Unsubstituted spiropyrans and spirooxazines

As for the multiplicity of the excited state from which the X isomer is generated, we have already mentioned that Krysanov and Alfimov8 proposed a mechanism in which the vibrationally excited triplet state of SPNo2 acts as the precursor to the cis-cisoid ring-opened photoproduct X. On the other hand, Kalisky et al.19 claimed that X is formed from SPNo2 in less than lOps. An excited singlet precursor SP is also postulated for the formation of X, with the 

From what has been so far reported, it would seem that the intermediacy of the cis-cisoid ring-opened nonplanar intermediate X is much better documented for nitrospiropyrans than for spiroazines, for which the matter is still controversial. No studies have so far been reported on the importance of the X isomer in the photodegradation mechanistic schemes. 

Only in the scheme of Malkin et al. does the X isomer seem to be involved in the formation of photodegradation products. 

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