Color Control in Bis-ethylenedioxythiophene Phenylene Anodically Coloring Electrochromes

This work assesses the steric, torsional, and electronic effects of substituents on anodically coloring electrochromic (ACE) molecules to gain insight toward constructing highly transmissive neutral states, while attaining vibrant colors in their cation radical and dication states. We examine seven thioalkyl-substituted bis(3,4-ethylenedioxthiophene)-1,4-phenylene (BEDOT-Ph) ACE molecules and compare their optoelectronic, geometric, and excited-state properties. We evaluate their redox properties and show that their first oxidation potential, forming the cation radical, can be varied over 0.70 V, with second oxidations ranging from 0.3 to 0.5 V versus Fc/Fc(+). Upon chemical solution doping with Fe(OTf)(3), the cation radical states absorb strongly in the visible, providing vibrantly colored solutions, while further oxidation with the stronger oxidant NOPF6 provides evidence for the formation of a dication state. Electrochemical oxidation in an optically transparent thin layer electrode (OTTLE) shows evidence of both the cation radical and the dication states, possibly forming via a disproportionation mechanism. Time-dependent density functional theory (TD-DFT) calculations carried out at the mPW1PBE/cc-PVDZ level are used for the excited state analysis of the neutral and charged states. These model results provide insight into how substituent identity impacts both a high energy absorption localized in the visible region and a lower energy peak that can be moved to the near-infrared region, thereby controlling the color. In tandem with experimental evaluation, we demonstrate how structural design modifications can control both redox and optical properties as a strategy to elucidate color, paving the way for electrochromic applications and fine manipulation of color control.