An electrically conducting 3D coronene-based metal–organic framework

A novel cubic mesoporous metal–organic framework (MOF), consisting of hexahydroxy-cata-hexabenzocoronene (c-HBC) and FeIII ions is presented. The highly crystalline and porous MOF features broad optical absorption over the whole visible and near infrared spectral regions. An electrical conductivity of 10−4 S cm−1 was measured on a pressed pellet.


X-ray diffraction
X-ray diffraction (XRD) analyses were performed on a Bruker D8 diffractometer in Bragg-Brentano geometry with Ni-filtered Cu Kα (λ = 1.54060Å) radiation operating at 40 kV and 30 mA with a position-sensitive detector (LynxEye).PXRD measurements of Fe-HBC-MOF batches were conducted in transmission mode on a STOE Stadi MP diffractometer with a Cu Kα1 radiation source (λ = 1.54060Å) operating at 40 kV and 40 mA.The diffractometer was equipped with a DECTRIS MYTHEN 1 K solid-state strip detector.

Scanning electron microscopy (SEM)
SEM images were recorded on a FEI Helios NanoLab G3 UC electron microscope with an acceleration voltage of 2 kV.Prior to SEM analysis, the samples were coated with a thin carbon layer by carbon fibre flash evaporation in high vacuum.

Transmission electron microscopy (TEM)
TEM images were collected on a FEI Titan Themis 60-300 microscope at an acceleration voltage of 300 kV.Powder samples were prepared by crushing the particles with a razor blade and subsequently depositing the powder onto a copper grid supporting a thin electron transparent carbon film.

Nitrogen sorption
Adsorption and desorption measurements were performed on an Autosorb 1 (Quantachrome instruments, Florida, USA) with nitrogen of 99.9999% purity at 77.3 K.The samples were activated under high vacuum at 120 °C for at least 12 h.Evaluation of adsorption and desorption isotherms was carried out with the AsiQwin v.3.01 (Quantachrome instruments, Florida, USA) software.

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For BET calculations, pressure ranges of the nitrogen isotherms were chosen with the help of the BET assistant in the AsiQwin software.In accordance with the ISO recommendations, multipoint BET tags equal to or below the maximum in V x (1-p/p0) in the semilogarithmic plot were chosen.

Thermogravimetric analysis (TGA)
Thermogravimetric analysis of the samples was performed using a NETZSCH STA 449C with a heating rate of 10 K min -1 and a heating range up to 900 °C under a stream of synthetic air with a gas flow rate of 25 mL min -1 .

Elemental analysis (EA)
The elemental analysis of the samples was performed on a Vario MICRO cube instrument (Elementar Analysensysteme GmbH, Germany).

Electrical conductivity measurements
Two-probe measurements of crystalline pellets were carried out under inert conditions with a Metrohm Autolab PGStat302N with an in house-built dc-conductivity measurement cell by recording I-V curves between −3 to +3 V by an Autolab 302N.The distance between the electrodes is equivalent to the thickness of the pellet, which was measured to be 130 μm.Van der Pauw measurements were conducted at room temperature under ambient conditions using an ECOPIA Model HMS-5300 Hall measurement setup at room temperature.Gold contact electrodes were placed in a square geometry with distances of about 2.4 mm on a pressed pellet of the MOF samples.Pellet thicknesses were measured with a slide gauge to be 130 μm.

Preparation of Fe-HBC-MOF pellets
MOF pellets with 1 cm diameter (obtained from several synthesis batches) for electrical conductivity measurements were fabricated with 60 mg of the respective MOF bulk material hand-tightened with a standard Paul-Weber KBr Press.An exemplary PXRD pattern of Fe-HBC-MOF pellet measured is shown in Fig. S4.

Infrared spectroscopy (FTIR)
ATR-Fourier-transform infrared (FTIR) spectra of powders were recorded on a Bruker Vertex S5 UV-Vis spectroscopy UV-Vis spectra were recorded using a Perkin Elmer UV Vis/NIR Lambda 1050 spectrophotometer equipped with a 150 mm InGaAs integrating sphere.Diffuse reflectance spectra were collected with a Praying Mantis (Harrick) accessory and were referenced to barium sulphate powder as white standard.

Total hemispherical reflectance in geometry 8°/di
The total hemispherical reflectance was determined with an Agilent Cary 5000 double-beam spectrophotometer equipped with an external diffuse reflectance accessory eDRA.This system collects the total reflectance of diffuse reflecting specimens with an integration sphere of diameter d = 150 mm.The influx angle of the collimated radiation with respect to sample normal is 8°; the measurements described were performed in the gloss component included scheme.As the sample diameter was only 20 mm, the standard sample port of the sphere was reduced by a self-made BaSO4-coated port reducer to 15 mm diameter and the eDRA optics for small spot measurements was used, resulting in an oval spot of about 10 mm height and 4 mm width.As a low spectral reflection of the sample was expected, it was measured in a twostep process relative to a calibrated PTFE white standard.In a first step a dark grey reflection standard with about 8% reflectance was measured against the white standard.Then the sample reflectance was determined against the grey standard.In both steps, the reference port was equipped with a nominal 25% reflectance standard.The zero-line of measurement was determined with open port by letting the influx radiation diffuse into an approx.80 cm deep black cabinet with virtual zero back-reflection, shielded against the shaded laboratory environment.

Zero-field 57 Fe Mössbauer spectroscopy
Zero-field 57 Fe-Mössbauer spectra were recorded on a WissEl Mössbauer spectrometer (MRG-500) at a temperature of 77 K in constant acceleration mode. 57Co/Rh was used as -radiation source.WinNormos for Igor Pro software was used for the quantitative evaluation of the spectral parameters (least-squares fitting to Lorentzian peaks).The minimum experimental line widths were 0.21 mm s -1 (full width at half maximum, FWHM).The temperature of the sample was controlled by an MBBC-HE0106 Mössbauer He/N2 cryostat within an accuracy of +/-0.3K. Least-square fitting of the Lorentzian signals was carried out with the "Mfit" S6 software, developed by Dr. Eckhard Bill (MPI Chemical Energy Conversion, Mülheim/Ruhr).
The isomer shifts were reported relative to -iron reference at 300 K. [1] Electron paramagnetic resonance (EPR) spectroscopy EPR spectra were recorded on a JEOL continuous-wave spectrometer JES-FA200, equipped with an X-band Gunn diode oscillator bridge, a cylindric mode cavity, and a helium cryostat.
The samples were measured as a solid under nitrogen in quartz glass EPR tubes at 95, and 293 K.The spectra shown were measured using the following parameters: microwave frequency = 8.959 GHz, modulation amplitude 1.00 mT, microwave power 1.0 mW, modulation frequency 100 kHz, time constant of 0.1 s.Data analysis and simulation of the data was performed using the software eview and esim, written by Dr. Eckhard Bill (MPI Chemical Energy Conversion, Mülheim/Ruhr) [2] , based on a spin-Hamiltonian description of the electronic ground state: Here,  represents the total spin quantum number of the coupled system,  and / are the axial and rhombic zero-field parameters, respectively, and  is the g-matrix.Calculations are based on the S = 5/2 routines developed by Gaffney and Silverstone. [3]EPR line widths, W, are given in units of • 10 −4 cm −1 / GHz at full-width-half-maximum (FWHM).

X-ray photoelectron spectroscopy
XPS was performed using an ESCALAB 250 Xi instrument (Thermo Fisher, East Grinstead, UK) with monochromatized Al Kα (hν = 1486.6eV) radiation focused to a spot of 500 µm diameter at the surface of samples.Spectra were measured with pass energies of 200 eV for survey scans and 10 eV for high-resolution regions.Charging was compensated by the use of an internal electron flood gun.Peak fitting was performed by the software Avantage, version 5.9904 (Thermo Fisher) using a Shirley background ("Smart Shirley") and a convolution of Gaussian and Lorentzian functions for each signal component. [4]All spectra were referenced to remaining adventitious carbon at 284.8 eV.S7

Experimental General
All materials were purchased from Sigma Aldrich, Acros or TCI Europe in the common purities purum, puriss or reagent grade.The materials were used as received without additional purification and handled in air unless otherwise noted.
The water utilized in the synthesis was subjected to a Merck-Milipore Mili-Q purification system prior to use.

Synthesis procedures
2,3,10,11,18,19-hexahydroxy-cata-hexabenzocoronene (c-HBC) was synthesized according to a reported procedure. [5]2,3,10,11,18,19-hexamethoxy-cata-hexabenzocoronene (30 mg, 0.038 mmol) was dissolved in 3 mL of dry dichloromethane.The solution was cooled down to 0 °C and 3 mL of BBr3 (1M solution in dichloromethane) were added.Afterwards, the solution was allowed to warm up to room temperature and stirred under argon overnight.After the reaction was completed, water was added to quench the reaction and the solvent was partly removed using an argon flow.The resulting precipitate was filtered and finally dried under reduced pressure to give c-HBC as a light green solid in a quantitative yield.

General handling and stability tests
The crystallinity of the Fe-HBC-MOF was found to be dependent on the quality of the c-HBC linker and on the exposure time to air.We note that highly crystalline batches remain stable for at least one month under ambient conditions in a closed vessel.

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The C 1s spectra contain the prominent peak for the unsubstituted aromatic carbon atoms in the center of the catecholate ligand (component A).The shake-up feature for aromatic compounds (component E) is clearly visible.Spectral features of the substituted carbon atoms of the ligand occur at higher binding energies.The exact assignment is challenging due to the occurrence of solvent residues and ubiquitous carbon-containing surface contamination for samples handled under atmosphere, which contribute to similar binding energies as expected for the organic ligand.Due to the surface-sensitive character of XPS, this thin contamination layer contributes over-proportionally to the observed spectrum.

Fig. S2 13 C
Fig. S2 13 C NMR spectrum of c-HBC in acetone-d6 at room temperature.

Fig. S3
Fig. S3 PXRD pattern of a Fe-HBC-MOF sample, freshly prepared (black) and aged under ambient air in a closed container (red).

Fig. S5
Fig. S5 TEM image of the Fe-HBC-MOF, showing an interplanar distance of 2.5 nm, which is in good accordance with the 111 reflection in the PXRD (A), EDS spectrum of Fe-HBC-MOF showing the presence of Fe, O and C (B).

Fig. S7
Fig. S7 Images of the simulated model structure of the Fe-HBC-MOF viewed along the c-axis (A), display of the open supertetrahedron (B), view on the pore system (C) and the arrangement of c-HBC ligands in connected supertetrahedra units (D).

Fig. S6
Fig. S6 Indexed PXRD pattern of the Fe-HBC-MOF measured as powder at room temperature.

Fig. S9
Fig. S9Thermogravimetric analysis (TGA) of Fe-HBC-MOF under a stream of synthetic air.First mass loss observed at 110 °C is attributed to the loss of solvent molecules and structural degradation of the framework followed by a second mass loss observed at 300 °C attributed to the final framework decomposition, to give a metal oxide mass residue.

Fig. S11
Fig. S11 High-resolution XP spectra of the N 1s region of tetrabutylammonium nitrate (A) and Fe-HBC-MOF (B).The N 1s components in Fe-HBC-MOF correspond to nitrate (406.0 eV) and the quaternary N-atom of the tetrabutylammonium ion (402.0 eV).The signal components at 399.0 eV and 400.0 eV are unknown contaminations of tetrabutylammonium nitrate that do not occur in the MOF (B).The two components in the N 1s spectrum of Fe-HBC-MOF are attributed to residual DMF and NMP used in the synthesis.

Fig. S12
Fig. S12 High-resolution XP spectra of the C 1s and O 1s regions of the Fe-HBC-MOF.

Fig. S13
Fig. S13 Low magnification SEM image of truncated-octahedron crystallites of Fe-HBC-MOF showing the overall homogeneity of the MOF powder.

Fig. S15
Fig. S15 UV-Vis-NIR spectrum of the c-HBC ligand (A).Tauc plot of the c-HBC ligand (B), indicating a direct band gap of 2.7 eV.