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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