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.