In this demonstration, it is not the addition of a new chemical, or heat, which starts the reaction. It is brought about by the introduction of a light source and is therefore called a photochemical reaction.
Thionin is an organic compound that can exist in two forms; a purple oxidized form and a colourless reduced form. When a reducing agent such as iron(II) (Fe2+) ions is added to an acidified thionin solution, light induces the reduction of the protonated thionin molecule to its colourless form by the Fe2+ ions.
This is an example of the conversion of light energy to chemical energy.
Thionin – Risk Assessment