The distinct optical absorption spectra of the two layers allowed spectroelectrochemical measurements to probe the electrochemical activity of the individual layers during the switching of the Prussian blue layer.We found that for producing layers of equal optical density, the thickness of the layers had to be different due to a difference in oscillator strength for the metal-to-metal charge-transfer transition. The films used here had a thickness of 300 AE 70 nm and 30 AE 15 nm for the FeCr and FeFe sub-layers, respectively.The colouration efficiency was found to be 147.8 AE 0.8 cm 2 C À1 for the multilayered film. These resultsshow that it is possible to obtain bilayers of Prussian blues that, with a unique optical spectral fingerprint of each sub-layer, retain the electrochromic functionality and therefore enable layer-sensitive studies of charge-transfer processes in thin film heterostructures of multifunctional materials.

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