On–Off Switching of Singlet Self‐Trapped Exciton Emission Endows Antimony‐Doped Indium Halides with Excitation‐Wavelength‐Dependent Luminescence

Excitation-wavelength-dependent (Ex-De) emitters are a fascinating category of luminescent materials whose emission properties vary with the wavelength of the light used for excitation. Antimony (Sb³⁺)-doped indium (In)-based metal halides are efficient light emitters; however, the peak fluorescence emission of most Sb³⁺-activated In-halide remains independent of the excitation wavelength. Here, the study introduces a new Sb³⁺-doped In-halide cluster, (BDPA)₂InCl₅:Sb (BDPA⁺ = C₁₅H₁₈N⁺, benzyldimethylphenylammonium), which demonstrates efficient Ex-De emission originating from the on-off switchable fluorescence behavior of singlet self-trapped exciton (STE) in 5-coordinate Sb³⁺ dopant. Interestingly, when excited within the range of 240-370 nm, photoluminescence (PL) spectra of (BDPA)₂InCl₅:Sb show both singlet and triplet STE emission. However, under excitation wavelengths of 370 to 420 nm, the singlet STE emission is absent, resulting in a noticeable correlated color temperature change from 1700 to 3800 K. The study provides a new approach to designing color-tunable Sb³⁺-based luminophores, and also presents a novel application scenario for the widely recognized Sb³⁺ doping strategy.