[60]Fullerene is oxidized to its radical cation at preparative scale by means of photoinduced electron transfer (PET). In general, C-60(.+) reacts with various H-donors such as alcohols, aldehydes, ethers etc., under formation of 1,2-dihydro[60]fullerenes. In case of tert-butanol, propionic acid and glycol, for example, product formation is also initiated by H-abstraction from the OH group. Both time-resolved laser flash photolysis studies and ESR spectroscopic investigations clearly demonstrate the formation of C-60(.+) using PET conditions. Upon addition of H-donors such as methanol the signal of C-60(.+) is quenched as monitored both by laser flash photolysis and ESR spectroscopy. Based on these studies we assume the oxidation of C-60 followed by abstraction of hydrogen from the H-donor.
UV/Vis-spectroscopic control of the photolysis of 3,6-diphenyl-1,2-dithiine shows formation of the corresponding thiophene derivative and elemental sulfur. The spin-forbidden sulfur extrusion can be influenced by addition of elemental sulfur or appropriate electrophiles, but also with cyanide ions. A dark reaction could be observed leading to partial regeneration of the starting 1,2-dithiine.
A new and highly selective methodology for the construction of a large variety of mono- and polysubstituted pyridines is described. The synthesis is efficient under irradiation employing artificial light as well as with the sun as light source. In particular the latter method allows the preparation of substituted pyridines under mild conditions and affords the possiblity to scale up the reaction. Deduced from our mechanistic investigations conditions for an efficient control of the selectivity could be identified including the reaction in water as reaction medium. The new method is also useful for the synthesis of pyridines with central or axial chirality and requires therefore ongoing attention.
The photoinduced ringopening of cyclohexadiene to hexatriene occurs on a femtosecond time scale. The observed fast non-radiative transition from the excited state Si to the ground state S-0 is only possible when the molecular system leaves the Woddward-Hoffmann defined reactionpath. To gain physical insight of this process on a microscopic scale we investigate the molecular motion on the excited S-1 state by means of wavepacket propagations.
2-Morpholinoporpenenitrile (2) is added with high regioselectivity to electronically excited 1-naphthoic acid (4a) with formation of a [2+2]-cycloadduct 5a bearing an exo-oriented morpholino ring as well as a [4+2]-cycloadduct 6a with an endo-oriented morpholino ring only. Esterification with diazomethane allows for the isolation of products 5b and 6b, and at the same time hydrolysis products 7 and 8 are built up. For methyl naphthalene-1-carboxylate (4b) an analogous product pattern was observed. Under otherwise the same irradiation conditions 2-naphthoic acid does not react with 2. In a [2+2]-photocycloaddition naphthalene-1,4-dicarboxylic acid (9a) and 2 give rise to the [2+2]-exo-product 10 only. Surprisingly, a product has been observed in very low yield, which is probably a follow-up product from a Paterno-Buchi-reaction of 2 to one of the carboxylic acid groups. The excited singlet and triplet state properties of the three naphthalene carboxylic acids are described, the triplet quantum yields are substantial and the triplet energies at 234-248 kJ mol(-1). Both states are readily quenched by 2, the rate constants are greater than or equal to 1 x 10(10) M-1 s(-1) and less than or equal to 2 x 10(9) M-1 s(-1) for the excited singlet and triplet state, respectively.
An alternative possibility to prepare non-erasable laser recording medium, analog to "Drexon", by vacuum deposition of Ag particles and their treatment in an electroless silver plating bath is demonstrated. The infuence of the amount of the deposited Ag particles, their deposition rate as well as the treating conditions in the electroless silver plating bath on the structure, optical and laser recording properties of the mirror-like films obtained are investigated. The deposition and processing conditions of Ag particles for preparing of a fine-grain laser recording medium with uniform and high absorption, moderate reflection and low electrical resistivity are established. The possibilities of the proposed method to prepare mirror-like laser recording films satisfying the requirements to non-erasable laser recording medium are shown.
The irradiation of alkyl-aryl-ketones bearing a leaving group adjacent to the carbonyl C-atom affords 1-hydroxy-1, n-biradicals, which undergo within its lifetime a very rapid elimination of acid giving 1-oxo-2, n-biradicals. We have called this process spin centre shift due to the shortening of the original biradical. In the current publication we present a first synthetic application of this approach. Starting with alpha -sulfonyloxy-alkyl-arylketones a short and straightforward method for preparation of highly functionalized cyclopropanes was developed. The method is tolerant towards many functional groups and it even allows the synthesis of highly strained hydrocarbons such as bicyclo[2.1.0] pentanes.
Upon photolysis in a bromine-doped xenon matrix with lambda = 254 nm propane (3) can be fragmented despite the fact that 3 does not absorb light of this wavelength. A mechanistic model is developed, which can explain this type of indirect photoexcitation. In addition, the behavior of propane (3) in a pure xenon matrix on irradiation with a laser and a mercury lamp is repel-red. The saturated hydrocarbon 3 is not photostable under these experimental conditions, and the corresponding mechanisms leading to its fragmentation are discussed.
It is known that the some stabilizers added in certain concentration increase the sensitivity of the silver halide photographic emulsion. This work presents some results obtained for two tetraazaindene derivatives applied to a double-jet growth, monodisperse AgBr(I) core-shell emulsion crystals with an iodide-rich core. The following three types of chemical ripening were investigated: type S + Au, type CNS + Au and type S + CNS + Au. The study identifies a correlation between the sensitivity of the photographic emulsions and the concentration of the stabilizer. Discussion of processes, which are contributing to the effects of two selected stabilizers on the sensitivity of photographic emulsions, is presented.
The technical applications for titanium dioxide are widespread and reach from one of the most efficient photocatalysts for the oxidative degradation of organically charged waste water up to electrode materials in novel solar cells. Here we want to focus the attention on the influence of preilluminating titania on the photocatalytic activity in gas phase reactions. This topic is mainly discussed especially in Japanese literature [1,2]. In order to study this proceeding effects of UV-preirradiation on physical properties (charge carrier lifetime, photocatalytic activity, surface morphology) are examined. Degradation experiments are done with the test substance trichloroethylene and correlated to physical properties.
The irradiation of aminoketones 1 gives the corresponding azetidinols 2 and 3. The diastereoselectivity and the yields are found to depend on the substituents, especially on R-1. The photocyclization of an analogous aminoketone 6 is used as a key step in the synthesis of 3-hydroxy-3-azetidinecarboxylic acid 4, starting from adrenaline 5.
Visible light photosensitization of TiO2 is achieved by platinum(IV)chloride complexes. Such modified materials can be successfully applied for water detoxification and photocurrent generation. The mechanism of the sensitization process is described and the role of hydroxyl radicals acid chlorine atoms discussed.
Rate constants k(iso) of the photoinduced thermal cis-trans isomerization of azo dyes are known to be dependent on polarity and viscosity of the medium. In poly(ethylene oxide) 600/water mixtures with varying water content, k(iso)-values of 4,4'-nitro-anilinoazobenzene show an appearently stronger dependence on bulk viscosity which depends on the water content, than in water-free aprotic solvents. Nearly constant UV/Vis absorption maxima lambda (max) of the dye (trans) however indicated almost constant micropolarity and hydrogen bonding effects on a time average. This effect of water on k(iso) and the stronger kinetic solvent isotope effect on exchange of H2O by D2O compared to polarity and viscosity isotope effects, is assumed to be due to the dynamics of hydrogen bonding on cis-trans isomerization.
The radical cation of methionine exists in several forms (open-chain protonated, open-chain deprotonated, and cyclic with a two-center three-electron bond between sulfur and nitrogen). These intermediates were generated by photoinduced electron transfer, and the equilibrium between them was investigated by CIDNP (chemically induced dynamic nuclear polarization) spectroscopy. From the pH dependence of the polarizations, the deprotonation kinetics and the equilibrium constant of cyclization were obtained. Evaluation of the polarization pattern yielded the spin-density distribution in the cyclic radical cation.
The influence of pH-values on the fluorescence behaviour of various perylene-tetracarboxydiimides and 4,5-dimethoxyphthalimides was analyzed. The shifts in fluorescence wavelength maxima and intensities were rationalized by hydrogen bonding and by a photoinduced electron transfer model, respectively.
Photoionizations by the sequence absorption-electron transfer-absorption were studied by one-pulse and two-pulse laser flash photolysis with optical detection. The efficiency of this cyclic mechanism is substantially higher than that of sequential two-photon ionization of the same substrates via the triplet state, i.e., without intervening chemical reaction. The intensity dependence of the electron yield in single-pulse experiments was analyzed.
Due to the toxicity of halogenated organic compounds, there is a need for their elimination in natural, drinking and industrial waste waters. The reductive dehalogenation by photoelectrolysis may be an alternative procedure for the remediation of organic halides from aqueous solution. In this work a semiconductor electrode (SiC) was used as photocathode for the reduction and cleavage of the carbon-halogen-bond, and the photo-electrochemical properties of single crystalline p-doped SiC are described. Chlorinated acetic acids were chosen as model compounds for the dehalogenation at irradiated cathodes. Trichloroacetic acid was step by step selectively reduced to di- and monochloroacetic acid under the given experimental conditions. Dichloroacetic acid was reduced to monochloroacetic acid, while the reductive dehalogenation of monochloroacetic acid revealed to be rather difficult.
Irradiation of cyclohexenones 1 in the presence of 2 in benzene affords naphthalenones 3 in a-formally - stepwise [4+2]-cycloaddition. In methanol a mixture of the diastereomeric ethers 5 and 6 is obtained.
Measurements of chemically induced dynamic nuclear polarization (CIDNP) were used to study the photoreactions of triphenylsulfonium hexafluoroantimonate with the singlet sensitizers naphthalene and 9,10-dimethylanthracene Me(2)An. Electron transfer quenching followed by cleavage of the neutral triphenylsulfonium radical leads to phenyl radicals (i.e., initiators for radical polymerizations). With Me(2)An, subsequent in-cage hydrogen abstraction from the sensitizer radical cation produces a carbocation (i.e., initiates cationic polymerization). Escaping phenyl radicals decay through hydrogen abstraction from the solvent and in the case of Me(2)An also add to surplus ground-state sensitizer. This competition was studied by using the polarizations as labels.
Broad band u.v. irradiation of 3,5,6-triphenyl-1,2,4-triazine (1) in neat triethylamine as solvent leads to rapid degradation of the starting material and N-dealkylation of the amine accompanied by build-up of the reduction product 2,5-dihydro-3,5,6-triphenyl-1,2,4-triazine (5) as well as the products of reductive ring contraction: 3,5-diphenyl-1,2,4-triazole (8) and 2,3-di-(3,5-diphenyl-1,2,4-triazol-1-yl)butane (13), the latter as a mixture of rac- and meso- diastereomers. In addition, a hitherto unidentified labile photo reduction product gives rise to formation of benzil during chromatographic work-up. A rationale to explain all observations is presented.
