Electron hopping, and more recently to a "stepping-stone mechanism" (= modified tunneling process) were proposed. Assemblies having reversible electron transfer to unidirectional current flows, and even charge trapping have been demonstrated. Reversible changes in the metal oxidation state, along with concurrent large variations in their light absorption efficiency, have resulted in potentially useful electrochromic coatings. Electrocatalysis, sensing, and antibacterial properties have been reported as well as supercapacitors, electrochromic devices, and inverted solar cells. Such films have also been shown to mimic the characteristics of conventional electronic circuits, including flip-flops. The properties of metal-organic assemblies can be controlled by sequence-dependent assembly, i.e., depositing nanoscale layers consisting of isostructural complexes with different metal cations. We demonstrate here a solid-state material with tailored charge-transfer properties. The key to these properties is a thin layer consisting of isostructural ruthenium (1) and cobalt (2) complexes. This composite layer is bound to the working electrode and an electrochromic active layer composed of iron complexes (3) is deposited on top (Scheme 1A,B). Depending on the ratio of complexes 1 and 2, an electrical charge can be stored in the top layer. The positive charge can be released chemically, demonstrating electrochemical-to-chemical electron transfer. The charge storage is reflected in the coloration of our assemblies. The charge-transfer layer is a new entry for the formation of charge storage and electrochromic materials that cannot be achieved otherwise. The design of the composite layer is based on the distinctly different redox properties of the two isostructural complexes of ruthenium (1) and cobalt (2). The colorless complex 2 was used because it is electrochemically inactive to indium-tin oxide (ITO) (Figure ); however, it shows oneelectron oxidation of Co 2+/3+ mediated by complex 1. Depending on the ratios of these two