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Nov 2, 2017 - (TCNQ) and the closely related guest tetracyanoethylene (TCNE) into the host porous framework [Cu3(BTC)2] (BTC = 1,3,5-benzenetricarboxy...
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Cite This: J. Phys. Chem. C 2017, 121, 26330-26339

Guest−Host Complexes of TCNQ and TCNE with Cu3(1,3,5benzenetricarboxylate)2 Pavel M. Usov,† Henry Jiang,† Hubert Chevreau,‡ Vanessa K. Peterson,‡ Chanel F. Leong,† and Deanna M. D’Alessandro*,† †

School of Chemistry, The University of Sydney, Sydney, New South Wales 2006, Australia Australian Nuclear Science and Technology Organization, Lucas Heights 2234, Australia

‡

S Supporting Information *

ABSTRACT: A combined spectroscopic and structural study was undertaken to investigate the nature of the incorporation of the electron acceptor guest 7,7,8,8-tetracyanoquinodimethane (TCNQ) and the closely related guest tetracyanoethylene (TCNE) into the host porous framework [Cu3(BTC)2] (BTC = 1,3,5-benzenetricarboxylate)a guest−host system recently shown to be highly conductive. We find that the guest concentration in the system can be modulated via the synthesis reaction time and temperature. A suite of spectroscopic, X-ray and neutron powder diffraction, and density functional theory techniques revealed the mechanism of guest binding within the framework host, including the guest redox states. This work provides insights into the way that electrical conductivity arises in porous framework host−guest systems and contributes to understanding how fine-tuning framework properties influences conductivity.

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oxidation potential.17−19 More recently, it was reported that solution phase infiltration of the well-known framework [Cu3(BTC)2] (BTC = 1,3,5-benzenetricarboxylate) with 7,7,8,8-tetracyanoquinodimethane (TCNQ) resulted in a dramatic increase in conductivity of several orders of magnitude.20 [Cu3(BTC)2] is composed of dicopper acetate units connected to BTC units, each with two coordinatively unsaturated Cu sites. While the framework is neutral overall, a partial positive charge at the Cu atoms exists with the remaining negative charge localized on the carboxylate units. Three pores are present in the structure: a cuboctahedral pore ∼10.7 Å in diameter lined with BTC units, an ∼12.7 Å diameter pore into which positively charged CuII ions protrude, and an ∼5 Å diameter pore in which four benzene rings form a tetrahedron. The smallest pore cannot be accessed via the ∼10.7 Å pore. Using computational modeling, the conductivity enhancement of [Cu 3(BTC)2 ] induced by TCNQ was attributed to coordination of TCNQ between Cu2II paddlewheel units to form a bridging motif, promoting a conductive pathway that spirals through the framework pores. The appearance of a new optical absorption band in the visible region provided evidence of a partial degree of charge transfer from the host framework (donor) to TCNQ (acceptor). An intriguing question was the origin of the charge transfer, as X-ray photoelectron spectroscopy (XPS) showed that the CuII centers did not undergo a redox state change upon incorporation of TCNQ, while electron paramagnetic resonance (EPR) found evidence of the TCNQ radical

INTRODUCTION The incorporation of guest molecules into the voids of porous framework hosts such as metal−organic frameworks (MOFs) has been extensively explored for the purpose of modulating their physical and chemical properties1−3 as well as expanding their functionalities.4−6 The host−guest behavior depends on a number of interactions including dipole−dipole effects, π−π stacking, and steric interactions, among others. As MOFs contain both void spaces and functionalities capable of intermolecular interactions, such as coordinatively unsaturated metal sites and hydrogen bond donors/acceptors, they are particularly attractive candidates as host materials.7 Host−guest interactions can also promote charge transfer (CT) between the host framework and interstitial guest molecules. This process occurs via molecular orbital hybridization between the guest molecules and the framework to produce partial charge delocalization between the components, resulting in a charge transfer system.8,9 For an electroactive framework, treatment with oxidizing or reducing agents can induce redox processes that result in the concomitant introduction or expulsion of counterion guests.10−12 One of the possible applications of this effect is in molecular sensing, since the formation of CT systems would modify properties of the framework such as luminescence13,14 as well as enhancing conductivity due to the generation of charge carriers.3,15,16 Guest incorporation has been extensively used to improve the conductive properties of framework materials. In this case, guests are usually employed as redox-active chemical agents to modify the redox state of the host, resulting in higher conductivity. Elemental iodine has been particularly popular for this application because of its small size and moderately high © 2017 American Chemical Society

Received: August 6, 2017 Revised: November 1, 2017 Published: November 2, 2017 26330

DOI: 10.1021/acs.jpcc.7b07807 J. Phys. Chem. C 2017, 121, 26330−26339

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The Journal of Physical Chemistry C

40 000 cm−1 with a scan rate of 6000 cm−1 min−1. The background was collected from dry, finely ground BaSO4. FT-IR Spectroscopy. IR spectra were collected between 400 and 3000 cm−1 using a Bruker Vertex 80v spectrometer equipped with an attenuated total reflectance attachment. The powdered samples were mounted onto a Ge crystal, and measurements were performed under vacuum. Raman Spectroscopy. Raman spectra were collected using an inVia Renishaw confocal Raman microscope employing 514 and 633 nm excitation lasers. Electron Paramagnetic Resonance (EPR) Spectroscopy. Continuous wave solid-state EPR spectra were collected at ambient temperature (295 ± 1 K) using a Bruker Elexsys 500 spectrometer equipped with an X-band microwave bridge. Spectra were referenced against strong pitch. Cyclic Voltammetry. Electrochemical experiments were undertaken in a three-electrode configuration at ambient temperature (295 ± 1 K) with the measurements collected using a BASi Epsilon electrochemical analyzer. 0.1 M [(nC4H9)4N]PF6/CH2Cl2 was used as a supporting electrolyte after degassing with high-purity Ar. The working electrodes were prepared by dropcasting and drying of a CH2Cl2 suspension of the powdered material onto a graphite paper strip (20 mm2) with a Pt wire current collector (0.075 mm diameter; connected using silver paint). A high surface area Pt wire was used as a counter electrode and Ag wire as a quasi-reference electrode. The potential of the quasi-reference electrode was calibrated vs the reversible potential of the ferrocene/ferrocenium (Fc0/+) couple measured in the same solution before or after the experiment. Neutron Powder Diffraction (NPD). NPD data were measured for desolvated [Cu3(BTC)2] and [Cu3(BTC)2] with TCNE prepared at 60 and 100 °C for 72 h. Prior to measurements, the samples were placed under vacuum for 16 h at room temperature, then loaded into a 6 mm diameter vanadium can in a helium glovebox, and sealed with an indium gasket. The samples were attached to a center stick that was inserted into a top-loading cryofurnace and cooled to 15 K, where data were recorded using the high-resolution neutron powder diffractometer ECHIDNA24 at the OPAL reactor facility at the Australian Nuclear Science and Technology Organization. Data were obtained using a neutron wavelength of 2.4395(5) Å, determined using the National Institute of Standards and Technology LaB116 standard reference material 660b, over the angular range 6.5°−163° in 2θ with an 11 h collection time. Data were analyzed using the Rietveld method within the GSAS22 suite of programs and the EXPGUI23 interface. The refined model for the desolvated [Cu3(BTC)2] was used as a starting model to examine the guest−host system, with Fourierdifference methods used to produce nuclear density maps to directly identify the location of the guest molecule atoms localized within the pores. The obtained structural models for the guest−host systems were used as the basis for density functional theory (DFT) calculations, the details of which are provided in the Supporting Information.

anion. Computational studies pointed toward a superexchange mechanism whereby significant electronic coupling between the CuII paddlewheel units and TCNQ was likely to originate from favorable overlap of TCNQ and Cu dz2 orbitals.15,20 Herein, we present a combined spectroscopic and structural study investigating the binding mechanism of TCNQ and the closely related acceptor tetracyanoethylene (TCNE) in [Cu3(BTC)2], obtained from vapor-phase infiltration. As part of this investigation, the synthesis conditions (both reaction time and temperature) were varied to modulate the loading of guests into the pores. Cyclic voltammetry results were correlated with structural insights obtained from a suite of characterization methods including ultraviolet−visible−near-infrared (UV−vis− NIR), vibrational (infrared and Raman), and EPR spectroscopies as well as neutron powder diffraction (NPD) and density functional theory (DFT) calculations which were aimed at understanding the structure of the guest−host systems and the guest oxidation states within these.

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EXPERIMENTAL SECTION Materials. All reagents and solvents were purchased from commercial sources and used without further purification unless otherwise stated. TCNQ, TCNE, and dichloromethane (CH2Cl2, anhydrous, Sure/Seal) were purchased from SigmaAldrich, and [(n-C4H9)4N]PF6 (recrystallized from ethanol, EtOH) was obtained from Alfa Aesar. Synthesis. [Cu3(BTC)2] was synthesized following a previously reported literature procedure.21 The as-synthesized MOF was soaked in CH2Cl2 for 6 days, during which time the CH2Cl2 was decanted and replenished three times. Samples of the MOF used in infiltrations by sublimation were washed with hot ethanol for 3 days using a Soxhlet apparatus before desolvation. The desolvated framework was prepared by heating the material in a Schlenk flask under dynamic vacuum at 170 °C overnight and was stored under argon. Infiltration by Sublimation. In general, the desolvated MOF (80 mg) was loaded into glass pipettes, which had been sealed at one end, in addition to TCNQ (50 mg) or TCNE (60 mg). The vessels were then evacuated under dynamic vacuum and flame-sealed, forming a closed evacuated vessel. Pressures inside the reaction vessels ranged between (4−6) × 10−2 mbar. The vessels were heated at 80 and 120 °C for TCNQ, and at 60 and 100 °C for TCNE, for varying amounts of time (12, 24, 48, and 72 h) to achieve different guest loadings. The materials were subsequently washed with CH2Cl2 to remove any excess TCNQ or TCNE. All the guest infiltrated samples were handled under ambient atmosphere. Elemental Analysis. C, H, and N (CHN) analyses were conducted at the Chemical Microanalysis Facility at the Department of Chemistry & Biomolecular Sciences, Macquarie University, Australia. X-ray Powder Diffraction (XRPD). Measurements were performed over the 2θ range 5°−50° with a 0.02° step size and 2° min−1 scan rate on a PANalytical X’Pert Pro diffractometer fitted with a solid-state PIXcel detector (40 kV, 30 mA, 1° divergence and antiscatter slits, and 0.3 mm receiver and detector slits) using Cu Kα (λ = 1.5406 Å) radiation. Le Bail fitting was performed on GSAS22 using the EXPGUI23 graphical user interface. UV−Vis−NIR Spectroscopy. Solid-state diffuse reflectance spectra were measured using a Agilent CARY 5000 UV−vis− NIR spectrophotometer with a Praying Mantis attachment and Agilent WinUV software V3.0. Data were recorded from 5000 to

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RESULTS AND DISCUSSION Guest Loading. [Cu3(BTC)2] was heated at 170 °C under vacuum to remove water molecules coordinated to the CuII sites and create coordinatively unsaturated metal sites.25 TCNQ and TCNE guests were incorporated into the activated framework by sublimation under vacuum, a method that avoids the use of solvents which could interfere with the guest infiltration by competitive binding. Adjustment of the reaction temperature 26331

DOI: 10.1021/acs.jpcc.7b07807 J. Phys. Chem. C 2017, 121, 26330−26339

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The Journal of Physical Chemistry C controls the incorporation rate as it determines the partial vapor pressure of the guests inside the reaction vessel. Two temperatures were selected for each guest (80 and 120 °C for TCNQ and 60 and 100 °C for TCNE), and the infiltration process was monitored over time (12, 24, 48, and 72 h). Following infiltration, the samples were washed with CH2Cl2 multiple times to remove unreacted and surface-bound guest molecules. Importantly, washing of the infiltrated products with CH2Cl2 did not result in a color change in the materials, which strongly suggests that adsorbed TCNQ and TCNE guests were not washed out during this process. Elemental analysis (CHN) was performed on the activated framework and infiltrated samples to determine the guest loading. As the activated [Cu3(BTC)2] framework does not contain nitrogen, and TCNQ and TCNE are both nitrogen rich, the N% is directly proportional to the number of incorporated guest molecules. Calculations of the amount of TCNQ or TCNE infiltrated were performed using the N:C ratio as it provided an accurate point of comparison to the host framework. H% results were not considered due to inaccuracies introduced by the variation in H2O uptake from the atmosphere between the activated and infiltrated frameworks. The number of incorporated guests per [Cu3(BTC)2] formula unit was determined using eq 1 (Supporting Information). no. of guests =

216(N%/C%) 14(no. of N per guest) − 12(N%/C%)(no. of C per guest)

(1)

Elemental analysis performed on the activated [Cu3(BTC)2] confirmed the absence of any nitrogen (65 h), whereas at higher temperature saturation is reached after 48 h. From these results it was evident that guest infiltration is governed by the diffusion of molecules through the pores of the framework. The nitrile groups on TCNQ may interact with open CuII sites impeding further penetration into the crystallites, resulting in slow kinetics of uptake of TCNQ, which increases with temperature. The number of incorporated guests plateaued at 0.98 ± 0.05 TCNQ molecules per [Cu3(BTC)2] or 16 per unit cell, compared to the 8 TCNQ per unit cell previously reported following infiltration in CH2Cl2.20 As such, vapor-phase infiltration gives rise to improved uptake, which is attributed to the higher reaction temperature and reduced competition with solvent. TCNE exhibited different uptake behavior compared to TCNQ (Figure 1b). The guest loading rose sharply in the first 12 h, after which a higher reaction time resulted in only a minor increase in uptake. The fast saturation behavior was found to occur at both 60 and 100 °C and can be explained by the smaller molecular size of TCNE, enabling more rapid diffusion through the framework pores. The guest loadings stabilized at 0.82 ± 0.05 and 1.8 ± 0.06 TCNE per [Cu3(BTC)2] formula unit (13 and 29 TCNE per unit cell) at 60 and 100 °C, respectively, suggesting the presence of at least two possible binding modes for TCNE inside the framework, one of which predominates at higher temperature. Overall, the differing loading profiles of TCNE and

Figure 1. Guest uptake of [Cu3(BTC)2] versus reaction time for (a) TCNQ at 80 °C (blue) and 120 °C (red) and (b) TCNE at 60 °C (green) and 100 °C (orange). Lines are intended to guide the eye.

TCNQ are largely attributed to the molecular size difference, which also accounts for the higher number of TCNE molecules incorporated into each unit cell of the framework. Preliminary Structural Characterization. XRPD of [Cu3(BTC)2] infiltrated with TCNQ and TCNE at different temperatures revealed retention of the initial framework structure (Figure 2). Le Bail refinements performed on the activated and infiltrated derivatives of [Cu3(BTC)2] showed unit cell parameters similar to that of as-synthesized [Cu3(BTC)2] (Supporting Information). Samples infiltrated with TCNQ did not exhibit additional reflections in the XRPD data at either of the temperature, indicating that the guests were disordered inside the pores (Figure 2a,b). On the other hand, several new reflections were detected at low angles (2θ < 20°) in the XRPD data of [Cu3(BTC)2] loaded with TCNE (Figure 2c,d). At lower reaction temperatures, a peak at 2θ = 6° due to the (111) reflection appeared in the XRPD data regardless of the reaction time, further evidencing the fast incorporation of TCNE. This reflection has previously been attributed to ordered binding of small molecules at the open CuII sites within the dicopper acetate unit. 20,25 The XRPD data here therefore support the coordination of TCNE at the metal centers. Interestingly, at the higher reaction temperatures this reflection was absent, suggesting a lack of ordering of the guests, and low-intensity reflections at 2θ = 8° and 16° were observed. These reflections are attributed to the appearance of a new phase, which was unable to be identified from these data. XRPD data for the guest infiltrated [Cu3(BTC)2] featured a background that was no 26332

DOI: 10.1021/acs.jpcc.7b07807 J. Phys. Chem. C 2017, 121, 26330−26339

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Figure 2. XRPD data for [Cu3(BTC)2] infiltrated with TCNQ at (a) 80 and (b) 120 °C and TCNE at (c) 60 and (d) 100 °C (infiltration time = 12 (blue), 24 (red), 48 (green), and 72 h (orange)). The pattern for [Cu3(BTC)2] is shown in black. Data are offset in y for clarity. Asterisks (∗) indicate new reflections in the TCNE infiltrated [Cu3(BTC)2].

Figure 3. Diffuse reflectance UV−vis−NIR spectra of [Cu3(BTC)2] infiltrated with TCNQ at (a) 80 and (b) 120 °C and TCNE at (c) 60 and (d) 100 °C (infiltration time = 12 (blue), 24 (red), 48 (green), and 72 h (orange)). The spectrum of empty [Cu3(BTC)2] is shown in black.

longer flat, suggesting the presence of some structural disorder and decomposition. UV−Vis−NIR Spectroscopy. Diffuse reflectance UV−vis− NIR spectroscopy performed on the infiltrated [Cu3(BTC)2]

(Figure 3) revealed that spectra following the incorporation of TCNQ and TCNE were significantly different to that for the empty [Cu3(BTC)2] (Supporting Information). The absorption bands at 14 500 and 38 500 cm−1 correspond to the d−d 26333

DOI: 10.1021/acs.jpcc.7b07807 J. Phys. Chem. C 2017, 121, 26330−26339

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Figure 4. Attenuated total reflectance-IR spectra of [Cu3(BTC)2] infiltrated with TCNQ at (a) 80 and (b) 120 °C and TCNE at (c) 60 and (d) 100 °C (infiltration time = 12 (blue), 24 (red), 48 (green), and 72 h (orange)). The spectrum of empty [Cu3(BTC)2] is shown in black. Spectra are offset in y for clarity.

cm−1. Since this region of the spectrum is devoid of features corresponding to [Cu3(BTC)2] modes, the vibrational transitions could be unambiguously assigned to the nitrile functionalities on the TCNQ or TCNE guests. In previous studies of solid state systems which incorporate TCNQ and TCNE, IR spectroscopy has proven invaluable for interrogating electronic characteristics, and a large library of vibrational modes of TCNQ and TCNE in different chemical environments has been generated, providing a convenient point of comparison for the present study.32,33 Infiltration of [Cu3(BTC)2] with TCNQ resulted in the appearance of several stretches in the nitrile region of the IR spectrum indicating the presence of multiple TCNQ species. Upon infiltration at 80 °C a new peak appeared at 2170 cm−1 after 12 h (Figure 4a), which corresponds to the TCNQ•− species. This result differs from that reported previously where the infiltration was conducted in the solution-phase, and the appearance of a single peak at 2204 cm−1 was attributed to a coordinated TCNQ with an effective charge of 0.3 e−.20 This discrepancy suggests a different binding mode for TCNQ infiltrated by the current vapor-phase method compared to the previously reported solution-phase approach, with the former resulting in a higher degree of charge transfer between the framework and guests and giving rise to a red-shift in the nitrile stretching frequency. At higher TCNQ uptakes, another sharp peak appeared at 2225 cm−1 which gradually intensified with increasing infiltration time, indicating the formation of a second guest phase. The energy of this band correlates well with that for a neutral TCNQ species.34 Coordination of the guest to the metal centers could not be definitively determined from the energy of this band because opposing frequency shifts of the nitrile stretch can occur depending on the nature of metal−ligand

transitions of CuII and ligand-to-metal charge-transfer (BTC → CuII) transitions of the parent framework, respectively, which remained largely unchanged in the infiltrated analogues. The spectra of the materials infiltrated with TCNQ (Figure 3a,b) exhibited two closely overlapping peaks at 24 500 and 25 500 cm−1 which are ascribed to the radical and neutral TCNQ species, respectively.26 The relative intensities of the two bands suggest that the radical form dominates the sample infiltrated at the lower temperature of 80 °C, with the proportion of the neutral form increasing at longer reaction times and the higher temperature of 120 °C. The TCNQ radical is also expected to exhibit an absorption band in the visible region of the spectrum at 12 000 cm−1;27 however, it was difficult to clearly discern this due to overlap with the d−d transitions. The UV−vis−NIR spectra of [Cu3(BTC)2] infiltrated with TCNE (Figure 3c,d) revealed several new spectroscopic features. A low-intensity band appeared at 22 500 cm−1 in the material infiltrated at both temperatures and was attributed to TCNE in its radical anion form.28,29 In addition, a broad shoulder band was detected at 30 000 cm −1 which overlapped with the [Cu3(BTC)2] ligand-to-metal charge-transfer band. The energy of this band correlates to the neutral TCNE0 species, while the relative intensity of the band increases with infiltration temperature, accounting for the increased uptake of TCNE at 100 °C. Vibrational Spectroscopy. Vibrational spectroscopy (IR and Raman) measurements were performed on the infiltrated samples to gain insight into the nature of the guest molecules upon their incorporation into [Cu3(BTC)2]. The vibrational stretches of TCNQ and TCNE are highly sensitive to their redox state and coordination to metal centers,30,31 particularly nitrile stretching modes which typically occur between 2100 and 2300 26334

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The Journal of Physical Chemistry C bonding.35,36 As such, this band was tentatively assigned to uncoordinated neutral TCNQ subjected to confinement effects inside the pores of the framework. Upon incorporation of TCNQ at 120 °C, the IR spectrum of the framework exhibited several new features (Figure 4b). The presence of peaks at 2170 and 2225 cm−1 is in accordance with the IR spectra of the low-temperature infiltration products, with the latter peak intensifying over time. This finding indicates that the same TCNQ species are generated at 80 and 120 °C: the band at 2170 cm−1 corresponds to the TCNQ•− species coordinated to CuII, whereas the band at 2225 cm−1 is attributed to unbound TCNQ0 within the framework pores. In addition, the spectra of the high-temperature products exhibited another peak at 2205 cm−1 which is indicative of a third coordination mode of TCNQ inside [Cu3(BTC)2] that only forms at higher temperature. The energy of this peak coincides with that previously reported20 which was ascribed to TCNQ0.3− species coordinated to CuII. Another possibility for the origin of this peak is dimerization of the radical anion giving rise to a (TCNQ)22− species which exhibits a nitrile stretching mode at a similar energy (vide inf ra).32 Thus, it is difficult to conclusively determine the nature of the third guest phase from IR data alone, since several TCNQ species could generate the observed peak. The two largest pores within [Cu3(BTC)2] are accessible to TCNQ, one of which is lined with coordinatively unsaturated and positively charged CuII sites. It is therefore expected that the TCNQ•− species would coordinate to CuII centers within one pore while the neutral TCNQ would reside elsewhere. The IR spectra of [Cu3(BTC)2] incorporated with TCNE are shown in Figure 4c,d and reveal that the nature of the guest is influenced by the reaction temperature. The loading at 60 °C resulted in the appearance of two peaks at 2190 and 2225 cm−1, which are tentatively assigned to the μ1-TCNE species coordinated to CuII centers.34 The positions of these peaks are between those reported for μ1-TCNE0 and μ1-TCNE•− phases, indicating that TCNE molecules inside [Cu3(BTC)2] possesses a fractional charge. These two peaks also coincide with unbound TCNE•−; however, the NPD data (vide inf ra) suggest a close interaction between the TCNE guest and the CuII site consistent with coordination to the metal center. As a result, the degree of charge transfer between the framework and the guests is only partial, consistent with the result reported by Talin, Allendorf, and co-workers for TCNQ.20 This assignment is supported by the presence of a radical signal in the EPR spectrum (Supporting Information). As the geometry of TCNE is similar to TCNQ, similar coordination modes are expected within the framework. On the other hand, in the IR spectra of the infiltrated materials obtained at higher temperature, these two peaks shift to higher energy (2210 and 2230 cm−1) which is more consistent with the neutral μ1-TCNE species and suggests that a lower degree of charge transfer occurs at higher loadings. Similarly, these peaks may also be assigned to unbound TCNE; however, further NPD and DFT studies (vide inf ra) indicate binding of TCNE at the open metal sites. EPR spectra revealed a significant shift in gfactor of the radical peak (Supporting Information), indicating a change in electronic delocalization which may arise from a decrease in the degree of charge transfer. To assist in elucidating the orientation of TCNE within [Cu3(BTC)2], NPD measurements were performed (vide inf ra). To test the hypothesis of the formation of TCNQ dimers at higher temperature, resonance-enhanced Raman spectroscopy was performed on [Cu3(BTC)2] infiltrated with TCNQ at 120 °C. For these measurements, 514 and 633 nm laser excitations

were used. Since the (TCNQ)22− species is known to exhibit a strong absorption band at 674 nm in aqueous solution,37 its nitrile stretching mode is typically found to be resonantly enhanced when excited at that wavelength. 38 As such, comparison of Raman spectra obtained with 514 and 633 nm excitations could evidence the presence of a TCNQ radical dimer. As shown in Figure 5, Raman spectra at these two

Figure 5. Raman spectra of [Cu3(BTC)2] infiltrated with TCNQ at 120 °C for 12 (blue), 24 (red), 48 (green), and 72 h (orange) collected using (a) 514 and (b) 633 nm excitation. The spectrum of empty [Cu3(BTC)2] is shown in black. Spectra are offset in y for clarity.

wavelengths differ in the nitrile stretching region. The 514 nm excitation resulted in a single peak at 2219 cm−1 which corresponds to the TCNQ•− species coordinated to CuII (Figure 5a).39 This peak was shifted to 2225 cm−1 in the materials infiltrated for longer than 48 h due to the formation of neutral TCNQ inside the pores. Different behavior was observed in the Raman spectra obtained with 633 nm excitation (Figure 5b), whereby shorter infiltration times (12 and 24 h) gave rise to a single broad peak at 2220 cm−1 which intensified with increasing reaction time. At infiltration times exceeding 48 h, however, a shoulder appeared at 2196 cm−1 which was not observed in the 514 nm Raman spectrum and is in good agreement with a (TCNQ)22− species.38 This result further evidence the formation of dimeric TCNQ radicals inside the pores of [Cu3(BTC)2] at 120 °C. Therefore, IR and Raman spectroscopic measurements have identified several possible phases of TCNQ and TCNE within 26335

DOI: 10.1021/acs.jpcc.7b07807 J. Phys. Chem. C 2017, 121, 26330−26339

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Figure 6. Possible phases of (a) TCNQ and (b) TCNE identified within [Cu3(BTC)2].

[Cu3(BTC)2] which are summarized in Figure 6. One of the proposed TCNQ forms, the radical anion coordinated to CuII, has been reported previously and was postulated to be responsible for the pronounced conductivity enhancement of the framework.20 The present study has revealed the existence of at least two additional phases: neutral and dimeric TCNQ species. On the other hand, TCNE exclusively coordinates to open CuII sites with a varying degree of charge transfer.40 Electrochemistry. To gain an insight into the redox states of TCNQ and TCNE within the frameworks, cyclic voltammetry was performed on the infiltrated [Cu3(BTC)2]. For this experiment, 0.1 M [(n-C4H9)4N]PF6/CH2Cl2 was used as a supporting electrolyte due to the stability of the infiltrated materials in CH2Cl2. The empty framework itself exhibited a redox process at E1/2 = −0.1 V vs Fc0/+ (R1) which was assigned to the CuII/I redox couple (Figure 7a). Similar electrochemical behavior has been reported for [Cu3(BTC)2] in aqueous and acetonitrile-based electrolytes.41,42 The presence of additional, less pronounced redox peaks could be attributed to the rearrangement of the framework structure giving rise to different redox-active phases. In the case of infiltrated [Cu3(BTC)2], only the samples with the lowest and highest loading of TCNQ and TCNE were subjected to cyclic voltammetric investigation. Incorporation of TCNQ resulted in the appearance of several new redox processes (Figure 7b). At 0.15 ± 0.05 TCNQ per [Cu3(BTC)2] formula unit (12 h, 80 °C), the cyclic voltammogram resembled that of the empty framework. In particular, the process corresponding to the CuII/I couple could still be detected at E1/2 = −0.09 V vs Fc0/+, indicating that some CuII remained accessible to reduction. In addition, a new irreversible redox process was observed at 0.36 V vs Fc0/+ (O1) which was tentatively assigned to the oxidation of coordinated TCNQ•− units. The low reversibility of this process is likely due to blockage of framework pores by counterions during the course of the redox transformation. The lack of any distinguishable redox peaks on the cathodic scan indicates the absence of neutral TCNQ species at lower loading levels, which is consistent with the results from vibrational spectroscopy analysis. In contrast, the cyclic voltammetry data of [Cu3(BTC)2] with the highest TCNQ loading of 0.98 ± 0.05 TCNQ molecules per [Cu3(BTC)2] (72 h, 120 °C) exhibited additional redox peaks that indicate the formation of other TCNQ phases. The reductive side contained two quasi-reversible redox processes at E1/2 = −0.2 and −0.77 V vs Fc0/+ (R1 and R2) which are characteristic of two subsequent reduction processes of TCNQ0

Figure 7. Solid-state cyclic voltammograms of (a) [Cu3(BTC)2], (b) frameworks infiltrated with TCNQ (12 h, 80 °C (blue) and 72 h, 120 °C (red)), and (c) frameworks infiltrated with TCNE (12 h, 60 °C (green) and 72 h, 100 °C (orange)); 0.1 M [(n-C4H9)4N]PF6/CH2Cl2 was used as a supporting electrolyte. The potential was scanned in the cathodic direction first at a scan rate of 200 mV s−1.

to TCNQ•− and to TCNQ2−, respectively. This result confirms the uptake of neutral TCNQ by [Cu3(BTC)2] at longer reaction times and higher temperature. Further, a quasi-reversible redox peak was observed on the anodic scan at higher potentials of 0.6 V vs Fc0/+ (O1) compared to the lower loading sample and was tentatively assigned to the oxidation of the dimeric (TCNQ)22− 26336

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The Journal of Physical Chemistry C

TCNE molecules per [Cu3(BTC)2]. This result is in good agreement with elemental analysis results and indicates that all TCNE at the higher temperature is coordinated through the N atom to CuII, ruling out η2 coordination of TCNE (Supporting Information). [Cu3(BTC)2] demonstrated a strong structural response to the infiltration of TCNE, experiencing a volume reduction of ∼1.2% following treatment at 60 °C and ∼1.5% following treatment at 100 °C. This phenomenon is consistent with a relatively strong interaction between the framework and the TCNE ligand, as observed for other guests (Supporting Information).44 Density Functional Theory. To further understanding the interaction of the TCNE ligand with the coordinatively unsaturated CuII sites within [Cu3(BTC)2], DFT-D2 (dispersion-corrected) ground-state (0 K) calculations were performed to determine the lowest energy configuration of the TCNE ligand coordinated to the CuII (Supporting Information). The lowest energy configuration found is consistent with the NPD results, indicating a distance between the N atom of the TCNE and the CuII of ∼2.12 Å, in agreement with that of 2.16(6) Å found in the NPD measurement (Figure 8). The strength of

species. The shift to a higher potential compared to the monomeric form is ascribed to the stabilization of the TCNQ anion within the dimer. To the best of our knowledge, only a single report regarding the electrochemical investigation of a TCNQ dimer system has appeared in the literature, where the oxidation potential of (TCNQ)22− inside a charge transfer complex was determined to be 0.26 V vs the saturated calomel electrode.43 The discrepancy between the redox potentials is attributed to the confinement of the TCNQ dimer inside the framework. The large peak−peak separation of 0.49 V may be caused by the dissociation and re-formation of (TCNQ)22− during the redox transformation. The disappearance of the redox peak at 0.36 V vs Fc0/+ for the species incorporating 0.98 ± 0.05 TCNQ per [Cu3(BTC)2] (72 h, 120 °C) suggests that coordinated TCNQ•− species have been converted to the dimer. Alternatively, the radical may have become inaccessible to oxidation as a result of higher guest occupation. Cyclic voltammograms of [Cu3(BTC)2] infiltrated with TCNE also displayed additional redox processes not observed in the host framework (Figure 7c). The sample with the lowest guest uptake of 0.43 ± 0.05 TCNE molecules per [Cu3(BTC)2] (12 h, 60 °C) exhibited an irreversible redox peak at 0.07 V vs Fc0/+ (R1) which correlates with CuII/I reduction in the empty framework indicating the accessibility of the CuII centers, consistent with their partial occupancy as indicated in the NPD result (vide inf ra). In the cathodic scan, another irreversible redox process was detected at −0.88 V vs Fc0/+ (R1) which was tentatively assigned to the reduction of TCNE•− coordinated to CuII to its corresponding TCNE2− state. The low reversibility of these processes could be caused by structural rearrangement of the framework and trapping of counterions. In the product obtained at 100 °C which incorporated 1.8 ± 0.06 TCNE per [Cu3(BTC)2] formula unit (72 h, 100 °C), no reduction processes could be detected, possibly due to blocking of [Cu3(BTC)2] pores by the guest molecules, making them less available for reduction. The CuII/I process, however, can still be observed at 0.09 V vs Fc0/+, which may be due to oxidation of CuII at the surface of the particles. Overall, cyclic voltammetry measurements provided additional support for the assignment of TCNQ and TCNE species inside [Cu3(BTC)2]. Baseline measurements after the cyclic voltammetry experiments (Supporting Information) revealed that guests are retained inside the framework and do not leach into the electrolyte. Neutron Powder Diffraction. NPD measurements of [Cu 3(BTC)2] confirmed the presence of coordinatively unsaturated CuII open metal sites, and NPD measurements of the TCNE-infiltrated [Cu3(BTC)2] were performed to interrogate the nature of TCNE incorporation to the CuII site. Rietveld analysis and Fourier-difference methods using NPD data on [Cu3(BTC)2] infiltrated with TCNE at 60 °C for 72 h confirmed that the TCNE ligand was coordinated to the CuII, with a CuII···N distance of 2.45915(18) Å. The site occupancy factor of the N atom (at a crystallographic site with a multiplicity of 48) was 0.244(1), while the site occupancy for the Cu (at a site also with a multiplicity of 48) was 1, which corresponds to 0.732(3) TCNE ligands per [Cu3(BTC)2]. This result is in good agreement with the elemental analysis and indicates that all TCNE was coordinated to the CuII, in agreement with spectroscopic results. The same analysis was performed on [Cu3(BTC)2] incorporated with TCNE at 100 °C for 72 h and revealed a shorter distance between the N terminus of TCNE and the CuII of 2.16(6) Å, alongside a higher site occupancy factor for the N atom of 0.571(0), corresponding to 1.713(0)

Figure 8. Lowest energy configuration of the TCNE ligand coordinated to the CuII of [Cu3(BTC)2], where Cu are in turquoise, carbon in gray, oxygen in red, nitrogen in blue, and hydrogen in white.

the coordination of the TCNE to CuII in this configuration was calculated to be −76.1 kJ mol−1 (Supporting Information), which is significantly higher than the interaction with methane (−24 to −27 kJ mol−1) and carbon dioxide (−22 kJ mol−1).45,46

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CONCLUSIONS In the present study, redox-active TCNQ and TCNE species were incorporated into the framework [Cu3(BTC)2] which possesses coordinatively unsaturated CuII sites. The infiltration was performed in the vapor phase by subliming the guests under vacuum, and the effects of reaction temperature and time on the uptake were explored. It was found that TCNQ infiltration was kinetically controlled by coordination to the CuII and subsequent blockage of the pores. On the other hand, TCNE uptake was relatively fast due to its smaller molecular size, and the reaction temperature governed loading levels. UV−vis−NIR, IR, and Raman spectroscopies as well as cyclic voltammetry measurements demonstrated the presence of several TCNQ phases within [Cu3(BTC)2]. Upon infiltration into the framework, 26337

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The Journal of Physical Chemistry C coordinated TCNQ radical species were observed first, followed by the uptake of neutral TCNQ. At higher reaction temperature, another phase was observed which may correspond to the formation of the (TCNQ)22− dimer. TCNE incorporation resulted in the formation of coordinated TCNE radical species with a fractional charge transfer from [Cu3(BTC)2]. NPD data confirmed that all TCNE guests are coordinated to the CuII centers, whereby the CuII···N distance decreases with increasing loading of TCNE. Furthermore, the degree of charge transfer was found to decrease as the infiltration temperature increased. The application of this combined structural, spectroscopic, and electrochemical analysis may prove valuable for other guest@ MOF systems where either the guest and/or host are electroactive.

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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.jpcc.7b07807. Elemental analysis data for infiltrated [Cu3(BTC)2], derivation of eq 1, XRPD of [Cu3(BTC)2], Raman spectra of [Cu3(BTC)2] infiltrated with TCNQ at 80 °C, Raman spectra of [Cu3(BTC)2] infiltrated with TCNE at 60 °C, EPR spectra of [Cu 3 (BTC) 2 ] as-synthesized and infiltrated with TCNQ/TCNE, baseline cyclic voltammograms, Rietveld-refinement results for NPD data for [Cu3(BTC)2] as-synthesized and infiltrated with TCNE, NPD crystallographic structure for [Cu3(BTC)2] infiltrated with TCNE, NPD determined unit cell volume vs TCNE infiltration temperature, details of DFT-D2 calculations (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel +61 2 93513777; e-mail [email protected]. au (D.M.D.). ORCID

Vanessa K. Peterson: 0000-0002-5442-0591 Deanna M. D’Alessandro: 0000-0002-1497-2543 Present Addresses

P.M.U.: Virginia Tech, Blacksburg, VA 24061. H.J.: University of California, Berkeley, Berkeley, CA 94720. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Australian Research Council, the Vibrational Spectroscopy Core Facility and the Australian Institute of Nanoscale Science and Technology (AINST) at the University of Sydney. We gratefully acknowledge Dr Mark Allendorf (Sandia National Laboratories) for his helpful comments and suggestions on an early version of the manuscript.

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