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.

Automated summary

Four tetraphenylethene-extended phenyl/benzoxazole compounds (TPEPBZs 1-4) were synthesized, which exhibited aggregation-induced emission behavior and multi-stimuli-responsive properties. The modification of substituents on the aggregation-induced emission luminogens (AIEgens) can change their solid structures and further mechanochromic contrasts before and after grinding. Moreover, TPEPBZs 1-4 showed significant acidichromic behavior and the emission could change reversibly between their initial color and subsequent orange emission under stimuli.