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

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 Yb³⁺-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 Tm³ ⁺-based UV upconversion emission. Using cyanine dye Cy5 as a molecular antenna, 635 nm excitation selectively populates the Tm^{3+ 1}D₂ state, yielding intense emission at 361 and 451 nm. Mechanistic studies revealed a direct energy transfer from photo-excited Cy5 to Tm³⁺, in which a ³F_2,3-mediated two-photon upconversion process efficiently populates the ¹D₂ level. Subsequent radiative relaxation generates well-defined UV and blue emissions. Compared with conventional 980 nm excitation, this dye-sensitized Tm³⁺ UV emission shows a three-orders-of-magnitude enhancement from ultrasmall ∼7 nm UCNPs. Furthermore, we demonstrated that Cy5 sensitized Tm³⁺ 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.