Tailored Near‐Infrared Photoemission in Fluoride Perovskites through Activator Aggregation and Super‐Exchange between Divalent Manganese Ions

Biomedical imaging and labeling through luminescence microscopy requires materials that are active in the near‐infrared spectral range, i.e., within the transparency window of biological tissue. For this purpose, tailoring of Mn²⁺–Mn²⁺ activator aggregation is demonstrated within the ABF₃ fluoride perovskites. Such tailoring promotes distinct near‐infrared photoluminescence through antiferromagnetic super‐exchange across effective dimers. The crossover dopant concentrations for the occurrence of Mn²⁺ interaction within the first and second coordination shells comply well with experimental observations of concentration quenching of photoluminescence from isolated Mn²⁺ and from Mn²⁺–Mn²⁺ effective dimers, respectively. Tailoring of this procedure is achieved via adjusting the Mn–F–Mn angle and the Mn–F distance through substitution of the A⁺ and/or the B²⁺ species in the ABF₃ compound. Computational simulation and X‐ray absorption spectroscopy are employed to confirm this. The principle is applied to produce pure anti‐Stokes near‐infrared emission within the spectral range of ≈760–830 nm from codoped ABF₃:Yb³⁺,Mn²⁺ upon excitation with a 976 nm laser diode, challenging the classical viewpoint where Mn²⁺ is used only for visible photoluminescence: in the present case, intense and tunable near‐infrared emission is generated. This approach is highly promising for future applications in biomedical imaging and labeling.