NIR‐II Luminescence With A Recorded 76% EQE Through Energy Extraction From Dark Cr3+.

NIR‐II luminescent materials are the key enablers of smart spectroscopy‐based techniques, offering capabilities for nondestructive analysis and bio‐imaging. Here, a broadband NIR‐II luminescence with a recorded external quantum efficiency (EQE) of 76% in LiGa5O8 are demonstrated, using heavily doping Cr3+ ion (10%) as a light harvester and a controllable energy transfer (ET) from Cr3+ to Ni2+ (0.4%) ions. Even though the Cr3+ ion produces substantially weak luminescence due to concentration quenching, the introduction of Ni2+ ions effectively extract the excitation energy to generate NIR‐II luminescence. Complementary investigations using synchrotron‐based EXAFS fittings and structural refinement disclose a significant structural distortion in the LiGa5O8 compound, which facilitates the relaxation of the parity‐selection rules for Cr3+ and Ni2+ ions. Additionally, DFT calculations identify specific site occupations, which favor unidirectional ET from Cr3+ to Ni2+ ions. As a result, the significant absorption of excitation light by heavy‐doping Cr3+ ions and high radiative transition probability in Ni2+ ions synergistically result in a record high EQE. These findings provide pioneering insight into rational NIR‐II light generation by deliberated control of ET pathway in heavily doped systems, thereby with promising implications for NIR spectroscopy applications. [ABSTRACT FROM AUTHOR]