Bernhard Schäfer

Palladium versus Platinum: The Metal in the Catalytic Center of a Molecular Photocatalyst Determines the Mechanism of the Hydrogen Production with Visible Light

To develop highly efficient molecular photocatalysts for visible light-driven hydrogen production, a thorough understanding of the photophysical and chemical processes in the photocatalyst is of vital importance. In this context, in situ X-ray absorption spectroscopic (XAS) investigations show that the nature of the catalytically active metal center in a (N^N)MCl2 (M=Pd or Pt) coordination sphere has a significant impact on the mechanism of the hydrogen formation. Pd as the catalytic center showed a substantially altered chemical environment and a formation of metal colloids during catalysis, whereas no changes of the coordination sphere were observed for Pt as catalytic center. The high stability of the Pt center was confirmed by chloride addition and mercury poisoning experiments. Thus, for Pt a fundamentally different catalytic mechanism without the involvement of colloids is confirmed.

Incorporating Phosphaalkenes into Oligoacetylenes

Single-molecular electronics and organic material electronics are expanding research fields that ultimately aim for a vast variety of different applications, ranging from organic light-emitting diodes, to novel ways to improve the performance and decrease the size of electronics components. To achieve these goals, research has to be focused both on the development of functional molecules, but also on device fabrication. The work of this thesis is focused on the development of synthetic routes towards novel molecules for potential organic electronics applications, together with an investigation of their optical and electronic properties.The first part of the thesis describes the synthesis of butadiyne-substituted and diacetylenic phosphaalkenes. Theoretical, spectroscopic and electrochemical techniques have been used to understand key steps during their synthesis, and to gain further information on the conjugative properties of their π-systems. A mechanism is proposed for the formation of the butadiyne-substituted and diacetylenic phosphaalkenes and it is shown that the phosphorus heteroatom is an intrinsic part of the π-conjugated system. The incorporation of the phosphorus heteroatom leads to decreased HOMO-LUMO gaps compared to all-carbon based reference compounds.In the second part of the thesis, acetylenic phosphaalkenes are utilized for the preparation of phosphaalkene-substituted phospholes. A first step towards the exploration of the difference in reactivity of the σ2, λ3 phosphaalkene-P and the σ3, λ3 phosphole-P is presented as the oxidation of the compounds by sulfur proceeds selectively at the σ3, λ3–P. Spectroscopic and electrochemical investigations show that the phosphaalkene is an integral part of the compounds’ π-systems, and induces a HOMO-LUMO gap decrease compared to reference compounds that lack the P=C substituent.The third part of this thesis presents an exploratory study concerning the suitability of metathesis reactions for the assembly of alkene-bridged phosphaalkenes.

Phosphaalkenes in pi-conjugation with Acetylenic Arenes

Phosphaalkene inclusion at the periphery of acetylenic arenes results in decreased band gaps of the title compounds as verified by spectroscopic and electrochemical techniques. The electronic coupling between two 1-phosphahex-1-ene-3,5-diyne units is mediated by all para-substituted arenes and increases from 4b to 4d.

Multi-modal sensing in spin crossover compounds

We exploited the solvatochromic spin-state switching in a spin crossover (SCO) compound based on the FeII complex and the simultaneous change of spectroscopic properties for selective multimodal sensing of methanol and ethanol. We demonstrate that sensing capabilities are due to the inclusion of methanol or ethanol molecules into the crystalline structure, which tailors simultaneously the transition temperature, colour, birefringence and vibrational modes. We exploited this capability by integrating a neutral compound, switchable at room temperature, into a micrometric TAG sensitive to the colour and birefringence. The system was characterised by optical microscopy, magnetic susceptibility, Raman spectroscopy and X-ray diffraction.

Divergent Coordination Chemistry: Parallel Synthesis of [2×2] Iron(II) Grid‐Complex Tauto‐Conformers

Abstract The coordination of iron(II) ions by a homoditopic ligand L with two tridentate chelates leads to the tautomerism‐driven emergence of complexity, with isomeric tetramers and trimers as the coordination products. The structures of the two dominant [FeII 4 L 4]8+ complexes were determined by X‐ray diffraction, and the distinctness of the products was confirmed by ion‐mobility mass spectrometry. Moreover, these two isomers display contrasting magnetic properties (FeII spin crossover vs. a blocked FeII high‐spin state). These results demonstrate how the coordination of a metal ion to a ligand that can undergo tautomerization can increase, at a higher hierarchical level, complexity, here expressed by the formation of isomeric molecular assemblies with distinct physical properties. Such results are of importance for improving our understanding of the emergence of complexity in chemistry and biology.

Structural Properties of Ruthenium Biimidazole Complexes Determining the Stability of their Supramolecular Aggregates

The results of a detailed investigation of the influence of substituents in a variety of ruthenium biimidazole-type complexes [Ru(R-bpy)2(R´-bi(bz)imH2)]2+ (R = H, tBu; R´ = H,Me; bi(bz)imH2 = 2,2´-bi(benz)imidazole) on selected structural and photophysical properties is reported. The photophysical properties are only marginally influenced by the substituents at the bipyridine and the biimidazole core. All complexes show intense absorptions in the visible range of the spectrum with maxima around 475 nm, and emission from the formed excited state occurs at wavelengths between 650 and 670 nm. The comparison of structural properties determined by X-ray analysis within a series of related complexes shows that the Ru-N bond lengths to the coordinated bipyridines are not significantly influenced by the substituents, but slight differences in the Ru-N bond lengths to the biimidazole-type ligands can be detected. The reactions between ruthenium complexes containing different biimidazole-type ligands with the sulfate dianion, however, show a strong correlation between the substituents at the biimidazole core and the solubility of the product. The bibenzimidazolecontaining complexes precipitate from aqueous solution whereas the ruthenium complex containing unsubstituted biimidazole stays in solution. The solid-state structure of one example of the sulfatecontaining products (2b) shows that strong hydrogen bonds between the secondary amine function of the bibenzimidazole and the oxygen functionalities of the sulfate contribute to this unexpected behavior. Graphical Abstract Structural Properties of Ruthenium Biimidazole Complexes Determining the Stability of their Supramolecular Aggregates

Spin-dependent electronic structure of the Co/Al(OP)3 interface

We have studied the spin-dependent electronic properties of the interface formed between epitaxial Co thin films deposited on Cu(001) and the experimental molecule tris-(9-oxidophenalenone)-aluminum (III) (Al(OP)3), created as a variation of the prototypical organic semiconductor Alq 3 to tailor the spin filtering properties by modifying chemisorption with cobalt. The interfaces have been grown under ultra-high vacuum conditions by progressive deposition of 0.5-5nm Al(OP)3 on the freshly prepared cobalt substrate. For every growth step we have monitored the energy level alignment at the interface as well as the spin polarization of the occupied manifold by spin-resolved photoemission spectroscopy. We identify two hybrid interface states in the energy window of 2eV below the Fermi energy. The first is at 0.9eV below EF and shows an 8% higher spin polarization than Co, while the second is at 1.6eV below EF and shows a spin polarization reduced by 4%.

Characterization of Nonanuclear Europium and Gadolinium Complexes by Gas-Phase Luminescence Spectroscopy

Gas-phase measurements using mass-spectrometric techniques allow determination of the luminescence properties of selected molecular systems with knowledge of their exact composition. Furthermore, isolated luminophores are unaffected by matrix effects like solvent interactions or crystal packing. As a result, the system complexity is reduced relative to the condensed phase and a direct comparison with theory is facilitated. Herein, we report the intrinsic luminescence properties of nonanuclear europium(III) and gadolinium(III) 9-hydroxyphenalen-1-one (HPLN)-hydroxo complexes. Luminescence spectra of [Eu9(PLN)16(OH)10](+) ions reveal an europium-centered emission dominated by a 4-fold split Eu(III) hypersensitive transition. The corresponding Gd(III) complex, [Gd9(PLN)16(OH)10](+), shows a broad emission from a ligand based triplet state with an onset of about 1000 wavenumbers in excess of the europium emission. As supported by photoluminescence lifetime measurements for both complexes, we deduce an efficient europium sensitization via PLN-based triplet states. The luminescence spectra of the complexes are discussed in terms of a square antiprismatic europium/gadolinium core structure as suggested by density functional computations.

Charge carrier mobility and electronic properties of Al(Op)3: impact of excimer formation

Summary We have studied the electronic properties and the charge carrier mobility of the organic semiconductor tris(1-oxo-1H-phenalen-9-olate)aluminium(III) (Al(Op)3) both experimentally and theoretically. We experimentally estimated the HOMO and LUMO energy levels to be −5.93 and −3.26 eV, respectively, which were close to the corresponding calculated values. Al(Op)3 was successfully evaporated onto quartz substrates and was clearly identified in the absorption spectra of both the solution and the thin film. A structured steady state fluorescence emission was detected in solution, whereas a broad, red-shifted emission was observed in the thin film. This indicates the formation of excimers in the solid state, which is crucial for the transport properties. The incorporation of Al(Op)3 into organic thin film transistors (TFTs) was performed in order to measure the charge carrier mobility. The experimental setup detected no electron mobility, while a hole mobility between 0.6 × 10−6 and 2.1 × 10−6 cm2·V−1·s−1 was measured. Theoretical simulations, on the other hand, predicted an electron mobility of 9.5 × 10−6 cm2·V−1·s−1 and a hole mobility of 1.4 × 10−4 cm2·V−1·s−1. The theoretical simulation for the hole mobility predicted an approximately one order of magnitude higher hole mobility than was observed in the experiment, which is considered to be in good agreement. The result for the electron mobility was, on the other hand, unexpected, as both the calculated electron mobility and chemical common sense (based on the capability of extended aromatic structures to efficiently accept and delocalize additional electrons) suggest more robust electron charge transport properties. This discrepancy is explained by the excimer formation, whose inclusion in the multiscale simulation workflow is expected to bring the theoretical simulation and experiment into agreement.

Photoluminescence Spectroscopy of Mass-Selected Electrosprayed Ions Embedded in Cryogenic Rare-Gas Matrixes

An apparatus is presented which combines nanoelectrospray ionization for isolation of large molecular ions from solution, mass-to-charge ratio selection in gas-phase, low-energy-ion-beam deposition into a (co-condensed) inert gas matrix and UV laser-induced visible-region photoluminescence (PL) of the matrix isolated ions. Performance is tested by depositing three different types of lanthanoid diketonate cations including also a dissociation product species not directly accessible by chemical synthesis. For these strongly photoluminescent ions, accumulation of some femto- to picomoles in a neon matrix (over a time scale of tens of minutes to several hours) is sufficient to obtain well-resolved dispersed emission spectra. We have ruled out contributions to these spectra due to charge neutralization or fragmentation during deposition by also acquiring photoluminescence spectra of the same ionic species in the gas phase.