Highly Sensitive Reversible Bidirectional Piezochromic and Hydrochromic Fluorescent Materials

ABSTRACT Achieving the bidirectional response of highly sensitive piezochromic fluorescence (PCF) and hydrochromic fluorescence in a single molecular system is of great significance for the development of a new generation of multifunctional intelligent materials. This study reports three carbazole quaternary ammonium derivatives (M‐PF 6 , M‐3F‐PF 6 , and M‐5F‐PF 6 ), which can undergo a crystalline‐to‐amorphous phase transition (CAPT) under external pressure, and realize an amorphous‐to‐crystalline phase transition (ACPT) via self‐assembly in aqueous solutions, exhibiting reversible dual fluorescence emission. Among them, M‐PF 6 exhibits a highly sensitive PCF behavior with a response threshold as low as 0.67 MPa and faster self‐assembly kinetics. Experimental and theoretical calculations confirm that its reversible chromic fluorescent performance stems from the low phase transition activation energy. Based on these characteristics, M‐PF 6 has been successfully applied to rewritable paper, which can even be drawn with a Chinese brush. By utilizing the differential assembly kinetics of the three derivatives in the aqueous solution, time‐dependent information encryption patterns have been successfully constructed. This study not only developed highly sensitive reversible bidirectional piezochromic and hydrochromic fluorescent materials, but also provided a new perspective for the design of effective stimuli‐responsive luminescent materials.