Solution chemistry of the lanthanide(III) ions is unexplored and relevant: extraction and recycling processes exclusively operate in solution, MRI is a solution-phase method, and bioassays are done in solution. However, the molecular structure of the lanthanide(III) ions in solution is poorly described, especially for the near-IR (NIR)-emitting lanthanides, as these are difficult to investigate using optical tools, which has limited the availability of experimental data. Here we report a custom-built spectrometer dedicated to investigation of 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 distributions 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 structures of the ground state and the emitting state of neodymium(III) in five different solution complexes. This is the first step toward correlating optical spectra with chemical structure in solution for NIR-emitting lanthanide complexes.

Technology and innovation requires new high performance materials, however our knowledge of how crystalline materials are formed is limited.1-5 We can control the growth of single crystals from melts and make semiconductors, optical fibers, and jet turbine blades.6 And we can with atomic precision make molecular architectures in the lab,7 or by exploiting biotechnological advances.8 However, where we for the creation of molecular species have a detailed mechanistic understanding, we do not have a succinct model that describes the formation mechanism of crystalline materials from solutions containing the molecular building blocks.9-11 Here we show that the formation of lanthanide(III) hydroxide crystallites does not take place in solutions,12-17 but happens upon dehydration on the filter. The data reveals that no solids are present in solution and that no crystallization occur. In solution a strong, washable gel phase is formed. Using total scattering,18-21 it is documented that no long range order is present in solutions, and that the gel is constructed from clusters of lanthanide(III) ions. Despite the emergence of studies of the structure of crystallites in solutions and in melts,22-31 the early stages of formation of solids from ions and molecules in solution remains poorly understood.21, 32-34 The process from solvated ion to cluster to agglomerate can be described like any other chemical transformation, and must follow a reaction profile with energy barriers, intermediates, and transition states—and will be governed by reaction kinetics and thermodynamics. The first step is to map all species involved between solute and nanocrystal, and here studying materials made from lanthanide(III) ions are useful as luminescence, electron miscopy and scattering is strong with these ions.