Ultrafast photophysics and photochemistry of iron hexacyanides in solution: Infrared to X-ray spectroscopic studies

We review recent results from the literature and from our work on ferrous and ferric hexacyanides in solution using a combination ultrafast spectroscopic tools, spanning from the infrared to the hard X-ray domain: transient infrared and visible spectroscopy, 2-dimensional deep-ultraviolet spectroscopy, vacuum ultraviolet photoelectron spectroscopy of solutions and hard X-ray absorption spectroscopy. The photoinduced dynamics in the case of the ferrous complex includes photoionization and photoaquation processes. The latter shows a sequence of events that includes: ultrafast intramolecular relaxation, dissociation with geminate recombination and solvent cage escape, formation of a penta-coordinated photo-product, conformational changes of the latter, binding of a water solvent molecule. The case of the ferric complex is different where upon excitation of the lowest LMCT state, a prompt change from Fe3+ to Fe2+ occurs followed by a back electron transfer, whose time scale is solvent-dependent. Furthermore, an electronic-to-vibrational energy transfer occurs as the molecule is found with a population reaching v = 3 of the CN stretch mode vibrational level. A weak photochemical channel is identified by IR and X-ray spectroscopy, showing formation of the same aquated species as in the ferrous case.