Hasan K. Arslan

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

Mechanical properties of metal-organic frameworks: An indentation study on epitaxial thin films

We have determined the hardness and Youngs modulus of a highly porous metal-organic framework (MOF) using a standard nanoindentation technique. Despite the very low density of these films, 1.22 g cm−3, Youngs modulus reaches values of almost 10 GPa for HKUST-1, demonstrating that this porous coordination polymer is substantially stiffer than normal polymers. This progress in characterizing mechanical properties of MOFs has been made possible by the use of high quality, oriented thin films grown using liquid phase epitaxy on modified Au substrates.

Surface-Anchored MOF-Based Photonic Antennae

The loading of a metal-organic framework (MOF), [Cu(3)(btc)(2)xH(2)O] HKUST-1, with europium β-diketonate complexes is studied with the goal to using the porous molecular framework as a photonic antenna. Whereas loading of HKUST-1 powder particles produced via the conventional solvothermal synthesis method was strongly hindered, for HKUST-1 SURMOFs, thin MOF films fabricated using the liquid phase epitaxy method, a high filling factor can be achieved. The optical properties of the HKUST-1-MOFs before and after loading were analysed with the aid of luminescence spectroscopy. Careful analysis of the absorption spectra reveals the presence of an effective energy transfer between the HKUST-1 framework and the Eu(3+) centers.

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.

Surface anchored metal-organic frameworks as stimulus responsive antifouling coatings

Surface-anchored, crystalline and oriented metal organic frameworks (SURMOFs) have huge potential for biological applications due to their well-defined and highly-porous structure. In this work we describe a MOF-based, fully autonomous system, which combines sensing, a specific response, and the release of an antimicrobial agent. The Cu-containing SURMOF, Cu-SURMOF 2, is stable in artificial seawater and shows stimulus-responsive anti-fouling properties against marine bacteria. When Cobetia marina adheres on the SURMOF, the framework’s response is lethal to the adhering microorganism. A thorough analysis reveals that this response is induced by agents secreted from the microbes after adhesion to the substrate, and includes a release of Cu ions resulting from a degradation of the SURMOF. The stimulus-responsive antifouling effect of Cu-SURMOF 2 demonstrates the first application of Cu-SURMOF 2 as autonomous system with great potential for further microbiological and cell culture applications.