A Synergy Between Push-Pull Electronic Effect and Twisted Conformation for High-Contrast Mechanochromic AIEgens

Mechanochromic (MC) luminogens in response to external stimulus have shown promising applications as pressure sensors and memory devices. While, research on their underlying mechanism is still in the initial stage. Here, three pyridinium-functionalized tetraphenylethylenes bearing n-pentyloxy group, hydrogen, nitro group, namely TPE-OP, TPE-H and TPE-NO, are designed to systematically investigate the influence of push-pull electronic effect and molecular conformation on MC luminescence. Upon anisotropic grinding and isotropic hydrostatic compression, TPE-OP with strong intramolecular charge transfer (ICT) affords the best MC behavior among them. Analysis of three polymorphs of TPE-H clearly indicates that planarization of molecular conformation is responsible for their bathochromic shift under mechanical stimuli. Theoretical calculations also verify that high twisting stress of AIEgens can be released under high pressure. This study presents a clear picture for MC behaviour and proposes an effective strategy to achieve high-contrast MC luminescence.