3.6 Chemiluminescence

Chemiluminescence is the production of light by chemical substances without the help from other energy sources; in effect it is the conversion of energy stored in chemical bonds into light. Mechanistically, chemiluminescence involves several consecutive processes. The first step is the chemical reaction to form a product, which can undergo further highly exothermic transformations of sufficient energy to emit light in the visible region (400 700 nm) of the spectrum. For this to occur the reaction enthalpy must be in excess of 40 kcal mol ^?1. In the second step, chemical potential energy is transformed into electron excitation energy by a reaction intermediate entering an electronically excited state. The final step is the loss of energy from this excited state, as fluorescence or phosphorescence depending on whether the orientation of the spin in the excited intermediate is singlet or triplet (see Figure 3.1). This is called direct Chemiluminescence as opposed to indirect Chemiluminescence in which the electronically excited state transfers energy to a suitable acceptor which itself emits light.