spectra of the H3L1 ligand and the excitation spectra of compounds 1−4 are shown in Figure S14. Because the Zn(II) and Cd(II) ions have d10 electronic configurations, they are difficult to oxide or to reduce. Therefore, the emissions of compounds 1−4 may be caused by the intraligand or interligand transitions.3,34,53 After irradiation by visible light for 1 min, the emission peaks of 1, 2, and 4 displayed different extents of hypochromatic shifts of 12, 15, and 3 nm, respectively. The hypochromatic shifts are corresponding to the emergence of viologen radicals.3,54 However, the position of the emission peak for compound 3 did not change after irradiation by visible light even for 20 min. With the colors of the samples gradually deepened, the

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CONCLUSIONS In conclusion, four coordination polymers based on the tricarboxylic acid 3,5-(dicarboxyl-phenyl)-(4-(2’-carboxyl-phenyl)-benzyl)ether and the viologen ligands have been synthesized hydrothermally. These compounds display photochromic behaviors from pale yellow to green under UV light, visible light, or sunlight. The photochromic mechanism is the formation of viologen radical caused by the photoinduced electron transfer, which has been confirmed by the EPR spectra, UV−vis absorption spectra, DFT calculations, and XPS spectra. Compounds 1 and 2 can be used for inkless and erasable printing, and the printing and erasing process can be repeated for at least 10 cycles for 1 and 5 cycles for 2 without obvious loss in contrast with the background. The emission spectra of compounds 1−4 with different irradiation times indicate that H


Article

Inorganic Chemistry the fluorescence of the compounds can be modulated by the irradiation time.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02206. Selected bond lengths and angles for 1−4, hydrogen bonds information for 1−3, figures of IR, PXRD, electron-density distributions, XPS spectra, UV−vis absorption spectra, rate plots, TGA, and crystal structures of 1−4 (PDF) Accession Codes

CCDC 1567072−1567075 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.-Z.W.). *E-mail: [email protected] (Y.-C.H.). ORCID

Wei-Qiu Kan: 0000-0002-2054-2769 Shi-Zheng Wen: 0000-0002-0316-1323 Yuan-Chun He: 0000-0002-1162-0067 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21401063, 21403081, and 21601104) and the Natural Science Foundation of Jiangsu Province (BK20140452 and BK20140453). The authors acknowledge the help of theoretical simulation by Prof. Su (NENU). The figures of electron-density distributions of HOMO and LUMO were drawn using the VASPMO program developed by Yang Wang with JMol viewer.

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Viologen-Based Photochromic Coordination Polymers for Inkless and

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