Structural Plasticity of [SbCl6]3− Octahedra in 0D Metal Halides Enables Broadband Near-Infrared Emission and Reversible Thermochromism

Low-dimensional lead-free antimony(III)-based organic−inorganic metal halides (OIMHs) are promising for near-infrared (NIR) optoelectronics, yet balancing high efficiency, structural robustness, and environmental responsiveness remains challenging. Herein, we strategically modulate the local coordination environment and excited-state relaxation of intrinsic Sb(III) centers via organic ligand steric hindrance. Specifically, a previously reported zero-dimensional (0D) hexacoordinated (PPh4)3SbCl6 compound was synthesized via a low-cost, solvent-free melt-quenching method. To elucidate its unique photophysical behaviors, the previously documented pentacoordinated (PPh3)2SbCl5 square pyramid was systematically evaluated as a side-by-side comparative baseline. Compared to the moderate distortion under rigid hydrogen-bonding confinement in (PPh3)2SbCl5, the overcrowded coordination shell of (PPh4)3SbCl6 displays an exceptionally high room-temperature bond length dispersion (△d = 5.31 × 10−3) within a flexible, soft lattice matrix. Photoexcitation of (PPh4)3SbCl6 triggers strong electron−phonon coupling and unrestrained excited-state over-relaxation, yielding a broadband NIR emission at 715 nm with a large full-width at half-maximum (FWHM) of 189 nm and a high photoluminescence quantum yield (PLQY) of 78%. Benefiting from coordination saturation, (PPh4)3SbCl6 exhibits exceptional stability against ambient exposure (4 weeks) and thermal decomposition (300 °C). Furthermore, leveraging the structural plasticity of [SbCl6]3− octahedra, (PPh4)3SbCl6 exhibits a dramatic, fully reversible thermochromic transition from deep red to bright yellow at 180 °C via thermally induced symmetry restoration. Prototypical phosphor-converted LEDs based on (PPh4)3SbCl6 deliver stable broadband emission with an outstanding external quantum efficiency (EQE) of 16.16% at 47.24 mA cm−2, highlighting its immense potential for next-generation smart lighting and thermal sensing.