Porphyrin based metal-organic frameworks with record sensitivity in optical oxygen sensing

The optical oxygen sensing capabilities of the porphyrin-based metal-organic frameworks, PCN-224, Pt(II)PCN-224 and Pd(II)PCN-224 were investigated. The bimolecular quenching constants (kq) of 37000 (PCN-224), 6700 (Pd(II)PCN-224) and 3900 Pa-1s-1 (Pt(II)PCN-224) were found and reveal an exceptionally high oxygen-permeability for these materials. A fast gas transport within the network, large pore sizes, electronic and spatial isolation of the porphyrin indicator in the framework are held responsible for the unprecedentedly high kq values. PCN-224 shows 6.7 ns fluorescence lifetime and the fluorescence in air is quenched by 4.2-fold. The metal-organic frameworks based on phosphorescent Pt(II) and Pd(II) porphyrins possess significantly longer decay times of 18.6 and 390 µs, respectively, and are suited to detect oxygen in trace and ultra-trace ranges with limits of detection of 1 and 0.015 Pa, respectively. Apart from free-standing crystals, also metal-organic frameworks supported on different fibrous substrates (poly(acrylonitrile) nanofibers, glass fibres), and flat substrates (TLC silica-gel, poly(amide) filter) were prepared in order to provide oxygen sensor materials of practical use. Electrospun and thermally treated poly(acrylonitrile) nanofibers were proven to be particularly favourable and the resulting composite material exhibited the same sensitivity as the free crystals. All sensing materials show reversible cross-talk to humidity at levels up to 53 % relative humidity but demonstrate a drastic decrease of oxygen sensitivity at high humidity levels and when exposed to water.

S3 100 mg crude Pt(II)TMCPP were dissolved in 50 mL THF and 3 mL 1M aq. NaOH were added. The solution was stirred at 65 °C overnight and cooled to room temperature. After reaction completion indicated by TLC, the product was precipitated by dropwise addition of 1M HCl. The precipitate was separated by centrifugation and washed 2x with water. After drying at 65 °C in vacuum, platinum(II) 5,10,15,20-tetrakis-(4-carboxyphenyl)-porphyrin (Pt(II)TCPP) was obtained as a red powder (60 mg, 73%).
TMCPP: 1  Pd(II)TCPP was synthesized according to a reported procedure. 3 65 mg (82 µmol) of 5,10,15,20-tetrakis-(4-carboxyphenyl)-21,23H-porphyrin were dissolved in 4 mL DMF in a 10 mL microwave-suitable borosilicate vial. 60 mg (338µmol) of palladium dichloride was added, the reaction vessel was closed and heated in a synthesis reactor at 155 °C for 15 minutes. Completion of the reaction was controlled after cooling to room temperature by UV-VIS spectroscopy. The resulting solution was filtered to remove colloidal palladium, diluted with THF : diethyl ether (2:1 v/v), filtered and washed with 3x 20 mL water. All solvents were removed and the product was dried at 65°C in vacuum. 55 mg product (74.8 %) were obtained as a red powder. 1

Synthesis of poly(trimethylsilylpropyne) (PTMSP)
Polymerization of TMSP was carried out in a dry box as follows. 4 TaCl5 (0.5 g, 1.4 mmol) was dissolved in 0.1 L of toluene. The mixture was stirred for 30 min at room temperature. 12 g S4 (107 mmol) of TMSP were added to this catalyst solution. The mixture immediately turned dark brown and the solution solidified within 30 min. After 24 h the polymerization mixture was worked up by precipitation of the polymer in rapidly stirred 200 mL hot methanol. The polymer was washed several times with hot methanol and then dried to a constant weight. A yield of 95% (11.4 g) was achieved.

Analysis of the PXRD Measurements of MOF PCN-224 and Pt(II)PCN-224
Powder x-ray diffraction (PXRD) measurements were performed at the XRD1 beamline at the Elettra Synchrotron in Triest. This beamline has an operating wavelength of 1.4 Å with a beam size of 200 x 200 µm 2 . The data was collected on a stationary Dectris Pilatus 2M detector which was mounted 400 mm away from the sample. The sample itself consisted of the lab-synthesized MOF PCN-224 powder which was filled in a glass capillary (1.5 mm diameter and 0.02 mm wall thickness). All data were transformed to reciprocal space for analysis. All data conversion, treatment and analysis steps were performed with GIDVis. 5 Two samples were prepared and characterized: (i) Pt(II)PCN-224 and (ii) PCN-224.

Pt(II)PCN-224
The measured data for this system were compared to known structures for two PCN-224 based MOFs, one MOF consisting of Ni(II) metalated TCPP (Ni(II)TCPP) and the other based on the metal-free porphyrin. 6 The reported structures (see Fig. S1) are cubic and belong to space group Im-3m (no. 229). To mimic the Pt(II)PCN-224 MOF we replaced the Ni atom in the literature structure by Pt. In Fig. S2 one can see the PXRD pattern of Pt(II)PCN-224 as measured at the synchrotron. The peak positions of PCN-224 are overlaid as dashed black lines. One can see that the peak positions agree well for these two structures. This agreement also allows the indexation of corresponding peaks. Replacing the Ni atoms by Pt atoms in our model structure one can calculate new peak positions that agree with the ones of the measured data within the experimental accuracy, as expected (see Fig. S3). Please note that the measured data presented in Fig. S2 and S3 is the same, only the model structure, the measurements are compared to, changes.

S6
In the next step also the relative intensities of the peaks are compared between the model structures (Ni(II)PCN-224 , Pt(II)PCN-224) and the measured data. For this comparison we integrated the intensity over the entire PXRD pattern (shown in Fig. S2 and S3) and these data are then normalized to the maximum intensity. As a logical next step these data are compared to calculated PXRD patterns of the model structures. These calculations were performed using the Mercury software package. [7][8][9][10][11] The obtained results are shown in Fig. S4 and S5. One can see that for both model systems Ni(II)PCN-224 and Pt(II)PCN-224 the relative intensities of all peaks agree well with the relative intensities determined in the experiment. Two peaks with rather weak experimental intensities (013 and 123) better agree to the Pt(II)PCN-224 data which can be directly explained by the larger atomic number of Pt and the resulting larger scattering factor. These peaks are hardly detectable for the Ni containing system, but can be measured for the Pt system, which is what we see in the experiment. Based on these observations one can conclude that the newly synthesized Pt(II)PCN-224 MOF adopts the structure of Ni(II)PCN-224 , with Ni atoms being replaced by Pt atoms. Nevertheless, a refinement of the unit cell parameter (lattice vectors a=b=c) has been performed by using the program PowderCell. 12 The lattice parameter was varied and the resulting PXRD pattern was compared to the experimental one. This refinement procedure resulted in a lattice vector of 38.62 Å. For this lattice parameter we find a good agreement between experiment and model (see Fig. S6). Some structural information for this system is given in Table S1.

PCN-224:
The results for PCN-224 have been analyzed analogously to the steps described above for Pt(II)PCN-224. The synchrotron PXRD pattern is shown in Fig. S7 and the extracted line graph is presented in Fig. S8. By comparing peak positions and relative intensities between the measured data and the data calculated for the literature structure one can conclude that the synthesized MOF exhibits the S8 structure reported for PCN-224 in Ref [ 6 ]. Despite this good agreement it seems that all the experimental peaks are slightly shifted towards lower angles (see Fig. S8)which would correspond to larger unit cell parameters. As this is true for all observed peaks and as the system has cubic symmetry this could for example be rationalized by different temperatures of the measurement here and in Ref [ 6 ]. Isotropic volumetric expansion could lead to such shifts of the peaks. Refining the unit cell parameter based on the measured data by using the program PowderCell 12 and employing the same methodology as described above one can actually find a slightly enlarged lattice parameter of 38.61 Å. Nevertheless, we can safely conclude on the synthesized material exhibiting the literature reported structure, 6 with the refined lattice parameter of 38.61 Å. In Table S2 some structural information can be found for PCN-224.

Electrospinning of PAN micro/nanofibers
A 10 % w/w solution of PAN in DMF was electrospun using a spinneret with a diameter of 0.8 mm. The humidity level was kept at 35 % relative humidity throughout the electrospinning procedure and as a collector an aluminum foil covered square copper plate with an area of 16 cm² was used. Electrospinning was done using 25kV DC voltage for 15 minutes. The nonwoven fibrous material was collected as a white, stable fiber matt. The PAN fiber matt was easily removed from the substrate with a pair of tweezers.
The PAN micro/nanofibers were then cross-linked by heating them with a controlled temperature increase of 2 °C/min from room temperature to 280 °C. The fibers were kept at 280 °C for 2 h. After cross-linking, the PAN nanofibers were slightly darker and completely insoluble in DMF. PXRD of PCN-224 on grown on PAN fibers were compared to two samples PCN-224 to confirm the crystal structure of PCN-224 grown on the fibers (Fig. S17).

Immobilization of TMCPP in poly(1-trimethylsilyl-1-propyne) (PTMSP)
A stock solution of PTMSP with a concentration of 10 mg mL -1 in toluene was prepared. It was used to prepare the polymer "cocktail" containing 0.5 %w/w of TMCPP in respect to the polymer which was knife-coated (25 µm-thick wet layer) on a clean PET support.

Immobilization of TCPP on aluminum supported TLC silica gel
10 mg silica gel was stirred with 0.5 mg TCPP in 5mL THF for 20 minutes. Afterwards the silica gel was filtered and washed 3x with 5mL THF and 3x with 5mL acetone until no TCPP was found in the filtrate. The slightly colored silica gel was dried at 70°C under reduced pressure overnight. Calibrations were recorded as described.