Luminescence (Molecular Biology)

Luminescence is the emission of light that does not result from high temperatures (cf. incandescence). In general, luminescence occurs when an atom or molecule is excited into a high energy state, and then decays to the ground state. Since electronic energy levels are quantized, the decay to the ground state is accompanied by the emission of a photon of a specific wavelength. Luminescence is categorized by the mode of excitation that produces the high energy excited state. Chemiluminescence is luminescence resulting from a chemical reaction, and bioluminescence is luminescence resulting from a biological (enzyme-catalyzed) chemical reaction, as in the firefly (see Luciferases And Luciferins). Fluorescence occurs when an atom or molecule is excited (either by an electric discharge or by absorption of a photon) into the singlet excited state, which then decays to the ground state. The lifetime of this excited state is very short (on the order of picoseconds), resulting in rapid emission. Some molecules may undergo intersystem crossing where the singlet excited state becomes a triplet state. Since the rules of quantum mechanics forbid a transition from a triplet excited state directly to the ground state, the triplet excited state has a long lifetime (seconds to hours), resulting in a weak but long-lived glow called phosphorescence.