Role of alkyl chain length and branching of substituents in diglycolamides on the complexation of Nd³⁺ and Eu³⁺ ions in ionic liquids: Spectroscopic, electrochemical and theoretical investigations

The effect of alkyl chain branching of diglycolamide (DGA) ligands on the complexation of the trivalent lanthanides Nd³⁺ and Eu³⁺was investigated systematically in ionic liquid mediums using UV–Vis absorption spectroscopy, photoluminescence, cyclic voltammetry, infrared spectroscopy, etc. Density functional theory (DFT) studies were also carried out to explain the obtained results. The branched tetraalkyl DGA ligands used in this study are N,N,N′,N′-tetra-iso-propyl DGA(TIPDGA), N,N,N′,N′-tetra-iso-butyl DGA (TIBDGA), N,N,N′,N′-tetra-iso-amyl DGA (TIADGA), while the ionic liquid mainly used was C₆mim•NTf₂, some studies were also carried out with a lower and a higher alkyl chain ionic liquid, viz., C₄mim•NTf₂and C₈mim•NTf₂. A predominant presence of ML₁,ML₂ and ML₃ species was found with complexation constants following the trend: β_TIPDGA > β_TIBDGA > β_TIADGA, which was attributed to a steric effect. Judd-Ofelt parameters, which give a quantitative measure of the metal-O bond covalency were evaluated for the ligands as TIPDGA (Ω₂ = 2.81 × 10⁻²⁰ and Ω₄ = 1.17 × 10⁻²⁰), TIBDGA (Ω₂ = 2.72 × 10⁻²⁰ and Ω₄ = 1.13 × 10⁻²⁰), and TIADGA (Ω₂ = 2.94 × 10⁻²⁰ and Ω₄ = 1.14 × 10⁻²⁰). The excited state lifetime measurement for the luminescence studies revealed the formation of inner sphere complexes with the removal of all the water molecules from the primary coordination sphere of the Eu³⁺ ion. Upon complexation, the asymmetry around the Eu³⁺ ions enhanced which, however, remained almost similar for all the DGAs. The β values in the three different ionic liquids follow the trend:C₄mim•NTf₂ > C₆mim•NTf₂ > C₈mim•NTf₂. The exothermic nature of the complexation was evident by the trend in the β values obtained at different temperatures: β_{300 K} > β_{313 K} > β_{323 K}.