Christoph Ulbricht

Reversible supramolecular functionalization of surfaces: terpyridine ligands as versatile building blocks for noncovalent architectures.

We report on the reversible and selective functionalization of surfaces by utilizing supramolecular building blocks. The reversible formation of terpyridine bis-complexes, based on a terpyridine ligand-functionalized monolayer, is used as a versatile supramolecular binding motif. Thereby, click chemistry was applied to covalently bind an acetylene functionalized Fe(II) bis-complex onto azide-terminated self-assembled monolayers. By decomplexation of the formed supramolecular complex, the ligand modified monolayer could be obtained. These monolayers were subsequently used for additional complexation reactions, resulting in the reversible functionalization of the substrates. The proper choice of the coordinating transition metal ions allows the tuning of the binding strength, as well as the physicochemical properties of the formed complexes and thus an engineering of the surface properties.

Synthesis, Microwave‐Assisted Polymerization, and Polymer Properties of Fluorinated 2‐Phenyl‐2‐oxazolines: A Systematic Study

We present a detailed systematic study of the synthesis and ability of fluorinated 2-phenyl-2-oxazolines to undergo polymerization. The synthesis of these compounds is based on a two-step procedure that gives the desired 2-oxazolines in moderate-to-good yields. All the compounds were fully characterized by IR and NMR ((1)H, (13)C, and (19)F) spectroscopy, mass spectrometry, and elemental analysis. The 2-oxazolines were subsequently used as monomers for living cationic ring-opening polymerization (CROP) with microwave irradiation as the heat source (T=140 degrees C), nitromethane as the solvent, and methyl tosylate as the initiator. The linear first-order kinetic plots of the polymerizations accompanied by a linear increase of the molecular weight with conversion and low polydispersity index (PDI) values (generally below 1.30) indicate a living polymerization mechanism. The resulting polymerization rates reflect a strong sensitivity to the quantity of fluorine substituents in general and the presence or absence of ortho-fluoro substituents of the phenyl ring in particular. All the polymers were isolated and characterized by size-exclusion chromatography and MALDI-TOF mass spectrometry. Finally, a detailed investigation of selected polymer properties was performed by using differential scanning calorimetry, thermogravimetric analysis, and contact-angle measurements, thus resulting in structure-property relationships. Whereas the thermal properties of the polymers are mostly influenced by the presence of ortho-fluoro substituents, the surface properties are mainly determined by the presence of para- and/or meta-fluoro substituents.

Morphology evaluation of a polymer–fullerene bulk heterojunction ensemble generated by the fullerene derivatization

We investigated a series of bulk heterojunction solar cells and corresponding thin films by blending of an anthracene-containing poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene) (PPE–PPV) copolymer with various fullerene derivatives. The so-called AnE-PVab copolymer was equipped with octyloxy side-chains at the PPE part and ethylhexyloxy side-chains at the PPV part, and has been chosen for this investigation due to its previously demonstrated superior performance in combination with phenylene-C60-butyric-acid-methyl-ester (PCBM). It is shown that the performance of bulk heterojunction solar cells strongly depends on the nature of the fullerene derivative, in particular the side-chain attached to the fullerene cage. In fact the intermolecular interaction between the polymer and fullerene derivative controls the blend morphology to a large extent, which has been interpreted on the basis of a novel approach for combining structural, topographical, photophysical, electrical and electro-optical characterizations.

Metallo-supramolecular materials based on amine-grafting onto polypentafluorostyrene

The post-modification of polypentafluorostyrene via the selective substitution of the para- fluorine by amine-functionalized molecules represents a versatile method for the preparation of functional materials. This communication presents the extension of this synthetic approach towards metallosupramolecular materials. First, the incorporation of terpyridines as metallo-supramolecular binding motifs is demonstrated, which enables the formation of a supramolecular crosslinked gel upon the addition of Fe(II) ions. Second, the linkage of phosphorescent iridium(III) complexes is reported yielding a copolymer that combines the excellent photophysical properties of the iridium(III) complex and the good solubility and processability of the polymer.

Optical and electronic properties of mixed halide (X = I, Cl, Br) methylammonium lead perovskite solar cells

We report on the fabrication and opto-electronic characterization of solution-processed planar heterojunction perovskite solar cells based on methylammonium (MA) lead halide derivatives, MAPbI3−xYx (Y = Cl, Br, I). Dissolving equimolar amounts of lead iodide (PbI2) and methylammonium iodide (H3CNH3I) together with various amounts of additional methylammonium halides, perovskite precursor solutions were obtained, which were used in the fabrication of three perovskite systems, MAPbI3, MAPbI3−xClx and MAPbI3−xBrx. The effect of the halide ratio in the perovskite formulations processed via a one-step deposition technique on optoelectronic properties and on photovoltaic performance of the formed perovskites was investigated. The perovskite film morphology, temperature-dependent photoluminescence properties, hysteresis behaviour in current–voltage characteristics and the photovoltaic performance as a function of chemical composition were studied using microscopic, spectroscopic and photovoltaic characterization techniques. The power conversion efficiency was found to be dependent on MAPbI3−xYx (Y = Cl, Br, I) perovskite film morphology. By controlling perovskite precursor composition and stoichiometry, highest power conversion efficiencies of 9.2, 12.5 and 5.4% were obtained for MAPbI3, MAPbI3−xClx and MAPbI3−xBrx devices, respectively. In addition, the physical parameters of the mixed halide perovskites such as the exciton binding energy, exciton–phonon interaction and bandgap were determined via temperature-dependent photoluminescence spectroscopy. Exciton binding and optical phonon energies of MAPbI3−xYx (Y = Cl, Br, I) were found to be in the ranges of 49–68 meV and 29–32 meV respectively. The solution-processed MA lead halide derivatives form highly crystalline materials with chemical versatility allowing the tuning of their optical and electronic properties depending on the nature and the ratio of the halides employed.

Unusual Terpyridines as Ligands for Novel Light-Emitting Iridium(iii) Complexes: Synthesis and Characterization

Based on S-shaped terpyridines, a series of yellow, orange, and red-orange light-emitting iridium(iii) complexes has been synthesized. The respective compounds have been prepared by the bridge-splitting method starting from the dimeric precursor complexes [(ppy)2Ir-μ-Cl]2, [(ppy-CHO)2Ir-μ-Cl]2, and [(c6)2Ir-μ-Cl]2. The products have been fully characterized by one- and two-dimensional (1H–1H correlation) NMR spectroscopy, elemental analysis, and MALDI-TOF mass spectrometry revealing the successful coordination of the iridium(iii) centres to the S-shaped terpyridine ligands. Furthermore, the quantitative coordination has been verified by the photophysical and electrochemical properties of the mononuclear iridium(iii) complexes. The photoluminescence spectra have shown strong emissions with maxima between 538 and 600 nm. The study of the optical properties of these novel complexes has indicated that the colour shifts are mainly depending on the nature of the cyclometallating ligands.

RAFT Polymerization Meets Coordination Chemistry: Synthesis of a Polymer-Based Iridium(III) Emitter.

A synthetic approach toward the synthesis of well-defined copolymers with attached phosphorescent iridium(III) emitters is presented. A reactive µ-hydroxy-bridged precursor complex has been utilized to coordinate suitable ligand sites of a methacrylate-based copolymer. The starting copolymer has been synthesized via the reversible addition fragmentation chain transfer (RAFT) polymerization technique. Using a reactive complex species, the coordination reaction at the copolymer could be performed under very mild conditions in the absence of any supporting additives.

Material structure–composite morphology–photovoltaic performance relationship for organic bulk heterojunction solar cells

Conjugated PPV-PPE copolymer has been investigated in organic solar cells in combination with twelve different fullerene derivatives. It was shown that the length of solubilizing alkyl chains in the fullerene derivative structures correlates well with the performance of photovoltaic cells.

Unexpected metal-mediated oxidation of hydroxymethyl groups to coordinated carboxylate groups by bis-cyclometalated iridium(III) centers

Two different procedures of the ‘click’ reaction were applied to synthesize a library of 1-aryl- and 4-aryl-functionalized 1H-[1,2,3]triazoles as new ligands for phosphorescent iridium(III) complexes. For three examples, single crystal X-ray analysis was carried out and the structural properties were discussed. The reactive μ-dihydroxy-bridged iridium(III) precursor complex [(ppy)2Ir-μ-(OH)]2 (ppy = 2-phenylpyridinato) was prepared for the complexation of the herein described ligands. During these complexation studies, an unexpected metal-assisted oxidation pathway was observed for the hydroxymethyl-substituted 1-aryl-1H-[1,2,3]triazoles 2d–f leading selectively to a [carboxylate-N3,O]-coordination of the ligands to the iridium(III) centers.

Confining metal-halide perovskites in nanoporous thin films

In situ perovskite nanocrystal formation within nanoporous thin films allows emission color tuning in optoelectronic devices. Controlling the size and shape of semiconducting nanocrystals advances nanoelectronics and photonics. Quantum-confined, inexpensive, solution-derived metal halide perovskites offer narrowband, color-pure emitters as integral parts of next-generation displays and optoelectronic devices. We use nanoporous silicon and alumina thin films as templates for the growth of perovskite nanocrystallites directly within device-relevant architectures without the use of colloidal stabilization. We find significantly blue-shifted photoluminescence emission by reducing the pore size; normally infrared-emitting materials become visibly red, and green-emitting materials become cyan and blue. Confining perovskite nanocrystals within porous oxide thin films drastically increases photoluminescence stability because the templates auspiciously serve as encapsulation. We quantify the template-induced size of the perovskite crystals in nanoporous silicon with microfocus high-energy x-ray depth profiling in transmission geometry, verifying the growth of perovskite nanocrystals throughout the entire thickness of the nanoporous films. Low-voltage electroluminescent diodes with narrow, blue-shifted emission fabricated from nanocrystalline perovskites grown in embedded nanoporous alumina thin films substantiate our general concept for next-generation photonic devices.