Thermo-chromism and photo-chromism in a Prussian blue analogue

We report on the photo-magnetic Prussian blue analogue series CsxCo[Fe(CN)(6)](y) (Cs-PBA). They undergo both a valence tautomeric conversion at high temperature (similar to 250 K) and a photo-excitation at low temperature, according to the equation already established for K-PBA and Rb-PBA series: Fe-II + Co-III(LS) --> Fe-III + Co-II(HS). The high-temperature state can be trapped by thermal quenching. The photo-magnetic data are not analysed quantitatively because of the strong effects of bulk absorption of light. However, the quantitative investigation could be completed by diffuse reflectivity, which only probes the top (= photo-excited) layers of the sample. We thus observed: (i) the bleaching effect of the photo-process, due to the weakening of the charge-transfer band in the red and near IR; (ii) the complete character of the photo-transformation, compared to the spontaneous tautomeric conversion; (iii) the similitude between the photo-excited and the high-temperature states, deduced from the comparison of absorption spectra and relaxation kinetics. We have focussed on the "optimised" composition Cs0.175Co[Fe(CN)(6)](j). Generic features of photo-induced magnetic structures (e.g. magnetic metastability) are also illustrated. A promising feature is the lifetime of the electronic excited state, which up to similar to 130 K remains long enough to allow sizeable photo-excitation experiments on a rather long time scale. This is a promising step on the route of futuristic applications to optical information storage. (C) 2001 Elsevier Science Ltd. All rights reserved.