Light-Induced Mixed-Valence State of Fe^II[Fe(CN)₅NO]·xH₂O

A long-living metastable state is produced in Fe^II[Fe(CN)₅NO]·xH₂O by an irradiation of 488 nm light at 200 K. A strong near-infrared band of about 10000 cm^-1 and a v(CN) band at 2070 cm^-1 that newly appear in the metastable state point to development of the mixed-valence state analogous to Prussian blue. An iron-to-nitrosyl charge transfer in pentacyanonitrosylferrate(2-), [Fe(CN)₅NO]^2-, explains the formation of the mixed-valence state, because the charge transfer causes a pair of iron(II) and iron(III) atoms bridged by CN^- that has the same local structure as Prussian blue. In addition to the formation of the metastable state, the prolonged irradiation causes an irreversible change of Fe^II[Fe(CN)₅NO]·H₂O and leads to the formation of a photoproduct. The photoproduct also has infrared spectra characteristic of the mixed-valence state. Mössbauer spectra of the photoproduct show that a low-spin iron ion in pentacyanonitrosylferrate(2-) is divalent and a high-spin iron outside of pentacyanonitrosylferrate(2-) is trivalent. Therefore, the charge-transfer between the two non-equivalent iron ions is thought to follow the intra-molecular charge transfer in pentacyanonitrosylferrate(2-).