Localization of Guest Molecules in Nanopores by ... - ACS Publications

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Cite This: J. Phys. Chem. C 2018, 122, 5376−5384

Localization of Guest Molecules in Nanopores by Pulsed EPR Spectroscopy Andrey Pivtsov, Martin Wessig, Viktoriia Klovak,† Sebastian Polarz, and Malte Drescher* University of Konstanz, Universitätsstraße 10, 78464 Konstanz, Germany S Supporting Information *

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-2,2,6,6-tetramethylpiperidine-1-oxyl (4-amino-TEMPO) and 4-carboxy-TEMPO in combination with a deuteration strategy. Applying a complementary set of different pulsed electron paramagnetic resonance methods, we obtained information about the dimensionality of the spatial distribution and local concentration via double electron−electron resonance experiments, orientation of the guest molecules with respect to the pore walls via electron spin echo envelope modulation spectroscopy, and about the distance between guest molecules and pore walls via electron nuclear double resonance spectroscopy. This allowed localizing the guest molecules and shows that their spatial distribution in nanopores strongly depends on their polarity.

1. INTRODUCTION Mesoporous materials (pore diameters between 2 and 50 nm) have many applications in different chemical technological processes,1 as for example, in molecular sieves,2−5 chromatography,6−9 catalysis,10−16 and for drug delivery and biomedical applications.17−19 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 their inner surface, and well-known methods for introduction 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 UKON2a11,24,27−29 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 NMR spectroscopy has been used.30−36 2Hsolid-state NMR spectroscopy was used for localization of benzene-d6 in mesoporous silica SBA-1530,31 and in mesoporous cellular foams31 at different temperatures. Distances and relative orientations between pyridine-15N and pyridine-4-d1 guest molecules and the inner surfaces of MCM-41 and SBA-15 materials were measured by different methods of solid-state NMR spectroscopy.32 The distribution of the guest molecules of the ionic liquid [C2Py][BTA]-d10 inside MCM-41 and SBA-15 © 2018 American Chemical Society

Scheme 1. (a) Structure of a Bridging Phenyl Entity with Carboxy Functional Group Located on the Inner Surface of the Pores of UKON2a; Structures of Nitroxide Radicals 4Carboxy- 2,2,6,6-Tetramethylpiperidine-1-oxyl (4-carboxyTEMPO) (b) and 4-Amino-TEMPO (c) Used as Guest Molecules

was estimated using 2H and 19F solid-state NMR methods.36 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 solidstate NMR methods and quantum chemical calculations was used.33 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 Received: October 31, 2017 Revised: February 13, 2018 Published: February 13, 2018 5376

DOI: 10.1021/acs.jpcc.7b10758 J. Phys. Chem. C 2018, 122, 5376−5384

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The Journal of Physical Chemistry C

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 (Temperature Controller TC-H02; Magnettech GmbH). In a series of temperature-dependent experiments, the temperature was continuously decreased starting from 293 K down to 103 K. To avoid EPR line shape 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 that of 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 3P-ESEEM and DEER experiments, a split-ring Bruker resonator ER 4118 X-MS3 was used. All of these experiments were performed at 30 K, but Mims-ENDOR experiments were performed at 10 K to increase T1, relaxation time. For the description of pulse sequences and the magnetic fields, see the Section S2, Supporting Information. 2.3. EPR Data Analysis. DEER data were analyzed using DEER Analysis 2016.55 The local spin concentration of a sample in UKON2a was determined by analyzing the DEER decays (eq 1) of this sample and of the corresponding sample in bulk ethanol with known homogeneous spin concentration. In the case of homogeneously distributed electron spins in space with the dimensionality d, the DEER signal decays by the following exponential law38,56

dynamics and localization of different guest molecules in MCM41, SBA-15, and zeolites studied by solid-state NMR.34 In summary, the localization with solid-state NMR spectroscopy is feasible for guest molecules and mesoporous materials with simple structures but still challenging due to the NMRactive environment.37 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 molecules 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 electron.39 Continuous wave (CW) EPR spectroscopy in combination with paramagnetically labeled guest molecules has been applied40−46 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 labeled 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 guest molecules 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 nanoscale. Applying amongst others three-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 labeled guest molecules we used 4-carboxy-TEMPO and 4-amino-TEMPO (Scheme 1b,c, respectively) dissolved in ethanol. We used different polar groups because 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 materials.24

⎛ 8π 2 2 ⎞ γ ℏpC·t d /3⎟ V (t ) = V (0) exp⎜ − ⎝ 9 3 ⎠

(1)

where p is the number of spins excited by the pump pulse, C is the spin concentration, γ is the gyromagnetic ratio, and ℏ is Planck’s constant. The time t = 0 corresponds to the case when the position of the pump pulse coincides with the position of the first echo in the DEER pulse pattern. V(0) is the DEER signal at t = 0. Spectral and ENDOR data were simulated using Easyspin.57 Spectral simulations were performed using the pepper function. For the simulation of the ENDOR spectra, we assumed an hfi tensor38

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. UKON2a material was synthesized as described in the literature.29 For its characterization, see Section S1, Supporting Information. For ESEEM and ENDOR experiments, we used nondeuterated 4-amino-TEMPO and 4-carboxy-TEMPO (SigmaAldrich) dissolved in ethanol-d6. For other EPR experiments, we used nondeuterated guest molecules and nondeuterated ethanol. For additional ENDOR experiments, we used fully deuterated 4amino-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. The

Aij = A isoδij + 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 = −

3xixj ⎞ gegNβeβN ⎛ ⎜δij − 2 ⎟ 3 ⎝ R R ⎠

(3)

Here, ge is the g factor of free electron, gN is the g factor of the nucleus, βe and βN are the Bohr magneton and the nuclear magneton, respectively, and xi,j are Cartesian components of the vector R⃗ connecting electron and nuclear spin, where R is the distance between the electron and the nuclear spin. In the 5377


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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 eq 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 eq 1 and DEER Analysis 2016. The insets schematically depict distributions of the guest molecules inside a pore.

diagonal form, the principal values of the tensor Dij can be

R∼

expressed as ⎧ g g ββ ⎪ DXX = DYY = − e N 3e N ⎪ R ⎨ gegNβeβN ⎪ ⎪ DZZ = 2 ⎩ R3

5.405 (R in Å, DZZ in MHz) DZZ

3

(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 S11 are obtained upon subtraction of these monoexponential decay functions. 3PESEEM spectra were acquired by subtraction of these exponential decays and then fast Fourier transformations.

(4)

From eq 4, the distance R can be determined as 5378


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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 radio frequency offset corresponds to the Larmor frequency of protons.

3. RESULTS AND DISCUSSION UKON2a was obtained as a highly ordered material with cylindrical, hexagonally aligned pores. A rather homogeneous pore-size distribution of D p ∼ 6 nm was determined independently via N2 physisorption and transmission electron microscopy (Figure S5). 3.1. Rotational Mobility. We analyzed temperaturedependent 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 (Tmp ∼ 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. For 4-amino-TEMPO in ethanol within UKON2a, we observe for the entire temperature range between Tmp and room temperature, EPR spectra showing that 4-amino-TEMPO contains a larger fraction of immobilized guest molecules in UKON2 compared to that in 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,c was explained by confinement conditions inside the pores.24 Using CW EPR spectroscopy, we obtained information about the rotational dynamics of the guest molecules both in the 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. To determine local concentration of the guest molecules inside the pores of UKON2a, we measured the distance-dependent electronic dipole−dipole interaction between the guest molecules via DEER spectroscopy.53 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 nonaveraged 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 that in a 1 mM bulk solution. This distribution is schematically depicted in the inset on Figure 1a. Analogous DEER experiments were performed for 4-aminoTEMPO. Figure 1b shows the DEER decays of 4-aminoTEMPO 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 1 mM 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 5379


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Figure 3. Mims-ENDOR spectra of 4-amino-TEMPO-d17 (see the inset) in ethanol-d6 inside UKON2a (black) upon subtraction of the corresponding spectrum in bulk solution and the simulation (red). Zero radio frequency 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).

for fitting the experimental data (root mean square, rms ∼ 0.00148) resulted in significant improvement with respect to d = 2 (rms ∼ 0.003216) or d = 3 (rms ∼ 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 angstrom thick.59 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. 3.3. Distance from the Pore Wall. To localize the guest molecules inside the pores, we prepared an isotope contrast between the protonated UKON2a pore walls and ethanol-d6 as solvent. This enabled us to determine the distances between the guest molecules and the pore wall by exploiting the distancedependent 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-carboxyTEMPO 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 (νH ∼ 14.9 MHz at 3500 G) corresponding to zero radio frequency offset. These peaks can be allocated to the hfi with protons of the nitroxide. The ENDOR spectrum in UKON2a features almost the same line shape and small difference only in the ENDOR intensities around 1HLarmor 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 experiment.58 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-amino-TEMPO. To eliminate these peaks, additional experiments were performed using 4-aminoTEMPO-d17 with deuterated methyl groups (whereas the amino protons are still present; see the inset in Figure 3). The Davies-ENDOR spectrum of 4-amino-TEMPO-d17 in bulk ethanol-d6 and the corresponding spectral simulation is shown in Figure S8. To increase the signal-to-noise ratio of the Davies-ENDOR spectrum of this solution inside UKON2a despite unfavorable electron spin relaxation times (T1 ∼ 120 μs and T2 ∼ 250 ns at T = 10 K), we performed Mims-ENDOR with 4-amino-TEMPOd17 in bulk ethanol-d6 and in ethanol-d6 inside UKON2a (Figure S9). 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 analyzing 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 the 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 orientations of the hfi tensor corresponding to the minimal possible distance Rmin (see 5380


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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). (b) 3P-ESEEM spectra of 4amino-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.

eq 5). This tensor features AZZ = 5.25 MHz, which corresponds to a distance between the electron spin of the labeled 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 ±0.3 Å interval (Figure S13). The agreement between our simple model and the experimental ENDOR spectra suggests that 4-amino-TEMPOd17 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 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. To determine the relative orientation of the guest molecules with respect to that of 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 that of 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 to high frequencies,38,50 as observed for protons. Figure 4a (4b) shows the 3P-ESEEM spectra of 4-carboxyTEMPO (4-amino-TEMPO) in bulk ethanol-d6 (black) and in ethanol-d6 inside UKON2a (red). All spectra (time domain data are shown in Figure S11) can be described as doublets centered at 2.28 MHz (Larmor frequency of 2H at 3487 G) and splitted by the value of the quadrupole interaction of 2H nuclei.50 For 4carboxy-TEMPO, the 3P-ESEEM spectra in bulk ethanol as well as in UKON2a have similar line shapes (Figure 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 4carboxy-TEMPO is uniformly distributed in the pores. 5381


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Scheme 2. Proposed Model of the Spatial Distribution of the Guest Molecules in Cross Sections of the Pores of UKON2aa

a

(a) 4-Carboxy-TEMPO molecules are homogeneously distributed inside the pores of UKON2a without defined orientations relative to the pore walls. (b) 4-Amino-TEMPO molecules are localized close to the pore walls. The brown ring depicts the area with localized electron spins of the guest molecules that are oriented with the NO bond toward the pore walls.

mesoporous materials. Our results are in full agreement with indirect conclusions in this work analyzing the host−guest interaction of similar molecules in terms of rotational mobility using CW EPR.24 In summary, we demonstrated the use of a combination of pulsed EPR methods for precise localization of spin-labeled guest molecules in nanopores 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.

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 that 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 center 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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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b10758. Characterization of UKON2a (Section S1); pulse sequences (Section S2); ENDOR simulation (Section S3); echo-detected (ED) EPR spectra of 4-carboxyTEMPO and spectral simulation (Figure S4); porosity of UKON2a (Figure S5); CW EPR spectra of 4-carboxyTEMPO and 4-amino-TEMPO (Figure S6); DaviesENDOR spectra of 4-amino-TEMPO (Figure S7); Davies-ENDOR spectrum of 4-amino-TEMPO-d17 (Figure S8); Mims-ENDOR spectra of 4-amino-TEMPO-d17 (Figure S9); ED EPR spectrum of 4-amino-TEMPO-d17 and spectral simulation (Figure S10); 3P-ESEEM decays of 4-carboxy-TEMPO and 4-amino-TEMPO (Figure S11); Mims-ENDOR simulations (Figure S12); MimsENDOR simulations at the different distances (Figure S13); a cylindrical pore of UKON2a (Scheme S14) (PDF)

4. CONCLUSIONS In this study, we aimed for precise localization of paramagnetically labeled guest molecules in nanopores. Therefore, we applied different methods of pulsed EPR spectroscopy to investigate 4-amino-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 that 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 suggests a minimum distance of ∼3.6 Å. Combining this information, we conclude that the unpaired electrons of the guest molecules are 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 toward 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

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sebastian Polarz: 0000-0003-1651-4906 Malte Drescher: 0000-0002-3571-3452 Present Address †

National University of Kyiv, Volodymyrska street 60, 01601 Kyiv, Ukraine (V.K.). 5382


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The Journal of Physical Chemistry C Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the DFG within the SPP 1570 is gratefully acknowledged. REFERENCES

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