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?
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.