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.