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.