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. This spatial proximity allows for enhanced catalyst activity that may not be observed using small molecules. Herein, we describe a triphenylpyrylium (TPT)-based visible-light active single-chain polymer nanoparticle (SCNP) that facilitates the radical cation [4+2]-cycloaddition. We find that the catalytic activity is highly dependent on the styrylarene comonomer used, wherein it can act as a redox mediator under confinement, increasing the catalytic turnover (TON) by up to 30 times in comparison to free TPT in solution. Mechanistic studies indicate that TPT excited states are quenched by the acene, with the resultant radical cation formed from naphthalene-based SCNPs able to proceed in oxidizing the dienophile in the elementary step of the reaction, while leading to near quantitative yields of the cycloadduct. The TPT-SCNP demonstrates enhanced photocatalyst efficiency compared to small molecule TPT, and is also able to be recycled and reused in three iterations of the reaction prior to decreased performance caused by photobleaching. Our results overall suggest that SCNPs feature multiple subtle design elements that can impart enhanced reactivity under confinement, boding well for use as catalysts in diffusion-limited photoredox reactions.