Hypersensitive f–f Transition Intensities of Lanthanide Trihalide Complexes

The absorption spectra of lanthanide trihalide complexes were calculated using the multiconfigurational SO-CIS(PT2) method, which accounts for spin–orbit coupling, dynamic electron correlation, and the dynamic coupling with the ligands. These complexes exhibit well-characterized hypersensitive transitions whose oscillator strengths depend sensitively on the nature of the ligands. The good agreement among the calculated transition energies, absorption intensities, and experimental spectra highlights the potential of this method for predicting absorption spectra of lanthanide complexes with larger ligands from first principles. Judd–Ofelt parameters were derived from the spin-free manifolds (i.e., without spin–orbit coupling) to avoid state mixing arising from intermediate coupling. Only transitions between manifolds with Δ L = −2 exhibit hypersensitivity. The Judd–Ofelt parameters display a decreasing trend across the lanthanide series and, as expected, an increasing trend along the halide series. In this work, the dynamic coupling model is not expressed in terms of polarizability but is formulated in an orbital framework, facilitating a microscopic rationalization of the observed results.