Luminescent Eu3+ complexes in acetonitrile solution: Anion sensing and effect of water on the speciation

In this paper the trifluoromethansulphonate (CF3SO3-, OTf-, triflate) Eu3+ complexes of a new family of imine-based ligands with 1:1 ligand to metal mole ratio have been employed for luminescence sensing of NO3-, Cl-, Br-, I-, CH3COO- and ClO4-, in non-anhydrous acetonitrile (wet AN = wAN) solution. The ligands show different donor ability and stereochemistry. In particular, they consist of the trans racemic isomers containing pyridine or furan as donating ring [N,N'-bis(2-pyridylmethylidene)-1,2-(R,R+S,S)-cyclohexanediamine, L1; N,N'-bis(2-furanylmethylidene)-1,2-(R,R+S,S)-cyclohexanediamine, L2 and the cis (meso) isomer containing the pyridine ring N,N'-bis(2-pyridylmethylidene)-1,2-(R,S)-cyclohexanediamine, L3]. As far as the sensitivity and the selectivity towards the different anions are concerned, the donor ability of the heteroaromatic ring within the ligand backbone, plays a crucial role: the poorly donating furan-based ligand L2 shows the best sensitivity and selectivity towards nitrate. On the other hand, the stereochemistry of the pyridine-based ligand shows a significant effect on the bromide sensing. The effects of the water content on the stability of these complexes have been also studied. Even though the speciation in non-anhydrous is the same as in anhydrous AN (1:1 EuL and 1:2 EuL2 species), their stability is significantly lower. The reasons for this behavior are proposed to be mainly found in the different solvation of both Eu3+ ion and the ligands. (C) 2016 Elsevier B.V. All rights reserved.