Porous Molecular Conductor: Electrochemical Fabrication of Through

Apr 17, 2019 - The NDI cores were partially reduced into radical anions to form conductive π-stacked columns, yielding (1.0–3.3) × 10–3S cm–1 ...
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Porous Molecular Conductor: Electrochemical Fabrication of ThroughSpace Conduction Pathways among Linear Coordination Polymers Liyuan Qu, Hiroaki Iguchi, Shinya Takaishi, Faiza Habib, Chanel F. Leong, Deanna M. D'Alessandro, Takefumi Yoshida, Hitoshi Abe, Eiji Nishibori, and Masahiro Yamashita J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b01717 • Publication Date (Web): 17 Apr 2019 Downloaded from http://pubs.acs.org on April 17, 2019

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Porous Molecular Conductor: Electrochemical Fabrication of ThroughSpace Conduction Pathways among Linear Coordination Polymers Liyuan Qu,† Hiroaki Iguchi,*,† Shinya Takaishi,† Faiza Habib,† Chanel F. Leong,‡ Deanna M. D’Alessandro,‡ Takefumi Yoshida,§ Hitoshi Abe,‖,┴ Eiji Nishibori,# Masahiro Yamashita*,†,¶,⊗ †Department

of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aza-Aoba, Aramaki, Sendai 980-8578, Japan ‡School of Chemistry, The University of Sydney, New South Wales 2006, Australia §Electronic Functional Macromolecules Group, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‖Institute of Materials Structure Science High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan ┴Department of Materials Structure Science, School of High Energy Accelerator Science, SOKENDAI (the Graduate University for Advanced Studies), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan #Division of Physics, Faculty of Pure and Applied Sciences & Tsukuba Research Center for Energy Materials Science (TREMS), University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan ¶Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ⊗School of Materials Science and Engineering Nankai University, Tianjin 300350, China

Supporting Information Placeholder ABSTRACT: The first porous molecular conductor (PMC) which exhibits porosity, a through-space conduction pathway and rich charge carriers (electrons)was prepared through electrocrystallization from Cd2+ and N,N’-di(4-pyridyl)-1,4,5,8naphthalenetetracarboxdiimide (NDI-py). [Cd(NDIpy)(OH2)4](NO3)1.3±0.1∙nDMA (PMC-1) was assembled by - stacking among one-dimensional (1D) linear coordination polymers. The NDI cores were partially reduced into radical anions to form conductive -stacked columns, yielding (1.0–3.3) × 10–3 S cm–1 at room temperature. Moreover, the electrical conductivity was significantly enhanced by removing the solvent molecules from PMC-1, indicating that PMCs are promising as molecule-responsive conductive materials.

Metal-organic frameworks (MOFs)1 or porous coordination polymers (PCPs)2 have recently been anticipated as a new group of electron-conductive porous materials3 that are promising materials as the basis for batteries4,5 and supercapacitors5,6 amongst other energy storage and conversion technologies.7 Essential to engendering electrical conductivity () in MOFs is to build charge transport pathways. To date, the synthetic strategies used have been categorized as either “through-bond” or “throughspace”. The former approach is common among charge transport in conductive coordination polymers (CPs).8,9 Some of them exhibited a change of  upon desolvation.10,11 The field of electron-conductive MOFs has more recently developed through the use of organic linkers. In the through-bond approach, mixed valency in metal ions and/or ligands has been key to obtaining high ,12–17 and particularly two-dimensional (2D) -conjugated

frameworks6,18–20 show relatively high  (> 10 S cm–1). However, the high covalent bonding character in the through-bond approach makes it difficult to study their structures and electronic states by single-crystal X-ray diffraction (SXRD) analyses, especially in 2D -conjugated frameworks. Moreover, the design of the linkers providing the electron delocalization has been confined to only a few systems such as dithiolate,12,18 semiquinonate,14 iminobenzene6,19,20 and azolate.16,17 Meanwhile, the through-space approach requires a relatively rare -stacked columnar structure of organic linkers suitable for electron conduction.21,22 However, the  of these MOFs is generally moderate (up to the order of 10−3 S cm–1) despite their high charge mobility.21a This indicates that developing methodologies for increasing the carrier density is important in order to increase . Recently, Kawano et al. reported the formation of a conductive MOF with a -stacked columnar structure induced by a trace amount of radical species.23 The  was gradually enhanced from 10−6 to 10−1 S cm–1 over several months by further oxidation in air. This result suggests that the introduction of radical species can relatively quickly lead to stacked columns with high carrier densities. Such rapid radical generation and the formation of -stacked conducting columns is common in electrocrystallization of molecular conductors.24 Therefore, we propose a new electron-conductive porous materials, named porous molecular conductors (PMCs), created by the fusion of MOFs and molecular conductors. The application of electrochemical reactions to the syntheses of MOFs can provide not only high conductivity but also various physical properties derived from molecular conductors. Considering the opposite perspective, porosity can provide an interface to control the electronic states and physical properties of molecular

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conductors by chemical stimuli. These concepts of PMCs open a new avenue for research in both electron-conducive MOFs and molecular conductors. Herein, we report the first PMC synthesized by the in situ electrochemical reaction of an organic linker. We focused on N,N’-di(4-pyridyl)-1,4,5,8naphthalenetetracarboxdiimide (NDI-py) as an organic linker which is capable of forming infinite -stacked columnar architectures. The NDI core is a well-known electron acceptor used in various fields in chemistry.25 Its large -conjugated plane and the ease of modification at the core and imide-N sites are suitable for preparing various molecular conductors. A few NDIbased molecular conductors have already been reported,26 encouraging us to use NDI-py in this study. To date, several MOFs with mixed -stacking between NDI-based ligands and donor molecules have been reported.27,28 However, to the best of our knowledge, a conductive MOF with -stacked columns consisting solely of NDI-based ligands has not reported. In the present work, the electrocrystallization was carried out by applying a constant direct current of 30 μA to a solution of NDI-py and Cd(NO3)2·4H2O in N,N-dimethylacetamide (DMA) at room temperature (RT). Rod-like dark brown single crystals of [Cd(NDI-py)(OH2)4](NO3)x∙nDMA (PMC-1) were obtained from the cathode (PtIr alloy wire) after two days. The SXRD analysis revealed that PMC-1 crystallized as conglomerates with the hexagonal space groups P6222 and P6422. [Cd(OH2)4]2+ ions are bridged by NDI-py ligands to form linear CPs (Figure 1a). The NDI-py ligands form infinite helical -stacked columns along the crystallographic c axis (Figures 1b and 1c), thus the linear CPs align along the a, b and a+b axes (Figure 1d). The interplanar distance between the NDI-py ligands is 3.18 Å, which is

(d) Perspective view of PMC-1 along c axis. Yellow, Cd; Blue, N; Grey, C; Red, O. considerably shorter than those (> 3.3 Å) observed in typical NDI analogues,29 suggesting the presence of radical anion (NDI•−) species. However, the formula for PMC-1 found for the framework is [Cd(NDI-py)(OH2)4], suggesting that the charge of NDI-py is −2 (i.e., no NDI•− species). The above formula also conflicts with the fact that a dianionic NDI2− state is very unstable under aerobic conditions. Therefore, some residual electron density in the pores, which is too dispersed to establish a meaningful molecular structure, should be assigned to the NO3− ions (and some solvent molecules). Indeed, the infrared (IR) absorption peak at 1383 cm−1 observed in PMC-1 (Figure S1) proves the presence of NO3− ions.30 Therefore, a chemical formula for PMC-1 of [Cd(NDI-py)(OH2)4](NO3)x∙nDMA was tentatively assigned. To investigate the thermal stability of PMC-1, we performed thermogravimetry (TG). As shown in Figure S2, gradual weight loss started at RT due to the liberation of DMA and a plateau was observed around 210 °C. To remove all DMA and coordinated water molecules, PMC-1 was heated at 210 °C under an N2 atmosphere for 30 min and then cooled to RT. This solid readsorbed a small amount of water from the air to afford the heated compound, [Cd(NDI-py)](NO3)x∙mH2O (PMC-1h). The absence of DMA was confirmed by the disappearance of its signals from the NMR spectrum (Figure S3). Finally, the number of NO3− ions was calculated as x = 1.3 ± 0.1 from the results of elemental analyses for PMC-1h, as discussed in the Supporting Information (Table S2). Therefore, the mean charge of NDI was calculated as −0.7 ± 0.1, indicating that a part of the NDI-py ligand in PMC-1 was reduced to its NDI•− state. The solid-state absorption spectra of PMC-1 and NDI-py (Figure 2) indicate that the absorption bands above 3.0 eV correspond to the * transition for the neutral NDI core. The broad bands around 2.7 and 2.0 eV observed only in PMC-1 can be assigned to the intramolecular transitions of the NDI•− core according to previous reports.27a,31 Moreover, PMC-1 shows other broad bands in the near-IR to IR region. Disregarding the O−H stretching mode from water molecules around 0.4 eV (3200 cm–1), a broad absorption band was observed with a maximum at approximately 0.3 eV. This transition can be assigned to charge transfer (CT) between NDI•− and NDI0 cores in PMC-1. A very weak broad band around 1 eV can also be regarded as CT between two NDI•− cores. These bands in the near-IR to IR region only appear in the presence of intermolecular interactions.32–34 In addition, a sharp signal (g = 2.003) observed in the ESR spectrum confirms the existence of NDI•− species (Figure S6). The solid-state cyclic voltammogram of PMC-1 (Figure 3a) showed the dependency of the redox states on the electrolyte. The

Figure 1. Crystal structure of PMC-1 (space group P6222). (a) Thermal ellipsoid plot of the linear CP in PMC-1. The four water molecules coordinating to the Cd2+ ion are disordered in eight positions. (b) The -stacked NDI-py column projected along the c axis. The pyridyl groups are oriented in 60° increments. (c) The columnar structure of NDI-py showing the interplanar distances.

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Figure 2. Solid-state absorption spectra of NDI-py (orange) and PMC-1 (black) dispersed in KBr pellet. reduction of PMC-1 in 0.1 M LiClO4/CH3CN was chemically reversible and its reduction potential (E1/2red = −0.77 V vs. Fc/Fc+) was comparable to the typical value for NDI/NDI•− reduction,35 indicating that the remaining neutral NDI cores are reduced. In contrast, the reduction in 0.1 M n-Bu4NPF6/CH3CN was irreversible (Figure S7) and the current was small. The reduction peak potential (Epred) in n-Bu4NPF6/CH3CN (−1.12 V vs. Fc/Fc+) is more negative than that in LiClO4/CH3CN (−0.88 V vs. Fc/Fc+). These results indicate that the Li+ ions can readily enter the void space of PMC-1 under reduction, although the relatively bulky nBu4N+ ions are impeded from diffusion into the structure. This selectivity supports the porosity of PMC-1. It is also suggested that the NO3 ions remain in the void spaces probably close to the framework during the reduction process. The areas of the reduction and oxidation waves are approximately equal, indicating that the companion oxidation process ceases at the initial reduction state (0.7±0.1 e–). To gain further insight into the redox processes, a solid state spectroelectrochemical (SEC) measurement of PMC-1 was performed under similar conditions to the cyclic voltammetry (Figure 3b).32 Both bands for the radical species (2.7 and 2.0 eV) and for the CT between two NDI•− cores (1 eV) were detected in the spectrum for the initial state (E = 0 V) owing to the partially reduced NDI cores, in agreement with Figure 2. Upon reduction, the

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reflectance mode, F(R), of PMC-1 in contact with 0.1 M LiBF4 in CH3CN. The applied potential was scanned from 0 V (black) to 0.9 V (red). Grey lines show the spectral transition. intensity of these absorption bands gradually increased and finally saturated at 0.9 V, corresponding to the increase of NDI•− species. In the reverse oxidation process, the intensity of these absorption bands decreased and returned to their initial intensities at 0 V (Figure S8), supporting the fact that oxidation does not proceed past the initial reduction state of the NDI core. These results for the SEC measurement are in good agreement with the electrochemical data (Figure 3a). The inactivity of the material towards oxidation suggests that the initial amount of radical species is necessary to maintain the well-packed columnar structure of PMC-1. A single crystal direct-current (DC) conductivity measurement was conducted along the -stacked columnar axis (c axis) by using a two probe method (Figure S9). The  measured at 300 K (300K) of five crystals ranged from 1.0 × 10–3 to 3.3 × 10–3 S cm–1 (Table S3). These values for 300K are comparable to the most conductive NDI-based molecular conductors.26a The  increased with rising temperature, revealing semiconducting behavior (Figure 4), whereas the non-integer charge on the NDI cores and the crystallographically equal interplanar distances promise to induce metallic conduction in PMC-1. This is likely due to charge localization that occurs randomly in the -stacked columns, owing to the electrostatic interaction with NO3− ions accommodated in the random sites of the pores. The removal of DMA molecules from the pores induced a structural change and the broadening of peaks in the powder Xray diffraction (PXRD) pattern in PMC-1h (Figure S10). As shown in the absorption spectrum of PMC-1h (Figure S11), however, the radical and CT bands still exist. Unexpectedly, the pressed pellet of PMC-1h showed a 300K of (1.2–3.7)  10–2 S cm1, which is a 104 times higher than the 300K of the PMC-1 pellet ((1.5–7.6)  10–6 S cm1) and higher than that of the single crystal. The structure of PMC-1h can be estimated from the isomorphic and more crystalline complex, PMC-1s, which was prepared by soaking PMC-1 in toluene for a week. The results of extended X-ray absorption fine structure (EXAFS) (Figure S15) and high resolution PXRD measurement (Figure S16) suggest that linear CPs were retained in PMC-1s and PMC-1h but -stacked structures were changed to 2D-like packing modes,36 which can provide high carrier mobility and low grain boundary resistivity compared with 1D columnar structure (see the Supporting Information). The enhancement of  is likely due to this structural change.

Figure 3. (a) Cyclic voltammograms obtained at a scan rate of 100 mV/s for reduction of PMC-1-modified glassy carbon electrode in contact with 0.1 M LiClO4 (black) and n-Bu4NPF6 (red) in CH3CN. (b) Solid-state spectroelectrochemistry (SEC) in diffuse

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Figure 4. Temperature dependence of electrical conductivity () of single crystalline PMC-1 measured along the -stacked column (filled, black), in addition to pressed pellets of PMC-1 (hollow, gray) and PMC-1h (hollow, red). In conclusion, we have demonstrated the electrochemical synthesis of PMC-1, which shares features of both MOFs and molecular conductors. The linear CPs formed face-to-face helical -stacked columns of NDI cores, yielding a through-space conduction pathways. The removal of lattice DMA molecules induced a structural change and an enhancement in the conductivity, demonstrating that PMCs are promising as molecule-responsive conductive materials, i.e., conductive soft crystals.37 The chemical reduction method using hydrazine monohydrate gave PMC-1 with numerous unisolable by-products. Thus, we conclude that the electrochemical method is an effective technique for fabricating PMCs as new electroactive frameworks. The concepts of PMCs will stimulate new researches in further functionalization of MOFs and in postsynthetic control of electronic states of molecular conductors.

ASSOCIATED CONTENT Supporting Information Additional experimental details for the synthesis and characterization in pdf file and X-ray crystallographic information file in CIF format. These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] (H.I.) *[email protected] (M.Y.)

ORCID Hiroaki Iguchi: 0000-0001-5368-3157 Shinya Takaishi: 0000-0002-6739-8119 Deanna M. D’Alessandro: 0000-0002-1497-2543 Takefumi Yoshida: 0000-0003-3479-7890 Hitoshi Abe: 0000-0001-6970-3642

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We acknowledge Prof. Akutagawa and Dr. Hoshino at Tohoku University for acquiring the ESR spectra. This work was performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2018G083, beamline NW2A and No. 2017P016, beamline NW10A). The synchrotron radiation experiments for PXRD were performed at BL02B2 of SPring-8 (Proposal No. 2018B0074). This work was partly supported by JSPS KAKENHI Grant Numbers JP18H04498 (H.I.), 18H04499 (E.N.) and JP18K14233 (H.I.), by the CASIO Science Promotion Foundation (H.I.), by the Ogasawara Foundation for the Promotion of Science and Engineering (H.I.), by the Australian Research Council Grant FT170100283 (D.M.D.) and by the Program for Interdisciplinary Research in Tohoku University Frontier Research Institute for Interdisciplinary Sciences (H.I.). M. Yamashita thanks the support by the 111 project (B18030) from China.

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