Photooxidation of Spirooxazines Singlet-Oxygen Reactivity

Mechanism of Photooxidation of Spirooxazines Singlet-Oxygen Reactivity 

No emission at 1269 nm was detected, which was taken to mean that no 02 was formed or that its formation quantum yield, fl A, was lower than the sensitivity limit of the detector (5 x 10-3). In two-laser, two-color experiments, after excitation at 355 nm the merocyanine formed was excited (2-ns delay) with a green layer (532 nm), and again no 02 was detected, a clear indication that the (photo)merocyanine did not participate in the sensitized formation of singlet oxygen, the species that could induce oxidative degradation of the photochromes. The only noticeable exception was found for 11 for which fl Awas measured to be 0.15. 

When spirooxazines were present in toluene (or acetonitrile) solutions of the endoperoxide, after periods of time longer than ca. 20 half-lives, no appreciable degradation of spirooxazines was observed. This would seem to indicate that even when 02 is independently generated, SOs quench it through a mosdy physical deactivation process, and the chemical reaction component is small or exceedingly small. 

We have shown38 that 8-methoxy-6-nitro-BIPS (11) is the only photochrome able to sensitize the formation of (h with a quantum yield A = 0.15 that is about one-fourth that of a commonly used singlet-oxygen sensitizer, i.e. methylene blue, for which 0A = 0.57.39 This benzopyran behaves as a good quencher of l02 as well. Unfortunately, in the paper by Guglielmetti and co-workers an analysis of the 

When the polarity of the medium increases, due to the enhanced polarization of the Cspiro-0 bond, its cleavage is thought to be almost completely heterolytic. According to Guglielmetti and co-workers43 the only viable reaction pathway is 

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