We synthesized a porous twofold interpenetrated MOF [Zn 2 (NDC) 2 (1)] (coined CAU-5) using 3-azo-phenyl-4,4¢-bipyridine (1), 2,6-naphthalenedicarboxylic acid, and Zn(NO 3 ) 2 ·6H 2 O. The azo-functionality protrudes into the pores, and can be switched, by irradiation with UV light (365 nm), from the thermodynamically stable trans-isomer to the cis-isomer. Back-switching was achieved thermally and with an irradiation wavelength of l max = 440 nm.

For the first time an azo functionality was covalently introduced into a MOF by post-synthetic modification. The reaction of Cr-MIL-101-NH 2 with p-phenylazobenzoylchloride (1) and 4-( phenylazo)-phenylisocyanate (2) as the reactants led to the compounds Cr-MIL-101_amide and Cr-MIL-101_urea, with the azo groups protruding into the mesoporous cages. XRPD and N 2 sorption measurements confirm the intactness of the framework and the successful covalent modification was proven by IR-and NMRspectroscopy. Furthermore, cis/trans isomerisation upon irradiation with light was demonstrated by UV/ Vis spectroscopy. More distinct changes in the UV/Vis spectra were observed for Cr-MIL-101_amide compared to Cr-MIL-101_urea, while the degree of functionalization, i.e. the number of reacted NH 2 -groups, seems to have a less pronounced effect. The variation of the sorption properties due to the cis/trans isomerisation was proven by methane adsorption measurements.

The photoisomerization of self-assembled monolayers of azobenzene-containing alkanethiols, as well as of mixed monolayers of these substances with n-alkanethiol spacer molecules on Au surfaces, was studied by photoelectrochemical measurements and surface plasmon resonance spectroscopy. A strong dependence on the molecular structure of the adsorbates was found, specifically on the linker between the azobenzene moiety and the alkanethiol: while molecules with an amide group were photoinactive, those with an ether group exhibited pronounced, reversible photoisomerization in pure and mixed adlayers. Both trans-cis and cis-trans isomerization followed first-order kinetics with time constants that suggest high quantum efficiencies for these processes.

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We present a new concept of light-controlled conductance switching based on metal/polymer nanocomposites with dissolved chromophores that do not have intrinsic current switching ability. Photoswitchable metal/PMMA nanocomposites were prepared by physical vapor deposition of Au and Pt clusters, respectively, onto spin-coated thin poly(methylmethacrylate) films doped with azo-dye molecules. High dye concentrations were achieved by functionalizing the azo groups with tails and branches, thus enhancing solubility. The composites show completely reversible optical switching of the absorption bands upon alternating irradiation with UV and blue light. We also demonstrate reversible light-controlled conductance switching. This is attributed to changes in the metal cluster separation upon isomerization based on model experiments where analogous conductance changes were induced by swelling of the composite films in organic vapors and by tensile stress.

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