Laser flash photolysis, radical

Photolytic methods are used to generate atoms, radicals, or other highly reactive molecules and ions for the purpose of studying their chemical reactivity. Along with pulse radiolysis, described in the next section, laser flash photolysis is capable of generating electronically excited molecules in an instant, although there are of course a few chemical reactions that do so at ordinary rates. To illustrate but a fraction of the capabilities, consider the following photochemical processes ... 

Lu, C. Y. Lui, Y.Y. (2002). Electron transfer oxidation of tryptophan and tyrosine by triplet states and oxidized radicals of flavin sensitizers a laser flash photolysis study. Biochimica et Biophysica Acta (BBA) - General Subjects, Vol. 1571, No.l, (May 2002), pp. 71-76, ISSN 0304-4165... 

Skibsted and coworkers (Mortensen and Skibsted 1996) have shown that upon the laser flash photolysis of carotenoids in chloroform bleaching of the ground state absorption is observed and there is formation of two near infrared-absorbing species ()tmax 920 and lOOOnm for 0-CAR). The species absorbing at about lOOOnm is 0-CAR + and, as with the carotenoid/CCl302 system noted earlier, the 0-CAR,+ is formed from the other species. The nature of the other species is not defined although an adduct or a neutral carotenoid radical is proposed. 

El-Agamey, A. and McGarvey, D.J. 2003. Evidence for a lack of reactivity of carotenoid radicals towards oxygen A laser flash photolysis study of the reactions of carotenoids with acylperoxyl radicals in polar and non-polar solvents. J. Am. Chem. Soc. 125 3330-3340. 

Mortensen, A. 2000. Mechanism and kinetics of scavenging of the phenylthiyl radical by carotenoids. A laser flash photolysis study. Asian Chem. Lett. 4 135-143. 

Laser flash photolysis of phenylated Group 14 catenates followed by trapping of the radical intermediates indicates that homolytic cleavage of the metal-metal bond is the... 

The chemical properties of BA have been studied in detail (Lapin et al., 1984). Low temperature epr spectroscopy shows clearly that the ground state of BA is the triplet (3BA). The zero field parameters (Table 3) reveal some details of this structure. When the irradiation is performed at 4.6 K in a 2-methyltetrahydrofuran glass no epr signals from radical species are apparent. The optical spectrum under these conditions shows absorptions (Table 4) which disappear when the glass is warmed. From these findings the absorption bands are assigned tentatively to 3BA. This conclusion is strongly supported by results from laser flash photolysis experiments. 

Radical cations of the most popular spin traps PBN and DMPO have been generated by the methods of ionizing radiolysis and laser flash-photolysis in solid matrices (435-437). As a polar solvent with high solvating ability for... 

For the sake of comparison and mutual validation of methods for measuring large follow-up reaction rate constants, it is interesting to apply different methods to the same system. Such a comparison between high-scan-rate ultramicroelectrode cyclic voltammetry, redox catalysis, and laser flash photolysis has been carried out for the system depicted in Scheme 2.25, where methylacridan is oxidized in acetonitrile, generating a cation radical that is deprotonated by a base present in the reaction medium.20... 

FIGURE 2.28. Comparison of high-scan-rate ultramicroelectrode cyclic voltammetry (A), redoc catalysis (A), and laser flash photolysis (x) for the determination of the rate constant of deprotonation of methylacridan cation radical by bases of increasing pKa. Adapted from Figure 6 in reference 20, with permission from the American Chemical Society. 

The laser flash photolysis of aromatic diisocyanate based polyurethanes in solution provides evidence for a dual mechanism for photodegradation. One of the processes, an N-C bond cleavage, is common to both TDI (toluene diisocyanate) and MDI (methylene 4,4 -diphenyldiisocyanate) based polyurethanes. The second process, exclusive to MDI based polyurethanes, involves formation of a substituted diphenylmethyl radical. The diphenylmethyl radical, which readily reacts with oxygen, is generated either by direct excitation (248 nm) or indirectly by reaction with a tert-butoxy radical produced upon excitation of tert-butyl peroxide at 351 nm. 

The results and discussion section is divided into two parts. The first part deals with direct laser flash photolysis of the MDI-PUE polymer and appropriate small molecule models. The transient spectra generated by direct excitation of the polyurethane are interpreted by consideration of the primary photochemical reactions of the carbamate moiety. The second part describes results obtained by production of a radical transient species which is capable of abstracting labile hydrogens from the polyurethane. This latter procedure represents an alternative method for production of the transient species which were obtained by direct excitation. 

In order to interpret the results for MDI-PUE and TDI-PU, the laser flash photolysis measurements of several model systems were performed. The transient spectra of the p-toluidinyl radical, recorded upon laser flash photolysis (Aex=248 nm) of p-toluidine (1.4 X 10 M in THF), has a distinct maximum at approximately 310 nm and a broad, diffuse absorbance above 400 nm (Figure 3). The results for p-toluidine are in agreement with previously reported spectra for anilinyl type radicals (12.13). Comparing the transient spectra for p-toluidine with TDI-PU and MDI-PUE, it is quite obvious that the p-toluidinyl radical in THF (Figure 3) is essentially identical to the 300-330 nm and > 400 nm portions of the transient spectra of MDI-PUE and TDI-PU in Figures 1 and 2. 

Laser Flash Photolysis at 351 nm of tert-Butyl Peroxide/Benzene Solutions Containing MDI-PUE and Model Compounds. Photolysis of tert-butyl peroxide (TBP) results in a highly efficient production of tert-butoxy radicals. It has recently been shown (15) that tert-butoxy radicals generated by the laser flash photolysis of TBP can rapidly extract hydrogen atoms from appropriate substrates such as aniline and diphenylamine (Scheme III). 

The results of an experiment for the laser flash photolysis (Xex=351 nm) of a 6.0 X 10 M solution of diphenylmethane in a 60/40 mixture of TBP and benzene (Figure 6) shows a distinct absorbance peak maximum at 340 nm characteristic of the unsubstituted diphenylmethyl radical. The results in Figure 6 illustrate the utility of TBP in indirect generation of diphenylmethyl radicals. 

The laser flash photolysis (Xex=351 nm) of a TBP/BP-MDI solution in benzene (Figure 7) yields a transient spectra with distinct maximum at 370 nm which can most likely be attributed to a substituted diphenylmethyl radical. (Similar results are obtained in other solvents such as DMF). No detectable transient species were generated above 350 nm by the laser flash photolysis (Xex=351 nm) of the 60/40 mixture of TBP and benzene alone. Results for the TBP/MDI-PU (7.0 X 10 2 g/dL) system in Figure 8 show, as in the case of the model BP-MDI (Figure 7), that the transient spectrum of MDI-PU obtained indirectly through tert-butoxy radicals has a maximum at 370 nm. This provides additional support for assignment of the transient species responsible for the 370 nm absorbance to a diphenylmethyl radical. 

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