Intra‐Configurational Spin‐Flip <i>d</i> →$ \to $ <i>d</i> Transition of Mo (III) Doped Perovskite for Ultra‐Narrow Near Infrared‐II Emission in Ambient Conditions

Abstract Sharp near‐infrared‐II (NIR‐II) emissions are typically achieved through electronic transitions of rare‐earth ions, while transitions in transition metal ions are broad due to electron–ligand interactions. An exception is the intra‐configurational spin‐flip (ICSF) transition like t 2g 3 t 2g 3 of Mo 3+ emitting sharp NIR‐II emission, but only at cryogenic temperatures under vacuum. The high oxophilicity of Mo 3+ created defects during the synthesis, quenching the emission at room temperature. Herein, we overcome this issue by synthesizing Mo 3+ − doped Cs 2 NaInCl 6 double perovskites in a reducing H 3 PO 2 environment. [MoCl 6 ] 3− octahedra are formed, exhibiting ultra‐narrow ICSF ( 2 T 1g / 2 E g 4 A 2g ) NIR‐II emission at 1095 nm in ambient conditions. In addition, a second ICSF 2 T 2g 4 A 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 Mo 3+ –based NIR‐II optoelectronic device, opening opportunities for applications like optical fibers and lasing.