Linda Stegemann

Cobalt(III)‐Catalyzed Directed CH Coupling with Diazo Compounds: Straightforward Access towards Extended π‐Systems

The first highly efficient and scalable cobalt-catalyzed directed C-H functionalization with carbene precursors is presented. This methodology provides a modular route towards a new class of conjugated polycyclic hydrocarbons with tunable emission wavelengths both in solution and in the solid state.

RhIII‐Catalyzed CH Activation with Pyridotriazoles: Direct Access to Fluorophores for Metal‐Ion Detection

The first C-H bond activation with pyridotriazoles as coupling partners is presented using a Rh(III) catalyst. The pyridotriazoles can be used as new carbene precursors in C-H activation for direct access to novel fluorescent scaffolds. These tunable fluorophores can be applied for the detection of metal ions.

Colour-tunable asymmetric cyclometalated Pt(II) complexes and STM-assisted stability assessment of ancillary ligands for OLEDs

We combined an asymmetric cyclometalated C^N^N Pt(II) complex with three different classes of monodentate ancillary ligands, namely a bulky phosphine, a tilted pyridine, and either a tilted or a coplanar isocyanide. The controlled planarization of the coordination environment yields green, orange or red phosphors. Electronic structure calculations using DFT reveal the excimeric intermolecular interactions in the excited triplet state, resulting in tunable photophysical properties. Finally, we assess the suitability of the ancillary ligands for optoelectronic applications by investigating the molecular integrity upon sublimation with the aid of scanning tunneling microscopy (STM). The ancillary ligand controls the surface-induced chiral self-assembly of the molecules on metallic substrates, as well as the availability of loose coordination sites. The concordance between the photoluminescence spectra and electroluminescence profiles for solution-processed and vapour-deposited devices confirm the predictions attained by STM regarding molecular integrity and processability.

Facile surface engineering of CuInS2/ZnS quantum dots for LED down-converters

We present a facile one-pot synthesis and simple surface modification strategies to prepare ternary CuInS2/ZnS quantum dots (CIS/ZnS QDs) functionalized with alkyl-, carboxyl- and hydroxyl-terminated ligands. Photoluminescence quantum yields (PLQY) up to 60% can be achieved for CIS/ZnS QDs synthesized on a gram scale. Aqueous mercaptopropionic acid (MPA) capped QDs are used as a base for further surface modification. On the one hand ligand–solvent interactions have been found to play a major role in the luminescence quenching mechanism of aqueous CIS/ZnS QDs. On the other hand, solvent extraction, immobilization in a polymer matrix or phase transfer greatly enhances the photoluminescence. Quantum yields in solution and in solid matrices differ greatly in the case of aqueous QDs both in polymers and in sol–gel silica. Surface engineered QDs are assessed for their usefulness in LED down-conversion experiments. We demonstrate that down-converter layer properties are not only dependent on the QD type and PLQY, but also on the matrix and the type of ligand shell. Despite significant (up to a factor of two) differences in PLQY between the hydrophobic and hydrophilic dots, their LED conversion efficiencies are comparable and up to 49% and 46% for hydrophobic and for hydrophilic QDs, respectively. Additionally, sol–gel nanocomposites incorporating CuInS2/ZnS QDs with OH-terminated ligands that can withstand 200 h high power blue LED irradiation are presented for the first time.

Agglutination of bacteria using polyvalent nanoparticles of aggregation-induced emissive thiophthalonitrile dyes

A novel class of aggregation-induced emissive bis(phenylthio)phthalonitrile dyes were synthesized. These dyes assembled into nanoparticles that were equipped with mannose units. The nanoparticles underwent selective interactions with lectins and bacteria. The bright fluorescent aggregates aid in the visualization of the agglutination of bacteria.

Effect of the C(3)-Substituent in Verdazyl Radicals on their Profluorescent Behavior.

Methods for the detection of reactive intermediates such as transient radicals are important in organic chemistry, polymer chemistry, biology or medicine. Along these lines we recently reported that 1,5-diphenyl-6-oxo verdazyl radicals can be used as fluorescent spin sensors. In situ generated C-centered radicals are efficiently trapped by the verdazyls, which in turn undergo transformation from a paramagnetic non-fluorescent state to a diamagnetic fluorescent state. Whereas the N-phenyl substituent in the spin probes is of high importance for obtaining profluorescent behavior, the effect of the C(3)-substituent has not been investigated to date. We herein present the synthesis and characterization of various 1,5-diphenyl-6-oxo-verdazyl radicals bearing differently hybridized C-substituents at the C(3) position. Steady-state and time-resolved fluorescence spectroscopy in solution and in the solid state along with time-dependent density functional theory (TDDFT) calculations reveal that a C(3)-aryl substituent is crucial for obtaining fluorescence after spin trapping. In addition, it is shown that the emission wavelength of the C(3)-aryl substituted verdazyl derivatives can be tuned by selective destabilization of the HOMO and the LUMO.

Formation of Silver Nanoclusters from a DNA Template Containing Ag(I)-Mediated Base Pairs

A series of DNA double helices containing different numbers of silver(I)-mediated base pairs involving the artificial nucleobases imidazole or 2-methylimidazole has been applied for the generation of DNA-templated silver nanoclusters. The original Ag(I)-containing nucleic acids as well as the resulting nanoclusters and nanoparticles have been characterized by means of UV/Vis spectroscopy, circular dichroism (CD) spectroscopy, fluorescence spectroscopy, and transmission electron microscopy (TEM). The results show for the first time that metal-mediated base pairs can be used for the templated growth of metal nanoclusters.

Photofunctional Surfaces for Quantitative Fluorescence Microscopy: Monitoring the Effects of Photogenerated Reactive Oxygen Species at Single Cell Level with Spatiotemporal Resolution

Herein, we report on the implementation of photofunctional surfaces for the investigation of cellular responses by means of quantitative fluorescence microscopy. The developed substrates are able to produce reactive oxygen species under the fluorescence microscope upon irradiation with visible light, and the behavior of cells grown on these surfaces can be consequently investigated in situ and in real time. Moreover, a suitable methodology is presented to simultaneously monitor phototriggered morphological changes and the associated molecular pathways with spatiotemporal resolution employing time-resolved fluorescence anisotropy at the single cell level. The results showed that morphological changes can be complemented with a quantitative evaluation of the associated molecular signaling cascades for the unambiguous assignment of reactive oxygen species-related photoinduced apoptosis. Indeed, similar phenotypes are associated with different cellular processes. Our methodology facilitates the in vitro design and evaluation of photosensitizers for the treatment of cancer and infectious diseases with the aid of functional fluorescence microscopy.

Photobleaching-resistant ternary quantum dots embedded in a polymer-coated silica matrix

Herein we present the preparation of ultrastable CuInS2/ZnS-silica composites able to withstand high density irradiation for more than 100 h without degradation using a blue LED operated at 20 mA forward current. The high photostability is obtained by employing a novel polymer-infused silica/QD composite. The versatility of the (3-mercaptopropyl)trimethoxy-silane/tetraethyl orthosilicate matrix used in this study is demonstrated by incorporating QDs with positively (amine) and negatively (carboxyl) charged ligands yielding transparent and highly luminescent gels with up to 20% solid loading. Quantum yields in solution (40–50%) are retained or even slightly improved in the silica matrix further proving efficient encapsulation without damaging the nanocrystal surface. White LED fabricated from green and orange ternary QDs exhibit a luminous efficacy of 30 lm W−1 and a color rendering index of 87 as compared to a blackbody radiator with 3850 K color temperature.

Toward Tunable Electroluminescent Devices by Correlating Function and Submolecular Structure in 3D Crystals, 2D-Confined Monolayers, and Dimers

The synthesis of new Pt(II) complexes bearing tailored cyclometalated C^N*N^C luminophores is reported along with their photophysical properties. The emission of the monomeric species can be blue shifted upon formal isosteric replacement of two C-H units by N atoms at the two cyclometalating rings. Their remarkable stability upon sublimation was demonstrated by means of scanning tunneling microscopy, which also revealed a defined self-assembly behavior leading to supramolecular arrays, showing a 3-fold symmetry in 2D-confined monolayers. The supramolecular organization is driven by van der Waals interactions of the side chains and does not depend on the nature of the luminophores, as also observed in the crystalline phases showing no significant Pt-Pt interactions in 3D. Conversely, the luminescence properties in glassy matrices at 77 K and in amorphous solids are indicative of intermolecular interactions with sizable intermetallic coupling, which was demonstrated by reproducing the emission spectra of dimeric species by means of (TD)DFT calculations. The tendency toward aggregation was also traceable by cyclic voltammetry, whereas thermogravimetric analyses confirmed their stability. Solution-processed and vacuum-deposited OLED devices showed a concentration-dependent electroluminescence that red shifts with increasing doping ratios. Due to the stability of the complexes, solution-processed and vacuum-deposited devices showed identical electroluminescence spectra. Besides favoring aggregation, introduction of two N atoms has a detrimental effect on the device performance, due to the prolonged excited-state lifetimes favoring triplet-triplet annihilation.