Energy‐Level‐Selective Dye Sensitization Enables Enhanced Ultraviolet Upconversion Emission

ABSTRACT Ultraviolet (UV) upconversion emission is attractive because high‐energy photons can initiate photophysical and photochemical transformations that are inaccessible with longer‐wavelength irradiation. However, conventional Yb 3+ ‐sensitized upconversion nanoparticles (UCNPs) exhibit intrinsically weak 4f‐4f absorption and significant energy dispersion among multiple energy levels, limiting their ability to generate intense, spectrally focused UV emission. Here, we introduce a direct dye‐sensitization strategy that dramatically enhances Tm 3 + ‐based UV upconversion emission. Using cyanine dye Cy5 as a molecular antenna, 635 nm excitation selectively populates the Tm 3+ 1 D 2 state, yielding intense emission at 361 and 451 nm. Mechanistic studies revealed a direct energy transfer from photo‐excited Cy5 to Tm 3+ , in which a 3 F 2,3 ‐mediated two‐photon upconversion process efficiently populates the 1 D 2 level. Subsequent radiative relaxation generates well‐defined UV and blue emissions. Compared with conventional 980 nm excitation, this dye‐sensitized Tm 3+ UV emission shows a three‐orders‐of‐magnitude enhancement from ultrasmall ∼7 nm UCNPs. Furthermore, we demonstrated that Cy5 sensitized Tm 3+ UV emission can facilitate the photochemical reaction in a microreactor, underscoring its practical utility. This generalizable approach provides a versatile platform for creating bright, spectrally concentrated UV upconversion systems for photochemical, photocatalytic, and photonic applications.