Marcel Albrecht

Red/Near-Infrared Luminescence Tuning of Group-14 Element Complexes of Dipyrrins Based on a Central Atom

A dipyrrin complex has been one of the most utilized fluorescent dyes, and a variety of dipyrrin complexes show intriguing functions based on the various coordination structures of the central element. We now report the synthesis, structure, and photophysical properties of germanium and stannane complexes of the N2O2-type tetradentate dipyrrin, L·Ge and L·Sn, which are heavier analogues of the previously reported dipyrrin silicon complex, L·Si. The central group-14 atoms of the monomeric complexes have geometries close to trigonal bipyramidal (TBP), in which the contribution of the square-pyramidal (SP) character becomes higher as the central atom is heavier. Interestingly, L·Sn formed a dimeric structure in the crystal. All complexes L·Si, L·Ge, and L·Sn showed a fluorescence in the red/NIR region. Fluorescence quantum yields of L·Ge and L·Sn are higher than that of L·Si. These results indicated that the central atom on the dipyrrin complexes contributes not only to the geometry difference but also to tuning the fluorescence properties.

Blue Fluorescent Amino Acids as In Vivo Building Blocks for Proteins

In vivo expression of colored proteins without post‐translational modification or chemical functionalization is highly desired for protein studies and cell biology. Cell‐permeable tryptophan analogues, such as azatryptophans, have proved to be almost ideal isosteric substitutes for natural tryptophan in cellular proteins. Their unique spectral features, such as markedly red‐shifted fluorescence, are transmitted into protein structures upon incorporation. Among the azaindoles under study (2‐, 4‐, 5‐, 6‐, and 7‐azaindole) 4‐azaindole has exhibited the largest Stokes shift (∼130 nm) in steady‐state fluorescence measurements. It is also highly biocompatible and as 4‐azatryptophan it can be translated into target protein sequences. However, its quantum yield and fluorescence intensity are still significantly lower when compared with natural indole/tryptophan. Since azatryptophans are hydrophilic, their presence in the hydrophobic core of proteins could be harmful. In order to overcome these limitations we have performed nitrogen methylation of azaindoles and generated mono‐ and dimethylated azaindoles. Some of these methyl derivatives retain the pronounced red shift present in the parent 4‐azaindole, but with much higher fluorescence intensity (reaching the level of indole/tryptophan). Therefore, the blue fluorescence of azaindole‐containing proteins could be further enhanced by the use of methylated analogues. Further substitution of any azaindole ring with either endo‐ or exocyclic nitrogen will not yield a spectral fluorescence maximum shift beyond 450 nm under steady‐state conditions in the physiological milieu. However, green fluorescence is a special feature of tautomeric species of azaindoles in various nonaqueous solvents. Thus, the design or evolution of the protein interior combined with the incorporation of these azaindoles might lead to the generation of specific chromophore microenvironments that facilitate tautomeric or protonated/deprotoned states associated with green fluorescence.

Redox active donor-substituted punicin derivatives

The redox active plant material punicin from Punica granatum, 2-hydroxy-1-(pyridinium-1-yl)-5-olate, and some derivatives were modified by substitution with 2-oxochromen-4-olate moieties to give donor-substituted molecules which form distinct types of atropisomeric mesomeric betaines and tetrapolar substances. Oxidation to new negatively-charged solvatochromic quinones was achieved on treatment of these betaines with CAN, taking advantage of the stabilizing properties of the electron-donating 2-oxochromen-4-olate partial structure. The quinones can be employed in model redox reactions. Cyclovoltametric studies have been performed.

Pyridinium salts and ylides as partial structures of photoresponsive Merrifield resins

Merrifield resin was treated with 4,4′-bipyridine and 2,4′-bipyridine, respectively, to give photochromic materials. On exposure to light, reversible color changes are observable. These resins also serve as indicators because reversible color changes are observable on addition of bases, which deprotonate the benzylic position of the N-benzylpyridinium salts to ylides. These can be detected spectroscopically and by trapping reactions on monomeric model compounds. EPR and HYSCORE measurements in combination with DFT calculations prove that radical species are formed in either case. Evidence is found that the carbanionic centers of the ylide partial structures serve as electron donators in these reversible color changes, thus setting them apart from known photochromic materials.

Studies on photocatalytically active materials containing structure elements of a pyridinium alkaloid from Punica granatum

The alkaloid punicin from Punica granatum, N-(2′,5′-dihydroxyphenyl)pyridinium chloride, forms heterocyclic mesomeric betaines and radicals, which were examined by ESR measurements and DFT calculations, in aqueous solution. By reaction of poly(4-vinylpyridine) with p-benzoquinone under two different reaction conditions, polymeric structures with punicin constituents were prepared and characterized. The ESR signals of these polymers were calculated and discussed. The 4,4′-bipyridine derivative of these polymers was prepared starting from Merrifield resin which was subsequently reacted with 4,4′-bipyridine and p-benzoquinone, or, alternatively, with N-(2″-5″-dihydroxyphenyl)-4-(4′-pyridine)pyridinium chloride. The properties of these new materials in reversible photocatalytic processes via radical species were examined. Therefore, solutions or suspensions were irradiated in water in the presence of proflavinium as a sensitizer, and EDTA as a sacrificial donor under an inert atomsphere. Oxygen (air) recovers the starting materials by formation of hydroxide ions which were determined by pH measurements.

Friction Mediated by Redox-Active Supramolecular Connector Molecules

We report on a friction study at the nanometer scale using atomic force microscopy under electrochemical control. Friction arises from the interaction between two surfaces functionalized with cyclodextrin molecules. The interaction is mediated by connector molecules with (ferrocenylmethyl)ammonium end groups forming supramolecular complexes with the cyclodextrin molecules. With ferrocene connector molecules in solution, the friction increases by a factor of up to 12 compared to control experiments without connector molecules. The electrochemical oxidation of ferrocene to ferrocenium causes a decrease in friction owing to the lower stability of ferrocenium-cyclodextrin complex. Upon switching between oxidative and reduction potentials, a change in friction by a factor of 1.2-1.8 is observed. Isothermal titration calorimetry reveals fast dissociation and rebinding kinetics and thus an equilibrium regime for the friction experiments.

Interactions between shape-persistent macromolecules as probed by AFM

Water-soluble shape-persistent cyclodextrin (CD) polymers with amino-functionalized end groups were prepared starting from diacetylene-modified cyclodextrin monomers by a combined Glaser coupling/click chemistry approach. Structural perfection of the neutral CD polymers and inclusion complex formation with ditopic and monotopic guest molecules were proven by MALDI–TOF and UV–vis measurements. Small-angle neutron and X-ray (SANS/SAXS) scattering experiments confirm the stiffness of the polymer chains with an apparent contour length of about 130 Å. Surface modification of planar silicon wafers as well as AFM tips was realized by covalent bound formation between the terminal amino groups of the CD polymer and a reactive isothiocyanate–silane monolayer. Atomic force measurements of CD polymer decorated surfaces show enhanced supramolecular interaction energies which can be attributed to multiple inclusion complexes based on the rigidity of the polymer backbone and the regular configuration of the CD moieties. Depending on the geometrical configuration of attachment anisotropic adhesion characteristics of the polymer system can be distinguished between a peeling and a shearing mechanism.

Discrimination of proteins through interaction with pyrene-labelled polymer aggregates

A pyrene-labelled PDMAEMA (2-(dimethylamino)ethyl methacrylate) polymer was synthesized through a controlled radical RAFT polymerisation approach. An average pyrene content of 3.65% was determined by UV/Vis and 1H NMR measurements. DLS measurements reveal the formation of polymer aggregates with an average size of 172 nm in aqueous phosphate buffer indicating the presence of hydrophobic interactions between pyrene and/or DMAEMA moieties of adjacent polymer chains. Furthermore, this aggregation results in the appearance of two characteristic emission bands at 394 and 488 nm analyzed by fluorescence measurements. Based on spectral changes of the so-called monomer and excimer emission intensity, the specific discrimination of various non-metallo- and metallo proteins was realized using an optical fingerprint approach. DLS and fluorescence measurements show a significant dependence of the structural characteristics of the analytes on the presence of different binding modes between the hydrophilic DMAEMA side groups of the polymer and the proteins, resulting in a molecular disassembly of the aggregates and/or fluorescence quenching. Furthermore, pH, temperature and ionic strength dependence of the sensor polymer with BSA was investigated to optimise the external parameters. Based on these results, the most specific discrimination of the analyzed proteins was obtained using a sodium chloride concentration of 50 mM and a pH of 7.0. This study gives fundamental insights into the sensing performance of a novel one-component pyrene-based polymer system and its application as a protein sensor.

A Metal-Organic Framework Constructed of 1,2-Di(pyridin-4-yl)ethyne, Terephthalic Acid, and Zinc(II)Nitrate

A metal-organic framework (MOF) was prepared from 1,2-di(pyridin-4-yl)ethyne, terephthalic acid and zinc(II) nitrate in dimethylformamide, water and ethanol at 80 °C. The cavities of the MOF are occupied by disordered molecules of dimethylformamide. The crystals are monoclinic, space group P21/c with Z = 4. Graphical Abstract A Metal-Organic Framework Constructed of 1,2-Di(pyridin-4-yl)ethyne, Terephthalic Acid, and Zinc(II) Nitrate

A Metal-Organic Framework Constructed of 1,4-Di(pyridin-4-yl)- buta-1,3-diyne and Nickel(II) Nitrate

A metal-organic framework (MOF) was prepared from 1,4-di(pyridin-4-yl)buta-1,3-diyne and nickel(II) nitrate hexahydrate in methanol and dichloromethane at room temperature. The crystals are orthorhombic, space group C2221, Z = 4. The rhombic cavities of the MOF are occupied by disordered molecules of dichloromethane Graphical Abstract A Metal-Organic Framework Constructed of 1,4-Di(pyridin-4-yl)- buta-1,3-diyne and Nickel(II) Nitrate