Vibronically Coupled Near-Infrared Emission and Excitation from <i>d</i>−<i>d</i> 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