Oxygen Tolerant Green Light Photopolymerizations with Type I Boron-Alkylated BODIPYs: The Power of a Methylene

Conventional Type I photoinitiators require high-energy UV light to generate radicals, while most visible-light systems rely on slower, oxygen-sensitive Type II photoredox pathways. We report boron-alkylated BODIPYs as visible-light Type I photoinitiators that operate efficiently under green LEDs and ambient conditions. Ethyl substitution, relative to methyl, at the boron site accelerates photolysis, lowers the B-C bond dissociation energy, and suppresses oxygen sensitization. Consequently, the ethyl derivative doubles the polymerization rate and decreases oxygen inhibition times over an order of magnitude compared to the methyl analogue. Relative to Ivocerin^®, a leading blue-light Type I photoinitiator, the BODIPY system achieves a higher external quantum yield due to stronger absorption. As proof of concept, the ethyl derivative enabled rapid ambient green-light DLP 3D printing of commercial acrylate resins, achieving 2.5 s exposures per 50 μm layer at 3 mW cm^-2. This oxygen-tolerant platform provides a practical, sustainable route for photocurable technologies.