Multi‐Stimuli‐Responsive Amphiphilic Pyridinium Salt and Its Application in the Visualization of Level 3 Details in Latent Fingerprints

Organic luminescent materials that can simultaneously achieve multimode mechanochromism and its water-vapor-induced recovery are desirable for practical applications but rarely reported. Herein, an amphiphilic compound, 4-(9H-carbazol-9-yl)-1-(2-hydroxyethyl)pyridin-1-ium bromide (CPAB), is designed by integrating a lipophilic aromatic unit and hydrophilic end in the molecular architecture. Self-recovered mechanochromism from brown to cyan is observed upon mechanical grinding in air. Comprehensive research by X-ray diffraction, infrared spectroscopy, and single-crystal analysis reveals that the photoluminescence switch originates from the variation in intermolecular hydrogen bonds and molecular packing mode. The amphiphilic nature of CPAB allows water molecules to enter the crystalline lattice, forming two polymorphs of the crystalline phase, namely CPAB-D and CPAB-W. The hydrosoluble CPAB exhibits excellent capability in probing the level 3 details of fingerprints because its lipophilic part can target the fatty acid residues of fingerprints, leading to strong aggregation-induced fluorescence. The research may inspire the design of latent fingerprint developers and application in forensics/anti-counterfeiting.