Excited‐State Pathway Switching via Reversible Structural Phase Transitions in Sb 3+ ‐Doped Cadmium Halides

ABSTRACT Reversible control of structural phase transitions and luminescence remains a key challenge in organic–inorganic hybrid metal halides for stimuli‐responsive photonic applications. Here, we report two new zero‐dimensional (0D) Cd‐based metal halides, (DFPD) 6 CdCl 8 and (DFPD) 2 CdCl 4 ·H 2 O (DFPD + = 4,4‐difluoropiperidine), in which Sb 3+ doping enables distinct emission behaviors governed by coordination geometry. Combined spectroscopic studies and theoretical calculations reveal that Sb 3+ ‐doped (DFPD) 6 CdCl 8 exhibits yellow emission with a large Stokes shift arising from triplet self‐trapped exciton ( 3 STE) emission, whereas Sb 3+ ‐doped (DFPD) 2 CdCl 4 ·H 2 O displays excitation‐dependent emission due to competing singlet STE ( 1 STE) and 3 STE states. This contrast originates from the different Cd–Cl coordination environments (octahedral vs. tetrahedral), which modulate the energy levels and transition dipole moments. Importantly, hydrochloric acid (HCl) and 4,4‐difluoropiperidine induce fully reversible interconversion between the two structures, allowing dynamic switching between yellow and deep‐orange emission. Based on this reversible luminescence, we further demonstrated applications in dynamic anti‐counterfeiting and multilevel information encryption. This work establishes a coordination‐structure‐driven strategy for programmable emission in 0D hybrid metal halides.