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Urea-containing metal-organic frameworks for carbonyl compounds sensing
Accepted Manuscript Title: Urea-containing Metal-organic Frameworks for Carbonyl Compounds Sensing Authors: Alireza Azhdari Tehrani, Hamed Abbasi, Leili Esrafili, Ali Morsali PII: DOI: Reference:
S0925-4005(17)31878-6 https://doi.org/10.1016/j.snb.2017.09.211 SNB 23302
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
27-7-2017 28-9-2017 29-9-2017
Please cite this article as: Alireza Azhdari Tehrani, Hamed Abbasi, Leili Esrafili, Ali Morsali, Urea-containing Metal-organic Frameworks for Carbonyl Compounds Sensing, Sensors and Actuators B: Chemical https://doi.org/10.1016/j.snb.2017.09.211 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Urea-containing
Metal-organic
Frameworks
for
Carbonyl
Compounds Sensing
Alireza Azhdari Tehrani, Hamed Abbasi, Leili Esrafili, Ali Morsali*
Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iran. E-mail: [email protected]
AUTHOR INFORMATION Corresponding Author [email protected]
Graphical abstract
Two urea-containing MOFs, namely TMU-35 and TMU-36, were synthesized and characterized using different techniques. Since these compounds contain urea functional group, they can interact favorably with carbonyl compounds and thus they are potential candidates for carbonyl compounds sensing. This
result is interesting as it represents the first example of the application of functionalized MOFs for carbonyl compounds sensing.
Highlights
Two urea-containing MOFs were synthesized and characterized using X-ray crystallography and different spectroscopic techniques.
Since these compounds contain urea functional groups, they can interact favorably with carbonyl compounds.
These two frameworks gave selective responses when exposed to different carbonyl compounds.
This result is interesting as it represents the first example of the application of functionalized MOFs for carbonyl compounds sensing.
Abstract Two urea-containing MOFs, namely TMU-35 and TMU-36, were synthesized and characterized using Xray crystallography and different spectroscopic techniques. Since these compounds contain urea functional group, they can interact favorably through hydrogen bonding with carbonyl compounds and thus they are potential candidates for carbonyl compounds sensing. Our study reveals that, when the pillaring linker containing benzene core (TMU-35) is replaced by pillaring linker containing naphthalene core (TMU-36), fluorescence sensing ability towards carbonyl compounds is dramatically quenched. It can be concluded that in comparison with TMU-35, TMU-36 provides a more electron-rich and hydrophobic pore walls which can interact favorably with organic analytes. This result is interesting as it represents the first example of the application of functionalized MOFs for carbonyl compounds sensing.
Keywords: Metal-organic framework; urea-containing MOF; carbonyl compounds sensing
Introduction Metal-organic frameworks (MOFs) are a class of crystalline hybrid materials that can be assembled from a wide range of inorganic and organic building blocks.[1] The main advantage of MOFs over other porous materials is the ability to tailor their pore size, chemical functionality and even the topology of the framework by judicious selection of molecular building blocks.[2, 3] Owing to this feature, these materials are considered to be potential candidates for many emerging applications such as gas storage, catalysis, molecular sensing and biomedical.[4-9] Selective molecular recognition and detection of small molecules, cations and anions by MOFs has been recently subject of intense research, since it was proved that incorporating supramolecular recognition units into the MOF backbone has enabled the docking of specific analytes with high affinity and selectivity based on their size, shape and fit to the recognition site.[7, 10-12] Molecular recognition arises from non-covalent intermolecular interactions such as electrostatic, hydrogen bonding, π-π stacking, as well as hydrophobic effects between receptor and substrate.[13, 14] Therefore, it can be envisioned that MOFs containing functional groups could be used to discriminate different substrates with high efficiency and selectivity through complementary intermolecular interactions.[15] Urea functional group is known to have a strong tendency to form hydrogen bonds with the molecules containing hydrogen bond acceptor group(s).[16-21] This type of intermolecular hydrogen bonding has been well-documented and was the subject of many research studies in the field of catalysis and molecular sensing.[22-24] The idea of incorporating urea functional groups into a MOF structure to exclude the possibility of urea self-association has been suggested by Farha,
Hupp, Scheidt and their co-workers.[16] Soon after this report, the design, synthesis and utilization of MOFs containing urea, thiourea and squaramide groups has become a fascinating area of research for supramolecular chemists because of their well-established potentials of this family of hydrogen bond donating (HBD) functional groups for catalysis and sensing.[18-20, 25-29] As part of our research program aimed at evaluating the potential of MOFs for molecular sensing,[30, 31] we have recently reported the potential of urea-functionalized MOFs for nitro-substituted compounds recognition. In continuation of this interest, herein, we report the synthesis of two urea-containing MOFs, namely TMU-35 and TMU-36, which exhibit potential in sensing carbonyl compounds.
Results and Discussion Structural analysis of TMU-35 and TMU-36 [Zn(L1)(L2) 0.5]•DMF•2H2O (TMU-35) and [Zn(L1)(L3)0.5]•DMF•2H2O (TMU-36) were synthesized by combining 4,4'-(carbonylbis(azanediyl))dibenzoic acid (H2L1), N,N'-(1,4-phenylene)bis(1-(pyridin-4yl)methanimine) (L2) and N,N'-(naphthalene-1,5-diyl)bis(1-(pyridin-4-yl)methanimine) (L3) ligands, respectively, and Zn(NO3)2·6H2O using the solvothermal method at 90˚C for 72 h to give suitable X-ray quality crystals, Scheme 1. X-ray crystallography analyses reveal that TMU-35 and TMU-36 crystallize in monoclinic P21/c and P21 space groups, respectively, Figure S1. In these structures, the Zn atoms are in a distorted square pyramidal geometry, surrounded in basal-plane by four oxygen atoms from four different L1 ligands; the apical position is taken by pyridine nitrogen atom of L2 and L3 ligands for TMU-35 and TMU-36, respectively. Both compounds are based on binuclear paddle-wheel secondary building unit [Zn⋯Zn separation of 2.9422(6) Å for TMU-35 and 2.9347(6) Å for TMU-36] that acts as a structural node held in place by the L1 ligands to form a two-dimensional (2D) square grid. In the third dimension, the 2D square grids are linked to each other by pillaring L2/L3 ligands, Figures 1a and 1d. Topological analyses give rise to quadruply interpenetrated 3D pillared-layer frameworks with uninodal
six-connected α-Po primitive cubic (pcu) topology, Figures 1b and 1e. TMU-35 and TMU-36 are microporous framework possess channels along the crystallographic a-direction with pore window of 7.2 Å × 5.1 Å (the van der Waals radii of the atoms at opposite sites of the channel have been subtracted) and free void spaces per unit cell of 45.8% (1558.5 Å3) and 43.9% (1541.9 Å3) for TMU-35 and TMU-.36, respectively, Figures 1c and 1f.[32] Hirshfeld surface analysis carried out for the purpose of exploring and studying the nature of intermolecular interactions in the crystal structures of these two compounds.[33] The results showed that, when the L2 ligand has been replaced by L3 ligand, the contribution percentages of π-π stacking and C-H⋯π interactions increase while that of H⋯H and N⋯H interactions decreases. Selected contribution percentages of different non-covalent interactions to the Hirshfeld surface areas are shown as a histogram in Figure S2. In accordance with the Hirshfeld surface analysis, contact angle measurements show that the wettability of the frameworks [with contact angle (CA) left and CA right of 69.1˚ and 67.5˚ for TMU-35 and 89.2˚ and 88.6˚ for TMU-36, respectively] can be decreased by changing the pillaring linker from L2 to L3, Figure S3. On the basis of these results it can be concluded that compared to TMU-35, the TMU-36 provides a more hydrophobic and electron-rich pore walls that can interact favorably with organic analytes. It is to be noted that the more hydrophobic nature of linker L3 relative to L2 has been previously studied by some of us.[31, 34] Thermogravimetric analysis (TGA) exhibited two weight loss steps at around 100˚C and 158˚C for TMU35 and 104˚C and 165˚C for TMU-36 which corresponds to the loss of water and DMF guest molecules, respectively. The third weight loss starting at 265˚C (for TMU-35) and 275˚C (for TMU-36) is due to the decomposition of network, Figure S4. The as-synthesized crystals of these frameworks were activated by solvent exchange with dry acetonitrile for 2 days followed by evacuation at 80˚C for 4 h. The removal of guest molecules inside the frameworks was confirmed using FT-IR spectroscopy, powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA) and elemental analysis, Figures S5-S9. Carbonyl compounds Sensing
As volatile organic compounds (VOCs),[35, 36] including carbonyl compounds (CCs),[37-39] are a major group of air pollutants, development and design of materials for detection and sensing of VOCs is of great significance in both environmental and analytical sciences. CCs are widespread environmental contaminants formed as a result of incomplete combustion of fossil fuels and industrial processes.[40, 41] Recently, luminescent MOFs have attracted increasing attention for their potential prospects in chemical sensing of a variety of VOCs.[42] It is expected that through the incorporation of the supramolecular recognition unit into luminescent MOF, the sensitivity and selectivity of the MOF sensors can be enhanced by taking advantage of specific guest recognition through supramolecular complementarity.[14, 31, 43, 44] Urea is an interesting functional group for incorportation into a MOF backbone owing to its ability to donate two hydrogen bonds in a parallel fashion to the molecules with hydrogen-bond acceptor sites.[45, 46] A search of the Cambridge Structural Database (CSD) revealed that urea function has a strong tendency to self-assemble to one-dimensional (1D) hydrogen bonded chain via bifurcated N-H⋯O hydrogen bonds.[47] Since urea-containing MOFs also exhibit a tendency for the formation of N-H⋯O hydrogen bonds with the carbonyl oxygen atom of N,N-dimethylformamide, it is reasonable to assume that such a complementary interaction is likely to occur through interactions of carbonyl groups on a variety of organic substrates with urea sites on the receptor. The solid state UV-visible spectra showed absorption bands in the range of 270-400 nm and 250-400 nm for TMU-35 and TMU-36, respectively, Figures S10-S11. The excitation wavelength was obtained by scanning wavelength in 10-nm steps from 250 to 700 nm. The Fluorescence emission spectra of these two frameworks upon excitation at four different wavelengths were shown in Figures S12 and S13. Both frameworks show fluorescence at 460 nm upon excitation at 320 nm for TMU-35 and TMU-36, respectively. As a first step in evaluating the potential of urea-containing MOFs, namely TMU-35 and TMU-36, we have studied the fluorescence properties of these frameworks in different solvent systems, Figures S14 and S15. Benzoquinone, 1,4naphthoquinone, 2-cyclopentenone, 2-cyclohexenone, and 4,4-dimethyl-2-cyclohexenone were selected
to study the response of these frameworks toward carbonyl compounds. Our study demonstrated that the quenching efficiency is in the order of 1,4-naphthoquinone> benzoquinone> 2-cyclopentenone> 2cyclohexenone> 4,4-dimethyl-2-cyclohexenone, Figures S16-S25. The Stern-Volmer (SV) quenching rate constants were measured by plotting I0/I vs. concentration of the quencher [Q], where I0 and I are the maximum fluorescence intensity of MOFs before and after the addition of the analytes, respectively, Figures 2a and S26-S27. For both compounds, the Stern-Volmer plots show linear relationship fluorescence in quenching reagent concentration, with KSV values of 5.87×103, 3.67×103, 2.52×103, 2.34×103 and 1.58×103 for 1,4-naphthoquinone, benzoquinone, 2-cyclopentenone, 2-cyclohexenone and 4,4-dimethyl-2-cyclohexenone for TMU-35. For TMU-36, the KSV values are 8.29×103 (1,4naphthoquinone), 6.92×103 (benzoquinone), 3.46×103 (2-cyclopentenone), 3.28×103 (2-cyclohexenone) and 1.90×103 (4,4-dimethyl-2-cyclohexenone). The higher KSV values for the TMU-36 signify that carbonyl compounds quench TMU-36 fluorescence more efficiently and faster than TMU-35. This may be associated with the more hydrophobic walls of TMU-36 as compared to the TMU-35 which can lead to more favorable interactions with organic guests.[31, 34] The results suggest that a photo-induced electron-transfer (PET) mechanism may be responsible for fluorescence quenching. The PET mechanism could be confirmed by observing the following: (i) linear SV behavior or (ii) no overlap between the absorption band of carbonyl compounds and the emission band of these MOFs.[48, 49] Furthermore, it is of our interest to evaluate the sensing ability of both TMU-35 and TMU-36 in a solvent system which contains carbonyl group (CO) as hydrogen bond acceptor. Thus, the fluorescence quenching properties of both frameworks toward benzoquinone (KSV=9.84 ×102 and 1.26×103 M-1 for TMU-35 and TMU-36, respectively) and 2-cyclopentenone (KSV=5.33 ×102 and 6.49×102 M-1 for TMU-35 and TMU-36, respectively) were studied in acetone as solvent. Notably, in the presence of acetone, TMU-35 and TMU-36 responses less efficient and slower to carbonyl compounds compared to the experiments in which ethanol was used as solvent, Figures S28-
S33. It may be concluded that in the presence of acetone, there is a competition between molecules of carbonyl compounds and acetone and acetone has an inhibition effect for the recognition of carbonyl compounds. The sensor stability was determined using different analysis. PXRD pattern, scanning electron microscopy (SEM) images, Inductively coupled plasma (ICP) analysis and atomic absorption spectroscopy (AAS) taken after the experiment revealed that both frameworks are stable during carbonyl compounds sensing. PXRD and SEM analyses indicate the retention of crystallinity and morphology, Figures S34 and S35. The ICP and AAS analyses showed that almost no leaching ( <1%) of the zinc into the solution happened for both compounds, which confirmed the stability of frameworks, Figures S7-S8. These MOFs were recovered and reused for three consecutive cycles by centrifugation of the dispersed solution after use and washing several times with fresh ethanol, Figure 2b. Also, the fluorescence sensing responses of TMU-35 and TMU-36 were evaluated by the addition of the same concentration of different organic analytes including benzene, chlorobenzene, tetrahydrofuran, acetonitrile, nitrobenzene, toluene and phenol, Figures 2c and S36. In practice, the maximum change in the fluorescence intensity of both compounds was observed in the presence of carbonyl compounds and nitrobenzene, which has been reported by some of us previously.[30] In order to find out if the adsorption of analytes takes place inside the pores by a diffusion mechanism, both frameworks were used for the extraction and preconcentration of 1,4-naphthoquinone and 4,4-dimethyl-2-cyclohexenone followed by their determination by gas chromatography (GC-MS) with mass spectrometry, according to the procedure described in the Supporting Information, Figure S37. To further address this issue, the interaction between TMU35/TMU-36 and the carbonyl compounds were investigated using the adsorption locator module implemented in the Material Studio package.[50] The Monte Carlo simulations reveal that the most probable adsorption site for carbonyl compounds guests in both frameworks are near the urea functional group of ligand L1, the central benzene and naphthalene core of ligand L2 and Ligand L3 in TMU-35 and TMU-36, respectively, Table S1. Also, the binding energy between urea functional group of ligand L1 and
carbonyl compounds were evaluated using the density functional theory (DFT) method with B3LYP/631++G** basis set with diffuse basis functions. The counterpoise corrected binding energies are displayed along with each complex in Figure S38, from which it may be revealed that the
ureaN-H⋯Ocarbonyl comp.
[with the 𝑅12 (6) graph-set motif] hydrogen bond energy for ligand L1/1,4-naphthoquinone, ligand L1/benzoquinone,
L1/2-cyclopentenone,
L1/2-cyclohexenone
and
ligand
L1/4,4-dimethyl-2-
cyclohexenone are -8.6, -7.8, -11.8, -11.8 and -12.0 kcal/mol, respectively. The molecular electrostatic potentials (MEPs) of the ligand L1 and carbonyl compounds have been calculated and studied in order to qualitatively compare the electronegative and electropositive regions of the molecules. As depicted in Figure S39, the presence of two electron-withdrawing carboxylic groups in ligand L1 strongly polarized the urea NH bonds, as a result of which the hydrogen bond donating ability of urea group of ligand L1 is enhanced. Also, MEP results show a large negative region on carbonyl oxygen atom of the analytes, which may complement the receptor's binding site.
Conclusion In summary, we present the structures of two isoreticular urea-containing MOFs, namely TMU-35 and TMU-36, which are constructed by combining zinc ion, a ditopic oxygen-donor ligand containing a urea functional group (H2L1) and pillaring linkers containing different central benzene (L2) and naphthalene (L3) core. Since these compounds contain urea functional groups, they can interact favorably with carbonyl compounds and thus they are potential candidates for carbonyl compounds sensing. Inspired by our previous studies, the ability of these two frameworks was evaluated in sensing five different carbonyl containing analytes.[30, 31] Interestingly, these two frameworks gave selective responses when exposed to carbonyl compounds. It can be concluded that the match of the complementary binding sites of receptor and analytes allows the formation of tight hydrogen bonding and may account for the sensitivity of these frameworks to carbonyl compounds. Also, our study reveals that, when the ligand L2 is replaced
by ligand L3, the interaction between carbonyl compounds and sensors increases. In comparison with TMU-35, TMU-36 provides a more electron-rich and hydrophobic pore walls which can interact favorably with organic analytes and favors the diffusion of organic substrates to the binding sites. The results presented here provide some support for the approach taken by the chemists to broaden the applicability of MOFs through rational functionalization of the linking units. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was funded by Tarbiat Modares University
ASSOCIATED CONTENT Supporting Information Experimental details, PXRD patterns, TGA, FT-IR spectroscopy and sensing graphs and details (PDF) Crystallographic data (CIF)
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Alireza Azhdari Tehrani, Inorganic chemistry PhD, Post-Doctoral fellow, Tarbiat Modares University, Tehran, Iran Hamed Abbasi, MSc student, Tarbiat Modares University, Tehran, Iran Leili Esrafili, MSc student, Tarbiat Modares University, Tehran, Iran Ali Morsali, Inorganic Chemistry PhD, Full professor of inorganic chemistry at Tarbiat Modares University
Figure 1. Representation of TMU-35 (a) and TMU-36 (d) crystal packing along the crystallographic a-direction. The oxygen-donor ligand and pillaring struts are shown in blue and red, respectively. quadruply interpenetrated 3D pillared-layer frameworks with uninodal six-connected pcu topology of TMU-35 (b) and TMU-36 (e). Different chains are shown in different colors. Connolly surfaces of TMU35 (c) and TMU-36 (f), along the pore channels, with a probe radius of 1.2 Å. Connolly surfaces were generated using Mercury software module of Cambridge Structural Database.
Figure 2. Fluorescence emission spectra of TMU-35 and TMU-36 dispersed in ethanolic solution of carbonyl compounds at different concentrations, excited at 460 nm upon excitation at 316 and 370 nm for TMU-35 and TMU-36, respectively. Comparison of KSV of TMU-35 and TMU-36 toward different carbonyl compounds (a). Recovering and reusing of TMU-35 and TMU-36 for three consecutive cycles of carbonyl compounds sensing (b). Relative fluorescence response of TMU-35 and TMU-36 to 50 ppm of different organics in ethanol (c).
Scheme 1 . Chemical Structure of the Oxygen Donor Ligand (H2L1) and Pillaring Struts, Namely, L2 and L3, and the Binuclear Secondary Building Unit in TMU-35 and TMU-36
S0925-4005(17)31878-6 https://doi.org/10.1016/j.snb.2017.09.211 SNB 23302
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
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Please cite this article as: Alireza Azhdari Tehrani, Hamed Abbasi, Leili Esrafili, Ali Morsali, Urea-containing Metal-organic Frameworks for Carbonyl Compounds Sensing, Sensors and Actuators B: Chemical https://doi.org/10.1016/j.snb.2017.09.211 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Urea-containing
Metal-organic
Frameworks
for
Carbonyl
Compounds Sensing
Alireza Azhdari Tehrani, Hamed Abbasi, Leili Esrafili, Ali Morsali*
Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iran. E-mail: [email protected]
AUTHOR INFORMATION Corresponding Author [email protected]
Graphical abstract
Two urea-containing MOFs, namely TMU-35 and TMU-36, were synthesized and characterized using different techniques. Since these compounds contain urea functional group, they can interact favorably with carbonyl compounds and thus they are potential candidates for carbonyl compounds sensing. This
result is interesting as it represents the first example of the application of functionalized MOFs for carbonyl compounds sensing.
Highlights
Two urea-containing MOFs were synthesized and characterized using X-ray crystallography and different spectroscopic techniques.
Since these compounds contain urea functional groups, they can interact favorably with carbonyl compounds.
These two frameworks gave selective responses when exposed to different carbonyl compounds.
This result is interesting as it represents the first example of the application of functionalized MOFs for carbonyl compounds sensing.
Abstract Two urea-containing MOFs, namely TMU-35 and TMU-36, were synthesized and characterized using Xray crystallography and different spectroscopic techniques. Since these compounds contain urea functional group, they can interact favorably through hydrogen bonding with carbonyl compounds and thus they are potential candidates for carbonyl compounds sensing. Our study reveals that, when the pillaring linker containing benzene core (TMU-35) is replaced by pillaring linker containing naphthalene core (TMU-36), fluorescence sensing ability towards carbonyl compounds is dramatically quenched. It can be concluded that in comparison with TMU-35, TMU-36 provides a more electron-rich and hydrophobic pore walls which can interact favorably with organic analytes. This result is interesting as it represents the first example of the application of functionalized MOFs for carbonyl compounds sensing.
Keywords: Metal-organic framework; urea-containing MOF; carbonyl compounds sensing
Introduction Metal-organic frameworks (MOFs) are a class of crystalline hybrid materials that can be assembled from a wide range of inorganic and organic building blocks.[1] The main advantage of MOFs over other porous materials is the ability to tailor their pore size, chemical functionality and even the topology of the framework by judicious selection of molecular building blocks.[2, 3] Owing to this feature, these materials are considered to be potential candidates for many emerging applications such as gas storage, catalysis, molecular sensing and biomedical.[4-9] Selective molecular recognition and detection of small molecules, cations and anions by MOFs has been recently subject of intense research, since it was proved that incorporating supramolecular recognition units into the MOF backbone has enabled the docking of specific analytes with high affinity and selectivity based on their size, shape and fit to the recognition site.[7, 10-12] Molecular recognition arises from non-covalent intermolecular interactions such as electrostatic, hydrogen bonding, π-π stacking, as well as hydrophobic effects between receptor and substrate.[13, 14] Therefore, it can be envisioned that MOFs containing functional groups could be used to discriminate different substrates with high efficiency and selectivity through complementary intermolecular interactions.[15] Urea functional group is known to have a strong tendency to form hydrogen bonds with the molecules containing hydrogen bond acceptor group(s).[16-21] This type of intermolecular hydrogen bonding has been well-documented and was the subject of many research studies in the field of catalysis and molecular sensing.[22-24] The idea of incorporating urea functional groups into a MOF structure to exclude the possibility of urea self-association has been suggested by Farha,
Hupp, Scheidt and their co-workers.[16] Soon after this report, the design, synthesis and utilization of MOFs containing urea, thiourea and squaramide groups has become a fascinating area of research for supramolecular chemists because of their well-established potentials of this family of hydrogen bond donating (HBD) functional groups for catalysis and sensing.[18-20, 25-29] As part of our research program aimed at evaluating the potential of MOFs for molecular sensing,[30, 31] we have recently reported the potential of urea-functionalized MOFs for nitro-substituted compounds recognition. In continuation of this interest, herein, we report the synthesis of two urea-containing MOFs, namely TMU-35 and TMU-36, which exhibit potential in sensing carbonyl compounds.
Results and Discussion Structural analysis of TMU-35 and TMU-36 [Zn(L1)(L2) 0.5]•DMF•2H2O (TMU-35) and [Zn(L1)(L3)0.5]•DMF•2H2O (TMU-36) were synthesized by combining 4,4'-(carbonylbis(azanediyl))dibenzoic acid (H2L1), N,N'-(1,4-phenylene)bis(1-(pyridin-4yl)methanimine) (L2) and N,N'-(naphthalene-1,5-diyl)bis(1-(pyridin-4-yl)methanimine) (L3) ligands, respectively, and Zn(NO3)2·6H2O using the solvothermal method at 90˚C for 72 h to give suitable X-ray quality crystals, Scheme 1. X-ray crystallography analyses reveal that TMU-35 and TMU-36 crystallize in monoclinic P21/c and P21 space groups, respectively, Figure S1. In these structures, the Zn atoms are in a distorted square pyramidal geometry, surrounded in basal-plane by four oxygen atoms from four different L1 ligands; the apical position is taken by pyridine nitrogen atom of L2 and L3 ligands for TMU-35 and TMU-36, respectively. Both compounds are based on binuclear paddle-wheel secondary building unit [Zn⋯Zn separation of 2.9422(6) Å for TMU-35 and 2.9347(6) Å for TMU-36] that acts as a structural node held in place by the L1 ligands to form a two-dimensional (2D) square grid. In the third dimension, the 2D square grids are linked to each other by pillaring L2/L3 ligands, Figures 1a and 1d. Topological analyses give rise to quadruply interpenetrated 3D pillared-layer frameworks with uninodal
six-connected α-Po primitive cubic (pcu) topology, Figures 1b and 1e. TMU-35 and TMU-36 are microporous framework possess channels along the crystallographic a-direction with pore window of 7.2 Å × 5.1 Å (the van der Waals radii of the atoms at opposite sites of the channel have been subtracted) and free void spaces per unit cell of 45.8% (1558.5 Å3) and 43.9% (1541.9 Å3) for TMU-35 and TMU-.36, respectively, Figures 1c and 1f.[32] Hirshfeld surface analysis carried out for the purpose of exploring and studying the nature of intermolecular interactions in the crystal structures of these two compounds.[33] The results showed that, when the L2 ligand has been replaced by L3 ligand, the contribution percentages of π-π stacking and C-H⋯π interactions increase while that of H⋯H and N⋯H interactions decreases. Selected contribution percentages of different non-covalent interactions to the Hirshfeld surface areas are shown as a histogram in Figure S2. In accordance with the Hirshfeld surface analysis, contact angle measurements show that the wettability of the frameworks [with contact angle (CA) left and CA right of 69.1˚ and 67.5˚ for TMU-35 and 89.2˚ and 88.6˚ for TMU-36, respectively] can be decreased by changing the pillaring linker from L2 to L3, Figure S3. On the basis of these results it can be concluded that compared to TMU-35, the TMU-36 provides a more hydrophobic and electron-rich pore walls that can interact favorably with organic analytes. It is to be noted that the more hydrophobic nature of linker L3 relative to L2 has been previously studied by some of us.[31, 34] Thermogravimetric analysis (TGA) exhibited two weight loss steps at around 100˚C and 158˚C for TMU35 and 104˚C and 165˚C for TMU-36 which corresponds to the loss of water and DMF guest molecules, respectively. The third weight loss starting at 265˚C (for TMU-35) and 275˚C (for TMU-36) is due to the decomposition of network, Figure S4. The as-synthesized crystals of these frameworks were activated by solvent exchange with dry acetonitrile for 2 days followed by evacuation at 80˚C for 4 h. The removal of guest molecules inside the frameworks was confirmed using FT-IR spectroscopy, powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA) and elemental analysis, Figures S5-S9. Carbonyl compounds Sensing
As volatile organic compounds (VOCs),[35, 36] including carbonyl compounds (CCs),[37-39] are a major group of air pollutants, development and design of materials for detection and sensing of VOCs is of great significance in both environmental and analytical sciences. CCs are widespread environmental contaminants formed as a result of incomplete combustion of fossil fuels and industrial processes.[40, 41] Recently, luminescent MOFs have attracted increasing attention for their potential prospects in chemical sensing of a variety of VOCs.[42] It is expected that through the incorporation of the supramolecular recognition unit into luminescent MOF, the sensitivity and selectivity of the MOF sensors can be enhanced by taking advantage of specific guest recognition through supramolecular complementarity.[14, 31, 43, 44] Urea is an interesting functional group for incorportation into a MOF backbone owing to its ability to donate two hydrogen bonds in a parallel fashion to the molecules with hydrogen-bond acceptor sites.[45, 46] A search of the Cambridge Structural Database (CSD) revealed that urea function has a strong tendency to self-assemble to one-dimensional (1D) hydrogen bonded chain via bifurcated N-H⋯O hydrogen bonds.[47] Since urea-containing MOFs also exhibit a tendency for the formation of N-H⋯O hydrogen bonds with the carbonyl oxygen atom of N,N-dimethylformamide, it is reasonable to assume that such a complementary interaction is likely to occur through interactions of carbonyl groups on a variety of organic substrates with urea sites on the receptor. The solid state UV-visible spectra showed absorption bands in the range of 270-400 nm and 250-400 nm for TMU-35 and TMU-36, respectively, Figures S10-S11. The excitation wavelength was obtained by scanning wavelength in 10-nm steps from 250 to 700 nm. The Fluorescence emission spectra of these two frameworks upon excitation at four different wavelengths were shown in Figures S12 and S13. Both frameworks show fluorescence at 460 nm upon excitation at 320 nm for TMU-35 and TMU-36, respectively. As a first step in evaluating the potential of urea-containing MOFs, namely TMU-35 and TMU-36, we have studied the fluorescence properties of these frameworks in different solvent systems, Figures S14 and S15. Benzoquinone, 1,4naphthoquinone, 2-cyclopentenone, 2-cyclohexenone, and 4,4-dimethyl-2-cyclohexenone were selected
to study the response of these frameworks toward carbonyl compounds. Our study demonstrated that the quenching efficiency is in the order of 1,4-naphthoquinone> benzoquinone> 2-cyclopentenone> 2cyclohexenone> 4,4-dimethyl-2-cyclohexenone, Figures S16-S25. The Stern-Volmer (SV) quenching rate constants were measured by plotting I0/I vs. concentration of the quencher [Q], where I0 and I are the maximum fluorescence intensity of MOFs before and after the addition of the analytes, respectively, Figures 2a and S26-S27. For both compounds, the Stern-Volmer plots show linear relationship fluorescence in quenching reagent concentration, with KSV values of 5.87×103, 3.67×103, 2.52×103, 2.34×103 and 1.58×103 for 1,4-naphthoquinone, benzoquinone, 2-cyclopentenone, 2-cyclohexenone and 4,4-dimethyl-2-cyclohexenone for TMU-35. For TMU-36, the KSV values are 8.29×103 (1,4naphthoquinone), 6.92×103 (benzoquinone), 3.46×103 (2-cyclopentenone), 3.28×103 (2-cyclohexenone) and 1.90×103 (4,4-dimethyl-2-cyclohexenone). The higher KSV values for the TMU-36 signify that carbonyl compounds quench TMU-36 fluorescence more efficiently and faster than TMU-35. This may be associated with the more hydrophobic walls of TMU-36 as compared to the TMU-35 which can lead to more favorable interactions with organic guests.[31, 34] The results suggest that a photo-induced electron-transfer (PET) mechanism may be responsible for fluorescence quenching. The PET mechanism could be confirmed by observing the following: (i) linear SV behavior or (ii) no overlap between the absorption band of carbonyl compounds and the emission band of these MOFs.[48, 49] Furthermore, it is of our interest to evaluate the sensing ability of both TMU-35 and TMU-36 in a solvent system which contains carbonyl group (CO) as hydrogen bond acceptor. Thus, the fluorescence quenching properties of both frameworks toward benzoquinone (KSV=9.84 ×102 and 1.26×103 M-1 for TMU-35 and TMU-36, respectively) and 2-cyclopentenone (KSV=5.33 ×102 and 6.49×102 M-1 for TMU-35 and TMU-36, respectively) were studied in acetone as solvent. Notably, in the presence of acetone, TMU-35 and TMU-36 responses less efficient and slower to carbonyl compounds compared to the experiments in which ethanol was used as solvent, Figures S28-
S33. It may be concluded that in the presence of acetone, there is a competition between molecules of carbonyl compounds and acetone and acetone has an inhibition effect for the recognition of carbonyl compounds. The sensor stability was determined using different analysis. PXRD pattern, scanning electron microscopy (SEM) images, Inductively coupled plasma (ICP) analysis and atomic absorption spectroscopy (AAS) taken after the experiment revealed that both frameworks are stable during carbonyl compounds sensing. PXRD and SEM analyses indicate the retention of crystallinity and morphology, Figures S34 and S35. The ICP and AAS analyses showed that almost no leaching ( <1%) of the zinc into the solution happened for both compounds, which confirmed the stability of frameworks, Figures S7-S8. These MOFs were recovered and reused for three consecutive cycles by centrifugation of the dispersed solution after use and washing several times with fresh ethanol, Figure 2b. Also, the fluorescence sensing responses of TMU-35 and TMU-36 were evaluated by the addition of the same concentration of different organic analytes including benzene, chlorobenzene, tetrahydrofuran, acetonitrile, nitrobenzene, toluene and phenol, Figures 2c and S36. In practice, the maximum change in the fluorescence intensity of both compounds was observed in the presence of carbonyl compounds and nitrobenzene, which has been reported by some of us previously.[30] In order to find out if the adsorption of analytes takes place inside the pores by a diffusion mechanism, both frameworks were used for the extraction and preconcentration of 1,4-naphthoquinone and 4,4-dimethyl-2-cyclohexenone followed by their determination by gas chromatography (GC-MS) with mass spectrometry, according to the procedure described in the Supporting Information, Figure S37. To further address this issue, the interaction between TMU35/TMU-36 and the carbonyl compounds were investigated using the adsorption locator module implemented in the Material Studio package.[50] The Monte Carlo simulations reveal that the most probable adsorption site for carbonyl compounds guests in both frameworks are near the urea functional group of ligand L1, the central benzene and naphthalene core of ligand L2 and Ligand L3 in TMU-35 and TMU-36, respectively, Table S1. Also, the binding energy between urea functional group of ligand L1 and
carbonyl compounds were evaluated using the density functional theory (DFT) method with B3LYP/631++G** basis set with diffuse basis functions. The counterpoise corrected binding energies are displayed along with each complex in Figure S38, from which it may be revealed that the
ureaN-H⋯Ocarbonyl comp.
[with the 𝑅12 (6) graph-set motif] hydrogen bond energy for ligand L1/1,4-naphthoquinone, ligand L1/benzoquinone,
L1/2-cyclopentenone,
L1/2-cyclohexenone
and
ligand
L1/4,4-dimethyl-2-
cyclohexenone are -8.6, -7.8, -11.8, -11.8 and -12.0 kcal/mol, respectively. The molecular electrostatic potentials (MEPs) of the ligand L1 and carbonyl compounds have been calculated and studied in order to qualitatively compare the electronegative and electropositive regions of the molecules. As depicted in Figure S39, the presence of two electron-withdrawing carboxylic groups in ligand L1 strongly polarized the urea NH bonds, as a result of which the hydrogen bond donating ability of urea group of ligand L1 is enhanced. Also, MEP results show a large negative region on carbonyl oxygen atom of the analytes, which may complement the receptor's binding site.
Conclusion In summary, we present the structures of two isoreticular urea-containing MOFs, namely TMU-35 and TMU-36, which are constructed by combining zinc ion, a ditopic oxygen-donor ligand containing a urea functional group (H2L1) and pillaring linkers containing different central benzene (L2) and naphthalene (L3) core. Since these compounds contain urea functional groups, they can interact favorably with carbonyl compounds and thus they are potential candidates for carbonyl compounds sensing. Inspired by our previous studies, the ability of these two frameworks was evaluated in sensing five different carbonyl containing analytes.[30, 31] Interestingly, these two frameworks gave selective responses when exposed to carbonyl compounds. It can be concluded that the match of the complementary binding sites of receptor and analytes allows the formation of tight hydrogen bonding and may account for the sensitivity of these frameworks to carbonyl compounds. Also, our study reveals that, when the ligand L2 is replaced
by ligand L3, the interaction between carbonyl compounds and sensors increases. In comparison with TMU-35, TMU-36 provides a more electron-rich and hydrophobic pore walls which can interact favorably with organic analytes and favors the diffusion of organic substrates to the binding sites. The results presented here provide some support for the approach taken by the chemists to broaden the applicability of MOFs through rational functionalization of the linking units. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was funded by Tarbiat Modares University
ASSOCIATED CONTENT Supporting Information Experimental details, PXRD patterns, TGA, FT-IR spectroscopy and sensing graphs and details (PDF) Crystallographic data (CIF)
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Alireza Azhdari Tehrani, Inorganic chemistry PhD, Post-Doctoral fellow, Tarbiat Modares University, Tehran, Iran Hamed Abbasi, MSc student, Tarbiat Modares University, Tehran, Iran Leili Esrafili, MSc student, Tarbiat Modares University, Tehran, Iran Ali Morsali, Inorganic Chemistry PhD, Full professor of inorganic chemistry at Tarbiat Modares University
Figure 1. Representation of TMU-35 (a) and TMU-36 (d) crystal packing along the crystallographic a-direction. The oxygen-donor ligand and pillaring struts are shown in blue and red, respectively. quadruply interpenetrated 3D pillared-layer frameworks with uninodal six-connected pcu topology of TMU-35 (b) and TMU-36 (e). Different chains are shown in different colors. Connolly surfaces of TMU35 (c) and TMU-36 (f), along the pore channels, with a probe radius of 1.2 Å. Connolly surfaces were generated using Mercury software module of Cambridge Structural Database.
Figure 2. Fluorescence emission spectra of TMU-35 and TMU-36 dispersed in ethanolic solution of carbonyl compounds at different concentrations, excited at 460 nm upon excitation at 316 and 370 nm for TMU-35 and TMU-36, respectively. Comparison of KSV of TMU-35 and TMU-36 toward different carbonyl compounds (a). Recovering and reusing of TMU-35 and TMU-36 for three consecutive cycles of carbonyl compounds sensing (b). Relative fluorescence response of TMU-35 and TMU-36 to 50 ppm of different organics in ethanol (c).
Scheme 1 . Chemical Structure of the Oxygen Donor Ligand (H2L1) and Pillaring Struts, Namely, L2 and L3, and the Binuclear Secondary Building Unit in TMU-35 and TMU-36