Design, synthesis, and applications of Naphthalimide-based Mechanofluorophores for multimodal security encryption and...

Two novel donor-acceptor (D-A) type naphthalimide-based π-conjugated mechanofluorophores, NMF1 and NMF2, exhibiting solvatochromic, aggregation-caused quenching (ACQ), and reversible mechanofluorochromic (MFC) properties, were designed and synthesized. A systematic investigation of their solvatochromic behavior revealed bathochromic shifts in absorption maxima (λ_abs from 411 to 441 nm (NMF1) and 415 to 444 nm (NMF2)) across a series of solvents of increasing polarity. Correspondingly, fluorescence emission maxima (λ_em) exhibited significant redshifts, accompanied by diminished emission intensity and quantum yield based on increasing solvent polarization. Aggregation-induced emission (AIE) studies showed that raising the water portion (fw) from 0 to 90 % caused bathochromic changes in both λ_abs and λ_em and a significant decrease in emission intensity (approximately 85 % for NMF1 and 55 % for NMF2) compared to the initial intensity in pure DMF. This AIE process is attributed to the formation of crystalline aggregates, evidenced by morphological changes in SEM and DLS. Furthermore, mechanical stimulation of the pristine NMF1 and NMF2 induced a striking mechanochromic transition from a bright greenish-yellow to a yellow emissive state. This transition was reversible, as exposure to diethyl ether vapor restored the original greenish-yellow emission. A powder X-ray diffraction (PXRD) study elucidated the mechanism of this mechanochromism, revealing that the color change is related to a change in molecular packing order induced by mechanical grinding. Finally, the potential of these mechanofluorophores for practical applications was demonstrated through inkless writing and solvent vapor-based erasing, highlighting their promise for advanced anti-counterfeiting strategies.