Thiophene-fused boron dipyrromethenes as energy efficient near-infrared photocatalysts for radical polymerizations

Abstract

Rapid near-infrared (NIR) light driven polymerizations have the potential to enable energy-efficient, benign, and multimaterial manufacturing for applications ranging from tissue engineering to soft robotics. However, achieving photopolymerization rates with NIR light that are viable for emergent additive manufacturing technologies (∼0.1–1.0 M s⁻¹ at light intensities <40 mW cm⁻²) has proven challenging due to its inherently low energy. Herein, we begin to take down these barriers through a systematic investigation of four distinct thiophene-fused boron dipyrromethene (BODIPY) photocatalysts. Through extended π-conjugation, the thiophene-fused BODIPYs effectively absorb NIR (>780 nm) light and drive efficient acrylate polymerizations upon exposure to low-intensity (2.5–40 mW cm⁻²) NIR light emitting diodes (LEDs). The installation of bromine atoms is shown to further improve NIR light-fueled photopolymerization efficiency by a factor of ∼1.5×. This enhancement was rationalized by the formation of long-lived spin-triplet excited states via intersystem crossing, as observed using ultrafast transient absorption spectroscopy. However, the triplet yields are modest (∼6–14%), suggesting efficient charge transfer from singlet excited states also occurs within the present photosystem. Finally, optimization of resin formulations containing non-halogenated thiophene-fused BODIPY photocatalysts is shown to provide unprecedented polymerization rates (0.33 M s⁻¹) upon exposure to an 850 nm LED at an intensity of 20 mW cm⁻². The structure–reactivity relationships identified herein provide key insights that will inform further design and optimization of energy-efficient, NIR light-driven polymerizations with utility in photocurables for coatings, adhesives, and 3D printing.