Why are transitions from triplet states to singlet states slow?

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energy levels of a luminescent centre — luminescence Figure 1: Energy levels of a luminescent centre (see text).

Transitions from triplet states to singlet states are slow because they require a change in the intrinsic spin of an electron.1

In a singlet state, electrons are spin-paired with opposing orientations, whereas in a triplet state, the unpaired electrons have parallel spins. According to the rules of quantum mechanics, a change in electron spin is highly improbable. Because this spin flip is forbidden by standard selection rules, the conversion between states of different multiplicities—such as from a triplet to a singlet—is significantly slower than other molecular processes. This low probability of a spin change is precisely what results in the long-lived nature of phosphorescence.12