Tuning the 5<i>d</i> State of Pr³⁺ in Oxyhalides for Efficient Deep Ultraviolet Upconversion

Abstract 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 (Pr 3+ )‐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:Pr 3+ is determined to be 56.7 times stronger than that of the well‐established Lu 7 O 6 F 9 :Pr 3+ . When employed as a photon‐converter to activate photocatalytic water splitting reactions, upconverted deep UV emission enables H 2 generation under visible light ( λ &gt; 420 nm) excitation from a xenon lamp. The efficient deep UV upconversion stems from tuning 4 f 1 5 d 1 state of Pr 3+ by oxyhalide constituent which both facilitates the absorption of excitation photons in long‐lived intermediate 4 f 2 states and suppress the probability of nonradiative relaxation from 4 f 1 5 d 1 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.