Excited-State Pathway Switching via Reversible Structural Phase Transitions in Sb³⁺-Doped Cadmium Halides

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)₆CdCl₈ and (DFPD)₂CdCl₄·H₂O (DFPD⁺ = 4,4-difluoropiperidine), in which Sb³⁺ doping enables distinct emission behaviors governed by coordination geometry. Combined spectroscopic studies and theoretical calculations reveal that Sb³⁺-doped (DFPD)₆CdCl₈ exhibits yellow emission with a large Stokes shift arising from triplet self-trapped exciton (³STE) emission, whereas Sb³⁺-doped (DFPD)₂CdCl₄·H₂O displays excitation-dependent emission due to competing singlet STE (¹STE) and ³STE 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.