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 ( 7 F 6 ) and emitting ( 5 D 4 ) 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)(H 2 O)] - ), ethylenediaminetetraacetic acid (EDTA, [Tb(EDTA)(H 2 O) 3 ] - ) and diethylenetriaminepentaacetic acid (DTPA, [Tb(DTPA)(H 2 O)] 2- ) as well as the Tb(III) aqua ion ([Tb(H 2 O) 9 ] 3+ ) were recorded at room temperature and in frozen solution. Using these data the electronic structure of the 5 D 4 multiplets of Tb(III) was mapped by considering the transitions to the singly degenerate 7 F 0 state. A detailed spectroscopic investigation was performed and it was found that the 5 D 4 multiplet could accurately be described as a single band for [Tb(H 2 O) 9 ] 3+ , [Tb(DOTA)(H 2 O)] - and [Tb(EDTA)(H 2 O) 3 ] - . In contrast, for [Tb(DTPA)(H 2 O)] 2- two bands were needed. These results demonstrated the ability of describing the electronic structure of the emitting 5 D 4 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.