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Organisationseinheit der BAM
Although the concept of structural water that is bound inside hydrophobic pockets and helps to stabilize protein structures is well established, water has rarely found a similar role in supramolecular polymers. Water is often used as a solvent for supramolecular polymerization, however without taking the role of a comonomer for the supramolecular polymer structure. We report a low–molecular weight monomer whose supramolecular polymerization is triggered by the incorporation of water. The presence of water molecules as comonomers is essential to the polymerization process. The supramolecular polymeric material exhibits strong adhesion to surfaces, such as glass and paper. It can be used as a water-activated glue, which can be released at higher temperatures and reused many times without losing its performance.
Coordination chemistry was applied to deposit pyridine-functionalized gold nanoparticles on silicon substrates. The particles were synthesized through the Brust/Schiffrin route with a subsequent ligand exchange reaction yielding well-defined particles of two different sizes. Multilayer deposition was carried out on a pyridine-terminated SAM, anchored on a hydroxyl-terminated silicon surface. Analogously, Hunter/Vögtle-type tetralactam macrocycle multilayers were deposited as well as mixed layers containing both either in an alternating sequence or as a macrocycle multilayer with a terminating nanoparticle layer. These composite layers were examined with respect to their ability to bind squaraine axles in the macrocycle cavities. The amount of guest bound is higher for the composite layer with alternating macrocycles and nanoparticles.
A novel photoswitchable rotaxane was synthesised and its switching behaviour in solution was analysed with NMR and UV-Vis. A monolayer of rotaxanes was deposited on glass surfaces and the on-surface photoswitching was investigated. Angle-resolved NEXAFS spectra revealed a preferential orientation that reversibly changes upon switching.
Experimental N K-edge NEXAFS data of surface immobilized azide, 1,2,3-triazole and terpyridyl groups are interpreted with the help of DFT spectrum simulations. Assignments of π* resonances in experimental N K-edge NEXAFS spectra to nitrogen atoms within these functional groups have been made. The azide was immobilized on gold as the head group of a thiol SAM, 1,2,3-triazole was formed on this SAM by click reaction and terpyridyl groups were introduced as substituents of the acetylene used for the click reaction. For azide-terminated molecules, DFT spectrum simulations are found to be useful to find measurement conditions delivering experimental N K-edge NEXAFS data with negligible X-ray damage. The 1,2,3-triazole group is found to be rather stable under X-ray irradiation.
Organic fluorophores, particularly stimuli-responsive molecules, are very interesting for biological and material sciences applications, but frequently limited by aggregation- and rotation-caused photoluminescence quenching. A series of easily accessible bipyridinium fluorophores, whose emission is quenched by a twisted intramolecular charge-transfer (TICT) mechanism, is reported.
Encapsulation in a cucurbit[7]uril host gave a 1:1 complex exhibiting a moderate emission increase due to destabilization of the TICT state inside the apolar cucurbituril cavity. A much stronger fluorescence enhancement is observed in 2:2 complexes with the larger cucurbit[8]uril, which is caused by additional conformational restriction of rotations around the aryl/aryl bonds. Because the cucurbituril complexes are pH switchable, this system represents an efficient supramolecular ON/OFF fluorescence switch.
The chemical composition of surfaces functionalized with self-assembled monolayers (SAMs) is an important parameter that determines their performance in a broad range of applications, from immobilizing molecular machines to initiation and growth control of MOFs (Metal-Organic Frameworks). In this article, a critical Survey of XPS (X-ray photoelectron spectroscopy) and NEXAFS (near edge X-ray absorption fine structure) spectroscopy data for pyridine-functionalized monolayers on gold surfaces is presented to compile correlations that have been indiscernible before. Monolayers with aromatic backbones are compared to monolayers with aliphatic backbones.
Monolayers with pyridine end-groups are compared to mixed monolayers formed terpyridine-functionalized by molecules end-groups and non-functionalized molecules. Thiol-oxidation during ageing of the SAMs in air is addressed. And finally, the addressability of the SAMs for the deposition of metal-ions and organic molecules is investigated and compared. This work consequently delivers a comprehensive set of spectroscopic data of (ter-)pyridine-terminated SAMs and their performance as template for the preparation of functional multilayers of macrocycles exhibiting a preferential orientation.
The production of high-quality self-assembled monolayers (SAMs) followed by layer-by-layer (LbL) self-assembly of macrocycles is essential for nanotechnology applications based on functional surface films. To help interpret the large amount of data generated by a standard ToF-SIMS measurement, principal component analysis (PCA) was used. For two examples, the advantages of a combination of ToF-SIMS and PCA for quality control and for the optimization of layer-by-layer self-assembly are shown. The first example investigates how different cleaning methods influence the quality of SAM template formation. The second example focuses on the LbL self-assembly of macrocycles and the corresponding stepwise surface modification.
The deposition of tetralactam macrocycles and the corresponding benzyl ether rotaxanes on gold substrates is investigated for the first time exploiting metallo-supramolecular chemistry. Two pyridine-terminated self-assembled monolayers (SAMs) are developed that are used as well-ordered template layers. The two SAMs differ with respect to the rigidity of the terminal pyridines as shown by angle-resolved near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The template layers are then used for the metal-mediated self-assembly of macrocylces and rotaxanes on solid supports. The SAM with the more rigid terminal pyridine shows a higher coverage with the macrocycles and is therefore preferable. Angle-resolved NEXAFS spectroscopy also shows the deposited supramolecules to be oriented preferentially upright. This order is only achieved for the macrocycles through the deposition on the more rigid SAM template, whereas rotaxanes form oriented layers on both SAMs. Time-of-flight secondary-ion mass spectrometry analysis was used to determine the deposition time required for the self-assembly process.
The deposition of palladium on a novel, reversibly protonatable, pyridyl-terminated self-assembled monolayer on gold substrates has been studied by X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure spectroscopy (NEXAFS spectroscopy) and time of flight-secondary ion mass spectrometry (ToF-SIMS). For this purpose, 12-(pyridin-4-yl)dodecane-1-thiol, consisting of a surface-active head group, an unfunctionalized hydrocarbon backbone and a terminal pyridyl group, has been synthesized and deposited on gold surfaces. Coordination of Pd(II) ions to the pyridyl group was examined. Furthermore, a reversible protonation/deprotonation cycle has been applied, and the relation between protonation and the amount of complexed palladium was studied. Investigation of the SAM by angle-resolved NEXAFS spectroscopy revealed the aliphatic backbone to be preferentially upright oriented with the aromatic head group being not preferentially oriented. The palladium layer was further coordinated with a CF3-labeled terpyridine ligand in order to prove the accessibility of the Pd(II) ions to further complexation and the platform useful for deposition of further layers toward a multi-layered system.