Burke, J.H.; Bae, D.Y.; Wallick, R.F.; Dykstra, C.P.; Rossi, T.C.; Smith, L.E.; Leahy, C.A.; Schaller, R.D.; Mirica, L.M.; Vura-Weis, J.; van der Veen, R.M.: High-Spin State of a Ferrocene Electron Donor Revealed by Optical and X-ray Transient Absorption Spectroscopy. Journal of the American Chemical Society 146 (2024), p. 21651-21663
10.1021/jacs.4c05646
Open Access Version
Abstract:
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.