Photoinitiating System

This chapter discusses the characteristics and the role of the photoinitiating system, the approach of the (photo)chemical reactivity through an analysis of the excited state processes and molecular orbital calculations. The properties that should fulfill a PIS falls into two categories: practical properties oriented to the end uses, and molecular properties that govern the reactivity of PIS in terms of quantum yields. The evolution of the excited states after light absorption is depicted by the familiar Jablonski’s diagram. The chapter consider radical photopolymerizations as examples. The initiating species generated through the different kinds of excited state processes react with the monomer/oligomer matrix and form the first reactive monomer species. The investigation of the mechanisms involved in photosensitive systems is mainly carried out using steady-state and time-resolved techniques. A general discussion of the photoinitiator reactivity is nowadays based on both experimental and theoretical considerations.