Xanthones, radicals from

Oxidative cleavage of ring B of anthraquinones yields benzophenone derivatives which are transformed to xanthones. Both hydroxyl groups of the symmetrically substituted benzene nucleus of the benzophenone carboxyhc acid intermediate can participate in the closure of the xanthone ring. From the xanthones dimeric products are formed probably via phenol oxidase or peroxidase-derived radicals (C 2.3.1 and C 2.4.1). 

Figure B 1.16.9 shows background-free, pseudo-steady-state CIDNP spectra of the photoreaction of triethylamine with (a) anthroquinone as sensitizer and (b) and (c) xanthone as sensitizer. Details of the pseudo-steady-state CIDNP method are given elsewhere [22]. In trace (a), no signals from the p protons of products 1 (recombination) or 2 (escape) are observed, indicating that the products observed result from the radical ion pair. Traces (b) and (c) illustrate a usefiil feature of pulsed CIDNP net and multiplet effects may be separated on the basis of their radiofrequency (RF) pulse tip angle dependence [21]. Net effects are shown in trace (b) while multiplet effects can...

Figure Bl.16.19. (a) CIDEP spectrum observed in die photolysis of xanthone (1.0 x 10 M) in cyclohexanol at room temperature. The stick spectra of the ketyl and cyclohexanol radicals with RPM polarization are presented, (b) CIDEP spectrum after the addition of hydrochloric acid (4.1 vol% HCl 0.50 M) to the solution above. The stick spectra of the ketyl and cyclohexanol radicals with absorptive TM polarization are presented. The bold lines of the stick spectra of the cyclohexanol radical show the broadened lines due to ring motion of the radical. Reprinted from [62].

Based on steady-state and time-resolved emission studies, Scaiano and coworkers have concluded that silicalite (a pentasil zeolite) provides at least two types of sites for guest molecules [234-236], The triplet states of several arylalkyl ketones and diaryl ketones (benzophenone, xanthone, and benzil) have been used as probes. Phosphorescence from each molecule included in silicalite was observed. With the help of time-resolved diffuse reflectance spectroscopy, it has been possible to show that these triplet decays follow complex kinetics and extend over long periods of time. Experiments with benzophenone and arylalkyl ketones demonstrate that some sites are more easily accessed by the small quencher molecule oxygen. Also, diffuse reflectance studies in Na + -X showed that diphenylmethyl radicals in various sites decay over time periods differing by seven orders of magnitude (t varies between 20/is and 30 min) [237]. 

The affect of variations in chemical structure of amines on the photo-initiating ability of 4- -propoxythio-xanthone was also investigated in photopolymerizations of n-butyl methacrylate and a commercial triacrylate resin in isopropyl alcohol solution. The conclusion from that study is that the activity is highly dependent on the ionization potential of the particular amine, the formation of a triplet exciplex and an electron transfer process. Analyses of chloroform extracts of the cured resin confirmed that the alkylamino radical is the initiating radical. ... 

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