Photobleachable cinnamoyl dyes for radical visible photoinitiators

Three new photobleachable cinnamoyl dyes for radical visible photoinitiators (CAs) were prepared by aldol condensation reaction of 2, 4-bis(diethylamino) benzaldehyde and esters (including ethyl acetate, diethyl malonate and ditertbutyl malonate), in comparison with CAs, the ditertbutyl 2-(4-(diphenylamino)benzylidene) malonate (TA) was also prepared. Their structures were confirmed by ¹H NMR, ¹³C NMR, HR-MS. The mechanisms of the radicals generated from CAs and TA were proposed through electron paramagnetic resonance (EPR) analysis and steady state photolysis experiments. The investigated CAs and TA can effectively initiate the photopolymerization of 1, 6-hexanediol diacrylate (HDDA) under LED@405 nm exposure. Among these photoinitiators, CA-2 showed the best visible light photoinitiating efficiency and photobleachable characteristic. Furthermore, the residual initiators in cured HDDA polymers were proved to be as low as 0.243% and the obtained colorless polymer was up to 2.8 mm thickness when CA-2 was employed as the photoinitiator. These efficient photobleachable visible initiators show great potentials in deep curing materials.