Catalyst Halogenation Enables Rapid and Efficient Polymerizations with Visible to Far-Red Light
Alex Stafford, Dowon Ahn, Emily K. Raulerson, Kun‐You Chung, Kaihong Sun, Danielle M. Cadena, Elena Forrister, Shane R. Yost, Sean T. Roberts, Zachariah A. Page
Abstract
The driving of rapid polymerizations with visible to near-infrared light will enable nascent technologies in the emerging fields of bio- and composite-printing. However, current photopolymerization strategies are limited by long reaction times, high light intensities, and/or large catalyst loadings. The improvement of efficiency remains elusive without a comprehensive, mechanistic evaluation of photocatalysis to better understand how composition relates to polymerization metrics. With this objective in mind, a series of methine- and aza-bridged boron dipyrromethene (BODIPY) derivatives were synthesized and systematically characterized to elucidate key structure–property relationships that facilitate efficient photopolymerization driven by visible to far-red light. For both BODIPY scaffolds, halogenation was shown as a general method to increase polymerization rate, quantitatively characterized using a custom real-time infrared spectroscopy setup. Furthermore, a combination of steady-state emission quenching experiments, electronic structure calculations, and ultrafast transient absorption revealed that efficient intersystem crossing to the lowest excited triplet state upon halogenation was a key mechanistic step to achieving rapid photopolymerization reactions. Unprecedented polymerization rates were achieved with extremely low light intensities (<1 mW/cm 2 ) and catalyst loadings (<50 μM), exemplified by reaction completion within 60 s of irradiation using green, red, and far-red light-emitting diodes. Halogenated BODIPY photoredox catalysts were additionally employed to produce complex 3D structures using high-resolution visible light 3D printing, demonstrating the broad utility of these catalysts in additive manufacturing.