Effect of the ligand polarizability on the hypersensitive 5D0→7F2 transition of Eu(III) in nitrate complexes with nitrogen-based ligands

In this contribution, the f-f spectroscopy of two racemic nitrate Eu(III) complexes containing an imine-based ( i.e. [N,N’-bis(2-pyridylmethylidene)-1,2-(R,R)+(S,S)-cyclohexanediamine]) ( complex 1 ) and an amine-based ( i.e. [N,N’-bis(2-pyridylmethylene)-1,2-(R,R+S,S)-cyclohexanediamine]) ( complexes 2 ) ligands has been revisited, in the light of the recent redefinition of the “asymmetry ratio” R2 [ as “hypersensitive ratio”, with regard to the luminescence in the visible spectral range of Eu(III) ion, stemming from the 5 D 0 level. We demonstrate, by means of computational calculations, that R2 directly connected to the Ω 2 intensity parameter, in turn dominated by a dynamic ligand polarization mechanism (dynamic coupling, DC), is significantly affected by the polarizability of the oxygen and nitrogen ligating atoms, more than by the geometric environment around Eu(III). The larger R2 value of complex 1 is ascribable to the lower polarizability of the sp 2 hybridized nitrogens of the iminic ligand involved in the resonance phenomenon together with the hetero-aromatic pyridine rings. • f-f spectroscopy of two racemic nitrate Eu(III) complexes containing tetradentate chiral nitrogen ligands has been revisited • “hypersensitive ratio” R2 is not a simple marker of Eu(III) site/point symmetry • R2 is significantly affected by the polarizability of the donating atoms of the ligands. • Polarizability of the donating atoms is closely connected with their hybridization ( sp 2 or sp 3 )