Development of semicarbazide-based fluorene as an AIE-active fluorescent sensor for Cu2+ ions: a validation through experimental and DFT studies

Abstract

A fluorene-based fluorescent sensor, FHT, was rationally designed and synthesized, which exhibited prominent solvatochromic behavior and aggregation-induced emission (AIE) effects in aqueous medium. Upon aggregation, the FHT sensor showed an enhancement in emission intensity accompanied by a bathochromic shift, indicating the formation of J-aggregates. Owing to its unique photophysical properties, FHT was developed as a highly selective fluorescent sensor for Cu²⁺. Selective binding of FHT to Cu²⁺ resulted in significant fluorescence quenching via a photo-induced electron transfer (PET) process. The binding interactions of FHT with Cu²⁺ were studied in detail by ¹H NMR titration, liquid chromatography-mass spectrometry (LC-MS), dynamic light scattering (DLS), Fourier-transform infrared (FTIR) spectroscopy, and density functional theory (DFT) analysis. Upon interaction with Cu²⁺, the FHT sensor exhibited strong fluorescence quenching with a low detection limit of 82 nM and quenching efficiency of 97%. In addition to its solution-phase sensing capability, FHT demonstrated effective performance in the solid state and enabled naked-eye colorimetric identification of Cu²⁺. Furthermore, the FHT sensor was successfully integrated into molecular logic-gate operations, emphasizing its promise for on-site environmental monitoring and the development of molecular electronic devices.