Luminescence is a process by which a molecule loses energy through the emission of visible or invisible radiation, unaccompanied by high temperature. The emission may result from the absorption of exciting energy in the form of photons, charged particles, or chemical change.
Chemiluminescence is a special case of luminescence in which the excitation source is a chemical reaction. One important chemiluminescent processes discovered during the middle of the last century involves the oxidation of the organic molecule luminol by hydrogen peroxide (H2O2) in the presence of Fe(CN)63-. A green-blue light is emitted. Other well known chemiluminescent systems are based on the decomposition of esters of oxalic acid.Hydrogen peroxide acts as a catalyst to form an energy-rich decomposition product of the ester. This energy can be transferred to a fluorescer, which then emits light. In the case of green lightsticks, an oxalate ester and fluorescer are present in an outer plastic tube, with H2O2 held in a breakable inner tube.
Because the emission of radiation involves a loss of energy, chemiluminescent reactions must be exothermic. A general expression for these reactions shows that the photon of light given off has an energy equal to Planck's constant times the frequency of the light emitted. This energy needs to be on the order of 40-70 kcal per mole if the emitted light is to be visble to the human eye. This corresponds to light with wavelengths between 400 and 700 nanometers.
Chemiluminescent systems often involve electrochemical reactions. In electrochemiluminescent reactions, the emitting species, the luminophore, is excited to a higher electronic state by a charge-transfer reaction occurring at an electrode. One possible reaction involves one luminophore molecule being oxidized (losing an electron), and another being reduced (gaining an electron). A charge transfer reaction occurs in which the reduced and oxidized species combine to form two neutral molecules, one of which is in an excited state. The excited luminophore emits light as it move to its ground state.
Electrogenerated chemiluminescence can be employed as an analytical tool. The light intensity given off by the chemiluminescent reaction is proportional to the luminophore concentration. Certain luminophores can be bound to biological materials, such as DNA, proteins, or antibodies, and in this way act as labels. The concentration of the biological species can be ascertained by measuring the chemiluminescent light intensity. Chemiluminescence has also been used as a detection method in chromatography.
Besides chemiluminescence, other types of luminescence include photoluminescence (with photon excitation), electroluminescence (with electric field excitation), triboluminescence (with mechanical excitation) and bioluminescence (with biochemical excitation). The glow of the firefly, which is due to the oxidation of the molecule luciferin, with the enzyme luciferase acting as a catalyst in the oxidation process and oxygen acting as the electron acceptor, is an example of bioluminescence.
The terms fluorescence and phosphorescence refer to specific characteristics of luminescent transitions. In fluorescence, the glow accompanying the emission of energy is very short (10-8 to 10-3 seconds), while in phosphorescence, the glow may last for several hours. This is because fluorescence involves an electronic transition from a higher to a lower electronic state, e.g., a triplet to singlet emission; whereas the electronic transition in phosphoresence is a same-state transition, e.g., a singlet to singlet emission. The afore-mentioned light from the firefly is an example of phosphorescence.
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