The Norrish type I reaction is the photochemical cleavage or
homolysis of aldehydes and ketones into two
free radical intermediates (α-scission). The carbonyl group accepts a photon and is
excited to a photochemical
singlet state. Through
intersystem crossing the
triplet state can be obtained. On cleavage of the
α-carbon bond from either state, two radical fragments are obtained. The size and nature of these fragments depends upon the stability of the generated radicals; for instance, the cleavage of
2-butanone largely yields ethyl radicals in favor of less stable methyl radicals. Several secondary reaction modes are open to these fragments depending on the exact molecular structure. • The fragments can simply recombine to the original carbonyl compound, with
racemisation at the α-carbon. • The acyl radical can lose a molecule of
carbon monoxide, forming a new carbon radical at the other α-carbon, followed by formation of a new carbon–carbon bond between the radicals. • The abstraction of an α-
proton from the carbonyl fragment may form a
ketene and an alkane. • The abstraction of a β-proton from the alkyl fragment may form an
aldehyde and an
alkene. The synthetic utility of this reaction type is limited, for instance it often is a
side reaction in the
Paternò–Büchi reaction. One
organic synthesis based on this reaction is that of bicyclohexylidene. The Norrish Type I reaction plays a crucial role in the field of photopolymerization, particularly in the development of
photoinitiators used for
two-photon polymerization (2PP). The Norrish Type I reaction is particularly significant here because it involves the cleavage of a carbon-carbon bond in a photoinitiator molecule upon excitation by UV or visible light, leading to the formation of two radical species. These radicals are highly reactive and can effectively initiate the
polymerization of
monomers in a localized region, allowing for the precise 3D structuring required in two-photon polymerization processes. This makes the Norrish Type I reaction a fundamental mechanism for designing photoinitiators that are capable of driving high-resolution additive manufacturing at the microscale. ==Type II==