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This question already has an answer here:

Why is in the reaction of hydrogen peroxide and sodium hypochlorite, singlet oxygen obtained? Why isn't triplet oxygen produced instead, as it is thermodynamically more stable?

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marked as duplicate by Jan, andselisk, airhuff, Pritt Balagopal, Mithoron Sep 30 '17 at 11:53

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  • $\begingroup$ Isn't the other question a duplicate of this one? $\endgroup$ – RBW Oct 8 '17 at 9:25
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You are starting with spin-paired starting materials (an even number of electrons, with half in the up state and half in the down state), and they react (exothermically) to create an oxygen molecule. Initially, spin is conserved so that the oxygen molecule also has to be spin paired (singlet), despite the fact that the triplet state of oxygen is thermodynamically more stable. It will then react with a substrate or eventually decay over minutes to hours (most likely thermally) to the triplet state.

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  • $\begingroup$ What else starting materials could be used? $\endgroup$ – RBW Mar 27 '15 at 20:37
  • $\begingroup$ (1) Irradiate oxygen in the presence of a photosensitizing dye. $\endgroup$ – iad22agp Mar 30 '15 at 16:01
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    $\begingroup$ (2) You can also use a trialkyl phosphite, treat it with ozone to make a phosphite ozonide, and then use a basic catalyst (pyridine) to decompose the ozonide to give singlet oxygen. $\endgroup$ – iad22agp Mar 30 '15 at 16:04

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