Publications by authors named "Lars Heinke"

52 Publications

Chirality Remote Control in Nanoporous Materials by Circularly Polarized Light.

J Am Chem Soc 2021 May 29;143(18):7059-7068. Epub 2021 Apr 29.

Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.

The ability to dynamically control chirality remains a grand challenge in chemistry. Although many molecules possess chiral isomers, lacking their isolation, for instance during photoisomerization, results in racemic mixtures with suppressed enantiospecific chiral properties. Here, we present a nanoporous solid in which chirality and enantioselective enrichment is induced by circularly polarized light (CPL). The material is based on photoswitchable fluorinated azobenzenes attached to the scaffold of a crystalline metal-organic framework (MOF). The azobenzene undergoes -to--photoisomerization upon irradiation with green light and reverts back to upon violet light. While each moiety in conformation is chiral, we show the isomer also possesses a nonplanar, chiral conformation. During photoisomerization with unpolarized light, no enantiomeric enrichment is observed and both isomers, - and - as well as and -, respectively, are formed in identical quantities. In contrast, CPL causes chiral photoresolution, resulting in an optically active material. Right-CPL selectively excites - and - enantiomers, producing a MOF with enriched -enantiomers, and . The induction of optical activity is reversible and only depends on the light-handedness. As shown by first-principle DFT calculations, while both, and , are stabilized in nonplanar, chiral conformations in the MOF, the isomer adopts a planar, achiral form in solution, as verified experimentally. This shows that the chiral photoresolution is enabled by the linker reticulation in the MOF. Our study demonstrates the induction of chirality and optical activity in solid materials by CPL and opens new opportunities for chiral resolution and information storage with CPL.
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http://dx.doi.org/10.1021/jacs.1c01693DOI Listing
May 2021

Structural and Dynamic Insights into the Conduction of Lithium-Ionic-Liquid Mixtures in Nanoporous Metal-Organic Frameworks as Solid-State Electrolytes.

ACS Appl Mater Interfaces 2021 May 27;13(18):21166-21174. Epub 2021 Apr 27.

Karlsruhe Institute of Technology (KIT), Institute of Functional Interfaces (IFG), Hermann-von-Helmholtz-Platz 1, Eggenstein-Leopoldshafen 76344, Germany.

Metal-organic framework (MOF)-based separators in Li-ion batteries (LIBs) have the potential to improve the battery performance. The mobility and conduction of lithium and organic ionic liquids (ILs) in these materials acting as (quasi) solid-state electrolytes are crucial for the battery power output. Here, we investigate the mobility of a Li-based IL in MOF nanopores and unveil the details of the conduction mechanism by molecular dynamics (MD) simulations. A complex conductivity depending on the Li-IL loading and on the IL composition is observed. Most importantly, the presence of Li prevents the collapse of the conductivity at high IL loadings. The fully atomistic MD simulations including guest-guest and guest-host interactions elucidate the competing mechanisms: Li follows a Grotthuss-like conduction mechanism with large mobility. While at small pore fillings, the Li conduction is limited by the large distance between the anions facilitating the Grotthuss-like conduction; the conduction at high pore fillings is governed by field-induced concentration inhomogeneities. Because of the small MOF pore windows, which hinders the simultaneous passage of the large IL cations and anions in opposite directions, the IL shows field-induced MOF pore blocking and ion bunching. The regions of low anion concentration and high cation concentration represent barriers for Li, decreasing its mobility. In comparison to Li-free IL, the IL bunching effect is attenuated by the formation of charge-neutral Li-anion complexes, resulting in a tremendously increased conductivity at maximum pore filling. The exploitation of this mechanism may enhance the development of advanced batteries based on IL and nanoporous separators.
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http://dx.doi.org/10.1021/acsami.1c00366DOI Listing
May 2021

Programmed Molecular Assembly of Abrupt Crystalline Organic/Organic Heterointerfaces Yielding Metal-Organic Framework Diodes with Large On-Off Ratios.

Adv Sci (Weinh) 2021 Apr 21;8(7):2001884. Epub 2021 Jan 21.

Karlsruhe Institute of Technology (KIT) Institute of Functional Interfaces (IFG) Hermann-von-Helmholtz-Platz 1 Eggenstein-Leopoldshafen 76344 Germany.

Structurally well-defined, crystalline organic/organic heterojunctions between C- and anthracene-based semiconductors are realized via layer-by-layer deposition of metal-organic framework, MOF, thin films. As demonstrated by X-ray diffraction, perfect epitaxy is achieved by adjusting the lattice constants of the two different MOFs. Deposition of top electrodes allows to fabricate p-n as well as n-p devices. Measurements of the electrical properties reveal the presence of high-performance diodes, with a current on/off ratio of up to 6 orders of magnitude and an ideality factor close to unity. The crystalline nature of the abrupt organic/organic heterojunction provides the basis for a rational, simulation-based optimization and tailoring of such organic semiconductor interfaces.
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http://dx.doi.org/10.1002/advs.202001884DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8024988PMC
April 2021

Photoswitchable Metal-Organic Framework Thin Films: From Spectroscopy to Remote-Controllable Membrane Separation and Switchable Conduction.

Langmuir 2021 Jan 21;37(1):2-15. Epub 2020 Dec 21.

Karlsruhe Institute of Technology (KIT), Institute of Functional Interfaces (IFG), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.

The preparation of functional materials from photoswitchable molecules where the molecular changes multiply to macroscopic effects presents a great challenge in material science. An attractive approach is the incorporation of the photoswitches in nanoporous, crystalline metal-organic frameworks, MOFs, often showing remote-controllable chemical and physical properties. Because of the short light-penetration depth, thin MOF films are particularly interesting, allowing the entire illumination of the material. In the present progress report, we review and discuss the status of photoswitchable MOF films. These films may serve as model systems for quantifying the isomer switching yield by infrared and UV-vis spectroscopy as well as for uptake experiments exploring the switching effects on the host-guest interaction, especially on guest adsorption and diffusion. In addition, the straightforward device integration facilitates various experiments. In this way, unique features were demonstrated, such as photoswitchable membrane separation with continuously tunable selectivity, light-switchable proton conductivity of the guests in the pores, and remote-controllable electronic conduction.
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http://dx.doi.org/10.1021/acs.langmuir.0c02859DOI Listing
January 2021

Introducing electrical conductivity to metal-organic framework thin films by templated polymerization of methyl propiolate.

Nanoscale 2020 Dec;12(48):24419-24428

Institute of Functional Interfaces, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344, Eggenstein-Leopoldshafen, Germany.

We herein present a case study on the templated, Pd-catalyzed polymerization reaction of methyl propiolate in the confined pore space of three different surface anchored metal-organic framework (SURMOF) systems in order to introduce electrical conductivity to MOF thin films and provide predictions for potential device integrations. To gain comprehensive insight into the influence of the template on polymerization, we chose Cu(bpdc), Cu2(bdc)2(dabco) and HKUST-1 because of their different types of pore channels, 1D, quasi-1D and 3D, and their free pore volumes. Well-defined MOF thin films were prepared using layer-by-layer deposition, which allows for the application of several characterization techniques not applicable for conventional powder MOFs. With SEM, AFM, XRD, MALDI-ToF/MS, ToF-SIMS and QCM, we were able to investigate the behaviour of the polymer formation. For lower dimensional pore channels, we find a depot-like release of monomeric units leading to top-layer formation determined by desorption kinetics, whereas for the 3D channels, quick release of an excess amount of monomers was observed and polymerization proceeds perfectly. Despite polymerization issues, control over the maximum chain lengths and the molecular weight distribution was achieved depending on the dimensionality of the pore systems. For the HKUST-1 system, polymerization was optimized and we were able to measure the electrical conductivity introduced by the conjugated polymer inside the channels.
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http://dx.doi.org/10.1039/d0nr06848aDOI Listing
December 2020

An Enantioselective e-Nose: An Array of Nanoporous Homochiral MOF Films for Stereospecific Sensing of Chiral Odors.

Angew Chem Int Ed Engl 2021 02 15;60(7):3566-3571. Epub 2020 Dec 15.

Karlsruhe Institute of Technology (KIT), Institute of Functional Interfaces (IFG), Hermann-von-Helmholtz-Platz 1, 76344, Eggenstein-Leopoldshafen, Germany.

Chirality is essential in nature and often pivotal for biological information transfer, for example, via odor messenger molecules. While the human nose can distinguish the enantiomers of many chiral odors, the technical realization by an artificial sensor or an electronic nose, e-nose, remains a challenge. Herein, we present an array of six sensors coated with nanoporous metal-organic framework (MOF) films of different homochiral and achiral structures, working as an enantioselective e-nose. While the achiral-MOF-film sensors show identical responses for both isomers of one chiral odor molecule, the responses of the homochiral MOF films differ for different enantiomers. By machine learning algorithms, the combined array data allow the stereoselective identification of all compounds, here tested for five pairs of chiral odor molecules. We foresee the chiral-MOF-e-nose, able to enantioselectively detect and discriminate chiral odors, to be a powerful approach towards advanced odor sensing.
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http://dx.doi.org/10.1002/anie.202013227DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7898876PMC
February 2021

Tuning Optical Properties by Controlled Aggregation: Electroluminescence Assisted by Thermally-Activated Delayed Fluorescence from Thin Films of Crystalline Chromophores.

Chemistry 2020 Dec 17;26(71):17016-17020. Epub 2020 Nov 17.

Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz Platz-1, 76344, Eggenstein-Leopoldshafen, Germany.

Several photophysical properties of chromophores depend crucially on intermolecular interactions. Thermally-activated delayed fluorescence (TADF) is often influenced by close packing of the chromophore assembly. In this context, the metal-organic framework (MOF) approach has several advantages: it can be used to steer aggregation such that the orientation within aggregated structures can be predicted using rational approaches. We demonstrate this design concept for a DPA-TPE (diphenylamine-tetraphenylethylene) chromophore, which is non-emissive in its solvated state due to vibrational quenching. Turning this DPA-TPE into a ditopic linker makes it possible to grow oriented MOF thin films exhibiting pronounced green electroluminescence with low onset voltages. Measurements at different temperatures clearly demonstrate the presence of TADF. Finally, this work reports that the layer-by-layer process used for MOF thin film deposition allows the integration of the TADF-DPA-TPE in a functioning LED device.
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http://dx.doi.org/10.1002/chem.202003712DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7839528PMC
December 2020

Conductive Metal-Organic Framework Thin Film Hybrids by Electropolymerization of Monosubstituted Acetylenes.

ACS Appl Mater Interfaces 2020 Jul 23;12(27):30972-30979. Epub 2020 Jun 23.

Institute of Nanotechnology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, Eggenstein-Leopoldshafen 76344, Germany.

1-Hexyne monomers were potentiostatically electropolymerized upon confinement in 1D channels of a surface-mounted metal-organic framework Cu(BDC) (SURMOF-2). A layer-by-layer deposition method allowed for SURMOF depostition on substrates with prepatterned electrodes, making it possible to characterize electrical conductivity in situ, i.e., without having to delaminate the conductive polymer thin film. Successful polymerization was evidenced by mass spectroscopy, and the electrical measurements demonstrated an increase of the electrical conductivity of the MOF material by 8 orders of magnitude. Extensive DFT calculations revealed that the final conductivity is limited by electron hopping between the conductive oligomers.
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http://dx.doi.org/10.1021/acsami.0c07036DOI Listing
July 2020

Interplay of Electronic and Steric Effects to Yield Low-Temperature CO Oxidation at Metal Single Sites in Defect-Engineered HKUST-1.

Angew Chem Int Ed Engl 2020 Jun 17;59(26):10514-10518. Epub 2020 Apr 17.

Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), 76344, Eggenstein-Leopoldshafen, Germany.

In contrast to catalytically active metal single atoms deposited on oxide nanoparticles, the crystalline nature of metal-organic frameworks (MOFs) allows for a thorough characterization of reaction mechanisms. Using defect-free HKUST-1 MOF thin films, we demonstrate that Cu /Cu dimer defects, created in a controlled fashion by reducing the pristine Cu /Cu pairs of the intact framework, account for the high catalytic activity in low-temperature CO oxidation. Combining advanced IR spectroscopy and density functional theory we propose a new reaction mechanism where the key intermediate is an uncharged O species, weakly bound to Cu /Cu . Our results reveal a complex interplay between electronic and steric effects at defect sites in MOFs and provide important guidelines for tailoring and exploiting the catalytic activity of single metal atom sites.
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http://dx.doi.org/10.1002/anie.202000385DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7318571PMC
June 2020

Advanced Photoresponsive Materials Using the Metal-Organic Framework Approach.

Adv Mater 2020 May 25;32(20):e1905227. Epub 2019 Nov 25.

Karlsruher Institut für Technologie (KIT), Institut für Funktionelle Grenzflächen (IFG), Hermann-von-Helmholtz-Platz 1, 76344, Eggenstein-Leopoldshafen, Germany.

When fabricating macroscopic devices exploiting the properties of organic chromophores, the corresponding molecules need to be condensed into a solid material. Since optical absorption properties are often strongly affected by interchromophore interactions, solids with a well-defined structure carry substantial advantages over amorphous materials. Here, the metal-organic framework (MOF)-based approach is presented. By appropriate functionalization, most organic chromophores can be converted to function as linkers, which can coordinate to metal or metal-oxo centers so as to yield stable, crystalline frameworks. Photoexcitations in such chromophore-based MOFs are surveyed, with a special emphasis on light-switchable MOFs from photochromic molecules. The conventional powder form of MOFs obtained using solvothermal approaches carries certain disadvantages for optical applications, such as limited efficiency resulting from absorption and light scattering caused by the (micrometer-sized) powder particles. How these problems can be avoided by using MOF thin films is demonstrated.
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http://dx.doi.org/10.1002/adma.201905227DOI Listing
May 2020

Light-Switchable One-Dimensional Photonic Crystals Based on MOFs with Photomodulatable Refractive Index.

J Phys Chem Lett 2019 Nov 16;10(21):6626-6633. Epub 2019 Oct 16.

Department of Chemistry and IRIS Adlershof , Humboldt-Universität zu Berlin , 12489 Berlin , Germany.

Photonic crystals are solids with regular structures having periodicities comparable to the wavelength of light. Here, we showcase the photomodulation of the refractive index of a crystalline material and present a quasi-one-dimensional photonic crystal with remote-controllable optical properties. The photonic material is composed of layers of TiO and films of a nanoporous metal-organic framework (MOF) with azobenzene side groups. While the rigid MOF lattice is unaffected, the optical density is reversibly modified by the light-induced --azobenzene isomerization. Spectroscopic ellipsometry and precise DFT calculations show the optical-density change results from the different orbital localizations of the azobenzene isomers and their tremendously different oscillator strengths. The photomodulation of the MOF refractive index controls the optical properties of the quasi-one-dimensional photonic crystal with Bragg reflexes reversibly shifted by more than 4 nm. This study may path the way to photoswitchable photonic materials applied in advanced, tunable optical components and lens coatings and in light-based information processing.
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http://dx.doi.org/10.1021/acs.jpclett.9b02614DOI Listing
November 2019

Switching the enantioselectivity of nanoporous host materials by light.

Chem Commun (Camb) 2019 Jul;55(60):8776-8779

Karlsruhe Institute of Technology (KIT), Institute of Functional Interfaces (IFG), 76344 Eggenstein-Leopoldshafen, Germany.

A chiral photoswitchable nanoporous material with remote-controllable enantioselective adsorption capacity is presented. This metal-organic framework possesses both homochiral d-camphoric acid and light-responsive azobenzene moieties. Although the structure at the chiral moieties is unaffected, the trans-cis-azobenzene-photoisomerization changes the pore environment and, thus, switches the enantioselective adsorption behavior of the homochiral MOF.
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http://dx.doi.org/10.1039/c9cc02849hDOI Listing
July 2019

Photoconductivity in Metal-Organic Framework (MOF) Thin Films.

Angew Chem Int Ed Engl 2019 Jul 30;58(28):9590-9595. Epub 2019 May 30.

Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), 76344, Eggenstein-Leopoldshafen, Germany.

Photoconductivity is a characteristic property of semi-conductors. Herein, we present a photo-conducting crystalline metal-organic framework (MOF) thin film with an on-off photocurrent ratio of two orders of magnitude. These oriented, surface-mounted MOF thin films (SURMOFs), contain porphyrin in the framework backbone and C guests, loaded in the pores using a layer-by-layer process. By comparison with results obtained for reference MOF structures and based on DFT calculations, we conclude that donor-acceptor interactions between the porphyrin of the host MOF and the C guests give rise to a rapid charge separation. Subsequently, holes and electrons are transported through separate channels formed by porphyrin and by C , respectively. The ability to tune the properties and energy levels of the porphyrin and fullerene, along with the controlled organization of donor-acceptor pairs in this regular framework offers potential to increase the photoconduction on-off ratio.
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http://dx.doi.org/10.1002/anie.201904475DOI Listing
July 2019

Bunching and Immobilization of Ionic Liquids in Nanoporous Metal-Organic Framework.

Nano Lett 2019 03 8;19(3):2114-2120. Epub 2019 Mar 8.

Karlsruhe Institute of Technology (KIT) , Institute of Functional Interfaces (IFG) , Hermann-von-Helmholtz-Platz 1 , 76344 Eggenstein-Leopoldshafen , Germany.

Room-temperature ionic liquids (ILs) are a unique, novel class of designer solvents and materials with exclusive properties, attracting substantial attention in fields like energy storage and supercapacitors as well as in ion-based signal processing and electronics. For most applications, ILs need to be incorporated or embedded in solid materials like porous hosts. We investigate the dynamic structure of ILs embedded in well-defined pores of metal-organic frameworks (MOFs). The experimental data combined with molecular dynamics simulations unveil astonishing dynamic properties of the IL in the MOF nanoconfinement. At low IL loadings, the ions drift in the pores along the electric field, whereas at high IL loadings, collective field-induced interactions of the cations and anions lead to blocking the transport, thus suppressing the ionic mobility and tremendously decreasing the conductivity. The mutual pore blockage causes immobilized ions in the pores, resulting in a highly inhomogeneous IL density and bunched-up ions at the clogged pores. These results provide novel molecular-level insights into the dynamics of ILs in nanoconfinement, significantly enhancing the tunability of IL material properties.
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http://dx.doi.org/10.1021/acs.nanolett.8b04694DOI Listing
March 2019

Dissolving uptake-hindering surface defects in metal-organic frameworks.

Chem Sci 2019 Jan 10;10(1):153-160. Epub 2018 Oct 10.

Institute of Functional Interfaces (IFG) , Karlsruhe Institute of Technology (KIT) , Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen , Germany . Email:

Metal-organic frameworks (MOFs) have unique properties which make them perfectly suited for various adsorption and separation applications; however, their uses and efficiencies are often hindered by their limited stability. When most MOFs are exposed to water or humid air, the MOF structure, in particular at the surface, is destroyed, creating surface defects. These surface defects are surface barriers which tremendously hinder the uptake and release of guest molecules and, thus, massively decrease the performance in any application of MOFs. Here, the destruction by exposure to water vapor is investigated by using well-defined MOF films of type HKUST-1 as a model system for uptake experiments with different-sized probe molecules as well as for spectroscopic investigations, complemented by density functional theory calculations of the defect structure. In addition to the characterization of the surface defects, it is found that the pristine MOF structure can be regenerated. We show that the surface defects can be dissolved by exposure to the synthesis solvent, here ethanol, enabling fast uptake and release of guest molecules. These findings show that the storage of MOF materials in a synthesis solvent results in healing of surface defects and enables ideal performance of MOF materials.
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http://dx.doi.org/10.1039/c8sc03735cDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6328000PMC
January 2019

Surface-Mounted Metal-Organic Frameworks: Crystalline and Porous Molecular Assemblies for Fundamental Insights and Advanced Applications.

Adv Mater 2019 Jun 30;31(26):e1806324. Epub 2019 Jan 30.

Karlsruher Institut für Technologie (KIT), Institut für Funktionelle Grenzflächen (IFG), Hermann-von-Helmholtz-Platz 1, 76344, Eggenstein-Leopoldshafen, Germany.

Metal-organic frameworks (MOFs) are crystalline coordination polymers, assembled from inorganic nodes connected by organic linker molecules. An enormous surface area, huge compositional variety, regular structure, and favorable mechanical properties are among their outstanding properties. Monolithic MOF thin films, i.e., surface-mounted metal-organic frameworks (SURMOFs), with high degree of structural order and adjustable defect density, can be prepared on solid substrates using layer-by-layer techniques. Recent studies where SURMOFs served as model systems for quantitative studies of molecular interactions in porous media, including diffusion, are reviewed. Moreover, SURMOFs are ideally suited for the incorporation of photoactive molecules as well as to study electrical transport through crystalline molecular assemblies. Recent work has demonstrated that the realization of crystalline chromophore assemblies via the SURMOF approach allows the study of fundamental aspects of exciton transport, exciton channeling, and photon upconversion at internal interfaces in organic semiconductor materials. Due to their crystalline nature, MOF materials are well suited for quantitative comparisons with theoretical results; especially, since defect densities and types can be characterized and varied in a straightforward fashion. The active role of these nanoporous films in advanced applications, like for remote-controlled release of molecules, membranes with photoswitchable selectivity, and ion-conductors with adjustable conductivity, are also emphasized.
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http://dx.doi.org/10.1002/adma.201806324DOI Listing
June 2019

Conductance Photoswitching of Metal-Organic Frameworks with Embedded Spiropyran.

Angew Chem Int Ed Engl 2019 Jan 20;58(4):1193-1197. Epub 2018 Dec 20.

Karlsruhe Institute of Technology (KIT), Institute of Functional Interfaces (IFG), Hermann-von-Helmholtz-Platz 1, 76344, Eggenstein-Leopoldshafen, Germany.

Conductive metal-organic frameworks (MOFs) as well as smart, stimuli-responsive MOF materials have attracted considerable attention with respect to advanced applications in energy harvesting and storage as well as in signal processing. Here, the conductance of MOF films of type UiO-67 with embedded photoswitchable nitro-substituted spiropyrans was investigated. Under UV irradiation, the spiropyran (SP) reversibly isomerizes to the open merocyanine (MC) form, a zwitterionic molecule with an extended conjugated π-system. The light-induced SP-MC isomerization allows for remote control over the conductance of the [email protected] MOF film, and the conductance can be increased by one order of magnitude. This research has the potential to contribute to the development of a new generation of photoelectronic devices based on smart hybrid materials.
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http://dx.doi.org/10.1002/anie.201811458DOI Listing
January 2019

Water as a modulator in the synthesis of surface-mounted metal-organic framework films of type HKUST-1.

Dalton Trans 2018 Nov;47(46):16474-16479

Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), 76344 Eggenstein-Leopoldshafen, Germany.

Thin films of nanoporous metal-organic frameworks, MOFs, are widely used for various purposes, ranging from molecular separation to electronic and optical applications. Ultrathin MOF films prepared in a controlled layer-by-layer fashion, also referred to as surface-mounted MOFs or SURMOFs, have attracted particular attention. While many aspects of SURMOF synthesis have been investigated and optimized, the impact of growth modulators, which are well established in bulk MOF synthesis, has so far received only a little attention in the context of layer-by-layer approaches. Here, we investigate the impact of water as a growth modulator during the synthesis of thin MOF films of type HKUST-1. We find that water has a tremendous impact on the crystallinity and on the defect-density of the prepared MOF films. The use of the optimized water concentrations allows considerable improvement in the SURMOF crystallinity and at the same time reduces their defect density. This study shows that water is an important modulator in MOF thin film growth and can be used to tune the material from a low defect to high defect state.
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http://dx.doi.org/10.1039/c8dt03310bDOI Listing
November 2018

Switching the Proton Conduction in Nanoporous, Crystalline Materials by Light.

Adv Mater 2018 Feb 8;30(8). Epub 2018 Jan 8.

Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), 76344, Eggenstein-Leopoldshafen, Germany.

Proton conducting nanoporous materials attract substantial attention with respect to applications in fuel cells, supercapacitors, chemical sensors, and information processing devices inspired by biological systems. Here, a crystalline, nanoporous material which offers dynamic remote-control over the proton conduction is presented. This is realized by using surface-mounted metal-organic frameworks (SURMOFs) with azobenzene side groups that can undergo light-induced reversible isomerization between the stable trans and cis states. The trans-cis photoisomerization results in the modulation of the interaction between MOF and guest molecules, 1,4-butanediol and 1,2,3-triazole; enabling the switching between the states with significantly increased (trans) and reduced (cis) conductivity. Quantum chemical calculations show that the trans-to-cis isomerization results in the formation of stronger hydrogen bridges of the guest molecules with the azo groups, causing stronger bonding of the guest molecules and, as a result, smaller proton conductivity. It is foreseen that photoswitchable proton-conducting materials may find its application in advanced, remote-controllable chemical sensors, and a variety of devices based on the conductivity of protons or other charged molecules, which can be interfaced with biological systems.
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http://dx.doi.org/10.1002/adma.201706551DOI Listing
February 2018

Multi-Component Uptake of Dye Molecules by Films of Nanoporous Metal-Organic Frameworks.

Chemphyschem 2017 Dec 15;18(24):3548-3552. Epub 2017 Nov 15.

Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344, Eggenstein-Leopoldshafen, Germany.

Nanoporous materials, such as metal-organic frameworks (MOFs), enable the separation of various molecular mixtures. Thus, detailed knowledge of multi-component diffusion properties of the guest molecules in the pores is vital. Here, UV/Vis absorption spectroscopy combined with step-by-step-synthesized MOF films with high transparency are used to measure the multi-component dye uptake and diffusion in a straightforward fashion. The time-resolved single-component, binary- and ternary-mixture uptakes of methylene blue (MB), nuclear fast red (NFR) and acid yellow 17 (AY) by MOF films of type HKUST-1 from ethanolic solutions with low dye concentrations were investigated. It was found that the diffusion of MB is approximately one order of magnitude slower than the diffusion of NFR or AY, independent of whether it is a single-component uptake or a mixture uptake.
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http://dx.doi.org/10.1002/cphc.201701023DOI Listing
December 2017

Defects as Color Centers: The Apparent Color of Metal-Organic Frameworks Containing Cu-Based Paddle-Wheel Units.

ACS Appl Mater Interfaces 2017 Oct 16;9(42):37463-37467. Epub 2017 Oct 16.

Institute of Functional Interfaces (IFG) and ‡Institute for Nanotechnology (INT), Karlsruhe Institute of Technology , Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.

As in the case of other semiconducting materials, optical and electronic properties of metal-organic frameworks (MOFs) depend critically on defect densities and defect types. We demonstrate here that, in addition to the influence of imperfections on MOF chemical properties like guest binding energies and catalytic activity, the optical properties of these crystalline molecular solids also crucially depend on deviations from the perfect crystalline structure. By recording UV-vis absorption spectra for MOF thin films of particularly high quality, we demonstrate that low-defect samples of an important MOF, HKUST-1, are virtually colorless. Electronic structure calculations of the excited states by employing complete active space self-consistent field (CASSCF) calculations show that the d-d excitations in defects result in the typical green color of the MOF material synthesized by conventional methods.
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http://dx.doi.org/10.1021/acsami.7b12045DOI Listing
October 2017

Effects of blood flow restriction during moderate-intensity eccentric knee extensions.

J Physiol Sci 2018 Sep 9;68(5):589-599. Epub 2017 Sep 9.

German Research Centre of Elite Sport-Momentum, German Sport University Cologne, Am Sportpark Müngersdorf 6, 50933, Cologne, Germany.

We investigated if blood flow restriction (BFR, cuff pressure 20 mmHG below individual occlusion pressure) increases metabolic stress, hormonal response, release of muscle damage markers, and muscle swelling induced by moderate-intensity eccentric contractions. In a randomized, matched-pair design, 20 male subjects (25.3 ± 3.3 years) performed four sets of unilateral eccentric knee extensions (75% 1RM) to volitional failure with (IG) or without (CG) femoral BFR. Despite significant differences of performed repetitions between IG (85.6 ± 15.4 repetitions) and CG (142.3 ± 44.1 repetitions), peak values of lactate (IG 7.0 ± 1.4 mmol l, CG 6.9 ± 2.7 mmol l), growth-hormone (IG 4.9 ± 4.8 ng ml, CG 5.2 ± 3.5 ng ml), insulin-like growth factor 1 (IG 172.1 ± 41.9 ng ml, CG 178.7 ± 82.1 ng ml), creatine-kinase (IG 625.5 ± 464.8 U l, CG 510.7 ± 443.5 U l), the absolute neutrophil count (IG 7.9 ± 1.3 10 µl, CG 8.7 ± 2.0 10 µl), induced muscle swelling of rectus femoris and vastus lateralis and perceived pain did not differ. The present data indicate that BFR is suitable to intensify eccentric exercises.
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http://dx.doi.org/10.1007/s12576-017-0568-2DOI Listing
September 2018

Stimuli-Responsive Metal-Organic Frameworks with Photoswitchable Azobenzene Side Groups.

Macromol Rapid Commun 2018 Jan 31;39(1). Epub 2017 Jul 31.

Karlsruhe Institute of Technology (KIT), Institute of Functional Interfaces (IFG), Hermann-von-Helmholtz-Platz 1, 76344, Eggenstein-Leopoldshafen, Germany.

Metal-organic frameworks (MOFs) are nanoporous, crystalline hybrid materials, which enable various functionalities by incorporating functional organic molecules. By using organic linker molecules that possess photoswitchable azobenzene side groups, the remote control over certain properties was introduced to MOFs. Different MOF materials in the form of powders and thin films have been used to demonstrate the photoswitching. The applications of these stimuli-responsive nanoporous solids range from switching the adsorption capacity of various gases over remote-controlled release of guest molecules to continuously tunable membrane separation of molecular mixtures. A particular focus of this review is the effect of the azobenzene photoswitching on the host-guest interaction, enabling smart applications of the material. Steric hindrance, which may suppress the photoswitching in some MOF structures, is also discussed.
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http://dx.doi.org/10.1002/marc.201700239DOI Listing
January 2018

Photoswitchable nanoporous films by loading azobenzene in metal-organic frameworks of type HKUST-1.

Chem Commun (Camb) 2017 Jul;53(57):8070-8073

Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.

Photoswitchable metal-organic frameworks (MOFs) enable the dynamic remote control of their key properties. Here, a readily producible approach is presented where photochromic molecules, i.e. azobenzene (AB) and o-tetrafluoroazobenzene (tfAB), are loaded in MOF films of type HKUST-1. These nanoporous films, which can be reversibly switched with UV/visible or only visible light, have remote-controllable guest uptake properties.
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http://dx.doi.org/10.1039/c7cc00961eDOI Listing
July 2017

Switching Thin Films of Azobenzene-Containing Metal-Organic Frameworks with Visible Light.

Chemistry 2017 Apr 3;23(23):5434-5438. Epub 2017 Apr 3.

Chemistry of Oxydic and Organic Interfaces, Institute of Functional Interfaces, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344, Eggenstein-Leopoldshafen, Germany.

Stimuli-responsive molecules change their properties when exposed to external signals, such as light, and enable the preparation of smart materials. UV light, which often destroys organic materials, is typically required for activating the desired response of photoswitchable compounds, significantly limiting the potential applications of light-operated smart materials. Herein, we present the first metal-organic framework (MOF), which enables reversible modulation of key properties upon irradiation with visible light only. The fluorinated azobenzene side groups in the MOF structure can be reversibly switched between the trans and cis state by green and violet light, avoiding UV light. It was demonstrated that the uptake of guest molecules by these MOF films can be switched in a fully remote-controlled way. The membrane separation of hydrogen/hydrocarbon mixtures was investigated. The light-induced changes of the MOF pore size result in the switching of the permeation and of the selection factor.
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http://dx.doi.org/10.1002/chem.201700989DOI Listing
April 2017

Sprayable, Large-Area Metal-Organic Framework Films and Membranes of Varying Thickness.

Chemistry 2017 Feb 20;23(10):2294-2298. Epub 2017 Jan 20.

Karlsruhe Institute of Technology (KIT), Institute of Functional Interfaces (IFG), Hermann-von-Helmholtz-Platz 1, 76344, Eggenstein-Leopoldshafen, Germany.

Despite their huge potential for efficient molecular separation, the fabrication of membranes from metal-organic frameworks (MOFs) remains a major challenge. The powders obtained by the conventional solvothermal MOF syntheses are difficult to process, and as a result the fabrication of well-performing, large-area MOF-based membranes is still awaiting success. The deposition of MOF thin films suited for membrane applications is demonstrated by employing a step-by-step spray method. This method can be scaled up to obtain industrially relevant membrane areas and a continuous process is also possible. The performance of sprayed HKUST-1-based membranes by the separation of a binary H /CO mixture is also demonstrated. Furthermore, this approach enables the control of the MOF film thickness, and thus controlling the permeance and the selectivity of the membrane.
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http://dx.doi.org/10.1002/chem.201606056DOI Listing
February 2017

Tunable molecular separation by nanoporous membranes.

Nat Commun 2016 12 20;7:13872. Epub 2016 Dec 20.

Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.

Metal-organic frameworks offer tremendous potential for efficient separation of molecular mixtures. Different pore sizes and suitable functionalizations of the framework allow for an adjustment of the static selectivity. Here we report membranes which offer dynamic control of the selectivity by remote signals, thus enabling a continuous adjustment of the permeate flux. This is realized by assembling linkers containing photoresponsive azobenzene-side-groups into monolithic, crystalline membranes of metal-organic frameworks. The azobenzene moieties can be switched from the trans to the cis configuration and vice versa by irradiation with ultraviolet or visible light, resulting in a substantial modification of the membrane permeability and separation factor. The precise control of the cis:trans azobenzene ratio, for example, by controlled irradiation times or by simultaneous irradiation with ultraviolet and visible light, enables the continuous tuning of the separation. For hydrogen:carbon-dioxide, the separation factor of this smart membrane can be steplessly adjusted between 3 and 8.
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http://dx.doi.org/10.1038/ncomms13872DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5187437PMC
December 2016

Photoswitchable Adsorption in Metal-Organic Frameworks Based on Polar Guest-Host Interactions.

Chemphyschem 2015 Dec 26;16(18):3779-83. Epub 2015 Oct 26.

Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344, Eggenstein-Leopoldshafen, Germany.

Reversible remote-controlled switching of the properties of nanoporous metal-organic frameworks (MOFs) is enabled by incorporating photoswitchable azobenzene. The interaction of the host material with different guest molecules, which is crucial for all applications, is precisely studied using thin MOF films of the type Cu2 (BDC)2 (AzoBipyB). A molecule-specific effect of the photoswitching, based on dipole-dipole interactions, is found.
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http://dx.doi.org/10.1002/cphc.201500829DOI Listing
December 2015

Transport in Nanoporous Materials Including MOFs: The Applicability of Fick's Laws.

Angew Chem Int Ed Engl 2015 Nov 8;54(48):14580-3. Epub 2015 Oct 8.

Department of Interface Physics, University of Leipzig, Linnéstrasse 5, 04103 Leipzig (Germany).

Diffusion in nanoporous host-guest systems is often considered to be too complicated to comply with such "simple" relationships as Fick's first and second law of diffusion. However, it is shown herein that the microscopic techniques of diffusion measurement, notably the pulsed field gradient (PFG) technique of NMR spectroscopy and microimaging by interference microscopy (IFM) and IR microscopy (IRM), provide direct experimental evidence of the applicability of Fick's laws to such systems. This remains true in many situations, even when the detailed mechanism is complex. The limitations of the diffusion model are also discussed with reference to the extensive literature on this subject.
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http://dx.doi.org/10.1002/anie.201506954DOI Listing
November 2015

cis-to-trans isomerization of azobenzene investigated by using thin films of metal-organic frameworks.

Phys Chem Chem Phys 2015 Sep 10;17(35):22721-5. Epub 2015 Aug 10.

Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.

The activation barrier for cis-to-trans isomerization is a key parameter for governing the properties of photoswitchable molecules. This quantity can be computed by using theoretical methods, but experimental determination is not straightforward. Photoswitchable molecules typically do not change their conformation in the pure crystalline state. When the molecules are in solution, the switching is affected by the viscosity and polarity of the solvent and when embedded in polymers, the conformational change is affected by the polymer matrix. Here, we describe a novel approach where the photoswitchable group is integrated in a highly crystalline, porous molecular framework. Sufficiently large pore sizes in such metal-organic frameworks, MOFs, allow unhindered switching and the strictly periodic structure of the lattice eliminates virtually all contributions from inhomogeneities. Using IR spectroscopy to probe the conformational state of azobenzene, the energy barrier separating the cis and the trans state could be determined by an Arrhenius analysis of the data accumulated in a temperature regime between 314 K and 385 K. The result, 1.09 ± 0.09 eV, is in very good agreement with the activation energy reported for the thermal cis-to-trans isomerization of free azobenzene as computed by DFT calculations.
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http://dx.doi.org/10.1039/c5cp03091aDOI Listing
September 2015