title: 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.
authors.affiliation: Karlsruher Institut für Technologie (KIT), Institut für Funktionelle Grenzflächen (IFG), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany. Lars.Heinke@KIT.edu.
authors.forename: Zhengbang
authors.initials: Z
authors.lastname: Wang
authors.forename: Lars
authors.initials: L
authors.lastname: Heinke
authors.forename: Jelena
authors.initials: J
authors.lastname: Jelic
authors.forename: Murat
authors.initials: M
authors.lastname: Cakici
authors.forename: Marcel
authors.initials: M
authors.lastname: Dommaschk
authors.forename: Reinhard J
authors.initials: RJ
authors.lastname: Maurer
authors.forename: Harald
authors.initials: H
authors.lastname: Oberhofer
authors.forename: Sylvain
authors.initials: S
authors.lastname: Grosjean
authors.forename: Rainer
authors.initials: R
authors.lastname: Herges
authors.forename: Stefan
authors.initials: S
authors.lastname: Bräse
authors.forename: Karsten
authors.initials: K
authors.lastname: Reuter
authors.forename: Christof
authors.initials: C
authors.lastname: Wöll
mesh_terms: D001391:Azo Compounds / Q000737:chemistry* / Q000528:radiation effects*; D003198:Computer Simulation; D003432:Cross-Linking Reagents / Q000737:chemistry / Q000528:radiation effects; D003460:Crystallization; D007536:Isomerism; D008027:Light; D008422:Materials Testing; D008956:Models, Chemical; D056004:Molecular Dynamics Simulation; D053758:Nanoparticles / Q000737:chemistry* / Q000528:radiation effects / Q000648:ultrastructure*; D058608:Nanopores / Q000648:ultrastructure*
chemical_list: D001391:Azo Compounds; D003432:Cross-Linking Reagents; C009850:azobenzene