Cleavable Unimolecular Photoinitiators Based on Oxime‐Ester Chemistry for Two‐Photon Three‐Dimensional Printing

Abstract

The development of efficient two‐photon initiators (2PIs) is critical to promote two‐photon three‐dimensional (3D) printing for market applications. On the basis of the photocleavage concept, several unimolecular 2PIs that contain coumarin moieties (as two‐photon absorption chromophores) and oxime‐ester groups (as initiation functional groups) were synthesized. Under visible light irradiation, the investigated oxime‐esters, sensitive to a broad wavelength range from 400 to 460 nm, undergo direct photocleavage followed by decarboxylation, forming active free radicals to effectively induce polymerization. Interestingly, the photoinitiators exhibit good photobleaching properties, which makes them applicable for photopolymerization of thick materials and dental restorative materials. In two‐photon 3D printing tests with acrylate formulations, the oxime‐ester derivatives were used to build complex 3D microstructures with submicrometer or even nanometer spatial resolution. The initiator comprised of a conjugated coumarin and an oxime ester attached at the 3‐position exhibited a low threshold laser power required for polymerization and had a broad processing window similar to a highly efficient 2PI in the literature.