Engineering Photo‐Functionalities of OrganicInorganic Hybrid Metal Halides Toward Repeatable Optical Writing/Erasing and Multilevel Information Encryption

Abstract Organic‐inorganic hybrid halides (OIHH) have garnered considerable attention as promising photo‐functional materials. Nevertheless, the rational design of OIHH with tailored optical properties remains a formidable challenge. Herein, the synthesis of Sb³⁺‐doped (benzamidine)₂InCl₅(H₂O) crystals exhibiting intense visible‐light emission is reported. Furthermore, photochromism (PC) and afterglow are achieved by exploiting intermolecular charge transfer between guest water and C═N⁺─H group within the crystalline lattice. Radicals with a characteristic absorption band at 520 nm are generated by photo‐irradiation, which causes PC, while the energy stored in the long‐lived radicals gradually transfers to Sb³⁺ centers, yielding an afterglow effect. Repeatable optical writing/erasing processes are demonstrated based on the reversible PC effect. In addition, a multi‐level information encryption scheme is proposed based on the multiplexed integration of fluorescence color, PC, and afterglow. Therefore, this work not only develops a novel OIHH system but also offers a universal strategy for engineering photo‐responsive functionalities by intermolecular charge transfer. While water promotes free radical reactions, it simultaneously accelerates PC bleaching, presenting a challenge that requires further research to resolve this dichotomy.