Highly Sensitive Reversible Bidirectional Piezochromic and Hydrochromic Fluorescent Materials

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-PF₆, M-3F-PF₆, and M-5F-PF₆), 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-PF₆ 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-PF₆ 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.