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Two azo dyes, ( E )-3-(benzo[ d ]thiazol-2-yldiazenyl)-4-hydroxy-2 H -chromen-2-one (3CBD) and ( E )-4-hydroxy-3-(quinolin-2-yldiazenyl)-2 H -chromen-2-one (3CQD), were designed and synthesized using facile methods. The structures were validated through FTIR and NMR spectroscopy. The photophysical property analyses were further studied using UV-Vis and fluorescence spectrophotometers. Consequently, the absorption and emission spectra of 3CBD confirmed its selectivity of Hg 2+ and turn-off response to Fe 3+ . On the other hand, the absorption spectra analysis of 3CQD demonstrated selectivity in the presence of Hg 2+ . The colorimetric investigations demonstrated a significant visual response specifically for Hg 2+ , enabling real-time analysis in the corresponding solutions. The presence of other coexisting metal ions does not interfere with the detection of the target metal ion. The fluorescence studies of the two probes revealed that 3CBD was highly fluorescent, which was significantly quenched by Fe 3+ , upon excitation at 340 nm. Utilizing Job plot analyses, it was determined that the complexes 3CBD-Hg 2+ and 3CQD-Hg 2+ exhibit a binding stoichiometry of 1 : 1. The association constants for these complexes were measured to be 7.48 × 10 5 and 9.12 × 10 5 M -1 , respectively, indicating a strong association between both probes and their respective metal ions. Both chemosensors exhibited comparable limits of detection (LOD) and limits of quantification (LOQ) of 0.03 μM and 0.10 μM, respectively. Reversible studies confirmed that only chemosensor 3CQD could serve as a secondary sensor for EDTA. The theoretical studies calculated using Density Functional Theory (DFT) program at B3LYP/6-31G** (Spartan '10 package) level., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

A ditopic multi-colorimetric probe based on the phenylpridyl-thioic moiety (EN) was synthesized via a Schiff base reaction mechanism and characterized using 1 H NMR and UV-vis spectroscopy. The colorimetric analyses carried out revealed that EN was capable of discriminating between a number of heavy metal cations via coordination induced charge transfer, as well as between anions through hydrogen bonding induced charge transfer, in DMSO-H 2 O (9 : 1). In particular, the ditopic probe could spectrally and colorimetrically recognize the most toxic heavy metal cations of Cd 2+ , Pb 2+ and Hg 2+ , among others, in DMSO-H 2 O. Additionally, EN was selective and sensitive to the presence of CN - , F - , AcO - and H 2 PO 4 - in the same solvent system as cations. The reversibility and reproducibility studies showed that EN exhibited complementary IMP/INH logic functions, based on colour and spectral switching (ON/OFF), modulated by F - /Al 3+ . The real time application of the probe was tested on food grade products to detect the presence of F - in toothpastes and mouthwash dissolved in water, as well as cations in underground water (normally saline), which displayed substantial responses. Thus, EN displayed an excellent scope of response and can thus be developed for real time sensing kits, which could be used instantly in on-field analysis. Theoretical studies were conducted to complement the experimental work., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)