Driving rapid polymerizations with visible to
near-infrared (NIR) 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. Improving 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 NIR 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<sup>2</sup>) and catalyst loadings (< 50 μM),
exemplified by reaction completion within 60 seconds of irradiation using
green, red, and NIR light-emitting diodes.
