Ligand-to-Ligand Charge Transfer within Metal–Organic Frameworks

Article pubs.acs.org/IC

Ligand-to-Ligand Charge Transfer within Metal−Organic Frameworks Based on Manganese Coordination Polymers with Tetrathiafulvalene-Bicarboxylate and Bipyridine Ligands Peng Huo, Ting Chen, Jin-Le Hou, Lei Yu, 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: A systematic study on ligand-to-ligand charge-transfer (LLCT) properties of three closely related metal−organic frameworks (MOFs) is presented. These compounds are formulated as [MnL(4,4′bpy)(H2O)]n·nCH3CN (1), [MnL(bpe)0.5(DMF)]n·2nH2O (2), and [MnL(bpa)(H2O)]n·2nH2O (3) (L = dimethylthio-tetrathiafulvalenebicarboxylate, 4,4′-bpy = 4,4′-bipyridine, bpe = 1,2-bis(4-pyridyl)ethene, bpa = 1,2-bis(4-pyridyl)ethane). The X-ray single-crystal diffractions show that complexes 1−3 are all two-dimensional (2-D) coordination polymers with different frameworks in crystal lattices. Charge-transfer (CT) interactions within these MOFs are visually apparent in colors and vary according to the conjugated states of the bipyridine ligands (4,4′-bpy, bpe, and bpa). Theoretical calculations show that the charge transfer occurs from ligand L to bipyridine. The intensity of the LLCT is in the order of 2 > 1 > 3 investigated by theoretical calculations and ESR, which indicates that the intensity of CT is related to the bipyridyl conjugated state. Photocurrent responses of these compounds are consequently studied, and the results are in agreement with the intensity of charge transfer and linearly related to the LLCT energy.

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INTRODUCTION Metal−organic frameworks (MOFs) have been developing rapidly in the last two decades for their intriguing structures, properties, and potential applications as porous materials.1 Though it still remains a great challenge to rationally design and construct the prospective structures, very recently, MOFs with specific electron-transfer properties attract much attention for their applications in electrochromic materials, electrocatalysts, and electronic devices.2 These new applications require intrinsic charge transport within the MOFs,3 which have not been fully explored. The charge transfer (CT) involving partial electron transfer from electron donor (D) to electron acceptor (A) possibly generates charge separation that is directly related to the physical properties of electron/hole transports.4 Some examples concerning the CT interactions of MOFs are those named host frameworks which can act as an interaction site to provide selective accommodation for the guest through the combination of CT and van der Waals interactions.5 MOFs with intraframework CT have only been reported very recently using electroactive ligands, such as conductive MOFs with ligand-to-metal charge transfer (LMCT) or metal-to-ligand charge transfer (MLCT).6 One example of a metal-to-metal charge-transfer (MMCT) MOF is an Fe- and Co-included system with a 4,4′-bipyridine pillar and a cyanide bridge.7 In the development of functional CT MOF materials, research on ligand-to-ligand charge-transfer (LLCT) MOFs is still less advanced. This might be because several © XXXX American Chemical Society

factors must be controlled at the same time. It must be a mixedligand system in which the metal coordination with electron donor and electron acceptor should be prior to the D−A combination and the M−A or M−D separation.8 The target of this work is to obtain LLCT MOFs through carefully selecting the donor and acceptor linkers. As a wellknown electron-rich donor, tetrathiafulvalene (TTF) and its derivatives have been successfully used as building blocks for the formation of charge-transfer compounds.9 Liu has recently reviewed the evolution of TTF fused D−A systems and their potential applications in areas such as solar cells, OFETs, molecular wires, and optoelectronics.10 Although metal coordination compounds and polymers based on TTF ligands such as TTF-bicarboxylate11,12 have been intensively studied for some years, the study of LLCT within MOFs is a new perspective for TTF-metal coordination polymers. Therefore, redox-active TTF-bicarboxylate is chosen as an electron-donor linker. Different conjugated bipyridine ligands, 4,4′-bipyridine (4,4′-bpy), 1,2-bis(4-pyridyl)ethene (bpe), and 1,2-bis(4pyridyl)ethane (bpa), are selected as the second linkers, mainly considering that, since 4,4′-bpy, bpe, or bpa can act as an electron acceptor through alkylation,9h their electron-accepting property could be also achieved by metal coordination (see Chart 1). Herein, three manganese complex frameworks with Received: March 6, 2016

A

DOI: 10.1021/acs.inorgchem.6b00571 Inorg. Chem. XXXX, XXX, XXX−XXX

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the compounds was carried out on a D/MAX-3C X-ray diffraction meter with CuKα (λ = 1.5406 Å) radiation. Room-temperature optical diffuse reflectance spectra of the microcrystal samples were recorded on a Shimadzu UV-3150 spectrometer. The absorption (α/S) data were calculated from the reflectance using the Kubelka−Munk function, α/S = (1 − R)2/2R, where R is the reflectance at a given energy, α is the absorption, and S is the scattering coefficient.14 ESR spectra were carried out at 110 K on a Bruker ER-420 spectrometer with a 100 kHz magnetic field in X band. The magnetic susceptibility of polycrystalline samples was measured from 2 to 300 K by a Quantum Design SQUID magnetometer on the MPMS-7 system. Preparation of Compounds. [MnL(4,4′-bpy)(H2O)]n·nCH3CN (1). Orange-red crystals of 1 were obtained using a method reported previously.11e [MnL(bpe)0.5(DMF)]n·2nH2O (2). To 1 mL aqueous solution of Na2L (2.2 mg, 0.005 mmol) was added 1 mL of an aqueous solution of MnCl2·4H2O (2.0 mg, 0.01 mmol). The mixture was stirred for 0.5 h at room temperature. bpe (1.8 mg, 0.01 mmol) in 2 mL of DMF was dropwise added to the solution. The final mixture was stirred for 0.5 h and filtered; black block single crystals of 2 were obtained by controlled evaporation of the solvent for 7 days (yield: 0.9 mg, 28.2% based on Na2L). Anal. Calcd for C19H22MnN2O7S6 (MW 637.68): C, 35.78; H, 3.48; N, 4.39%. Found: C, 35.93; H, 3.29; N, 4.47%. IR data (cm−1): 1633(s), 1604(vs), 1563(s), 1509(m), 1421(m), 1351(s), 1322(vs), 1084(m), 1012(m), 959(m), 831(m), 765(m), 551(m). [MnL(bpa)(H2O)]n·2nH2O (3). To a 2 mL methanol solution of Na2L (4.3 mg, 0.01 mmol) was added a 2 mL aqueous solution of

Chart 1. Structures of Dimethylthio-tetrathiafulvalenebicarboxylate (L), 4,4′-bpy (4,4′-Bipyridine), bpe (1,2-Bis(4pyridyl)ethene), and bpa (1,2-Bis(4-pyridyl)ethane)

TTF-bicarboxylate and bipyridine ligands, [MnL(4,4′-bpy)(H2O)]n·nCH3CN (1), [MnL(bpe)0.5(DMF)]n·2nH2O (2), and [MnL(bpa)(H2O)]n·2nH2O (3), are reported and their structures and LLCT properties are studied.

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EXPERIMENTAL SECTION

General Remarks. The sodium salt of dimethylthio-tetrathiafulvalene-bicarboxylate (Na2L) was prepared using a previously reported method.13 All analytically pure starting materials were purchased and used without additional purification. FT-IR spectra were recorded with a Nicolet Magna-IR 550 spectrometer in dry KBr pellets. Elemental analysis was carried out on an EA 1110 elemental analyzer. PXRD of

Figure 1. (a) An infinite −Mn−(O−C−O)2−Mn− 1-D chain along the b axis constructed by carboxylate bridges of 2. bpe and DMF except the coordinated atoms and all hydrogen atoms are omitted for clarity. (b) 2-D network structure of 2. DMF except the coordinated oxygen atom and all hydrogen atoms are omitted for clarity. B

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Figure 2. (a) The orientations of bpe and L, DMF and all hydrogen atoms are omitted for clarity. (b) Short S···S and C···C interactions within the TTF couple in 2. All atoms except L and coordinated atoms are omitted for clarity. MnCl2·4H2O (4.0 mg, 0.02 mmol). The mixture was stirred for 0.5 h at room temperature. bpa (2.0 mg, 0.01 mmol) in 2 mL of DMF was dropwise added to the solution. The final mixture was stirred for 0.5 h and filtered; orange-yellow color block single crystals of 3 were obtained by controlled evaporation of the solvent for 7 days (yield: 2.4 mg, 35.6% based on Na2L). Anal. Calcd for C22H24MnN2O7S6 (MW 675.76): C, 39.10; H, 3.58; N, 4.15%. Found: C, 39.49; H, 3.39; N, 4.22%. IR data (cm−1): 1610(s), 1567(s), 1426(m), 1377(s), 1226(w), 1014(w), 817(w), 772(m), 539(w). X-ray Crystallographic Study. The X-ray single-crystal analysis and the crystallographic data collection and processing are the same as those reported previously.11g,15 SHELXS-14 is used to solve the structure by direct methods,16 and SHELXL-14 is used to perform the refinement against all reflections of the compound.17 All of the nonhydrogen atoms were refined anisotropically, and hydrogen atoms were added theoretically. The crystal data and structural refinement parameters of 2 and 3 are listed in Table S1. Crystallographic data CCDC 1451766 and 1451765 contain the supplementary crystallographic data for 2 and 3. The data can be obtained from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. Electrode Preparation and Photocurrent Measurement. The methods of electrode preparation and photocurrent measurement have been reported previously.11g In short, a powder coating method is used to prepare the photoelectrodes of the compounds. The photocurrent responses were measured on a CHI650E electrochemistry workstation using a 150 W high-pressure xenon lamp as a full-wavelength light source. Theoretical Calculations. Density functional theory (DFT) calculations were performed using the Gaussian 09 program package at the B3LYP level.18a The basis set used for C, N, O, S, and H atoms was 6-31G while effective core potentials with a LanL2DZ basis set was employed for the Mn atom.18b,c The crystal structures of 1−3 were used as the initial structures, edited into a minor similar model

using the HyperChem program in order to cut the computational cost, and further optimized to the minimum energy configurations (Figure S1).

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RESULTS AND DISCUSSION Synthesis and Characterization. The reaction of MnCl2· 4H2O with ligand L in the presence of a bipyridyl coligand (4,4′-bpy, bpe, and bpa) generated three TTF manganese complexes. These complexes are stable in ambient condition and insoluble in common polar or nonpolar solvents such as acetone, chloroform, benzene, H2O, methanol, CH3CN, etc. The elemental analyses indicate that the components were in good accordance with the results of the single-crystal X-ray diffraction analysis. The XRD patterns of the microcrystal samples are in agreement with the patterns simulated from the crystal data of the compounds (Figure S2) to confirm the identity and purity of all the samples. Structures of Compounds 2 and 3. [MnL(bpe)0.5(DMF)]n·2nH2O (2). Single-crystal X-ray structural analysis shows that complex 2 is a 2-D coordination polymer crystallized in the monoclinic C2/m space group. The asymmetric unit of 2 includes one manganese atom, one ligand L, a half bpe, one coordinated solvent molecule DMF, and two co-crystallized water molecules. The manganese(II) atom is located in a somewhat distorted octahedral environment constituted by four oxygen atoms from three different L ligands forming the equatorial plane, one nitrogen atom from bpe, and one oxygen atom from DMF occupying the axial positions (Figure S3). The Mn−O bond lengths are in the range of 2.006(18)−2.175(5) Å and the Mn−N bond length is 2.280(8) Å. C

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Figure 3. (a) An infinite −Mn−O−C−O−Mn− 1-D chain along the b axis constructed by carboxylate bridges of 3. bpa except the coordinated nitrogen atoms and all hydrogen atoms are omitted for clarity. (b) 2-D network structure of 3. L except the coordinated carboxylate groups and all hydrogen atoms are omitted for clarity.

with a Mn−Mn distance of 5.19 Å, longer than that in 2 (Figure 3a). One of the two carboxylate groups takes the μ2-η1:η1 bidentate coordination mode and the other takes a monodentate coordination mode. The adjacent 1-D chains of 3 are further connected by bpa molecules, forming a coordinated 2-D network structure (Figure 3b and Figure S4b). There are weak C11···S1 (3.432 Å) contacts between the TTF moiety and the pyridine ring of bpa in 3. Crystal Packing of 1−3. The structure of 1 has been reported previously.11e The 2-D MOF structures of 1−3 can be viewed in Figure 4a−c, if the TTF moiety is ignored. The Mn(II) ions are bridged by double carboxylate bridges in 2 and a single carboxylate bridge in 3 to form a 1-D string, whereas, in 1, the Mn(II) ions are paired in one direction by a double bridged carboxylate group. These Mn(II) centers are further linked in another direction by bipyridine bridges to form 2-D MOF. The 4,4′-bpy in 1 and bpe in 2 are completely coplanar, but the two pyridine planes of bpa in 3 are not conjugated (dihedral angle 79.94°). The bpe molecules in 2 are all parallelly arranged, while the 4,4′-bpy molecules in 1 are arranged alternately in two directions. The topologies can be described as (6, 3) net for 1 and 2, and (4, 4) net for 3, respectively. Charge-Transfer Properties. The colors of these crystals varied from orange-yellow (3) to orange-red (1) to black (2) as the bis(pyridine) component changed from bpa to 4,4′-bpy to bpe (Figure S5). Coloration may indicate varying levels of charge-transfer interaction in the solid state between the

The Mn(II) atoms in 2 are bridged together by carboxylate groups to form an infinite −Mn−(O−C−O)2−Mn− onedimensional (1-D) chain along the b axis with a Mn−Mn(i) (i = 0.5 + x, 0.5 + y, 1 + z) distance of 4.91 Å (Figure 1a). Each of the two carboxylate groups takes the μ2-η1:η1 bidentate coordination mode. The 1-D chains are linked to each other by bpe bridges to form a 2-D network structure (Figure 1b). All the bpe ligands are coordinated parallelly within the ac plane in the same mode, and the TTF ligands are situated up and down the plane (Figure 2a). There are short interactions within the TTF couple, S1···S1 (3.542 Å) and C2···C2 (3.354 Å) (Figure 2b), but there is not any interaction between the TTF moiety and bpe in 2. [MnL(bpa)(H2O)]n·2nH2O (3). Using bpa as a replacement of bpe also forms a 2-D coordination polymer 3. Compound 3 is crystallized in the monoclinic P21/n space group, and the asymmetric unit consists of one manganese atom, one ligand L, one bpa, one coordinated water molecule, and two cocrystallized water molecules. Each Mn(II) ion is surrounded by three oxygen atoms from two L ligands and one oxygen atom from the water molecule forming the equatorial plane, and two nitrogen atoms from two bridging bpa ligands occupying the axial positions, giving a slightly distorted octahedral geometry (Figure S4a). The Mn−O and Mn−N bonds are in the ranges of 2.131(5)−2.155(5) and 2.288(5)− 2.291(5) Å, respectively. The Mn(II) atoms in 3 are bridged together by carboxylate groups to form an infinite −Mn−O−C−O−Mn− 1-D chain D

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the molecular orbitals involved in the third excited state for 1− 3. It can be found that the low energy molecular orbitals (162A for 1, 169A for 2, and 171A for 3) are mainly localized on the TTF moiety, whereas the high energy molecular orbitals (164A for 1, 171A for 2, and 172A for 3) are totally localized on the bipyridine ligand with almost no distribution on the central metal ion. The complete spatial separation of these frontier orbitals means that the electronic transition possesses significant ligand-to-ligand charge-transfer character.19 However, considering the small value of the oscillator strength of 3, the LLCT of 3 should be very weak. Notably, the high energy orbital populations of 1 and 2 are evenly located on the whole conjugated bipyridine molecules, whereas most of 3 is only located on the pyridine moiety which is directly coordinated to metal ion and a small part is located on the other pyridine moiety (unconjugated). The theoretical results strongly support the understanding that LLCT occurs in the MOFs of 1−3, but is interrupted in 3 by the unconjugated −CH2CH2−. Optical diffuse-reflection spectra of 1−3 and the starting materials Na2L were measured at room temperature using BaSO4 as a standard reference (Figure 6). As shown in Figure

Figure 4. View of the two-dimensional MOFs of 1 (a), 2 (b), and 3 (c). L except the coordinated carboxylate groups and hydrogen atoms were omitted for clarity.

electron-donator L and the electron acceptor of metal coordinated bipyridyl species with different conjugated structures and hence different electron-accepting ability. In order to investigate the possibility of CT, the density functional theory (DFT) and TDDFT calculations were performed on the simplified models (Figure S1). The results show that the oscillator strengths of the third excited state of all the three models, 162A→164A ( f = 0.0013) for 1, 169A→ 171A (f = 0.0014) for 2, and 171A→172A (f = 0.0002) for 3, are high in comparison with those of the first and the second excited states (Table S2). Therefore, this electronic transition should be the main contributor to the lowest energy intense experimental band. Figure 5 shows the distribution patterns of

Figure 6. Optical diffuse-reflection spectra of 1−3 and the starting compound Na2L for comparison in the solid state. Inset: Optical diffuse-reflection spectra of 2 with peak separation.

S6, the lowest broad absorption can be peak-separated to two peaks for 1 and 3 and three peaks for 2. The peaks (blue curves in Figure S6) of 456 nm for 1, 494 nm for 2, and 444 nm for 3 should originate from the shift of the 402 nm band of the TTF ligand, which is affected by the coordination with the Mnbipyridine moiety. In comparison with the spectra of the ligand and based on the calculated results mentioned above, the lowest energy bands (purple curves in Figure S6), 532 nm for 1, 582 nm for 2, and 498 nm for 3, should be ascribed to the ligand (TTF)-to-ligand (bipyridine) charge transfer (LLCT). The calculated energy gap of the third excited state (3 > 1 > 2, Table S2) reasonably agrees with the experimental chargetransfer absorption band. The order of the LLCT band is in accordance with the bipyridyl conjugated state. The additional peak at 663 nm of 2 can be assigned to the interaction of the TTF dimer,20 because the dimer is only found in compound 2, which forms an electron state with low energy gap. Electron spin resonance (ESR) studies of compounds 1−3 were carried out at 110 K (Figure 7), and the results are interesting. The solid-state ESR spectra of all the three compounds 1−3 show a broadened signal with a center point

Figure 5. Frontier molecular orbitals involved in the third excited state of the calculation models of 1−3. Manganese atom is purple; nitrogen, blue; oxygen, red; carbon, pale-gray; sulfur, yellow; and hydrogen, white. E

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ences of χm and χmT of 2 are given in Figure 8. The value of χmT at room temperature is 4.14 cm3·K·mol−1, which is close to the expected value for an isolated Mn(II) ion S = 5/2 assuming g = 2.0 (4.37 cm3·K·mol−1). The χmT value drastically drops below 100 K and decreases to 0.322 cm3·K·mol−1 at 2.0 K. The susceptibility (χm) increases with the temperature cooling, reaching the value of 0.181 cm3·mol−1 at 6.45 K, and quickly decreases until 2.0 K. Compound 2 is a carboxylate bridged 1-D structure if the long-range coupling along the bpe bridge is ignored and TTF radical concentration cannot be detected; therefore, Fisher’s uniform 1-D model was used to fit the susceptibility data.23 The best fit for the experiment data of 2 is given by the following parameters: J = −1.14 cm−1 and g = 1.98A. Similarly, compound 3 is also a carboxylate bridged 1-D structure. The temperature dependences of χm and χmT of 3 (Figure 8) are similar to those of 2. The best fit for the experiment data of 3 is given by the following parameters: J = −0.42 cm−1 and g = 2.00. All of those results indicate that only weak antiferromagnetic coupling between the Mn(II) ions and the data are in agreement with the magnetic property of a 1-D chloride bridged TTF complex reported previously.24 Photoelectric Response Properties. TTF derivatives, especially their charge-transfer compounds, usually possess electroactivity and photoelectroactivity.25 However, the influences of LLCT within the TTF-metal coordination MOFs on the photocurrent response properties have never been studied. Therefore, a three-electrode photoelectrochemical cell consisting of a microcrystal sample pressed ITO electrode was constructed to study the photocurrent response properties of 1−3. As these compounds are the products of the coassembly of the electron donor and the electron acceptor, hence only a sodium sulfate solution was used as the supporting electrolyte. Photocurrent responses were observed for all of these three compounds based on the results shown in Figure 9a. Upon irradiation with xenon light, a current generated in the μA scale. The current intensity is comparable with those of chargetransfer systems of MnTTF-MV compounds.11g The photocurrent was stable without a decrease in intensity, while the response kinetics is not so good (not a square wave). The current density is in the order of 2 > 1 > 3, in agreement with the order of LLCT property. The correlogram of current density and charge-transfer peak demonstrated in Figure 9b exhibits a quasi-linear relationship. The possible electrontransfer mechanism in the MOF with LLCT interaction is

Figure 7. ESR spectra of 1−3 recorded at 110 K.

located at g = 2.0006, similar to those of charge-transfer systems of MnTTF-MV compounds.11g The broad signal consists of the coupling of two resonance signals, charge-transfer induced radical resonance of the TTF•+ and the unpaired electrons of the paramagnetic center of the Mn(II) atom.21 The peak intensities are in the order of 2 > 1 > 3 (1.000:0.939:0.567), which is consistent with the energy of the LLCT peak in spectra. Although the structures of these three compounds are different, their basic coordination spheres are similar to each other. Moreover, magnetic studies on all of the three complexes showed that their magnetic coupling parameters are very weak (discussed below). For these reasons, it is reasonable to consider that the intensity of ESR peaks is related to the partial electron transfer.21 The narrower the charge-transfer gap is, the easier the electron transfer should be. Magnetic Properties of Compounds 1−3. The correlation of the oxidation states of TTF derivatives with the bond lengths of central CC bonds has been well characterized.22 The TTF ligand in these compounds is not oxidized according to the correlation. The result shows that, although charge transfer or partial electron transfer occurs in the compounds, the radical concentration is very low and can only be detected by high sensitive ESR spectra. Measurement of the magnetic susceptibility is also a useful technique to evaluate the electronic state. The effective magnetic moment was performed in the temperature range of 2−300 K under the field of 1 kOe. Magnetic properties for 2 and 3 are similar to those of 1 reported previously.11e The temperature depend-

Figure 8. Experimental χm versus T and χmT versus T curves of complexes 2 and 3. The black curves were plotted based on the best fitting parameters illustrated in the text. F

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lations show that complete spatial separation of the frontier orbitals exists, which means that ligand-to-ligand (TTF-tobipyridine) charge transfer occurs. Although 1−3 are similar 2D coordination polymers, the LLCT peak energies evaluated by electronic spectra quantitatively reflect that the order of the intensity of charge transfer in the MOFs is 2 > 1 > 3, which is in agreement with their conjugated states of the bipyridines. Complexes 1−3 are inherent LLCT MOFs and hence could be viable materials for photoelectronics. Photocurrent responses of these compounds are in accordance with the intensity of charge transfer and linearly related to the LLCT energy.

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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.6b00571. Optimized structures of the calculation models of 1−3; PXRD of 2 and 3; coordination environment of 2 and 3; UV−vis absorption spectra of 1 and 3 with peak separation; photos of the colors of three crystals 1−3; crystal data and structural refinement parameters for compounds 2 and 3; and the results of TDDFT theoretical calculations for simplified models of 1−3 (PDF) X-ray crystallographic data for complexes 2 and 3 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Q.-Y.Z.). *E-mail: [email protected] (J.D.). Figure 9. (a) Photocurrent responses of 1−3 in the presence of a 0.1 mol·L−1 Na2SO4 aqueous solution. (b) Relationship of photocurrent density and LLCT peak energy.

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.

shown in Figure 10. At first, upon irradiation, the photoinduced LLCT happened, which generates charge separation to form

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REFERENCES

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Figure 10. Sketch map of the possible electron-transfer mechanism in the MOF with LLCT interaction.

electron/hole excitons. Under the applied potential, the electron and hole move oppositely to generate current.

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CONCLUSIONS In summary, three manganese coordination 2-D MOFs (1−3) are synthesized to study the LLCT within the MOFs in which TTF-bicarboxylate is an electron-donor linker and different conjugated bipyridines (bpy, bpe, and bpa) act as an electron acceptor achieved by metal coordination. Theoretical calcuG

DOI: 10.1021/acs.inorgchem.6b00571 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.6b00571 Inorg. Chem. XXXX, XXX, XXX−XXX