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

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 ps and 500 ps lifetimes. Using a combination of ultraviolet, visible, nearinfrared, and short-wave infrared probe pulses, we capture the spectral features of these states over an ultrabroadband range spanning 320 nm to 2200 nm. Time-dependent density functional theory 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 signatures of bond-length expansion and a Jahn-Teller distortion, consistent with an Fe(II) high-spin state. No transient signal is detected at the Co K-edge, confirming the ferrocene-centered character of the excited state. Altogether, these results demonstrate that the high-spin state of ferrocene should be considered when designing donor-acceptor assemblies for photocatalysis and photovoltaics.