Identification of the dominant photochemical pathways and mechanistic insights to the ultrafast ligand exchange of Fe(CO)₅ to Fe(CO)₄EtOH

We utilized femtosecond time-resolved resonant inelastic X-ray scattering and ab initio theory to study the transient electronic structure and the photoinduced molecular dynamics of a model metal carbonyl photocatalyst Fe(CO)₅ in ethanol solution. We propose mechanistic explanation for the parallel ultrafast intra-molecular spin crossover and ligation of the Fe(CO)₄ which are observed following a charge transfer photoexcitation of Fe(CO)₅ as reported in our previous study [Wernet et al., Nature 520, 78 (2015)]. We find that branching of the reaction pathway likely happens in the ¹A₁ state of Fe(CO)₄. A sub-picosecond time constant of the spin crossover from ¹B₂ to ³B₂ is rationalized by the proposed ¹B₂ → ¹A₁ → ³B₂ mechanism. Ultrafast ligation of the ¹B₂ Fe(CO)₄ state is significantly faster than the spin-forbidden and diffusion limited ligation process occurring from the ³B₂ Fe(CO)₄ ground state that has been observed in the previous studies. We propose that the ultrafast ligation occurs via ¹B₂ → ¹A₁ → ¹A' Fe(CO)₄EtOH pathway and the time scale of the ¹A₁ Fe(CO)₄ state ligation is governed by the solute-solvent collision frequency. Our study emphasizes the importance of understanding the interaction of molecular excited states with the surrounding environment to explain the relaxation pathways of photoexcited metal carbonyls in solution.