Mechanism of Excited State Formation from Dioxetans

The large number of investigations into dioxetans was provoked by the suggestion [3] that if the reaction were concerted, then an anti-aromatic transition state would require the formation of an excited state product. After much calculation and experiment, essentially two proposals for the excitation mechanism exist. These are the concerted and diradical intermediate mechanisms, each with its attendant explanations for the high excitation yields characteristic of dioxetans. 

Although the idea of a concerted decomposition initiated work in the area, as the number of simple dioxetans increased and methods for titrating singlets and triplets appeared, it became obvious that triplet states predominated overwhelmingly in the products. A concerted route with no opportunity for spin inversion should of course produce only singlets as the primary product. It is still not clear. 

However other evidence particularly that produced by Richardson and his coworkers [15,30] is very much in accord with a two step, diradical process. A variety of calculations also supports this conclusion, as does evidence drawn from the stability of sterically hindered dioxetans [14]. The main features of the argument which have emerged over the last ten years are briefly summarised. 

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V.5 Mechanism of Excited State Formation from Dioxetans... 

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