Copper‐Iodide Cluster Ink for Fast and Robust Anti‐Counterfeiting and Erasable Encryption

Abstract Copper‐iodide clusters hold great potentials in anti‐counterfeiting, information encryption, reversible writing, and X‐ray imaging due to their excellent luminescence and stimuli‐responsive properties. However, developing advanced copper‐iodide stimuli‐responsive materials still faces challenges of long photoluminescence response time and poor stability. In this work, green‐, yellow‐, and orange‐emissive copper‐iodide clusters based on the same tri(m‐toly)phosphine (P‐(m‐tol) 3 ) ligand are reported. The luminescence difference originates from different crystal structure or crystallinity. Either heat or solvent stimulus can trigger reversible structure transition and then as well the corresponding emission colors. Notably, the clusters can be dissolved in solvents as printing or coating inks, and the patterns exhibit fast color change within 1.5 s with robust write‐erase‐write reversibility more than 100 cycles and information maintenance in water for one month. Based on these fascinating properties, quick response code anti‐counterfeiting labels and rapid‐erased rewritable encryption applications are demonstrated. This work breaks through the dilemma of response rate and stability in traditional stimuli‐responsive materials and opens up new avenues for developing high‐security dynamic encryption systems.