Polarity profiling of porous architectures: solvatochromic dye encapsulation in metal–organic frameworks

Metal–organic frameworks (MOFs) have gathered significant interest due to their tunable porosity leading to diverse potential applications. In this study, we investigate the incorporation of the fluorosolvatochromic dye 2-butyl-5,6-dimethoxyisoindoline-1,3-dione ( 
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 Phth) into various MOF structures as a means to assess the polarity of these porous materials. As a purely inorganic compound, zeolite Y was tested for comparison. The fluorosolvatochromic behavior of Phth, which manifests as changes in its emission spectra in response to solvent polarity, provides a sensitive probe for characterizing the local environment within the MOF pores. Through systematic variation of the MOF frameworks, we demonstrate the feasibility of using (fluoro-)solvatochromic dyes as probes for assessing the polarity gradients within MOF structures. Additionally, the fluorosolvatochromic response was studied as a function of loading amount. Our findings not only offer insights into the interplay between MOF architecture and guest molecule interactions but also present a promising approach for the rational design and classification of porous materials based on their polarity properties.

Table S2.Results of the Le Bail fit of high-resolution synchrotron powder diffraction data of Phth@MIL-53(Al) (4) compared to the structural data of MIL-53(Al) ht.
Tables S3 to S4. Calculations of the composition of 1 to 10 via XPS.
Table S5.Ratio of Phth per formula unit of the PM in 1 to 10 as calculated from the XPS data.
Tables S6 to S7. Calculation of the composition of the dilution series of Phth@MOF-5 and Phth@MIL-68(Ga) via XPS.

References.
Electronic Supplementary Material (ESI) for Journal of Materials Chemistry C.This journal is © The Royal Society of Chemistry 2024           For MIL-53(Al), a change of the diffraction pattern was expected upon guest loading due to the known "breathing effect" of the host material.When trying to upload larger amounts of spiropyrans or spirooxazines as guest molecules, no additional peaks or changes in the diffraction patterns were observed.Since photochromic behavior of the respective dye was still observed, surface adsorption of the photoswitchable molecule as an amorphous film was assumed and confirmed by means of XPS measurements. 1,2In contrast, for Phth@MIL-53(Al) (4), a modulated diffraction pattern is found compared to MIL-53(Al) ht with minor amounts of solid (crystalline) Phth being present (Figure S5).After excluding these reflections, the unit cell and possible space group of 4 was determined with the program package Topas 3 and fitted with JANA2006 4 .The results are listed in Table S2 and

Table S2).
For 2, (Figure S3), additional reflections e.g. in the low angle region are the result of nonembedded Phth, which is assumed to arrange itself in an ordered fashion on the surface of the MOF host.This assumption is corroborated by the XPS measurements, where an additional broad feature is present at ~ 397 eV (see Figure S14).Even after heating the sample to high temperatures (150 °C), the additional reflections in the PXRD pattern remained.Table S1.Solvents used in this investigation (see Fig. S1) with their elution power  0 according to Snyder 4 and λmax of 2-butyl-5,6-dimethoxyisoindoline-1,3-dione (= Phth) dissolved in these solvents.Table S2.Results of the Le Bail fit of high-resolution synchrotron powder diffraction data of Phth@MIL-53(Al) (4) compared to the published structural data of MIL-53(Al) ht. 3,4th@MIL-53(Al) ( Composition.For compounds 1 to 10, XPS measurements were conducted to determine the composition of the Phth@PM hybrid systems (PM: porous material), i.e. the degree of guest loading.The calculations for the fits are listed in Tables S3 (1-5) and S4 (6-10), the results of these calculations are summarized in Table S5.In Tables S6 and S7 the respective calculations and results are given for the dilution series of Phth@MOF-5 and Phth@MIL-68(Ga).The fits for the XPS measurements of all compounds are presented in Figures S13 to S24.For compound 8, a significantly higher amount of Phth is found compared to the weights applied in the synthesis.As known from previous studies, 6 the guest molecules within UiO-66 prefer positions close to the [Zr6O4(OH)4] metal-nodes.This leads to shielding effects of the Zr-cations and thus a reduced XPS signal of these cations, which, on the other hand, leads to an overestimation of the guest molecule loading.

Figures
Figures S2 to S11.PXRD patterns of Phth@PM systems (1 -10) in comparison to measured or simulated patterns of the unloaded porous material (PM).

Figure S12 .
Figure S12.LeBail fit of 4. Figures S13 to S22.XPS spectra of the Phth@PM systems (1 -10) with fits of the characteristic core level and N 1s peak.

Figures
Figures S25 to S34.Emission and excitation spectra of the MOF host materials and Zeolite Y.
the Le Bail fit is shown in Figure S12.Guest-free MIL-53(Al) ht crystallizes in the space group Imma (no.74) with a unit cell volume of 1411.95Å 3 . 5Upon loading with Phth, the unit cell volume of Phth@MIL-53(Al) (4) increases to 1445.5 Å 3 .The symmetry (i.e.space group Imma, no.74) does not change, but the lattice parameters b and c (the open pores are aligned parallel to c) change significantly (cp.

Figure S25 .
Figure S25.Emission (top) and excitation (bottom) spectra of MOF-5 with respective excitation and emission wavelengths given within the figure.

Figure S26 .
Figure S26.Emission spectra of MIL-68(In) with respective excitation wavelengths given within the figure.

Figure S27 .
Figure S27.Emission spectra of MIL-68(Ga) with respective excitation wavelengths given within the figure.

Figure S28 .
Figure S28.Emission (top) and excitation (bottom) spectra of MIL-53(Al) with respective excitation and emission wavelengths given within the figure.

Figure S29 .
Figure S29.Emission (top) and excitation (bottom) spectra of MFM-300(Ga2) with respective excitation and emission wavelengths given within the figure.

Figure S30 .
Figure S30.Emission (top) and excitation (bottom) spectra of UoC-2(1F) with respective excitation and emission wavelengths given within the figure.

Figure S31 .
Figure S31.Emission (top) and excitation (bottom) spectra of UoC-2(2F) with respective excitation and emission wavelengths given within the figure.

Figure S32 .
Figure S32.Emission (top) and excitation (bottom) spectra of UiO-66 with respective excitation and emission wavelengths given within the figure.

Figure S33 .
Figure S33.Emission (top) and excitation (bottom) spectra of ZIF-8 with respective excitation and emission wavelengths given within the figure.

Figure S34 .
Figure S34.Emission spectra of Zeolite Y with respective excitation wavelengths given within the figure.

Figure S35 .
Figure S35.Time evolution of the nearest-neighbor distances between Phth and bdc groups of the Phth1@MOF-5 host system analyzed based on the centroids of the aromatic units.The latter were determined as an average over the respective carbon atoms.Snapshots displaying the key configurations encountered along the DFTB MD simulations are marked S1, S2 and S3.The colors of the nearest-neighbor distances (bottom, purple, blue and green lines) reflect the distances between the inserted Phth to the similarly colored phenyl-rings of the bdc linkers (top, purple, blue and green colored phenyl rings).

Figure S36 .
Figure S36.Time evolution of the nearest-neighbor distances between Phth and bdc groups of the Phth3@MOF-5 host system analyzed based on the centroids of the aromatic units.The latter were determined as an average over the respective carbon atoms.Snapshots displaying the key configurations encountered along the DFTB MD simulations are marked S1 and S2.The colors of the nearest-neighbor distances (left, purple, blue, green and red lines) reflect the distances between the inserted Phth to the similarly colored phenyl-rings of the bdc linkers (right, purple, blue, green and red colored phenyl rings).

Figure S37 .
Figure S37.Time evolution of the nearest-neighbor distances between Phth and bdc groups of the Phth1@MIL-68(Ga) host system analyzed based on the centroids of the aromatic units.The latter were determined as an average over the respective carbon atoms.Snapshots along two viewing directions (bottom left: side view on the MOF channel; bottom right: front view on the MOF channel) displaying the key configuration encountered along the DFTB MD simulations are marked S1.The red color of the nearest-neighbor distances (top) reflects the distances between the inserted Phth to the also red colored phenyl-ring of the bdc linker (bottom).

Figure S38 .
Figure S38.Time evolution of the nearest-neighbor distances between Phth and bdc groups of the Phth3@MIL-68(Ga) host system analyzed based on the centroids of the aromatic units.The latter were determined as an average over the respective carbon atoms.Snapshots displaying the key configurations encountered along the DFTB MD simulations are marked S1 (S1 to S3 (bottom left): side view on the MOF channel; S1 (bottom right): front view on the MOF channel), S2 and S3.The colors of the nearest-neighbor distances (top) reflect the distances between the inserted Phth to the similarly colored phenyl-rings of the bdc linkers (bottom).

Table S4 .
Calculation of the composition of 6 to 10 via XPS (PM: porous material).

Table S5 .
Ratio of Phth per formula unit of the PM in 1 to 10 as calculated from the XPS data.

Table S6 .
Calculation of the composition of the dilution series of Phth@MOF-5 via XPS.

Table S7 .
Calculation of the composition of the dilution series of Phth@MIL-68(Ga) via XPS.