Structural Effects of Cinnamoyl-Indanone-Based Photobleachable Free Radical Visible Initiators

In order to study the photobleaching property of photoinitiators sensitive to visible light, in this work, a series of visible photoinitiators cinnamoyl-indanones (CA-IDs) were designed and synthesized by the Knoevenagel condensation reaction. Their D-π-A structures possess wide absorptions in the visible region (400–600 nm) as well as high molar extinction coefficients. The results of electron paramagnetic resonance (EPR) and steady-state photolysis showed that CA-ID-3–7 can yield α-aminoalkyl radicals and lead to photolysis. In addition, the photopolymerization kinetics indicated that CA-ID-3–7 could successfully initiate the photopolymerization of HDDA (1,6-hexanedioldiacrylate), and CA-ID-4 exhibited the highest photoinitiating efficiency among these photoinitiators. In addition, CA-ID-4/MDEA (N-methyldiethanolamine) demonstrated a comparable initiating efficiency as CQ (camphorquinone)/MDEA. The results of theoretical calculation experiments illustrated that the introduction of ortho groups (alkyl or aminoalkyl) to double bonds on the benzene ring in cinnamyl-indanedione is an effective way to develop photoinitiators with high light absorption in the visible light region for radical polymerization. Combined with the analysis of EPR, the possible mechanism for generating active radicals upon the appropriate wavelength LED exposure was proposed. The results of the curing depth experiment implied that CA-IDs have great potential applications in deep curing materials.