Precise peripheral design enables propeller-like squaraine dye with highly sensitive and wide-range piezochromism

Piezochromic fluorescent (PCF) materials that feature high sensitivity and wide-range switching are attractive in intelligent optoelectronic applications but their fabrication remains a significant challenge. Here we present a propeller-like squaraine dye SQ-NMe(2) decorated with four peripheral dimethylamines acting as electron donors and spatial obstacles. This precise peripheral design is expected to loosen the molecular packing pattern and facilitate more substantial intramolecular charge transfer (ICT) switching caused by conformational planarization under mechanical stimuli. As such, the pristine SQ-NMe(2) microcrystal exhibits significant fluorescence changes from yellow (λ(em) = 554 nm) to orange (λ(em) = 590 nm) upon slight mechanical grinding and further to deep red (λ(em) = 648 nm) upon heavy mechanical grinding. Single-crystal X-ray diffraction structural analysis of two SQ-NMe(2) polymorphs provides direct evidence to illustrate the design concept of such a piezochromic molecule. The piezochromic behavior of SQ-NMe(2) microcrystals is sensitive, high-contrast, and easily reversible, enabling cryptographic applications.