Dual Metal Ion Sensing by an Azo Compound Derived from Thiazolo[3,2-a] Indole: A Selective and Sensitive Approach

In this study, we report the synthesis and photophysical evaluation of hydrazone 7 and azo 9 compounds of a thiazolo[3,2-a] indole derivative 5; latter synthesized as per our earlier report. Of the two compounds, the azo compound 9 showed exceptional selectivity and sensitivity as a dual sensor for detecting Fe²⁺ and Cu²⁺ ions via a turn-off fluorescence mechanism with limit of detection 0.09 µM and 0.14 µM respectively which are much lower than the US Environmental Protection Agency (EPA) guideline, 5.36 µM for Fe²⁺ and WHO guideline, 31.5 µM for Cu²⁺ in the drinking water. DFT calculations revealed that both Fe²⁺ and Cu²⁺ complexes with compound 9, and possess low HOMO-LUMO gap of 1.63 eV and 2.94 eV respectively. The binding stoichiometry for both metals was determined to be 1:1 by Job's plot. Stern-Volmer plot (plotted after applying correction to emission intensity), which showed a linear pattern, and fluorescence lifetime measurements indicated primarily a static quenching mechanism for both ions. The chemosensor 9 demonstrated exceptional reversibility and restorability in detecting both Fe²⁺ and Cu²⁺, highlighting its potential for practical applications.