Michael Schmittel

Reversible phase transitions in self-assembled monolayers at the liquid-solid interface: temperature-controlled opening and closing of nanopores.

We present a variable-temperature study of monolayer self-assembly at the liquid-solid interface. By means of in situ scanning tunneling microscopy (STM), reversible phase transitions from a nanoporous low-temperature phase to a more densely packed high-temperature phase are observed. The occurrence of the phase transition and the respective transition temperature were found to depend on the type of solvent and solute concentration. Estimates of the entropic cost and enthalpic gain upon monolayer self-assembly suggest that coadsorption of solvent molecules within the cavities of the nanoporous structure renders this polymorph thermodynamically stable at low temperatures. At elevated temperatures, however, desorption of these relatively weakly bound solvent molecules destabilizes the nanoporous polymorph, and the densely packed polymorph becomes thermodynamically favored. Interestingly, the structural phase transition provides external control over the monolayer morphology and, for the system under discussion, results in an effective opening and closing of supramolecular nanopores in a two-dimensional molecular monolayer.

Thermodynamical equilibrium of binary supramolecular networks at the liquid-solid interface.

Coadsorption of two different carboxylic acids, benzenetribenzoic acid and trimesic acid, was studied at the liquid-solid interface in two different solvents (heptanoic and nonanoic acid). Independent alteration of both concentrations in binary solutions resulted in six nondensely packed monolayer phases with different structures and stoichiometries, as revealed by means of scanning tunneling microscopy (STM). All of these structures are stabilized by intermolecular hydrogen bonding between the carboxylic acid functional groups. Moreover, phase transitions of the monolayer structures, accompanied by an alteration of the size and shape of cavity voids in the 2D molecular assembly, could be achieved by in situ dilution. The emergence of the various phases could be described by a simple thermodynamic model.

In vitro studies of ferritin iron release and neurotoxicity.

Abstract: The increase in brain iron associated with several neurodegenerative diseases may lead to an increased production of free radicals via the Fenton reaction. Intracellular iron is usually tightly regulated, being bound by ferritin in an insoluble ferrihydrite core. The neurotoxin 6‐hydroxydopamine (6‐OHDA) releases iron from the ferritin core by reducing it to the ferrous form. Iron release induced by 6‐OHDA and structurally related compounds and two other dopaminergic neurotoxins, 1‐methyl‐4‐phenylpyridinium iodide (MPP+) and 1‐trichloromethyl‐1,2,3,4‐tetrahydro‐β‐carboline (TaClo), were compared, to identify the structural characteristics important for such release. 1,2,4‐Trihydroxybenzene (THB) was most effective in releasing ferritin‐bound iron, followed by 6‐OHDA, dopamine, catechol, and hydroquinone. Resorcinol, MPP+, and TaClo were ineffective. The ability to release iron was associated with a low oxidation potential. It is proposed that a low oxidation potential and an ortho‐dihydroxyphenyl structure are important in the mechanism by which ferritin iron is mobilized. In the presence of ferritin, both 6‐OHDA and THB strongly stimulated lipid peroxidation, an effect abolished by the addition of the iron chelator deferoxamine. These results suggest that ferritin iron release contributes to free radical‐induced cell damage in vivo.

From 2-Fold Completive to Integrative Self-Sorting: A Five-Component Supramolecular Trapezoid

The amalgamation of two incomplete self-sorting processes into a process that makes quantitative use of all members of the library is described by 2-fold completive self-sorting. Toward this goal, individual metal-ligand binding scenarios were optimized for high thermodynamic stability and best selectivity, by screening a variety of factors, such as steric and electronic effects, pi-pi interactions, and metal-ion specifics. Using optimized, heteroleptic metal-ligand binding motifs, a library of four different ligands (1, 2, 3, 4) and two different metal ions (Zn(2+), Cu(+)) was set up to assess 2-fold completive self-sorting. Out of 20 different combinations, the self-sorting library ended up with only two metal-ligand complexes in basically quantitative yield. To demonstrate the value of 2-fold completive self-sorting for the formation of nanostructures, the optimized, highly selective binding motifs were implemented into three polyfunctional ligands. Their integrative self-sorting in the presence of Zn(2+) and Cu(+) led to the clean formation of the supramolecular trapezoid T, a simple but still unknown supramolecular architecture. The dynamic trapezoid T consists of three different ligands with four different donor-acceptor interactions. Its structure was established by (1)H NMR spectroscopy, electrospray ionization mass spectroscopy, and differential pulse voltammetry (DPV) and by exclusion of alternative structures.

From an Eight-Component Self-Sorting Algorithm to a Trisheterometallic Scalene Triangle

Using motifs from 3-fold completive self-sorting in an eight-component library, we report on the design and fabrication of a fully dynamic trisheterometallic scalene triangle, a demanding supramolecular structure that complements the so far known triangular structures.

Reversible ON/OFF nanoswitch for organocatalysis: mimicking the locking and unlocking operation of CaMKII.

Flip a switch: a nanoswitch uses chemical inputs to turn an organocatalytic Knoevenagel reaction on and off (see scheme: R=reactant, P=product). To stop catalysis the chemical input (pink and green) wraps around the inhibitory segment of the nanoswitch to effect release or unlocking of the switch. The process can run reversibly over three cycles without loss of activity.

Rapid and Highly Sensitive Dual-Channel Detection of Cyanide by Bis-heteroleptic Ruthenium(II) Complexes

Two new ruthenium complexes [Ru(bipy)(2)(PDA)](2+) (1) and [Ru(phen)(2)(PDA)](2+) (2) (PDA = 1,10-phenanthroline-4,7-dicarboxaldehyde) have been synthesized to detect cyanide based on the well-known formation of cyanohydrins. Both 1[PF(6)](2) and 2[PF(6)](2) were fully characterized by various spectroscopic techniques and their solid state structures determined by single-crystal X-ray diffraction. Their anion binding properties in pure and aqueous acetonitrile were thoroughly examined using two different channels, i.e., UV-vis absorption and photoluminescence (PL). After addition of only 2 equiv of CN(-), the PL intensity of 1[PF(6)](2) and 2[PF(6)](2) was enhanced ∼55-fold within 15 s along with a diagnostic blue shift of the emission by more than 100 nm. PL titrations of 1[PF(6)](2) and 2[PF(6)](2) with CN(-) in CH(3)CN furnished the very high overall cyanohydrin formation constants log β([CN(-)]) = 15.36 ± 0.44 (β([CN(-)]) = 2.3 × 10(15) M(-2)) and log β([CN(-)]) = 16.37 ± 0.53 (β([CN(-)]) = 2.3 × 10(16) M(-2)), respectively. For both probes, the second constant, K(2), is about 57-84 times less than K(1), suggesting that the cyanohydrin reaction is stepwise. The stepwise mechanism is further supported by results of a (1)H NMR titration of 2[PF(6)](2) with CN(-). The high selectivity of 2[PF(6)](2) for CN(-) was established by PL in the presence of other competing anions. Furthermore, the color change from orange-red to yellow and the appearance of a orange luminescence, which can be observed by the naked eye, provides a simple real-time method for cyanide detection. Finally, theoretical calculations were carried out to elucidate the details of the electronic structure and transitions involved in the ruthenium probes and their cyanide adducts.

On-surface Ullmann coupling: the influence of kinetic reaction parameters on the morphology and quality of covalent networks.

On-surface Ullmann coupling is a versatile and appropriate approach for the bottom-up fabrication of covalent organic nanostructures. In two-dimensional networks, however, the kinetically controlled and irreversible coupling leads to high defect densities and a lack of long-range order. To derive general guidelines for optimizing reaction parameters, the structural quality of 2D porous covalent networks was evaluated for different preparation protocols. For this purpose, polymerization of an iodine- and bromine-functionalized precursor was studied on Au(111) by scanning tunneling microscopy under ultrahigh vacuum conditions. By taking advantage of the vastly different temperature thresholds for C-Br and C-I cleavage, two different polymerization routes were compared - hierarchical and direct polymerization. The structural quality of the covalent networks was evaluated for different reaction parameters, such as surface temperatures, heating rates, and deposition rates by statistical analysis of STM data. Experimental results are compared to Monte Carlo simulations.

Switching from the Myers reaction to a new thermal cyclization mode in enyne-allenes

Abstract Through variation of the substituents at the acetylene terminus in enyne-allenes the reaction mode may be changed from the Myers- to a novel C-2-C-6 cyclization reaction. This thermal cyclization of acyclic enyne-allenes in high yields allows the synthesis of complex indene derivates.

Supramolecular Multicomponent Self-Assembly of Shape-Adaptive Nanoprisms: Wrapping up C60 with Three Porphyrin Units

Self-assembly of a C(3v) symmetric trisphenanthroline and linear bisterpyridines in the presence of Cu(+) did not furnish the expected supramolecular nanoprisms in quantitative yield. With an accurately sized tripyridine as a stabilizing template, the nanoprism formed exclusively. Furthermore, an adaptive constriction of the nanoprism was seen with C(60) as template: as a result of the smaller size of C(60) the nanoframework wrapped up around the guest like an accordion-type host system.