Regulation of the luminescence efficiency of europium benzoates by -OH and -NO^sub 2^ substituents and alkali metal counterions

Methods for the design of visible-light excitable europium nitrosalicylates having a high efficiency and "monochromaticity" of the luminescence are proposed. A series of europium-alkali metal 3,5-dinitrosalicylates and 5-nitrosalicylates with the general formula M5Eu(Lig)4-nH2O (M = Li+, Na+, K+, Cs+), as well as 3-nitrosalicylates, was investigated using optical spectroscopy. The optimization of relative energies of the lowest excited singlet and triplet states of the ligands to achieve the best photophysical characteristics is provided by the proper arrangement of -OH and -NO2 groups on the aromatic ring and a suitable choice of the counterion. The excitation energy transfer processes to the Eu3+ ion were analyzed. The effect of the triplets lying in close proximity to the 5D0 state for 5- and 3-nitrosalicylates and to the 5D1 state for 3,5-dinitrosalicylates on the Eu3+ luminescence efficiency was demonstrated. The europium-lithium 3,5-dinitrosalicylate has an optimum triplet position relative to the 5D1 state to supply the most efficient energy transfer to the 5D0 state of Eu3+ ion and the brightest red emission. The luminescence intensity of this compound exceeds that for the europium benzoate ~60 times at 77 K and ~25 times at 295 K. Thermal quenching properties of the compounds shown by the temperature behavior of the luminescence intensity and the 5D0 lifetime are entirely similar. The proper combination of the sizes of the Ln3+ ion, ligand and counterion is necessary for the formation of a perfect structure of the complex with the most symmetrical Eu3+ luminescence center. It was found that the compound Li5Eu(5-NSal)4-4H2O is characterized by an intense "monochromatic" red-light emission with the 7 cm-1 half-width for the 612.2 nm line related to the 5D0-7F2 transition.