Theory-Driven Spectral Control of Bis-EDOT Arylene Radical Cation Chromophores

The electronic structure of π-conjugated materials is responsible for their distinct optoelectronic properties and can be manipulated either by incorporating substituents with varying steric or electronic interactions to influence torsional twist, noncovalent electrostatic interactions, and the degree of electron richness. The structure–property relationship governing the electronic transitions of neutral conjugated molecules is fairly well understood, but this same relationship for the corresponding radical cation/anion has received little attention and the observable transitions can be difficult to predict a priori. Here, we use quantum calculations, specifically density functional theory (DFT) and time-dependent DFT (TD-DFT), to predict how the choice and placement of functional substituents on dioxythiophene-based ter-heterocycles influence the optical properties of the corresponding radical cation and dications states. Specifically, we examine seven thioalkyl-substituted bis­(3,4-ethylenedioxythiophene)-1,4-phenylene (BEDOT-B) molecules with varying alkoxy groups around the central phenylene and compare their optoelectronic, geometric, and excited-state properties by TD-DFT. We then verify the theoretical results with experiments using three model compounds BEDOT-benzene (BEDOT-B), BEDOT-methoxyphenylene (BEDOT-MOB), and BEDOT-2,5-dimethoxyphenylene (BEDOT-2,5-DMOB). Single-crystal X-ray diffraction, ultraviolet–visible (UV–vis) spectroscopy, spectroelectrochemistry, and chemical doping experiments are performed to understand the evolution of the geometric, optical, and electronic properties as the neutral trimers are converted to the radical cation, and to some extent, the dication state. We find that changes in geometric conformation in the radical cations as a result of methoxy substituents are reflected in changes in the dominant high-energy absorbance peak that are associated mainly with the singly occupied molecular orbital (SOMO) having the α electron spin state transition to the lowest unoccupied molecular orbital (LUMO) having the same electron spin state, Sα → Lα. However, the low-energy peak associated with the Sβ → Lβ transition remains fairly unaffected by the choice/placement of substituent. Herein, we demonstrate how this observation can be utilized in electrochromic applications as a strategy for fine-tuning the hue and saturation of molecular electrochromes that switch between colorless neutral states and vibrantly colored radical cation states. Fundamentally, this study deepens our understanding of how to synthetically control the optoelectronic properties of conjugated materials in their charged states, guided by TD-DFT to elucidate the electronic transitions at the heart of these properties.