Facile Fabrication of a Multifunctional MetalâOrganic Framework

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Functional Inorganic Materials and Devices

Facile Fabrication of a Multi-Functional MetalOrganic Framework based Sensor Exhibiting Exclusive Solvochromic Behaviors towards Ketone Molecules Bao Li, Xi Chen, Peng Hu, Angelo Kirchon, Yu-Meng Zhao, Jiandong Pang, Tianle Zhang, and Hong-Cai Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19815 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 6, 2019

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ACS Applied Materials & Interfaces

Facile Fabrication of a Multi-Functional Metal-Organic Framework based Sensor Exhibiting Exclusive Solvochromic Behaviors towards Ketone Molecules Bao Li,1, 2* Xi Chen, 1,‡ Peng Hu, 1,‡ Angelo Kirchon,2,‡ Yu-Meng Zhao,1,‡ Jian-Dong Pang,2 Tianle Zhang1 and Hong-cai Zhou2,3,** 1

Key laboratory of Material Chemistry for Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China. 2

Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States.

3

Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77842, United States.

ABSTRACT: To probe the efficient strategy for preparing multi-functional sensing material, the facile synthesis strategies and successful examples are urgently required. Through the utilization of a hexa-dentate ligand derived from cyclotriphosphazene which displays spiral configurations and multiple connection modes, a novel Metal-Organic Framework (MOF) has been constructed via one-step synthesis from low-cost raw materials. The presence of multiple interaction sites decorating the helical channels of the reported MOF gives rise to exclusive solvochromic sensing behavior for small ketone molecules, such as acetone, acetophenone and 2,5-diketohexane. Additionally, the helical structure of manganese-carboxylate chain allows for the pore volume to not only be available for the adsorption of large organic molecules, but enables the enantiopure selective separation of 1-phenylethanol (ee. 35.99 %). Furthermore, structural analysis of the acetophenone-encapsulated sample allowed the solvochromic mechanism to be elucidated, which should be ascribed to the strong hydrogen bonding interaction between the guest molecules and specific sites on host matrix. The experimental results have not only clearly manifested the vital role of the starting materials of MOFs, including the connection modes and spatial configuration, but also have provided very valuable insight for the future assembly of novel multifunctional sensing materials.

Keywords: Metal-Organic Framework; Solvochromic Behaviors; Multi-function ; Enantiopure selective separation; Sensing mechanism

Introduction In recent years, there has been a major push within the science and technology communities in order to develop novel materials that have multi-functional properties and can be fabricated from low-cost raw materials using simple and straightforward synthetic routines.1-3 Over the last decade, Metal-Organic frameworks (MOFs) have emerged as one of the leading classes of materials that could successfully meet all of these demands. The benefits of MOFs display over other porous materials such as zeolites, porous carbons, and porous silicas, is that MOFs can be systemically designed, tuned and functionalized using a wide variety of facile synthetic methods for a wide range of applications.4-6

MOFs have recently begun to be investigated for sensing based applications towards substances such as toxic organic compounds (TOCs), metal ions, drugs and other biological relevant molecules.7-11 Although MOFs have been shown to be effective sensors, many of their applications have been limited to fluorescence-based sensing.12-14 The extensive attention that has been given to the development of fluorescence-based MOF sensors has in turn limited the development of MOF-based sensors that present other signal transductions such as solvochromic or vaporchromic behaviors. Materials that exhibit solvochromic behavior, refers to a material that shows a response to different guest molecules captured in their matrixes/pores. This chemical response in turn yields a visible color change of the material which corresponds to the change of peak position or shape in the UV-Vis absorption

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spectrum.15-17 Solvochromic materials have gained more attention over the last 5 years but due to the difficulty associated with designing such materials there has not been great success. The difficulty associated with solvochromic material synthesis stems from a lack of understanding of the complicated solvochromic mechanisms that can occur. However, due to their crystalline nature, the corresponding mechanistic studies for the solvochromic behavior in the proper MOF systems might be fully explored via the versatile measurements. Using techniques such as x-ray crystallography, EPR, magnetic studies and much more, the sensing mechanism can be studied, understood and potentially used in order to further develop better and more specific solvochromic sensor materials.18-22 Additionally, the wide variety of available synthetic techniques that can be used to tune the chemical environments of MOFs, such as ligand/cluster design, ligand/cluster exchange and ligand/cluster insertions, yield the opportunity to design a material that could exhibit solvochromic behaviors specific to targeted guest molecules. Specifically, chirality, acted as one important function, can be also designed into MOFs via the proper strategies. The traditional strategy for synthesizing chiral MOFs focused on the utilization of homo-chiral ligands, which typically makes the material cost skyrocket to unreasonable values. On the other hand, an area which is less advanced is the development of chiral MOFs that are assembled from achiral components via spontaneous resolution during the crystal growth process.23-29 Unfortunately, the generation of helical features from only achiral building blocks is still very unpredictable due to the fact that the crystallization process is inclined to obtain centro-symmetry. Despite the challenges, the fact that crystal growth processes can be affected by many factors yields a potential for controlling the process in favor of yielding enantio-pure crystals. In this regard, one approach that has been shown to be successful for generating chiral MOFs is using intrinsically spiral ligands with specific connection modes.30-37 Of course, MOF materials constructed using spiral ligands cannot be said to be homo-chiral, due to the limitation of X-ray structural studies and Circular dichroism (CD) spectral. However, this strategy can be used for the preparation of chirality-enriched MOF materials, which is similar to the method of introducing chiral dopants in the synthesis process.38-41

Figure 1. the perspective view of inter-connection mode of 1D Mn chain of 1. Asymmetric codes: A, -x+1, y, -z+1/2; B, -x+1, -y+1, z+1/2; C, x, -y+1, -z.

With all the aforementioned aspects in mind, we set out to explore the possibilities of generating novel sensing materials by combining solvochromic behaviors and enantiopure selective separations. After a comprehensive analysis of a wide variety of possible structural features, the semi-rigid ligand, hexakis(4-formylphenoxy) cyclotriphosphazene (H6L1), was selected as the starting materials for the following reasons: 1)The substituted benzoate groups can twisted around the central six memebered P3N3 ring resulting in a spiral configuration with the designated metallic nodes ( Figure S1 ), which is very vital for the construction of helical frameworks;4243 2) The ligand allows for the generation of a permanently porous, yet flexible framework; 3) The resulting framework would possess multiple interaction sites and respiratory frameworks, which are the basic requirements for chemosensor materials. In accordance with our assumption, a novel 3D porous MOF, [Mn6(L1)2(H2O)5]n, was obtained via a onestep synthesis from low-cost raw materials. The structure displays helical channels which endows the title MOF as a solvochromic sensor to ketone molecules and as well as the ability to perform enantiopure selective separation of 1phenylethanol. The interesting performances illustrate a rare example of a MOF exhibiting color-change sensing behavior and chiral characteristics, which satisfies the standard of multifunctional properties needed from modern materials. In addition, the synthesis, description of crystal structure and investigation of mechanism of solvochromic phenomenon are reported below.


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Figure 2. the perspective view of 1D helical Mn chains (a), 1D helical channels along c axis (b) and the 3D packing mode of 1 (c) .

Results and discussion Crystal structure of 1 Solvothermal reaction of MnCl2·4H2O and H6L1 in the presence of DMF and H2O leads to the generation of triangular colorless crystals, which was further characterized by single-crystal X-ray diffraction (SCXRD), elemental analysis, Powder X-ray diffraction (PXRD) and TGA anaylsis. IR spectra illustrates the asymmetric and symmetric stretching vibrations of the de-protonated carboxylate groups ( Figure S7 ).44-46 Crystal data and selected structural parameters had been gathered and listed in Table S1-S4. SCXRD analysis reveals that 1 crystallizes in orthorhombic chiral space group C2221, and its asymmetric unit contains three Mn(II) centers, one hexa-carboxylate ligand and three coordinated water molecules (shown in Figure S1). The 3D porous framework consists of 1D helical Mn-chains and hexa-carboxylate bridges. In 1, Mn(1) atom is six-coordinate and surrounded by six oxygen atoms. Four of the oxygen atoms are from different carbonyl groups of individual L ligands via three syn-syn O,O’-

bridges and one μ2-2:1 coordination modes, while the remaining two coordination sites are occupied by one terminally-coordinated water molecules and one μ2-bridged water molecule. The coordination geometry around the Mn(1) center can be described as a distorted octahedral geometry. Mn(2) atom is also octahedrally-coordinated by five carboxylate oxygen atoms (from four different hexacarboxylate ligands through three syn-syn O,O’-bridges and one μ2-2:1 coordination modes) and one bridged water molecule to form a highly distorted octahedral geometry. In contrast, Mn(3) is five-coordinate and surrounded by four carboxylate atoms from distinct syn-syn carbonyl groups of individual hexa-carboxylate ligands and one bridged aqua molecule, respectively. All of the Mn-O distances for the three Mn ions range from 2.010(3) to 2.460(2) Å, corresponding to the normal MnII-O values previously reported.47-49 Three crystallographically distinct Mn atoms are inter-connected together to form a trinuclear Mn(II) cluster. Mn(1) ion is linked with Mn(2) ion by two carboxyl groups from two different ligands via syn-syn O,O’-bridge and μ2-2:1 coordination modes, and Mn(2) atom is linked with Mn(3) atom via two syn–syn carboxyl groups of distinct ligands and one μ2-bridged aqua molecule. In addition, Mn(1) atom interconnects with its symmetric Mn(1) ion in different trinuclear units via two syn–syn carboxylates of distinct ligands and one μ2-bridged aqua molecule, while Mn(3) ion is linked to its symmetric Mn(3) ion via two syn–syn carboxylates of different ligands(Figure 1). The trinuclear Mn(II) clusters are further connected with each other through the head-to-tail connection mode to form the 1D helical-pillared secondary building units which assemble the final framework. In 1, the hexa-carboxylate ligand adopts a twelve-dentate coordination mode which acts as the struts in the final framework. The structural parameters of hexa-carboxylate ligand in 1 are similar to other systems.42 The ligand presents the spiral configuration due to the synergy effect of metallic nodes and semi-rigid ligands during the process of crystallization (Figure S1), which plays an important role in spontaneously deriving the final helical framework. Similar spontaneously introducing helical characteristic into MOFs from achiral ligands and metallic nodes had been discovered and summarized, which emphasizes the vital role of the synergy effect of reactants.50-53 The six carboxyl groups exhibit two types of coordination modes as syn–syn O,O’-bridge and μ2-2:1 coordination modes which connect ten independent MnII ions assigned to two helical chains, with the range of distorted Ccarboxyl-Osubstituted-Pcentral angles from 120.2 to 132.5º, as shown in Figure S5. Furthermore, each chain is interlinked with four parallel chains via the hexa-carboxylate struts to form the 3D porous MOF (Figure 2b,c). 1D helical channel with the approximate diameter of 1.3 nm along the c-axial direction was left in the structure, which are occupied by guest molecules



such as DMF and water that could not be located via X-Ray studies. The solvent accessible volume of the unit cell in the dehydrated structure of 1 was estimated to be 13301.7 Å3 (64.1% ) which was calculated using the PLATON routine. The PXRD patterns of freshly synthesized sample (see Figures S9) match the patterns simulated from the crystal structure. Slight differences can be assigned to the fact that the enclosed solvent molecules could not be taken into account for the calculation of the theoretical patterns. The presence of DMF molecules could be also verified by the peak around 1650 cm-1 in IR spectra(Figure S7). TGA curve displays a rapid loss of solvent or coordinated water molecules below 220 ℃. The resulting dehydrated sample began to decompose at 400 ℃.

Figure 3. Photographs of solvent-loaded compounds with the sequence of fresh sample, methanol, acetonitrile, acetone, phenol in CH2Cl2, CHCl3.

Figure 4. Photographs of solvent-loaded compounds with the sequence of fresh sample, acetophenone, 1-(4iodophenyl)ethanone in CH2Cl2, 3-Acetylindole in CH2Cl2, 2Acetonaphthone in CH2Cl2, 2,5-Diketohexane, 1-Phenyl-1,3butanedione in CH2Cl2, 1,4-Cyclohexanedione in CH2Cl2.

Host-guest interactions and sensing phenomenon To prove the permanent porosity and accessibility of 1, three different dye molecules were adsorbed in solution. Once activated, 1 was soaked in DMF solutions of different kinds of dyes. Dye molecules could be efficiently adsorbed over a period of time and the colorless crystals rapidly turned into the corresponding color of the absorbed dye (Figure S10). The adsorption of dye molecules was further evaluated by UV-Vis spectroscopy, which confirms the incorporation of dye

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molecules into the parent structure of 1. In this way, the porous structure of 1 in solution could be validated. Inspired by the reported solvochromism of porous frameworks and stable pore structure of 1 in solution, the solvochromic properties of 1 was also investigated by immersing fresh samples in different solvents. Firstly, colorless fresh samples of 1 were immersed in solvents such as methanol, acetonitrile, acetone, phenol and CHCl3. No obvious solvochromic phenomenon was initially observed in these solvents. Interestingly, the samples of 1 immersed in acetone changed its color to a light red after 1 h and the color became deeper over time. After three days, the light red samples had turned dark red. In contrast, the samples immersed in methanol, acetonitrile, phenol and CHCl3 remained colorless, as well as other solvents such as ethyl acetate, THF, CHCl2, DMSO, and toluene. Photographs of the solvochromic behavior related with the solvent inclusions are shown in Figure 3, and the spectrum of UV-Vis absorption is consistent with their colors. Infrared spectroscopy also confirmed the successful incorporation of acetone. Absorption bands at about 1715 cm-1 were observed which are characterized to the vibration of the C=O group of acetone (Figure S7).54-56 As observed by PXRD, the solvent-containing samples maintain their crystallinity (see Figures S9). With the consideration of the structural difference of these solvents, acetone molecules interact with the host framework via C=O bond. Encouraged by this fantastic solvochromic phenomenon and to validate the role of C=O bond, other ketone molecules with different sizes and configurations, such as acetophenone, 1-(4iodophenyl)ethanone, 3-acetylindole, 2-acetonaphthone, 2,5diketohexane, 1-phenyl-1,3-butanedione and 1,4cyclohexanedione, were investigated for color-changed behaviors. In accordance with our assumption, obvious colorchanged behaviors for liquid ketone species were observed as shown in Figure 4. For acetophenone, the sample started to change the color within thirty minutes and the sample became dark red after two days. For 2,5-diketohexane, the colorless sample also changed to very deep dark red. In contrast, the samples changed to light yellow in 1-(4-iodophenyl)ethanone and 1-phenyl-1,3-butanedione; the colorless samples had turned light gray in 3-acetylindole, 2-acetonaphthone and 1,4cyclohexanedione. It was concluded from the color-changed phenomenon, the liquid ketone molecules as acetone, acetophenone and 2,5-diketohexane trigger the obvious solvochromic phenomenon. However, light color-change behaviors could be observed in the CH2Cl2 solutions of solid ketone molecules (Figure 4 and S15), which might be caused by the interruption of CH2Cl2 molecules in the cavity. In order to investigate the nature of the color change phenomenon, acetone- and acetophenone-encapsulated crystal samples were placed in methanol for 7 days, and the red color slightly fades for these samples. Moreover, even when the solvochromic


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samples were desolvated at 80 ℃ overnight, the red color maintained. TGA curves for acetone-encapsulated sample clearly indicate the loss of acetone molecules at high temperatures (Figure S18). All experimental results strongly indicate the existence of strong interactions between ketone molecules and host framework. Normally, the exchanged guest molecules are easily desorbed via solvent exchange or thermal treatment, especially volatile acetone molecules. Such stable solvochromic behavior is extremely rare, and the properties of 1 are of interest for sensing of ketone molecules observable by the naked eye.

The mechanism of solvochromic phenomenon of 1 It must be noted that the crystallinity of sample after the adsorption of ketone molecules is still retained, which is suitable for investigating the structural characteristics of the color-changed samples. Due to the quick and apparent colorchange phenomenon compared to other ketone species, the detailed crystal structure of 1 with encapsulated acetophenone, labeled as 1’, was selected to investigate the specific structureactivity relationship of solvochromic behaviors via SCXRD, as shown in Figure 5. The detailed crystal data and selected structural parameters have been gathered and listed in Table S1-S4. The Mn-O bond lengths in 1’ are in consistent with the typical values of Mn(II) state, which was also validated by XPS (Figure S11) and EPR (Figure S12) spectra. In addition, LC detection for the extraction of acetophenone from its encapsulated sample manifests the reservation of the original state. In this way, the change of Mn(II) state and catalytic conversion of ketone molecules can be excluded from the mechanism of solvochromic behaviors for 1.

Figure 5. a) perspective view of the hydrogen bonding interaction between acetophenone molecule (blue color) and host framework; b and c) the perspective view of the 3D packing mode of 1’ after inclusion of acetophenone molecules along a- and c- axial directions.

Analyzed from the crystal structure, no obvious changes of bond length and angles was observed, excepted for the distorted octahedral coordination environment of Mn1 atom and the π-π interaction between the benzoate rings of O18 and O12. The inclusion of acetophenone molecules contacts with the μ2-bridged water atom (O21) on the surface of the 1D channels via hydrogen bonding interactions, which subsequently affects the steric configuration of framework skeleton. The bond length of the hydrogen bonding interactions is 2.827(2) Å between O21 and O23 of acetophenone, along with the specific conformation of the channels responsible for stabilizing the guest molecules and the quasi-irreversible solvochromic behaviors. The steric hindrance of guest inclusions promotes the further deformation of coordination environment of Mn1 centers and hexa-carboxlyate ligands, which is reflected in the corresponding structural parameters of 1’ compared with 1. For example, the distance between the π···π conjugated benzene rings (benzeneO1-O3 and benzeneO13-O15) changes from 4.002(3)Å to 3.940(2)Å along with the dihedral angle varies from 27.01º to 21.04º. Thus, the mechanism of solvatochromic phenomenon of title MOF to ketone molecules is most likely due to the hydrogen interactions between the host matrix and encapsulated ketone molecules, along with partial effect of the



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transitions of π-π* transition in ligands.57-59 The introduction of ketone molecules would cause the distortion of parent framework including the octahedron of metal ions and conformation of ligands, and subsequently to some extent bring the changes of d-d transition into visible region. Although there are also hydrogen bonding interactions between the framework and the other solvent molecules, the unsuitable steric configuration and bond intensities would be responsible for the absence of color change phenomenon.

molecules from the helical channels. Chiral HPLC analysis of the released PEA from the alcohol-encapsulated solid sample yielded an ee value of 35.99% ( shown in Figure S17 ). The low ee value for the separation of R/S-PEA within the activated sample is attributed to its large helical pore environments and the residual racemic PEA molecules on the surface commonly observed in MOF systems, which have limited its high enantioselective separation abilities.60-62 However, the title MOF material with the properties of

With this interpretation in mind, the large ketone molecules are considered to be improper to interact with the MOF skeleton due to the large steric hindrance effect. In addition, the inhibition of CH2Cl2 molecules for dissolving solid ketone molecules also hold back the interactions between MOF matrix and guest molecules. Comparably, only the small volume ketone molecules such as acetone, acetophenone and 2,5-diketohexane could effectively interact with the framework, and could cause the apparent solvochromic behavior.

solvochromic behaviors does exhibit the ability of enantiopure selective separation, illustrating the potential as a composite functional sensing material.

Enantiopure selective separation of 1-phenylethanol The expression of chiral characteristic in as-synthesized samples of 1 could be validated by the spectra of CD in the solid state ( Figure S16 ). Ten crystal samples in the same batch of products with the scale ranged from 1-5 μm had been randomly picked. The spectrum exhibits the positive cotton effect, and the peaks in CD signal are in consistent with the positions in UV-Vis spectra. The shape of CD spectra indicates the possibility of chiral configuration in the porous structure, caused by the utilization of helical ligand. Limited to the existing experimental conditions, strictly speaking, it cannot be said that the introduction of helical ligand could construct the framework with homo-chirality. However, concluding from the presented results, chirality-enriched MOF materials could be effectively fabricated via the introduction of specific linkers with spiral configuration. In an attempt to further explain the production of chirality-enriched MOF materials, chiral dopants (R)-(-)-2-Amino-1-butanol or (S)-(+)-2-Amino-1butanol had been individually introduced into the original reaction systems. The resulted crystal samples also exhibit the similar positive cotton effect and positions of peaks in the solid CD spectra ( Figure S16 ). Therefore, the spiral configuration of ligands and specific bind characteristic between Mn ions and the spiral ligands must play the more important role in the formation of the chirality-enriched MOFs. The helical channels within 1 also encouraged the exploration of its potential application in enantioselective separation of small alcohol molecules, such as 1-phenylethanol (1-PEA). The as-synthesized crystal sample was firstly solventexchanged with methanol molecules, and then immersed in the racemic mixture of 1-PEA at room temperature for two days. The alcohol-encapsulated solid sample was filtered and washed with diethyl ether several times, and then immersed in methanol again in order to release the encapsulated PEA

Conclusion In conclusion, a novel multi-functional sensing material has been constructed via facile synthesis method and low-cost raw materials. The series of experimental results illustrates the reported Mn-MOF as multi-functional material displaying properties such as the exclusive solvochromic sensing phenomenon towards ketone molecules and enantiopure selective separation of 1-PEA. The successful integration of solvochromic behaviors and chiral characteristic into one material has been rarely reported in MOF-based sensors. Structural analysis of acetophenone-encapsulated sample enables elucidation of the solvochromic mechanism, which was ascribed to the strong hydrogen bonding interactions between the guest ketone molecules and the host matrix. Furthermore, the related results powerfully emphasize the importance of rationally designing the structures of MOFs for building multifunctional materials, which should be served as one superior platform for exploring novel sensing materials.

Experimental Section The hexa-carboxylate ligand according to the referent.43

had

been

synthesized

Synthesis of [Mn6(C42H24O18P3N3)2(H2O)5]n. A mixture of MnCl2 (10mg) and H6L ( 20mg ) were added to a solution of DMF/H2O ( 10/0.5 mL ), which was further stirred for 30 minutes. And then 5 drops of concentrated nitric acid were added. The mixture solution was sealed in a 20 mL vial, then heat at 80 ℃ for 3 days. Colorless block crystals of 1 were obtained in a yield of 45% ( based on L ). Anal. Calcd for C84H58Mn6N6P6O51: C, 45.05; H, 4.32; N, 11.26. Found: C, 44.69; H, 3.98; N, 11.62. IR ( KBr pellet, cm−1 ): 3377, 3231, 2924, 1606, 1532, 1397, 1311, 1081, 995. Synthesis of ketone-encapsulated sample. The fresh crystal samples of 1 had been directly immersed into the different ketone solvents for several days, without the additional activation. Then the color-changed sample could be


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obtained, which reserve the high crystallinity. The crystal structure of acetophenone-encapsulated sample, 1’, had been investigated in order to observe the structural characteristics. Crystal data and structural parameters of 1 and 1’ had been gathered in Table S1-S4 for clear comparisons.

ASSOCIATED CONTENT Supporting Information. Experimental section, crystal data, XRD and supporting spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (B.L.). * E-mail: [email protected] (H.-C. Z.).

ORCID Bao Li: 0000-0003-1154-6423 Angelo Kirchon: 0000-0003-1082-9739 Hong-Cai Zhou: 0000-0002-9029-3788

Author Contributions ‡These authors contributed equally.

Notes The authors declare no competing financial interests..

ACKNOWLEDGMENT The authors acknowledge the financial supports of National Science Foundation of China ( 21471062 ), the Center for Gas Separations Relevant to Clean Energy Technologies, an Energy Frontier Research Center (EFRC) funded by the U.S. Department of Energy (DOE), Office of Science, and Office of Basic Energy Sciences (DESC0001015), Office of Fossil Energy, the National Energy Technology Laboratory (DE-FE0026472), and the Robert A. Welch Foundation through a Welch Endowed Chair to HJZ (A-0030).

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