Optical spectroscopy of europium 3,5-dinitrosalicylates—Intense red luminophores

It was found, that alkali metal-europium dinitrosalicylates of composition M3Eu(3,5-NO2-Sal)3·nH2O (M = Li, Na, K, Cs) are intense red luminophores with wide excitation band. Using methods of optical spectroscopy we studied the influence of nitrogroups and alkali metal counterions on Eu3+ luminescence efficiency and on processes of excitation energy transfer to Eu3+ ion in compounds synthesized. The Eu3+ luminescence and Eu3+ luminescence excitation spectra, as well as vibrational IR and Raman spectra were investigated. Details of the structure of compounds were discussed. The network of hydrogen bonds in lanthanide dinitrosalicylates is weakening at introduction of large alkali metal ions in compounds and at the increase of the temperature. As a consequence, the long-wavelength shift of the intraligand charge transfer (ILCT) band in Eu3+ excitation spectra arises at inclusion of Cs+ cations instead of Li+ in the crystal lattice of europium dinitrosalicylates and at heating of these compounds. To obtain the energy of the lowest excited triplet state the phosphorescence spectra of alkali metal-gadolinium compounds M3Gd(3,5-NO2-Sal)3·nH2O, of alkali metal dinitrosalicylate and salicylate salts were measured with time delay. Change of the energies of ligand electronic states and ligand–metal charge transfer state (LM CTS) can give a two-three orders of magnitude enhancement of the Eu3+ luminescence efficiency in dinitrosalicylates in comparison with salicylates and ten-fold enhancement at the substitution of Li+ and Na+ for Cs+ in dinitrosalicylates.