Sustainable Visible-Light Photoinitiation Enabled by a Bio-Based Photosensitizer/Oxime Ester Combination via Triplet–Triplet Energy Transfer

AC:AC:Visible-light photopolymerization offers safer processing, lower energy input, and broader compatibility with advanced manufacturing than thermally initiated curing or short-wavelength UV processes. However, sustainable photoinitiating systems that operate efficiently under visible light remain limited. Here, a sustainable two-component strategy is reported by combining bio-derived oxime esters with natural or bio-inspired photosensitizers for 405 nm curing. Among the systems investigated, the synthesized oxime ester (OXE1) and menadione (MD, vitamin K3) pair show the best performance, enabling rapid photopolymerization of TMPTA with good formulation homogeneity, low coloration, and good storage stability. Electrochemical analysis and excited-state energetics indicate that direct single-electron transfer between MD and OXE1 is thermodynamically unfavorable. In contrast, steady-state photolysis, laser flash photolysis, CO 2 evolution, and ESR spin trapping support a triplet–triplet energy transfer pathway, followed by N–O bond cleavage and decarboxylation, to generate initiating radicals. This work establishes a simple sensitization principle for converting UV-responsive bio-derived oxime esters into visible-light photoinitiators and provides a sustainable design strategy for photopolymerization and 3D microfabrication.