Cationic tetraphenylethylene-based AIE-active acrylonitriles: investigating the regioisomeric effect, mechanochromism, and wash-free bioimaging

Developing regioisomeric organic fluorescent materials with tunable photophysical properties is particularly important in diverse applications. Here we synthesized three cationic regioisomeric aggregation-induced emission (AIE)-active acrylonitriles (o-TPE-ANPy⁺, m-TPE-ANPy⁺, and p-TPE-ANPy⁺) by conjugation with tetraphenylethylene at different ortho/meta/para positions. The regioisomerism remarkably impacted their photophysical characteristics and solid-state intermolecular interactions. The regioisomeric AIE luminogens (AIEgens) exhibited faint emissions in solution but boosted fluorescence behaviors in the solid state. o-TPE-ANPy⁺ and m-TPE-ANPy⁺ showed similar yellow solid-state emissions with a fluorescence quantum yield (QY) of 14.3% and 1.8%, respectively. However, due to the strong intermolecular charge transfer effect, p-TPE-ANPy⁺ exhibited a red-shifted solid-state emission of 616 nm (QY of 17.2%). In addition, m-TPE-ANPy⁺ and p-TPE-ANPy⁺ exhibited larger fluorescence enhancements toward viscosity than o-TPE-ANPy⁺. Moreover, o-TPE-ANPy⁺ was applied for achieving reversible mechanochromic performance with fluorescence changing from yellow to orange. Further use of these biocompatible AIEgens for wash-free and light-up imaging of mitochondria was successfully demonstrated. These interesting results facilitate a better understanding of the structure-property relationships in fluorescent regioisomers.