Jinxuan Liu

Enantiopure metal-organic framework thin films: oriented SURMOF growth and enantioselective adsorption.

Growing effort is being paid to metal–organic frameworks (MOFs), in the form of microcrystalline powder materials, for the storage, capture and separation of gases and for applications in catalysis. The demand of integrating MOFs into analytical sensing devices and smart membranes is stimulating the development of MOF thin-film possessing techniques of various kinds. In this respect, the layer-by-layer (LBL) liquid-phase epitaxial (LPE) growth method is quite attractive for depositing multilayers or small crystallites of surfaceattached MOFs (SURMOFs) in an automatic and thus very controlled fashion. This stepwise MOF synthesis and deposition scheme can be coupled with in-situ process monitoring by UV/Vis spectroscopy, surface plasmon resonance spectroscopy (SPR), or by quartz crystal microbalance techniques (QCM) and provides unique opportunities to study the (SUR)MOF growth mechanism and is advantageous for MOF-based sensor fabrication. LPE is also well suited for deposition of MOF (hetero-)structures, for suppressing interpenetration, and for tailoring the chemical functionality of the external SURMOF surface, tasks which are quite difficult to achieve for MOF thin films grown by other deposition techniques. In particular SURMOFs of HKUST-1 allow the monitoring of adsorption/desorption of guest molecules at ultra thin and very homogeneous coatings and allow the determination of the corresponding diffusion constants. Using these concepts we now demonstrate LPE growth of [{Zn2((+)cam)2(dabco)}n] ((+)cam= (1R,3S)(+)-camphoric acid, dabco= 1,4-diazabicyclo(2.2.2)octane)) and the application of this very first example of an enantiopure SURMOF to the direct QCM monitoring of the uptake of a pair of enantiomeric guest molecules, namely (2R,5R)2,5-hexanediol (R-HDO) and (2S,5S)-2,5-hexanediol (SHDO) from the gas phase under flow conditions. Microcrystalline MOF powder materials have been explored as stationary phases in both gas and liquid-phase chromatography and related theoretical and experimental studies on the diffusion in MOF single crystals have been reported. Quite recently, the LBL growth scheme was adopted for coating fused silica capillaries with MOF-5 for the first time. Accordingly, enantiopure MOFs are highly promising for the separation of enantiomers, a result of their high porosity, functional diversity, flexibility, and size and shape selectivity, surpassing other porous materials. The technological challenge is to achieve LPE growth of enantiopure SURMOFs as a model to study in detail enantioselective adsorptions on well-defined MOF coatings. Multicomponent layer-based MOFs of the general formula [{M2L2P}n] (M: Cu , Zn; L: dicarboxylate linker; P: dinitrogen pillar ligand) have been shown to be favorable for step-by-step LPE. Our test case, [{Zn2(cam)2(dabco)}n] (+)-1 for (+)cam and ( )-1 for ( )cam) with an anisotropic tetragonal crystal system is such a layer-based MOF containing the binuclear “paddle wheel” zinc carboxylate unit {Zn2(COO)4N2} with distorted octahedral geometry, in which chiral camphorate bridge the dimeric zinc units into infinite planar layers {Zn2cam2}n. Linear N-donor ligands dabco occupy the axial Zn sites, perpendicularly to these {Zn2cam2}n layers, leading to a scaffold-like 3D structure. [16] The structure allows two principle growth directions depending on carboxylate and pyridine groups location (Figure 1). Typically, the enantiopure SURMOFs (+)-1 or ( )-1 are grown (20–40 cycles) by dipping the QCM substrate alternately in ethanol solutions of Zn(Ac)2·H2O and equimolar ( )cam/dabco mixtures, each step followed by immediately rinsing with pure ethanol, according to the procedure developed in our group (Figure 1). The growth process was monitored in situ by QCM as shown in Figure S1 in the Supporting Information. The crystallite orientation of the samples can be controlled by applying self-assembled monolayer (SAM) modified QCM substrates with different functional head groups (pyridyl or carboxylate) and appropriate growth conditions. As examined by surface X-ray diffraction in an out of plane mode (Figure 2), SURMOF (+)-1 was grown in (110) and (001) orientation on SAMs of MHDA and PPMT (MHDA= 16-mercaptohexadecanoic acid; PPMT= (4,(4pyridyl)phenyl)methanethiol)) on Au-coated QCM substrates. The (110) and (001) X-ray diffraction (XRD) peak positions are very close to each other at 9.288 and 9.228 which is in accord with the corresponding single-crystal X-ray diffraction data. To accurately distinguish these two peaks, the XRDpeak positions were calibrated by referencing to the XRD peak positions of the Au substrate. Accordingly, [*] Dr. B. Liu, Prof. Dr. R. A. Fischer Chair of Inorganic Chemistry II— Organometallics and Materials Chemistry Ruhr-Universit t Bochum, 44870 Bochum (Germany) E-mail: roland.fischer@rub.de

A novel series of isoreticular metal organic frameworks: realizing metastable structures by liquid phase epitaxy

A novel class of metal organic frameworks (MOFs) has been synthesized from Cu-acetate and dicarboxylic acids using liquid phase epitaxy. The SURMOF-2 isoreticular series exhibits P4 symmetry, for the longest linker a channel-size of 3 × 3 nm2 is obtained, one of the largest values reported for any MOF so far. High quality, ab-initio electronic structure calculations confirm the stability of a regular packing of (Cu++)2- carboxylate paddle-wheel planes with P4 symmetry and reveal, that the SURMOF-2 structures are in fact metastable, with a fairly large activation barrier for the transition to the bulk MOF-2 structures exhibiting a lower, twofold (P2 or C2) symmetry. The theoretical calculations also allow identifying the mechanism for the low-temperature epitaxial growth process and to explain, why a synthesis of this highly interesting, new class of high-symmetry, metastable MOFs is not possible using the conventional solvothermal process.

Patterned deposition of metal-organic frameworks onto plastic, paper, and textile substrates by inkjet printing of a precursor solution.

Flexible in many aspects: inkjet printing of metal-organic frameworks permits their larger area, high-resolution deposition in any desired pattern, even in the form of gradients or shades. When flexible substrates are used, many applications can be envisioned, such as sensing and capture of hazardous gases for personal safety measures.

Defects in MOFs: a thorough characterization.

As indicated by nearly perfect XRD data, but challenged by a two-signal IR spectrum of CO guest molecules, it is confirmed by computer simulations and XPS experiments that the most defect-free SURMOFs contain about 4% defective Cu sites.

On the dielectric and optical properties of surface-anchored metal-organic frameworks: A study on epitaxially grown thin films

We determine the optical constants of two highly porous, crystalline metal-organic frameworks (MOFs). Since it is problematic to determine the optical constants for the standard powder modification of these porous solids, we instead use surface-anchored metal-organic frameworks (SURMOFs). These MOF thin films are grown using liquid phase epitaxy (LPE) on modified silicon substrates. The produced SURMOF thin films exhibit good optical properties; these porous coatings are smooth as well as crack-free, they do not scatter visible light, and they have a homogenous interference color over the entire sample. Therefore, spectroscopic ellipsometry (SE) can be used in a straightforward fashion to determine the corresponding SURMOF optical properties. After careful removal of the solvent molecules used in the fabrication process as well as the residual water adsorbed in the voids of this highly porous solid, we determine an optical constant of n = 1.39 at a wavelength of 750 nm for HKUST-1 (stands for Hong Kong Un...

Nanoporous designer solids with huge lattice constant gradients: multiheteroepitaxy of metal-organic frameworks.

We demonstrate the realization of hierarchically organized MOF (metal-organic framework) multilayer systems with pronounced differences in the size of the nanoscale pores. Unusually large values for the lattice constant mismatch at the MOF-MOF heterojunctions are made possible by a particular liquid-phase epitaxy process. The multiheteroepitaxy is demonstrated for the isoreticular SURMOF-2 series [ Liu et al. Sci. Rep. 2012 , 2 , 921 ] by fabricating trilayer systems with lattice constants of 1.12, 1.34, and 1.55 nm. Despite these large (20%) lattice mismatches, highly crystalline, oriented multilayers were obtained. A thorough theoretical analysis of the MOF-on-MOF heterojunction structure and energetics allows us to identify the two main reasons for this unexpected tolerance of large lattice mismatch: the healing of vacancies with acetate groups and the low elastic constant of MOF materials.

Fabrication of highly uniform gel coatings by the conversion of surface-anchored metal-organic frameworks

We report the fabrication of 3D, highly porous, covalently bound polymer films of homogeneous thickness. These surface-bound gels combine the advantages of metal-organic framework (MOF) materials, namely, the enormous flexibility and the large size of the maximum pore structures and, in particular, the possibility to grow them epitaxially on modified substrates, with those of covalently connected gel materials, namely, the absence of metal ions in the deposited material, a robust framework consisting of covalent bonds, and, most importantly, pronounced stability under biological conditions. The conversion of a SURMOF (surface-mounted MOF) yields a surface-grafted gel. These SURGELs can be loaded with bioactive compounds and applied as bioactive coatings and provide a drug-release platform in in vitro cell culture studies.

Post-Synthetic Modification of Metal–Organic Framework Thin Films Using Click Chemistry: The Importance of Strained C–C Triple Bonds

In this work, we demonstrate that strain-promoted azide-alkyne cycloaddition (SPAAC) yields virtually complete conversion in the context of the post-synthetic modification (PSM) of metal-organic frameworks (MOFs). We use surface-anchored MOF (SURMOF) thin films, [Zn2(N3-bdc)2(dabco)], grown on modified Au substrates using liquid-phase epitaxy (LPE) as a model system to first show that, with standard click chemistry, presently, the most popular method for rendering additional functionality to MOFs via PSM, quantitative conversion yields, cannot be reached. In addition, it is virtually impossible to avoid contaminations of the product by the cytotoxic Cu(I) ions used as a catalyst, a substantial problem for applications in life sciences. Both problems could be overcome by SPAAC, where a metal catalyst is not needed. After optimization of reaction conditions, conversion yields of nearly 100% could be achieved. The consequences of these results for various applications of PSM-modified SURMOFs in the fields of membranes, optical coatings, catalysis, selective gas separation, and chemical sensing are briefly discussed.

Deposition of Metal-Organic Frameworks by Liquid-Phase Epitaxy: The Influence of Substrate Functional Group Density on Film Orientation

The liquid phase epitaxy (LPE) of the metal-organic framework (MOF) HKUST-1 has been studied for three different COOH-terminated templating organic surfaces prepared by the adsorption of self-assembled monolayers (SAMs) on gold substrates. Three different SAMs were used, mercaptohexadecanoic acid (MHDA), 4’-carboxyterphenyl-4-methanethiol (TPMTA) and 9-carboxy-10-(mercaptomethyl)triptycene (CMMT). The XRD data demonstrate that highly oriented HKUST-1 SURMOFs with an orientation along the (100) direction was obtained on MHDA-SAMs. In the case of the TPMTA-SAM, the quality of the deposited SURMOF films was found to be substantially inferior. Surprisingly, for the CMMT-SAMs, a different growth direction was obtained; XRD data reveal the deposition of highly oriented HKUST-1 SURMOFs grown along the (111) direction.

Oriented Circular Dichroism Analysis of Chiral Surface‐Anchored Metal–Organic Frameworks Grown by Liquid‐Phase Epitaxy and upon Loading with Chiral Guest Compounds

Oriented circular dichroism (OCD) is explored and successfully applied to investigate chiral surface-anchored metal-organic frameworks (SURMOFs) based on camphoric acid (D- and Lcam) with the composition [Cu2(Dcam)(2x)(Lcam)(2-2x)(dabco)]n (dabco = 1,4-diazabicyclo-[2.2.2]-octane). The three-dimensional chiral SURMOFs with high-quality orientation were grown on quartz glass plates by using a layer-by-layer liquid-phase epitaxy method. The growth orientation, as determined by X-ray diffraction (XRD), could be switched between the [001] and [110] direction by using either OH- or COOH-terminated substrates. These SURMOFs were characterized by using OCD, which confirmed the ratio as well as the orientation of the enantiomeric linker molecules. Theoretical computations demonstrate that the OCD band intensities of the enantiopure [Cu2(Dcam)2(dabco)]n grown in different orientations are a direct result of the anisotropic nature of the chiral SURMOFs. Finally, the enantiopure [Cu2(Dcam)2(dabco)]n and [Cu2(Lcam)2(dabco)]n SURMOFs were loaded with the two chiral forms of ethyl lactate [(+)-ethyl-D-lactate and (-)-ethyl-L-lactate)]. An enantioselective enrichment of >60 % was observed by OCD when the chiral host scaffold was loaded from the racemic mixture.