Mapping the distribution of electronic states within the 5 D 4 and 7 F 6 levels of Tb 3+ 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 (⁷F₆) and emitting (⁵D₄) 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)(H₂O)]^-), ethylenediaminetetraacetic acid (EDTA, [Tb(EDTA)(H₂O)₃]^-) and diethylenetriaminepentaacetic acid (DTPA, [Tb(DTPA)(H₂O)]^2-) as well as the Tb(III) aqua ion ([Tb(H₂O)₉]³⁺) were recorded at room temperature and in frozen solution. Using these data the electronic structure of the ⁵D₄ multiplets of Tb(III) was mapped by considering the transitions to the singly degenerate ⁷F₀ state. A detailed spectroscopic investigation was performed and it was found that the ⁵D₄ multiplet could accurately be described as a single band for [Tb(H₂O)₉]³⁺, [Tb(DOTA)(H₂O)]^- and [Tb(EDTA)(H₂O)₃]^-. In contrast, for [Tb(DTPA)(H₂O)]^2- two bands were needed. These results demonstrated the ability of describing the electronic structure of the emitting ⁵D₄ 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.