New anti-counterfeiting materials based on hemicyanine dyes: HSO3−/SO32− encryption and UV light decryption

Abstract

Information security is part of our everyday lives, with items as small as diplomas, certificates and passports, as well as bills and national currency, in constant use. With the development of information decryption and fraud technology, higher requirements are needed for encryption methods and anti-counterfeiting materials. In view of this, it is necessary to strengthen the research on anti-counterfeiting encryption materials. The work in this study reports the design and synthesis of two dye molecules, namely TPA-S-Cy3 and MOTPE-S-Cy3, which incorporate the heptamethine cyanine structure and possess a D–π–A configuration. The “CC” double bond in TPA-S-Cy3 and MOTPE-S-Cy3 reacts via a Michael addition with HSO₃⁻/SO₃²⁻, suppressing the ICT (intramolecular charge transfer) process and disrupting the D–π–A structure, thus rendering the molecules colorless. Upon exposure to ultraviolet light, the HSO₃⁻/SO₃²⁻ complex generates reactive oxygen species (ROS). These ROS oxidize HSO₃⁻/SO₃²⁻, thereby restoring the original structure and returning the system to its original color. This mechanism enables a synergistic chromatic effect by combining chemochromism and photochromism. By leveraging this color-changing mechanism, we have developed a color-changing ink, which has then been applied to stamps, markers, and inkjet printers, allowing for the encryption and decryption of information on paper-based substrates.