Chemiluminescence is considered as the inverse of an elementary photochemical reaction. Nine elementary processes which could lead to luminescence are distinguished on this basis, and illustrated by examples.
It is shown that the energy of the emitted quanta can only result from four combinations of bond dissociation energies; the most frequent are association and exchange processes. Moreover, in solution, a blue, violet, or ultra-violet chemiluminescence can only be due to a reaction involving two free radicals. The quantum yield of such a reaction is necessarily small.
The particular case of luminol (3-aminophtalhydrazide) is considered.
The luminescence of alcaline solutions of this substance, even in absence of H2O2, is enhanced by substances known to produce OH and O2H radicals in H2O2; for example, oxidizing and reducing agents and diazo-compounds. This effect practically disappears on thorough outgassing, and the same treatment suppresses completely the spontaneous faint emission of the alcaline solutions (visual observation of estimated sensitivity less than 105 quanta per sec); the presence of O2 is thus essential for the reaction to take place.
The quantum yield of the luminescence, Φ, induced by Fe(CN)6K3 and by NaOCl has been determined in the absence and in presence of H2O2. The light intensities were measured by a photographic method, yielding a mean value Φ = 2,5.10−3 quanta per molecule of luminol.
A mechanism of the oxidation of luminol by monovalent steps is suggested in which the luminescent reaction consists in an association of two different radicals; a dismutation is less probable. One of these radicals, derived from luminol, must be stable and should have semi-quinonic structure, the other radical must have hydroperoxidic character (HO2). The analogy with autoxydation reactions is pointed out.
Previous data on the chemiluminescence of luminol are in agreement with this mechanism.