Controlling Charged State Colors in Triphenylamine-Based Anodically Coloring Electrochromes

In this work, we have designed a series of anodically coloring electrochromic (ACE) molecules comprised of thioalkyl-substituted 3,4-ethylenedioxythiophenes coupled to triphenylamine units (EDOT-TPA) that vary in the position and degree of electron richness of substituents, which influences the molecules geometric, electrochemical, optoelectronic, and excited-state properties. We evaluated their redox properties and discovered that modulation of both the first and second oxidation potential, formation of the cation radical and dication, can be varied from 0.03 to 0.18 V and 0.32 to 0.46 V versus Fc/Fc⁺ respectively. For the first time in ACE-based molecular systems, we demonstrated the ability to vary the electrochemical potential separation between successive charge states, which is directly involved in the generation of color. We use the chemical oxidant, Fe(OTf)₃, to visualize the saturation and contrast of the vibrantly colored cation radical solutions at 1 equivalent, followed by a second equivalents that opens a new and differing set in the color palette for the dication state. Optical transitions were probed during electrochemical oxidation using an optically transparent thin layer electrode (OTTLE) demonstrating selective control in generating successive charge states. We couple our findings with Density Functional Theory (TD-DFT) simulations to show how modulating electron richness and steric interactions control the optical transitions. Specifically, excited state analysis is performed to elucidate how substituent identity affects the neutral, cation radical, and dication transitions in the visible and near infrared, and thereby the resulting color.