High-Spin State of a Ferrocene Electron Donor Revealed by Optical and X‑ray Transient Absorption Spectroscopy

• Published in: Journal of the American Chemical Society Vol. 146; no. 31; pp. 21651 - 21663
• Main Authors: Burke, John H., Bae, Dae Young, Wallick, Rachel F., Dykstra, Conner P., Rossi, Thomas C., Smith, Laura E., Leahy, Clare A., Schaller, Richard D., Mirica, Liviu M., Vura-Weis, Josh, van der Veen, Renske M.
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 07-08-2024; American Chemical Society (ACS)
• Subjects: Excited States; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Probes; Quantum Mechanics; Sandwich Compounds; Time Dependant Density Functional Theory
• ISSN: 0002-7863; 1520-5126; 1520-5126
• DOI: 10.1021/jacs.4c05646

Ferrocene is one of the most common electron donors, and mapping its ligand-field excited states is critical to designing donor–acceptor (D–A) molecules with long-lived charge transfer states. Although 3(d–d) states are commonly invoked in the photophysics of ferrocene complexes, mention of the high-spin 5(d–d) state is scarce. Here, we provide clear evidence of 5(d–d) formation in a bimetallic D–A molecule, ferrocenyl cobaltocenium hexafluorophosphate ([FcCc]­PF6). Femtosecond optical transient absorption (OTA) spectroscopy reveals two distinct electronic excited states with 30 and 500 ps lifetimes. Using a combination of ultraviolet, visible, near-infrared, and short-wave infrared probe pulses, we capture the spectral features of these states over an ultrabroadband range spanning 320 to 2200 nm. Time-dependent density functional theory (DFT) calculations of the lowest triplet and quintet states, both primarily Fe­(II) (d–d) in character, qualitatively agree with the experimental OTA spectra, allowing us to assign the 30 ps state as the 3(d–d) state and the 500 ps state as the high-spin 5(d–d) state. To confirm the ferrocene-centered high-spin character of the 500 ps state, we performed X-ray transient absorption (XTA) spectroscopy at the Fe and Co K edges. The Fe K-edge XTA spectrum at 150 ps shows a red shift of the absorption edge that is consistent with an Fe­(II) high-spin state, as supported by ab initio calculations. The transient signal detected at the Co K-edge is 50× weaker, confirming the ferrocene-centered character of the excited state. Fitting of the transient extended X-ray absorption fine structure region yields an Fe–C bond length increase of 0.25 ± 0.1 Å in the excited state, as expected for the high-spin state based on DFT. Altogether, these results demonstrate that the high-spin state of ferrocene should be considered when designing donor–acceptor assemblies for photocatalysis and photovoltaics.