Visible light activated coumarin photocages: an interplay between radical and organobase generation to govern thiol-ene polymerizations

Photobase generators (PBGs) are an attractive tool for the latent and spatially governed formation of polymer networks with uniform topologies due to the anionic step-growth mechanistic pathway they unlock. Despite the significant advances made in PBG frameworks, utility in rapid, visible light driven polymer formation and associated structure-reactivity relationships remain scarce. Herein, five coumarinylmethyl PBGs bearing caged tetramethylguanidine (TMG) were synthesized and systematically examined for inducing thiol-ene polymerizations, while benchmarking against the classic ortho-nitrobenzyl (oNB) PBG framework. Quantification of photopolymerization kinetics and bond scission quantum yields with real-time Fourier transform infrared and steady-state UV-vis absorption spectroscopies revealed an increase in the disparity between C=C and S-H conversion for derivatives halogenated at the 3-position. Alternatively, incorporation of a pi-extended styryl moiety at the same position decreased the conversion gap and enabled uncaging with a blue LED (470 nm). This gap was attributed to the concurrent activation of radical chain-growth and base-catalyzed step-growth mechanisms, which was tempered through the (sub-)stoichiometric addition of tetramethylpiperidinyloxy (TEMPO). These findings paint a detailed picture of PBGs for thiol-ene polymerizations that will inform the selection and optimization of future light-activated catalysts and enable advanced manufacturing of tailored soft materials.

Automated summary

In this study, five photobase generators (PBGs) with photo-protective properties were synthesized. Through the investigation of thiol-ene polymerization reactions, it was found that halogenation at the 3-position increased the disparity between C=C and S-H conversion, while the incorporation of an extended phenyl moiety at the same position reduced the conversion gap and enabled uncaging with a blue LED. These findings provide important insights for the selection and optimization of future light-activated catalysts and enable advanced manufacturing of tailored soft materials.