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 Cs 2 MX 6 (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 d 2 electronic configuration of M 4+ in isolated [MX 6 ] 2− octahedra. Interestingly, isolated [MX 6 ] 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 Cs 2 MoCl 6 on commercial 730 nm LED chips without requiring UV-blue chips.