Synthesis and Evaluation of Analyte Recognition Properties of Fluorenone Appended Schiff Bases

Abstract

Two novel Schiff base chemosensors 1 and 2 based on the 9-fluorenone hydrazone scaffold were designed, synthesised, and evaluated for the selective colorimetric detection of analytes that are both biologically and environmentally hazardous. The chemosensors 1 and 2 were prepared by the condensation of 9-fluorenone hydrazone with 3-methoxysalicylaldehyde and 1H-indole-3-carbaldehyde, respectively, and were characterised using ¹H NMR, ¹³C NMR, and HRMS. The evalution of photophysical properties of chemosensors 1 and 2 was done via colorimetric and UV–visible spectroscopic techniques. Upon the addition of cyanide (CN⁻) ions, the absorption band of chemosensor 1 at 345 nm underwent a bathochromic shift to 475 nm, in CH₃CN/H₂O (1:1, v/v), accompanied by a distinct colour change from colourless to yellow. The chemosensor exhibited excellent selectivity towards CN⁻ over competing anions and metal ions, with a limit of detection of 144 μM and a binding constant of 3.5 × 10³ M⁻¹. Additionally, chemosensor 2 was evaluated in CH₃CN/H₂O (9:1, v/v) and displayed selective recognition of diethyl chlorophosphate (DCP). The interaction with DCP produced a pronounced bathochromic shift in the absorption spectrum from 375 nm to 472 nm, along with a visible colour change from colourless to yellow, while no appreciable spectral changes were observed with other competing analytes. The binding constant of 2 was evaluated to be 6.2 × 104 M-1, with an LOD value of 18.75 μM.