Time-resolved X-ray absorption spectroscopy(TRXAS) at the Co L3-edge was used to identify metal-centered(MC) character in the S1 excited state of cyanocobalamin (CNCbl).Cobalamins have UV/visible spectra that are dominated by intensecorrin-based excitations, but these ligand-centered states energeti-cally overlap with charge transfer and MC excited states that may bepopulated following photoexcitation. Ultrafast optical and hard X-ray spectroscopy have shown that CNCbl forms a structurallydistorted S1 state, but these probes lack a clear signature of the S1electronic identity, which theory has suggested is a ligand-to-metalcharge transfer (LMCT) state. Femtosecond soft X-ray TRXASoffers greater state-selectivity than many optical or hard X-rayprobes but has, so far, been limited to highly concentrated (≥100mM) samples. A new experimental setup at the European X-ray Free Electron Laser (EuXFEL) that enables studies of sub-10 mMsamples and provides ∼100 fs time-resolution is used to measure the TRXAS of CNCbl at the Co L3-edge. Comparison of the L3-edge XAS spectrum measured at 0.8 ps with ligand field multiplet simulations indicates that the S1 state is primarily an MC excitedstate. The sub-20 μOD detection sensitivity achieved in this study demonstrates the possibility of applying this method to a widerange of naturally occurring and synthetic transition metal complexes.
Ghodrati et al. (Thu,) studied this question.