Selenium-Atom-Enhanced Triplet Exciton Kinetics in MR-TADF Photocatalysts for Ultrafast, High-Resolution, and Open-Air 3D Printing.

The photoinitiating system is a decisive factor governing the speed, resolution, and operational robustness of digital light processing (DLP) 3D printing. However, most existing systems rely on inert atmospheres, high irradiation intensities, or prolonged exposure times, severely limiting printing efficiency and practical applicability. Here we report a molecular design paradigm that fundamentally redefines the functional role of multiple-resonance thermally activated delayed fluorescence (MR-TADF) materials, transforming them from efficient light emitters into highly active triplet photocatalysts through selenium-atom engineering. By integrating carbonyl-assisted n-π*/π-π* state coupling with selenium-induced spin-orbit enhancement, the resulting photocatalyst QPSO achieves a near-unity intersystem crossing quantum yield together with an exceptionally large forward-to-reverse intersystem crossing rate constant ratio, thus efficiently channeling exciton flux into long-lived, redox-active triplet states. When combined with a hypervalent iodonium co-initiator, this system enables rapid photopolymerization in ambient air using low-intensity blue light. As a result, single-layer curing is completed within only 1.5-2 s across a broad thickness range, affording a printing resolution down to 10 µm and a record-high build speed of up to 72 cm h-1 at 400 µm layer thickness. The platform supports the fabrication of complex hierarchical architectures while exhibiting excellent biocompatibility.