Achieving High Quantum Efficiency in Mn5+ Activated Phosphors for NIR‐II Deep Bioimaging Application

Abstract Mn 5+ emission is a promising candidate for imaging deep tissue structures (e.g., vessels, tumors) in the second near‐infrared (NIR‐II, 1000–1350 nm) region. However, its practical application is impeded by the limited quantum efficiency of the available phosphors due to the unstable valence state of Mn 5+ . Herein, a novel strategy involving site competition is proposed to stabilize the Mn 5+ state by the introduction of valence‐unstable Bi 2+/3+ . The results demonstrate that Bi 3+ ions tend to occupy two different Ca 2+ ion sites within the Ca 6 Ba(PO 4 ) 4 O lattice. The incorporation of a small amount of Bi 3+ effectively suppresses the amount of Mn 2+ in Ca 2+ sites. This is also confirmed by spectroscopic experiments and density function theory calculations. Notably, an ultra‐high internal quantum efficiency of 82.3% is achieved under excitation at 653 nm, surpassing more than twofold the previously reported value of 37.5% in Ca 6 Ba(PO 4 ) 4 O: Mn 5+ . As a proof of concept, deep tissue imaging with a penetration depth of ≈2.8 cm is achieved using a self‐produced NIR‐II light‐emitting diodes device embedded with Ca 6 Ba(PO 4 ) 4 O: 0.003Mn 5+ /0.003Bi 3+ powder. These findings provide valuable insights into improving the luminescent properties associated with Mn 5+ ions and pave the way for deep tissue imaging with high spatiotemporal resolution.