How do singlet states and triplet states affect fluorescence and phosphorescence?

In the study of luminescence, the distinction between fluorescence and phosphorescence is defined by the electronic states involved and the nature of the transitions between them.12

Singlet States and Fluorescence

When a molecule absorbs energy, an electron is promoted to a higher energy level without changing its spin, resulting in an excited singlet state. Because the ground state is also typically a singlet, the transition back to the ground state involves no change in spin multiplicity. These transitions are quantum mechanically "allowed," resulting in an almost instantaneous emission of light, typically occurring within ${10}^{-8}$ seconds.132

Triplet States and Phosphorescence

An electron may occasionally undergo a spin flip during excitation or through intersystem crossing, resulting in a triplet state where two electrons have parallel spins. Transitions from a triplet state back to a singlet ground state require a change in spin, which is quantum mechanically "forbidden". Consequently, these transitions are much less probable and significantly slower, leading to the long-lived afterglow characteristic of phosphorescence, which can persist from milliseconds to seconds.132