Star-Shaped ESIPT-Active Mechanoresponsive Luminescent AIEgen and Its On–Off–On Emissive Response to Cu²⁺/S^2–

Design and development of multifunctional materials have drawn incredible attraction in recent years. Herein, we report the design and construction of versatile star-shaped intramolecular charge transfer (ICT)-coupled excited-state intramolecular proton transfer (ESIPT)-active mechanoresponsive and aggregation-induced emissive (AIE) luminogen triaminoguanidine-diethylaminophenol (LH₃) conjugate from simple precursors triaminoguanidine hydrochloride and 4-(N,N-diethylamino)­salicylaldehyde. Solvent-dependent dual emission in nonpolar to polar protic solvents implies the presence of ICT-coupled ESIPT features in the excited state. Aggregation-enhanced emissive feature of LH₃ was established in the CH₃CN/water mixture. Furthermore, this compound exhibits mechanochromic fluorescence behavior upon external grinding. Fluorescence microscopy images of pristine, crystal, and crushed crystals confirm the naked-eye mechanoresponsive characteristics of LH₃. In addition, LH₃ selectively sensed a Cu²⁺ ion through a colorimetric and fluorescence “turn-off” route, and subsequently, the LH₃-Cu²⁺ ensemble could act as a selective and sensitive sensor for S^2– in a “turn-on” fluorescence manner via a metal displacement approach. Reversible “turn-off–turn-on” features of LH₃ with Cu²⁺/S^2– ions were efficiently demonstrated to construct the IMPLICATION logic gate function. The Cu²⁺/S^2–-responsive sensing behavior of LH₃ was established in the paper strip experiment also, which can easily be characterized by the naked eye under daylight as well as a UV lamp (λ = 365 nm).