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Sensitiser

For the '"Sensitiser" used in explosives see explosive.

A sensitiser in chemoluminescence is a chemical compound, capable of light emission after it has received energy from a molecule, which became excited previously in the chemical reaction. A good example is this:

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When an alkaline solution of sodium hypochlorite and a concentrated solution of hydrogen peroxide are mixed, a reaction occurs:

ClO^-(aq) + H₂O₂(aq) → O₂*(g) + H⁺(aq) + Cl^-(aq) + OH^-(aq)

O₂* is excited oxygen - meaning, one or more electrons in the O₂ molecule have been promoted to higher-energy molecular orbitals. Hence, oxygen produced by this chemical reaction somehow 'absorbed' the energy released by the reaction and became excited. This energy state is unstable, therefore it will return to the ground state by lowering its energy. It can do that in more than one way:

it can react further, without any light emission
it can lose energy without emission, for example, giving off heat to the surroundings or transferring energy to another molecule
it can emit light

The intensity, duration and colour of emitted light depend on quantum and kinetical factors. However, excited molecules are frequently less capable of light emission in terms of brightness and duration when compared to sensitisers. This is because sensitisers can store energy (that is, be excited) for longer periods of time than other excited molecules. The energy is stored through means of quantum vibration, so sensitisers are usually compounds which either include systems of aromatic rings or many conjugated double and triple bonds in their structure. Hence, if an excited molecule transfers its energy to a sensitiser thus exciting it, longer and easier to quantify light emission is often observed.

The colour (that is, the wavelength), brightness and duration of emission depend upon the sensitiser used. Usually, for a certain chemical reaction, many different sensitisers can be used.