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

Sharp near-infrared-II (NIR-II) emissions are typically achieved through <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>f</mml:mi> <mml:mo>→</mml:mo> <mml:mi>f</mml:mi></mml:mrow> <mml:annotation>$f \to f$</mml:annotation></mml:semantics> </mml:math> electronic transitions of rare-earth ions, while <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>d</mml:mi> <mml:mo>→</mml:mo> <mml:mi>d</mml:mi></mml:mrow> <mml:annotation>$d \to d$</mml:annotation></mml:semantics> </mml:math> transitions in transition metal ions are broad due to electron-ligand interactions. An exception is the intra-configurational spin-flip (ICSF) <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>d</mml:mi> <mml:mo>→</mml:mo> <mml:mi>d</mml:mi></mml:mrow> <mml:annotation>$d \to d$</mml:annotation></mml:semantics> </mml:math> transition like t_2g ³ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:semantics><mml:mo>→</mml:mo> <mml:annotation>$ \to $</mml:annotation></mml:semantics> </mml:math> t_2g ³ of Mo³⁺ emitting sharp NIR-II emission, but only at cryogenic temperatures under vacuum. The high oxophilicity of Mo³⁺ created defects during the synthesis, quenching the emission at room temperature. Herein, we overcome this issue by synthesizing Mo³⁺- doped Cs₂NaInCl₆ double perovskites in a reducing H₃PO₂ environment. [MoCl₆]^3- octahedra are formed, exhibiting ultra-narrow ICSF <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>d</mml:mi> <mml:mo>→</mml:mo> <mml:mi>d</mml:mi></mml:mrow> <mml:annotation>$d \to d$</mml:annotation></mml:semantics> </mml:math> (²T_1g/²E_g <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:semantics><mml:mo>→</mml:mo> <mml:annotation>$ \to $</mml:annotation></mml:semantics> </mml:math> ⁴A_2g) NIR-II emission at 1095 nm in ambient conditions. In addition, a second ICSF ²T_2g <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:semantics><mml:mo>→</mml:mo> <mml:annotation>$ \to $</mml:annotation></mml:semantics> </mml:math> 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.