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 (T-5) 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