Chemiluminescence in solution
Most existing evidence points to the fact that visible chemiluminescent emission from reactions in solution arises from the production ofmolecules in their lowest singlet excited states, such molecules being of types capable of fluorescence. Chemiluminescence is, in fact, fluorescence excited by chemical action. The emitting molecules must have absorption bands in the visible or near ultraviolet region, and the reaction by which they are formed must liberate energy of at least 40-80 kcalfmol. Triplet excited states have lower energies than the corresponding singlet states, but because of their long radiational life they are usually very efficiently quenched by collisions in liquid media before radiating. For enough energy to be located on individual molecules to permit visible radiation to be emitted the reaction must be a very energetic one. Oxidation reactions with molecular oxygen or hydrogen peroxide, or the breakdown of organic hydroperoxides, form practically all types of chemiluminescent reactions. Radical intermediates are involved, and the course of the reaction is usually a matter of speculation only. The identification of the emitter is also a less simple problem than might appear, because fluorescent bands are not always sharply diagnostic. These are the two central problems of chemiluminescence. The chemical structures of molecules primarily involved in luminescence are usually known; even those in living organisms are yielding to intensive attack by organic chemists. Their reaction mechanisms on oxidation, however, arestill in most instances obscure. The bimolecular recombination of radicals such as C6H5 + CsH5 -+ diphenyl does not seem usually to lead to emission. The evolved energy is largely concentrated in the newly-formed bond and is quickly removed by thermal equilibration. A reaction, due to Kurtz, which may have a simple mechanism is as followsl. Dibenzanthrone (R) in chloroform, treated with chlorine, forms the cation R +. When aqueous hydrogen peroxide and alkali are added, and more chlorine passed in, a bright red chemiluminescence is emitted, ap· parently the same as the fluorescence of dibenzanthrone, while scarcely any permanent oxidation of the dye occurs. This could be explained by the reaction, R + + 02--+ R * + 02, followed by R *-+ R + hv. There is no certainty, however, that the actual mechanism takes this course instead of a more complex one. The oxidation of luminol, 3-amino phthalic cyclic hydrazide, because of the brillance of its chemiluminescence, has given rise to a great nurober of investigations, which, however, have mostly served merely to show the complexity of the mechanism. The reaction is usually carried out in