Efficient Near-Infrared Photopolymerizations Using azaBODIPYs with Electron-Donating Groups and Intramolecular Charge Transfer

Polymerizations driven by low-energy, far-red to near-infrared (NIR) light can enable mild, biocompatible, and wavelength-selective photocuring for the fabrication of advanced soft materials. This arises from a reduction in light scattering and photodamage by long-wavelength light relative to commonly employed UV light. However, photopolymerizations with far-red to NIR light are slow and inefficient and thus require long reaction times, high light intensities, and/or large catalyst loadings relative to analogous UV light-driven approaches. To overcome these limitations, a comprehensive evaluation of aza-boron dipyrromethene (azaBODIPY) derivatives as far-red to NIR reactive catalysts to efficiently induce polymerizations is presented. Herein, we examine how introducing electron-donating groups into azaBODIPY scaffolds affects the polymerization rate (r(p)). The systematic introduction of methoxy functionality resulted in a similar to 5x increase in r(p) upon exposure to a far-red (740 nm) LED, along with unprecedented sensitivity (0.7 mW/cm(2), r(p) = 0.1 M/s), while maintaining good temporal control over photopolymerization. Replacing methoxy groups with more electron-donating dimethylamino groups enhanced the charge transfer (CT) character of photoexcited azaBODIPY molecules, which enabled NIR light-driven polymerizations with low intensity LED exposure (850 nm, similar to 55 mW/cm(2), r(p) = 0.1 M/s). Furthermore, transient absorption spectroscopy revealed a direct relationship between the lifetime of azaBODIPY CT states and r(p), which revealed that azaBODIPYs bearing dimethylamino functionality at the aza-bridgehead side of the molecule are optimal for photopolymerization. The unveiled catalyst design principles will inform next-generation NIR light-fueled photopolymerizations and enable integration with emergent additive manufacturing technologies.