New AIE+ESIPT β-Naphthol Azine-based Chemosensor of Cu2+ ions, An Experimental and In Silico Analysis

A straightforward imine condensation reaction successfully yielded a novel and promising fluorescent chemosensor for Cu2+ ions, derived from the β-naphthol azine moiety, with a 26% chemical yield of a yellowish solid. Subsequent spectroscopic analyses employing FTIR, ToF-HRMS, 1H-NMR, and 13C-NMR elucidated the structure as 1-((((4-bromothiophen-2-yl)methylene)hydrazono)methyl)naphthalen-2-ol (1). The photophysical properties of the compound were characterized by a maximum UV/Visible absorption in a DMF:H2O (1:9, v/v, c: 1.0×10-5 mol dm-3) at 414 nm (ε: 6.80×103 mol-1 dm3 cm-1) and emission at 565 nm (λex 360 nm), exhibiting a significant Stokes shift of 151 nm. Furthermore, at elevated concentrations (1.0×10-4 mol dm-3), the compound exhibited aggregation-induced emission (AIE) in response to increased water content in the organic solvent mixtures. The compound functioned as a moderately reversible on-off chemosensor for Cu2+, demonstrating a 1:1 molar ratio binding mode and a 96% quenching efficiency ((ϕem of 1 = 0.252 ± 0.030; 1+Cu2+ = 0.012 ± 0.004), as determined by a sensing experiment of 1 against various cations in DMF:H2O (1:9, v/v). The Limits of Detection (LoD) and Quantification (LoQ) for 1 against Cu2+ ions were identified as 0.606 µM and 1.836 µM, respectively. Additionally, computational studies utilizing DFT and TD-DFT, accompanied by frontier molecular orbital (FMO) analysis, confirmed the presence of excited-state intramolecular proton transfer (ESIPT) in 1, which was further inhibited upon chelation with Cu2+ ions.

HIGHLIGHTS

Aggregation-induced emission (AIE) of Schiff base b-naphthol azine

A prominent Stokes shift probe (151 nm)

Cu2+ ions chemosensor in an aqueous system

ESIPT Inhibition of Cu2+ ions chemosensor

Chelating quenched fluorescence (CHQF) chemosensor (Dϕem = 96%)

GRAPHICAL ABSTRACT