Confinement Effects of Metal–Organic Framework on the Formation of

Article pubs.acs.org/IC

Confinement Effects of Metal−Organic Framework on the Formation of Charge-Transfer Tetrathiafulvalene Dimers Ting Chen, Peng Huo, Jin-Le Hou, Jing Xu, Qin-Yu Zhu,* and Jie Dai* College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P. R. China S Supporting Information *

ABSTRACT: Three transition metal coordination polymers (CPs) based on the redoxactive dimethylthio-tetrathiafulvalene-bicarboxylate (L) and 1,3-bi(4-pyridyl)propane (bpp) ligands, formulated as [MnL(bpp)]n (1), [CdL(bpp)]n (2), and [Cd2L(bpp)2(H2O)(C2O4)0.5]n·n(ClO4)·n(H2O) (3), are crystallographically characterized. Complexes 1 and 2 are isostructural 2-D polymers, and 3 features an unusual 3-D metal− organic framework (MOF). The 3-D MOF is constructed from tetranuclear cluster nodes built through the μ2-O bridge of the TTF ligand, which is first found for TTF coordination polymers. It is found that the channel generated by the 3-D MOF exerts a confinement effect on the formation of TTF dimers. The TTF dimers show strong intradimer interaction with partial electron transfer or charge transfer, and hence, the Cd compound 3 has relatively good photocurrent response property in comparison with that of 2-D Cd compound 2.

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that TTF dimers, including (TTF2)0, (TTF2)1+, and (TTF2)2+, play an important role in these applications. Several attempts have been made to understand the nature of the intermolecular interactions between the different redox states of the TTF dimers.12 Different strategies have been attempted to study these TTF dimers including (i) covalent attachment of the TTF units in a specific geometry to facilitate the formation of the dimers,12d,13 and (ii) stabilization of the TTF radical dimers using appropriate host molecules.14 On the other hand, many coordination complexes with TTF ligands have been prepared in order to combine the electronic and magnetic properties of the organic and inorganic moieties in the resulting solids.15 However, despite the progresses in the construction of TTF coordination polymers (CPs), the incorporation of TTFs into three-dimensional (3-D) MOFs has rarely been reported.16−18 Furthermore, the confinement effects of MOFs on the formation of TTF dimers have never been discussed. Herein, two 2-D CPs, [MnL(bpp)]n (1) and [CdL(bpp)]n (2), and a 3-D MOF [Cd2L(bpp)2(H2O)(C2O4)0.5]n·n(ClO4)· n(H2O) (3) were obtained, using dimethylthio-tetrathiafulvalene-bicarboxylate (TTF-bicarboxylate, L), together with a flexible bipyridine derivative 1,3-bi(4-pyridyl)propane (bpp). We found that TTF moieties are confined in the pores of the 3D MOF 3 to form charge-transfer (CT) TTF dimers, but no such phenomena occur in 2-D 1 and 2. The confinement effects of MOF 3 are discussed based on structural analysis, UV−vis, and ESR spectra data. To understand photoelectro behaviors of the MOF confined TTF dimer, photocurrent response properties of the three compounds with confined and unconfined TTF dimers are studied.

INTRODUCTION The construction of metal−organic frameworks (MOFs) from metal ions and functional organic ligands has become an exciting approach to novel solid-state materials.1−5 The interest is stimulated not only by the MOFs’ intriguing structural features but also by their potential applications in a wide range of research fields such as catalysis,2 gas sorption and storage,3 electronic materials,4 and light-harvesting and energy transfer.5 The functions of MOF materials can be generated from pore spaces, pore surfaces, and the nodes.6 Structural confinement effects based on the pore spaces make great contribution to the functions of MOFs.7,8 For example, uniform nanometal particles can be prepared in the cavity of MOFs.7 An isotactic polymer was synthesized by [2 + 2] cycloaddition reaction under UV light from the diene ligand that was embedded inside single crystals of MOFs.8 Therefore, the specific confinement effects of MOFs can be utilized for diverse applications. Although the adopted explanation of “confinement effect” of MOFs is to regulate the arrangement of guests or the growth of particles within a preformed pore space, it is a similar situation that the guest moieties were squeezed into a pore space when the MOF framework is in situ formed. The energy needed for the close arrangement of molecules can be compensated by the coordination bonding energy in the formation of the framework. However, such a phenomenon has not been valued and discussed based on our knowledge. In this work, we find such a confinement effect of a 3-D MOF on the formation of charge-transfer tetrathiafulvalene (TTF) dimers. TTF and its derivatives (TTFs) are well-known electron donors and can be reversibly oxidized into their corresponding radical cation (TTF•+) and dictation (TTF2+). Accordingly, TTFs have been widely used in the construction of diverse functional materials such as molecular conductors,9 molecular switches,10 and solar-energy transformation.11 It has been found © XXXX American Chemical Society

Received: August 25, 2016

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DOI: 10.1021/acs.inorgchem.6b02062 Inorg. Chem. XXXX, XXX, XXX−XXX

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Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/ cif. Electrode Preparation and Photoelectrochemical Measurement. The photoelectrodes of the compounds were prepared by a powder coating method. As a typical procedure, the crystals of compounds (0.005 mmol) were ground and pressed uniformly on the cleaned ITO glass (100 Ω/□). A 150 W high-pressure xenon lamp, located 20 cm away from the surface of the ITO electrode, was employed as a full-wavelength light source. The photocurrent experiments were performed on a CHI650E electrochemistry workstation in a three-electrode system, the sample coated ITO glass as the working electrode mounted on the window with an area of 0.385 cm2, a Pt foil as auxiliary electrode, and a saturated calomel electrode (SCE) as reference electrode. The supporting electrolyte solution was 0.1 mol·L−1 sodium sulfate aqueous solution. The lamp was kept on continuously, and a manual shutter was used to block exposure of the sample to the light. The sample was typically irradiated at intervals of 20 s.

EXPERIMENTAL SECTION

General Remarks. The compound, dimethylthio-tetrathiafulvalene-bicarboxylate sodium salt (Na2L), was prepared using the method reported previously.19 All other reagents were purchased from commercial suppliers (Alfa Aesar). The IR spectra were recorded as KBr pellets on a Nicolet Magna 550 FT-IR spectrometer. Elemental analyses of C, H, and N were performed using an EA1110 elemental analyzer. Electronic absorption spectra were measured on a Shimadzu UV-3150 spectrometer. PXRD of the compounds was carried out on a D/MAX-3C X-ray diffraction meter with Cu Kα (λ = 1.5406 Å) radiation. ESR spectra were carried out at 110 K on a Bruker ER-420 spectrometer with a 100 kHz magnetic field in X band. Cyclic voltammetry (CV) experiments of the solid state compounds were performed in acetonitrile with 0.10 mol·L−1 tetrabutylamonium perchloride on a CHI600 electrochemistry workstation in a threeelectrode system, a surface-modified Pt-plate working electrode, a Pt wire auxiliary electrode, and a saturated calomel electrode (SCE) as reference. Preparation of Compounds. Caution! All metal perchlorates must be regarded as potentially explosive. Only a small amount of compound should be prepared, and it should be handled with caution. [MnL(bpp)]n (1): Reactants MnCl2·4H2O (2.0 mg, 0.01 mmol) or Mn(ClO4)2·6H2O (3.6 mg, 0.01 mmol), bpp (2.0 mg, 0.01 mmol), and Na2L (2.2 mg, 0.005 mmol) were mixed in 0.2 mL of CH3OH. The mixture was sealed in a Pyrex tube (7 mm dia., 180 mm length) under ambient atmosphere and pressure. The tube was heated to 80 °C for 7 days. After cooling to room temperature, bright yellow single crystals of 1 were obtained (yield: 1.1 mg, 34.6% based on Na2L). Anal. Calcd for C23H20MnN2O4S6 (1) (MW 635.8): C, 43.45; H, 3.17; N, 4.41%. Found: C, 43.36; H, 3.14; N, 4.46%. IR data (cm−1): 1647(s), 1612(m), 1574(m), 1556(s), 1543(s), 1507(w), 1427(w), 1375(m), 1356(s), 1227(w), 1107(w), 1067(w), 1012(m), 813(w), 791(w), 767(m), 744(m), 693(m). [CdL(bpp)]n (2): Complex 2 was obtained by following a similar procedure to that of 1, but CdCl2·4H2O (2.6 mg, 0.01 mmol) was used instead of Mn(II) salts. Yellow single crystals of 2 appeared for 7 days and were used for all measurements (yield: 1.3 mg, 37.5% based on Na2L). Anal. Calcd for C23H20CdN2O4S6 (2) (MW 693.2): C, 39.85; H, 2.91; N, 4.04%. Found: C, 39.95; H, 2.86; N, 3.95%. IR data (cm−1): 1641(s), 1611(m), 1571(m), 1559(m), 1540(s), 1507(w), 1427(w), 1375(m), 1354(s), 1223(w), 1100(w), 1067(w), 1014(m), 816(w), 791(w), 767(m), 739(m), 693(m). [Cd2L(bpp)2(H2O)(C2O4)0.5]n·n(ClO4)·n(H2O) (3): Reactants Cd(ClO4)2·6H2O (4.2 mg, 0.01 mmol), bpp (2.0 mg, 0.01 mmol), Na2L (2.2 mg, 0.005 mmol), and K2C2O4·H2O (1.8 mg, 0.01 mmol) were mixed in 0.2 mL of CH3OH. The mixture was sealed in a Pyrex tube (7 mm dia., 180 mm length) under ambient atmosphere and pressure. The tube was heated to 80 °C for 14 days. After cooling to room temperature, black single crystals of 3 (yield: 1.3 mg, 21.9% based on Na2L) and yellow single crystals of 2 (yield: 0.4 mg, 11.5% based on Na2L) were simultaneously obtained. Anal. Calcd for C37H38Cd2ClN4O12S6 (3) (MW 1183.3): C, 37.55; H, 3.24; N, 4.73%. Found: C, 37.61; H, 3.12; N, 4.81%. IR data (cm−1): 1642(s), 1614(s), 1582(s), 1558(s), 1535(shoulder), 1507(w), 1428(m), 1385(m), 1354(s), 1303(s), 1227(w), 1089(b,s), 1016(m), 1014(m), 968(w), 892(w), 860(w), 817(w), 791(w), 774(m), 760(s), 623(m). X-ray Crystallographic Study. The measurement was carried out on a Rigaku Mercury CCD diffractometer at room temperature with graphite monochromated Mo Kα (λ□ = 0.71073 Å) radiation. X-ray crystallographic data for all compounds were collected and processed using CrystalClear (Rigaku).20 The structure was solved by direct methods using SHELXS-97 for 1 and 2 and SHELXS-14 for 3,21 and the refinement against all reflections of the compound was performed using SHELXL-97 for 1 and 2 and SHELXL-14 for 3.22 All of the nonhydrogen atoms were refined anisotropically, and hydrogen atoms were added theoretically. Relevant crystal data, collection parameters, and refinement results can be found in Table S1. Crystallographic data CCDC 1499910−1499912 contain the supplementary crystallographic data for 1−3. The data can be obtained from the Cambridge

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RESULTS AND DISCUSSION Synthesis and Characterization. The controllable preparation of MOFs is still a challenge because many factors influence the crystallization process, such as metal ion, solvent, pH, as well as the synthetic method.23 The bpp ligand has a flexible propane linker between two pyridyl groups, and thus it is expected to facilitate the formation of high dimensional structures. When the mixture of MnCl2·4H2O or Mn(ClO4)2· 6H2O, bpp, Na2L, and solvent CH3OH was heated at 80 °C for 7 days, bright yellow 2-D crystals of 1 were obtained. Following a similar procedure to that of 1, but CdCl2·4H2O was used instead of Mn(II) salts, bright yellow 2-D crystals of 2 were obtained in good yield. The crystal of 2 is isomorphous to that of 1 with very similar cell dimensions. If Cd(ClO4)2·6H2O is used instead of CdCl2·4H2O, both bright yellow crystals of 2 and a few black crystals of 3-D MOF 3 were obtained. Oxalate anion was found unexpectedly in the crystal of 3, which is perhaps generated by the oxidation and decomposition of L. Then, potassium oxalate is pre-added into the system on purpose. The yield of black crystals of 3 indeed largely increases, but crystals of 2 still simultaneously appear. Crystals 2 and 3 were manually separated based on different colors under a microscope. Furthermore, if potassium oxalate is preadded into the Mn(II) reaction system, there are only crystals of 1 grown and no 3-D crystals are isolated. These experimental results indicate that the metal ions play a crucial role in the formation of different dimensional crystal structures in this reaction system. These three complexes are insoluble in common solvents such as methanol, CH3CN, acetone, chloroform, benzene, etc. The elemental analyses indicate that the components were in good agreement with the results of the X-ray structure analysis. The PXRD patterns of the microcrystal samples are in accordance with the simulated patterns from the crystal data of the compounds (Figure S1) to confirm the identity and purity of all the samples. PXRD of the freshly prepared crystals, samples kept under ambient condition for 3 months, and after CV measurement of compounds 1−3 were carried out (Figure S2). The results show that there is no change and, therefore, these compounds are stable. The IR spectra of compounds 1−3 are similar to each other, with the vibration of the central CC bond, 1535−1540 cm−1, which is in agreement with that of neutral TTF compounds.24 Strong IR bands of 3 at 1089 and 1303 cm−1 are assigned to perchlorate groups and oxalate anion, respectively. The Raman spectra of compounds 1−3 are also similar to each other (Figure S3). B

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Figure 1. (a) Structure of [MnL(bpp)]n (1) showing the coordination environment of Mn(II) (i: 1 − x, 1 − y, 1 − z; ii: x, −1 + y, z; iii:1 − x, −1 + y, 1.5 − z). bpp in left and TTF in right are omitted for clarity. (b) 2-D structure of 1. (c) Topological network of 1 (red and green lines: bpp ligand; orange line: carboxylate groups of L). (d) Packing structure of 1 showing that TTF dimers exist between the neighboring 2-D networks.

Description of Structures. [MnL(bpp)]n (1) and [CdL(bpp)]n (2). Compounds 1 and 2 are isostructural, and 1 is used as a representative in the description of structures. Compound 1 is a 2-D coordination polymer crystallized in the monoclinic C2/c space group, and the asymmetric unit consists of one manganese atom, one ligand L, and one bpp. The Mn(II) ion is

six-coordinated by four carboxylate oxygen atoms from three different ligands L and two nitrogen atoms from two different bpp with a cis orientation of the pyridine rings (N1−Mn1−N2′ = 88.5°) (Figure 1a). In contrast to the reported similar complex,25 the Mn−O bonds trans to the nitrogens are slightly shortened (Mn1−O2 = 2.139(3) Å and Mn1−O3 = 2.169(3) C

DOI: 10.1021/acs.inorgchem.6b02062 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Å) when compared to the axial Mn−O bonds (Mn1−O1 = 2.174(3) Å and Mn1−O4 = 2.211(3) Å). The carboxylate groups of L show a μ2-η1:η1 coordination mode and link three Mn(II) centers to form an infinite 1-D zigzag chain (Figure S4) with Mn−Mn distances of 4.61 and 4.86 Å. The TTF moieties are regularly arranged on both sides of the chain. These chains are then linked by the coordination of bpp with TG conformations26 to form a 2-D network (Figure 1b,c). A paired structure of TTF moieties is found between the 2-D layers (Figure 1d), but there is not strong intrapair interaction. [Cd2L(bpp)2(H2O)(C2O4)0.5]n·n(ClO4)·n(H2O) (3). Compound 3 crystallizes in the triclinic space group P21/n. The asymmetric unit contains two crystallographically independent Cd(II) ions, one L ligand, two bpp, one coordinated water molecule, a half oxalate anion, and one perchlorate anion as well as cocrystallized one water molecule. As depicted in Figure 2a and Figure S5a, each Cd(II) atom is in a pentagonal bipyramid geometry, which is different from that in 2. The Cd1 is coordinated by five oxygen atoms, four of which from two different L and the other one from oxalate anion, constructed an equatorial plane, and two nitrogen atoms from two different bpp molecules, occupied the axial positions. The Cd2 is also located in an O5N2 environment, but the equatorial plane is constructed by one nitrogen atom from bpp and four oxygen atoms, two of which from one L and the other two from oxalate anion, and the axial positions are occupied by one oxygen atom from a water molecule and one nitrogen atom from bpp. The average Cd1− N bond distance is 2.285 Å, slightly shorter than that of Cd2− N, 2.314 Å. The Cd1−O bond distances are from 2.343 to 2.489 Å, slightly longer than those of Cd2−O, from 2.320 to 2.431 Å. The central CC bond distance of 3, 1.351(2) Å, is somewhat longer than those of 1 and 2, 1.343(6) and 1.344(2) Å, due to partial electron transfer or charge transfer, but still falls in the range of neutral TTF compounds, 1.33−1.35 Å.27 It is noteworthy that all the carboxylate groups of L in 3 show μ2-η2:η1 coordination modes and four Cd(II) ions are interlinked to form a tetranuclear cluster (Cd2−Cd1−Cd1− Cd2) with the short Cd−Cd distance of 3.9 Å (Figure 2b, Figure S5b,c). The Cd1 and Cd2 ions are connected to form a subunit through double μ2-O bridges, one of which is from L and the other one is from oxalate anion. Two such subunits are further linked to form a centrosymmetric tetranuclear cluster through double μ2-O bridges from two different L. The oxalate anions connect adjacent tetranuclear clusters to form a steptype 1-D chain along the a axis (Figure 2b). These 1-D chains are further linked by bpp along two different directions to generate a 3-D MOF structure (Figure 2c,d). The conformations of bpp in 3 are alterable, and the TG and GG conformations coexist. The channels of the 3-D MOF are occupied by TTF dimers that grafted on the network, as well as perchlorate anions and cocrystallized water molecules (Figure 2d,e, Figure S5d). It should be pointed out that the notable characteristics of 3 are the formation of the tetranuclear cluster and charge-transfer TTF dimer (discussed below). The clusters coordinated by redox-active TTF ligands have been already reported,28 but TTF grafted MOF with a polynuclear cluster node (built through the μ2-O bridge) is first found in the structure of 3. The TTF dimers in 3 are confined in a narrow space formed by the 3-D network. MOF’s Confinement Effects on the Formation of Charge-Transfer TTF Dimers. Since compounds 1 and 2

Figure 2. (a) Coordination environment of Cd(II) in [Cd2L(bpp)2(H2O)(C2O4)0.5]n·n(ClO4)·n(H2O) (3) (i: 1 − x, −y, −z; ii: 1/2 + x, −1/2 − y, −1/2 + z). (b) The cluster-chain structure (polyhedral representation of the coordination centers) along the a axis. In (a) and D

DOI: 10.1021/acs.inorgchem.6b02062 Inorg. Chem. XXXX, XXX, XXX−XXX

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However, the TTF moieties in 3 are basically neutral based on the bond distance and IR and Raman spectra data of the central CC bond of the TTF moiety. The ESR spectra are highly sensitive to partial electron transfer or small quantity oxidation of the TTF moieties; therefore, only a trace amount of TTF was oxidized during synthesis. There are some such CT examples in which the TTF moieties are oxidized in trace amounts, showing an ESR signal and exhibiting a black color,30 whereas the ESR spectrum of 2 shows no such signal. The ESR spectrum of 1 displays broadened signals with g = 2.005 (Figure 4) that can be attributed to the unpaired electrons of the paramagnetic center of the Mn(II) atom. The paired arrangement of TTF moieties is a common characteristic for TTF compounds, but the interaction within the TTF dimer is crucially related to the electronic, conjugated states and distance of the TTF moieties. Although paired arrangements of TTF moieties are found in all three structures of 1−3, only the 3-D MOF compound shows a strong absorption band of the TTF dimer. The ESR results have indicated the partial oxidation of the TTF moiety in 3. On the basis of the structural analysis, the conjugated state of the TTF moiety in 3 is far better than that in 2, reflected from the following two aspects. The dihedral angles of the TTF moiety in 3 are 4.38° and 12.66° bent along the S···S axis (Figure 5a),

Figure 2. continued (b), bpp, except the coordinated nitrogen atom, and L, except the coordinated carboxylate group, are omitted for clarity. (c) The schematic view of the 3-D MOF structure (red line: bpp ligand; orange line: carboxylate groups of L). (d) 3-D MOF structure showing that TTF dimers are occupied in the pores. (e) MOF structure of 3 showing that TTF dimers, perchlorate anions, and water molecules are occupied in the pores.

are yellow crystals and Cd(II) oxalate is colorless, then the black color of crystals of 3 is an interesting phenomenon that suggests the presence of TTF dimers. Optical diffuse-reflection spectra of 1−3 together with the starting material Na2L were measured at room temperature using BaSO4 as a standard reference. As shown in Figure 3, the lowest intense absorption

Figure 3. Optical diffuse-reflection spectra of 1−3 as well as the precursor Na2L in the solid state. Inset: Optical diffuse-reflection spectra of 3 with peak separation.

band at 402 nm for the TTF ligand shifts to 442 nm for 1 and 2 due to the metal coordination, while the spectrum of 3 shows a broad range absorption in the range of 400−750 nm, which can be peak-separated to two peaks: 480 and 584 nm. The peak at 480 nm is attributed to the TTF ligand, and the large shift attributes to the strong μ2-η2:η1 coordination that is different from μ2-η1:η1 coordination in 1 and 2. The new lowest energy band at 584 nm could be assigned to the absorption of the TTF dimer of 3.12a,d Solid state ESR spectrum of 3 shows a sharp weak signal at g = 2.008 (Figure 4) that agrees with the characteristic ESR signal of the TTF•+ radical cation.29

Figure 5. Dimers of the TTF moiety in 3 (a) and 2 (b), showing the dihedral angles of TTF moieties and average intermolecular distances.

whereas those in 2 are 18.42° and 20.55° (Figure 5b). Besides, the carboxylate groups of L in 3 are conjugated with the TTF moiety but not in 2. The average distance of the TTF pair is 3.393 Å in 3, shorter than that in 2 (3.453 Å) (Figure 5). The

Figure 4. ESR spectra of 3 and 1 recorded at 110 K. E

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(current density of 1 > 2) indicates that the redox-active metal coordination center Mn(II) is in favor of the photocurrent conversion, in agreement with the previous report.31 The photocurrent response of compound 3 with confined TTF dimers shows an obvious enhancement compared with that of 2 with unconfined TTF dimers. Upon irradiation, the photoinduced charge separation happens within the TTF dimer. Under the applied bias, the electron and hole move oppositely toward electrodes to generate current. Although the conductive TTF dimers are spatially separated by the MOF framework, the quick photocurrent response property (see current curve) demonstrates that the MOF net does not hinder the carrier mobility. The charge transport is dominated by charge hopping between TTF dimers.

close and conjugated paired arrangement benefits from the confinement effects of the MOF pore in 3. The TTF dimers are confined in a compressed channel, where the perchlorate ions and cocrystallized water molecules are also filled in (Figure 2d,e). Electrochemical and Photoelectrochemical Properties. Compounds 1−3 are redox-active CPs or MOF due to the functional TTF moieties. Their redox properties in the solid state were studied by cyclic voltammetry (CV) using surfacemodified electrodes, and the results are shown in Figure 6. Two

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CONCLUSIONS In summary, three coordination polymers with TTF-bicarboxylate and 1,3-bi(4-pyridyl)propane ligands are synthesized, and their structures are solved by single-crystal X-ray analysis. Compounds 1 and 2 are isostructural 2-D polymers, and 3 is an unusual 3-D MOF. The 3-D MOF is constructed from tetranuclear cluster nodes built through the μ2-O bridge of the TTF ligand that is first found for TTF coordination polymers. Although paired arrangements of TTF moieties are found in all the structures of 1−3, only the dimers in 3 are confined in the pores of 3-D MOF and show intradimer charge transfer or partial electron transfer. The charge-transfer compound 3 hence has relatively good photocurrent response properties in comparison with those of 2-D Cd compound 2. The confinement effect of MOF on the TTF dimer discussed here should enrich the chemistry and applications of MOFs.

Figure 6. Cyclic voltammograms of complexes 1−3 in the solid state (CH3CN, 0.1 mol·L−1 Bu4NClO4, 100 mV s−1).

sets of redox waves, corresponding to the TTF/TTF•+ and TTF•+/TTF2+ redox couples, were observed. Although metal ions and topologies of the compounds are different, their redox potentials E1/2(1) and E1/2(2) are roughly approximate at about 0.46 and 0.77 V with slight shifts of E1/2(1): 1(2-D-Mn) < 2(2D-Cd) ≈ 3(3-D-Cd). These electrochemical results confirm that these polymeric compounds are easy to be oxidized under specific conditions that is an essential feature for photoconductive materials. Thereby, photocurrent response properties of these TTFmetal coordination polymers were investigated, and the results are shown in Figure 7. The intensity of photocurrent densities of these polymers are below 1.0 μA, smaller than that of similar bipyridine polymers (MTTF-bpy),30 which is reasonable because bpp is not a conjugated ligand, and thereby, compounds 1−3 (MTTF-bpp) are not real donor−acceptor systems compared with the MTTF-bpy system. The comparison of photocurrent intensities of isostructural 1 and 2

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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.6b02062. PXRD of 1−3; Raman spectra for compounds 2 and 3; chain structure of 1; structure, tetranuclear cluster, and 3D MOF of 3; and crystal data and structural refinement parameters for compounds 1−3 (PDF) X-ray crystallographic data for complex 1 (CIF) X-ray crystallographic data for complex 2 (CIF) X-ray crystallographic data for complex 3 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Q.-Y.Z.). *E-mail: [email protected] (J.D.). ORCID

Qin-Yu Zhu: 0000-0003-1864-1175 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support by the NSF of China (21571136), the Priority Academic Program Development of Jiangsu Higher Education Institutions, and by the State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials.

Figure 7. Photocurrent responses of 1−3 in the presence of a 0.1 mol· L−1 Na2SO4 aqueous solution with 0.5 V bias. F

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