How to Avoid the Back Electron Transfer in Type II Photoinitiators

Benzophenone (BP) and camphorquinone (CQ) are long known Type II photoinitiators (PIs) for the radiation curing of Coatings, printing inks, etc. After excitation, the P1 is able to abstract hydrogens from the resin such as from ether functionalities, thus generating reactive radicals on the backbone of the oligomers. Nowadays, amine-based coinitiators (Col) are applied in industrial applications because highly reactive radicals are formed. After fast electron transfer to the excited ketone the slow and rate determining proton transfer occure. Compared to cz-cleavable Type I Pls, these initiators suffer from bimolecularity of the reaction.