Optical properties and electronic structure of Eu(III) complexes with HMPA and TPPO.

Abstract Two adducts of Eu(III) tris-hexofluoroacetylacetonate with HMPA (OP(N(CH3)2)3, hexamethylphosphotriamide) and TPPO (OP(C6H5)3, triphenylphosphine oxide) were studied by optical spectroscopy and quantum chemistry (DFT/TD-DFT). The structure of the higher occupied molecular orbitals (MO) of the two adducts determines differences in the position of the excitation band maximum of hfac ligands. According to the calculation data, all excited states are caused by the transition to 3 vacant π4* MOs of hfac ligands. Optical spectra of absorption, excitation and luminescence are obtained and interpreted. The peculiarity of the HOMO-LUMO structure, the low value of the energy gap, and the broadened absorption region of hfac ligands compensate the low absorbance ability of the groups N(CH3)2 in a region of 220–360 nm for the adduct Eu(hfac)3(HMPA)2, reducing the luminescence intensity by only 5–10% relative to the adduct of the Eu(III) complex with TPPO ligands.