Photochemistry with Circularly Polarized Light

The reactions induced by circularly polarized light are reviewed and a general discussion of the induction of optical activity by circularly polarized light is presented. It is concluded that three different mechanisms, or their combinations, can cause asymmetric induction. In each case the optical yield is dependent on the optical anisotropy factor g, the ratio between the circular dichroism (Δ?) and the extinction coefficient (?). The results are discussed in terms of the presumed mechanisms, and particular attention is paid to the asymmetric synthesis of helicenes. It is concluded that this kind of photochemistry may have analytical importance.     

Photochemical Rearrangements and Fragmentations of Benzene Derivatives and Annelated Arenes

In this article the numerous intramolecular reactions of electronically excited benezene derivatives, which in many instances are only mentioned in original papers, are systematically analyzed and arranged according to reaction types. All known reaction types can be classified, and subdivided into reactions of the benzene ring (ionization, ring opening, ring alteration), reactions with participation of side chains (α-, β-, γ-cleavage, homolysis, heterolysis), reactions of substituents with side chains (cyclization, dealkylation, cleavage of protective groups), and reactions of side chains with the aromatic ring (substitution, addition, dearomatization, cyclization). The selectivity of the energetically feasible competing reactions is primarily determined by geometric factors. Applications of the empirical effects are numerous and varied in preparative organic chemistry. Many of the reactions under discussion are already utilized industrially (e.g. in photochromism, UV stabilization, photography, information storage, printing, coating and polymer technology, and pharmacy).     

Synthesis and Photochemistry of syn- and anti-9,10-Epoxy-1, 4-dihydro-1,4-ethanonaphthalene-2,3-diones

The title epoxydiketones were prepared stereoselectively, direct epoxidation of 3a-c with MCPBA produced syn isomers la-c whereas epoxidation of 4a-d followed by saponification of the spirolactone rings gave anti isomers 2a-c. The stereochemistry of 1a and 2a was established by X-ray diffraction, whereas that of the remaining epoxydiketones was determined from the correlation of visible, ¹H, and ¹³C NMR spectral data; the differences between spectra of the corresponding syn and anti isomers are explained in terms of through-space interaction and steric effects. Photolysis of syn isomers 1a-c afforded the corresponding naphthalenes 8a-c in almost quantitative yields; in contrast, irradiation of anti isomers 2a-c gave complicated mixtures. The quantum yield of disappearance of 1a was 52 times that of 2a. Reaction mechanisms are proposed to account for the product formation. The differences in the photochemical behavior of the title syn and anti isomers are rationalized in terms of stereoelectronic effects of the epoxy rings in the syn isomers.     

Electrochemical approach to evaluate the mechanism of photocatalytic water splitting on oxide photocatalysts

Photoelectrochemical measurements of TiO₂, NaTaO₃, and Cr or Sb doped TiO₂ and SrTiO₃ photocatalysts were carried out in H₂ and O₂ saturated electrolytes in order to evaluate the reverse reactions during water photolysis. The poor activity of TiO₂ as a result of reverse photoreactions of O₂ reduction and H₂ oxidation was revealed with the respective high cathodic and anodic photocurrents. The rise in the photocurrents at NaTaO₃ after La doping was in harmony with the doping-induced increase in the photocatalytic activity. NiO loading suppresses the O₂ photoreverse reactions, which declines photocatalytic activity, and/or promotes the photo-oxidation of water, because the O₂ photo-reduction current was scarcely observed near the flatband potential. Photocurrents of O₂ reduction and H₂ oxidation were observed under visible light for the Cr and Sb doped SrTiO₃ and TiO₂, respectively. These phenomena are in harmony with the previous reports on the photocatalysts examined with sacrificial reagents.     

Laser Photochemistry at Surfaces—Laser-Induced Chemical Vapor Deposition and Related Phenomena

The structural characterization of materials and the tailoring of their properties is an important area of chemical research. A new trend in this area is the recourse to lasers both for analytical as well as preparative purposes, exploiting the fact that lasers, by virtue of their properties (sharp energy, spatial and temporal resolution etc.), offer the most precise and selective interaction of energy and matter that we know. Furthermore, photochemical syntheses and material transformations can proceed “cold” and without causing damage to surface structures. Laser chemistry finds application in thin film deposition, in the formation of surface layers, as well as in (structuring) ablation or etching, and in the initiation and enforcing of reactions at surfaces. In the present paper an introduction to these new possibilities on the general basis of molecule-surface interactions is followed by a brief characterization of lasers suitable for such purposes. Thereafter, four examples are discussed: gas phase deposition from volatile organometallic compounds with photoelectric activation at the surface or photochemical activation of the gaseous species (e.g. by employing molecular beams). In this way (noble) metal contacts can be deposited on various substrates. Instead of surface deposition, nucleation can occur in the gaseous medium, yielding highly disperse powders, e.g. of silicon carbide. Finally, an etching reaction is discussed where the laser does not act as an energy source but as an analytical instrument to provide diagnostic and mechanistic information.     

Comparative Studies of the Redox Behaviour and the Antioxidative Activity of Arylnaphthoquinones

Summary.  Comparative studies of redox behaviour and antioxidative activity (AOA) were carried out with two series of arylnaphthoquinone derivatives. For the electrochemical investigations, a voltammetric procedure combined with a glassy carbon electrode was applied. The AOA was examined using the photo-chemiluminometric method with the Photochem^? measuring device according to Popov et al. and the chemical procedure with the radical quencher DPPH (diphenylpicrylhydrazyl). Both the redox potential and the antioxidative activity of the investigated compounds are influenced by their substituents. Compounds with the OH-group at position 3 are easiest to oxidize (E _p = 0.18 V; pH 7.4). The lag phases (AOA) of these compounds are five to ten times greater than those of the strong antioxidants ascorbic acid and Trolox^?. As suggested for other natural compounds, we also found a good correlation between the oxidation potential and the AOA. Therefore, it seems that a good antioxidative activity requires a low redox potential. Corresponding authors. E-mail: wurglics@pharmchem.uni-frankfurt.de Received March 1, 2002; accepted March 28, 2002     

Reducing the complexity of photoreaction pathway of pyrazoles: photochemistry of 4,5-cycloalkanopyrazoles

Products from photoirradiation of 4,5-cycloalkanopyrazoles have been isolated and characterized. The simpler product mixtures relative to the parent systems are in agreement with the hypothesis that the extra carbocyclic ring could reduce the possible number of reaction pathways. Relative reactivities and secondary photoreaction pathways have also been determined.     

The Beginnings of Organic Photochemistry

Although sunlight induced photochemistry must have occurred on the planet Earth for billions of years, the chemical changes caused by light have attracted systematic scientific scrutiny only relatively recently. How did scientists first conceive the idea that the interaction of materials with light could not only cause physical phenomena, but could also alter their chemical nature? When sunlight began to be employed as a heat source for distillation, the eventual discovery of photochemical reactions was assured. One can envision three types of changes that would have aroused the curiosity of laboratory chemists: color changes; the evolution of gas bubbles (oxygen in photosynthesis); and the precipitation of a photoproduct less soluble than its precursor. Less predictable was the observation that sunlight caused crystalline santonin to burst because it is converted into a product with a different crystal lattice. In the course of the eighteenth and nineteenth centuries a variety of photochemical reactions, some observed by chance, others uncovered in carefully planned studies, ultimately led to a major systematic investigation that established photochemistry as a viable branch of chemistry.     

The photochemical synthesis of 1- and 3-acetylphenoxazines

1- and 3-Acetylphenoxazine were prepared by the photochemical reaction of 10-acetylphenoxazine.

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Photochemische Synthese von 1- und 3-Acetylphenoxazinen (Kurze Mitteilung)

Zusammenfassung 1- und 3-Acetylphenoxazin wurden durch photochemische Reaktion aus 10-Acetylphenoxazin dargestellt.

     

Spectroscopic and DFT evidence for a nonclassical radical cation derived from 7-benzhydrylidenenorbornene

Nanosecond time-resolved UV/vis absorption spectroscopy on laser flash photolysis was conducted for photoinduced electron-transfer reactions of 7-benzhydrylidenenorbornene (1) and 7-benzhydrylidenenorbornane (5). The differences in the observed absorption bands and the structures of 1⁺ and 5⁺ were evaluated successfully using calculations based on (time dependent) density functional theory, confirming the nonclassical nature of 1⁺.