SPECTROSCOPIC STUDY OF LANTHANIDE SQUARATE HYDRATES

In the present investigation some spectroscopic properties of several lanthanide squarate hydrates are reported. The Raman spectra show the same distinctive Jahn-Teller intensity pattern for non-totally symmetr ic modes, as previously observed for the free anion. In the case of th e terbium salt, the Tb3+ emission is very intense even at room tempera ture, revealing an efficient excitation via the ligand electronic leve ls. The Tb3+ dilution in Gd3+ or La3+ hosts increases this excitation efficiency without any appreciable variation in the D-5(4) excited-sta te lifetime. However, the Eu3+ emission is very weak, with excited sta tes located above the D-5(2) level (ca. 21 550 cm(-1)) being completel y quenched at room temperature. At lower temperatures higher-lying lev els are not so efficiently quenched. The broad band observed in the UV excitation spectra of Eu3+ and Tb3+ is easily assigned to an intra-li gand transition leading to ligand-to-lanthanide ion energy transfer pr ocesses. As observed for Tb3+, Eu3+ dilution in Gd3+ or La3+ hosts als o increases the relative emission intensity mediated by the ligand, wi thout variation in the D-5(0) excited-state lifetime. The Eu3+ D-5(0) excitation spectra show vibronic structures that can be interpreted on the basis of the data available from the vibrational spectra. An incr ease in the vibronic intensities is observed as the lanthanide concent ration is increased.