Follow the Yellow Brick Road: Structural Optimization of Vibrant Yellow-to-Transmissive
Electrochromic Conjugated Polymers
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Abstract
A series of conjugated polymers were
designed and synthesized to
extract structure–property relationships with the goal of yielding
yellow-to-transmissive switching electrochromes. The polymers are
based on repeat units of propylenedioxythiophene (ProDOT) in alternation
with a variety of arylenes including 1,4-phenylene (ProDOT-Ph), 2,7-fluorene
(ProDOT-Fl), 2,7-carbazole (ProDOT-Cbz), 2,5-dimethoxy-1,4-phenylene
(ProDOT-Ph(MeO)<sub>2</sub>), and 2,7-pyrene (ProDOT-Py). Additionally,
a random copolymer containing ProDOT and two different arylene units
was produced: ProDOT-phenylene-ProDOT-dimethoxyphenylene (R-ProDOT-Ph/Ph(MeO)<sub>2</sub>) and two polymers with a ProDOT dimer in alternation with
pyrene and phenylene composed ProDOT<sub>2</sub>-pyrene (ProDOT<sub>2</sub>-Py) and ProDOT<sub>2</sub>-phenylene (ProDOT<sub>2</sub>-Ph),
respectively. The polymers were synthesized using Suzuki polycondensation.
Examinations of the optoelectronic properties via UV–vis–NIR
spectroscopy, differential pulse voltammetry, and spectroelectrochemistry
show that varying the electron richness of the polymer by utilizing
more electron rich arylenes, dimers of ProDOT, or less electron rich
arylenes, the oxidation potential could be decreased or increased,
respectively, ranging from 270 to 650 mV. Through subtle C–H <i>ortho</i> interactions from the arylene unit, yellow neutral
state colors were maintained with transmissive or near-transmissive
oxidized states. Colorimetry utilizing <i>L</i>*<i>a</i>*<i>b</i>*, where <i>a</i>*<i>b</i>* values correlate to the chroma or saturation of a color
(note: −<i>a</i>* and +<i>a</i>* correspond
to green and red and −<i>b</i>* and +<i>b</i>* correspond to blue and yellow, respectively) and <i>L</i>* represents the lightness, was used to show the maintenance of yellow
colors in the neutral states. Herein, the yellow polymers had <i>L</i>* values above 84.0, <i>a</i>* values ranging
from −11.6 to 24.8, and <i>b</i>* values greater
than 47.6. In the oxidized states, the most transmissive forms had <i>L</i>* values above 70.0, <i>a</i>* values ranging
from −2.1 to 2.0, and <i>b</i>* values ranging from
−6.8 to −0.1. These structure–property relationships
grant access to conjugated polymers with high energy absorbance in
the visible, while allowing variability in redox potentials, providing
a deeper understanding in yielding yellow-to-transmissive electrochromic
polymers