Host-Guest Interactions in Metal-Organic Frameworks Doped with Acceptor Molecules as Revealed by Resonance Raman Spectroscopy.

Metal-organic frameworks (MOFs) represent a class of porous materials whose properties can be altered by doping with redox-active molecules. Despite advanced properties such as enhanced electrical conduction that doped MOFs exhibit, understanding physical mechanisms remains challenging because of their heterogeneous nature hindering experimental observations of host-guest interactions. Here, we show a study of charge transfer between Mn-MOF-74 and electron acceptors, 7,7,8,8-tetracyanoquinodimethane (TCNQ) and XeF2, employing selective enhancement of Raman scattering of different moieties under various optical-resonance conditions. We identify Raman modes of molecular components and elucidate that TCNQ gets oxidized into dicyano-p-toluoyl cyanide (DCTC-) while XeF2 fluorinates the MOF upon infiltration. The framework's linker in both cases acts as an electron donor as deduced from blue shifts of the C-O stretching mode accompanied by the emergence of a quinone-like mode. This work demonstrates a generally applicable methodology for investigating charge transfer in various donor-acceptor systems by means of resonance Raman spectroscopy.

Introduction

Metal–organic frameworks (MOFs) are a group of porous materials composed of metal nodes and organic linkers. High porosity and low density constitute MOFs’ excellent applicability in gas storage, separation, and catalysis. Because of the demands of semiconductor industries, MOFs are considered as materials for electronic devices, such as field effect transistors, photodetectors, radiation detectors, solar cells, supercapacitors, or chemical sensors.[1−9] Various synthetic design strategies were developed in the last decade in order to produce frameworks with permanent porosity and long-range charge transport. Whereas intrinsically conducting MOFs can be prepared using a limited number of specific linkers containing sulfur atoms, quinone groups, or aromatic amines,[5−7,9] another strategy consists in doping of nonconducting MOFs with redox-active conjugated guest molecules.

The host–guest interaction is in specific cases accompanied by a formation of donor–acceptor pair between the MOFs’ linker and the guest molecule, which may result in guest-induced emergent properties of newly formed materials.[10] It was reported that the electrical conductivity of HKUST-1 can be increased by six orders of magnitude to 7 S/m by doping with 7,7,8,8-tetracyanoquinodimethane (TCNQ), a redox-active organic molecule.[11] Since this pioneering work, much effort has been devoted to the investigation of emergent properties of TCNQ-doped HKUST-1[10,12−21] and some other MOFs[22−24] including Co-MOF-74.[25,26] Regarding the principles governing charge transport in TCNQ-doped HKUST-1, superexchange electron transport was suggested to explain the low activation energy.[13]

M-MOF-74 (M = Mg, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, or Zn), also known as M2(DOBDC) or CPO-27-M, is a MOF with honeycomb pores composed of metal and 2,5-dihydroxyterephthalic acid. One coordination site of the metal remains unoccupied and can interact with other ligands.[27] Host–guest interaction of M-MOF-74 with various molecules has been previously investigated,[28−36] including molecular doping with TCNQ[25,26] or tetrathiafulvalene.[26] It was shown that the Co-MOF-74 changes its physical properties by accommodating TCNQ; strong intermolecular charge transfer reduces the optical band gap down to 1.5 eV of divalent TCNQ and enhances the electrical conduction, which allowed the Co-MOF-74 to be utilized for resistive gas- and photo-sensing.[25,26]

In this contribution, Mn-MOF-74 is doped with two different electron acceptors: TCNQ and xenon difluoride (XeF2). Resonance Raman spectroscopy using multiple laser lines elucidates the presence of dicyano-p-toluoyl cyanide (DCTC–), an oxidation product of TCNQ2–, in Mn-MOF-74, while exposure to vapors of XeF2, a powerful oxidative fluorinating agent,[37] leads to fluorination of Mn-MOF-74 (see Figure ). Both TCNQ-doped and fluorinated Mn-MOF-74 are found to exhibit consistent changes in the Raman modes of the hosting network, which can be attributed to oxidation of the linker. The methodology for the identification of charge states of MOFs’ linkers and molecular dopants by means of resonance Raman spectroscopy provides the basis for a better understanding of physical properties of doped MOFs such as enhanced conductivity.

Figure 1

Schematics for DCTC–-doped and fluorinated Mn-MOF-74.

Results and Discussion

The UV–visible absorption spectra of as-prepared nanocrystalline Mn-MOF-74, TCNQ-infiltrated Mn-MOF-74, and neat TCNQ films are presented in Figure . X-ray diffraction analysis confirms the Mn-MOF-74 structure unchanged by the infiltration. See Figure S1 in the Supporting Information. The spectrum of TCNQ shows intense absorption bands at 356, 423, and 603 nm. The bands at 356 and 423 nm can be of TCNQ0, while the band at 603 nm can be assigned to the TCNQ– dimer.[38] The spectrum of Mn-MOF-74 (Figure ) exhibits a strong absorption band at 417 nm. In turn, the spectrum of TCNQ-doped Mn-MOF-74 exhibits bands at 424, 480, and 500 nm. Whereas the broad band absorption in the region around 424 nm can be a superposition of the band of Mn-MOF-74 at 417 nm and those of neutral TCNQ, the emerging absorption bands at 480 and 500 nm indicate that TCNQ acts as an electron acceptor in the TCNQ-doped Mn-MOF-74 system. According to the literature, trianion TCNQ3– was reported to exhibit absorption bands in this wavelength region.[39] Another candidate species is dicyano-p-toluoyl cyanide (DCTC–), an oxidation product of TCNQ2– (Scheme ), exhibiting a broad absorption around 480 nm.[40,41] Note that in the previous report,[25] TCNQ-doped Co-MOF-74 nanocrystals in toluene showed an intense absorption response at 660 nm that was attributed to TCNQ2– of disproportionated TCNQ– dimer. In our case, no such absorption is observed in the respective wavelength range.

Figure 2

UV–visible absorption spectra of as-prepared Mn-MOF-74, TCNQ-infiltrated Mn-MOF-74, XeF2-altered Mn-MOF-74, and neat TCNQ. Recorded for films on glass slides.

Scheme 1

Reduction of TCNQ to TCNQ2– Followed by Oxidation to DCTC–

Raman spectra of TCNQ-doped Mn-MOF-74 recorded at excitation wavelengths λexc = 458, 488, 514.5, 568, and 633 nm are plotted in Figure (see the full-range spectra in Figure S8 in the Supporting Information) together with those of Mn-MOF-74 and TCNQ recorded at λexc = 633 nm. TCNQ has four intense Raman modes at 1207, 1455, 1602, and 2227 cm–1, which can be assigned to C–H bending (in-plane-deformation), exocyclic C=C stretching, ring C=C stretching, and C≡N stretching modes,[42] respectively. The two most intense bands of Mn-MOF-74 located at 1275 and 1404 cm–1 can be assigned to the C–O stretching[29,36,43] and O–C–O symmetric stretching[29,36,43−45] modes of the linker (2,5-dihydroxyterephthalic acid), respectively. In addition, three bands are discerned at 1499, 1556, and 1613 cm–1. The band at 1499 cm–1 belongs to the O–C–O asymmetric stretching mode of the carboxylate group,[36,43,45] and the bands at 1556 and 1613 cm–1 belong to the benzene ring-stretching vibrations of the linker.[43,45] All these modes of TCNQ and Mn-MOF-74 are invisible or largely shifted in the spectra of TCNQ-doped Mn-MOF-74. The band located in the range 1183–1193 cm–1, depending on the laser wavelength, can be assigned to the C–H bending mode of TCNQ[42] red-shifted from 1207 cm–1 by 10–20 cm–1, which is toward the frequency range typical of C–H bending in a benzenoid ring.[46] Multiple bands are observed at frequencies marked by the vertical bars in the range 1600–1650 cm–1 (for peak analysis, see Figures S12–S16 in the Supporting Information). These bands can be associated with the C=C ring-stretching modes of TCNQ[42] blue-shifted and split into two components. Note that these modes overlap with the benzene ring-stretching modes of the linker in Mn-MOF-74[43,45] observed at 1613 cm–1.

Figure 3

Raman spectra of TCNQ-doped Mn-MOF-74 recorded with excitation wavelengths λexc = 458, 488, 514.5, 568, and 633 nm. The spectra of as-prepared Mn-MOF-74 and TCNQ recorded with λexc = 633 nm are presented for comparison.

In the frequency range between the C–O stretching and O–C–O symmetric stretching of Mn-MOF-74, 1275–1404 cm–1, multiple modes are distinguished at various frequencies (marked by the vertical bars in Figure ) depending largely on the laser wavelength (for peak analysis, see Figure S17–S21 in the Supporting Information). Two possible candidates for the origin of these bands are a blue-shifted C–O stretching of the linker and a red-shifted exocyclic C=C stretching mode of TCNQ. The lowest-frequency mode around 1285 cm–1 could be of the former. This point will be discussed later in conjunction with results for fluorinated Mn-MOF-74. The exocyclic C=C stretching mode of TCNQ, located at 1454 cm–1 for TCNQ0, is known to be susceptible to the charge state of TCNQ. Hence, all these modes in the range 1275–1404 cm–1 can be attributed to the exocyclic C=C stretching mode of TCNQ molecules at different charge states. The resonance enhancement in Raman spectroscopy is so strong that even a small amount of minor species can be observed as major Raman bands when the laser energy matches their absorption band. The major absorption bands unique to TCNQ-doped Mn-MOF-74, not observed for neat TCNQ and nondoped Mn-MOF-74, are those located at 480 and 500 nm, attributed to DCTC– or trianion TCNQ3– (Figure ). This means that the Raman spectra measured at λexc = 458, 488, and 514.5 nm represent the TCNQ encapsulated in the MOF. They all exhibit two major Raman bands within the range of interest: one at about 1285 cm–1 and the other at about 1330 cm–1. First of all, they do not correspond to those of TCNQ3–[39] or A1g Raman modes of any TCNQ species in D4 symmetry.[47,48] According to the literature,[40,49−51] these peaks can be assigned to nonsymmetric B1g modes of DCTC–, the oxidation product of TCNQ2–. The red-shifted C–H bending mode is also characteristic of DCTC–. As observed in Figure , the ring C=C stretching mode of DCTC– at about 1640 cm–1, the higher component of the doublet, is resonance enhanced at λexc = 458, 488, and 514.5 nm. Note that the exocyclic C=C stretching mode can also be red-shifted to frequencies as low as 1300 cm–1 because of the chemisorption of TCNQ onto a metal atom or cluster.[52] One or two of TCNQ’s four nitrogen atoms bound to a metal leads to a fractional charge transfer smaller than one electron per TCNQ. However, a red shift of the C≡N stretching mode because of chemisorption is not observed in the present study.[52] Hence, it is more likely that neutral TCNQ0 is reduced to TCNQ2– because of host–guest charge transfer, then the TCNQ2– is oxidized to DCTC– by molecular oxygen present in the framework.

The presence of DCTC– means that electrons are extracted from the framework. However, it is challenging to distinguish Raman modes of the linker because of the strong optical resonance of the TCNQ species as well as spectral overlaps and frequency shifts as results of the charge transfer. In order to discuss the charge state of the linker that is crucial for understanding electrical conduction mechanisms in doped MOFs, we employ another acceptor molecule, XeF2, as an oxidative fluorination agent.[37,53] X-ray diffraction analysis confirms that the Mn-MOF-74 structure is unchanged after exposure to XeF2 vapors. See Figure S1 in the Supporting Information.

Figure shows the X-ray photoemission spectra of Mn-MOF-74 after exposure to XeF2 vapors measured in the F 1s region (left) and the Mn 2p region (right). The Mn 2p spectral shape and binding energy are consistent with the presence of Mn(II).[54−56] Fluorine is present in two chemical states with F 1s binding energies of 685.0 and 687.9 eV. The dominating (84%) lower binding energy component belongs to fluorine bound to manganese, while the less-intense (16%) higher binding energy component belongs to fluorine bound covalently to carbon (see the structure model in Figure right). The atomic concentration ratio, calculated from intensities of the low binding energy component in the F 1s region and the Mn 2p spectrum, is F/Mn = 1.0. This indicates that all manganese ions in the framework are fluorinated.

Figure 4

X-ray photoemission spectra of XeF2-altered Mn-MOF-74 in the F 1s region (left) and the Mn 2p region (right). The dashed curves are the Shirley background profiles. The two Gaussian–Lorentzian product components, peak 1 and peak 2, are plotted in the F 1s region.

The Raman spectra of Mn-MOF-74 before and after exposure to XeF2 vapors are presented in Figure (for full-range spectra at λexc = 514.5 and 633 nm, see Figure S10 in the Supporting Information). No Raman modes of gaseous XeF2[37,57,58] are observed, indicating that XeF2 has reacted to fluorinate the framework. We observe three characteristic changes upon the fluorination: (1) The C–O stretching mode[29,36,43] is blue-shifted from 1275 to 1283 cm–1 by +8 cm–1. (2) A band emerges at 1448 cm–1. (3) A broad intense band at 1608 cm–1 appears and overlaps the band of benzene ring stretching vibrations[43] at 1613 cm–1. It was reported that an oxidative chlorination of Mn-MOF-74 (or M2DOBDC) with C6H5ICl2 yielded an oxidized material Cl2M2DOBDC.[59] According to magnetic measurements, X-ray absorption, and infrared spectroscopic data, it was proposed that the benzene-like linker of a formal oxidation state −4 is oxidized to a quinone-like linker of a formal oxidation state −2 through the chlorination of the open-metal sites, while the manganese ions maintain a formal oxidation state of +2. In our case, the X-ray photoelectron spectroscopy analysis indicates that all manganese ions are divalent and fluorinated. The emergent Raman band at 1608 cm–1 in Figure can be attributed to Raman-active bands of the quinone-like linker as a result of the oxidation (for peak analysis, see Figures S12–S16 in the Supporting Information).

Figure 5

Raman spectra of as-prepared Mn-MOF-74 and XeF2-altered Mn-MOF-74, recorded with λexc = 633 nm, and p-benzoquinone recorded with λexc = 514.5 nm.

Based on the above knowledge obtained for the oxidized framework, we now re-evaluate the resonance Raman data of the TCNQ@Mn-MOF-74 in Figure . The intense peak of the TCNQ@Mn-MOF-74 located at 1605 cm–1 at λexc = 633 nm (off resonance to DCTC–) can be attributed to the oxidized quinone-like linker. The C–O stretching mode of the linker is hardly visible because of the overlap with the intense DCTC– modes. The peak analysis reveals a mode in the range 1284–1285 cm–1 (see Figures S20–S21 in the Supporting Information) that intensifies as λexc goes from 568 to 633 nm toward the off resonance to DCTC–, that could be of the C–O stretching mode shifted by ca. 10 cm–1 from 1275 cm–1 for the neutral TCNQ. This blue shift is slightly larger than +8 cm–1 observed upon the fluorination, indicating a charge transfer achieved by the infiltration of the framework by TCNQ.

Conclusions

To summarize, Mn-MOF-74 has been doped with TCNQ. The resonance Raman spectroscopy has revealed the presence of DCTC–, an oxidation product of TCNQ2–, inside the framework, that can be understood as a result of strong host–guest charge transfer. The emergent quinone-like Raman mode at 1608 cm–1 and the blue-shifted C–O stretching mode at 1283 cm–1 can be associated with the quinone ring of the oxidized linker, that is inline with the Raman response of the TCNQ@Mn-MOF-74 off the resonance to the DCTC– at λexc = 633 nm. This work demonstrates that for the identification of the local charge density in doped MOFs, the optical resonance effect in Raman spectroscopy needs to be taken into account, and halogenated MOFs exhibiting no resonance effect can be a good reference material. Both the resonance Raman spectroscopy and halogenated MOFs are elemental components for the study of charge transfer in molecule-doped MOFs.