Triplet Energy Transfer Strategy for Visible Light Activation of Bio‐Based Oxime Esters in Photopolymerization and 3D Printing

Visible-light and solar photopolymerization require photoinitiating systems that combine high reactivity, formulation stability, and biological safety. Here, we present a triplet energy transfer strategy that couples bio-based benzoyl oxime esters, prepared from bio-derived aldehydes, with the high-triplet-energy visible sensitizer 2-isopropylthioxanthone (ITX), converting UV-absorbing oxime ester scaffolds into efficient metal-free photoinitiators for 405 nm LED and sunlight irradiation. Quantum chemical calculations, electrochemical analysis, laser flash photolysis, ESR spin trapping, and in situ FTIR reveal that Dexter-type triplet-triplet energy transfer from ITX selectively populates the reactive triplet states of the oxime esters, while the electron pathway is disfavored. Subsequent N─O bond cleavage and rapid decarboxylation generate initiating phenyl radicals. The BE2+ITX formulation affords faster curing, higher conversion, and deeper through-cure than BE2 or ITX alone, while matching or surpassing benchmark onium and amine systems with favorable cytocompatibility and storage stability. This triplet-energy-driven design rule enables modular development of greener photoinitiators for high-resolution 3D printing and ambient curing.