Correlating Valence and 2p3d RIXS Spectroscopies: A Ligand-Field Study of Spin-Crossover Iron(II)

The molecular spin-crossover phenomenon between high-spin (HS) and low-spin (LS) states is a promising route to next-generation information storage, sensing applications, and molecular spintronics. Spin-crossover complexes also provide a unique opportunity to study the ligand field properties of a system in both HS and LS states while maintaining the same ligand environment. Presently, we employ complementing valence and core-level spectroscopic methods to probe the electronic excited state manifolds of the spin-crossover complex [FeII(H2B(pz)2)2phen]0. Light-induced excited spin state trapping (LIESST) at liquid He temperatures is exploited to characterize magnetic and spectroscopic properties of the photo-induced HS state using SQUID and MCD spectroscopy. In parallel, Fe 2p3d RIXS spectroscopy is employed to examine the S = 0, 1 excited ligand-field (LF) states. These experimental studies are combined with state-of-the-art CASSCF/NEVPT2 and CASCI/NEVPT2 calculations characterizing the ground and LF excited states. Analysis of the acquired LF information further supports that the spin-crossover of [FeII(H2B(pz)2)2phen]0 is asymmetric, evidenced by a decrease in e in the LS state. The results demonstrate the power of cross-correlating spectroscopic techniques with high and low LF information content to make accurate excited state assignments as well as the current capabilities of ab initio theory in interpreting these electronic properties.