Localization of Guest Molecules in Nanopores by Pulsed EPR

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Localization of Guest Molecules in Nanopores by Pulsed EPR Spectroscopy Andrey Pivtsov, Martin Wessig, Viktoriia Klovak, Sebastian Polarz, and Malte Drescher J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10758 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 28, 2018

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The Journal of Physical Chemistry

Localization of Guest Molecules in Nanopores by Pulsed EPR Spectroscopy Andrey Pivtsov1, Martin Wessig1, Viktoriia Klovak1,2, Sebastian Polarz1 and Malte Drescher1* 1

University of Konstanz, Universitätsstraße 10, 78464 Konstanz, Germany

2

present address: National University of Kyiv, Volodymyrska street 60, 01601 Kyiv, Ukraine

ABSTRACT: The localization of guest molecules at the molecular scale in mesoporous host materials is crucial for applications in heterogeneous catalysis, chromatography, drug delivery, and in different biomedical applications. Here, we present for the first time the precise localization of different guest molecules inside the mesoporous organosilica material UKON2a with a pore size of 6 nm. We exploited paramagnetic probe molecules 4-Amino-TEMPO and 4-CarboxyTEMPO in combination with a deuteration strategy. Applying a complementary set of different pulsed EPR methods, we obtained information about the dimensionality of the spatial distribution and local concentration via DEER experiments, orientation of the guest molecules with respect to the pore walls via ESEEM spectroscopy, and about the distance between guest molecules and pore walls via ENDOR spectroscopy.

This allowed localizing the guest molecules and shows that

their spatial distribution in nanopores strongly depends on their polarity.

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1. INTRODUCTION Mesoporous materials (pore diameters between 2 and 50 nm) have many applications in different chemical technological processes1, as for example molecular sieves2-5, in chromatography6-9, in catalysis10-16, for drug delivery and biomedical applications17-19. In these processes, the spatial distribution of molecular species within the nanopores plays an important role. However, the localization in situ with resolution at the molecular scale (~0.1-1 nm) is still challenging. Periodically ordered mesoporous (PMO) organosilica materials20-23 prepared using sol−gel precursors with a bridging organic group ((R′O)3Si-R-Si(OR′)3) feature well-defined pore structure, large size of its inner surface and well-known methods for introducing of different functional groups inside the pores. UKON materials11,24-29 are PMO materials containing phenyl entities enabling the incorporation of functional groups. For example, the inner surface of UKON2a 11,24,27-29 contains –COOH functional groups (Scheme 1a) with a surface density27 of 1.2 groups/nm2. For the localization of guest molecules inside mesoporous materials, solid-state nuclear magnetic resonance (NMR) spectroscopy has been used30-36. 2H-solid-state NMR spectroscopy was used for localization of benzene-d6 in mesoporous silica SBA-1530,31 and in MCF31 at different temperatures. Distances and relative orientations between pyridine-15N and pyridine-4d1 guest molecules and the inner surfaces of MCM-41 and SBA-15 materials were measured by


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different methods of solid-state NMR spectroscopy32. The distribution of the guest molecules of the ionic liquid [C2Py][BTA]-d10 inside MCM-41 and SBA-15 was estimated using 2H and 19F solid-state NMR methods36. For the investigation of the interaction of small guest molecules such as water, benzene and pyridine with the pore surface of different mesoporous carbon materials a combination of different solid-state NMR methods and quantum chemical calculations was used33. Moreover, the combination of solid-state NMR and molecular dynamic simulations35 allowed to determine the distributions of the molecules of isobutyric acid and water in SBA-15. Buntkowsky et al. gave a nice overview describing dynamics and localization of different guest molecules in MCM-41, SBA-15 and zeolites studied by solid-state NMR34.

Scheme 1. (a) The structure of a bridging phenyl entity with carboxy functional group located on the inner surface of the pores of UKON2a. The structures of nitroxide radicals 4-CarboxyTEMPO (b) and 4-Amino-TEMPO (c) used as guest molecules. In summary, the localization with solid-state NMR spectroscopy is feasible for guest molecules und mesoporous materials with simple structures but still challenging due to the NMR-active environment37.


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Also electron paramagnetic resonance (EPR) spectroscopy38 can be applied for the investigation of guest molecules inside mesoporous materials. As compared with NMR methods, EPR spectroscopy is virtually ''background-free'' as long as the porous hosts and solvents are not EPR active, which is usually the case. The use of nitroxide based paramagnetic probe moelcules is advantageous owing to their stability, small size (~0.7-0.9 Å) of the nitroxide moiety and rather localized spin density of the EPR active unpaired electron39. Continuous wave (CW) EPR spectroscopy in combination with paramagnetically labelled guest molecules has been applied4046

for the investigation of dynamics of the guest molecules inside MCM-41 by Okazaki et. al.

Also, CW EPR has been applied24 for investigation of dynamics of spin labelled guest molecules inside UKON2a showing that the host-guest interactions in UKON materials strongly depend on the interplay of the polarities of solvent, guest molecules and the surface functionalization. However, CW EPR does not provide direct access to the spatial distribution of the guestmolecules inside the pores. EPR imaging has been used47-49 to localize paramagnetically guest molecules inside nanoporous organosilica on a micro meter scale. In this work we demonstrate how a combination of different sophisticated pulsed methods of EPR spectroscopy can be used to precisely localize guest molecules inside nanopores on the nano-scale. Applying amongst others 3-pulse electron spin echo envelope modulation (ESEEM)50, electron nuclear double resonance (Mims- and Davies-ENDOR)51,52 , and double electron-electron resonance (DEER)53 spectroscopy, we precisely localize guest molecules in UKON2a pores with a resolution in the sub-nanometer range. We investigated the localization of two different guest molecules in UKON2a. As paramagnetically labelled guest molecules we used 4-Carboxy-TEMPO and 4-Amino-TEMPO (Scheme 1b and 1c, respectively) dissolved in ethanol. We used different polar groups because


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we had recently shown that the host-guest interaction in UKON materials strongly depends on polarities of solvent, guest molecule, and functional groups of the mesoporous materials24.

2. EXPERIMENTAL SECTION 2.1 Sample preparation. UKON2a material was synthesized as described in the literature29. For its characterization, see supporting information (S1). For ESEEM and ENDOR experiments we used non-deuterated 4-Amino-TEMPO and 4Carboxy-TEMPO (Sigma-Aldrich) dissolved in ethanol-d6. For other EPR experiments, we used non-deuterated guest molecules and non-deuterated ethanol. For additional ENDOR experiments we used fully deuterated 4-Amino-TEMPO-d17 (D, ≥98 %) dissolved in ethanol-d6 (D, ≥99.5 %) (both Sigma-Aldrich). For sample preparations for the EPR experiments, UKON2a (~8 mg) powder was placed into a quartz flask connected with a vacuum pump. Then the material was degassed during several hours at ~5∙10-2 mbar to remove adsorbed water out of the pores. All guest molecules were dissolved in ethanol or ethanol-d6 with a concentration of 1 mM. The solutions were added into the flask containing UKON2a and were infiltrated overnight. Next day the supernatants were removed and the porous materials were washed three times with ethanol or ethanol-d6 to remove adsorbed nitroxides from the outer surface of UKON2a. After the washing procedure the samples were dried and transferred into quartz tubes (Bruker) with 3 mm outer diameter. 2.2 EPR experiments. X-band CW EPR measurements were performed using a Miniscope spectrometer (MS200, Magnettech GmbH) equipped with a variable temperature unit


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(Temperature Controller TC-H02, Magnettech GmbH). In series of temperature dependent experiments, the temperature was continuously decreased starting from 293 K down to 103 K. In order to avoid EPR lineshape distortions and saturation of the samples by strong microwave power, the microwave power of 0.1 mW was used. The modulation amplitude was 1 G. For pulsed EPR experiments at cryogenic temperatures, the samples were shock frozen in liquid nitrogen and then quickly inserted into the cold cryostat at a temperature below the ethanol glass transition (Tg~95 K)54. Pulsed EPR experiments were implemented on an X-band Bruker ELEXSYS E580 spectrometer equipped with an Oxford Instruments CF 935 cryostat. For ENDOR experiments a dielectric Bruker resonator ER 4118 X-MD4 was used, whereas for 3PESEEM and DEER experiments a split-ring Bruker resonator ER 4118 X-MS3 was used. All these experiments were performed at 30 K, but Mims-ENDOR experiments were at 10 K to increase T1-relaxation time. For the description of pulse sequences and the magnetic fields, see supporting information (S2). 2.3 EPR data analysis. DEER data were analysed using DEER Analysis 201655. The local spin concentration of a sample in UKON2a was determined by analysing the DEER decays (eqn. (1)) of this sample and of the corresponding sample in bulk ethanol with known homogeneous spin concentration. In case of homogeneously distributed electron spins in space with the dimensionality d , the DEER signal decays by the following exponential law38,56:  8 2 2  V (t )  V (0) exp    hpC  t d /3  ,  9 3 

(1)

where p - is the number of spins excited by the pump pulse, C - is the spin concentration,  is gyromagnetic ratio and h - is Planck constant. The time t=0 corresponds to the case, when the


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position of the pump pulse coincides with the position of the first echo in DEER pulse pattern.

V (0) - is the DEER signal at t=0. Spectral and ENDOR data were simulated using Easyspin57. Spectral simulations were performed using the pepper function. For the simulation of the ENDOR spectra, we assumed an hfi tensor 38: Aij  Aiso ij  Dij ,

(2)

where Aiso is the isotropic and Dij is the anisotropic hfi component. The distance between electron and nuclear spins can be determined38,58 via the anisotropic part of the corresponding hfi tensor:

Dij  

ge g N e  N R3

3 xi x j    ij  2 R 

 . 

(3)

Here, g e - is the g-factor of free electron, g N - is the g-factor of the nucleus,  e and  N - are the Bohr magneton and the nuclear magneton, respectively; xi , j - are Cartesian components of r the vector R connecting electron and nuclear spin, where R is the distance between the electron

and the nuclear spin. In the diagonal form the principal values of the tensor Dij can be expressed as: g g   DXX  DYY   e N 3 e N   R .  g g   e N e N D  2 ZZ   R3

(4)

From eqn. (4) the distance R can be determined as:


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R~

o 5.405 ( R in A , DZZ in MHz) . 3 D ZZ

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(5)

3P-ESEEM signals are superimposed by T1-relaxation. Therefore, the global maxima were normalized to 1 for each sample individually, and the data were fitted using monoexponential decay functions. The data shown in figure S-11 are upon subtraction of these monoexponential decay functions. 3P-ESEEM spectra were acquired by subtraction of these exponential decays and then fast Fourier transformations.

3. RESULTS AND DISCUSSION UKON2a was obtained as a highly ordered material with cylindrical, hexagonally aligned pores. A rather homogeneous pore-size distribution around D p ~ 6 nm was determined independently via N2-physisorption and transmission electron microscopy (Figure S5). 3.1 Rotational mobility. We analysed temperature dependent CW EPR spectra (see Figure S6) in full analogy to ref. 24 to monitor the rotational diffusion of the guest molecules in bulk ethanol and in ethanol inside UKON2a from 293 K down to 103 K. The EPR spectra report on the rotational mobility of the guest molecules. We find powder pattern like EPR spectra below the melting point of ethanol (Tm.p.~159 K). In the liquid phase we observe for 4-Carboxy-TEMPO in UKON2a a superposition of a fast spectral component as for 4-Carboxy-TEMPO in bulk ethanol at room temperature and a slow spectral component. The corresponding fractions vary depending on temperature. These two components can be allocated to guest molecules with and without interactions with the pore walls.


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For 4-Amino-TEMPO in ethanol within UKON2a we observe for the entire temperature range between Tm.p. and room temperature EPR spectra showing that 4-Amino-TEMPO contains a larger fraction of immobilized guest molecules in UKON2 compared to 4-Carboxy-TEMPO. Therefore, we conclude that 4-Amino-TEMPO undergoes strong interactions with the pore walls. The longer rotation correlation time of the guest molecules inside UKON2a as compared with the free solutions on Figure S6a and S6c was explained by confinement conditions inside the pores24. Using CW EPR spectroscopy, we obtained information about the rotational dynamics of the guest molecules both in case of bulk solution and inside UKON2a. One can speculate that immobilization occurs due to adsorption at the walls but it is worth mentioning that the data on rotational mobility is if at all an indirect way of localization. Therefore, we applied different methods of pulsed EPR spectroscopy for direct and precise localization of the guest molecules within the pores. Using a variety of different pulse EPR techniques and a systematic deuteration strategy, we established an approach to localize the paramagnetic guest molecules within the nanopores on a molecular scale. 3.2 Local concentration. The local concentration of guest molecules in porous media is crucial for the characterization of their spatial distribution. In order to determine local concentration of the guest molecules inside the pores of UKON2a we measured the distancedependent electronic dipole-dipole interaction between the guest molecules via DEER spectroscopy53. Figure 1a shows the DEER decays of 4-Carboxy-TEMPO in bulk ethanol (black line) and in ethanol in UKON2a (red line). The oscillations visible in the experimental data are non-


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averaged proton modulations. A simulated decay (green line) takes into account a homogeneous distribution in space with d  3 . Because the DEER decays do not show significant differences with respect to each other, and the data can be described by a model assuming a homogeneous spatial distribution with d  3 , we conclude that the guest molecules are uniformly distributed inside the pores of UKON2a and feature the same intermolecular distance distribution as in a 1 mM bulk solution. This distribution is schematically depicted in the inset on Figure 1a. Analogous DEER experiments were performed for 4-Amino-TEMPO. Figure 1b shows the DEER decays of 4-Amino-TEMPO in bulk ethanol (black line) and in ethanol in UKON2a (red line). These curves were simulated (green lines), where for bulk ethanol a homogeneous spatial distribution with d  3 was used, and the simulation of the DEER decay in UKON2a revealed a local concentration C ~ 4.2 mM (compared to 1mM of the stock solution). . The DEER experiment is sensitive to both relative spatial locations of the guest molecules and the dimensionality d of the space occupied by them. Using d=2.4 for fitting the experimental data (r.m.s. ~0.00148) resulted in significant improvement with respect to d=2 (r.m.s. ~0.003216) or d=3 (r.m.s. ~0.005206). The increase of the local concentration inside UKON2a suggests an inhomogeneous distribution of 4-Amino-TEMPO in the pore volume, most likely due to interactions with the pore walls. From the reduced dimensionality d ~ 2.4 we concluded that the detected spins are neither distributed in the entire pore volume nor completely confined to an ideal cylindrical inner surface but rather occupy a shell of space a few Å thick59. The inset on Figure 1b schematically depicts an inhomogeneous distribution of the same number of the guest molecules as in the inset on Figure 1a inside the pore.


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Figure 1. (a) DEER decays for 4-Carboxy-TEMPO dissolved in bulk ethanol (black) and in ethanol inside UKON2a (red). The latter was simulated (green) using eqn. (1) and DEER Analysis 2016. (b) DEER decays for 4-Amino-TEMPO dissolved in bulk ethanol (black) and in ethanol inside UKON2a (red) and corresponding simulations (green) using eqn. (1) and DEER Analysis 2016. The insets schematically depict distributions of the guest molecules inside a pore. 3.3 Distance from the pore wall. In order to localize the guest molecules inside the pores, we prepared an isotope-contrast between the protonated UKON2a pore walls and ethanold6 as solvent. This enabled us to determine the distances between the guest molecules and the pore wall by exploiting the distance dependent hfi between the electron spins and the proton nuclear spins inside the pore wall. The hfi was determined by ENDOR spectroscopy. Figure 2 shows 1H-Davies-ENDOR spectra of 4-Carboxy-TEMPO dissolved in bulk ethanol-d6 (black) and in ethanol-d6 inside UKON2a (red). The ENDOR spectrum in bulk ethanol consists of two main peaks symmetrically located with respect to the Larmor frequency of protons ( H ~ 14.9 MHz at 3500 G) corresponding to zero radiofrequency offset. These peaks can be allocated to the hfi with protons of the nitroxide. The ENDOR spectrum in UKON2a features almost the same lineshape and small difference only in the ENDOR intensities around 1

H-Larmor frequency. This difference can be explained by the fact that some guest molecules

(albeit only a small number) are located close to the walls at sub nanometer distances, so that a small hfi is detected in the ENDOR experiment58.


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Figure 2. 1H-Davies-ENDOR spectra of 4-Carboxy-TEMPO in bulk ethanol-d6 (black) and in ethanol-d6 inside UKON2a (red). Zero radiofrequency offset corresponds to the Larmor frequency of protons. For 4-Amino-TEMPO we performed similar Davies-ENDOR experiments (Figure S7) and observed strong differences between the spectra in bulk ethanol-d6 and in ethanol-d6 inside UKON2a in the region close to the 1H-Larmor frequency that can be attributed to hfi with protons in the pore walls. However, the analysis of this hfi is hampered by the presence of the peaks at frequencies ±2 MHz (as in Figure 2) allocated to the hfi with the protons of 4-AminoTEMPO. In order to eliminate these peaks, additional experiments were performed using 4Amino-TEMPO-d17 with deuterated methyl groups (while the amino protons are still present, see inset Figure 3).


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The Davies-ENDOR spectrum of 4-Amino-TEMPO-d17 in bulk ethanol-d6 and the corresponding spectral simulation is shown in Figure S8. In order to increase the signal to noise ratio of the Davies-ENDOR spectrum of this solution inside UKON2a despite from unfavourable electron spin relaxation times (T1~120 µs, T2~250 ns at T = 10 K) we performed Mims-ENDOR with 4-Amino-TEMPO-d17 in bulk ethanol-d6 and in ethanol-d6 inside UKON2a (Figure S9). In order to get rid of the hfi with 14N (below 12 MHz) and the hfi due to protons in the –NH2 group of the nitroxide, we present the difference of the ENDOR spectra in UKON2a and the bulk solution, respectively (Figure 3). This difference can be allocated to the hyperfine interactions between the nitroxide electron spin and protons within the pore walls. We assume that there is a comparable H-D-exchange between the amino group and the deuterated ethanol in both environments (i.e. within UKON2a and in bulk ethanol), so analysing the difference spectrum in Figure 3 corrects for intramolecular interactions. However, this intramolecular contribution is small (see Figure S9).

The ENDOR spectrum in Figure 3 can be interpreted as a superposition of many individual ENDOR spectra corresponding to couplings to individual protons in the pore wall (see Scheme 1a), where each coupling corresponds to an individual hfi tensor. A corresponding simulation is shown in Figure 3 (red). The principal values Axx = Ayy = 0.22 MHz of the hyperfine tensor describe the maxima of the ENDOR spectrum close to the 1HLarmor frequency. The AZZ -values derived from our model (see Supporting Information) assuming a homogeneous distribution of protons on the inner surface of the pore wall describe the width of the spectrum. The vertical arrows show the positions of the X, Y and Z canonical


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orientations of the hfi-tensor corresponding to the minimal possible distance Rmin (see eqn. (5)). This tensor features AZZ  5.25 MHz, which corresponds to a distance between the electron spin of the labelled guest molecule and the pore wall of Rmin ~ 3.6 Å. From simulations with varying Rmin we estimated the accuracy of Rmin being within a +/- 0.3Å interval (Figure S-13).

Figure 3. Mims-ENDOR spectra of 4-Amino-TEMPO-d17 (see inset) in ethanol-d6 inside UKON2a (black) upon subtraction of the corresponding spectrum in bulk solution and the simulation (red). Zero radiofrequency offset corresponds to the Larmor frequency of protons. The vertical arrows show the positions of the X, Y and Z canonical orientations of an hfi-tensor corresponding to the closest distance (Rmin~3.6 Å). The agreement between our simple model and the experimental ENDOR spectra suggests that 4-Amino-TEMPO-d17 molecules are localized close to the pore wall in such a way that the distance between the unpaired electron spin of the nitroxide moiety and the pore wall amount to


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3.6 Å. Therefore, we concluded that 4-Amino-TEMPO is adsorbed at the pore walls of UKON2a. 3.4 Orientation of the guest molecules with respect to the pore walls. In order to determine the relative orientation of the guest molecules with respect to the pore walls we used 3P-ESEEM spectroscopy to measure the hfi between the unpaired electrons of the guest molecules and the 2H-nuclei of ethanol-d6. ESEEM features a better sensitivity than ENDOR in the low-frequency range (2H-Larmor frequency is ~2.29 MHz at 3500 G), where the modulation depths are more pronounced as compared with high frequencies38,50 as observed for protons. Figure 4a (4b) shows the 3P-ESEEM spectra of 4-Carboxy-TEMPO (4-Amino-TEMPO) in bulk ethanol-d6 (black) and in ethanol-d6 inside UKON2a (red). All spectra (time domain data is shown in Figure S-11) can be described as doublets centred at 2.28 MHz (Larmor frequency of 2

H at 3487 G) and splitted by the value of the quadrupole interaction of 2H nuclei50. For 4-

Carboxy-TEMPO the 3P-ESEEM spectra in bulk ethanol as well as in UKON2a have similar lineshapes (Fig. 4a). This suggests that 4-Carboxy-TEMPO is rather distributed completely within the deuterated solvent than interacting with the protonated pore walls. This corresponds to our findings regarding above that 4-Carboxy-TEMPO is uniformly distributed in the pores.


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Figure 4. (a) 3P-ESEEM spectra of 4-Carboxy-TEMPO in bulk ethanol-d6 (black) and in ethanol-d6 inside UKON2a (red). (c) 3P-ESEEM spectra of 4-Amino-TEMPO in bulk ethanol-d6 (black) and in ethanol-d6 inside UKON2a (red curve). The vertical dashed line shows the position of 2H-Larmor frequency, ESEEM decays are shown in Figure S11. Comparing the experimental results for 4-Amino-TEMPO (Figure 4b) in bulk ethanol-d6 and in ethanol-d6 inside UKON2a, we find that the 3P-ESEEM spectra for 4-Amino-TEMPO inside UKON2a has much smaller intensity. This suggests that a significant fraction of electron spins do not fully contribute to the detected interaction with the deuterated solvent. If the distance between the electron spin and the 2H nuclear spin is larger than 1 nm a much smaller ESEEM modulation is expected60 than for the distances of 3-5 Å. A possible explanation is, that NO-bonds of a significant number of 4-Amino-TEMPO molecules are localized close to the walls and their -NH2 groups are oriented into the centre of the pores (size of the piperidine nitroxides is ~8 Å) and the ethanol-d6 molecules are mainly localized farther from the surface, than the -NH2 group.


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Scheme 2. Proposed model of the spatial distribution of the guest molecules in cross sections of the pores of UKON2a. (a) 4-Carboxy-TEMPO molecules are homogeneously distributed inside the pores of UKON2a without defined orientations relative to the pore walls. (b) 4-AminoTEMPO molecules are localized close to the pore walls. The brown ring depicts the area with localized electron spins of the guest molecules, which are oriented with the NO-bond towards to the pore walls.

4. CONCLUSION In this study we aimed for precise localization of paramagnetically labelled guest molecules in nanopores. Therefore, we applied different methods of pulsed EPR spectroscopy to investigate 4Amino-TEMPO and 4-Carboxy-TEMPO in UKON2a. The results are summarized in Scheme 2. For 4-Carboxy-TEMPO in UKON2a we found a local concentration determined by DEER which corresponds to the averaged concentration of the solution which was used for infiltration. Full access of the solvent to the NO-bond was shown by ESEEM, and no significant hfi with the pore walls was detected by ENDOR. Therefore, we concluded that 4-CarboxyTEMPO molecules were homogeneously distributed in the entire pore volume as depicted in Scheme 2a. For 4-Amino-TEMPO the local concentration inside UKON2a is increased by a factor of 4.2 indicating an inhomogeneous distribution of the guest molecules in the pores. The analysis of the hyperfine interaction to the protonated pore walls suggest a minimum distance of ~3.6 Å. Combining this information we conclude that the unpaired electrons of the guest molecules are


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localized in a spherical shell with a thickness of ~3 Å as depicted in Scheme 2b. The indicated orientation of the guest molecules, i.e. the NO-bond pointing towards the pore surface is deduced from the analysis of the hyperfine interaction with the deuterated solvent. The precise localizations of all guest molecules show that the spatial distribution in nanopores strongly depends on polarities of solvent, guest molecule, and functional groups of the mesoporous materials. Our results are in full agreement with indirect conclusions in this work analysing the host-guest interaction of similar molecules in terms of rotational mobility using CW EPR24. In summary, we demonstrated the use of a combination of pulsed EPR methods for precise localization of spin-labeled guest molecules in nano-pores for the first time. This opens the avenue for systematic studies of the localization of guest molecules in nanopores, e.g. for optimization of porous media for applications in catalysis or chromatography.

ASSOCIATED CONTENT Supporting Information. S1: Characterization of UKON2a. S2: Pulse sequences. S3: ENDOR simulation. Figure S4: ED EPR spectra of 4-Carboxy-TEMPO and spectral simulation. Figure S5: Porosity of UKON2a. Figure S6: CW EPR spectra of 4-Carboxy-TEMPO and 4-AminoTEMPO. Figure S7: Davies-ENDOR spectra of 4-Amino-TEMPO. Figure S8: Davies-ENDOR spectrum of 4-Amino-TEMPO-d17. Figure S9: Mims-ENDOR spectra of 4-Amino-TEMPO-d17. Figure S10: ED EPR spectrum of 4-Amino-TEMPO-d17 and spectral simulation. Figure S11: 3PESEEM decays of 4-Carboxy-TEMPO and 4-Amino-TEMPO. Figure S12: Mims-ENDOR


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Localization of Guest Molecules in Nanopores by Pulsed EPR

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