Phenyl/benzoxazole-extended tetraphenylethene derivatives for reversible mechanochromism and acidichromism

Four tetraphenylethene-extended phenyl/benzoxazole compounds (TPEPBZs 1–4) were synthesized, which exhibited aggregation-induced emission behavior and multi-stimuli-responsive properties. Single-crystal structural analyses revealed that TPEPBZ-2 possessed the most planar conformation and a unique hydrogen-bonded organic framework structure, resulting in a redshifted emission. After being ground in an agate mortar, the morphologies of TPEPBZs 1–4 transformed from crystalline to amorphous state, and their luminescence was simultaneously shifted bathochromically to a green emission. It was proposed that the slight modification of substituents on the aggregation-induced emission luminogens (AIEgens) was a facile strategy to change their solid structures and further mechanochromic contrasts before and after grinding. The initial emission of TPEPBZs 1–4 could be returned upon fuming with dichloromethane vapor, indicating reversible mechanochromic properties. Moreover, TPEPBZs 1–4 exhibited significant acidichromic behavior. The emission of TPEPBZs 1–4 could change reversibly between their initial color and subsequent orange emission under trifluoroacetic acid and triethylamine stimuli. Finally, TPEPBZ-1 with the best mechanochromic and acidichromic properties was successfully applied to paper for anti-counterfeiting applications.