On-demand regulation of photochromic behavior through various counterions for high-level security printing

Materials exhibiting reversible changes in optical properties upon light irradiation have shown great potential in diverse optoelectronic areas. In particular, the modulation of photochromic behavior on demand for such materials is of fundamental importance, but it remains a formidable challenge. Here, we report a facile and effective strategy to engineer controllable photochromic properties by varying the counterions in a series of zinc complexes consisting of a spirolactam-based photochromic ligand. Colorability and coloration rate can be finely tuned by conveniently changing their counterions. Through utilization of the reversible feature of the metal-ligand coordination bond between Zn²⁺ and the spirolactam-based ligand, dynamic manipulation of photochromic behavior was achieved. Furthermore, we demonstrated the practical applications of the tunable photochromic properties for these complexes by creating photochromic films and developing multilevel security printing. These findings show opportunities for the development of smart materials with dynamically controllable responsive behavior in advanced optoelectronic applications.