Aggregation induced emission and mechanochromism in pyrenoimidazoles

Pyrene-based solid state emitters 3a and 3b were designed and synthesized by the Pd-catalyzed Suzuki cross-coupling reaction of bromopyrenoimidazole 2 with 4-(1,2,2-triphenylvinyl)phenylboronic acid pinacol ester and 2-(4-pinacolatoboronphenyl)-3,3-diphenylacrylonitrile. The single crystal X-ray structure of 3a was reported and revealed the twisted conformation. Their photophysical, aggregation induced emission (AIE) and mechanochromic properties were studied. Pyrenoimidazoles 3a and 3b exhibit strong AIE. 3b shows different colored emission with varying water fraction. 3a and 3b exhibit reversible mechanochromic behavior with color contrast between blue and green. The enhanced conjugation and increased amorphous nature observed after grinding are associated with mechanochromism in pyrenoimidazoles 3a and 3b.