2021 Recap

As we recap 2021 we're happy to say it's been a successful year science-wise despite the on-going pandemic. The AMC chair is very proud about a total of 43 publications, 5 PhD dissertations, 5 completed Master Thesis', 7 new PhD candidates, and a new sub group within the chair (AK Halter). Additionally, Prof. Fischer celebrated his 60^th birthday this year with a small hike in the Bavarian Alps - see the photo!

In terms of research, the Fischer group continues to expand its range of chemical disciplines with insights into a wide variety of topics from MOF lithography (Nat. Mater.), semi-hydrogenation with Pd nanoparticles (ChemCatChem), configurational entropy (Angew. Chem. Int. Ed.), non-linear optical building blocks (Dyes Pigm.), confinement effects (ChemCatChem), MOF defect engineering (Catal. Sci. Technol.), advanced SURMOF fabrication and detection techniques (Adv. Mater., Angew. Chem. Int. Ed., Adv. Mater.), pyrolysis (Microporous Mesoporous Mater.), electrically conductive MOFs (Chem. Mater.), organometallic cluster studies and detection methods (Chem. Sci., Dalton Trans.), photocatalytic CO₂ reduction (Angew. Chem. Int. Ed.), MOF-encapsulated clusters (Mol. Syst. Des. Eng.), vectorial catalysis (Angew. Chem. Int. Ed.), and - last but not least - new MOF linker design (Inorg. Chem.). With such breadth of research, there's always something new to discover and exciting ideas to discuss in the lab. If you're interested to join us in the coming year, please reach out to the respective PI(s).

We're excited to see what the next year brings and wish everyone a Merry Christmas and a Happy New Year! :) 

Feature Article on SURMOFs

Abstract of the full article available here: 

Metal–organic frameworks (MOFs) are an emerging class of porous materials composed of organic linkers and metal centers/clusters. The integration of MOFs onto the solid surface as thin films/coatings has spurred great interest, thanks to leveraging control over their morphology (such as size- and shape-regulated crystals) and orientation, flexible processability, and easy recyclability. These aspects, in synergy, promise a wide range of applications, including but not limited to gas/liquid separations, chemical sensing, and electronics. Dozens of innovative methods have been developed to manipulate MOFs on various solid substrates for academic studies and potential industrial applications. Among the developed deposition methods, the liquid-phase epitaxial layer-by-layer (LPE-LbL) method has demonstrated its merits over precise control of the thickness, roughness, homogeneity, and orientations, among others. Herein, we discuss the major developments of surface-mounted MOFs (SURMOFs) in LbL process optimization, summarizing the SURMOFs’ performance in different applications, and put forward our perspective on the future of SURMOFs in terms of advances in the formulation, applications, and challenges. Finally, future prospects and challenges with respect to SURMOFs growth will be discussed, keeping the focus on their widening applications.

6 new AMC publications!

The published works span from metal-organic framework thin films, over organometallic cluster growth and their characterization, to molecular CO₂ reduction photocatalysts hosted by metal-organic frameworks! We're very excited to share these and congrats to everyone involved! Check out the publications here:

Open Framework Material Based Thin Films: Electrochemical Catalysis and State‐of‐the‐art Technologies, W. Li, S. Mukerjee, B. Ren, R. Cao, R. A. Fischer, Adv. Energy Mater.

Exploring Cu/Al cluster growth and reactivity: From embryonic building blocks to intermetalloid, open-shell superatoms, M. Schütz, C. Gemel, M. Muhr, C. Jandl, S. Kahlal, J.-Y. Saillard, R. A. Fischer, Chem. Sci.

High‐Quality Thin Films of UiO‐66‐NH₂ by Coordination Modulated Layer‐by‐Layer Liquid Phase Epitaxy, A. L. Semrau, R. A. Fischer, Chem. Eur. J.

Understanding entrapped molecular photosystem and metal-organic framework synergy for improved solar fuel production, P. M. Stanley, M. Parkulab, B. Rieger, J. Warnan, R. A. Fischer, Faraday Discuss.

Enabling LIFDI-MS measurements of highly air sensitive organometallic compounds: a combined MS/glovebox technique, M. Muhr, P. Heiß, M. Schütz, R. Bühler, C. Gemel, M. H. Linden, H. B. Linden, R. A. Fischer, Dalton Trans.

Host-Guest Interactions in Metal-Organic Framework Isoreticular Series for Molecular Photocatalytic CO₂ Reduction, P. M. Stanley, J. Haimerl, C. Thomas, A. Urstoeger, M. Schuster, N. B. Shustova, A. Casini, B. Rieger, J. Warnan, R. A. Fischer, Angew. Chem. Int. Ed.

Redox-Switchable Anthraquinone-Based Metal–Organic Frameworks

Abstract of the full publication available here:

A dipyridyl-substituted anthraquinone (2,6-di(pyridin-4-yl)-9,10-anthraquinone, DPAq) was incorporated as a redox-active linker molecule into crystalline coordination networks. The oxidation state of the organic linker can be selectively controlled prior to framework formation and furthermore be maintained in the solid state. Hydrogen bonding is identified to be a substantial stabilization factor. Additionally, it is shown that the anthraquinone–anthrahydroquinone redox pair can be switched reversibly even after incorporation in the solid state by a thermal treatment/soaking procedure—going along with the formation of hydrogen peroxide from molecular oxygen (air) during the oxidation process.

Tuning Conductivity in MOFs

Abstract of the full text available here:

Electrically conductive metal–organic frameworks (MOFs) exhibit a large potential as working medium in next-generation electronic devices where electronic transport is paired with one or more other physicochemical properties. An important pathway in introducing electronic transport in MOFs is the postsynthetic incorporation of guest molecules in the porous host structures, where resulting host–guest interactions facilitate the creation of charge transport pathways. Here, we report the in-depth analysis of the host–guest interactions in the system (TCNE)_xM₂TTFTB, (TTFTB^4– = tetrathiafulvalene tetrabenzoate, TCNE = tetracyanoethylene), rationalizing the electrical conductivity enhancement in the isostructural MOFs Zn₂TTFTB and Cd₂TTFTB via TCNE. Via vibrational spectroscopy, we show that the guest-infiltrated MOFs contain a mixture of neutral TCNE⁰ and radical–anionic TCNE^–, TTFTB⁰, radical–cationic TTFTB⁺, and dicationic TTFTB²⁺, which together serve as an indicator for the MOF reactivity toward guest infiltration. For (TCNE)_xZn₂TTFTB, an increase of the electrical conductivity of two magnitudes is observed compared to Zn₂TTFTB, while the increase of (TCNE)_xCd₂TTFTB compared to Cd₂TTFTB is significantly smaller due to the lower reactivity of Cd₂TTFTB toward TCNE. The results highlight the power of vibrational spectroscopy as a tool for accessing the chemical nature of host–guest interactions in MOFs, a crucial aspect on the road toward design principles of MOFs with high electrical conductivities.

In situ MOF studies published

Abstract of the full text available here:

Titanium based metal-organic frameworks (MOFs) are interesting self-sacrificial precursors to derive semiconducting porous nanocomposites for highly efficient heterogeneous catalysis. However, there is a lack of systematic and in-depth mechanistic understanding of the pyrolytic conversion of MOF precursors into the desired functional composite materials. In this work, TGA-MS and in situ STEM/EDX combined with other characterization techniques were employed to investigate the evolution of the structural, physicochemical, textural and morphological properties of NH₂-MIL-125(Ti) pyrolysis at different temperatures in an inert gaseous atmosphere. In situ thermal analysis of NH₂-MIL-125(Ti) reveals the presence of 3 rather defined stages of thermal transformation in the following order: phase-pure, highly porous and crystalline MOF → intermediate amorphous phase without accessible porosity → recrystallized porous phase. The three stages occur from room temperature till 300 °C, between 350 and 550 °C and above ∼550 °C respectively. It is found that the framework of NH₂-MIL-125(Ti) starts to collapse around 350 °C, accompanied with the cleavage of coordination and covalent bonds between organic linkers [O₂C–C₆H₃(NH₂)–CO₂]₆ and the Ti oxo-cluster Ti₈O₈(OH)₄. The organic linker continues fragmentation at 450 °C causing the shrinkage of particle sizes. The dominant pore size of 0.7 nm for NH₂-MIL-125(Ti) gradually expands to 1.4 nm at 800 °C along with the formation of mesopores. The derived disc-like particles exhibit an approximately 35% volume shrinkage compared to the pristine MOF precursor. Highly crystalline N and/or C self-doped TiO₂ nanoparticles are homogeneously distributed in the porous carbon matrix. The original 3D tetragonal disc-like morphology of the NH₂-MIL-125(Ti) remains preserved in derived N and/or C doped TiO₂/C composites. This study will provide an in-depth understanding of the thermal conversion behavior of MOFs to rationally select and design the derived composites for the relevant applications.

Expanding our work on defect engineering

Abstract of the full text in Catalysis Science & Technology available here:

A series of Cu(I)-enriched metal–organic frameworks (MOF) of the type CuBTC (BTC = benzene-1,3,5-tricarboxylate) was prepared by a mixed-linker defect engineering technique, namely substituting a portion of a parent linker with truncated pyridine-3,5-dicarboxylate (PyDC) in the synthesis process. The reduced carboxyl coordination sites and the emerged Lewis basic pyridyl sites of PyDC spawned mixed-valence Cu(I)–Cu(II) paddlewheels (PWs) in the defect-engineered CuBTC (DE-CuBTC) structure. Cu(I)-enriched DE-CuBTC shows significantly enhanced catalytic performance for the click reaction of azide–alkyne cycloaddition by accelerating the rate determining step of Cu(I)–acetylide intermediate formation. To further evaluate the catalytic activity of Cu(I)-enriched DE-CuBTC for reactions involving a Cu(I)–acetylide intermediate, the A³ coupling reaction of phenylacetylene, paraformaldehyde and piperidine was studied as well. This study shows that defect engineering is an effective editing tool of catalytic active sites in catalysts.

Roland Fischer's supercapacitors article highlighted by the TUM

A team working with Roland Fischer has developed a highly efficient supercapacitor. The basis of the energy storage device is a novel, powerful and also sustainable graphene hybrid material that has comparable performance data to currently utilized batteries. This work has been highlighted by the TUM as exciting research news and has received a respective article. 

Access the full news article here: https://www.tum.de/nc/en/about-tum/news/press-releases/details/36404/ 

Access the underlying publication in Advanced Materials here: https://onlinelibrary.wiley.com/doi/full/10.1002/adma.202004560 

New Article in ACS Catalysis on Entrapped Molecular Catalysts in MOF Nanoreactors for Carbon Dioxide Reduction

Abstract of the full article available here:

Herein, we report on a molecular catalyst embedding metal–organic framework (MOF) that enables enhanced photocatalytic CO₂ reduction activity. A benchmark photocatalyst fac-ReBr(CO)₃(4,4′-dcbpy) (dcbpy = dicarboxy-2,2′-bipyridine) and photosensitizer Ru(bpy)₂(5,5′-dcbpy)Cl₂ (bpy = 2,2′-bipyridine) were synergistically entrapped inside the cages of the nontoxic and inexpensive MIL-101-NH₂(Al) through noncovalent host–guest interactions. The heterogeneous material improved Re catalyst stabilization under photocatalytic CO₂ reduction conditions as selective CO evolution was prolonged from 1.5 to 40 h compared to the MOF-free photosystem upon reactivation with additional photosensitizer. By varying ratios of immobilized catalyst to photosensitizer, we demonstrated and evaluated the effect of reaction environment modulation in defined MOF cages acting as a nanoreactor. This illustrated the optimal efficiency for two photosensitizers and one catalyst per cage and further led to the determination of ad hoc relationships between molecular complex size, MOF pore windows, and number of hostable molecules per cage. Differing from typical homogeneous systems, photosensitizer—and not catalyst—degradation was identified as a major performance-limiting factor, providing a future route to higher turnover numbers via a rational choice of parameters.

Concept Article published on Intrinsic Confinement Effects of MOFs in Catalysis!

Abstract of the full concept article, available here:

In catalysis research the design of bio‐inspired, “artificial enzymes” is a field of huge interest. These catalysts are distinguished by their high catalytic efficiency resulting from a close proximity of several active sites and secondary substrate‐catalyst interactions enabled by functional groups in the catalytic pocket. A class of materials which meets these requirements are metal‐organic frameworks (MOFs). Here, the pores confine a reaction environment where several functionalities can be incorporated and spatially positioned in a tunable fashion. Recently, a number of reports revealed the importance of such confinement effects for the control of catalytic activity and selectivity by exploiting the intrinsic properties like pore size and neighboring group effects and the alignment and distance of different active sites within one MOF pore. Thus, this concept aims to accentuate the potential of the exploitation of those effects in MOFs for the design of sophisticated catalysts.

Cover page in ChemCatChem!

This paper shows that the reactivity of colloidal palladium nanoparticles can be tuned by addition of phosphanes. The size and the electronic properties of the phosphanes influence the particle surface and determine both the product selectivity and the catalytic activity. More information can be found in the Full Paper.

Control of nano-MOF positioning on functionalized materials

Abstract of the full text available here:

Herein, we describe the selective positioning of metal–organic framework (MOF) nanoparticles UiO-66 (Universitet i Oslo; Zr₆O₄(OH)₄(bdc)₆; bdc^2– = benzene-1,4-dicarboxylate) and MIL-101 (Matérial Institut Lavoisier, Cr₃O(OH) (H₂O)₂(bdc)₃) at defined positions on a patterned substrate. For this purpose, patterned alkyne- and carboxylic acid-terminated self-assembled organic monolayer (SAM)-modified silicon surfaces were prepared by liquid immersion and microcontact printing (μCP). Preformed UiO-66 and MIL-101 nanometer-sized MOFs (NMOFs) were synthesized by solvothermal synthesis, and the nanocrystallite particles’ exterior surface was functionalized in order to generate reactive sites (such as azides and amines) at the NMOFs. Copper-catalyzed alkyne azide cycloaddition and N-hydroxysuccinimide-mediated amide formation were used to selectively position the NMOFs at the surface of pre-patterned substrates. The resulting surfaces were thoroughly investigated by scanning electron microscopy, infrared spectroscopy, and X-ray photoelectron spectroscopy, confirming the validity of the presented approach. We hope that our research paves the way for microsystem integration of NMOFs, for example, in microfluidic devices/reactors, and further investigation of their enhanced catalytic activity.

Building Block for Non-Linear Optical Active Coordination Polymers

Highlights of the full text, which can be found here, include:

• Structural and photophysical study of a new carbazole based “push-pull” organic dye.

• In solution molecule exhibits an solvatochromic intramolecular charge transfer emission.

• In solid-state parallel packing of chromophore leads to excimer emission.

Metal-organic frameworks become flexible

Brief abstract of Pia's work:

Materials consisting of inorganic and organic components can combine the best of two worlds: under certain circumstances, the so-called MOFs – short for metal-organic frameworks – are structured in the same order as crystals and are at the same time porous and deformable. This opens up the prospect of intelligent materials for energy-saving technical applications. However, so far only a few flexible MOFs have been identified.

Intrigued? Follow this link to read the full press release and click here to read the respective publication in Angewandte Chemie!

Web of Science Recognition as a Highly Cited Researcher

After 2018, Roland A. Fischer has again been a 'Highly Cited researcher' of the Clarivate Web of Science in 2020. Congratulations also go to our colleague Hubert Gasteiger from the Dept. of Chemistry and the Catalysis Research Center for being highly cited the second year in a row. A total of thirteen researchers from TUM are among the most cited in their respective fields (see the official TUM press release for details).