Abstract: Through the observation on the quenching process of the nitrone/9-cyanoanthracenesystem.it was found that the process may have different mechanism, including electrontransfer, energy transfer, and competitive absorption etc.α, N-diphenyl nitrone quenchesthe fluorescence of 9-CNA through the electron transfer mechanism, Which forms ionradicals of quencher and sensitizer respectively.But the nitrone positive ion radical neverleads to the cyclization reaction.The results also show that the 1,3-dipolar structure is the most important moiety forquencher.When the cyclization takes place, the product, oxaziridine Which is without 1,3-dipolar structure, loses the quenching ability.So, if the quenched system is directlyexposed to light, the fluorescence will recover.

Key words: nitrone, sensitizer, photo cyclization reaction, electron transfer