Characterisation of redox states of metal–organic frameworks by growth on modified thin-film electrodes

Two different SURMOF films have been grown on a transparent conducting surface for spectro-electrochemical characterisation of radical species.


Experimental Section
Instrumentation. Electrochemical measurements were made using an Eco Chemie Autolab PGSTAT20 potentiostat. All solutions were purged with a stream of Ar prior to use. Cyclic voltammetry was performed using a three-electrode system, with a Pt wire secondary electrode and a saturated calomel reference electrode. For solution based cyclic voltammetry a glassy carbon working electrode was used and before each measurement the electrode was cleaned UV-visible spectroelectrochemical measurements on Et4L 1 and Et8L 2 were carried out using an optically transparent electrode mounted in a modified quartz cuvette with an optical path length of 0.5 mm. A three-electrode configuration consisting a Pt/Rh gauze working electrode, a Pt wire secondary electrode (in a fritted PTFE sleeve) and a saturated calomel electrode, chemically isolated from the test solution via a bridge containing electrolyte solution and terminated in a porous frit, was used in the cell. The potential at the working electrode was controlled by a Sycopel Scientific Ltd DD10M potentiostat. The UV-visible spectra were recorded on a Perkin Elmer Lambda 16 spectrophotometer. The spectrometer cavity was purged with N2 and temperature control at the sample was achieved by flowing cooled N2 across the surface of the cell. UV-visible spectroelectrochemical experiments on thin films of MOFs were carried out by using an Ocean Optics Jaz spectrometer equipped with tungsten and S3 deuterium light sources. X-band EPR spectra were recorded on a Bruker EMX spectrometer.
The simulations of the EPR spectra were performed using the Bruker WINEPR SimFonia package. Powder X-ray diffraction patterns were collected on a Panalytical diffractometer using Cu-Kα radiation (λ = 1.5418 Å) in reflection mode.
AFM measurements were recorded on films grown on the ITO surface by loading the MOFmodified-substrate into an Asylum Research Cypher-S AFM. Repulsive-mode amplitudemodulated AFM images were obtained using Olympus AC240-TS AFM cantilevers (Asylum research). AFM images were processed using the Gwyddion software package.

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Scheme S1: Synthesis of Et4L 1 and H4L 1 To a stirred solution of (3,5-dibromophenyl) boronic acid (10.0 g, 36 mmol) in toluene (170 mL) was added 1,8-diaminonapthalene (6.6 g, 42 mmol). The solution was heated to 100 o C for 1 h and the solvent removed under reduced pressure to give the crude product as a brown solid. The solid was dissolved in a minimum volume of boiling CH2Cl2, and the crude product precipitated by addition of petroleum ether (b.p 60-80 °C). The suspension was allowed to cool to room temperature, and the solid was separated by filtration and dried (80 °C) to obtain pure product as a bright yellow solid (11.3 g, 78%). Spectroscopic analysis and purity of the compound were in accordance to those found in literature. S2

Synthesis of 4-ethoxycarbonylphenyl boronic acid, 2
To a stirred solution of 4-carboxyphenylboronic acid (15.0 g, 90 mmol) in EtOH (375 mL) was added concentrated H2SO4 (4.5 mL). The solution was heated at reflux for 19 h and the solution concentrated under reduced pressure until a precipitate formed. An excess of water was added to the suspension, which was collected by filtration. The solid product was washed with water until the filtrate was pH 7, and then dried (80 °C) to give the product as a fine white powder (15.4 g, 88%).

4,4''-dicarboxylate diethyl ester, 3
To a stirring, degassed suspension of 1 (6.0 g, 15 mmol), 2 (8.13 g, 41 mmol) and K2CO3 (4.34 g, 44 mmol) in toluene (400 mL) and water (100 mL) at 60 °C was added tri-tert-butyl phosphine (1M in toluene, 2.4 mL, 2.4 mmol) and tris(dibenzylideneacetone)dipalladium (0) (1.34 g, 1.5 mmol). The reaction mixture was heated to 80 °C for 1.5 hours, and the resulting S6 suspension filtered while hot. The solid was extracted with CH2Cl2 (200 mL) and the organic phase washed with water (2 x 200 mL), dried over MgSO4 and filtered. The solvent was removed under reduced pressure and the resulting black oil was dissolved in a minimum volume of CH2Cl2 and the solution passed through a silica plug using ethyl acetate as eluent.
The filtrate was evaporated to dryness under reduced pressure to give a dark brown solid. The solid was dissolved in a minimum volume of boiling CH2Cl2, and precipitated by addition of petroleum ether (b.p. 60-80 o C). The product was separated by filtration and dried (80 °C) to obtain the pure product as a yellow/orange solid (5.98 g, 74%

Synthesis of 4,4''-bis(ethoxycarbonyl)-[1,1':3',1''-terphenyl]-5'-yl)boronic acid, 4
To a stirred solution of 3 (6.35 g, 11.8 mmol) in tetrahydrofuran (360 mL) was added H2SO4 (2M, 73 mL) and the solution heated under reflux for 4 h. The reaction mixture formed a suspension over this time, and this was filtered while hot to remove the solid impurities. The filtrate was reduced under reduced pressure until a precipitate formed. This suspension was re-solubilised by heating to 70 °C, and an excess of water was added to precipitate a solid.
The mixture was filtered and the solid collected and washed with a large volume of water until the filtrate was at pH = 7. The product was dried (80 °C) to give an off white solid (

Functionalisation of conducting surface.
Indium tin oxide (ITO) coated glasses and carbon paper was treated before each experiment by ultrasonic cleaning in ultrapure H2O for 15 mins, followed by ultrasonication for 30 min in

Cyclic Voltammetry (CV) and Bulk Electrolysis of MOF Films.
For all electrochemical studies, MOF films were synthesized from 10 dipping cycles. For cyclic voltammetry, the MOF films grown on carbon paper (1mm x 1mm) were used as working electrodes due to the superior conductivity of carbon paper over an ITO-coated glass.
Conversely, the ITO-based films were used as working electrodes for spectroelectrochemical studies to exploit the transparent nature of the ITO-coated glass. Prior to electrochemical experiments the MOF films were immersed in CH2Cl2 for 3-4 days, with solvent replaced every 24h. The films were then dried in air and re-immersed in the electrolyte solution of 0.4 M of in CH2Cl2 for 2-3 days to allow CH2Cl2 and electrolyte to fully exchange with S16 the other guests such as DMF trapped in the MOF pores. This acts as an "electrochemical conditioning" period.
We found that it is necessary to use thinner films (10 cycles of dipping) in order to record the voltammetric response. This can be explained by considering ion transfer through MOF pores.
The distance an ion can travel/diffuse during an electrochemical process is usually proportional to (Dt) 0.5 (where D = diffusion coefficient, t = time). S3 Therefore it is necessary for films to be thinner than the diffusion layer so that electrolyte can diffuse readily through the pores of the MOF to the electrode surface.

CV Scan Rate (ν) Dependence of Peak Currents (ip) and Separation (ΔEp) in MFM-186.
CV measurements of on MFM-186 films on carbon paper were carried out at a series of scan rates (ν = 2, 3, 4, 5, 6, 8, 10 mV/s). The resulting data are plotted in Figure S8. To understand the dependence of ip over scan rate a Ln(ip) vs Ln(scan rate) plot can be used.
ip α ν n where ν = scan rate; 1 ≥ n ≥ 0.5 Hence, Therefore, the slope for the data plotted as Ln(ip) vs Ln(ν) can indicate the order of the dependence with respect to scan rate. For a diffusion limited process, the slope of this plot should approach 0.5, while for a surface confined process the slope should be 1.

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For MFM-186 film the slope tends to be closer to 1 for slow scan rates although this deviates from ideal behaviour at the higher scan rate. Such behaviour is not expected from a surface confined species. This phenomenon has been observed in related literature and is explained by taking into account mass transport limitations and slower diffusion of electrolyte through the micro-porous channel. S3 The MOF modified electrode can be considered as a working electrode, modified by non-conducting crystals having isolated redox active species. However, due to the microporous MOF structure ions can diffuse through the crystals. Hence electrochemical process can be written as: where A-is the counter ion of an electrolyte solution. The sign "⊂" indicates supramolecular assembly involving host and guest (i.e. Guest ⊂ Host). If electron transfer is slower compared to diffusion of counter ion A -, the rate of electron transfer becomes the rate determining step, while the rate of diffusion of Awill not have any influence on the cyclic voltammogram. A similar situation can be seen at the low scan rate. However, when the rate of diffusion of Ais slower (or not significantly fast compare to electron transfer), the rate of diffusion of Abegins to influence the cyclic voltammogram. S18 Figure S8: Cyclic voltammetry of MFM-186 film at various scan rates (a) and related analyses (b-d). Dependence of ΔE and ip on scan rate strongly indicates that the electrochemical oxidation process of MFM-186 is not diffusion limited. This can be verified further by plotting Ln(i) vs Ln(scan rate); the slope is much higher than 0.5, as would be expected for a classical diffusion limited process. See preceding discussion for details. S19 Figure S9: Multiple cycles in the cyclic voltammetric scan for MFM-186 films on carbon paper at scan rate 2 mV s -1 . The Broken line represents 1 st scan while the solid line represents 3 rd scan.

EPR spectroscopy of [Et4L 1 ] •+
The experimental EPR spectrum of [Et4L 1 ] •+ can be reasonably reproduced by simulation using hyperfine couplings to 3 sets of hydrogen nuclei (see parameters in Fig. S10). This suggests that the unpaired electron is localised mainly on the anthracene core, a result consistent with DFT analysis, although the smallest hyperfine couplings may indicate that some electron density extends to the adjacent phenyl rings.