Would the solvent effect be the main cause of band shift in the theoretical absorption spectrum of large lanthanide complexes?

As most reactions take place in solution, the study of solvent effects on relevant molecular properties – either by experimental or theoretical methods – is crucial for the design of new processes and prediction of technological properties. In spite of this, only few works focusing the influence of the solvent nature specifically on the spectroscopic properties of lanthanide complexes can be found in the literature. The present work describes a theoretical study of the solvent effect on the prediction of the absorption spectra for lanthanide complexes, but other possible relevant factors have been also considered such as the molecular geometry and the excitation window used for interaction configuration (CI) calculations. The [Eu(ETA)2·nH2O]+1 complex has been chosen as an ideal candidate for this type of study due to its small number of atoms (only 49) and also because the absorption spectrum exhibits a single band. Two Monte Carlo simulations were performed, the first one considering the [Eu(ETA)2]+1 complex in 400 water molecules, evidencing that the complex presents four coordinated water molecules. The second simulation considered the [Eu(ETA)2·4H2O]+1 complex in 400 ethanol molecules, in order to evaluate the solvent effect on the shift of the maximum absorption in calculated spectra, compared to the experimental one. Quantum chemical studies were also performed in order to evaluate the effect of the accuracy of calculated ground state geometry on the prediction of absorption spectra. The influence of the excitation window used for CI calculations on the spectral shift was also evaluated. No significant solvent effect was found on the prediction of the absorption spectrum for [Eu(ETA)2·4H2O]+1 complex. A small but significant effect of the ground state geometry on the transition energy and oscillator strength was also observed. Finally it must be emphasized that the absorption spectra of lanthanide complexes can be predicted with great accuracy by the combined use of semiempirical Sparkle/AM1 and INDO/S-CIS as long as the largest possible excitation window is used in the configuration interaction calculation.

► We study the solvent effect in a europium complex.
► The influence of the excitation window in CI calculations on spectral shift was the more significant factor.
► The absorption spectra of europium complexes can be predicted accurately using semiempirical calculations.