Document security has played a significant role in military and in the economy, as well as in our daily lives. In particular, there is an increasing demand for paper document security since paper is still the most widespread medium for information storage and many important documents are still paper-based. Several critical challenges, such as multiple-time uses and high-level security, need to be addressed to improve the practical application of security printing in different scenarios. The development of smart luminescent materials is a promising way to achieve those goals.

Our group are interested in developing stimuli-responsive materials for rewritable information recording and data security protection. Recently, we proposed a strategy to realize water-jet rewritable security printing based on dynamic metal-ligand coordination (Nat. Commun. 2018, 9, 3). However, the resolution of printed texts or patterns on our fabricated paper is not high, which might be because the metal-ligand coordination of complexes on paper substrate cannot be completely broken by water. The ionic bond is a weaker interaction than coordination bond, thus it seems like that the luminescent ionic complexes are alternative candidates for water-jet security printing.

In this work, luminescent ionic Mn(II) complexes are selected to prepare water-jet security paper. The core idea is controlling the reversible ionic interaction of Mn(II) complexes to manipulate their photophysical properties. For example, the ionic bond of the Mn(II) complex can be easily destroyed by water, thus quenching the luminescence and realizing the reversible on/off switching. Based on this result, confidential information, which is only visible under UV light, can be printed on Mn(II) complex coated paper by using water as the ink for multiple times. Importantly, the obtained results are superior to our previous report in the aspect of resolution, contrast and reversible ability.

Additionally, we tried to improve the security level of recorded information. Previously, our group demonstrated that using emission lifetime as analysis signal, which can provide multiple-level dimensions, is an effective method to achieve data encryption and decryption (Nat. Commun. 2014, 5, 3601). We envisioned that combining the emission lifetime and security printing may be an interesting idea to improve the security level of stored information. It is found that changing the halide ions of Mn(II) complexes leads to remarkable differences in their emission lifetimes. Taking advantage of this feature, lifetime-based security printing has been successfully demonstrated. The recorded information is invisible under both ambient light and UV light. The data will only reveal when analyzed by a photoluminescence lifetime imaging (PLIM) technique, providing a higher security level of stored information.

Moreover, we are wondering that is there an effective way to further improve the performance of lifetime-based security printing? After very full discussions, we reached an agreement that promoting the emission lifetime manipulation from static to dynamic would endow security printing with revolutionary performance. Again, by utilizing the reversible ionic interaction of Mn(II) complexes, we realized the dynamic control of emission lifetime. More importantly, the uncrackable multi-level security printing has been demonstrated.

Look into the future, our strategy on the control of photophysical properties can be extended to various ionic complexes for different advanced photonic applications.

 More details can be found in our paper " Dynamic Luminescence Manipulation for Rewritable and Multi-level Security Printing" published in Matter.

 Link to article: https://www.cell.com/matter/fulltext/S2590-2385(19)30174-2