Photoswitching in nanoporous, crystalline solids: an experimental and theoretical study for azobenzene linkers incorporated in MOFs

Abstract

In this article, we use the popular photoswitchable molecule, azobenzene, to demonstrate that the embedding in a nanoporous, crystalline solid enables a precise understanding of light-induced, reversible molecular motion. We investigate two similar azobenzene-containing, pillared-layer metal-organic frameworks (MOFs): Cu2(AzoBPDC)2(BiPy) and Cu2(NDC)2(AzoBiPy). Experimental results from UV-vis spectroscopy and molecular uptake experiments as well as theoretical results based on density-functional theory (DFT) show that in the Cu2(AzoBPDC)2(BiPy) MOF structure, the azobenzene side groups undergo photoisomerization when irradiated with UV or visible light. In a very similar MOF structure, Cu2(NDC)2(AzoBiPy), the experimental studies show an unexpected absence of photoisomerization. The DFT calculations reveal that in both MOFs the initial and final states of the photoswitching process (the trans and the cis conformation) have similar energies, which strongly suggests that the reason for the effective blocking of photoswitching in the AzoBiPy-based MOFs must be related to the switching process itself. More detailed calculations show that in Cu2(NDC)2(AzoBiPy) a naphthalene linker from the molecular framework blocks the photoisomerization trajectory which leads from the trans to the cis conformation. For Cu2(AzoBPDC)2(BiPy), as a result of the different geometry, such a steric hindrance is absent.

Liquid- and Gas-Phase Diffusion of Ferrocene in Thin Films of Metal-Organic Frameworks

“…, the diffusion coefficient. Since the SURMOF pores which are activated in argon can be considered as empty, the QCM experiments allow a straightforward quantification of the uptake amount, as demonstrated in references [34,35,36]. The ferrocene uptake is determined to be 0.11 ± 0.02 µg·cm −2 , which corresponds to roughly 3/4 ferrocene compounds per HKUST-1 unit cell.…”

Tunable molecular separation by nanoporous membranes

“…The prepared SURMOF films have a pillared-layer structure of type Cu 2 (AzoBPDC) 2 (AzoBiPyB) (Fig. 2; AzoBPDC: 2-phenyldiazenyl-4,4′-biphenyldicarboxylic acid26, and AzoBiPyB: ( E )-4,4′-(2-(phenyldiazenyl)-1,4-phenylene)dipyridine (or dipyridylazobenzene)30. The samples used in this study were prepared by 90 (or 60) cycles of immersing the sample in ethanolic 1 mM copper(II)acetate solution and in ethanolic 0.1 mM AzoBiPyB and 0.1 mM AzoBPDC solution at 60 °C.…”

“…In summary, the layer-by-layer growth process consists of alternately immersing the substrate in the ethanolic solutions of the building units, that is, the metal knots (here: 1 mM copper acetate) and the organic linkers (here: 0.1 mM AzoBPDC and 0.1 mM AzoBiPyB; the synthesis of the linkers is explained in refs 26, 30). Between each immersion step, the substrates were rinsed thoroughly with ethanol.…”

“…Furthermore, photoswitchable molecular scaffolds20 and gel assemblies21 were realized utilising azobenzene photoisomerization. Also, for MOFs in the form of powders or thin films, photoswitchable moieties222324 were used to demonstrate the remote-control of the MOF properties like colour25, release262728 and adsorption2930313233 of the guest molecules, in particular of CO 2 (refs 32, 33). While in most cases illumination with ultraviolet-light was needed, linker molecules that can be switched with visible light were used for the MOF preparation recently34.…”

Thermal cis-to-trans Isomerization of Azobenzene Side Groups in Metal-Organic Frameworks investigated by Localized Surface Plasmon Resonance Spectroscopy

“…The experimental data of the well-defined, crystalline model system allowed us to compare the results with theoretical data in detail. There, steric hindrance of the azobenzene side groups was identified in certain MOF structures [29]. Recently, we used thin MOF films which possess virtually isolated azobenzene pendant groups to investigate the thermal cis-to-trans isomerization [30].…”