Stephan Landgraf

Graz University of Technology | TU Graz · Institute of Physical and Theoretical Chemistry

For more than 20 years, we have investigated electron transfer reactions of various types and under various conditions. According to the Marcus theory, solvent parameters have a significant influence on the observed kinetics. Temperature-dependent measurements, such as those used to obtain activation parameters, are especially strongly affected by...

Photophysical properties and photochemical reactivity for a series of bis-naphthols 4a-4e and bis-anthrols 5a and 5e were investigated by preparative irradiations in CH3OH, fluorescence spectroscopy and laser flash photolysis (LFP). Methanolysis taking place via photodehydration (bis-naphthols ΦR = 0.04-0.05) is in competition with symmetry breakin...

The Back Cover picture illustrates the interplay between acceleration of a chemical reaction by the temperature and the deceleration by the viscosity. In ionic liquids the temperature dependence of the reaction is often dominated by the high activation energy of the viscosity of the medium. Our viscosity independent measurements clearly show this e...

We introduce a binary system of two ionic liquids 1-ethyl-3-methyl-imidazolium and tri-octyl-methyl-ammonium bis-trifluoromethylsulfonyl-imide (emim and oma NTf2) with varying molar fractions. It allows to keep the dynamic viscosity constant over a theoretical range of almost 60 K which means that any activated process can be studied independent of...

Photooxidation kinetics of phenol, 1-naphthol, 2-naphthol, tyrosine (TyrOH) and N-acetyl-tyrosine (AcTyrOH), tryptophan (TrpH) by ruthenium(II) polypyridyl complexes: [Ru(bpy)3]Cl2 (1), [Ru(phen)3]Cl2 (2), [Ru(bpy)(phen)(bpg)]Cl2 (3), and [Ru(dpq)2(bxbg)]Cl2 (4) where bpy is 2,2′-bipyridine, phen – 1,10-phenanthroline, bpg – bipyridine-glycoluril,...

Current methods for the determination of molecular interactions are widely used in the analytical sciences. To identify new methods, we investigated as a model system the hybridisation of a short 7 nt oligonucleotide labelled with, structurally, very similar cyanine dyes CY3 and DY-547, respectively, to a 34 nt oligonucleotide probe immobilised in...

Derivatives of p-cresol 1-4 were synthesized and their photochemical reactivity, acid-base and photophysical properties were investigated. The photoreactivity of amines 1 and 3 is different from that for the corresponding ammonium salts 2 and 4. All compounds have low fluorescence quantum yields because the excited states undergo deamination reacti...

The simultaneous static and time-resolved fluorescence detection in high performance liquid chromatography (HPLC) systems using time-correlated single photon counting (TCSPC) and modulation techniques is a very recent topic in the area of analytical and physical chemistry. Whereas most commercial systems use simple intensity detection in fluorescen...

The diffusion properties of tetrathiafulvalene (TTF) and its radical cation (TTF.+) have been studied in organic solvents and in eight imidazolium-based ionic liquids (ILs). The values of the diffusion coefficients of TTF and TTF.+ were obtained by using cyclic and steady-state voltammetry (only for TTF) over a range of 293-343 K. The temperature-d...

Intramolecular electron spin exchange as a function of temperature, solvent viscosity and polarity has been studied by X-band electron paramagnetic resonance (EPR) spectroscopy for two nitroxide biradicals containing mercury-organic groups in the bridge connecting two 1-oxyl-2,2,6,6-tetramethylpiperidine-3,4-ene-nitroxide rings, R. The temperature...

Photoinduced electron transfer reactions from the triplet erythrosin (Ery(2-)) dianion to various quinones (1,4-benzoquinone, 2,5-di-t-butyl-1,4-benzoquinone, duroquinone, 2,5-dichloro-1,4-benzoquinone, chloranil, bromanil, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and 9,10-anthraquinone) and other organic acceptors like 7,7,8,8-tetracyanoquinodime...

We describe the experimental investigation of time-resolved magnetic field effects in exciplex forming organic donor-acceptor systems. In these systems, the photo-excited acceptor state is predominantly deactivated by bimolecular electron transfer reactions (yielding radical ion pairs) or by direct exciplex formation. The delayed fluorescence emitt...

Controlled tuning of material properties by external stimuli represents one of the major topics of current research in the field of functional materials. Electrochemically induced property tuning has recently emerged as a promising pathway in this direction making use of nanophase materials with a high fraction of electrode-electrolyte interfaces....

Current methods for the determination of molecular interactions are widely used in the analytical sciences. To identify new methods, we investigated as a model system the hybridisation of a short 7 nt oligonucleotide labelled with, structurally, very similar cyanine dyes CY3 and DY-547, respectively, to a 34 nt oligonucleotide probe immobilised in...

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 (k(het)) and diffusion coefficients (D) of ferroc...

The electrical resistance (R) of nanoporous platinum prepared by dealloying reversibly changes by 4% upon electrochemical surface charging in a regime where oxygen adsorption/desorption and surface oxidation/reduction occur. The variation of R with charging shows a sign inversion. Besides the usual behavior of increasing R with positive charging, a...

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 e...

The effect of preferential solvation on the exciplex luminescence detected magnetic field effect has been studied using magnetic-field-effect-on-reaction-yield (MARY) spectroscopy. By designing solvent mixtures which can provide a micro-environment around the magneto-sensitive radical ion pair (RIP) from highly heterogeneous to quasi-homogenous, th...

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...

Using photochemical electron transfer, N,N-dimethylnaphthylamine derivatives are added to α,β-unsaturated carboxylates. The addition takes place exclusively in the α-position of electron-deficient alkenes and mainly in the 4-position of N,N-dimethylnaphthalen-1-amine. A minor regioisomer results from the addition in the 5-position of this naphthyla...

AbstractThe lifetime of the lowest triplet state of the dianion erythrosin B depends on its concentration because of self-quenching. The self-quenching rate constants vary with the solution viscosity at room temperature. Dextrose was used to change the viscosity of aqueous solutions in the range 1≤η≤5.31cP. Photoinduced electron transfer reactions...

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 th...

The kinetics of rubrene (5,6,11,12-tetraphenylnaphthacene) fluorescence quenching by energy transfer to the radicals of N,N,N',N'-tetramethyl-para-phenylenediamine (TMPD+*), tetracyanoquinonedimethane (TCNQ-*) and galvinoxyl (2,6-di-tert-butyl-alpha-(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadien-1-ylidene)-p-tolyloxy; GAL*), have been measured in solve...

We present here some of the physicochemical properties of tetracyano-p-phenylenediamine (TCPPD). This is the second highly fluorescent p-phenylenediamine (PPD) presented in the literature. The molecule emits with a very long lifetime (20ns) and an oxygen insensitive high quantum yield (up to 0.9) for this class of molecules. Its emission peak is lo...

Rate constants k(hom) and k(het) are reported for the homogeneous electron-self-exchange and the heterogeneous electrochemical electron-transfer reactions, respectively, of the cyclooctatetraene/cyclooctatetraene(-) (COT/COT(.-)) redox couple. In acetonitrile, the values k(hom) (298 K)=(5+/-3) x 10(5) M(-1) s(-1) and k(het) (295 K)=8 x 10(-3) cm s(...

Benzyl radicals were generated photochemically directly in an electrochemical cell, using diphenylacetone as a precursor. The half-wave potentials and the plateau current of benzyl radical reduction and respectively oxidation were determined in acetonitrile, propylene carbonate, N,N-dimethylformamide and dimethylsulfoxide. While half-wave potential...

The driving-force dependence of bimolecular fluorescence quenching by electron transfer in solution, the Rehm-Weller experiment, is revisited. One of the three long-standing unsolved questions about the features of this experiment is carefully analysed here, that is, is there a diffusional plateau? New experimental quenching rates are compiled for...

The fluorescence quenching by electron transfer of a fluorophore, 2,5-bis(dimethylamino)-1,3-benzenedicarbonitrile, to 1,3-dimethyl-2-nitrobenzene, has been studied by means of time-resolved and steady-state experiments at different viscosities and up to large quencher concentrations. Differential Encounter Theory (DET) has been used to rationalize...

Photophysical properties of 2,6-dicyano-N,N,N ′,N ′-tetramethyl-p-phenylenediamine (DCTMPPD) are reported. Fluorescence quantum yields range from 0.37 to 0.58 and fluorescence lifetimes from 12.2 to 22.9 ns in 28 different solvents. The triplet energy is determined to be 16.8 kK. Steady-state and time-resolved emission anisotropies in glycerol are...

Electric field-induced tuning of material properties is usually restricted to nonmetals such as semiconductors and piezoelectric ceramics. We show that variations of the electrical resistance of a metal (Pt) in the range of several percent can be reversibly induced at low charging voltages making use of a nanocrystallite-electrolyte composite. The...

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 ef...

Photoinduced cycloadditions of N-methyl-1,8-naphthalenedicarboximide 1 with phenylacetylenes 2a–2c, cyclopropylacetylene 2d, diphenylacetylenes 2e–2f and 1-phenylpropyne 2g were investigated. In the case of phenylacetylenes 2a, 2b and cyclopropylacetylene 2c, photoreaction with 1 takes place at the naphthalene C(1)C(2) bond to give the cyclobutene...

Unprecedented ab initio simulations of exchange-affected magnetic field on reaction yield (MARY) spectra of real radical pair (RP) systems are presented which provide a novel, reliable method for the determination of electron self-exchange rate constants by MARY spectroscopy. The MARY technique is an interesting alternative to EPR in the study of s...

cis-Azobenzene was generated from trans-azobenzene directly inside an electrochemical cell by irradiation with light from a mercury lamp. The formation of the cis-isomer was proved by recording the UV–VIS absorption spectra of a trans-azobenzene solution in acetonitrile after different periods of irradiation. The reduction half-wave potentials of c...

ESR and ENDOR spectra are reported of several symmetrical substituted N,N,N,N-tetraalkyl-p-phenylenediamine radical cations in solution. Different N,N-alkyl substituted para-phenylenediamines, like the ethyl, n-propyl, and iso-propyl derivative are compared with the parent N,N,N,N-tetramethyl-p-phenylenediamine (Wursters Blue Cation). N,N,N,N-Tetra...

Ultrabright light emitting diodes (LEDs) are an inexpensive alternative to laser diodes (LDs) and other short wavelength emitting light sources. They have a high stability, a long lifetime, and a very low power consumption. A large number of publications are already available for fluorescence applications using this type of LEDs. Most of them are d...

The electron self-exchange between a neutral molecule and its charged radical, which is part of a spin-correlated radical ion pair, gives rise to line width effects in the fluorescence-detected MARY (magnetic field effect on reaction yield) spectrum similar to those observed in EPR spectroscopy. An increasing self-exchange rate (i.e., a higher conc...

Reduction half-wave potentials of several phenoxyl radicals have been measured by photomodulated voltammetry in acetonitrile. Two of the investigated radicals exhibit a reversible behavior, but for the others a rather quasi-reversible nature of the heterogeneous electron transfer has to be considered. The measured reduction half-wave potentials are...

The idea of building up a new method for the determination of fluorescence lifetime using semiconductor light sources, such as laser diodes (LDs) and ultrabright light emitting diodes (LEDs), was born in 1995. It had been shown earlier that LDs and LEDs can be used as pulsed light sources in a single photon timing (SPT) setup (LD:1, LED:2). The int...

The steady-state and time-resolved fluorescence of phenylene-1,4-ethylene bis(4-quinoline), PhQ, have been measured in some organic solvents. The fluorescence quantum yield and lifetime are highly sensitive to the solvent nature—they decrease markedly with increasing the hydrogen bond donating ability and polarity of the solvent. The radiative rate...

The rate constants of the oxidation of N,N,N',N'-tetramethyl-p-phenylenediamine (TMPPD) are measured in water by means of stopped-flow techniques using different inorganic ions, like Fe(CN)(6)(3-), MnO4, Co(NH3)(6)(3-), Ru(OH)(6)(3-) The rates do not depend on the different oxidation potentials, but can be correlated well with calculated rate const...

The influence of electron self-exchange on the magnetic field effect of various exciplex–radical ion pair (RIP) systems (1,2- and 1,4-dicyanobenzene in combination with the fluorophores pyrene, N-methylcarbazole and N-ethylcarbazole in tetrahydrofurane) has been investigated using magnetic-field-effect-on-reaction-yield (MARY) spectroscopy. By vary...

The influence of electron self-exchange on the magnetic field effect of various exciplex–radical ion pair (RIP) systems (1,2- and 1,4-dicyanobenzene in combination with the fluorophores pyrene, N-methylcarbazole and N-ethylcarbazole in tetrahydrofurane) has been investigated using magnetic-field-effect-on-reaction-yield (MARY) spectroscopy. By vary...

 The kinetics of the CrO(O2)2 formation by H2O2 and Cr2O7 2− in aqueous acidic media was measured at 293 ± 2 K in a pH range between 2.5 and 3.3. Using the stopped-flow method with rapid scan UV-VIS detection, the rate law of the formation of CrO(O2)2 was determined. For the media HClO4, HNO3 and CH3COOH, the reaction order in the Cr2O7 2− concentr...

Dimerization enthalpies and equilibrium constants have been determined for the radical anion of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), the radical cations of N,N,N',N'-tetramethyl-p-phenylenediamine, N,N-dimethyl-p-phenylenediamine, 2,3,5,6-tetramethyl-p-phenylenediamine, N,N,N',N'-tetraethyl-p-phenylenediamine, N,N-diethyl-p-phenylenediami...

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...

Dimerization enthalpies and equilibrium constants have been determined for the radical anion of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), the radical cations of N,N,N′,N′-tetramethyl-p-phenylenediamine, N,N-dimethyl-p-phenylenediamine, 2,3,5,6-tetramethyl-p-phenylenediamine, N,N,N′,N′-tetraethyl-p-phenylenediamine, N,N-diethyl-p-phenylenediami...

Two newly developed laboratory simulation chambers will be introduced: (a) Plan- etary surface/atmosphere/irradiation experiments - concerning processes relevant for Mars, Mercury and the Moon - will be performed by means of a chamber, made of high-grade steel. Numerous connections make it possible to carry out in-situ measure- ments under certain...

The inner-sphere reorganization energy of the electron self-exchange of the couple cyclooctatetraene/cyclooctatetraene radical anion has been investigated by quantum mechanical calculations. The more stable Jahn Teller distorted B2g conformation of the radical anion has been used in this study. Two different theories have been applied in this first...

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...

 Since the first application of laser diodes (LD) for the determination of fluorescence lifetimes in our group in 1995, great technical improvements have been made to increase the time resolution of the apparatus. Using the phase fluorometric method, measurements down to 100 ps are now possible. Values can be measured in steps of 10 ps with good re...

Host-guest principles are put into action advantageously by cyclodextrins that give remarkable sensor responses to halogenated hydrocarbons on mass-sensitive devices such as QMB (quartz micro balance) and SAW (surface acoustic wave) resonators. Modifications of the structure can tune the compound to different analytes and molecular modeling allows...

The fluorescence of three diolefinic laser dyes: phenylene-1,4-diethylenebis(N-methylpyridinium) diiodide, phenylene-1,4-diethylenebis-(N-methylquinolinium) diiodide and phenylene-1,4-diethylenebisquinoline by some aromatic and aliphatic amines with variable oxidation potentials, has been studied in acetonitrile (polar solvent) and cyclohexane (non...

The viscosity dependence of the photoinduced electron transfer reaction between perylene and two different aromatic amines, N,N-dimethylaniline and 2,4,6,N,N-pentamethylaniline, is studied. In order to increase the viscosity, mixtures of methanol and cyclohexanol, and of methanol and glycerol are used as solvents. The advantage of this particular m...

Rate constants of the electron self-exchange between 2,3-dicyano-5,6-dichloro-p-benzoquinone (DDQ) and its radical anion (DDQ˙–) are measured by means of EPR-line-broadening effects in different solvents at T = 293 K. Solvents of different polarity like CHCl3, CH2Cl2, CH3CN, benzonitrile and acetone were used. No correlation is found in the sense o...

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 St...

External magnetic field effects on the recombination fluorescence (MARY effect) in the photoionization reaction of N,N,N′,N′-Tetramethyl-p-phenylenediamine (TMPPD) in water and DMSO/water mixture are studied. Relatively large magnetic field effects (MFE), ∼ 2–4%, on the fluorescence yield are observed in the extremely polar water solvent under magn...

(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.

In order to investigate the viscosity dependence of the reaction of photoinduced perylene with tetracyanobenzene and N,N-2,4,6-pentamethylaniline in acetonitrile fluorescence quenching measurements have been performed, respectively. To increase the viscosity several highly viscous substances (ethyleneglycole, polyethyleneglycole, tetramethylurea) h...

Fluorescence lifetime measurements are often required for investigations of photoinduced electron transfer reactions. In general complicated and expensive experimental set-ups are necessary for these measurements. Using semiconductor light sources, such as laser diodes and ultrabright light emitting diodes, we have developed a new experimental meth...

Continuous wave laser diodes in the visible region can be used for the determination of fluorescence lifetimes using the phase fluorometric method. With moderate frequencies between 10 and 50 MHz and very high modulation in the excitation light a time resolution of less than 0.1 ns is possible. The method was applied for two laser dyes, nile blue a...

Electron self-exchange rates between the radical anions and the respective neutral molecules of the dicyanoben-zene isomers (DCB) have been determined applying field-modulated MARY (MAgnetic field effect on Reaction Yield) spectroscopy, using pyrene as electron donor. Due to lifetime uncertainty energy broadening of spin levels caused by electron s...

Diss. - Bibliogr. s. 195-199