Recent advances on ferrocene-based photoinitiating systems

Photoinitiators of polymerization with reversible electrochemical properties are actively researched as these molecules can advantageously be used in photocatalytic systems, enabling to reduce the photoinitiators content within the photocurable resins. In this field, ferrocene which is extensively used by electrochemists as a reference compound for cyclic voltammetry also exhibits a significant absorption in the visible range as well as a low oxidation potential so that this metallocene was used as a potential candidate for the design of photoinitiators of polymerization. Over the years, ferrocene has been examined in numerous polymerization processes going from anionic to cationic polymerizations, but also for the free-radical polymerization of acrylates or as a sacrificial electron donor in free radical polymerization processes. Parallel to this, the development of cheap, compact and energy-saving irradiation setups based on light-emitting diodes (LEDs) have clearly favored the development of visible light photoinitiators and this technology has nowadays the potential to replace the well-established UV photopolymerization. In this review, an overview of the recent development of the ferrocene-based photoinitiating systems is provided. To evidence the interest of the ferrocene derivatives recently developed, comparisons with benchmark photoinitiators will be provided.