Highly Efficient Cleavable Photoinitiators for Visible Light Curing of Aqueous Formulations

Curing water-based formulations with photoinitiators is a state of the art technique. However, the commercially available Norrish type II water-compatible photoinitiators disappeared from the market since the bimolecularity and the resulting solvent cage in a water-based solution lead to a loss in reactivity. Since a few decades there is only one cleavable water-compatible photoinitiator on the market but it has some disadvantages, like limited solubility and an absorption only up to 370 nm, which makes visible light curing impossible thus reducing the applicability. We describe in this work a series of alternative Norrish type I photoinitiators based on monoand bisacylphosphine structures with ionic or hydrophilic groups to guarantee good water solubility. Furthermore we investigated toxicity, photoreactivity with NIR-photorheometry, storage stability and absorption spectra. In addition we describe easy syntheses from cheap, commercially available starting materials