Solvent stimuli-responsive off-on fluorescence induced by synergistic effect of doping and phase transformation for Te4+ doped indium halide perovskite: Giving printable and colorless ink for information encryption and decryption.

Since traditional fluorescent materials are too easily observed by the eyes just under the UV light, off-on fluorescent materials are explored as the new generation of fluorescent labels. In the "off" state, such off-on fluorescent labels cannot be observed by naked eyes under either natural light or UV light. Only after a specific decryption treatment to make the fluorescent materials turning into the "on" state, the fluorescent labels can be observed under the UV light. Up to now, it is still a challenge to prepare fluorescent inks with aforementioned ideal properties by using halide perovskite materials. Herein, we reported the first example of Te4+ doped indium halide perovskite inks with both off-on fluorescence under solvent stimuli and invisible ink color by the naked eyes. The synergistic effect of doping/undoping of Te4+ together with the reversible phase transformation between Cs2InCl5(H2O) and Cs3InCl6 under solvent stimuli is key for the off-on fluorescence of crystals. Under acid solvent, the substitutional doping of Te4+ during the process of phase transformation from Cs3InCl6 to Cs2InCl5(H2O):Te4+ gives rise to "turning-on" orange emission from Te-induced self-trap emission (STE). Under the stimuli of methanol, the dissolution of Te4+ from the crystals destroys the structure of Te4+ in ligand-field and results in "turning-off" Te-induced emission during the process of phase transformation from Cs2InCl5(H2O):Te4+ to Cs3InCl6. On the basis of the Te4+ doped indium halide perovskite, printable and colorless ink can be prepared for the confidential information encryption and decryption. Since the mixture of Cs3InCl6 crystals and TeCl4 have no absorption in visible light scope, the printed encrypted information by such off-state fluorescent ink is colorless and invisible by the naked eyes under either ambient light or UV light. After decryption by acid solvent stimuli, the resulted Cs2InCl5(H2O):Te4+ doping crystals have a large Stokes shift with absorption below 450 nm from the excitation of Te4+ in ligand-field and emission around 570 nm from Te-induced STE. It makes the decryption information still colorless and invisible by the naked eyes under the ambient light but visible and readable under the UV light. In comparison to traditional undoped CsPbBr3/CsPb2Br5 perovskites with small Stokes shift and eye-visible ink color, the current colorless Te4+doped indium halide perovskites are no doubt providing better security level for both encrypted and decrypted information.