Abstract The complexes tris(4,4,4-Trifluoro-1-(1-naphthyl)-1,3-butanedionate) (2,2′-bipiridyl) Ln(III), Ln(tan) 3 bipy, where Ln(III)=Eu 3+ and Gd 3+ have been synthesized, characterized and their photophysical properties (absorption, excitation and luminescence spectra and emission quantum yield) investigated down to 4.2 K. The Eu(tan) 3 bipy complex has its molecular structure experimentally determined using X-ray crystallography and theoretically using the SMLC/AM1 method as well as their electronic singlet and triplet states were calculated, using the INDO/S-CI method with a point charge model to represent the Eu 3+ ion, where two values were adopted, +3.0 e and +3.5 e , to investigate the imperfect shielding of the 4f shells. The so calculated +3.5 e model electronic absorption spectrum and low lying triplet state energies agreed very well with the experimental ones. The emission quantum yield of the Eu 3+ complex is quite low at room temperature, namely 7%, probably due to the too low lying triplet state, 19,050 cm –1 , and increases by a factor of three when the temperature is lowered to 4.2 K. This strong thermal effect indicates the presence of a channel deactivating the main emitting state, what can be due to a LMCT state possibly lying in the same spectral region, as usually found in Eu 3+ compounds.