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 [I(5D0-7F2) /I(5D0-7F1)] as "hypersensitive ratio", with regard to the luminescence in the visible spectral range of Eu(III) ion, stemming from the 5D0 level. We demonstrate, by means of computational calculations, that R2 directly connected to the 522 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 sp2 hybridized nitrogens of the iminic ligand involved in the resonance phenomenon together with the heteroaromatic pyridine rings.