Guiding synthetic targets of anodically coloring electrochromes through density functional theory

Electrochromic devices offer many technological applications, including flexible displays, dimmable mirrors, and energy-efficient windows. Additionally, adsorbing electrochromic molecular assemblies onto mesoporous metal-oxide surfaces facilitates commercial and manufacturing potential (i.e., screen-printing and/or roll-to-roll processing). These systems also demonstrate synthetic versatility, thus making a wide array of colors accessible. In this work, using Time-Dependent Density Functional Theory (TD-DFT), we investigated ten different bi-aryl type molecules of 3,4-ethylendioxythiophene (EDOT) conjugated to various phenyl derivatives as potential anodically coloring electrochromes (ACEs). The non-substituted phenylene, hexylthiol-EDOT-phenyl-phosphonic acid, PA1, was synthesized and characterized as a means of model validity. PA1 absorbs in the UV region in its neutral state and upon oxidation absorbs within the visible, hence showcasing its potential as an ACE chromophore. The properties of PA1 inspired the designs of the other nine structural derivatives where the number and position of methoxy groups on the phenylene were varied. Using our DFT treatment, we assessed the impact of these modifications on the electronic structures, geometries, and excited-state properties. In particular, we examined stabilization intermolecular interactions (S-O and O-H) as they aid in molecule planarization, thus facilitating charge transport properties in devices. Additionally, destabilizing O-O forces were observed, thereby making some chromophores less desirable. A detailed excited state analysis was performed, which linked the simulated UV-Vis spectra to the dominant excited state transitions and their corresponding molecular orbitals. Based on these results, the nine chromophores were ranked ergo providing an ordered list of synthetic targets.