Ethanol‐Induced Reversible Phase Transition in Antimony Halides for Morse Code Anti‐Counterfeiting and Optical Logic Gates

Abstract Low‐dimensional hybrid organic‐inorganic metal halides (OIMHs) have attracted considerable attention in anti‐counterfeiting due to their non‐toxicity and high photoluminescence quantum yield (PLQY). However, many reported OIMHs are either not reversible or have a poor PLQY. In this study, two antimony‐based halides, (C 21 H 21 P) 2 SbCl 5 and (C 22 H 24 P) 2 SbCl 5 , are synthesized using different organic cations. Both compounds exhibit bright orange–yellow emissions with a PLQY of 82.6% and 83.5%, respectively. The orange–yellow emission of (C 21 H 21 P) 2 SbCl 5 and (C 22 H 24 P) 2 SbCl 5 are attributed to the radiative recombination of self‐trapping excitons. While (C 21 H 21 P) 2 SbCl 5 maintains stable orange–yellow luminescence when exposed to ethanol, (C 22 H 24 P) 2 SbCl 5 undergoes a structural transformation to non‐luminescent (C 22 H 24 P) 2 Sb 2 Cl 8 upon ethanol treatment, which can be reverted to its luminescent state by heating. Even after multiple cycles, the PLQY of (C 22 H 24 P) 2 SbCl 5 is still over 80%, demonstrating excellent cycling stability. (C 21 H 21 P) 2 SbCl 5 and (C 22 H 24 P) 2 SbCl 5 are also explored as fluorescent materials for Morse code anti‐counterfeiting and optical logic gate applications. This work offers a completely new option for fluorescent material used for security information.