Intra-Configurational Spin-Flip d → $ \to $ d Transition of Mo (III) Doped Perovskite for Ultra-Narrow Near...

Sharp near-infrared-II (NIR-II) emissions are typically achieved through $f\to f$ electronic transitions of rare-earth ions, while $d\to d$ transitions in transition metal ions are broad due to electron-ligand interactions. An exception is the intra-configurational spin-flip (ICSF) $d\to d$ transition like t_2g ³ $\to$ t_2g ³ of Mo³⁺ emitting sharp NIR-II emission, but only at cryogenic temperatures under vacuum. The high oxophilicity of Mo³⁺ created defects during the synthesis, quenching the emission at room temperature. Herein, we overcome this issue by synthesizing Mo³⁺- doped Cs₂NaInCl₆ double perovskites in a reducing H₃PO₂ environment. [MoCl₆]^3- octahedra are formed, exhibiting ultra-narrow ICSF $d\to d$ (²T_1g/²E_g $\to$ ⁴A_2g) NIR-II emission at 1095 nm in ambient conditions. In addition, a second ICSF ²T_2g $\to$ ⁴A_2g emission is observed at 700 nm, violating the Kasha's rule. The intensity of ICSF emissions increase with increasing temperature (7-350 K) due to vibronic coupling relaxing the Laporte selection rule. The samples are stable for more than 6 months in ambient conditions, allowing for a detailed study of fundamental photophysics and fabrications of phosphor-converted light emitting diodes. This is the first Mo³⁺-based NIR-II optoelectronic device, opening opportunities for applications like optical fibers and lasing.