Fully Sensitized Upconversion Nanoparticles as Efficient Catalysts for NIR‐Driven UV Photochemistry

Biological photosynthesis harnesses energy from multiple photons to drive complex chemical transformations. In contrast, chemical photocatalysis typically relies on single-photon excitation, limiting its applicability in high-energy-demanding reactions. Upconversion nanoparticles (UCNPs), which can convert multiple low-energy near-infrared (NIR) photons into a single higher-energy photon, offer a promising solution. We synthesized and systematically improved NaYbF₄:Tm@NaYF₄ nanoparticles, focusing on sensitizer concentration, dopant spacing, and shell thickness to enhance ultraviolet (UV) and blue emission. Compared to low doped NaYF₄:Yb, Tm systems, our nanoparticles exhibited significantly improved brightness, with a 210-fold enhancement in UV emission at 345 nm. Using these UCNPs as heterogeneous photocatalysts, we achieved efficient [2 + 2] photocycloadditions and Paternò-Büchi reactions under 980 nm excitation, with turnover numbers (TON) exceeding 290,000 and turnover frequencies (TOF) up to 8.52 s^-1. Additionally, the UCNP catalysts were readily recoverable. Our results provide a rational framework for tailoring UCNPs for energy-demanding photochemical reactions and establish their potential in synthetic and biomedical applications that require deep-tissue, low-phototoxicity excitation.