Switchable Metal-Organic Frameworks (MOF-Switches, FOR 2433)

Table of contents

1. 1. Duration:
   2. Principal Investigators:
   3. Summary:
   4. Objectives and Structure of the Research Unit:
   5. Project List:

Duration:

Total Duration: 6 Years

• 1^st Funding Phase: 1/2017-12/2019
• 2^st Funding Phase: 1/2020-12/2022

Principal Investigators:

• Prof. Dr. Stefan Kaskel, Department of Chemistry and Food Chemistry, Chair of Inorganic Chemistry I, TU Dresden (Speaker)
• Prof. Dr. Eike Brunner, Department of Chemistry and Food Chemistry, Chair of Bioanalytical Chemistry, TU Dresden
• Prof. Dr. Tina Düren, Department of Chemical Engineering, Full professor, University of Bath (Mercator Fellow)
• Prof. Dr. Roland Fischer, Department of Chemistry, Chair of Inorganic Chemistry, TU Munich
• Prof. Dr. Thomas Heine, Department of Chemistry and Food Chemistry, Chair of Bioanalytical Chemistry, TU Dresden
• Prof. Dr. Andreas Pöppl, Institute of Experimental Physics II, University of Leipzig
• Prof. Dr. Rochus Schmid, Computational Materials Chemistry Group, Chair of Inorganic Chemistry II, Ruhr-University Bochum

Summary:

Porous materials play a key role in gas and liquid phase separations, energy storage, as catalysts and for optical and chemical sensing. Metal-Organic Frameworks (MOFs) stand out among other porous materials due to their extremely high porosity and modular tunability. While the majority of porous solids (and MOFs) is rigid, a novel and unique class of switchable MOFs was discovered in recent years. These materials only open their pores dynamically, as a response to the presence of gases or liquids at a characteristic concentration associated with unprecedented, step-wise unit cell volume changes (more than 240 %) during gas uptake. Such switchable MOFs are able to specifically respond or even recognize certain types of molecular species by opening their pores, resulting in a step-wise change of physical (i.e. magnetism, optical density, bulk density, etc.) and chemical characteristics (catalytic activity, reactivity). Moreover, they reversibly close their pores in the absence of the respective species. A principle understanding of the dynamic phenomena in such materials would represent a unique technological basis for the design of switchable catalysts, filters, threshold sensors, or stimulus induced drug delivery by receptor systems with integrated key-lock functionality. However, so far the discovery of switchable MOFs (also named gating, or breathing MOFs) was essentially accidental. So far, it is impossible to rationally predict new switchable structures, because the underlying microstructural principles, responsible for such a high degree of flexibility, are not well understood. For the technological development of switchable MOFs in separation, catalysis, or sensing, a fundamental understanding of the underlying structural principles and gas-solid interaction mechanisms is needed.

Objectives and Structure of the Research Unit:

The research unit (FOR 2433) primarily addresses the fundamentals of porosity switching phenomena in the solid state and the underlying principles with an interdisciplinary team involving 7 PIs with a strong expertise in simulation (Th. Heine: T1, R. Schmid: T2, T. Düren: Mercator Fellow), synthesis (R. A. Fischer: S1, S. Kaskel: S 2) and physical characterization (E. Brunner: P1, A. Pöppl: P2).

To exploit the fascinating potential of switchable porous MOFs for applications in separation, catalysis and sensing, it is essential to achieve a fundamental understanding of physical principles responsible for switching in porous solids. The research unit addresses four fundamental questions/tasks in the first phase (1/2017-2/2019) and the individual projects and respective work packages are organized along these questions:

• Will it be possible to model porosity switching in MOFs and develop a theory guided (predictive) selection of structural building blocks with intrinsic flexibility to derive a general modular synthesis concept for switchable MOF construction? Which inorganic or organic subunits or connection schemes (bonds, hinges) between the subunits are important? Which inorganic or organic subunits show intrinsic bistability?
• Development of tailored in situ characterization techniques for monitoring structural transformations and adaptive processes, motion, flexibility of all kinds under controlled chemical (e.g. chemical potential of certain species) and physical environments or stimuli.
• How important are cooperative/correlated phenomena (CCP) in framework flexibility (responsive/adaptive)? How does the crystallite domain size regime (nano, meso, macro), interfacial contact, heterostructures (composites) and/or properties and modulation of the external surface of MOFs influence relevant parameters for functions, such as gate-opening pressure, thermoresponsivity, others, and in general allow modulation of the "gate-opening pressure"? In which way do disorder and defect structure (DDS) affect switchability and consequently responsivity / adaptivity of MOFs?
• Specific vs. non-specific responsivity: In which cases is the pore opening/closing only governed by non-specific van der Waals interactions (adsorption enthalpy) and what are the criteria to clearly distinguish selective recognition phenomena induced by shape or the presence/arrangement of functional groups interacting with the inner pore structure (key/lock mechanism)? In which cases is the phenomenon dominated by entropic factors?

Targeting idealized switchable MOF model materials (S1, S2: pillared-layer MOFs, Fig. 2), the role of network constituents on the degree of flexibility is studied in a collaborative and closely coordinated experimental and theoretical approach in order to derive a predictive theoretical model for framework flexibility (T1, T2).

Such materials undergo colossal step-wise volume changes in the presence of adsorbing gases leading to linker and or cluster deformation associated with gating type adsorption isotherms (Fig. 3).

Parallelized physical characterization tools are established enabling the application of in situ global scattering techniques (XRD) and in situ local probe spectroscopies (NMR, EPR, EXAFS) in order to analyze the microscopic structural transformations and dynamics induced by host/guest interactions during adsorption/desorption (P1, P2). In an interdisciplinary effort of the focused research unit we target the development of a predictive framework for switchable MOFs fostering an intense cooperation of theoreticians, synthetic chemists, and physicists.

Project List:

Project	Title	Principle Investigator
T1	Origin of Flexibility and Mechanisms of Responsivity from first principles	Thomas Heine (TUD)
T2	Force field simulation of guest induced gating phenomena	Rochus Schmid (RUB)
S1	Multivariate, flexible MOFs: The role of functionalized linkers, heterogeneity and defects	Roland Fischer (TUM)
S2	Synthesis of switchable MOFs and in situ X-ray diffraction studies of gas and liquid phase induced reversible structural transformations	Stefan Kaskel (TUD)
P1	Experimental Analysis of the Switching Process and its Dynamics by in situ NMR Spectroscopy	Eike Brunner (TUD)
P2	Monitoring adsorption induced structural transformations of MOFs at a molecular level by in situ EPR spectroscopy	Andreas Pöppl (UL)
Z1	Coordination of the Research Unit	Stefan Kaskel (TUD)