Why does phosphorescence have a longer wavelength than the light that excites it?

Phosphorescence involves the emission of light at a longer wavelength than the exciting radiation because the process involves a loss of energy before the light is emitted.1

When a molecule absorbs a photon, it is promoted to an excited state. In the case of phosphorescence, the molecule typically undergoes a process called intersystem crossing, where an electron's spin flips, moving the molecule from an excited singlet state to a lower-energy triplet state. Because the energy difference between this triplet state and the ground state is smaller than the energy difference between the initial excited singlet state and the ground state, the photon emitted during the transition back to the ground level has less energy.2

Since the energy of a photon is inversely proportional to its wavelength, this lower energy results in a longer wavelength. Additionally, some energy is often dissipated as non-radiating vibrational energy (heat) during the transition process, further contributing to the shift toward longer wavelengths.12