Dalton Transactions PAPER Cite this: Dalton Trans., 2015, 44, 182 Received 31st July 2014, Accepted 21st October 2014 DOI: 10.1039/c4dt02326a www.rsc.org/dalton Structural, optical and sensing properties of novel Eu(III) complexes with furan- and pyridine-based ligands† Fabio Piccinelli,* a Marco Bettinelli, a Andrea Melchior,* b Cristian Grazioli b and Marilena Tolazzi b A new family of imine and amine-based racemic ligands containing furan or pyridine as an aromatic donating ring [N,N’-bis(2-pyridylmethylidene)-1,2-(R,R + S,S)-cyclohexanediamine, L1; N,N’-bis(2-furanyl- methylidene)-1,2-(R,R + S,S)-cyclohexanediamine, L2; N,N’-bis(2-pyridylmethyl)-1,2-(R,R + S,S)-cyclo- hexanediamine, L3; and N,N’-bis(2-furanylmethyl)-1,2-(R,R + S,S)-cyclohexanediamine, L4] and their trifluoromethanesulphonate (CF 3 SO 3 - , OTf - ) and nitrate Eu(III) complexes is studied in acetonitrile (AN) solution. The stoichiometry and stabilities of the formed complexes are obtained by means of spectro- photometric titrations: when Eu(III) triflate is used as a starting salt, two mononuclear species (1 : 1 and 1 : 2) are detected, while only the 1 : 1 complex is observed when the nitrate salt is employed. The stability of these complexes, as well as the geometry of their Eu(III) environment, is significantly dependent on the nature of the ligand employed (imine or amine, furan or pyridine-based). DFT calculations show that all donor atoms are coordinated to the metal ion in the 1 : 1 EuL (L = L1–L4) species and suggest that the higher stability of the complexes with L1 and L2 with respect to L3 and L4 is mostly due to the higher degree of preorganization of the former species. The optical response of the imine-based L1 and L2 Eu complexes, produced by NO 3 - coordination, has been studied in order to assess their application as sensing devices. With both ligands, an increase of the emission intensity on the addition of the nitrate ion is observed. This is higher for the EuL2 complex and underlines the important role of the nature of the heteroaromatic ring. Finally, it is worth noting that an efficient energy transfer process from the ligand to the metal is present in the case of the 1 : 1 triflate Eu(III) complex with the ligand L1. Introduction Lanthanide complexes have been extensively used in the last few decades as luminescent chemosensors for medical diagnostics and optical cell imaging, 1–3 contrast reagents for magnetic resonance imaging, 4–6 shift reagents for NMR spec- troscopy, 7 as well as for applications in fundamental and applied science such as organic synthesis, bioorganic chem- istry and catalysis. 8 These applications have been made possible by the increased knowledge of fundamental properties (electronic, spectroscopic, thermodynamic, magnetic, and structural) of these ions, achieved as a consequence of the rapid develop- ment of fundamental studies on the lanthanide coordination chemistry. In this context, many solution studies have been carried out to obtain a detailed description of the speciation and formation thermodynamics of Ln(III) complexes in aqueous solutions 9,10 and organic solvents. 11–15 As far as biomedical imaging applications are concerned, Eu(III) (and Tb(III)) complexes exhibit several desirable charac- teristics when compared with conventional organic fluoro- phores, such as long excited state lifetimes (usually in the milliseconds range), large energy shift between absorbed and emitted radiations (in the case of ligand sensitization) and very narrow emission bands; these two effects allow the separ- ation of Ln(III) luminescence and the short-lived background fluorescence. 16 Eu(III) and Tb(III) complexes have also been extensively exploited as sensors of anions both in aqueous solution and in organic solvents. In this context, a possible approach to the anion recognition envisages the bonding of the anion at the metal center, displacing a coordinated solvent molecule or a weakly bound donor ligand. In the case of Eu(III) complexes the perturbation of the metal coordination environment can be evidenced by a change of the lumines- cence features of the metal ion. † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4dt02326a a Laboratorio Materiali Luminescenti, DB, Università di Verona, and INSTM, UdR Verona, Strada Le Grazie 15, 37134 Verona, Italy. E-mail: fabio.piccinelli@univr.it b Dipartimento di Chimica, Fisica e Ambiente, Università di Udine, via Cotonificio 108, 33100 Udine, Italy. E-mail: andrea.melchior@uniud.it 182 | Dalton Trans. , 2015, 44, 182–192 This journal is © The Royal Society of Chemistry 2015