Long-lived singlet spin states

The first excited singlet state, 2 Sq, is also metastable in the sense that a transition to the ground state is forbidden by the Af selection rule but, because the transition is not spin forbidden, this state is not so long-lived as the 2 Si metastable state. 

Long-lived luminescence. 2. Luminescence involving a change in spin multiplicity (e.g., triplet-to-singlet, singlet-to-triplet, quartet state-to-doublet state, etc.). See Fluorescence Jablonski Diagram... 

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