Ueber die Bestimmung der Bernsteins?ure

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Luminescence and circularly polarized luminescence of macrocyclic Eu(III) and Tb(III) complexes embedded in xerogel and sol–gel SiO2 glasses

Luminescence, time-resolved luminescence, circularly polarized luminescence (CPL) and decay profiles of Ln(III)(15-crownether-5) (Ln=Ce, Sm, Eu, Tb) and Tb(III)-(R),(S)-cyclen derivative complexes doped in xerogel and sol–gel silica glasses are measured at temperatures down to 10 K to characterize luminescence properties and the electronic structure in the excited states. Luminescence spectral profiles and calculation of crystal field parameters (B₀⁽²⁾,B₂⁽²⁾) in the ⁵D₀→⁷F_J(J=1,2) transition give evidence of the fact that the pentagonal and planar structure of Eu(III) (15-crownether-5) does hold in xerogel and sol–gel glasses prepared at temperatures below 100°C. As annealing temperatures are increased from 80°C to 750°C, Eu(III) complexes in sol–gel glasses are found to decompose gradually to SiO₂:Eu³⁺. Tb(III)-(R) and (S)-cyclen derivative complexes in xerogel reveal at room temperature and 10 K sharp CPL spectra with luminescence dissymmetry factors g_lum=−0.1 and 0.1, respectively. These complexes doped in sol–gel glasses represent luminescence characteristics of rare earth ions encapsulated in the nano-porous host.     

Multiple Triplet Metal-Centered Jahn-Teller Isomers Determine Temperature-Dependent Luminescence Lifetimes in [Ru(bpy)3]2+

Understanding the factors that determine the luminescence lifetime of transition metal compounds is key for applications in photocatalysis and photodynamic therapy. Here we show that for (bpy = 2,2’-bipyridine), the generally accepted idea that emission lifetimes can be controlled optimizing the energy barrier from the emissive triplet metal-to-ligand charge-transfer (³MLCT) state to the thermally-activated triplet metal-centered (³MC) state or the energy gap between both states is a misconception. Further, we demonstrate that considering a single relaxation pathway determined from the minimum that is lowest in energy leads to wrong temperature-dependent emission lifetimes predictions. Instead, we obtain excellent agreement with experimental temperature-dependent lifetimes when an extended kinetic model that includes all the pathways related to multiple Jahn–Teller isomers and their effective reaction barriers is employed. These concepts are essential to correctly design other luminescent transition metal complexes with tailored emission lifetimes based on theoretical predictions.     

Investigations on ternary metallocene-based catalyst systems

The influence of the cocatalyst triisobutyl aluminium (TiBA) on the two step reaction of the ternary activation of the metallocene precursor Ph₂C(CpFlu)ZrCl₂ with TiBA and the activator N,N-dimethylanilinium-tetrakis(pentafluorophenyl)borat (DMAB) was investigated by means of NMR spectroscopy. More than 5 equivalents of TiBA are required for a total conversion of Ph₂C(CpFlu)ZrCl₂. The reaction exclusiveley leads to the monoalkyl complex Ph₂C(CpFlu)ZrClBuⁱ, independent of the Al / Zr ratio. The reaction of the Ph₂C(CpFlu)ZrCl₂ / TiBA mixtures, using 10, 20 and 50 equivalents of TiBA with DMAB, generates two cationic metallocene species. Using 100 equivalents of TiBA exclusiveley leads to the formation of the cationic heterodinuclear metallocene complex [Ph₂C(CpFlu)Zr-(μ-H)-(μ-C₄H₇)-AlBu₂ⁱ]⁺. In no case was the degradation or complexation of [B(C₆F₅)₄]⁻ detected, whereas the direct reaction between TiBA and DMAB gives AlBuⁱ_3−x(C₆F₅)_x compounds. A mechanism was postulated to explain the formation of [Ph₂C(CpFlu)Zr-(μ-H)-(μ-C₄H₇)-AlBuⁱ₂]⁺. The catalyst system Ph₂C(CpFlu)ZrCl₂ / TiBA / DMAB was used for continuous high pressure polymerizations of ethylene at 150 MPa and 180°C. To investigate the influence of additional TiBA and triethyl aluminium (TEA) in the reactor, their concentrations were varied over a wide range. Highest productivities and molecular weights were obtained with low concentrations of TiBA in the reactor. Up to a concentration of 30 molppm Al in the reactor, unimodal polymers were formed with Mw / Mn between 2 and 3. With higher aluminium concentrations, the products formed contained small amounts of waxes. This was due to oligomerization catalyzed by the aluminium alkyl compounds. Using the Schulz-Zimm distribution all MWDs were analyzed with regard to the amount of waxes produced by ethylene oligomerization and with regard to the influence of chain transfer reactions to the aluminium. The rate constants of chain transfer to aluminium in relation to the rate constants of insertion of ethylene were estimated.