PCA-Assisted Spectral Reconstruction Reveals Speciation of Neodymium(III) DOTA and pDO3A Complexes

The solution structure of lanthanide(III) complexes remains a puzzle despite its importance for biomedical applications and the circular economy. We present a method for unravelling the solution structure of luminescent lanthanide(III) ions using high-quality emission and excitation spectra. By analyzing spectral variations in the emission spectra as a function of excitation wavelength, multiple emissive species can be resolved within a single sample. Principal component analysis (PCA) reveals the spectral variance and thus isolates the spectral shape of the emission of each species. This enables full spectral reconstruction of the excitation and emission spectra of these species and quantification of their relative populations. The approach is demonstrated on Nd.DOTA and Nd.pDO3A in water and DMSO, and the speciation results are in excellent agreement with those previously obtained from NMR spectroscopy. Furthermore, CASSCF electronic structure calculations relate the reconstructed spectral features to specific molecular structures. This combined optical and computational strategy provides a powerful and accurate route to determining lanthanide speciation in solution.