Visible-Light-Mediated Diels-Alder Reactions Under Single-Chain Polymer Confinement: Investigating the Role of the Crosslinking Moiety on Catalyst Activity

Macromolecular scaffolds are rapidly emerging in catalysis owing to the ability to control catalyst placement at precise locations in the framework. This often allows for cooperation in multi-catalytic reactions that may not be observed using small molecules, enabling high levels of reactivity. Herein, we describe a triphenylpyrylium (TPT)-based visible-light active single-chain polymer nanoparticle (SCNP) scaffold that facilitates the radical cation [4+2]-cycloaddition. In our studies, we find that the catalytic activity is highly dependent on the identity of the crosslinking group – incorporated in 5-10% overall – owing to its ability to act as a redox mediator in the Diels-Alder reaction. Mechanistic studies indicate that the vast majority of the TPT excited states are quenched by the acene, with the resultant radical cation formed from either naphthalene- or phenanthrene-based SCNPs able to proceed in oxidizing the dienophile in the elementary step of the reaction, leading to near quantitative yields of the cycloadduct. The TPT-SCNP demonstrates enhanced reactivity with low catalyst loadings when compared to small molecule TPT, and is also able to be recycled and reused in three iterations of the reaction prior to substantial drops in yield observed from photobleaching. Our results overall suggest that SCNPs feature multiple subtle design elements that can use as catalysts in diffusion-limited reactions.