Switching between photochromism and photoluminescence in Schiff base derivatives by molecular design of end groups

Abstract

Ensuring the security and reliability of information has increasingly become a key issue for modern society, placing greater demands on encryption technologies and anti-counterfeiting materials. Therefore, the development of intelligent responsive materials with multi-mode encryption and protection capabilities has become crucial. In this study, an aldehyde–amine exchange approach was applied based on the traditional ESIPT–ISO (Excited state intramolecular proton transfer and cis to trans isomerization) color-switching mechanism of salicylaldehyde-aniline Schiff bases. The reaction between tetraphenylethyl salicylaldehyde and aniline derivatives generated eight Schiff base compounds. Five of these compounds displayed UV-triggered color change in the solid phase, whereas the other three exhibited fluorescence emission in solid form. Spectroscopic analysis, theoretical computation, and crystal-structure characterization clarified the mechanisms responsible for both color variation and stability. The stable compounds showed characteristic stacking configurations, and the hydroxyl groups formed weak interactions with nearby atoms, restricting the ESIPT transition and preventing color change under UV irradiation. This work presents the first systematic report on the effect of substituent variation on the ESIPT–ISO process, illustrating how weak interactions and packing modes can inhibit ESIPT. It also represents the first study describing substituent influences on the reverse ESIPT–ISO reaction. Finally, five UV-responsive color-switching materials were developed into color-shifting inks. Through pad and screen printing, these inks enabled encrypted information and anti-counterfeiting features on paper, inorganic, and fiber substrates, maintaining long-term stability for up to three years. Moreover, by utilizing differences in fading times among the photochromic molecules, color-changing inks were overprinted in specific sequences. Upon UV activation, this approach allowed multi-level time-space encryption and anti-counterfeiting of data.