Tuning the 5d State of Pr³⁺ in Oxyhalides for Efficient Deep Ultraviolet Upconversion

Visible-to-ultraviolet (UV) upconversion provides a fascinating strategy to achieve deep UV emission through readily accessible visible light. However, the intensity of deep UV emission obtained through visible-to-UV upconversion progress is still far from satisfactory, severely constraining its practical applications. Herein, a novel class of praseodymium ion (Pr³⁺)-doped rare-earth oxyhalides (YOCl, YOBr, and LuOBr) to achieve efficient upconverted deep UV emission in the spectral range of 250–350 nm is developed. The upconverted UV emission intensity of LuOBr:Pr³⁺ is determined to be 56.7 times stronger than that of the well-established Lu₇O₆F₉:Pr³⁺. When employed as a photon-converter to activate photocatalytic water splitting reactions, upconverted deep UV emission enables H₂ generation under visible light (λ > 420 nm) excitation from a xenon lamp. The efficient deep UV upconversion stems from tuning 4f¹5d¹ state of Pr³⁺ by oxyhalide constituent which both facilitates the absorption of excitation photons in long-lived intermediate 4f² states and suppress the probability of nonradiative relaxation from 4f¹5d¹ state. These findings not only provide new insights into a mechanistic understanding of the host effect on upconversion process but also make a breakthrough in developing efficient deep upconversion materials that will expand their further applications. © 2024 Wiley-VCH GmbH.