Structure and luminescent investigation of new Ln(III)-TTA complexes containing N-methyl-ε-caprolactam as ligand Alex Santos Borges a,n , Ewerton Valadares Caliman b , José Diogo L. Dutra c , Jeferson G. Da Silva d , Maria Helena Araujo e,n a Coordenadoria de Química e Biologia, IFES, Vitória, ES 29040-780, Brazil b Coordenadoria de Engenharia Metalúrgica, IFES, Vitória, ES 29040-780, Brazil c Departamento de Química Fundamental, UFPE, Recife, PE 50590-470, Brazil d Departamento de Farmácia, UFJF, Governador Valadares, MG 35010-17, Brazil e Departamento de Química, UFMG, Belo Horizonte, MG 31270-901, Brazil article info Article history: Received 20 January 2015 Received in revised form 6 June 2015 Accepted 9 July 2015 Available online 17 July 2015 Keywords: Lanthanides 3-Thenoyltrifluoroacetonate N-methyl-ε-caprolactam Crystalline structure Luminescence quantum yield abstract The synthesis and photoluminescent properties of Ln(III)-TTA complexes (Ln ¼Eu(III) and Sm(III) ions; TTA ¼3-thenoyltrifluoroacetonate) with N-methyl-ε-caprolactam (NMC) are reported. The Ln complexes were characterized by elemental analysis, complexometric titration with EDTA and infrared spectroscopy. The molecular structures of the [Eu(TTA) 3 (NMC)(H 2 O)] and [Sm(TTA) 3 (NMC)(H 2 O)] Á H 2 O compounds were determined by single crystal X-ray crystallography. In these structures, the three TTA molecules are coordinated to the metal in anionic form as bidentate ligands, while the H 2 O and NMC molecules are coordinated to the metal in neutral form as monodentated ligands. The coordination polyhedron around the Ln(III) atom can be described as square antiprismatic molecular geometry. The geometry of the [Eu(TTA) 3 (NMC)(H 2 O)] complex was optimized with the Sparkle/RM1 model for Ln(III) complexes, allowing analysis of intramolecular energy transfer processes of the Eu(III) compound. The spectroscopic properties of the 4f 6 intraconfigurational transitions of the Eu(III) complex were then studied experi- mentally and theoretically. The low value of emission quantum efficiency of 5 D 0 emitting level (η) of Eu (III) ion (ca. 36%) is due to the vibrational modes of the water molecule that act as luminescence quenching. In addition, the luminescence decay curves, the experimental intensity parameters (Ω λ ), lifetimes (τ), radiative (A rad ) and non-radiative (A nrad ) decay rates, theoretical quantum yield (q cal ) were also determined and discussed. & 2015 Elsevier B.V. All rights reserved. 1. Introduction Since long time trivalent lanthanides ions, Ln(III), are becoming increasingly attractive in coordination chemistry. This interest is not only due their chemical properties that differ from those presented by the transition metal ions, but also because of their unique magnetic and spectroscopic properties [1–3]. When a lanthanide ion is coordinated to organic ligands, many of their properties are preserved owing to the shielding of 4f orbitals from the environment by the outer 5s and 5p orbitals, causing the minimal perturbation by the external field generated by the ligands [4,5]. For example, lanthanide compounds are known to exhibit narrow emitting and absorption bands. The applications of Ln(III) ions are many, such as in catalysis, as active components in organic light emitting diodes (OLEDs) and in optical fibers for data transmission, as magnetic materials, glasses, lasers, etc. [6–9]. In biological systems the use of Ln(III) ions has attracted a great deal of interest and are usually used as lumi- nescent probes in the investigations of binding sites in proteins and other biomolecules, labels in immunoassays and in non- invasive tests [10]. Lanthanide-based luminescence is usually sensitized by energy transfer from a nearly strongly absorbing chromophoric antenna group since the narrow 4f–4f transitions are parity-forbidden. The main role of the organic ligands is to collect the photons provided by the light source in order to allow an efficient energy transfer to the emitting levels of the Ln(III) ion. It is generally accepted that the energy transfer from ligand to Ln(III) ion in complexes occurs via: (a) strong absorption from the ground state to the excited singlet state (S 0 -S 1 ) of the ligand; (b) singlet state decays non- Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jlumin Journal of Luminescence http://dx.doi.org/10.1016/j.jlumin.2015.07.010 0022-2313/& 2015 Elsevier B.V. All rights reserved. n Corresponding authors: Tel.: þ55 27 3331 2198; fax: þ55 27 3331 2110. E-mail addresses: alexb@ifes.edu.br (A.S. Borges), maria.araujo@pq.cnpq.br (M.H. Araujo). Journal of Luminescence 170 (2016) 654–662