Photoluminescence: Fluorescence and Phosphorescence

When molecules absorb UV, visible, or IR EM radiation, it promotes their valence electrons to excited states.

If these electrons emit light while relaxing, it is called photoluminescence.

Photoluminescence spectra are recorded by measuring the intensity of emitted radiation in relation to either the excitation wavelength or the emission wavelength.

When the emission of light results from an electronic transition without changing the electron spin, the process is called fluorescence.

This process typically produces light with a longer wavelength than the absorbed light and is short-lived, taking place within nanoseconds.

Fluorescence is favorable for molecules exhibiting π→π* transitions.

In contrast, phosphorescence involves an electronic transition from an excited state to a lower energy state with a change in electron spin.

The spin alteration leads to a longer-lasting excited state and, subsequently, a delayed emission of light, ranging from seconds to minutes.

Phosphorescence occurs over a range of wavelengths at lower energies than their absorption bands and is most favorable for molecules that have n→π* transitions.