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

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, (C21H21P)2SbCl5 and (C22H24P)2SbCl5, 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 (C21H21P)2SbCl5 and (C22H24P)2SbCl5 are attributed to the radiative recombination of self‐trapping excitons. While (C21H21P)2SbCl5 maintains stable orange–yellow luminescence when exposed to ethanol, (C22H24P)2SbCl5 undergoes a structural transformation to non‐luminescent (C22H24P)2Sb2Cl8 upon ethanol treatment, which can be reverted to its luminescent state by heating. Even after multiple cycles, the PLQY of (C22H24P)2SbCl5 is still over 80%, demonstrating excellent cycling stability. (C21H21P)2SbCl5 and (C22H24P)2SbCl5 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. [ABSTRACT FROM AUTHOR]