Invisible strings. The first single crystal of the cTSAP form of [Eu(DOTA)(H 2 O)] - has an electronic structure similar to one of the reported cSAP forms.

The coordination geometry of lanthanide(III) ions is extremely sensitive to perturbation from the surrounding environment. Changes in the crystal field can be observed as spectral variations in the emission spectra of luminescent lanthanide(III) ions. Europium(III) ions are commonly used to correlate luminescence properties to the crystal field. In solution, kinetically inert complexes as [Ln(DOTA)(H ₂ O)] ^- can fluctuate and give rise to different diastereomers (H ₄ DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid). The [Ln(DOTA)(H ₂ O)] ^- complex adopts a capped square antiprism (cSAP) geometry but can rotate into a capped twisted square antiprism (cTSAP). The time scale of the solution dynamics in [Ln(DOTA)(H ₂ O)] ^- is shorter than that of luminescence emission, thus, structural averages are observed in the emission spectra. For the first time, we were able to crystallise both forms of the [Eu(DOTA)(H ₂ O)] ^- diastereomers. The single crystal structure was combined with single crystal luminescence spectroscopy to reveal the electronic structure of Eu(III) in each form of the complex. The coordination geometry of the crystallized cSAP and cTSAP forms of the complex was compared to ideal coordination polyhedra using a continuous symmetry measure in the AlignIt code. The diastereomers of [Eu(DOTA)(H ₂ O)] ^- all demonstrate very little deviation from ideal geometries yet nearly identical electronic properties were observed from the two different forms.