Abstract
The photoreduction of phthalic anhydride (PA), 3,6-dichlorophthalic anhydride (3,6-DCPA), 4,5-dichlorophthalic anhydride (4,5-DCPA) and tetrachlorophthalic anhydride (TCPA) in 2-propanol has been studied with time-resolved electron paramagnetic resonance. The chemically induced dynamic electron polarization spectra show that the reaction takes place through the excited triplet states. From PA, cyclohexadienyl-type hydrogen adduct and ketyl radicals were observed, whereas 3,6-DCPA produced the 3,6-DCPA anion and hydrogen adduct radicals. With 4,5-DCPA only the anion radical appeared, whilst the TCPA system showed no apparent anion and adduct radical formation. These data show that the hydrogen adduct formation occurs at the 4 position in the benzene ring, but that 4,5-DCPA and TCPA do not undergo this reaction. The anion radicals of PAs are formed in subsequent deprotonation reactions of the ketyl radicals. We propose that the hydrogen adduct radical formation in PA and 3,6-DCPA takes place through direct hydrogen abstraction by the excited triplet molecules, in competition with similar abstraction by the carbonyl group to form the ketyls.
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