Structural regularities and luminescence properties of dimeric europium and terbium carboxylates with 1,10-phenanthroline (C.N. = 9)

Abstract

A systematic comparison of the spectroscopic and structural data for series of related dimeric lanthanide carboxylates with 1,10-phenanthroline Ln(RCOO) 3·Phen (Ln = Eu, Gd, Tb): 1-naphthylcarboxylates, benzoates, 2-furancarboxylates, phenoxyacetates, caproates, acetates, propionates, 3-nitropropionates was undertaken in search for methods of regulation of quantum yield of the Ln 3+ luminescence and for paths of the control of excitation energy transfer. The crystal structures of europium propionate and europium and terbium 3-nitropropionates were solved by X-ray diffraction methods. Effect of stepwise distortions of Ln coordination polyhedron derived from X-ray data of the succession of compounds, which determine details of the charge distribution in the surroundings of the Ln 3+ ion, on luminescence spectra, on lifetimes of Ln 3+ states, and on efficiency of Ln 3+ luminescence was investigated. Polyhedron distortions are to a great extent caused by the range of the Ln–O bond lengths related to bridging-cyclic carboxylic group that is conditioned by the type and the size of carboxylate anion. Relative contributions of the rates of radiative and various nonradiative processes to the lifetimes of europium 5D 0 and terbium 5D 4 metastable states were estimated. Multiphonon relaxation is the main nonradiative process in europium compounds at temperatures from 77 to 295 K and in terbium compounds at low temperatures, but the radiative processes are appreciably prevailing. Dependence of the lifetimes due to radiative processes determined from the integral intensity of europium 5D 0– 7F J transitions related to the intensity of magnetic-dipole 5D 0– 7F 1 transition on distortions of Eu polyhedron in the sequence of compounds is in agreement with the same dependence obtained from the measured lifetimes, if nonradiative processes are taken into account. A back energy transfer from 5D 4 state of Tb 3+ ions to the lowest triplet state related to Phen molecule in most of compounds studied is another nonradiative process contributing to quenching in terbium compounds at high temperatures. An influence of this process on luminescence intrinsic quantum yield can be predominant. It leads to abrupt lowering the luminescence intensity of some terbium carboxylates, for example, benzoate at increasing the temperature. It was found, that lifetime of 5D 4 state and luminescence efficiency of terbium compounds at high temperatures depend on the bonding strength of Phen ligand with Tb 3+ ion. A range of the lowest triplet state energies of Phen ligand, from 20,850 to 21,750 cm −1, was revealed in lanthanide carboxylates under investigation. Closeness of the natural lifetimes of 5D 0 and 5D 4 states of Eu 3+ and Tb 3+ ions, respectively, in the same charge surroundings was demonstrated.