Femtosecond X-ray Spectroscopy Directly Quantifies Transient Excited State Mixed Valency and Subsequent Vibronic Dynamics

Quantifying charge delocalization and coupled nuclear motions on short-lived photoexcited states in solution remains experimentally challenging, requiring element specific femtosecond experimental probes of time-evolving electron transfer. Here, we use advanced X-ray spectroscopic probes and excited state calculations to measure the hole charge on the photoexcited charge transfer state of a prototypical mixed valence bimetallic (FeIIRuIII) complex in water. We find that the hole charge at the Fe atom is 0.75±0.18 on the metal-to-metal charge transfer excited state, which has a 58±10 fs lifetime. The X-ray emission data reveals that the rapid back electron transfer excites vibrations that modulate the Fe-Ru distance on the non-equilibrium ground state. Our combined experimental and computational approach provides a spectroscopic ruler to quantify excited state valency and sheds light on the low-frequency nuclear vibrations that are coupled to core level transitions