Electronic Structure of Neodymium(III) and Europium(III) Resolved in Solution using High Resolution Optical Spectroscopy and Population Analysis

Solution chemistry of the lanthanide(III) ions is unexplored and relevant: extraction and recycling processes exclusively operates in solution, MRI is a solution phase method, and bioassay are done in solution. However, the molecular structure of the lanthanide(III) ions in solution is poorly described, especially for the NIR emitting lanthanides as these are difficult to investigate using optical tools, which has limited the availability of experimental data. Here, we report on a custom built spectrometer dedicated to investigate lanthanide(III) luminescence in the NIR-region. Absorption, luminescence excitation, and luminescence spectra of five complexes of europium(III) and neodymium(III) were acquired. The obtained spectra display high spectral resolution and high signal to noise ratios. Using the high quality data, a method for determining the electronic structure for the thermal ground states and emitting states is proposed. It combines Boltzmann distribution with population analysis and uses the experimentally determined relative transition probabilities from both excitation and emission data. The method was tested on the five europium(III) complexes, and was used to resolve the electronic structure of the ground state and the emitting state of neodymium(III) in five different solution complexes. This is the first step towards correlating optical spectra with chemical structure in solution for NIR emitting lanthanide complexes.