Luminol is chemiluminescent because of the unique electron deexcitation that takes place in the oxidation product, 3-aminophthalate (3-APA). 3-APA is formed in its triplet state, which means that there are two excited electrons with the same spin. The electrons undergo intersystem crossing, which is a slow spin-flip process. Some energy is lost during in molecular rotations and vibrations. This singlet excited state decays to its ground state and emits a photon of light.
Luminol and 3-APA compounds emit UV radiation, which is invisible to humans. However, there is a broad, jagged peak in the 3-APA graph. 2 peaks within that broad peak can be explained by 3-APA interacting with its solution. One peak at 424 nm corresponds to 3-APA that is hydrogen bonded to water molecules in solution. The other peak at 485 nm corresponds to 3-APA that is not hydrogen bonded. Wavelengths between 424 nm and 485 nm correspond to green-blue light.
Molecular oxygen is in a triplet state. It has been suggested that adducts of oxygen exist as triplet states. Loss of nitrogen leads to a vibrationally excited triplet state of 3-APA which undergoes intersystem crossing to form a singlet state from which it can emit a photon.
The reaction of luminol in a basic solution with a metal catalyst and an oxidizer creates an almost instantaneous light. The intensity of the light is primarily dependent on the concentration of the metal and oxidizer. The light intensity-time decay data were taken immediately after mixing the reagents and for 60 seconds.
The graph below reveals that with most luminol reactions, the intensity of that light decreases by half (or has a half-intensity) of 8-9 seconds.
NOTE: (up)The multiplicity is often referred to as being a triplet or singlet state which are directly related to the total spin of the molecule, S (the multiplicity is 2S+1). Where the multiplicity is 1, the total spin, S is 0 and there is no net spin on the molecule. This effectively means that all electrons are coupled in orbitals. So He, electron configuration 1s2 has no net spin and a multiplicity of 1 (in fact all closed shells have multiplicity of 1). A multiplicity of 3 indicates a total spin of 1 – here there are two unpaired orbitals (each electron can be spin up or down, the lowest energy will be where the electrons have spin in the same direction, each contributes a ½ making a total of 1. Looking at the multiplicity of a molecule tells you something about its energy. A term symbol for a particular species with a higher multiplicity always has a larger energy than one with a lower multiplicity.