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Photochemistry excimer

Excimer lasers are of great importance for UV and vacuum UV (VUV) spectroscopy and photochemistry. They are also found in a wide range of applications. For example, they are used in micromachine medical devices, including refractive surgery, in photo-lithography for the microelectronics industry, for material processing, as optical pump sources for other type of lasers (dyes), and so on. More details about excimer lasers can be found in Rodhes (1979). [Pg.54]

In addition to the rather trivial differences mentioned above, laser irradiation can also lead to products as a result of reexcitaion of the carbenes. Thus, excitation of 30 in isooctane with a pulse of the 249-nm line from a KrF excimer laser results in the formation of 9,10-diphenylanthrancene (103), 9,10-diphenylphenanthrene (104), and fluorene, in addition to tetraphenylethylene (Scheme 9.31). Conventional lamp irradiation of 30 results in the formation of benzophenone azine as a major product. None of the products mentioned above are detected. Moreover, the yield of both 103 and fluorene increased markedly with increased laser power. While the details of the mechanism of this reaction are not certain yet, it is clear from the dependence on laser power that some of these products arise from carbene photochemistry. " ... [Pg.435]

Besides the valuable information obtained from the companies cited in Refs. 2 and 3, cooperation with Atochem Elf-Aquitaine (Paris and Pierre-Benite, France), E. I. DuPont de Nemours Company (Wilmington, Delaware, USA) and Ems-Dottikon AG (Dottikon, Switzerland) in the domain of industrial photochemistry, as well as with Asea Brown Boveri AG (Baden, Switzerland) for the technical application of excimer light sources in photochemistry, provided many interesting discussions, new insights in problems of industrial chemistry, and valuable new tools for their solution. [Pg.307]

All of the photochemical cycloaddition reactions of the stilbenes are presumed to occur via excited state ir-ir type complexes (excimers, exciplexes, or excited charge-transfer complexes). Both the ground state and excited state complexes of t-1 are more stable than expected on the basis of redox potentials and singlet energy. Exciplex formation helps overcome the entropic problems associated with a bimolecular cycloaddition process and predetermines the adduct stereochemistry. Formation of an excited state complex is a necessary, but not a sufficient condition for cycloaddition. In fact, increased exciplex stability can result in decreased quantum yields for cycloaddition, due to an increased barrier for covalent bond formation (Fig. 2). The cycloaddition reactions of t-1 proceed with complete retention of stilbene and alkene photochemistry, indicative of either a concerted or short-lived singlet biradical mechanism. The observation of acyclic adduct formation in the reactions of It with nonconjugated dienes supports the biradical mechanism. [Pg.223]

The present article reviews the photochemical deactivation modes and properties of electronically excited metallotetrapyrroles. Of the wide variety of complexes possessing a tetrapyrrole ligand and their highly structured systems, the subject of this survey is mainly synthetic complexes of porphyrins, chlorins, corrins, phthalocyanines, and naphthalocyanines. All known types of photochemical reactions of excited metallotetrapyrroles are classified. As criteria for the classification, both the nature of the primary photochemical step and the net overall chemical change, are taken. Each of the classes is exemplified by several recent results, and discussed. The data on exciplex and excimer formation processes involving excited metallotetrapyrroles are included. Various branches of practical utilization of the photochemical and photophysical properties of tetrapyrrole complexes are shown. Motives for further development and perspectives in photochemistry of metallotetrapyrroles are evaluated. [Pg.135]

It was stated in our previous review [1] that exciplexes and excimers are not very common in the photochemistry of coordination compounds, probably due to the fact that they have not been systematically looked for. This part can be concluded with a statement that only a small change has occurred in this matter. [Pg.147]

Photocycloaddition and photoaddition can be utilized for new carbon-carbon and carbon-heteroatom bond formation under mild conditions from synthetic viewpoints. In last three decades, a large number of these photoreactions between electron-donating and electron-accepting molecules have been appeared and discussed in the literature, reviews, and books [1-10]. In these photoreactions, a variety of reactive intermediates such as excimers, exciplexes, triplexes, radical ion pairs, and free-radical ions have been postulated and some of them have been detected as transient species to understand the reaction mechanism. Most of reactive species in solution have been already characterized by laser flash photolysis techniques, but still the prediction for the photochemical process is hard to visualize. In preparative organic photochemistry, the dilemma that the transient species including emission are hardly observed in the reaction system giving high chemical yields remains in most cases [11,12]. [Pg.127]

Griesbeck, A.G., Maptue, N., Bondock, S., and Oelgemoller, M. (2003) The excimer radiation system a powerful tool for preparative organic photochemistry. A technical note. Photochemical and Photobiological Sciences, 2, 450-451. [Pg.22]

The photochemistry of poly(di-n-hexylsilane) (PDHS) has been investigated by excimer laser flash photolysis20. Transient absorptions were found to be strongly dependent on the solvent employed and the near-UV absorptions at 385 and 360 run observed in cyclohexane and tetrahydrofuran, respectively, were ascribed to polysilylated silyl radicals, while that at 345 nm observed in dichloromethane was attributed to the radical cations of PDHS formed during the electron photoejection process. [Pg.347]

Although the phenomenon is more common in organic photochemistry, a coordination entity can also act in the process of excimer or exciplex formation as an excited molecule AB or quencher (Q).The second-sphere donor-acceptor interaction with an acceptor quencher causes oxidative quenching of AB, whereas interaction with a donor quencher yields reductive quenching. [Pg.57]

An important aspect of the photophysics of the Pt(diimine)(dithiolate) photochemistry that has received increasing attention is the ability of the excited-state complexes to undergo self-quenching. Initial work by Connick and Gray (111) showed that the lifetime of the complex Pt(bpy)(bdt) (bdt = benzene-1,2-dithiolate, 31) decreased with increasing solution concentration. The bimolecular self-quenching rate constant, calculated from a Stem-Volmer quenching analysis, was found to be 9.5 x 109 A/-1 s-1 in acetonitrile and 4 x 109 M 1 s 1 in chloroform. However, no evidence of excimer formation... [Pg.346]

The importance of bioexcimers (bioexciplexes) in the photochemistry of biological compounds has been also emphasized. Computation of potential energy curves modeling the complex pheophytin-quinone shows the relevance that stabilization caused by the formation of rr-stacked excited dimers, that is, excimers (exciplexes) and the corresponding presence of conical intersections, have to provide... [Pg.468]

During the last decade, there has been considerable interest in studying the interaction between ultraviolet radiation and polymers by the use of pulsed excimer laser (1-41. In fact, some attractive applications in microelectronics and surgery have been successfully implemented (5.), and further informations about the different mechanisms (photochemistry, thermal effect...) involved at the polymer surface have been invoked in order to elucidate their relative contributions. More recently, the attention has been focused on this type of polymer surface modifications to improve some surface properties like the adhesion in metallized polymer structures. [Pg.161]

Peeieer Roland R, Bolle M, Anderson RW (1998) Hydrogen Bromide Photochemistry Actinometry for Determination of Absolute Power Outputs of Xenon Excimer and other UV/VUV Light Sources, J. Phys. D ... [Pg.77]

Oppenlander T, Baum G, Egle W, Hennig T (1995) Novel Vacuum-UV-(VUV) and UV-Excimer Flow-through Photoreactors for Waste Water Treatment and for Wavelength-selective Photochemistry Proc. Indian Acad. Sci. (Chem. Sci.) 107 621-636. [Pg.99]

Mark G, Schuchmann H-P, Sonntag C VON (1994) Application of Excimer Inco-herent-UV Sources as a New Tool in Photochemistry Photodegradation of Chlorinated Dibenzodioxins in Solution and Adsorbed on Aqueous Pulp Sludge, J. Photochem. Photobiol. A Chem. 79 141—... [Pg.235]

A large number of papers has appeared on the subject of excimer laser exposure of polymer films (16-21). Most of these have dealt with the phenomenon of photoablation. A few have observed intensity dependent photochemistry (22,23). The latter authors were concerned with the effect of exposure intensity on resist development characteristics. The utility of nonlinear photochemistry for image modification has not been explored except in our earlier communication, in which strongly nonlinear irreversible bleaching was observed for KrF laser irradiation of acridine/PMMA films with lOnsec pulses (5). [Pg.232]

The discovery of two excimer emitting polymorphs of the stilbene derivative 6-XXXVIII (Cohen et al. 1975) provided an opportunity to determine directly the relationship between excimer structure and its emission properties. The structure of Form A was determined crystallographically and indicated a stacking arrangement, while the structure of Form B, inferred from the cell constants and the photochemistry consistent with the topochemical principles (Section 6.4), was a pairwise arrangement... [Pg.232]


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See also in sourсe #XX -- [ Pg.227 ]




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