Distinct Chromic and Magnetic Properties of MetalâOrganic

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Distinct Chromic and Magnetic Properties of Metal-Organic Frameworks with a Redox Ligand Teng Gong, Xiao Yang, Jia-Jia Fang, Qi Sui, Fu Gui Xi, and En-Qing Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15540 • Publication Date (Web): 13 Jan 2017 Downloaded from http://pubs.acs.org on January 18, 2017

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ACS Applied Materials & Interfaces

Distinct Chromic and Magnetic Properties of MetalOrganic Frameworks with a Redox Ligand Teng Gong, † Xiao Yang, † Jia-Jia Fang, † Qi Sui, † Fu-Gui Xi † and En-Qing Gao†,* †

Shanghai Key Laboratory of Green Chemistry and Chemical Processes, College of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062, China.

ACS Paragon Plus Environment

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ABSTRACT: An electron-deficient and potentially chromic ligand has been utilized to impart redox activity, photo-/hydrochromism, and solvotomagnetism to metal-organic frameworks (MOFs). Two MOFs were constructed from the flexible zwitterionic viologen-tethered tetracarboxylate linker N,N’-bis(3,5-dicarboxylatobenzyl)-4,4’-bipyridinium (L2-): [Co3(L)(N3)4] (1) and [Mn2(L)(N3)2(H2O)2]·3H2O (2). Both compounds show three-dimensional frameworks in which mixed azido- and carboxylato-bridged chains are connected through the electron-deficient viologen moieties. The chain in 1 is built from alternating bis(azide) and (azide)bis(carboxylate) bridges, while that in 2 contains uniform (azide)(carboxylate) bridges. The MOFs shows the characteristic redox properties of the viologen moieties. The redox activity affords the MOFs with different chromic properties, owing to subtle differences in chemical environments. 1 shows reversible photochromism, which is related to the radical formation through photo-induced electron transfer from azide/carboxylate to viologen according to UV-vis, X-ray photoelectron and electron spin resonance spectroscopy and DFT calculations. 2 is non-photochromic for lack of appropriate pathways for electron transfer. Unexpectedly, 2 shows a novel type of solid-state hydrochromism. Upon removal and reabsorption of water, the compound shows remarkable color change because of reversible electron transfer accompanying a reversible structural transformation. The radical mechanism is distinct from those for traditional hydrochromic inorganic and organic materials. Magnetic studies indicate ferro- and antiferromagnetic coupling in 1and 2, respectively. What’s more, 2 shows marked magnetic response to removal of water molecules owing to the formation of radicals. The compound illustrates a unique material exhibiting dual responses (color and magnetism) to water.

KEYWORDS:

metal-organic

frameworks,

viologens,

redox

activity,

photochromism,

hydrochromism, radicals, magnetism


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INTRODUCTION Smart materials capable of recognizing specific external stimuli and giving readable signals in response to the stimuli are a hot and fascinating area in materials science for their important technologic applications in various fields.1-4 Metal-organic frameworks (MOFs), composed of metal-containing units and organic bridging ligands, have recently emerged as versatile platforms to design responsive materials.5-8 By judicious selection and functionalization of the inorganic and organic components of the frameworks, a growing number of MOFs have been found to be responsive to physical or chemical stimuli through changes in optical,9 magnetic10 or electric properties. 11 The electron-deficient viologen derivatives (V2+, N,N’-disubstituted 4,4’-bipyridiniums) can undergo reversible redox chemistry to give rise to three differently colored oxidation states (V2+, V•+ and V0) in response to light,12 electricity13 or chemical stimuli. 14-15 Based on this character, viologens are frequently used as electrochemical/photochemical probes, as electron relays in electron-transport processes16 or oxidative damage in DNA17, as the key building block of supramolecular assembly18 such as mechanically interlocked molecules19 and molecular machines20. Recently, some viologen-tethered ligands have been used as building blocks for MOFs or coordination polymers.21-34 Most of these studies have been focused on photochromism24, while photo-switched luminescence23, thermochromism25, colorimetric sensing26, magnetic properties27-29 and gas absorption30 have been also demonstrated for some of these “viologen MOFs”. Nevertheless, the study is still in its infancy. The electrochemical behaviors are scarcely studied,31 and a better insight into the structural factors determining the photochromic properties is still needed, although some pathways for electron transfer to viologen units have been proposed.32-34 Considering the readiness of viologens to form colored radicals


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and the capability of MOFs to provide various chemical environments35 and physical functions36, viologen MOFs have great potentials in creating responsive smart materials. For instance, the combination of the redox and magnetic properties could lead to new responsive systems. In this context, using a three-in-one synthetic strategy with viologen-based ligands as redoxactive and potentially chromic components, paramagnetic metal ions as spin carriers, and azide and carboxylate as pathways for magnetic exchange, we constructed two 3D MOFs, [Co3(L)(N3)4]

(1)

and

[Mn2(L)(N3)2(H2O)2]·3H2O

(2),

where

L2-

is

N,N’-bis(3,5-

dicarboxylatobenzyl)-4,4’-bipyridinium (Chart 1). Interestingly, 1 is photochromic while 2 is not. The different photo-responsiveness has been related to intermolecular contacts. Notably, Complex 2 shows reversible changes in structure, in color and in magnetic properties upon removal and reabsorption of water molecules. The chromic and magnetic responses are owing to the formation of radicals through electron transfer. Although photochromism has been known for organic and hybrid compounds with viologen units, radical-related solid-state hydrochromism and solvotomagnetism have not yet been reported previously.

Chart 1. The L2- Ligand.


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EXPERIMENTAL SECTION Physical measurements. FT-IR spectra were measured with KBr pellets using a Nicolet NEXUS 670 spectrophotometer. NMR spectroscopy was carried out with a Bruker Advance 400 MHz spectrometer. CHN analysis was performed on an Elementar Vario ELIII analyzer. Powder X-ray diffraction (PXRD) patterns were obtained at a scan speed of 10 ○C/min using a Rigaku Ultima IV X-ray diffractometer (Cu-Kɑ radiation, λ = 1.54056 Å). UV-vis spectra were determined on a SHIMADZU UV-2700 spectrometer using finely ground samples coated on BaSO4 plates. Room-temperature X-band electron spin resonance (ESR) spectroscopy was performed using a Bruker Elexsys 580 spectrometer with a 100 kHz magnetic field. X-ray photoelectron spectra (XPS) were recorded on a PHI 5000 Versaprobe spectrometer (Al Kα radiation, λ = 8.357 Å). Thermal gravimetric analysis (TGA) was performed on a STA 449 F3 Simultaneous Thermal Analyzer in flowing air at 10 °C/min. Magnetic measurements with variable temperature/field were performed on a Quantum Design SQUID MPMS-5 magnetometer. Pascal’s constants were used for diamagnetic corrections. Photochromic tests were carried out using a 300W xenon lamp system (CEL-HXUV300), with the samples being placed at 30 cm from the lamp. Electrochemical studies. Cyclic voltammetry (CV) was conducted on a CHI 604E electrochemical analyzer (Shanghai) using the three-electrode method, wherein the working electrode, counter electrode and the reference electrode are respectively a modified glassy carbon electrode (GCE), a platinum wire and an Ag/AgCl electrode. The GCE was modified as follows. A given amount (1 mg) of the solid analyte was dispersed in 0.5 mL of ethanol and 40 µL of Nafion solution by sonication for 20 min. A portion of the above dispersion (10 µL) was dropcast on a pre-cleaned glassy carbon electrode. To avoid peeling of the material during the


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measurement, another 10µL of Nafion solution in ethanol was cast, then the working electrode was dried in air. Electrochemical measurements were performed in 0.1 M KCl solutions. Electronic structure calculation. Density functional theory (DFT) calculations on the periodic structure derived from the crystallographic data were carried out with the Dmol3 module37-38 of the Material Studio package.39 A fine accuracy for the numerical integration of the Hamiltonian and a fine (10-5 eV/atom) tolerance for SCF convergence were applied. The Perdew-Burke-Eruzerhof (PBE) functional with the generalized gradient approximation

40

was

used for exchange correlation. The Tkatchenko-Scheffler (TS) scheme was applied for dispersion corrections.41 All electrons were included in the computation and the double numerical plus polarization (DNP) basis set was used with a fine orbital cutoff quality. Synthesis. All the starting materials and solvents employed were commercially purchased and used as obtained. [H4L]Cl2, the hydrochloride salt of the ligand, was obtained according to the literature procedure with some modification.35 CAUTION! Metal compounds with azide are potentially explosive. Although not encountered in our experiments. Such compounds should be synthesized and handled carefully in a small quantity. [H4L]Cl2. A solution of 4,4’-bipyridine (1.00 g, 6.40 mmol) and dimethyl 5(bromomethyl)isopthalate (3.67 g, 12.8 mmol) in ACN (acetonitrile, 15 mL) was refluxed for 1 d. After cooled down, the yellow solid formed was collected by filtration out, washed with ACN, and then dried in vacuo to give the tetramethyl ester ([Me4L]Br2) as light yellow powder. Yield: 80% based on 4,4’-bipyridine. 1H NMR (400 MHz，DMSO-d6): δ = 9.57 (d, J = 6.8 Hz, 4H), 8.75 (d, J = 6.8 Hz, 4H), 8.57 (s, 4H), 8.51 (s, 2H), 6.09 (s, 4H), 3.91 (s, 12H).


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A solution of [Me4L]Br2 (1.00 g, 1.37 mmol) in concentrated HCl (30 ml) was refluxed for 2 d. The solution was slowly cooled to room temperature and placed in 2 °C refrigerator for 12 h. The white powder precipitated was filtrated, washed with water and acetone, and dried under vacuum [H4L]Cl2. Yield: 88%. 1H NMR (400 MHz，DMSO-d6): δ = 13.53 (s, 4H), 9.61 (d, J = 6.8 Hz, 4H), 8.77 (d, J = 6.8 Hz, 4H), 8.49 (s, 6H), 6.09 (s, 4H). Elemental analysis calcd. for C28H22N2O8Cl2 (M = 585.39): C, 57.45; H, 3.79; N, 4.79. Found: C, 57.85; H, 3.36; N, 5.11%. [Co3(L)(N3)4] (1). CoCl2·6H2O (4.1 mg, 17 µmol) and NaN3 (110 mg, 1.7 mmol) were dissolved in H2O/CH3OH (2 ml, v/v = 1:1). Then [H4L]Cl2 (5.0 mg, 8.5 µmol) was introduced and the mixture was sonicated for half an hour. The resulting green solution was kept in 80 °C for 6 d in a 15 mL screw-capped glass jar. After cooling, the red cubic crystals were rinsed by H2O for several times, separated by filtration, and dried in air. The yield was 15% based on CoCl2. Elemental analysis calcd. for C28H18N14O8Co3 (M = 855.33): C, 39.32; H, 2.12; N, 22.93. Found: C, 39.56; H, 1.88; N, 22.54%. IR (KBr pellet, cm-1): 3382m, 3336m 3126m 3058m 2075vs, 2057vs, 1637s, 1619s, 1572vs, 1442s, 1406s, 1342vs, 816s, 773s, 721s. [Mn2(L)(N3)2(H2O)2]·3H2O (2). Mn(ClO4)2·6H2O (0.30 g, 0.081 mmol) and NaN3 (0.052 g, 0.81 mmol) were mixed in H2O/CH3OH (4 ml, v/v = 3:1). After stirring for 10 min, [H4L]Cl2 (5.0 mg, 8.5 µmol) was added under stirring. The suspension was sealed in a Teflon-lined stainless-steel reactor (15 mL) and kept in 100 °C for 72 h. The orange cubic crystals were rinsed by H2O for several times, separated by filtration, and dried in air. The yield was 43% based on [H4L]Cl2. Elemental analysis calcd. for C28H28N8O13Mn2 (M = 794.46): C, 42.33; H, 3.55; N, 14.10. Found: C, 41.96; H, 3.28; N, 14.54%. IR (KBr pellet, cm-1): 3495br, 3380m, 3055m, 2068vs, 2053vs, 1638vs, 1550vs, 1440s, 1372vs, 1230m, 1190m.


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Table 1. Crystal data and refinement parameters of compounds 1 and 2. 1

2

Formula

C14H18N14O8Co3 C28H28N8O13Mn2

Fw

855.35

794.46

Crystal system

Triclinic

Monoclinic

Space group

P-1

C2/c

a, Å

9.223(2)

19.453(1)

b, Å

9.483(3)

11.361(3)

c, Å

9.983(9)

15.296(3)

α, deg

62.920(3)

90

β, deg

86.045(3)

115.636(2)

γ, deg

83.467(3)

90

V, Å3

772.33(9)

3047.8(7)

Z

1

4

ρcalcd, g cm-3

1.839

1.731

µ, mm-1

13.153

0.913

θ range collected

6.81 – 64.91

2.14- 28.28

data / unique

14033 /2549

20263/ 3741

Rint

0.0556

0.0261

S on F2

1.174

1.088

R1 [I > 2σ(I)]

0.1087

0.0366

wR2 (all data)

0.2599

0.1048

Single-Crystal X-ray Diffraction. Data collection for 1 was performed at 150 K using a Bruker D8 VENYURE diffractometer (Cu Kα radiation, λ = 1.54178 Å) equipped with a graphite monochromator and a CCD area detector. Data collection for 2 was conducted at 296 K


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using a Bruker APEX II diffractometer (Mo Kα radiation, λ = 0.71073 Å) equipped with a graphite-monochromated and a CCD area detector. Structure solution was carried out using the direct method and full-matrix least-squares refinements were performed on F2.42 Non-hydrogen atoms were all refined anisotropically. The H atoms attached to carbons were added geometrically and refined isotropically with the riding model. The H atoms of water molecules were assigned from the difference maps and refined isotropically. The crystallographic data and the parameters for the final refinemen are presented in Table 1. RESULTS AND DISCUSSION The two compounds were synthesized as crystals by solvothermal methods from metal salts, NaN3 and [H4L]Cl2 in aqueous methanol. NaN3 serves as weak base deprotonating the tetracarboxylic ligand and also as anionic ligand binding metal ions. Excessive NaN3 is needed for the synthesis of the compounds as pure crystalline phase. The bulk phase purity of the samples has been verified by PXRD (Figure S1). Structure Description. Crystallographic studies revealed different chain-based 3D networks for 1 and 2. Compound 1. Figure 1a depicts the structure of 1. There are two crystallographically independent Co(II) ions in different pseudo-octahedral environments. Co1 resides in the [N3O3] mer-octahedral surroundings completed by three azides, a chelating carboxylate, and an oxygen atom from another carboxylate. Co2 resides in the centrosymmetric [N2O4] trans-octahedral environments with two axial azide nitrogen atoms and four equatorial oxygen atoms arising from four carboxylates. The Co-N and Co-O bond lengths fall in the 2.014(3) - 2.331(6) Å range. The carboxylate groups assume two different coordination fashions. One is the syn,syn µ-η1:η1 bridge that binds Co1 and Co2 through different O atoms, and the other is the µ-η2:η1 bridge that


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chelates Co1 and binds Co2 through a µ-O atom. Each Co2 is connected to two Co1 ions through triple bridges consisting of µ-O (µ-η2:η1-COO), µ-η1:η1-COO and µ-1,1-N3, forming a linear trinuclear unit, with Co1···Co2 = 3.068(5) Å, Co1-O1-Co2 = 90.5(2)°, and Co1-N-Co2 = 92.9(3)°. Neighboring trinuclear clusters are related by inversion centers and doubly bridged by two µ-1,1 azides to generate a 1D [Co3(OCO)4(N3)4]n chain parallel to the a direction, along which the metal centers alternate in the Co1-Co1-Co2 sequence. The Co1···Co1B distance separated by the (µ-1,1-N3)2 bridges is 3.094(7) Å, with a Co1-N-Co1B angle of 95.6(3)°.

Figure 1. Structure of 1. (a) A presentation of the coordination surroundings and the chain structure formed from mixed azide and carboxylate bridges. Symmetry operation: A, 1-x, 1-y, 1z; B, -x, -y, 1-z; C, 2-x, 2-y, -z; D, 1-x, -y, 1-z; E, x, -1+y, z; F, 1+x, y, z. (b) The 3D framework formed by the organic ligand crosslinking the chains (viewed down the a direction).


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The L ligand in the structure takes a centrosymmetric zigzag conformation, with the two isopthalate groups trans to each other with respect to the planar N,N’-dimethylenebipyridinium moiety. The benzene and pyridinium rings have a dihedral angle of 82.5(1)°. Each ligand binds eight metal ions from four different [Co3(OCO)4(N3)4]n chains to generate a 3D coordination network (Figure 1b). The shortest interchain metal-to-metal separation is 9.483(3) Å. For a better understanding, the 3D framework may be described as follows. The isopthalate moieties of the L ligands interconnect the Co-N3-COO chains into formally negative layers along the ab plane, and the positive viologen moieties serve as slanted pillars between the layers (Figure S3). Compound 2. Two independent Mn(II) sites are present in this structure (Figure 2a). Mn1 lies on a C2 axis and is ligated by four carboxylate oxygens and two azide nitrogens in the cisoctahedral [N2O4] arrangement. Mn2 adopts the centrosymmetric trans-octahedral [N2O4] geometry furnished by two carboxylate oxygens, two azide nitrogens, and two water molecules. The Mn-N bond lengths [2.152(8)-2.188(1) Å] are generally longer than Mn-O [2.277(4)2.299(5) Å]. Neighboring Mn(II) centers are connected by an azide ion in the µ-1,1 mode and a carboxylate bridge in the syn,syn µ-η1:η1 mode to generate a uniform [Mn(N3)(OCO)]n chain parallel to the c axis. The Mn···Mn distance across the double bridges is 3.941(6) Å, with a MnN-Mn angle of 118.9(1)°. Adjacent MnN2O4 octahedrons down the chain share vertices (N2) and are relatively inclined, with an angle of 31.7(4)° between the [MnO4] and [MnNO3] equatorial planes


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Figure 2. Structure of 2. (a) The coordination surroundings and the chain formed from mixed azide and carboxylate bridges. Symmetry operation: A, 0.5-x, 1.5-y, 1-z; B, 0.5+x, 1.5-y, 0.5+z; C, 1-x, 1-y, 1-z; D, 1-x, y, 1.5-z; E, 1-x, 2-y, z. (b) The 3D structure.

The L ligand is centrosymmetric and assumes a zigzag conformation similar to that in 1 with the dihedral angle between benzene and pyridinium moieties being 87.2(1)°. The ligand adopts a coordination mode different from that in 1. Here each ligand binds six metal ions, with two carboxylate groups serving as µ-η1:η1 bridges and the other two adopting the monodentate mode. Each ligand connects four different [Mn(N3)(OCO)]n chains to produce a 3D coordination network (Figure 2b). The nearest metal-to-metal distance between the chains is 9.968(5) Å. The small apertures left in the structure are occupied by free water molecules.


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Cyclic Voltammetry. Considering the potential redox activity of the viologen moiety in the ligand, cyclic voltammetry was performed with GCEs modified with 1, 2 as well as the free ligand ([H4L]Cl2) (Figure 3). [H4L]Cl2 shows two pairs of quasi-reversible peaks with formal potentials (vs. Ag/AgCl) E = -0.52 and -0.98 V, corresponding to the two successive one-electron transfer processes of the viologen moiety (V2+ ↔ V•+ ↔ V0) 43-44. The first process occurs at less negative potential and the second one at more negative potential than the corresponding processes for MVCl2 (E = -0.64 and -0.92 V; MV2+ = dimethyl viologen). The observations suggest that the isopthalic substituent is beneficial to stabilization of the intermediate radical species.43-44 Compounds 1 and 2 show similar voltammetric behaviors, with E = -0.54 and -0.97 V for 1 and E = -0.52 and -1.0 V for 2. It is obvious that the redox activity is retained after the formation of these MOFs, with only minor changes in potentials.

Figure 3. Cyclic voltammograms of [H4L]Cl2, 1 and 2 (0.1 M KCl; scan rate, 100 mV s-1).

Photochromism of 1. As found for many viologen derivatives, compound 1 is photochromic. It turns from dark red to dark violet upon irradiation with a Xenon lamp (Figure 4a). The colored


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sample 1P can be bleached back to the original color by putting it in air at ambient temperature for several days. The bleaching process can be accelerated by heating in air. However, no appreciable color change was observed when 1P was kept in nitrogen atmosphere for two weeks, suggesting the involvement of O2 in the bleaching process. The color development and bleaching process can be repeated several times without appreciable variations in PXRD and IR spectra (Figure S1, S2), suggesting that the crystal and molecular structure is maintained during the photochromic and the reverse processes.

Figure 4. UV-vis reflectance spectra (a), photograph (inset) and solid ESR spectra (b, 298 K) of 1 before and after (1P) irradiation.


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To monitor the photochromic process, solid-state UV-vis reflectance and ESR spectra were measured (Figure 4). The UV-vis spectrum before irradiation shows an absorption envelope of multiple bands below 550 nm, with tailing in the longer wavelength region. The maximum at about 550 nm is attributable to the spin-allowed d-d transition 4T1g→4T1g(P) of high-spin octahedral Co(II), and the high-frequency bands should mainly arise from MLCT (metal-toligand charge transfer) transitions and/or intraligand transition. After irradiation, new absorption bands centered at about 412, 595 and 747 nm appear gradually and tend to saturate after irradiated for 80 min. The changes could indicate the photo-induced formation of free radicals. The compound is ESR silent before irradiation, owing to the very short spin-lattice relaxation time related to the strong spin-orbit coupling typical of the orbitally degenerate Co(II)-4T1g state. As shown in Figure 4b, the sample after irradiation (1P) shows a spectrum with small axial anisotropy in g values (g1 = g2 = 1.9994, g3 = 2.0002). The small deviation of gav (1.9997) from ge = 2.0023 and the small g anisotropy (∆g = g3-g1 = 0.0008) are within the ranges expected for organic radicals with weak spin-orbit coupling.45-47 The resonance signal cannot be due to the Co(II) center, which is usually undetectable at room temperature and should show significantly larger g-value deviation and anisotropy if observed. Nevertheless, the metal center could have contribution to the spin-orbit coupling of the radicals.48-49 The results suggest that the photochromism of 1 should originate from the generation of organic free radicals via one-electon transfer, consistent with the redox activity. The color fading in air can be due to the quenching of the free radicals by O2.


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Figure 5. XPS spectra of 1 (before irradiation) and 1P (after irradiation).

In order to decide the electron donor and acceptor, XPS spectra of 1 and 1P were measured (Figure 5). The Co 2p components show no appreciable change before and after Xe light irradiation, indicating that Co(II) ions are not involved in electron transfer. The O 1s peak of 1P is shifted towards higher energy compared with that of 1 (from 531.3 eV to 531.6 eV), indicating that the carboxylate oxygen atoms in L could have served as electron donors upon irradiation. The N 1s spectrum of 1 shows two bands at 398.9 eV and 402.9 eV. The former binding energy is attributable to the terminal N atoms of azide ions, while the latter one could be the envelope of the contributions from the pyridinium N atoms and the central N atoms of azides.50-51 The intensity ratio of the two bands (I403/I399) is 0.71, well correlated with the ratio of the N atoms in the compound (3/4). After Xe light irradiation, the low-energy band shows a minor shift toward higher binding energy, indicating that the terminal N atoms of azide ions could be another source of electron donors. The C 1s spectrum of 1 shows three peaks. The small band at 287.9 eV is assignable to the carboxylate C atoms. The main band at 284.9 eV and the shoulder at the higher energy side (286.1 eV) arise from the rest moieties of the ligand.52 The electron-deficient and positively-charged pyridinium moiety could have contribution to the shoulder peak. After


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irradiation, the relative intensities of the shoulder peak and the main peak are decreased and increased, respectively. The changes are in good agreement with the occurrence of electron transfer to the pyridinium moiety. In addition, the peak arising from carboxylate C atoms is broadened on the higher energy side, indicating the decreases of the electron density at some of the carboxylate groups.

Figure 6. LUMO (a) and HOMO (b) profiles for 1. The non-coordinated atoms of the azide ions are omitted in (a) for clarity.

To better understand the electron-transfer process, the density of states (DOS) and frontier orbital were analyzed through DFT calculations. Since Co(II) ions are not involved in electron transfer as confirmed by XPS, the calculations were performed on a periodical structure obtained


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by replacing Co(II) in 1 with Zn(II) to preclude the complex effects of the unpaired electrons in Co(II). As depicted in Figure S4, the DOS bands close to Fermi level (set to 0 eV) mainly arise from azide ions, with also some contributions from the carboxylate moiety, whereas the lowest conduction bands are predominated by the 4,4’-bipyridinium moiety. Frontier orbital calculations (Figure 6) show that the highest occupied molecular orbital (HOMO) is mainly composed of p orbitals of the terminal azide nitrogens and the carboxylate oxygens, and the lowest unoccupied molecular orbital (LUMO) arises almost solely from the π* orbitals of the 4,4’-bipyridinium moiety. These theoretical results support that the electron transfer is most likely from azide and/or carboxylate to 4,4’-bipyridinium, provided that there are appropriate pathways. Photoinduced electron transfer in solid-state viologen compounds has been related to the short contact between the pyridinium moiety and an electron donor (D), such as Npyridinium⋅⋅⋅D53-54, (CH)pyridinium⋅⋅⋅D25, 29, and πpyridinium⋅⋅⋅πD32-33. The appropriate N⋅⋅⋅O distance for electron transfer is usually suggested to be less than 3.5 Å. However, no unequivocal criterion has been established perhaps because the process could be influenced by the nature and various environmental factors of the donor and the acceptor. According to the structural data of 1, the shortest distance between carboxylate O atoms and pyridinium N atoms is 4.162(3) Å, which is quite far for electron transfer. However, there is a short contact between a carboxylate oxgygen (O1) and the hydrogen atom at the α position (C13) of the pyridinium nitrogen atom [d(H⋅⋅⋅O) = 2.484(1) Å, ∠C-H⋅⋅⋅O = 140.7°] (Figure S5). In addition, short contacts are present between the pyridinium N atoms (N7) and a terminal azide N atom (N6), with N7···N6C = 3.394(8) Å and N7···N6B = 3.414(1) Å. The (C-H)pyridinium⋅⋅⋅Ocarboxylate and Npyridinium⋅⋅⋅Nazide , contacts could be the favorable pathway for electron transfer, quite consistent with the XPS results and DFT calculations.


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Different from 1, compound 2 is not photohromic and shows no changes in UV-Vis spectrum even under prolonged irradiation with the Xe lamp (Figure S7). According to the crystallographic data, there are no short contacts between the pyridinium moiety and the azide ion in 2 (the shortest N⋅⋅⋅N distance is larger than 4 Å). The shortest distance between carboxylate O atoms and N atoms of pyridinium rings is 3.590(6) Å in 2. There is also weak hydrogen bonding between pyridinium α-C-H and carboxylate O, with H⋅⋅⋅O = 2.504(8) Å and C-H⋅⋅⋅O = 131.19°. It is noted that the hydrogen bonding contact is somewhat weaker than that in 1. The subtle difference may impose negative influences on electron transfer in 2. Hydrochromism of 2. Although not photochromic, compound 2 undergoes reversible color change upon releasing and reabsorbing water molecules. Upon heating to above 100 °C under vacuum or nitrogen atmosphere, the yellow compound turns brownish green. To monitor the thermal effect, Thermogravimetric analysis and in situ variable-temperature IR spectroscopy were performed with 2 (Figure S8). Four weight-loss steps were observed in the TG plot. The first weight loss (6.7%) occurring before 95 °C is related to the removal of the lattice water (calcd. 6.8%). Then the coordinated water molecules are released upon further heating to 150 °C (weight loss found 3.8%, calcd. 4.0%). The weight loss in the temperature range from 220 to 390 °C is due to azide decomposition (found 10%, calcd. 10.5%). The organic moiety decomposes in the range of 390−430 °C. The IR spectrum of 2 (Figure S6) displays two absorption bands at 3487 and 3537 cm-1. Upon heating, the former band decreases in intensity and disappears at 100 °C, indicating the release of lattice water. The latter band decreases above 100 °C and disappears at 150 °C, indicating that the coordinated water molecules are lost. The fact that the material does not change color until heated to 100 °C suggests that the chromic phenomenon is related to the removal of the coordinated water.


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Figure 6. UV-vis reflectance spectra (a), photographs (inset) and ESR spectra (b) of 2 before and after dehydration under vacuum.

The chromic process has been monitored using solid-state UV-vis diffuse reflectance spectroscopy (Figure 6a). The compound before dehydration exhibits strong absorption in the region of 230-450 nm, which should be due to ligand-centered transitions and charge-transfer transitions between Mn(II) and ligands. Upon heating at 100 °C in vacuum for 1 h, the absorption in the visible light region increases and new shoulder bands appear. After fully dehydrated at 150 °C, the compound shows four bands around 588, 632, 696 and 771 nm, indicating the formation of radical species. The bands show no significant change if the temperature is raised to 200 °C, indicating that chromic saturation has been achieved.


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X-band ESR spectroscopy was performed to confirm the radical mechanism of the chromic phenomenon (Figure 6b). The spectrum of the hydrate phase shows a strong symmetric signal centered at g = 2.0087, without signals of hyperfine splitting. The g value is typical of the isotropic high-spin Mn(II) ion, and the collapse of the hyperfine structure related to the Mn nucleus is due to magnetic interactions in magnetically undiluted systems.55-56 The fully dehydrated phase (2D) shows a symmetric signal at a similar position. However, the intensity is dramatically reduced, and the width at half maximum is much larger. The changes could be because of the magnetic interaction between the metal ion and the radical species induced by dehydration.

Figure 7. XPS spectra of 2 and 2D (dehydrated).

XPS spectroscopy was performed to gain clues about the radical formation. As illustrated in Figure 7, the Mn 2p spectra before and after dehydration are almost identical, which confirms that the oxidation state of Mn(II) remains unchanged. The variations of the O 1s, C 1s and N 1s peaks of 2 before and after dehydration are similar to those observed for compound 1. The O 1s peak for carboxylate and the N 1s peak for terminal N atoms of azide are both slightly shifted towards higher energy after dehydration, proving that the carboxylate and azide groups could be


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the source of electron donors. As for the C 1s spectrum, the change in the relative intensities of the main peak at 284.8 eV and the shoulder at 286.6 eV also indicates 4,4’-bipyridinium as electron acceptor. PXRD measurements suggest that 2D is a new crystalline phase different from the original hydrate phase (Figure S1). Unexpectedly, the radical phase is stable in dry air. The green sample can be kept in dry air for months, with no changes in color and ESR signals. This suggests a long-lived radical phase. However, the phase readily turns yellow when exposed to moist air or allowed to be in direct contact with liquid water, indicating the quenching of radical species. PXRD measurements suggest that the original hydrate phase is recovered. The dehydrationhydration switched color change has been repeated for 5 times. Notably, the reverse color change can occur in moist nitrogen atmosphere, so the radical quenching is not a process of oxygen oxidation but involves back electron transfer triggered by water absorption. Furthermore, the solvatochromism of 2D is highly selective to water, with negative response to nonaqueous solvents, such as ethanol, acetonitrile, DMF and DMSO. The reversible and ready interconversion in the solid state indicates that the structure does not undergo dramatic changes during dehydration and rehydration. A closer inspection of the structure of 2 revealed that the uncoordinated O atom arising from a monodentate carboxylate group of the L ligand forms a hydrogen bond with the coordinated water (Figure S9). It can be speculated that the vacant coordination site left after the removal of the water molecule is approached by the carboxylate oxygen atom. This speculation is supported by the variabletemperature IR spectra (Figure S6). The band at about 1640 cm-1 for the hydrated phase can be ascribed to the ν(C=O) absorption of the monodentate carboxylate moiety. The band remains nearly unchanged at 50 °C, but significant red shift occurs when heated up to 100 °C and above.


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The observation implies (weak) coordination of C=O upon release of the coordinated water. The coordination should be accompanied by changes in molecular conformation and intermolecular contacts. It is very likely that the contacts between electron donors (azide/carboxylate) and acceptors (bipyridinium) are changed in favor of electron transfer. In other words, the pathways of electron transfer to form radicals are switched on. Chromic phenomena related to dehydration/rehydration are very common for transition metal compounds, where the color change is due to the modulation of ligand-field d-d transitions. The mechanism for the phenomenon observed for 2 is completely different, which involves the reversible formation of radicals through electron transfer triggered by water molecules. To our knowledge, such reversible solid-state hydrochromic behaviors related to the redox-active organic species is unprecedented. Here the hydrochromism also differs from thermochromism observed for a few coordination compounds with other viologen ligands, where the radical formation (coloration) is induced by heat and decoloration occurs through oxygen quenching.25, 57-60

The chromic processes of 2 relies only on dehydration/rehydration. We have heated 2 at 150

°C in water vapor, no color change was observed. Therefore, heat alone cannot cause the color change, and it just acts to promote the release of coordination water. Furthermore, the reverse color change occurs only when water is re-absorbed, independent of oxygen.


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Figure 8 (a) χT versus T curve for 1 at 1 kOe (■). The inset presents the low-temperature χ versus T data (□) and the fit to the J1J1J2 Ising chain model (the solid line). (b) A scheme showing the alternating interactions along the chain in 1.

Magnetic properties. Compound 1. The magnetic susceptibility of 1 over 2-300 K is plotted in Figure 8a. The χT product at 300 K (3.35 emu K mol-1 per Co(II)) is well above the spin-only value expected for S = 3/2 (1.875 emu K mol-1), typical of the octahedral CoII-4T1g term with unquenched orbital contribution. As the temperature decreases, both χ and χT increase monotonically. The susceptibility above 20 K can be fitted to the Curie-Weiss law with Weiss temperature θ = 10 K and Curie constant C = 10 emu K mol-1. The behaviors suggest ferromagnetic (FM) interactions between Co(II) ions. The magnetization isotherm was measured at 2 K (Figure S10). The very rapid increase of magnetization in the low field region confirms the FM behavior. The magnetization at 5 T (2.49 Nβ) is consistent with the value (2-3 Nβ) expected for octahedral Co(II).


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Considering that the Co(II) ions are connected into a chain by the triple (µ-Ocarboxylate)(µ-1,3carboxylate)(µ-1,1-azide) (A) and double (µ-1,1-azide)2 (B) bridges alternating in the AAB sequence (Figure 8b), a J1J1J2 chain model has been utilized to estimate the exchange parameters. At low temperature, Co(II) can be taken as an Ising spin with effective spin of S = 1/2, the Hamiltonian for the J1J1J2-alternating chain is 61 H = −J1Σ(S3i,zS3i+1,z+ S3i+1,zS3i+2,z) − J2Σ(S3i-1,zS3i,z)

(1)

The parallel component of the susceptibility per Co(II) center for such a Ising chain has been deduced as61 2

χ // =

Nβ 2 g // 9 exp( J1 / 2kT ) + 2 exp(− J 2 / 2kT ) + exp(− J 1 / 2kT ) 12kT exp(J 1 / 2kT − J 2 / 2kT ) + exp(− J 1 / 2kT − J 2 / 2kT ) + 2

(2)

Fitting the susceptibility data of 1 over the 8 - 40 K range using the least-squares method gave satisfactory results with J1 = 3.6 cm-1and J2 = 24.0 cm-1 with g// = 10.6. The parameters confirm that the double and triple bridges both propagate FM coupling. The FM interaction through the double µ-1,1-azide

bridges is consistent with previous Co(II) compounds with the same

bridge.62-64 The (µ-Ocarboxylate)(µ-1,3-carboxylate)(µ-1,1-azide) mixed bridges are sparely encountered, and the much weaker interaction through the mixed bridges compared with the double µ-1,1-azide bridges is due to the compensation between the different bridges. Compound 2. The χT and χ versus T plots for compound 2 in the temperature range 2–300 K are given in Figure 9. The χT product at 300 K (4.09 emu K mol-1) is somewhat smaller than that expected for an isolated S = 5/2 ion (4.375 emu K mol-1). As the temperature decreases, χT decreases smoothly, whereas χ exhibits a maximum around 10 K and a very small rise below 3 K. Fitting the 1/χ vs. T plot above 20 K to the Curie-Weiss law led to θ = -17.5 K with C = 8.94 emu mol-1 K. Such behaviors indicate that antiferromagnetic (AF) coupling is operative between


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the metal ions. The small rise in χ below 3 K indicates a little contribution from paramagnetic impurity.

Figure 9 Plots of χ (○ for 2 and + for 2D) and χT (□ for 2 and × for 2D) versus T. The red lines is the best fit by the Fisher model.

The Fisher model 65 for classical-spin uniform chains (H = -JΣSiSi+1) was used to evaluate the coupling parameter (J) within the Mn(II) chain

χ chain = [ Ng 2 β 2 S ( S + 1) /(3kT )][(1 + u ) /(1 − u )]

(4)

wherein u = coth[ JS ( S + 1) / kT ] − kT [ JS ( S + 1)] with S = 5/2. Fitting the susceptibility data to the model yielded J = -1.7 cm-1 and g = 2.01. The fit has included a molar fraction (ρ = 0.0063) of the paramagnetic impurity. Isothermal measurements at 2 K (Figure S11) also confirms the AF interaction: the magnetization displays a quasilinear and slow increase with the field; the value at 5 T (1.43 Nβ) is well below the saturation value (5 Nβ) of an isolated Mn(II) ion. The (µ-1,1-N3)(µ-1,3-COO) bridges have been identified in several Mn(II) complexes, all exhibiting AF exchange interactions with J falling in the range from -1.29 to -5.38 cm-1.66-70 Some magnetic and structural data available for these compounds are summarized in Table S1. The main factors


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influencing the magnitude of AF interactions should be the M-N/O distances, the M-N-M bridging angles, and the M-O-C-O torsion angles. However, no unambiguous correlations of the J values to any of these factors can be drawn. The magnetic properties of the anhydrous phase 2D were also investigated. The data are plotted in Figures 9 and S11 for comparison with the original phase. The χT value of 2D at 300 K is 4.44 emu K mol-1, higher than that for 2. This is in agreement with the generation of organic radicals after dehydration. The temperature-dependent magnetic behaviors of 2D also indicate overall AF interactions. The isothermal magnetization of 2D also increases quasilinearly with field, but the values are higher than those of 2 in the whole field range owing to the contribution from radicals. The magnetization would show nonlinear variation with field if the radicals were magnetically isolated (i.e., as paramagnetic components superimposed on the AF coupled Mn(II) ions). The absence of appreciable nonlinear variation in the magnetization curve of 2D indicates that the radicals are AF coupled with each other and/or with Mn(II). Considering the ESR signal of 2D, one can be sure of the spin exchange between Mn(II) and the radical. Compound 2 represents a unique example of solovatomagnetic materials, where the marked magnetic change is related to reversible electron transfer induced by release and absorption of water molecules.

CONCLUSIONS We have described two redox-active MOFs with a viologen-based tetracarboxylate ligand, which show different 3D structures based on FM [Co(II)] or AF [Mn(II)] chains with simultaneous azide and carboxylate bridges. Because of the contacts between electron-poor viologen units and electron-rich sites (either carboxylate oxygen or azide nitrogen), 1 is


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photochromic, which is related to the formation of radicals via photo-induced electron transfer. 2 is not photochromic for lack of appropriate electron-transfer pathways. Nevertheless, 2 shows reversible and marked changes in color and magnetic properties upon removal and absorption of water molecules. The dual responses to water are attributed to reversible electron transfer promoted by reversible structural transformation during the dehydration-rehydration process. The solid-state solvochromism and solvotomagnetism through radical mechanisms have not yet been reported prior to our work and may find applications in snensing and switching devices. Our work could shed light on the design of new smart orgainic-inorganic hybrid materials using redox-active organic building blocks.

ASSOCIATED CONTENT Supporting Information. Structural diagrams, spectral (XRD, IR, UV-Vis), TGA, DOS and magnetic data (PDF); crystallographic data in the CIF format. The Supporting Information is available for free via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS. We acknowledge the financial support from the National Natural Science Foundation of China (NSFC Nos. 21471057, 91022017 and 21173083).


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