Unraveling the Mystery of Near-Infrared Emission in an Antimony Halide Dimer Driven by the Jahn–Teller Effect

Most zero-dimensional hybrid metal halides enable emission in the first near-infrared (NIR-I) region (650–950 nm), and the shorter wavelengths result in poor photon penetration depth. Here, we report the observation of an NIR-II emission signal (band gap = 1.13 eV) from a (BTBAC)2Sb2Cl8 single crystal at room temperature. Although a variety of mechanisms are involved in the explanation of such a broad band with a large Stokes shift, the exploration of the structural origin of the NIR optical activity remains challenging due to misconceptions about excitons and experimental limitations. A strategy that spatially confines 5s2 ions within edge-shared [Sb2Cl8]2– units with controlled lattice distortion is proposed to cause a Jahn–Teller effect (JTE) during the interconfiguration transition (3P1 → 1S0). A family of hybrid metal halides doped with Bi3+,In3+ or Bi3+/In3+ demonstrates that the fine interplay between the Jahn–Teller effect and spin–orbit coupling (SOC) takes into full consideration orbital degeneracy and electron flipping, thus boosting the photoluminescence quantum yield to 12%. TD-DFT calculations visualize the symmetry of the two lattices within a dimeric structure varying with specific modes, and the rational assignment of the energy gap (ground state: 3.69 eV; excited state: 1.22 eV) between 1S0 and 3P1 is highly consistent with the experimental findings (λex = 330 nm; λem = 1100 nm). Leveraging the (BTBAC)2Sb2Cl8 sample, electroluminescence pumped by a UV-LED chip yields a highly efficient NIR-II emission, and the phosphor displays diverse functions in night vision and anticounterfeiting.