About Photochemical Reactors

Photochemical reactions of organic and inorganic compounds provide the synthetic chemist with routes to new compounds and the mechanistic chemist with information on reaction kinetics and pathways and the properties and lifetimes of intermediate species.

Preparative methods using ultraviolet and visible radiation in place of heat enable chemical compounds to be synthesized often by novel reactions with high yields and in the absence of side reactions. Of the major reactions in organic photochemistry, photodissociation is a convenient source of reactive intermediates (e.g. benzyne, carbenes) often produced following extrusion of small molecules such as SO₂, CO₂, or N₂. Photoaddition is well represented by photochemical Diels-Alder reaction; photocyclic rearrangements have opened up the extensive field of intramolecular cyclisation reactions of cyclic polyenes and the ingenious production of 'cage' structures; photochemical substitutions have provided routes to isomers of substituted benzenoid compounds difficult to make by thermal procedures and photo-oxidation and photo-reduction allow molecules to incorporate molecular oxygen or hydrogen.

Quantum yield measurements in mechanistic studies of photochemical reactions have been used to obtain information on the nature of excited states and their energies and in determination of kinetic parameters such as lifetimes of excited states and quenching rate constants. Quantitative studies of the efficiency of a photochemical reaction (usually expressed in terms of the quantum yield) under various conditions of temperature, solvent concentration, etc. can provide a clearer understanding on the reaction mechanism. In particular, bimolecular quenching experiments expressed by a kinetic treatment known as the Sterm-Volmer plot can give valuable information on the lifetime and reactivity of a triplet state, without the needs for direct methods of observation (i.e. flash photolysis and phosphorimetry).

Immersion well reactors allow solutions of the reactant to be irradiated by a lamp located in a double-walled immersion well. These are general purpose reactors and perhaps the most popular for preparative work.