Chemical Origins of Line Broadening in the f–f Transitions of a Molecular Yb(III) Complex

Abstract

Lanthanide molecular complexes are promising platforms for quantum information science (QIS), where narrow optical line widths are essential for coherent light–matter interaction. Rational design of such color centers requires understanding how the chemical environment sets the line width, yet predictive modeling of lanthanide electronic structure remains challenging. Here we compute the energy gaps and line widths of f–f optical transitions in a model Yb(III) solvate, Yb(2Me–THF)3Cl3, using CASSCF/NEVPT2 and compare them with high-resolution absorption spectra. We simulate broadening by scanning the PES along all 150 normal modes and by conformational sampling of the ligand environment. The simulated envelopes reproduce the main features of the experimental spectrum and can be decomposed into contributions from individual nuclear motions. Our analysis shows that the broadening is set by fluctuations of the ligand field and is dominated by low-frequency collective motions of the first coordination sphere. We further find that 4f energy gaps are highly sensitive to the Yb-ligand bond distances, and that oscillator strength varies nonmonotonically with geometry, so that brighter transitions need not be broader. These results suggest that spectral broadening in molecular Yb(III) absorbers may be reduced by ligand designs that fix the first coordination sphere and limit conformational freedom.