We further conducted several control experiments to demonstrate that the polymerization of MA is induced by compound 1 and not by DPI or light alone. Specifically, we prepared samples containing only MA and DPI in MA, and irradiated them with 365 nm light (1.5 W), as well as with a combination of 365 nm (1.5 W) and 450 nm (400 mW) light. Within the timeframes where compound 1 induces polymerization, none of these sample mixtures alone induced polymerization (Supporting Table , Supporting Figure ). Therefore, aza-diarylethene 1 represents a very potent photoinitiator for radical polymerization reactions with highly interesting prospects for 3D printing applications.