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A new hydroxy-benzene modified thiazole-azo derivative was synthesized and its chemosensing towards heavy metal ions (Na + , Al 3+ , K + , Ca 2+ , Mn 2+ , Fe 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Ag + , Cd 2+ , Hg 2+ and Pb 2+ ) was characterized by UV–vis spectroscopy. The newly obtained compound produced cation-induced bathochromic shifts for Hg 2+ ion more than the other tested metal ions in acetonitrile solution. The interactions with cations were further studied by colorimetric, UV–vis and NMR studies. The presence of the hydroxyl moiety along with the electron rich nitrogen of thiazole ring acting as the binding site and the azo group acting as a signaling unit enables the system to display excellent sensitivity for Hg 2+ with a color change from light orange to red purple. The stoichiometric ratio and the association constant for the complex between Hg 2+ and the target compound assume the values of 1:1 and 2.1 × 10 5 mol −1 dm 3 , respectively.

A new chalcone-based probe containing coumarin and naphthol at both ends has been synthesized via aldol condensation. The uniqueness of the newly derived probe can be ascribed to the presence of naphthol and coumarin units acting as binding site and signaling element, respectively. The fluorogenic behaviors toward various anions were investigated. The probe was characterized by various spectroscopic techniques and the in-depth study led to show excellent selectivity and sensitivity for fluoride ions. The hydrogen bonding thus formed with fluoride anion provides remarkable fluorometric responses. The interactions of the probe with fluoride ions were determined by fluorescence, FT-IR and NMR spectroscopic techniques. The exploratory studies by fluorescent spectral changes augur well for the naked-eye sensing applications.

A new azobenzene chromophore (AZQU), appended with a bis-quinoline unit as a turn-on chemosensor for selective detection of Cd2+ ions, has been designed and synthesized. The fluorescent receptor AZQU has an excellent selectivity and sensitivity towards Cd2+ ions over other metal ions in CH3CN:H2O (80:20, v/v) solution. AZQU is red colored and non-fluorescent in the absence of Cd2+ ions, but upon addition of Cd2+ ions it turns colorless with appreciable fluorescence. This “off–on” type signaling behavior is attributed to the CHEF (chelation-enhanced fluorescence) effect of the quinoline unit and the change in color from red to yellow occurs by an ICT process with Cd2+. The association constant for AZQU + Cd2+ was calculated to be 1.68 × 105 L·mol−1 with binding in the 1:1 stoichiometric ratio.

A bis-pyridine unit containing azobenzene chemosensor for Cu2 + ion is designed and synthesized with good yield. The structure of the receptor I is determined by FT-IR, 1H NMR, 13C NMR, ESI-MS and single crystal XRD. The receptor I is constructed on the basis of internal charge transfer (ICT) mechanism with bis-pyridine unit acting as the binding part for Cu2 + ion. In solution, the proposed receptor I produces a cation induced 120 nm blue shift for Cu2 + ion from 457 nm to 337 nm with remarkable colour change from red to colourless. Whereas no significant colour change is observed upon addition of other metal ions in aqueous HEPES buffer (pH 7.0). Moreover, spectroscopic studies confirm the formation of 1: 1 stoichiometry between the receptor I and Cu2 + ion with an association constant of ca. 1.45 × 106 M− 1. The receptor I is highly specific to Cu2 + ions in aqueous solution attributed to the rational design of the molecular structure.

A Cu2+-specific colorimetric sensor of bis-triazole-appended azobenzene receptors I and II was designed and synthesized. Receptor II shows high selectivity toward Cu2+ in acetonitrile/water (80:20, v/v) solution based on the internal charge transfer (ICT) mechanism. The tridentate coordination behavior was proposed with 1:1 stoichiometry between receptor II and Cu2+. Herein, the Cu2+ gives rise to a large change in the absorption spectra (from red to pale yellow) that is clearly visible to the naked eye.

A new rhodamine-based fluorescent turn-on chemosensor (L) for selective detection of Al3+ ion has been developed and characterized. The fluorescent chemosensor L was synthesized by the reaction of intermediate (4) with 2,5-bis (4-phenylacyl chloride)-1,3,4-oxadiazole (3). The chemosensor L displays an excellent selective and sensitive response to Al3+ ion over other metal ions, in which the spirocyclic (non-fluorescent) to ring opened amide (fluorescent) process was utilized and a 1:2 stoichiometry for L–Al3+ complex was formed with an association constant of 2.03 × 103 M−1. Furthermore, chemosensor L can be applied as a fluorescent probe for monitoring Al3+ in living cells by performing cell imaging studies.