Photo- and dehydration-induced charge transfer processes accompanied with spin transition on CoFe(CN)₅NH₃·6H₂O

The coexistence of different valences of iron and cobalt in cobalt pentacyanoferrate, CoFe(CN)₅NH₃·6H₂O, was confirmed by the infrared spectrum, in which two distinct peaks for the C-N stretch at 2162 cm^-1 and 2117 cm^-1 were assigned to CN in Fe^III-CN-Co^II and Fe^II-CN-Co^III respectively, and by the Mössbauer spectrum, in which Fe^III and Fe^II exhibited doublets with different isomer shifts and different quadrupole splitting. Photo-irradiation of this compound with red light changed the population of these two electronic states due to the internal electron transfer from Fe^II to Co^III. As a result, the ferrimagnetic properties of this pentacyanometalate at low temperature can be changed by photo-irradiation similar to the case of the hexacyanometalate, K_0.2Co_1.4Fe(CN)₆·6.9H₂O reported previously (O. Sato et al., Science 272, 704 (1996)). Moreover, the fraction of these two electronic states in this pentacyanoferrate is sensitive to the number of water molecules, which induces a charge transfer from Co^II and Fe^III. As a result, dehydration increases the fraction of Fe^II-CN-Co^III and causes the compound to be paramagnetic even at low temperature.