Mechanofluorochromic behaviors and data security protection properties of salicylaldimine-based difluoroboron complexes with different aryl substituents.

To further explore the relationship between aryl substituents and mechanofluorochromic (MFC) behaviors, four salicylaldimine-based difluoroboron complexes (ts-Ph BF, ts-Ph-NA BF, ts-2NA BF, and ts-triphenylamine [TPA] BF), including aromatic substituents with different steric hindrance effects, were designed and successfully synthesized. Four complexes with twisted molecular conformation displayed intramolecular charge transfer and aggregation-induced emission properties. Under external mechanical stimuli, the as-synthesized powders of ts-Ph BF, ts-Ph-NA BF, and ts-TPA BF exhibited redshift fluorescence emission behaviors, and ts-Ph BF and ts-TPA BF could be recovered to original shifts by fuming, but ts-Ph-NA BF displayed irreversible switching. ts-2NA BF had no change during the grinding and fuming processes. The results indicated that the MFC behaviors could be attributed to the phase transformation between the well-defined crystalline and disordered amorphous states by X-ray diffraction measurement. Further research illustrated that ts-TPA BF with the most significant MFC phenomenon could be applied in data security protection in ink-free rewritable paper.