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 FeIII–CN–CoII and FeII–CN–CoIII respectively, and by the M?ssbauer spectrum, in which FeIII and FeII 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 FeII to CoIII. 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, K0.2Co1.4Fe(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 CoII and FeIII. As a result, dehydration increases the fraction of FeII–CN–CoIII and causes the compound to be paramagnetic even at low temperature.