Stephan Landgraf

Application of semiconductor light sources for investigations of photochemical reactions.

Semiconductor light sources, like laser diodes or ultrabright light emitting diodes, are widely used in optical spectroscopy. In this presentation an overview of applications in photochemistry is given. Since the beginning of the 1990s an increasing number of publications with the application of semiconductor light sources appeared. Three different techniques were used: single photon counting with short pulses, phase-modulation fluorometry using a conventional modulation spectrometer, or a lock-in amplifier. Using continuous wave laser diodes in the visible region, which are available from 690 to 630 nm (and, recently, down to 400 nm), a new compact fluorescence spectrometer was developed in our laboratory. Using the phase fluorometric method, measurements down to 100 ps are now possible. Values can be measured in steps of 10 ps with good reproducibility using a high-frequency signal generator and a GHz digital storage oscilloscope. Several investigations have been carried out applying this technique including time-resolved detection of crude oil as an example for possible practical applications.

Time-resolved magnetic field effects distinguish loose ion pairs from exciplexes.

We describe the experimental investigation of time-resolved magnetic field effects in exciplex-forming organic donor–acceptor systems. In these systems, the photoexcited acceptor state is predominantly deactivated by bimolecular electron transfer reactions (yielding radical ion pairs) or by direct exciplex formation. The delayed fluorescence emitted by the exciplex is magnetosensitive if the reaction pathway involves loose radical ion pair states. This magnetic field effect results from the coherent interconversion between the electronic singlet and triplet radical ion pair states as described by the radical pair mechanism. By monitoring the changes in the exciplex luminescence intensity when applying external magnetic fields, details of the reaction mechanism can be elucidated. In this work we present results obtained with the fluorophore-quencher pair 9,10-dimethylanthracene/N,N-dimethylaniline (DMA) in solvents of systematically varied permittivity. A simple theoretical model is introduced that allows discriminating the initial state of quenching, viz., the loose ion pair and the exciplex, based on the time-resolved magnetic field effect. The approach is validated by applying it to the isotopologous fluorophore-quencher pairs pyrene/DMA and pyrene-d10/DMA. We detect that both the exciplex and the radical ion pair are formed during the initial quenching stage. Upon increasing the solvent polarity, the relative importance of the distant electron transfer quenching increases. However, even in comparably polar media, the exciplex pathway remains remarkably significant. We discuss our results in relation to recent findings on the involvement of exciplexes in photoinduced electron transfer reactions.

C5H5)2TiF2 and (C5H5)2ZrF2 as catalyst precursors for the dehydropolymerisation of silanes

Abstract (C5H5)2TiF2 and (C5H5)2ZrF2 were found to be rapidly converted into catalytically-active species for the dehydrocoupling of silanes upon treatment with PhSiH3 or H2MeSiSiMeH2.

Adsorption-driven tuning of the electrical resistance of nanoporous gold

The electrical resistance of nanoporous gold prepared by dealloying is tuned by charging the surfaces of the porous structure in an electrolyte. Reversible variations in the resistance up to approximately 4% and 43% occur due to charging in the regimes of double layer charging and specific adsorption, respectively. Charging-induced variations in the electron density or of the volume cannot account for the resistance variation, indicating that this variation is primarily caused by charge-induced modifications of the charge carrier scattering at the solid-electrolyte interface. The relative resistance variation in nanoporous Au with surface charging is found to be much higher than reported for porous nanocrystalline Pt. This is due to the lesser resistance contribution from internal grain boundaries. The resistance variation in nanoporous Au is also higher than that found in thin films owing to the stronger surface scattering in the ligament structure compared to plan surfaces. We argue that the strong resi...

Cyclic Voltammetric Study of Heterogeneous Electron Transfer Rate Constants of Various Organic Compounds in Ionic liquids: Measurements at Room Temperature

Abstract Room temperature ionic liquids (RTILs) are of growing interest due to their outstanding solvent properties. The high conductivity and large electrochemical window of RTILs have enabled their use in electrochemistry without adding supporting electrolyte. Heterogeneous electron transfer rate constants (khet) and diffusion coefficients (D) of ferrocene, 2,6-dimethylbenzoquinone, bromanil, tetracyanoethylene, tetrathiofulvalene, methylviologen, and ethylviologen were determined in several RTILs such as [emim][BF4], [bmim][OTf], [bmim][BF4] and [bmim][PF6] using cyclic voltammetry. The results obtained for khet and D, range from 0.25–29.6 × 104 cm s-1 and 1.27–25.5 × 108 cm2 s-1 respectively. Both were significantly lower than those found in organic solvents like acetonitrile (MeCN), dimethylformamide (DMF), etc. It was found that khet and D were two to three orders of magnitude lower in more viscous RTILs. Diffusion coefficients were inversely proportional to the viscosity of the RTILs for all substances under investigation. Marcus theory was applied to compare the khet. The main problem arising is to understand the role of solvent reorganization energy (λo). Whereas Marcus theory describes λo in two parts of polarization, a fast electronic and a slower orientational contribution both expressed by the Pekar factor γ = (1/n2 - 1/ɛs), the solvent is treated as a continuum having a dielectric constant (ɛs) and a refractive index (n). Such a concept seems to be not applicable to ionic liquids.

Magnetic field effects on the pyrene—dicyanobenzene system: determination of electron self-exchange rates by MARY spectroscopy

An experimental determination has been made of electron self-exchange rates between the radical anions of 1,2-, 1,3- and 1,4-dicyanobenzene (DCB) and the respective neutral molecules applying steady-state field-modulated MARY (magnetic field effect on reaction yield) spectroscopy. For the first time this has been achieved successfully for compounds whose self-exchange rate constants can be obtained independently by alternative methods such as EPR linebroadening. In this study, pyrene was used as an electron donor to generate the spin-correlated radical ion pair (pyrene·+ DCB·−) essential for MARY spectroscopy. The radical ion pair is in equilibrium with an exciplex whose magnetic field affected fluorescence was recorded as a function of the magnetic field to yield the MARY spectrum. Due to lifetime uncertainty energy broadening of spin levels caused by electron self-exchange, the characteristic B 1/2 value increases with the concentration of DCB in the sample. The rate constant of self-exchange was obtained from the slope of the linear part in the plot of B1/2 versus DCB concentration. The values range between 6 x 108 M−1 s−1 and 1.4 x 1010 M−1 s−1, depending on the DCB isomer and solvent. Comparison with literature data from EPR linebroadening measurements shows good agreement.

ESR and ENDOR investigations of the degenerate electron exchange reactions of various viologens in solution. Solvent dynamical effects

The temperature dependences of the rates of the degenerate electron transfer of various viologens (1,1′-di(hydrocarbyl)-4,4′-bipyridinium salts) are measured in seven different solvents by means of electron spin resonance (ESR) line broadening. Rates vary between 1.7·108 and 1.1·109 M−1s−1 at room temperature and clearly show a solvent dynamical effect, which is inferred from the dependence of the rate constants on the longitudinal relaxation time of the solvent. Activation energies ranging from 5.3 to 24.4 kJ mol−1 are found. For the first time, hyperfine coupling constants are reported for the radical cations of the hydroxyethyl viologen and the amino viologen based on both continuous-wave ESR and electron-nuclear double resonance spectroscopy. Furthermore, the temperature and the solvent dependence of the hyperfine coupling constants of the methyl viologen radical cation are reported.

Fluorescence quenching of aromatic hydrocarbons by nitroxide radicals: a mechanismatic study

The fluorescence quenching of phenanthrene (Phen), 9-cyanophenanthrene (CPhen), 9-cyanoanthracene (CA), perylene (Per), 9,10-dicyanoanthracene (DCA), and 9,10-diphenylanthracene (DPA) using stable nitroxide radicals as quenchers has been studied by steady state and flash photolysis measurements. Both linearity and deviation from linearity in the Stern-Volmer plots have been observed. The active sphere model was used to discuss the upward curvature of the Stern-Volmer plots in case of Per, DCA, and DPA. The bimolecular quenching rate constant (kq) of Phen, CPhen and CA was found to be diffusion controlled while in other cases it is lower than the diffusion limit. On the basis of flash photolysis measurements as well as the overlap between the emission spectra of hydrocarbons and the absorption spectra of radicals, a resonance energy transfer mechanism is taken place in case of Per, DPA, DCA, and CA. For Phen and CPhen where the energy gap between the first excited singlet and the nearest lower triplet state is small, an induced intersystem crossing was suggested. Finally, the quenching process was discussed in terms of the free energy dependence (ΔG) of the electron transfer from nitroxide radicals to the excited hydrocarbons.

Conformational dynamics of some short-chain biradicals in solutions

Intramolecular electron spin exchange, as a function of temperature and the solvent nature, has been studied by X-band electron paramagnetic resonance (EPR) spectroscopy in five short-chain flexible nitroxide biradicals. Certain thermodynamic parameters of the conformational rearrangements were calculated from the EPR spectra. The process of spin exchange in short flexible biradicals has some peculiarities in comparison with that in long-chain molecules.

High pressure ESR studies of electron self-exchange reactions of organic radicals in solution.

Simple electron self-exchange reactions are often used to study the role of the reaction medium on a chemical process, commonly implying the use of various solvents with different physical properties. In principle, similar studies may be conducted using a single solvent, changing its physical properties by application of elevated pressures, but so far only little information is available on pressure dependent exchange reactions. In this work, we have used a recently constructed high pressure apparatus for use with electron spin resonance (ESR) spectroscopy to investigate simple electron self-exchange reactions involving 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) and tetracyanoethylene (TCNE) and their respective radical anions as well as TMPPD and its radical cation in three different solvents. The self-exchange was observed by ESR line broadening experiments, yielding rate constants and volumes of activation. The experimental results were compared to theoretical calculations based on Marcus theory and taking into account solvent dynamic effects. The use of elevated pressures has enabled the study of solvent effects without commonly encountered problems like solubility issues or chemical reactions between solvent and solute which sometimes limit the range of useable solvents.