A New Class of Bond-Length-Engineered Transition-Metal-Doped Nanocrystals with Tunable NIR-II Absorption and Emission

Developing luminescent probes with tunable absorption and emission in the second near-infrared (NIR-II) or shortwave infrared (SWIR) region (1000-2500 nm) is essential for advanced photonic technologies, as this region offers dramatically reduced photon scattering in complex media and intrinsic compatibility with optical communication bands. Here, we report a new class of NIR-II nanoprobes based on transition-metal Ni2+-doped fluoride nanocrystals, in which Ni2+ ions serve simultaneously as both sensitizer and activator. Theoretical analysis and experimental validation reveal that varying the matrix from NaMgF3 to MgF2, KMgF3, and NaMnF3 systematically modulates the local Ni-F bond length and the coordination environment of Ni2+. This bond-length engineering enables broadband and finely tunable NIR-II absorption (1318-1414 nm) and emission (1620-1780 nm) from d-d electronic transitions, with the optimized NaMgF3:9.6%Ni@NaMgF3 nanocrystals achieving a quantum yield of 20.7% under 1320 nm excitation. Using these distinctive excitation-emission fingerprints, we demonstrate low-crosstalk in vivo multiplexed imaging with high signal-to-noise ratios. This work establishes transition-metal-doped nanocrystals as a broadly tunable platform and expands the scope of inorganic luminescent materials for next-generation photonic applications.