In situ atomic force microscopy studies of reversible light-induced switching of surface roughness and adhesion in azobenzene-containing PMMA films

Thin films in the range 40-80 nm of a blend of PMMA with an azobenzene derivative have been studied directly during UV and blue light irradiation by atomic force microscopy (AFM), revealing highly reversible changes in the surface roughness and the film adhesion. UV light induces an ≈80% increase in surface roughness, whereas illumination by blue light completely reverses these changes. Based on the observed surface topography and transition kinetics a reversible mass flow mechanisms is suggested, where the polarity changes upon switching trigger a wetting-dewetting transition in a surface segregation layer of the chromophore. Similar AFM measurements of the pull-off force indicate a decrease upon UV and an increase after blue light illumination with a complex kinetic behavior: a rapid initial change, attributed to the change in the cis isomer fraction of the azobenzene derivative, and a more gradual change, indicative of slow structural reorganization.     

Reversible light-induced capacitance switching of azobenzene ether/PMMA blends

Fully reversible light-induced switching of the capacitance with changes larger than 50% has been achieved by using photo-active thin polymer films. The films consist of PMMA polymer blended with high concentrations of branched azobenzene ether dye molecules. Polymer thin sheet capacitors can be easily produced by spin coating and drop casting in various thicknesses ranging from 100 nm to 8 μm, respectively. Reversible capacitance switching is induced by illumination with UV and visible light, respectively. The capacitance change is correlated to the trans to cis isomerization of the azobenzene ether molecules and the alignment of the molecular dipoles in the film. Switching times depend on chromophore concentration, film thickness and light intensity used. The influence of wavelength of the light, of the temperature as well as of an electrical field applied during illumination on the capacitance change is addressed.     

Introducing a photo-switchable azo-functionality inside Cr-MIL-101-NH2 by covalent post-synthetic modification

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 NMR-spectroscopy. 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.     

Regioselective Bromination of Aromatic Amines and Phenols Using N-Benzyl-DABCO Tribromide

N-Benzyl-DABCO tribromide, a stable, solid organic ammonium tribromide, has been used as a bromine source for the regioselective and high-yielding bromination of aromatic amines and phenols. Mono-bromination proceeds well in the presence of a stoichiometric amount of bromine source at room temperature.     

N‐Benzyl‐DABCO‐tribromide as an efficient and mild reagent for deprotection of dithioacetals

N‐Benzyl‐DABCO‐ammonium tribromide was found to be an efficient and recyclable reagent for the deprotection of dithioacetals in dichloromethane/methanol at room temperature. The reaction can be performed cleanly, in short time, and in high yield.     

Gap-mode SERS studies of azobenzene-containing self-assembled monolayers on Au(1 1 1)

Self-assembled monolayers of azobenzene-containing thiols on smooth Au(1 1 1) surfaces were studied by gap-mode surface-enhanced Raman spectroscopy (gap-mode SERS). By adsorption of colloidal Au nanoparticles on top of the organic adlayer highly reproducible spectra with strongly enhanced intensities are obtained. The observed bands indicate a trans conformation of the azobenzene moieties and are in agreement with structural data for the molecular layer. A characteristic dependency on the terminal and the spacer groups of the molecules is found. Samples prepared during illumination with UV light show pronounced spectral differences that can be attributed to azobenzene in cis conformation.     

N‐Benzyl DABCO Tribromide–Promoted Oxidative Coupling of Benzyl Cyanides: A Convenient Procedure for the Synthesis of α, α′‐Dicyanostilbenes

A convenient and efficient procedure was developed for preparing α,α′‐icyanostilbenes through the oxidative coupling reaction of benzyl cyanide derivatives using N‐benzyl DABCO tribromide as the oxidative bromination reagent in the presence of K₂CO₃ as a base.     

The first porous MOF with photoswitchable linker molecules

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 λ(max) = 440 nm.