Why does fluorescence usually emit lower-energy light than the light a molecule absorbs?

Fluorescence typically emits light of lower energy than the light absorbed because of a process known as the Stokes shift.12

When a molecule absorbs a photon, it is promoted to a higher electronic state. Because electrons have a much smaller mass than the nuclei, this absorption occurs almost instantaneously, leaving the nuclei in their original ground-state positions. Following this excitation, the molecule undergoes a relaxation process where the nuclei move toward new, more stable positions corresponding to the excited state. This structural relaxation lowers the total energy of the molecule.1

Consequently, when the molecule eventually emits a photon to return to a lower electronic state, the energy of that emitted photon is reduced by the amount of energy lost during this internal relaxation. Because energy is directly related to wavelength, this loss of energy results in the emitted light having a longer wavelength than the original absorbed radiation. Additionally, some energy may be dissipated as heat through nonradiative processes, such as vibrational energy loss, further contributing to the lower energy of the emitted fluorescence.123