Mapping the distribution of electronic states within the D and F levels of Tb complexes with optical spectroscopy.

The Tb(III) ion has the most intense luminescence of the trivalent lanthanide(III) ions. In contrast to Eu(III), where the two levels only include a single state, the high number of electronic states in the ground (F) and emitting (D) levels makes detailed interpretations of the electronic structure-the crystal field-difficult. Here, luminescence emission and excitation spectra of Tb(III) complexes with 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA, [Tb(DOTA)(HO)]), ethylenediaminetetraacetic acid (EDTA, [Tb(EDTA)(HO)]) and diethylenetriaminepentaacetic acid (DTPA, [Tb(DTPA)(HO)]) as well as the Tb(III) aqua ion ([Tb(HO)]) were recorded at room temperature and in frozen solution. Using these data the electronic structure of the D multiplets of Tb(III) was mapped by considering the transitions to the singly degenerate F state. A detailed spectroscopic investigation was performed and it was found that the D multiplet could accurately be described as a single band for [Tb(HO)], [Tb(DOTA)(HO)] and [Tb(EDTA)(HO)]. In contrast, for [Tb(DTPA)(HO)] two bands were needed. These results demonstrated the ability of describing the electronic structure of the emitting D multiplet using emission spectra. This offers an avenue for investigating the relationship between molecular structure and luminescent properties in detailed photophysical studies of Tb(III) ion complexes.