Thiophene-Appended Benzothiazole Compounds for Ratiometric Detection of Copper and Cadmium Ions with Comparative Density Functional Theory Studies and Their Application in Real-Time Samples

A thirst for the development of a simple fluorescence probe for enhanced sensing application has been achieved by synthesizing a stupendous thiophene-appended benzothiazole-conjugated compound L2. The synthesized compound L2 was characterized using nuclear magnetic resonance and mass spectrometry techniques. Furthermore, a photophysical property of L1 and L2 reveals the enhanced emission spectrum of L2 because of a restricted spin–orbital coupling as a result of increased conjugation compared to the ligand L1. Therefore, comparative studies were undertaken for L1 and L2. Henceforth, L2 was deployed for the ratiometric detection of Cd²⁺ ions in THF:water and L1 for the detection of Cu²⁺ ions in THF medium. The chemosensor L2 shows an outstanding water tolerance up to 60% and is stable between pH 2 and 7. This level of water tolerance and stability make L2 a suitable probe for analyzing real-time and biological samples. While the cadmium ion was added to L2, there was a significant red shift in emission from 496 to 549 nm, which indicates the controlled ICT due to complex formation. The metal–ligand complexation was also confirmed by noticing a decreased band gap of metal complex compared to the ligand as calculated using Tauc’s plot with solid-phase UV data. The stoichiometric ratio was obtained by Job’s plot that exhibited a 1:1 ratio of L2 and Cd²⁺ ions, and the limit of detection (LOD) was found to be 2.25 nM by the photoluminescence spectroscopic technique. The fluorescence lifetime of both L2 and L2-Cd²⁺ was found to be 58.3 ps and 0.147 ns, respectively. Alongside, the colorimetric-assisted ratiometric detection of Cu²⁺ by L1 with 1:2 stoichiometric ratio having an LOD of 1.06 × 10^–7 M was also performed. Furthermore, the practical applicability of the probe L2 in sensing cadmium was tested in sewage water and vegetable extract; the recovery was approximately 98 and 99%, respectively. The experimental data were supported by theoretical investigation of structures of L1, L2, L1-Cu²⁺, and L2-Cd²⁺, complex formation, charge transfer mechanism, and band gap measurements done by quantum chemical density functional theory calculations.