5,10,15,20-Tetrakis(p-tolyl)porphyrin derived carbon dots as colorant in flexo and screen inks with multi-level covert features for security printing

Fluorescent inks with covert security features, which make it difficult for forgers to replicate, are highly sought after among the array of anti-counterfeit technologies. The present study focuses on the synthesis of Carbon dots (CDs) from 5,10,15,20-tetrakis(p-tolyl)porphyrin (TTP) via a solvothermal approach. The prepared CDs exhibit attractive emissive characteristics with an absolute fluorescence quantum yield of 38.85 %. Time-resolved fluorescence studies reveal the multiple exponential decay nature of the emitters, which could be due to the diverse chemical environment on the surface of the CDs. XPS analysis proves oxygen as the major element, along with carbon, sulfur, and nitrogen as minor elements. DFT studies on four optimized structures of CDs along with their respective UV and FT-IR plots are conducted. The theoretically derived photoluminescence plots corresponding to the 25-30th excited states coincide with the experimental emission peak. CDs demonstrate characteristic variations in fluorescence in the presence of water, attributed to it-it stacking induced aggregation. Aggregate formation is also influenced by pH changes, affecting the surface functional groups in proximity to the solvent medium. CDs are used as colorants in the fabrication of both flexo and screen inks. Flexo ink retains the fluorescence features of the CDs, while screen ink offers multilevel security features. Exposing UV-dull paper to 365 nm UV light allows the ink inscribed letters to appear fluorescent blue, whereas the use of UV/green optical filters facilitates the visualization of green/yellow emissions. The introduction of multilevel features makes the duplication of the print difficult, protecting the integrity of the product and could be used for real-life applications.