Electron transfer and spin transition in metal-hexacyanoferrates driven by anatase TiO2: electronic and structural order effects

57Fe Mossbauer spectroscopy was used to highlight a charge transfer mechanism in cobalt hexacyanoferrate (CoHCF) when synthetized in the presence of anatase TiO2. The electron transfer in TiO2–CoHCF composites along the Co–NC–Fe–CN–Co chain is confirmed by the unequivocal evolution of the typical quadrupole doublet of Fe(II) and Fe(III) in pristine CoHCF into the single line spectrum of the Fe(II) species when CoHCF is nanodispersed on TiO2 anatase. Parallel galvanostatic measurements also confirm the electron transfer mechanism induced by TiO2. To the best of our knowledge, this is one of the first examples of the triggering of spin-crossover phenomena in metal-hexacyanoferrate based-molecular solids by an external stimulus other than photoexcitation and most of all one of the first studies in which a completely pure low-spin Fe(II) is obtained starting from a mix of low-spin Fe(II) and Fe(III).