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

Mn5+ 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 Mn5+. Herein, a novel strategy involving site competition is proposed to stabilize the Mn5+ state by the introduction of valence-unstable Bi2+/3+. The results demonstrate that Bi3+ ions tend to occupy two different Ca2+ ion sites within the Ca6Ba(PO4)(4)O lattice. The incorporation of a small amount of Bi3+ effectively suppresses the amount of Mn2+ in Ca2+ 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 Ca6Ba(PO4)(4)O: Mn5+. As a proof of concept, deep tissue imaging with a penetration depth of approximate to 2.8 cm is achieved using a self-produced NIR-II light-emitting diodes device embedded with Ca6Ba(PO4)(4)O: 0.003Mn(5+)/0.003Bi(3+) powder. These findings provide valuable insights into improving the luminescent properties associated with Mn5+ ions and pave the way for deep tissue imaging with high spatiotemporal resolution.