Dynamic processes in porous films are crucial for their applications, which can range from sensor technology to molecular storage and separation. Furthermore, dynamic processes are the key to the production of smart materials which dynamically respond to external stimuli. A particularly interesting group of porous films are SURMOFs, i.e. surface mounted metal-organic frameworks. The interaction of the guest molecules with the framework and the diffusion of the guest molecules in the pores are among the research interests of this group. Another focus is the production and investigation of smart porous surface coatings, i.e. thin films which change their structure and therewith their properties due to external physical or chemical stimuli. The remote-controlled release of molecules from a porous surface coating is particularly interesting. For these studies, various techniques are used, such as quartz crystal microbalance, X-ray diffraction, IR and UV-VIS spectrometry.
Mass Transfer in Thin Films of Metal-Organic Frameworks
The loading with guest molecules is the key for most applications of metal-organic frameworks (MOFs). It was found in numerous studies that the rate of the mass transfer into a MOF material may differ by several orders of magnitude for different samples. This phenomenon is commonly attributed to the presence of so-called surface barriers, which are almost omnipresent, but unwanted and poorly understood. Using well-defined and highly crystalline epitaxially grown thin films of type MOF HKUST-1 as model system, we have been able to quantitatively study this phenomenon using a quartz crystal microbalance (QCM). These experiments clearly demonstrate that surface barriers are not an intrinsic feature of MOFs – perfect, pristine films do not show these limitations. Only by destroying the structure at the outer surface of the MOF films, e.g. by exposure to air or water vapor, the molecular uptake is reduced.
Figure: Model of the mass transfer in MOFs. Left side: unhindered mass transfer in the nanoporous crystal. Right side: Surface barriers hinder the guest molecules in entering the pore space and slows down the mass transfer significantly.
Heinke, L. / Gu, Z. / Wöll, C. (2014): „The surface barrier phenomenon at the loading of metal-organic frameworks“. In: Nat. Comm. 5 (2014).
Photoswitching in Metal-Organic Frameworks – Remote-controlled release from molecular containers
The remote control of physical and chemical properties is one of the key challenges for intelligent, functional materials. Here, the great potential of thin porous films of MOFs containing linkers with photoswitchable azobenzene side groups is shown. The state of the azobenzene side groups can be switched by UV light from trans to cis and vice versa by visible light. This photoswitching influences the mass transfer of the guest molecules in the pores, i.e., ideally, the pores are opened and closed by light. By means of liquid-phase epitaxy, SURMOFs (surface-mounted MOFs) with vertical compositional gradients can be synthesized. This enables the preparation of thin porous films with a passive bottom layer, which serves as a storage container, and a photoswitchable top layer serving as a valve, which opens and closes the container, fig. left. After loading the porous coating and closing the top layer, the release of the guest molecules is initiated by illumination with visible light and monitored by a quartz crystal microbalance (QCM), fig. right.
Heinke, L. / Cakici, M. / Dommaschk, M. / Grosjean, S. / Herges, R. / Bräse, S. / Wöll, C. (2014): „Photoswitching in two-component SURMOFs: Optically triggered release from a molecular container“. In: ACS Nano 8 (2014), 1463-1467.