Photochromic Properties of a Series of Zinc(II)âviologen Complexes

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Photochromic Properties of a Series of Zinc(II)–viologen Complexes with Structural Regulation by Anions Hai-Yang Li, Jing Xu, Lin-Ke Li, Xiang-Sha Du, Fu-An Li, Hong Xu, and Shuang-Quan Zang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00995 • Publication Date (Web): 18 Oct 2017 Downloaded from http://pubs.acs.org on October 23, 2017

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Crystal Growth & Design

Photochromic Properties of a Series of Zinc(II)–viologen Complexes with Structural Regulation by Anions

Hai-Yang Li,a Jing Xu,a Lin-Ke Li*,a Xiang-Sha Du,a Fu-An Li,b Hong Xu*,a and Shuang-Quan Zanga a

College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou,

450001, P. R. China b

College of Chemistry and Environment Engineering, Pingdingshan University,

Pingdingshan, 467000, P. R. China ABSTRACT The solvothermal reaction of 1-(3,5-dicarboxybenzyl)-4,4′-bipyridinium chloride (H2L+Cl−) with different anionic Zn(II) salts leads to three diverse metal–viologen complexes, formulated as {[Zn(HL)Cl2]2·2H2O} (1), {[Zn3(L)2(H2O)8]·2(SO4)·2(H2O)}n (2) and {[Zn(L)H2O]·NO3·H2O}n (3). Single-crystal X-ray analyses revealed that complex 1 displays dinuclear structure. Complex 2 features a 2D layer structure which consists of four-fold interpenetrating stacked layers with sulfate ions locating between the sheets. Complex 3 shows a 3D two-fold interpenetrating framework with nitrate anions interspersing the void space of the framework, and it features an uninodal 3-connected ThSi2 (ths) topology. The structural disparities reveal that the different geometries of counterions, coordination modes and conformations of ligand L−, together with the deprotonation degree of aromatic carboxylate have great influence on the formation of various structures. Interestingly, complexes 1-3 are all photoactive to UV-visible light, and undergo obvious and reversible color transformations from light yellow to blue/green. Structural analyses indicated that not only the carboxylate-O donor and N acceptor of the bipyridinium moiety can provide an effictive electron transfer pathway for photochromic behavior, the Cl→L electron transfer in complex 1 also has a crucial effect on the photochromic behavior of it.

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INTROUCTION In the past decades, multifunctional organic-inorganic hybrid materials have caught much attention, because the hybrid materials possess the advantages of both organic and inorganic materials and can be used in many fields, such as information storage,1 molecular switches,2 sensing,3 energy conversion,4 solid electrolytes,5 catalysis,6 and biomedical areas, etc.7 Recently, viologen/bipyridinium derivatives as a kind of photosensitive organic group have attracted much more interest, mainly because the viologen/bipyridinium moiety can undergo one-electron reduction to produce intensely colored free radicals and act as good electron acceptors in charge-transfer (CT) complexes.8 Based on the above consideration, many efforts have been directed towards the incorporation of viologen-functional organic photochromic groups into metal systems to design and assemble novel inorganic-organic hybrid materials. Through embedding this kind of photochromic fragment into a condensed metal organic framework, some interesting properties such as selective guest adsorption,9 photochromism,10 photomodulated luminescence,11 mechanochromic luminescence,12 etc. have been demonstrated by these “metal-viologen” coordination complexes.13 It indicates that this kind of complex may have more potential practical applications in the future. On the other hand, the easy fabrication and modification properties of the metal-organic frameworks can modulate the photochemical properties of the hybrid materials purposefully, which can be achieved by selection of suitable metal centers and photoactive functional organic linkers. Moreover, rational construction of metal-organic hybrid materials with tunable photosensitive characters also provides a unique platform to investigate the structure-photoresponse relationship and direct instruction for synthesizing functional crystalline materials. However, it has been well documented that in the process of assembling, not only the nature of metal ion and ligand but also the counterion, solvent effect, reaction temperature, metal-ligand ratio and pH, etc. all have unpredictable impact on the coordinating reaction or crystallization of coordination polymers,14 and further resulting in the disparities of the chemical/physical properties. As for the photochromic complexes, a variety of influencing factors for photochromic reactions based on electron-transfer (ET) processes have been researched.15 For example, the molecular

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structure, the packing type, distance, as well as the orientation between the electron donor and the electron acceptor all affect the rate of the ET reactions. But the specific reasons and mechanisms that influence the photoinduced ET processes are still unclear. Inspired by this, our group has been intensely studying the coordination chemistry of viologen/bipyridinium functional aromatic carboxylate derivatives, including the photochemical properties and the structure-photoresponse relationship of this kind of viologen-based metal-organic hybrid material. As a part of our continuous work in developing solid photochromic materials,16 we extend our focus on the fine-tuning of photochromic properties. In this way, three coordination

complexes,

formulated

as

{[Zn(HL)Cl2]2·2H2O}

(1),

{[Zn3(L)2(H2O)8]·2(SO4)·2(H2O)}n (2) and {[Zn(L)(H2O)2]·NO3·H2O}n (3) were acquired by the reaction of dicarboxylate bipyridinium with different anionic Zn(II) salts. Due to the existence of inorganic anions as structure directing factors, these complexes display dinuclear, four-fold interpenetrating 2D layer structure and 3D two-fold interpenetrating framework, respectively. And they all show reversible photochromic behaviors. Expediently, the three complexes with different anions provide a perfect opportunity to explore the key factor impacting on the photosensitivity. Additionally, IR spectra and PXRD patterns before and after the photoirradiation as well as the thermostability of them have also been discussed.

Scheme 1. 1-(3,5-dicarboxybenzyl)-4,4′-bipyridinium chloride (H2L+Cl−) EXPERIMENTAL SECTION Materials and general methods 1-(3,5-dicarboxybenzyl)-4,4′-bipyridinium chloride (H2L+Cl−) (Scheme 1) ligand was 3



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prepared according to the reported literature.16a All other commercially available reagents are used as received without further purification. Elemental analyses for C, H and N were determined using a Perkin-Elmer 240 elemental analyzer. IR spectra were recorded from KBr pellets in the range from 4000 to 400 cm−1 on a Bruker VECTOR 22 spectrometer. Thermogravimetric analyses were performed using a TA Q50 thermal analyzer from room temperature to 800 °C with a heating rate of 10 °C/min under nitrogen flow. Powder X-ray diffraction (PXRD) patterns for complexes 1-3 were recorded on a Rigaku D/max-3B diffractometer (Cu Kα, λ = 1.5418 Å) radiation in the 2θ range of 5-50°. The crushed single crystalline powder samples were prepared by crushing the crystals and scanning from 5 to 50 °C with a step of 0.1°/s. Solid UV-visible spectrum was obtained in the 200-800 nm range on a JASCOUVIDEC-660 spectrophotometer. Electron-spin resonance (ESR) signals were recorded on a Brucker A300 spectrometer at room temperature.

Syntheses of the complexes For complex 1, the reaction mixture of ZnCl2 (8 mg, 0.06 mmol), H2L+Cl− (0.0110 g, 0.03 mmol), acetonitrile (2 mL) and water (1 mL) was sealed in a 25 mL Teflon reactor autoclave and heated to 120 °C for 3 days. After cooling down to room temperature at a rate of 5 °C /h, The resulting pale yellow block crystals were filtered, washed with water, and dried in vacuo to provide 1 in 78% yield based on the ligand. Anal. Calcd for C38H32N4O10Cl4Zn2: C, 46.70; H, 3.30; N, 5.73%. Found: C, 46.50; H, 3.36; N, 5.93%. IR/cm-1 (KBr): 3434(s), 3406(w), 1723(s), 1640(s), 1617(vs), 1587(s), 1458(w), 1438(w), 1359(m), 1191(s), 1162(w), 1080(w), 920(w), 816(m), 850(s), 747(w), 677(w), 671(m), 596(m), 542(m). The preparation processes of complexes 2 and 3 are similar to that of complex 1, except that ZnCl2 was replaced by ZnSO4 (10 mg, 0.06 mmol) and Zn(NO3)2·6H2O (9 mg, 0.03 mmol), respectively. The pale yellow crystals were isolated in 81% and 80% yields for 2 and 3 (based on ligand), respectively. Anal. calcd. for 2 (C38H46N4O26S2Zn3) (%):C, 36.96; H, 3.75; N, 4.54%. Found: C, 40.05; H, 3.56; N, 4.63%. IR/cm-1 (KBr): 3407 (s), 3046 (s), 1624 (s), 1569 (m), 1502 (w), 1467 (w), 1434 (m), 1370 (s), 1335 (w), 1220 (w), 1205 (m), 1187 (s), 1097 (s), 1039 (m), 989 4


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(m), 835 (w), 829 (w), 756 (w), 740 (w), 720 (w), 640 (w), 569 (w). Anal. calcd. for 3 (C19H17N3O9Zn) (%):C, 45.94; H, 3.45; N, 8.46%. Found: C, 45.80; H, 3.30; N, 8.55%. IR/cm-1 (KBr): 3430 (s), 3105 (w), 1624 (s), 1575 (m), 1496 (w), 1448 (w), 1406 (m), 1382 (s), 1248 (w), 1120 (w), 1149 (w), 1081 (w), 1041 (w), 928 (w), 887 (w), 808 (w), 776 (w), 730 (w), 640 (w), 553 (w).

X-ray crystallography Single crystal X-ray analyses were conducted on a Bruker SMART APEX CCD diffractmeter using graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å) at room temperature using the ω-scan technique. Lorentz polarization and absorption corrections were applied. The structures were solved by direct methods with SHELXS-9717 and refined by full-matrix least-squares using the SHELXL-9718 program. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms of the ligands were included in the structure factor calculation at idealized positions using a riding model and refined isotropically. The hydrogen atoms of the solvent water molecules were located from the difference Fourier maps, and then restrained at fixed positions and refined isotropically. Analytical expressions of neutral atom scattering factors were employed, and anomalous dispersion corrections were incorporated. The crystallographic data and selected bond distances and angles for complexes 1-3 are listed in Tables S1 and S2.

RESULTS AND DISCUSSION Crystal structures of {[Zn(HL)Cl2]2·2H2O} (1) Single-crystal X-ray structure analysis revealed that complex 1 crystallizes in the monoclinic space group P21/c and exhibits a dinuclear structure (Figure 1a). The asymmetric unit contains one crystallographically independent Zn(II) center, one partially deprotonated ligand HL, two coordinated Cl− anions and one solvated water molecule (Figure 1b). Zn1 is four-coordinated to one nitrogen atom (N1), one monodentate coordinated O atom from a HL ligand and two terminal coordinated Cl− anions, displaying distorted tetrahedral geometry (τ4 = 0.92).19 For the partially deprotonated ligand HL, only 5



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one of the carboxylate groups takes part in coordination in monodentate mode to connect the center Zn(II) ion. Both the two HL ligands present V-shaped conformation when bridging the two Zn(II) ions in the end to end manner, furnishing a neutral binuclear unit [Zn(HL)Cl2]2 with a Zn-Zn distance of 12.298 Å (Figure 1b). The included angle of the V-shaped conformation is 111.7°, and the dihedral anlge between the dicarboxylate phenyl ring and the nearest pyridinium ring in the same ligand is 83.6°. The two pyridine rings of the same ligand are not in a plane, and the dihedral anlge of the two rings is 31.3°. The centroid to centroid distance between the nearest pyridyl rings of different HL ligands is 3.897 Å, indicating the existence of the relatively weaker π···π interaction. Meanwhile, the adjacent dinuclear units are connected by several kinds of H-bonding interactions: one is mediated by the protonated carboxylate oxygen atom from the HL ligand and the solvated water molecule (O4-H4···O1W), another one is constructed by the uncoordinated water molecules and the deprotonated carboxylate oxygen atom of the adjacent dinuclear units (O1W-H1WA···O3) and the third is the connection between the coordinated Cl− anion and the

uncoordinated

water

molecule

(O1W-H1WA···Cl1).

Besides

these,

the

hydrogen-bonding interactions of C-H···Cl (C9-H9B···Cl1 2.722Å, C9-H9A···Cl2 2.776Å; and C10-H10···Cl2 2.861Å) are also helpful to form the stable 3D supramolecular solid-state structure (Figure S1).

(a)

(b) 6


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Figure 1. (a) The coordination environment of the Zn(II) ion in 1. (b) Perspective view of the dinuclear structure of complex 1 (all hydrogen atoms and solvent molecules are omitted for clarity). Symmetry codes: #1 -x + 1, -y + 1, -z + 1.

Crystal structure of {[Zn3(L)2(H2O)8]·2(SO4)·2(H2O)}n (2) Complex 2 crystallizes in a monoclinic space group C2/c and features a 2D layered structure. In the asymmetric unit, there exist one and a half crystallographically independent Zn(II) ions, one completely deprotonated ligand L−, four aqua ligands, one free sulfate ion and one solvated water molecule. As shown in Figure 2a, the two Zn(II) ions show two different coordination manners i.e. penta-coordination and hexa-coordination (Zn1 and Zn2, respectively). The penta-coordinated Zn1 with a ZnO4N1 core shows trigonal bipyramidal geometry with 0.57 Addison parameter (τ5) value.20 The coordination is provided by one monodentate coordinated carboxylate oxygen atom (O1) of the ligand L−, one pyridyl nitrogen atom (N1) of another ligand L− and three aqua molecules. The three water molecules compose the equatorial plane (the mean derivation from the ideal ZnO3 plane is 0.0523 Å) and the other two atoms (O1 and N1) exist on the axial positions, the axial angle is 136.43(11). The hexa-coordinated Zn2 shows a distorted octahedral geometry with a ZnO6 environment, which is completed by four oxygen atoms from two carboxylate groups in chelating manners, and two O atoms from two aqua ligands in a cis configuration. Zn1 and Zn2 atoms are linked together by two carboxylate groups from one L− ligand in monodentate and chelating mode, respectively. Zn1 and Zn2 atoms are located on a straightline along b direction, respectively. The nearest Zn1···Zn2 distance connected by the same L− ligand is 8.972 Å. In this complex, the deprotonated L− ligand participates in coordination in µ3-η1:η2 conformation: one of the carboxylate groups takes part in coordinating in monodentate manner and the other is in chelating mode. The included angle of the V-shaped ligand is 111.1°, close to that of complex 1. The dihedral angle between the dicarboxylate phenyl ring and the nearest pyridinium ring in the same ligand is 106.6°, which is clearly larger than that of complex 1. The reason may be that in this complex, the two carboxylate groups 7



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of the same L− ligand simultaneously participate in coordination resulting in the bigger distortion. The dihedral anlge of the two pyridyl rings in the same ligand is 34.6°, which is close to that of complex 1 (31.3°). Interestingly, the chains lined by Zn1–L along b direction present helical chain structure, and the helical chains are linked by Zn2 atoms to form a 2D layer, the chirality of the chains in the same layer are unified, with one being left-handed, and the other one in the adjacent layer feature right-handed (Figure 2b). These two kinds of helical chains possess the same long pitches of 22.307 Å. And these two kinds of layers are interdigitated closely, forming the four-fold interpenetrating stacked layers (Figure 2c). The adjacent Zn1···Zn1 distance in the stacked layers is 11.153 Å. Since the nearest centroid to centroid distance between the adjacent phenyl rings of two different L− ligands is 3.948 Å, thus, the π···π interaction is much weaker than that of complex 1. The free sulfate ions locate between the layers, and connect with the adjacent stacked layers by plentiful hydrogen-bonding interactions. These hydrogen bondings are composed of Ow-Hw···O, Ow-H···S interactions (H-bonding bond distances are in the range of 2.650 ~ 3.289 Å and 3.500 ~ 3.677 Å, respectively). Just these weak interactions extend the 2D layers to 3D solid-state packed structure (Figure 2d).

(a)

8


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(b)

(c)

(d) Figure 2. (a) Perspective view of the coordination environment of the Zn(II) ion in 2 (hydrogen atoms, −

free SO42 ainons and free H2O molecules are omitted for clarity). (b) The layers structure composed of left-handed and right-handed helical chains. (c) Viewing of the 2D four-fold interpenetrating structure of complex 2 from b direction. (d) The 3D solid-state packed structure of complex 2 with sulfate ions located between the stacked layers. Symmetry codes: #1 -x + 2, y + 1, -z + 1/2; #2 –x + 2, y, -z - 1/2; #3 -x + 2, y - 1, -z + 1/2.

9



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Crystal structure of complex {[Zn(L)(H2O)2]·NO3·H2O}n (3) Single-crystal X-ray diffraction analysis reveals that complex 3 crystallizes in the monoclinic space group C2/c and features a 3D two-fold interpenetrating structure. As shown in Figure 3a, the asymmetric unit of 3 has one Zn(II) ion, one completely deprotonated L− ligand, one aqua ligand, one free nitrate anion and one guest water molecule. Zn1 is also four-coordinated to one pyridine N atom, two monodentate coordinated O atom from two carboxylate groups of different L− ligands and one O atom of one aqua ligand displaying distorted tetrahedral geometry (τ4 = 0.85).19 As for the deprotonated ligand L−, it also presents V-shaped conformation and participates in coordinating in µ2-η1:η1 manner to connect the three adjacent Zn(II) ions together. Viewed along b direction, the Zn(II) ions are connected by the carboxylate groups to form 2D layers. And then, through the bridging of the pyridine N atoms, the 3D stacking structure come into being (Figure S2). It is worth noting that the V-shaped L− ligands along b direction are parallel with each other, while, they are not parallel along c direction. And the neighbouring L− ligands have an include angle (66.2°) viewed alonlg c direction (Figure 3b). As for the internal angle of the V-shaped ligand, 113.0°, is close to those of the former two complexes. The dihedral anlge between the dicarboxylate phenyl ring and the nearest pyridinium ring in the same ligand is 68.2°, between the two pyridyl rings of the same ligand is 19.0°, both of them are the smallest among the three complexes. This can be ascribed to the extending of the adjacent ligand L− along different directions when it takes part in coordinating, which effectively decrease the steric hindrance of the organic ligands, thus leading to the smaller dihedral angles. The centroid to centroid distance between the phenyl ring and the nearest pyridyl ring of different L− ligands is 3.615 Å, indicating the existence of the face-to-face aromatic π···π interactions. Owing to a large accessible pore volume in the present framework, another set of identical framework intersected and parallel with each other to form a two-fold interpenetrating network with lattice H2O molecules and NO3− ions filling in the spaces (Figure 3c). Similarly, there are also plentiful hydrogen-bonding interactions existing between the coordinated H2O molecules and the O atoms of the nitrate ions and carboxylate units (O1w-Hw···O bond distances ranging from 2.655~3.088 Å), as well as between the solvate H2O molecules and the O atoms of the 10


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nitrate ions (O2w-Hw···O bond distances ranging from 3.028~3.071 Å).

(a)

(b)

(c) Figure 3. (a) Coordination environment of the Zn(II) ion in complex 3. (b) The 3D two-fold interpenetrating network of complex 3 viewed from c-direction. (c) The 3D two-fold interpenetrating

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−

network of complex 3 with lattice H2O molecules and NO3 ions filling in the spaces. The hydrogen atoms are not shown for clarity. Symmetry codes: #1 x, -y, z + 1/2; #2 x + 1/2, -y - 1/2, z + 1/2; #3 x, -y, z - 1/2; #4 x - 1/2, -y - 1/2, z - 1/2.

The structural disparities of 1-3 may be ascribed to the differences of counterions. In general, the influence of anions can be explained by their differences in size, concentration, coordination ability and the template effect. Halides (Cl−) in complex 1 adopt terminal coordination mode, thus, resulting in a dinuclear structure. While in complexes 2 and 3, SO42− and NO3− anions exist as uncoordinated counterions probably due to the steric-hinerance. Simultaneously, owing to the different rotation angles of the dicarboxylate phenyl rings with the pyridinium rings, the deprotonation effect and the flexing angles, three different conformations are presented in this paper. In the crystal structures, the methylene group in ligand HL (or L−) links the pyridinium ring and dicarboxylate phenyl unit which deviate the whole ligand from linearity and impart flexibility to the molecular framework. On the other hand, the relatively longer distance between the terminal carboxylate groups and the pyridyl unit in the ligand and its ditopic nature makes it suitable for the synthesis of macrocycles or extended coordination networks. Obviously, utilizing different geometric configuration anions to be counterions and taking advantange of the flexibility of the bicarboxylate bipyridinium moieties, the crystal structures can be significantly modified and interesting results could be obtained.

Thermostability, PXRD patterns and IR Powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA) and IR (Infrared Spectroscopy) patterns before and after light irradiation were determined to support the remarkable stability of these complexes. TGA measurements of complexes 1-3 were performed from room temperature to 800 °C under a nitrogen atmosphere to study the thermal stability of these coordination polymers (Figure S3). The TGA curve for 1 shows that in the range of 100-145 °C, the total weight loss of ca. 3.68% is attributed to the loss of lattice water molecule (3.78% calculated). And then, it followed by a flat area. Above 320 °C, the weight loss is ascribed to the collapse of the whole framework. In the case of 2, a gradual weight loss between 12


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30-70 °C can be attributed to the release of one coordinated water molecule and one free water molecule (observed, 6.15%; calculated, 6.26%). After that, a gradual weight loss of ~9.17% observed at around 70-150 °C is attributed to the departure of the other three coordinated water molecules (calculated, 9.62%). The further weight loss from 360 °C indicates decomposition of the framework. As for complex 3, the TGA shows a weight loss from 85 °C to 134 °C (found 3.69%, calculated 3.62%), which can be attributed to the loss of one free lattice water molecule. And the following gradual weightlessness from 134 to 220 °C can be ascribed to the leave of the other three coordinated aqua ligands (found 7.08%, calculated 7.24%). After 285°C, the weight loss indicates decomposition of the framework. In order to confirm the phase purity of these complexes, PXRD of complexes 1-3 were recorded at room temperature. As shown in Figures S4-S6, the peak positions of the theoretical and experimental PXRD patterns are in good agreement with each other, which clearly indicates the high purity and homogeneity of these samples. IR (Figure S7-S9, Supporting Information) spectra of complexes 1-3 before and after UV−vis light irradiation coincide well, indicating that the same coordination skeleton remains before and after heat-treatment and the photochromism are not caused by isomerization or dissociation. It is reasonable to think that the photochromism of 1-3 are based on an electron-transfer mechanism. Photochromism Complexes 1-3 are photosensitive. Upon irradiation by sunlight or 300W Xenon lamp at room temperature in air, the color change of crystals 1-3 could be clearly observed. For complex 1, it gives an eye-detectable change from pale yellow to pale green upon irradiation. The coloration of 1 tends to be saturated after illumination for 10 min (Figure 4a). Complexes 2 and 3 change from pale yellow to pale blue and dark green, respectively. Both of the two colorations tend to achieve saturation after irradiation for 20 min (Figures 4b and 4c). The photoproducts of 1-3 are stable in air. While after being placed in the dark or being heated for a period of time, all the colored samples can restore to their initial state. The photoproduct of 1 could be completely decolored in 1 day in the dark or in 30 min by 13



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heating at 120 °C in air, while decoloration of the photoproducts of 2 and 3 took about 2 days in the dark or 1 h by heating at 120 °C in air. The development and fading of the color can repeat many times, and no degradation of the efficiency in the color change was observed, which indicates the reversible photochromism of 1-3. The UV-vis reflectance spectra were measured before and after illumination (Figure 4). As shown in the figures, the original samples show strong and structured absorption bands below 370 nm, with rapidly decreased tailing in the visible light region. The absorption should be related to the π-π* and n-π* transitions of the conjugated ligand L−. The photochromic products of 1-3 show characteristic bands around 460 and 670 nm, with progressively enhanced intensity as the irradiation time increases, and tend to saturate after illumination for 10, 20 and 20 min, respectively. These characteristic spectral bands are similar to those observed for bipyridinium radicals, suggesting that the color change of complexes 1-3 presumably arises from generation of the viologen radical through photoinduced electron transfer.21-23 After the decoloration, the new emerging absorption peaks vanish (Figure 4). It is well documented in the literatures that under suitable conditions the viologen moiety can undergo one electron transfer and convert into viologen radicals.10b, 22,23

(a)

(b)

14


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(c) Fig. 4 UV-vis spectra and photographs showing the photochromic behaviours of complex 1 (a), 2 (b) and 3 (c). To further confirm the generation of the radicals, the electron spin resonance (ESR) spectra of 1-3 have been measured before and after irradiation (Figure 5). There are no ESR signals before irradiation but after irradiation a single line signal with g = 2.0025, 2.0014, and 2.0028 appears, respectively (Figure 5a,5b,5c). These g values are close to that of a free electron (2.0023) and similar to those found in bipyridinium and viologen complexes.21b,24,25 This indicates that the bipyridinum L− ligand is indeed reduced to generate L*− free radicals after the irradiation, and the photochromic processes arise from photoinduced free radical generation of the viologen units.10b,23-26

Figure 5. ESR spectra for 1 (a), 2 (b), and 3 (c) before and after irradiation. As previously reported, for the bipyridinium/viologen-based complexes, the electron transfer in the solid state is related to the short contact between the carboxylate O donor and the pyridinium N accepor,26,10b the face-to-face aromatic π···π interactions between the pyridinium rings can also provide an effective pathway for electron transfer.24-26 There is no doubt that in complexes 1-3, the organic radicals are mainly originated from the electron transfer from the oxygen atoms of the carboxylate groups to the nitrogen atoms of the 15



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pyridinium rings. In the solid-state structures, the O···N distances are 3.289 Å for complex 1 (Figure S10); 2.958, 3.158 and 3.297 Å for 2 (Figure S11); 2.983, 3.020 and 3.368 Å for 3 (Figure S12), and the angles between the O···N direction and the plane of the pyridinium ring are 122.47° for 1; 126.01°, 134.61° and145.77° for 2; 127.98°, 126.39° and 141.34° for 3, respectively. These distances and angles are all in favor of the intermolecular charge transfer between the carboxylate group donor and the viologen acceptor unit, and offer a reasonable electron-transfer pathway for the photochromic process.27,28 Besides this, the close condensed packing mode of the solid-state structure also makes the intramolecular electron transfer more facilitative and results in the viologen radicals.23,24b Therefore, the organic ligand aromatic dicarboxylate bipyridinium plays an important role on the photochromic properties in these complexes. It may not only be a powerful factor to stabilize the viologen monocation radical but also provides an effective path for electron transfer from the π-conjugated substituent to the viologen cation in the photochromic process. Moreover, for complex 1, except the electron transfer from the oxygen atoms of the carboxylate groups to pyridinium rings can produce the organic radicals, the Cl→L electron transfer is another key factor for the photoinduced chromism of 1. Some reports have proposed that Cl atom can be a suitable electron-donating ligand to design redox photochromic metal complexes. 21b,29 In the structure of complex 1, the distances between Cl···N are 3.393 and 3.461 Å, and the angles between the Cl···N direction and the plane of the pyridinium ring are about 136.59° and 138.66° (Figure S10), which can also mediate electron transfer effectively and is favourable to forming viologen radicals. Besides the distances between donor atoms and acceptors in the solid state, the dihedral angles between the two pyridinium rings may also affect the efficiency of the photoinduced reduction.30 It is proposed that the planar configuration of the two pyridinium rings are more favorable for the photoinduced reduction of viologens and the stability of free radicals,10b as the distortion between the intraligand pyridinium rings may destroy the large conjugated planar configuration, which leading to a decrease in the stability of free radicals. In complexes 1-3, the interplanar angles between the pyridinium rings are 31.3°, 34.6° and 19.0°, respectively, which are close to those of some reported viologen-based 16


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complexes,16,26 such geometries are all suitable for the photoinduced reduction of pyridinium rings and the stability of the free radicals. Comparing the photoresponse abilities of the three complexes, 1 is more photosensitive than complexes 2 and 3. Since the O···N distances in 1 are not the nearest and the interplanar angles between the pyridinium rings is not the smallest among the three complexes, the Cl→L electron transfer pathway may make great contribution to the fast photoresponse, because Cl atom is a strong electron-donating species has been documented previously.21b,29 Moreover, another possible reason for this observation is that the bridging coordination to ZnCl2 enhances the rigidity of the ligand L− in 1, which favors the stabilization of radicals.20a Compared to complexes 2 and 3, complex 1 has the advantage of being sensitive to the photoirradiation. Therefore, it is considered that the incorporation of halide plays a crucial role in influencing the efficiency of photoinduced electron transfer reactions and results in the different degrees of photoresponse abilities.

CONCLUSIONS In summary, utlizing a viologen-based ligand, and modulated by different counterions, three complexes with structures ranging from dinuclear to 3D two-fold interpenetrating framework have been successfully synthesized. The different geometries of counterions have dramatic influences on the final structure of the metal-viologen MOFs, and further result in their distinct photochromic behaviors upon irradiation. Detailed analysis of the structural parameters of these complexes has revealed that the electron transfer between the carboxylate-O donor and pyridinium-N acceptor are the dominating factor for photochromic behaviors. Moreover, the Cl→L electron transfer in the structure is also a key factor for the disparity photochromism of complex 1 with the other two complexes. The findings in this paper may provide new clues for the design of solid crystalline hybrid materials

with

tunable

photosensitivity.

Systematic

investigations

on

the

structure–photoresponse relationship of this kind of hybrid materials are still on the way to search for much more MOFs materials with useful photochromic properties.

17



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ASSOCIATED CONTENT Supporting information available Crystallographic data in CIF format, TGA, PXRD patterns and IR spectra of complexes 1-3 before and after irradiation. This material is available free of charge via the Internet at http://pubs.acs.org. Crystallographic data for the structures reported in this article have been deposited in the Cambridge Crystallographic Data Center with CCDC reference numbers 1562618-1562620 for complexes 1-3.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21371153, 20901070), Key Scientific Research Project Plan in Colleges and Universities of Henan Province (16A150045) and Scientific and technological project in Henan Province (162102410070).

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SYNOPSIS TOC

Photochromic Properties of a Series of Zinc(II)–viologen Complexes with Structural Regulation by Anions

Solvothermal

reaction

of

1-(3,5-dicarboxybenzyl)-4,4’-bipyridinium

chloride

(H2L+Cl−) with different anionic Zn(II) salts leads to three diverse metal–viologen complexes. These complexes display dinuclear, four-fold interpenetrating 2D layer structure and 3D two-fold interpenetrating framework, respectively. They all show reversible photochromic behaviors. Structural analysis indicated that not only the carboxylate-O donor and N acceptor of the bipyridinium moiety can provide an effictive electron transfer pathway for photochromic behaviors, the Cl→L electron transfer in complex 1 also has a crucial influence on its photochromic behavior.

23


Photochromic Properties of a Series of Zinc(II)âviologen Complexes

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Photochromic Properties of a Series of Zinc(II)âviologen Complexes