In luminol, an excited triplet dianion forms first, followed by intersystem crossing to an excited singlet state (forbidden, no emission of light yet). Then the excited singlet state goes to the singlet ground state (allowed) with emission of light.
The simple Jablonski diagrams imply that phosphorescence happens in one step when the molecule goes from an excited triplet state to the ground singlet state, but this is actually two processes: 1) change in spin multiplicity from triplet to singlet, and 2) relaxation from excited singlet to ground singlet. Some of the links below indicate these steps are occurring during the luminol phosphorescence, but the Jablonski diagram such as the one I put here earlier does not show these two steps in going from the triplet excited state to the singlet ground state. Where phosphorescence is concerned, it is often stated that this process, being forbidden, is not very probable (compared to fluorescence where the spin state does not change) but still occurs with emission of light without undergoing intersystem crossing to an excited singlet state before going to the ground singlet state. There are lots of sites dealing with research about the mechanism of phosphorescence, which I do not yet understand very well!
Steve
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