Squaraine-based AIEgens for reversible mechanochromism, sensitive and selective hypochlorite detection and photostable far-red fluorescence cell imaging

In this work, three new squaraine-based aggregation induced emission luminogens (AIEgens),TPE-SQ1-3, have been successfully developed with different functions by using TPE-functionalized diarylamine derivatives as the critical end-capping subunits. In particular, the powders of symmetricTPE-SQ1andTPE-SQ2exhibit reversible mechanochromic (MC) characteristics with a clear emission color change from yellow to orange-red, due to the morphology change from the loose crystalline state to the tight amorphous state. Differently, using indolenine as another end-capping subunit, the resulting asymmetricTPE-SQ3can exhibit intense far-red solid state emission, but does not show any mechanochromic characteristics, attributed to its amorphous nature in the pristine state. However, it can encouragingly serve as a reaction-based fluorescent probe to detect hypochlorite sensitively and selectively, and the detection limit is as low as 105 nM. The sensing mechanism is based on the oxidation cleavage of electron-deficient CC double bonds connecting to the indolenine side. Furthermore, amphiphilic polymer encapsulatedTPE-SQ3nanoparticles have further proven to be good live-cell imaging agents, providing bright intracellular far-red fluorescence with high photostability. Thus, our work has shown that integrating AIE characteristics into squaraines is an efficient design strategy to afford new functional materials for optoelectric and biological applications.