Sub-50-fs photoinduced spin crossover in [Fe(bpy)3]2+

It is known that excitation by visible light of the singlet metal-to-ligand charge-transfer (¹MLCT) states of Fe(II) complexes leads to population of the lowest-lying high-spin quintet state (⁵T) with unity quantum yield. Here we investigate this so-called spin crossover (SCO) transition in aqueous iron(II)tris(bipyridine). We use pump-probe transient absorption spectroscopy with a high time resolution of <60 fs in the ultraviolet probe range, in which the ⁵T state absorbs, and of <40 fs in the visible probe range, in which both the hot MLCT state and the ⁵T state absorb. Our results show that the ⁵T state is impulsively populated in less than 50 fs, which is the time we measured for the depopulation of the MCLT manifold. We propose that non-totally-symmetric modes mediate the process, possibly high-frequency modes of the bipyridine (bpy) ligand. These results show that even though the SCO process in Fe(II) complexes represents a strongly spin-forbidden (ΔS = 2) two-electron transition, spin flipping occurs at near subvibrational times and is intertwined with the electron and structural dynamics of the system.