Christof Wöll

Controlling interpenetration in metal|[ndash]|organic frameworks by liquid-phase epitaxy

Metal-organic frameworks (MOFs) are highly porous materials generally consisting of two building elements: inorganic coupling units and organic linkers. These frameworks offer an enormous porosity, which can be used to store large amounts of gases and, as demonstrated in more recent applications, makes these compounds suitable for drug release. The huge sizes of the pores inside MOFs, however, also give rise to a fundamental complication, namely the formation of sublattices occupying the same space. This interpenetration greatly reduces the pore size and thus the available space within the MOF structure. We demonstrate here that the formation of the second, interpenetrated framework can be suppressed by using liquid-phase epitaxy on an organic template. This success demonstrates the potential of the step-by-step method to synthesize new classes of MOFs not accessible by conventional solvothermal methods.

Growth of aromatic molecules on solid substrates for applications in organic electronics

The growth of molecular adlayers on solid substrates is reviewed with aspecial emphasis on molecules of relevance for organic electronics. In particular,we will consider planar molecules with extended π-systems, namely acenes (anthracene, tetracene, pentacene), perylene, coronenes, diindenoperylene, 3,4,9,10-perylene-tetracarboxylicacid-dianhydride, poly-phenylenes, oligothiophenes, and phthalocyanines. Special consideration is given to the importance of the formation of ordered molecular overlayers, which are compared with the structure of the corresponding bulk crystals. Whenever possible, aspects relevant for device fabrication (morphology of deposited films, mobilities of charge carriers) will be addressed.

Surface chemistry of metal-organic frameworks at the liquid-solid interface.

Metal-organic frameworks (MOFs) are a fascinating class of novel inorganic-organic hybrid materials. They are essentially based on classic coordination chemistry and hold much promise for unique applications ranging from gas storage and separation to chemical sensing, catalysis, and drug release. The evolution of the full innovative potential of MOFs, in particular for nanotechnology and device integration, however requires a fundamental understanding of the formation process of MOFs. Also necessary is the ability to control the growth of thin MOF films and the positioning of size- and shape-selected crystals as well as MOF heterostructures on a given surface in a well-defined and oriented fashion. MOFs are solid-state materials typically formed by solvothermal reactions and their crystallization from the liquid phase involves the surface chemistry of their building blocks. This Review brings together various key aspects of the surface chemistry of MOFs.

Growth Mechanism of Metal–Organic Frameworks: Insights into the Nucleation by Employing a Step‐by‐Step Route

One step at a time: The in situ monitoring of the step-by-step formation of metal-organic frameworks (MOFs) by using surface plasmon resonance (SPR), allows the nucleation process and the formation of the secondary building units to be investigated. Growth rates on functionalized organic surfaces with different crystallographic orientations can also be studied.

Charge-transfer-induced structural rearrangements at both sides of organic/metal interfaces

Organic/metal interfaces control the performance of many optoelectronic organic devices, including organic light-emitting diodes or field-effect transistors. Using scanning tunnelling microscopy, low-energy electron diffraction, X-ray photoemission spectroscopy, near-edge X-ray absorption fine structure spectroscopy and density functional theory calculations, we show that electron transfer at the interface between a metal surface and the organic electron acceptor tetracyano-p-quinodimethane leads to substantial structural rearrangements on both the organic and metallic sides of the interface. These structural modifications mediate new intermolecular interactions through the creation of stress fields that could not have been predicted on the basis of gas-phase neutral tetracyano-p-quinodimethane conformation.

Vacuum level alignment at organic/metal junctions: “Cushion” effect and the interface dipole

The electronic level alignment of various organic molecules on metal surfaces has been determined by a combined experimental and theoretical effort. Using ab initio electronic structure calculations, it is demonstrated that the commonly observed interface dipole is largely due to a quantum-mechanical phenomenon resulting from exchange repulsion. Surprisingly, this physical effect, also referred to as Pauli repulsion dominates even in the case of aromatic molecules on Cu and Au surfaces, i.e., on interfaces that are of key importance in molecular electronics.

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

Self-metalation of 2H-tetraphenylporphyrin on Cu(111): An x-ray spectroscopy study

The bonding and the temperature-driven metalation of 2H-tetraphenylporphyrin (2H-TPP) on the Cu(111) surface under ultrahigh vacuum conditions were investigated by a combination of x-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy with density functional theory calculations. Thin films were prepared by organic molecular beam epitaxy and subsequent annealing. Our systematic study provides an understanding of the changes of the spectroscopic signature during adsorption and metalation. Specifically, we achieved a detailed peak assignment of the 2H-TPP multilayer data of the C1s and the N1s region. After annealing to 420 K both XPS and NEXAFS show the signatures of a metalloporphyrin, which indicates self-metalation at the porphyrin-substrate interface, resulting in Cu-TPP. Furthermore, for 2H-TPP monolayer samples we show how the strong influence of the copper surface is reflected in the spectroscopic signatures. Adsorption results in a strongly deformed macrocycle and a quenching of the first NEXAFS resonance in the nitrogen edge suggesting electron transfer into the LUMO. For Cu-TPP the spectroscopic data indicate a reduced interaction of first-layer molecules with the substrate as demonstrated by the relaxed macrocycle geometry.

Light-Driven Water Splitting for (Bio-)Hydrogen Production: Photosystem 2 as the Central Part of a Bioelectrochemical Device

Abstract To establish a semiartificial device for (bio-)hydrogen production utilizing photosynthetic water oxidation, we report on the immobilization of a Photosystem 2 on electrode surfaces. For this purpose, an isolated Photosystem 2 with a genetically introduced His tag from the cyanobacterium Thermosynechococcus elongatus was attached onto gold electrodes modified with thiolates bearing terminal Ni(II)-nitrilotriacetic acid groups. Surface enhanced infrared absorption spectroscopy showed the binding kinetics of Photosystem 2, whereas surface plasmon resonance measurements allowed the amount of protein adsorbed to be quantified. On the basis of these data, the surface coverage was calculated to be 0.29 pmol protein cm−2, which is in agreement with the formation of a monomolecular film on the electrode surface. Upon illumination, the generation of a photocurrent was observed with current densities of up to 14 μA cm−2. This photocurrent is clearly dependent on light quality showing an action spectrum similar to an isolated Photosystem 2. The achieved current densities are equivalent to the highest reported oxygen evolution activities in solution under comparable conditions.

Liquid-Phase Epitaxy of Multicomponent Layer-Based Porous Coordination Polymer Thin Films of [M(L)(P)0.5] Type: Importance of Deposition Sequence on the Oriented Growth

The progressive liquid-phase layer-by-layer (LbL) growth of anisotropic multicomponent layer-based porous coordination polymers (PCPs) of the general formula [M(L)(P)(0.5)] (M: Cu(2+), Zn(2+); L: dicarboxylate linker; P: dinitrogen pillar ligand) was investigated by using either pyridyl- or carboxyl-terminated self-assembled monolayers (SAMs) on gold substrates as templates. It was found that the deposition of smooth, highly crystalline, and oriented multilayer films of these PCPs depends on the conditions at the early growth cycles. In the case of a two-step process with an equimolar mixture of L and P, growth along the [001] direction is strongly preferred. However, employing a three-step scheme with full separation of all components allows deposition along the [100] direction on carboxyl-terminated SAMs. Interestingly, the growth of additional layers on top of previously grown oriented seeding layers proved to be insensitive to the particular growth scheme and full retention of the initial orientation, either along the [001] or [100] direction, was observed. This homo- and heteroepitaxial LbL growth allows full control over the orientation and the layer sequence, including introduction of functionalized linkers and pillars.