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.
Zhao et al. (Tue,) studied this question.