Naphthyl-imidazo-anthraquinones as novel colorimetric and fluorimetric chemosensors for ion sensing

• Novel heterocyclic systems as chemosensors bearing anthraquinone, imidazole and naphthyl moieties.
• Synthesis of novel napthyl-imidazo-anthraquinones through straightforward methods using commercially available reagents.
• Chemosensors for selective “naked-eye” detection of cyanide in organic–aqueous medium.
• Novel fluorescent and colorimetric chemosensors for anion and cation detection.
• Study of the binding process followed by spectroscopic and 1H NMR titrations.

Novel colorimetric and fluorimetric chemosensors for F−, CN− and OH− containing anthraquinone and imidazole as signalling/binding sites have been synthesised and characterised. Upon addition of F−, CN− and OH− to acetonitrile solutions of compounds 1 and 2, a marked colour change from yellow to pink was observed and the fluorescence emission of 1 was switched “on”, whereas for 2 there was a fluorescence quenching. Considering recognition in organic aqueous mixture, it was found that selectivity for CN− was achieved for both receptors, with an easily detectable colour change from yellow to orange. Moreover, sensors 1 and 2 showed good sensitivity with μM-level detection limit for cyanide in acetonitrile as well as in acetonitrile/water (9:1). Additionally, compounds 1 and 2 in their deprotonated form, after fluoride addition, were studied as metal ion chemosensors and displayed a drastic change from pink to yellow after metal ion complexation giving a yellow–pink–yellow, reversible colorimetric reaction and a “on-off-on” fluorescence in acetonitrile. The binding stoichiometry between the receptors and the anions and cations was found to be 1:1 and 2:1, respectively. The binding process was also followed by 1H NMR titrations which corroborated the previous findings.

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