The absorption of light (usually of a specific wavelength) by a fluorescent molecule leads to its attaining an excited state. Relaxation to the ground state may occur upon losing energy either non-radiatively or by emission of light of longer wavelengths. Fluorescence is the light emitted by molecules in order to relax back to their ground state energy level. Changes in the spectral pattern of fluorescence upon specific environmental changes induced by the researcher lead to detailed knowledge of the structure-activity relationship of molecules. Hallmarks of fluorescence are its high sensitivity, speed and near non-invasiveness. For specific examples, see Principles of Fluorescence Spectroscopy by Joseph R. Lakowicz (New York: Plenum Press, 1983).

A stopped-flow attachment on a fluorimeter allows for monitoring changes in fluorescence in real-time upon biomolecular interaction.

Click to enlarge data examples: