Aggregation-induced-emission-active vinamidinium salts with tunable emissions, reversible mechanochromic response and the application in data-security protection

The development of aggregation-induced-emission (AIE)-active and reversible mechanochromic fluorescent organic materials has been of great interest in recent years because of their potential and practical application in many fields. Five vinamidinium salts that bear different aryl substitutes (Ar = phenyl (N1), p-methylphenyl (N2), p-bromophenyl (N3), p-nitrophenyl (N4) and naphthyl (N5)) were designed and synthesized. Their photophysical properties were investigated via a series of spectroscopic and theoretical methods. All compounds exhibit intense ¹π-π*/intramolecular charge-transfer (¹ICT) absorption bands in the region from ca. 300 nm–450 nm. All compounds except N5 display aggregation induced enhanced emission behavior in DMSO-CH₂Cl₂ mixtures, and relatively high quantum efficiency in solid-states (Φ_F = 0.06–0.22). Compounds N1 and N3 with yellow and orange solid fluorescence, respectively, show mechanochromic and vapochromic response. Upon grinding, the emission of N1 and N3 could be transformed to green–yellow and yellow, respectively, and restored when the ground powders were fumed with MeOH vapor. Powder X-ray diffraction spectroscopy indicates that the reversible mechanochromism is ascribed to the transformation of a crystalline state into an amorphous state. Compound N1 is used in data-security protection and is of assistance in the rational design of smart luminescent materials.