Vibronically Coupled Near-Infrared Emission and Excitation from d−d Transitions of Cs₂MX₆ (M = Mo/W, X = Cl/Br)

In near-infrared (NIR)-emitting phosphors, the emission typically originates from forbidden d−d or f−f electronic transitions of metal ion dopants. But the excitation happens through higher-energy (UV or blue) allowed transitions, resulting in energy loss for this UV- or blue-to-NIR conversion. Here we report Cs₂MX₆ (M = Mo/W, X = Cl/Br) 0D perovskite derivatives with NIR excitation and emission arising from the same pair of d-electronic states, showing small Stokes shifts. The samples show significant optical absorption at 787 nm (12700 cm⁻¹) and emission at 986 nm (10140 cm⁻¹) because of the d² electronic configuration of M⁴⁺ in isolated [MX₆]²⁻ octahedra. Interestingly, isolated [MX₆]²⁻ also leads to vibrational fine structures in the electronic excitation and emission spectra at cryogenic temperatures, which are rare for undoped inorganic crystals. Finally, phosphor-converted light-emitting diodes are fabricated by coating Cs₂MoCl₆ on commercial 730 nm LED chips without requiring UV-blue chips.