Synthesis, electrochromism and display-device application of electroactive ruthenium purple films prepared by 'Directed Assembly' and electrochemical precipitation techniques

Electrochemical reduction of electroactive solid films of Prussian blue (iron(III) hexacyanoferrate(II), PB), produces iron(II) hexacyanoferrate(II), which appears transparent as a thin film. Oxidation of PB yields iron(III) hexacyanoferrate(III), via intermediate Prussian green. The blue-transparent (anodically-colouring) transition in PB at one electrode has often been partnered with cathodically-colouring electrochromic materials at a second electrode, in ‘complementary’ electrochromic devices (ECD’s), where both films are coloured simultaneously. Electrochemical reduction of thin-film ruthenium purple (iron(III) hexacyanoruthenate(II), RP), produces the transparent iron(II) hexacyanoruthenate(II) redox state, but no oxidized form is available. By contrast with prototype PB-based ECD’s, systems based on RP have rarely been reported, likely due to the electrosynthetic challenge in preparing stable thin films. Thin-film PB is readily available through electrochemical reduction of solutions containing iron(III) and hexacyanoferrate(III) ions. Salts of the analogous hexacyanoruthenate(III) ion are not commercially available, and although preparation by chemical or electrochemical oxidation of hexacyanoruthenate(II) is possible, the resulting solution is unstable. Here we describe the ‘directed assembly’ and electrochemical precipitation of thin-film RP on ITO-glass substrates. In both techniques, the hexacyanoruthenate(II) redox state is used directly. For ‘directed assembly’, the synthesis involves exposure of ITO-glass to solutions containing, alternately, adsorbable iron(III) cations and hexacyanoruthenate(II) anions, leading to well-defined multilayer structures. In the electrochemical precipitation technique, a precisely formulated iron(III) hexacyanoruthenate(II) sol is used, film formation again relying on electrostatics. The purple-transparent (anodically-colouring) transition in RP directly contrasts with the transparent-purple (cathodically-colouring) transition in di-n-alkyl viologens and we have constructed transmissive ECD’s using these materials. Although both the viologen dication and viologen radical cation redox states are water soluble, fast colour-switching in displays is demonstrated through the use of a thin-layer device construction, with capillary-filling of electrolyte solution. In addition to spectroelectrochemical measurements of the materials and devices, we report the quantitative description of colour and relative transmissivity of the RP/viologen displays as sensed by the human eye using our Microsoft® Excel® spreadsheet for the accurate calculation of CIE (Commission Internationale de l’Eclairage) 1931 xy colour coordinates and luminance data from visible region absorption spectra.