3D Copper Tetrathiafulvalene Redox-Active Network with 8-Fold

Communication pubs.acs.org/IC

3D Copper Tetrathiafulvalene Redox-Active Network with 8‑Fold Interpenetrating Diamond-like Topology Zhong-Nan Yin, Yan-Hong Li, Yong-Gang Sun, Ting Chen, 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 *

whole conjugated py-TTF-py ligand is double (14.74 Å) that of 4,4′-bpy (7.13 Å). The sulfur-rich tetrathiafulvalene (TTF) moiety, the spacer between the pyridyl moieties, can be easily and reversibly oxidized into its corresponding cationic radical (TTF•+) and has been studied extensively as a component of electronic and optoelectronic materials.9 Although metal coordination compounds and polymers with various topologies based on TTF ligands have been intensively studied for some years, no TTF CP with an interpenetration dia-like structure was reported. To the best of our knowledge, compound {[Cu(pyTTF-py)2]ClO4·H2O}n (1) reported here is the first example not only with highly 8-fold interpenetration dia-like structure but also with redox-active properties. A photoelectric active electrode of 1 was prepared, and its redox-state-related photocurrent properties were studied. Complex 1 was found to be air-stable, dark-red crystals. The structure was characterized by single-crystal X-ray diffraction studies and further corroborated by elemental analysis. The phase purity of a polycrystalline sample was confirmed by powder X-ray diffraction (PXRD; Figure S1). The IR spectrum (Figure S2) shows a strong absorption band at 1081 cm−1 attributable to uncoordinated perchlorate groups. Single-crystal X-ray diffraction reveals that 1 is a 3D 8-fold interpenetrating diamond-like network and crystallizes in tetragonal space group P4/n. The asymmetric unit is composed of one-quarter CuI atom, half-ligand py-TTF-py, one-quarter perchlorate anion for charge balance, and one-quarter cocrystallized water molecule. As depicted in Figure S3, the Cu1 ion exhibits an almost regular tetrahedral coordination geometry completed by four N atoms from four different py-TTF-py ligands with the same Cu−N distances of 2.061(4) Å and bond angles of 108.5(2) and 109.9(1)°. One py-TTF-py bridges two CuI atoms, and the four-connected CuI nodes are connected by four ligands extending into a 3D network (Figure 1, left). The adjacent CuI nodes are separated by a distance of 18.58 Å. On the basis of the concept of topology, the whole structure can be simplified into a diamond-like network containing large adamantanoid cages (Figure S4). The Cu···Cu···Cu angles are 71.88 and 130.96°, which represent significant distortions from the ideal tetrahedral angle of 109.5° found in a diamond. The TTF moiety is almost planar with a mean deviation of 0.454(4) Å. The TTF ligands are stacked to form columns with face-toface interaction (3.33 Å) between TTF moieties (Figure S5). The shortest S···S distance is 4.87 Å in the TTF pair. Because of the spacious nature of the single network, the potential voids are

ABSTRACT: A tetrathiafulvalene derivative has been incorporated into a diamond-like structure for the first time. The coordination network shows highly unusual 8fold interpenetration with redox-active and photoelectric properties.

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uring the last 2 decades, coordination polymers (CPs) have been extensively studied because of their intriguing structures and properties.1,2 The architectures of CPs attracted overwhelming interest of multidisciplinary researchers because they play important roles in properties such as catalysis,2a adsorption,2b,c separation,2d biomedicine2e and so on. In early works of CPs, 4,4′-bipyridine (4,4′-bpy) was used in coordination with transition-metal ions to create a series of networks with various topologies, which played a significant role in establishing the foundation for the assortment of coordination networks.3,4 In 1995, a CuI-4,4-bpy network was first used in the description of CPs as metal−organic frameworks (MOFs) in Yaghi’s paper.3a Among the various architectures, one of the important types is an aesthetically pleasing diamond-like (dia) network, which is, in general, formed by propagating a tetrahedral node in four directions by coordination with topological linear bidentate ligands.5,6 The CuI-4,4-bpy network was one of the early important examples with dia networks in the realm of coordination chemistry.3b Changing the spacers between the pyridyl moieties can modulate the size and shape of the pores as well as the properties of the final materials.7 Recently, one of the exciting developments for CP or MOF materials is using functional molecules as linker ligands to construct materials with redox-active properties.8 However, in comparison with the adsorption properties, the redox properties of dia networks have been relatively less studied and the photoelectric response properties have not been reported. The incorporation of redox-active ligands into coordination frameworks offers an effective strategy for the synthesis of redoxcontrollable CP networks. In this work, 2,6-bis(4′-pyridyl)tetrathiafulvalene (py-TTF-py; Chart 1), an extended conjugated analogue of 4,4′-bpy, is selected to synthesize the CuI-4,4′-bpylike dia network with redox-active properties. The length of the Chart 1. Structure of py-TTF-py

Received: July 8, 2016

© XXXX American Chemical Society

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DOI: 10.1021/acs.inorgchem.6b01632 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

states, both of which include the transitions of HOMO−LUMO, HOMO−LUMO+1, HOMO−1−LUMO, and HOMO−1− LUMO+1 at 661 nm (1.876 eV) with an oscillator strength of 0.1602 and at 654 nm (1.897 eV) with an oscillator strength of 0.0273, respectively. Table S2 lists the energies of the frontier orbitals, which indicate that the highest occupied orbitals of HOMO and HOMO−1 are degenerate and so are the lowest unoccupied orbitals of LUMO and LUMO+1. Figure 3 shows Figure 1. (left) 3D coordination polymeric structure viewed along the a axis. (right) Schematic view of the 8-fold interpenetration dia-like structure viewed along the c axis of 1. Each color represents a discrete 3D dia network.

filled via mutual interpenetration of the other seven independent equivalent networks, generating an 8-fold interpenetrating architecture (Figure 1, right). In view of the neutral ligand, the network is cationic, and thus the ClO4− anions are dispersed in the interpenetrating network to balance the positive charge. It is the first example of a TTF CP with a highly unusual 8-fold interpenetration dia-like structure. Optical diffuse-reflectance spectra of crystals of 1, together with the ligand, were measured, and the absorption data in the visible−near-IR range were calculated from the reflectance.10 As shown in Figure 2a, an intense absorption band of 1 appears at

Figure 3. Distribution patterns of the molecular orbitals involved in the third and fourth excited states for 1.

the distribution patterns of the molecular orbitals involved in the third and fourth excited states for 1. It can be found that the largest coefficients in the HOMO and HOMO−1 orbitals are mainly located on the TTF moiety (the S2CCS2 fragment) with small contributions of the pyridyl group, while LUMO and LUMO+1 shift to the pyridyl group with small contributions of the TTF moiety. Therefore, the intense lowest-energy transition is also attributed to intraligand charge transfer. According to a comparison of the oscillator strengths, charge transfer is strengthened through metal coordination in comparison with that of the ligand. Both the absorption spectra and theoretical calculation results show that in the dia network charge transfer is intensified in comparison with that of the py-TTF-py ligand. Charge-transfer compounds of TTF derivatives usually exhibit photoelectroactivity.9 Therefore, the photocurrent response properties of crystals of 1 and the py-TTF-py ligand for comparison were measured using a three-electrode photoelectrochemical cell consisting of a microcrystal-sample-modified indium−tin oxide (ITO) electrode (see the experimental section in the Supporting Information). Upon irradiation with xenon light, a clear photocurrent response for 1 or py-TTF-py was observed with an applied potential of 0 V (Figure 4), and the photocurrent was stable without a decrease in the intensity. The current intensity of 1 (5 μA cm−2) is 2.5 times higher than that of py-TTF-py (2 μA cm−2), which indicates that coordination of the CuI ion in the dia network enhances the photocurrent response.

Figure 2. (a) Absorption spectra of 1 and py-TTF-py in the solid state. (b) Cyclic voltammogram of 1 using the microcrystal-modified electrode.

582 nm, red-shifted 45 nm in comparison with that of the ligand. The energy band gap of 1 was estimated to be 1.87 eV from the band edge (663 nm). The weak shoulder absorption band in the range of 670−845 nm for 1 and py-TTF-py is due to the face-toface interaction between TTF moieties.11 As is usually observed in TTF derivatives (TTFs), two pairs of one-electron redox waves of 1, E11/2 = 0.503 V (TTF•+/TTF) and E21/2 = 0.818 V (TTF2+/ TTF•+), are observed by cyclic voltammetry (CV) in the solid state (Figure 2b). All of the results indicate that the dia network of 1 is redox-active and electron transfer can occur in the network. To investigate the possibility of electron transitions, timedependent density functional theory (TDDFT) calculations were performed on the simplified model of the dia network of compound 1 and the py-TTF-py ligand. The theoretical calculations of the excitation bands are in agreement with the experimental results of the spectra and indicate that the energy gap of 1 is smaller than that of the ligand. For py-TTF-py, the lowest-energy transition is involved with the first excited state, including the transitions of highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) at 506 nm (2.448 eV) with an oscillator strength of 0.1265. The transition is ascribed to intramolecular charge transfer from the TTF moiety to the pyridyl group based on the distribution patterns of HOMO and LUMO (Figure S6). For 1, the lowestenergy transition is involved with the third and fourth excited

Figure 4. Photocurrent responses of py-TTF-py, 1, and oxidized 1. B

DOI: 10.1021/acs.inorgchem.6b01632 Inorg. Chem. XXXX, XXX, XXX−XXX

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Because compound 1 features a redox-active network, electrochemical oxidation of a 1-modified ITO electrode is carried out by the application of an oxidation potential at 0.6 V for 10.0 min using the above-mentioned electrochemical cell. After electrochemical oxidation, the color of the electrode changes from orange to black and the absorption spectrum shows that the low-energy absorption band of 1 was broadened and redshifted about 30 nm (Figure 2). Then the photocurrent response of the oxidized electrode was measured. In comparison with the unoxidized one, the photocurrent density was increased from 5 to 9 μA cm−2 (Figure 4). The band gaps estimated from the electronic spectra are 2.1 eV for py-TTF-py, 1.9 eV for 1, and 1.7 eV for oxidized 1, respectively. The HOMO energy can be estimated from the CV data, which show that the oxidation edges of py-TTF-py and 1 are comparable (0.40 V), while the oxidized 1 is 0.58 V. Figure S7 shows a schematic drawing of the band gaps of py-TTF-py, 1, and oxidized 1 along with the conductive level of the ITO electrode. The excellent photocurrent properties of oxidized 1 should be attributed to its narrow band gap and energy matching with the ITO electrode. The high photocurrent density of 1 compared with that of py-TTF-py should be attributed to the column arrangement of the TTF moieties in the dia and their short face-to-face stacking (3.33 Å; Figure S5). No such short interaction was found in a crystal of py-TTF-py.12 In summary, an extended analogue of 4,4′-bpy with a redoxactive TTF moiety is selected to assemble CPs. A unique 3D CuI4,4′-bpy-like dia network 1 with an 8-fold interpenetration structure was obtained. To the best of our knowledge, 1 is the first TTF coordination compound not only with a highinterpenetration dia-like structure but also with redox-active properties. Charge transfer is intensified in 1 in comparison with that of the ligand. A photoelectric active electrode of 1 was prepared, and the results suggest that the photocurrent intensity is related to the redox state of the network.

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REFERENCES

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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.6b01632. Experimental, PXRD, IR, coordination environment of CuI, a schematic view of the diamondoid network, short contacts in 1, distribution patterns of the frontier molecular orbitals for py-TTF-py, a schematic drawing of the band gaps, crystal data and structural parameters, and energies of the molecular orbitals (PDF) CIF file for 1 (CCDC 1486132) (CIF)

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Communication

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support by the NSF of China (Grant 21571136), the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Program of Innovative Research Team of Soochow University. C

DOI: 10.1021/acs.inorgchem.6b01632 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b01632 Inorg. Chem. XXXX, XXX, XXX−XXX