Multi‐Stimuli Programmable Ultralong Afterglow Polymer Films With Underwater Room Temperature Phosphorescence for Time‐Gated Information Encryption

ABSTRACT The expansion of global trade and digital information exchange has increased the risks of counterfeiting and data breaches, while conventional security measures remain limited by weak robustness and encryption. Introducing additional protection based on time and medium could improve security, but developing such materials is highly challenging. In this context, the intrinsic time‐gated afterglow of underwater room‐temperature phosphorescent (RTP) materials provides a promising solution.Conventional RTP films fail underwater due to hydrogen bonding with water and quenching by dissolved oxygen. Here, we report a simple and effective strategy to fabricate underwater RTP poly(vinyl alcohol) (PVA) films with enhanced afterglow. Crosslinking via immersion in concentrated boric acid (BA) forms B–O linkages with PVA hydroxyl groups, preventing water‐induced quenching. All five phosphors tested exhibited efficient underwater phosphorescence, with lifetimes up to 2350 ms, afterglows up to 23 s, and full‐spectrum emission. The BA‐crosslinked films also showed long‐term stability under ambient conditions and in diverse aqueous and organic media. By integrating time‐dependent phosphorescence with underwater stability, we demonstrate advanced information‐encryption prototypes that use time as an additional dimension (4D). This facile, synthesis‐free approach yields water‐stable RTP films suitable for underwater encryption and camouflage applications.