Energy transfer process in highly photoluminescent binuclear hydrocinnamate of europium, terbium and gadolinium containing 1,10-phenanthroline as ancillary ligand

New binuclear lanthanide (III) complexes of general formula [Ln2(hcin)6(phen)2] (where Ln = Eu 1; Gd 2; Tb 3; hcin = hydrocinnamate anion; phen = 1,10-phenanthroline) were synthesized and fully characterized by elemental analysis, vibrational spectroscopy (infrared and Raman), thermal analysis (TGA/DTA), CP/MAS 13C NMR and powder X-ray diffraction. The crystal description based on powder X-ray diffraction data reveals that all compounds are isostructural and that each lanthanide ion is nine coordinated by oxygen and nitrogen atoms to form distorted tricapped trigonal-prismatic coordination polyhedron. The photoluminescence behavior was studied based on the excitation and emission spectra and luminescence decay curves. The emission spectra of Eu(III) and Tb(III) complexes are composed of intense and typical red and green emissions, respectively. Phosphorescence data of Gd(III) complex showed that the triplet states (T1) of ligands have higher energy than the main emitting states of Eu(III) and Tb(III) indicating the possibility of intramolecular energy transfer for these metal ions. To elucidate the energy transfer process in the Eu(III) complex, spectroscopic properties as Ωλ intensity parameters (λ = 2 and 4), radiative (Arad) and nonradiative (Anrad) decay rates and quantum efficiency (η) of [Eu2(hcin)6(phen)2] were determined. Such spectroscopic properties were compared with [Eu2(hcin)6(bpy)2] complex properties recently reported. The high emission quantum efficiency (η = 72%) for Eu(III) complex 1 showed that it is a potential candidate as emitter in photonic systems.