Tuning the painter's palette: Subtle steric effects on spectra and colour in conjugated electrochromic polymers

Article abstract of DOI:10.1039/c4tc02685c

A series of vibrantly coloured π-conjugated electrochromic polymers (ECPs) were designed and synthesized with the goal of extracting structure-property relationships from subtle changes in steric strain or relaxation. These are soluble all donor, electron rich, alternating polymers based on repeat units of 3,4-ethylenedioxythiophene (EDOT), 3,4-propylenedioxythiophene (ProDOT), and an acyclic dioxythiophene (AcDOT) in varying combinations to tune steric interactions and the subsequent optical absorption for fine colour control. Two families of polymers were formed where ProDOT₂-EDOT, ProDOT-EDOT, and ProDOT₂-EDOT₂ constitute new shades of blues while AcDOT₂-ProDOT, AcDOT-ProDOT, and AcDOT-EDOT yield new hues of magentas with the homopolymers of ProDOT and AcDOT and the copolymer AcDOT₂-EDOT₂ serving as comparisons. The polymers were synthesized using direct (hetero)arylation polymerization. Examinations of the optoelectronic properties via cyclic voltammetry, spectroelectrochemistry, and colorimetry show that by subtly varying the level of steric relaxation or strain in the form of EDOT or AcDOT content, lower or higher energy absorption transitions are produced respectively. This increase in relaxation or strain allows more short or long wavelength light to transmit, giving new shades of blues or magentas respectively. Since these are all donating polymers, they exhibit changes in contrast no less than 70% at the λ_max with the exception of AcDOT-EDOT. The most desirable electrochromic properties were achieved with ProDOT₂-EDOT₂ and AcDOT₂-ProDOT, with band gaps (E_g) and neutral state Lab colour values (-a and +a correspond to green and red and -b and +b correspond to blue and yellow respectively and L represents the lightness) of 1.74 eV, 37, 12, -63 and 2.01 eV, 56, 59, -16 respectively. The highly transmissive oxidized state colour values for ProDOT₂-EDOT₂ and AcDOT₂-ProDOT are 92, -3, -3 and 91, -2, -1 respectively. These structure-property relationships grant a greater ability to tune light absorption across the visible, with colour properties similar to ECPs made through other methods without using donor-acceptor effects. This all donor steric tuning method leads to considerably higher levels of transparency when the polymers are fully oxidized.

Full text of DOI:10.1039/c4tc02685c

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Tuning the painter’s palette: subtle steric effects
on spectra and colour in conjugated
electrochromic polymers†
Cite this:DOI: 10.1039/c4tc02685c
J. A. Kerszulis,a K. E. Johnson,a M. Kuepfert,b D. Khoshabo,b A. L. Dyerac and
J. R. Reynolds*a
A
series of vibrantly coloured p-conjugated electrochromic polymers (ECPs) were designed and
synthesized with the goal of extracting structure–property relationships from subtle changes in steric strain
or relaxation. These are soluble all donor, electron rich, alternating polymers based on repeat units of 3,4-
ethylenedioxythiophene (EDOT), 3,4-propylenedioxythiophene (ProDOT), and an acyclic dioxythiophene
(AcDOT) in varying combinations to tune steric interactions and the subsequent optical absorption for
fine colour control. Two families of polymers were formed where ProDOT2–EDOT, ProDOT–EDOT, and
ProDOT2–EDOT2 constitute new shades of blues while AcDOT2–ProDOT, AcDOT–ProDOT, and
AcDOT–EDOT yield new hues of magentas with the homopolymers of ProDOT and AcDOT and the
copolymer AcDOT2–EDOT2 serving as comparisons. The polymers were synthesized using direct (hetero)-
arylation polymerization. Examinations of the optoelectronic properties via cyclic voltammetry, spectro-
electrochemistry, and colorimetry show that by subtly varying the level of steric relaxation or strain in the
form of EDOT or AcDOT content, lower or higher energy absorption transitions are produced respectively.
This increase in relaxation or strain allows more short or long wavelength light to transmit, giving new
shades of blues or magentas respectively. Since these are all donating polymers, they exhibit changes in
contrast no less than 70% at the lmax with the exception of AcDOT–EDOT. The most desirable electro-
chromic properties were achieved with ProDOT2–EDOT2 and AcDOT2–ProDOT, with band gaps (Eg) and
neutral state L*a*b* colour values (Àa* and +a* correspond to green and red and Àb* and +b* corre-
spond to blue and yellow respectively and L* represents the lightness) of 1.74 eV, 37, 12, À63 and 2.01 eV,
56, 59, À16 respectively. The highly transmissive oxidized state colour values for ProDOT2–EDOT2 and
AcDOT2–ProDOT are 92, À3, À3 and 91, À2, À1 respectively. These structure–property relationships grant
a greater ability to tune light absorption across the visible, with colour properties similar to ECPs made
through other methods without using donor–acceptor effects. This all donor steric tuning method leads
to considerably higher levels of transparency when the polymers are fully oxidized.
Received 24th November 2014,
Accepted 29th January 2015
DOI: 10.1039/c4tc02685c
www.rsc.org/MaterialsC
electrochromic polymers (ECPs) provide a full palette of colours
from which to develop devices. These polymers all attain highly
Introduction
When comparing electrochromic materials derived from metal transmissive states, high electrochromic contrast, rapid (sub-
oxides1 with molecular organic2–4 and polymeric5 electrochromes, second) switching and long cycle lives (410 000). These material
the latter stand out for the broad array of colours that can switch to properties have allowed ECPs to be incorporated into display and
highly transmissive states. Due to their solubility allowing window type devices (ECDs) with potential commercial viability.
them to be processed via spin,6–8 spray,9,10 blade,11 and slot-die12
Numerous techniques have been employed to control the
coating along with inkjet printing,11,13 fully conjugated visible absorption and colour of the neutral state in ECPs. These
range from utilizing donor–acceptor (D–A) effects, vinylene and
a School of Chemistry and Biochemistry, School of Materials Science and
Engineering, Center for Organic Photonics and Electronics, Georgia Institute of
Technology, Atlanta, Georgia, 30332, USA. E-mail: reynolds@chemistry.gatech.edu
b Re-Synthesis Laboratory, NC/IS, BASF Corporation, Tarrytown, NY, 10591, USA
c Department of Natural Sciences, Clayton State University, Morrow, GA, 30260,
USA
azomethine14 linkers, the interchange of atoms that constitute
heterocycles or reside on the b positions, the variation of comono-
mer feed ratios or the nature of pendant groups,5 and varying the
design of the metal–ligand in metallo-supramolecular polymers.15–18
The D–A method has proven especially useful as the donor
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4tc02685c moiety positions the HOMO while the acceptor positions the
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LUMO, retaining low oxidation potentials required for long
term redox switching.19–21
Examining the spectra for the transmissive oxidized states of
these D–A ECPs shows tailing of charge carrier absorption in the
near infrared back into the visible, giving some residual visible
absorption and inducing a blue tint in the films. This effect
is evident in poly(3,4-ethylenedioxythiophene) (PEDOT)22 and some-
what minimalised in poly(3,4-propylenedioxythiophene) (ProDOT)23
ECPs with long and bulky substituents. By retaining the highly
electron rich nature of the XDOT polymers (where X represents
ethylene, propylene, butylene, etc.), we hypothesize that steric con-
trol can be the sole ‘‘handle’’ for tuning colour while maintaining
outstanding electrochromic switching to highly transmissive states.
This is exemplified in the literature by the use of an acyclic
dioxythiophene (AcDOT) in developing orange and red-coloured
ECPs.24
Here, we report a family of EDOT, ProDOT, and AcDOT
copolymers, each having regular nonrandom repeat units, and
all prepared by direct arylation polymerisation (DAP) where
neutral colours can be varied from bright pink, to magenta,
purple, periwinkle, and blue solely by steric control. We find
these polymers to switch rapidly and reproducibly to highly
transmissive oxidised states that are more transmissive and less
coloured than previously reported polymers. These ECPs explore
the effects of subtle relaxation and torsional strain on spectra for
colour control and allow us to address the issue of colour break-in
by comparing as-sprayed films to those that have been electro-
chemically cycled as steric hindrance eliminates the break-in
effect making for improved colour consistency.
As illustrated by Scheme 1, steric interactions can be viewed
as inducing torsional strain and relaxation along a conjugated
polymer backbone, controlling pi-overlap. When AcDOT units
neighbor another AcDOT, strain induces conformational twist-
ing. This breaks conjugation and leads to shorter wavelength
absorption. As torsional strain is relaxed through the incorpora-
tion of ProDOT or EDOT, planarity is induced, conjugation is
extended, and longer wavelengths of light are absorbed.
Scheme 2 Family of EDOT-, ProDOT-, and AcDOT-based ECPs prepared
for this study using direct arylation.
to yield ECPs with repeat unit structures shown in Scheme 2.
With recent developments reviewed by Leclerc et al.,27 DAP was
chosen as it allows access to the desired electron rich polymer
structures without the need to synthesize and purify organotin
Results and discussion
1. Polymer synthesis
All polymers were prepared using optimized DAP25,26 from six monomers while maintaining an alternating repeat unit that
different monomeric and dimeric XDOTs, as shown in the ESI,† is not accessible via oxidative methods. Characterization by
Scheme 1 Cartoon depicting backbone torsional interactions between neighbouring 3,4-dioxythiophene rings. Strain decreases from left to right
through relaxation.
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Table 1 All polymer molecular weight, yield, and solid state spectroscopy
data
examining the effects of subtle relaxation and torsional strain
on spectra to explore colour control.
Prior to spectroelectrochemistry, the films were electro-
chemically conditioned (so called ‘‘broken in’’) with 25 cycles
via cyclic voltammetry (CV) to the point where the current
stabilized to become reproducible. As noted from Fig. S2 (ESI†)
there is a considerable change between the first and second cycle,
after which the current minimally decreases and is fully reproduc-
Molecular weight
(Mn, PDI, solvent (kDa)) (%) onset (eV) (nm)
Yield Absorption lmax
Polymer
ProDOT
12.3, 1.77, THF
ProDOT2–EDOT 14.5, 2.00, CHCl3
ProDOT–EDOT NA
ProDOT2–EDOT2 14.1, 4.50, CHCl3
279.7, 2.08, THF
AcDOT2–ProDOT 176.0, 2.20, THF
AcDOT–ProDOT 51.8, 2.37, THF
30.9, 1.21, THF
AcDOT2–EDOT2 24.2, 4.90, CHCl3
40
82
24
70
62
82
97
75
60
1.97
1.81
1.75
1.74
2.10
2.01
2.00
1.84
1.81
555, 606
590
597
606
490
AcDOT
536, 579 ible in all cases by the 25th cycle. All of the polymers oxidize
541, 587
553
557
easily and, as shown by differential pulse voltammetry (DPV), the
AcDOT–EDOT
onsets for the oxidation potentials range from À490 to 206 mV.
AcDOT (ECP-orange) is an exception and has an oxidation
potential of 260 mV, attributed to pendant group bulkiness.24
1H-NMR and comparative GPC demonstrates all polymers to The induction of steric relaxation along the conjugated back-
be correctly structured and of sufficient molecular weight bone via the alternating addition of EDOT moieties and mono-
(Mn ranging from 14 kDa to 176 kDa) to produce continuous mer repeats having large and bulky solubilizing substituents
films where the optical properties are in the polymer limit. A presents drastic changes between the pristine and broken-in
collection of polymerization yields and molecular weights is film states. As strain is introduced through AcDOT incorpora-
given in Table 1. All of the polymers were isolated in high yield tion, this effect is mitigated with a minimal change between
(460%) with the exception of ProDOT–EDOT as it was difficult the pristine and broken in state exhibited by AcDOT2–ProDOT
to solubilize. The solubility of the polymers that are composed (Fig. S2e, ESI†). Examining the differential pulse voltammetry
of EDOTs is relatively low (ca. o2 mg mLÀ1) but sufficient enough results in Fig. S3 (ESI†), the increase in strain raises oxidation
to conduct spray casting of solutions at room temperature.
potential onsets while relaxation lowers it.
Optical properties. The break-in effect observed results in
an alteration of the film spectra after switching as exemplified in
Fig. S4 (ESI†). In concert with the changes observed by CV, the film
2. Optoelectronic properties
Electrochemistry and break-in. Solutions of the polymers spectra redshift and increase in optical density as relaxation is
were prepared at a concentration of 5 mg mLÀ1 in chloroform induced, while the changes in the spectra are reduced with increas-
and then spray cast onto ITO glass at 10 psi yielding films of ing strain. This is exemplified with AcDOT2–ProDOT where there is
varying thickness based on optical density (from 0.8 a.u. to no spectral difference between the pristine and broken in state,
1.2 a.u.) which was then correlated to a physical thickness as which can be correlated to a minimal change during CV. To
measured and averaged by profilometry (see ESI,† Table S1). examine the optical properties of the broken-in films, the normal-
The polymers are divided into two families: the blue hues ized neutral state absorption spectra for the two families of the all
(comprised of ProDOT–EDOT, ProDOT2–EDOT, and ProDOT2– donor polymers are shown in Fig. 1 with the absorption onsets and
EDOT2) and the magenta hues (comprised of AcDOT–ProDOT, lmax values reported in Table 1. Examining Fig. 1a, as strain is
AcDOT2–ProDOT, and AcDOT–EDOT) while AcDOT2–EDOT2, decreased (relaxation) through increased EDOT content, long
ProDOT, and AcDOT (purple and orange hues respectively) stand wavelength absorption, relative to the ProDOT and AcDOT
as comparison structures between the two families. These families homopolymers, is increased. All of the copolymers possess
of blue and magenta ECPs were developed with the goal of considerably lower band gaps relative to ProDOT allowing the
Fig. 1 Normalized neutral state film spectra of blue (a) and magenta (b) hued ECPs post break-in.
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absorption of long wavelength light, making the materials
lmax respectively, and the least residual absorption in the visible
appear periwinkle-blue to the eye. With this red-shifted absorp- from the fully oxidized transmissive state, even for thicker films.
tion onsetting around 700–750 nm, these polymers can be Those that incorporate EDOT have a broadened potential range
thought of as organic soluble analogues of PEDOT. Turning over which the polymer switches with subtle changes over the
attention to Fig. 1b, as steric hindrance is increased via AcDOT duration of oxidation while those that utilize AcDOT have a
addition, shorter wavelength absorption is induced relative to narrowing of the potential window with larger changes upon
ProDOT. These materials with greater strain absorb more of the each potential step. As presented by the Leclerc group, this
high energy portion of the visible spectrum, reducing the phenomena has been attributed to twiston effects, which are
amount of blue light that reaches the eye, making the materials in essence delocalized conformational defects, seen in previous
appear as ‘‘reddened’’ or ‘‘hotter’’ magentas and pinks.
polymer systems.24,34,35 Because of the high twist (less planarity)
The repeat unit structure has an overall effect on the presence, induced in AcDOT–ProDOT and AcDOT2–ProDOT, there is a resis-
or lack thereof, of vibronic coupling in the spectra. Increasing the tance to planarization on initial oxidation. However, at increasing
amount of strain via shifting from AcDOT–ProDOT to AcDOT2– oxidation steps, localized planarization begins to occur, which has a
ProDOT only showed a slight decrease in the degree of vibronic rapid propagation along neighboring repeat units.
coupling, as evidenced by the lower definition in the multiple
Upon examination of the switching kinetics via chrono-
peaks, relative to ProDOT which exhibits a high degree of absorptometry in Fig. S6 (ESI†), all of the polymers synthesized
order.22,28–30 Following this, there is a decrease in the extent of for this study are shown to undergo their full neutral coloured
vibronic coupling observed for the EDOT containing polymers. to oxidized transmissive change with switch times as fast as
This indicates that, although AcDOT is considerably bulkier than 1 second showing minimal loss in contrast. Upon switching at
1
2
1
4
EDOT, the low steric hindrance of the latter allows the propylene fast speeds of and second cycles, the polymers exhibit an
bridge on neighboring ProDOT rings to have more freedom in expected loss in contrast due to diffusion limiting processes of
conformational changes, inducing greater degrees of disorder in electrolyte migration. After cycling at high speeds, the polymers
the polymer backbone, thereby reducing the extent of vibronic are all capable of regaining high levels of contrast with no
coupling and broadening the spectra.31–33
apparent negative effects (delaminating, blistering, etc.), demon-
strating robustness during rapid changes in switching speeds.
3. Spectroelectrochemistry
These all-donor polymers exhibit a high electrochromic contrast,
switching from a vibrant coloured state to highly transmissive, as
illustrated in Fig. 2 for ProDOT2–EDOT2 and AcDOT2–ProDOT,
along with the remaining polymers in Fig. S5 (ESI†). The entire
family of materials studied exhibit changes in contrast above
60% at lmax as shown in Table 2. ProDOT2–EDOT2 and AcDOT2–
ProDOT exhibit the highest contrast values, 75 and 73 D%T at
4. Colorimetry
Upon the completion of spectroelectrochemistry the colorimetric
properties of each polymer were examined and are presented as
a*b* colour tracks in Fig. 3 (including photographs), while the
colour values of each polymer in the neutral and fully oxidized
states are shown in Table 2. The colour trend for the blue hued
Fig. 2 Electrochromic properties of films sprayed to an optical density of B1.2 a.u. of ProDOT2–EDOT2 (a) and AcDOT2–ProDOT (b). Spectro-
electrochemistry performed at potential steps of 50 mV with 0.5 M TBAPF6 in PC with a Pt wire as counter electrode; Ag/Ag+ reference electrode
calibrated to Fc/Fc+ with a value of 82 mV.
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Table 2 L*a*b* colour coordinates for all polymers in the neutral and transmissive states and total change in contrast upon switching between the
extremes
D%T
(at lmax
Neutral state
L*, a*, b*
Oxidized state
L*, a*, b*
Strained
polymers
D%T
(at lmax
Neutral state
L*, a*, b*
Oxidized state
L*, a*, b*
Relaxed polymers
)
)
AcDOT
ProDOT
ProDOT2–EDOT2
ProDOT2–EDOT
ProDOT–EDOT
ProDOT2–EDOT2
48
71
70
71
68
75
72, 42, 53
81, À2, À7
88, À1, À2
91, À2, À4
90, À2, À3
85, À4, À5
92, À3, À3
AcDOT2–EDOT2
AcDOT–EDOT
ProDOT
AcDOT–ProDOT
AcDOT2–ProDOT
AcDOT
70
63
71
72
73
48
38, 38, À44
49, 41, À35
50, 51, À37
47, 70, À36
56, 59, À16
72, 42, 53
91, À2, À4
88, À3, À4
88, À1, À2
91, À2, À3
91, À2, À1
81, À2, À7
50, 51, À37
38, 38, À44
33, 32, À63
40, 16, À43
37, 12, À63
Fig. 3 Colorimetry of blue and magenta hue ECPs. All films sprayed to B1.0 a.u. Direction of the arrow indicate increasing relaxation or strain.
ECPs with increasing relaxation progressing from orange, or passing of more blue and less red light. The colour trend for
through purple, to blue a*b* values are as follows:
the magenta hued ECPs with increasing strain progressing
from purple, through pink, to orange a*b* values are as follows:
AcDOT (42, 53) - ProDOT (51, À37) -
AcDOT2–EDOT2 (38, À44) - ProDOT2–EDOT (32, À63) -
ProDOT–EDOT (16, À43) - ProDOT2–EDOT2 (12, À63)
AcDOT2–EDOT2 (38, À44) - AcDOT–EDOT (41, À35) -
ProDOT (51, À37) - AcDOT–ProDOT (70, À36) -
AcDOT2–ProDOT (59, À16) - AcDOT (42, 53)
Referring to the neutral state spectra in Fig. 1a, as long
wavelength absorption transitions are induced through relaxa-
Again, connecting to the neutral state spectra in Fig. 1b, as
tion from the increased content of EDOT, colour values in the moderate and short wavelength light absorption is induced
neutral state progress to lower a*, indicating less purple colour through the increase in strain from the addition of AcDOT, colour
and more blue hue. This is due to the progressive transmission values in the neutral state progress to higher b*, decreasing
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Fig. 4 Most transmissive oxidized state absorption for all polymers across the visible with spectrum as inset (380–730 nm) (a). Lightness values (L*) for all
polymers as a function of voltage (b). All films sprayed to B1.0 a.u.
‘‘purpleness’’ and yielding truer magenta and pink colours due to onset was achieved (1.53 eV) with a window of transmission at
more absorption of blue light and increasing transmission or 431 nm between the two bands. The two different absorption
passing of red light. Importantly, AcDOT2–ProDOT nearly matches spectra give two unique neutral state colours. As shown earlier,
the colour values of a standard magenta as defined by Munsell ProDOT2–EDOT2 allows a majority of blue light, and a minimal
(L*, a*, b* = 52, 50, À15)36 whereas ProDOT is a bit too ‘‘purple’’ amount of red light, to transmit, giving a saturated blue colour.
after break-in. This will allow for more accurate colour mixing in With ECP-blue, its more redshifted onset from the CT absorp-
the cyan-magenta-yellow (CMY) subtractive regime using these tion allows more green than red light to transmit however, the
soluble polymers.37 Complete colorimetry and arrays of photo- higher energy p to p* filters out blue and violet light, making it
graphs for each polymer at a variety of thicknesses can be found a less saturated sky-blue colour. Though they have similar
in Fig. S7 (ESI†).
numerical colour values, ProDOT2–EDOT2 appears more satu-
The minimal hue and saturation of colour in the oxidized rated in colour because more blue light is reaching the highly
form for all of these polymers comes from their ability to sensitive S cones in the human eye.
exhibit minimal residual absorption in the visible spectrum
The electron rich character of the all donor polymers devel-
because of their electron rich, all-donating character, as shown oped here allow them to be fully bleached (oxidized to a point
by Fig. 4a and Fig. S8 (ESI†). The L* for all of the polymers as a where a*b* colour values approach zero and L* is high) at
function of voltage during switching can be seen in Fig. 4b. potentials that are lower than those exhibited by donor–acceptor
Upon oxidation all values of lightness are high.
systems. In the oxidized transmissive states, for the all donor
polymer (ProDOT2–EDOT2), the p-electron donating character of
the oxygens residing on the b-position of thiophene impart
5. Comparing all-donor and donor–acceptor systems
Through relaxation, the subtle steric tuning in all donor poly- stabilization to the oxidized form, red-shifting the overall absorp-
mers can yield materials with low band gaps (ProDOT2–EDOT2) tion (see Fig. 5a) and leading to a repression of the absorption
that are blue in the neutral state with colour values that are between 700–1000 nm relative to the donor–acceptor system
comparable to those of a blue-to-transmissive ECP designed by (ECP-blue). The ability for S and O to interact between neigh-
donor–acceptor (D–A) methods.20 A comparison between the boring dioxythiophene units and induce planarization may also
neutral states for ProDOT2–EDOT2 and the previously reported assist in this redshift.32,33 In Fig. 5a it can be seen that when
ECP-blue (ProDOT–BTD) is presented in Fig. 5a. This compar- ECP-blue is fully oxidized to the transmissive state there is
ison was chosen as the two materials possess similar colour a significant absorption tailing into the long wavelength portion
values while employing distinctly different structures to tune of the visible (600–720 nm) with a peak in the NIR at B1460 nm,
optical transitions. From Fig. 5a it can be seen that using a D–A showing a blue tint when oxidized (Fig. 5c) while ProDOT2–
approach between alternating ProDOT and BTD (benzothiadia- EDOT2 exhibits minimal tailing in the same portion with a peak
zole) a dual band absorption results where the shorter wave- beyond 1600 nm, being nearly colourless when oxidized.
length absorption is attributed to the p to p* and the longer
From the comparisons presented in Fig. 5, one can see that
wavelength absorption is from the donor–acceptor charge lower gaps can be attained with donor–acceptor effects, but at
transfer interactions.21 Using the method of steric relaxation the cost of electrochromic contrast as a consequence of tailing
in ProDOT2–EDOT2, an absorption was attained with an onset absorption due to the incorporation of electron deficient acceptors.
at shorter wavelengths (1.74 eV), remaining at the edge of the All donor systems would require significant synthetic design of
visible spectrum. Through the D–A approach, a much lower new relaxed rings to achieve comparably low gaps, but as the state
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Fig. 5 Comparison between ProDOT2–EDOT2 and ECP-blue (ProDOT–BTD) spectra of the extreme states (a), colorimetry (b), and structurally and
photographically (L*a*b* colour values below photos, Munsell blue: 29, 14, À50) (c). Both polymers sprayed to B1 a.u.
Fig. 6 One electrode comparison of optical properties of all donor and donor–acceptor polymer films in 0.04 M TBAPF4 in acetonitrile (ACN). Counter
electrode is an ITO coated glass pane of equal size with reference electrode that is the same used for other studies herein.
of the art stands with DOTs, the attainable gaps are sufficient for similar voltage window, as desired for simultaneously colouring-
colour applications with high electrochromic contrast from all bleaching displays. The ProDOT–BTD remains coloured until
donor structures nearly eliminating residual visible absorption higher voltages are applied due to its higher oxidation potential.
when fully oxidized.
Also it is worth noting in Fig. 6, the polymer that makes up the
For improved visualization of the enhanced properties of the stem and leaves of the flower, is also a donor acceptor system,
all donor systems, Fig. 6 shows a patterned film comparing ProDOT4–BTD (structure shown in Fig. S9, ESI†); however, it
several of the all donor polymers examined here (ProDOT2–EDOT2, qualitatively shows behavior that is comparable with the all donor
ProDOT, AcDOT2–ProDOT, and AcDOT) side by side with the systems. This can be explained by the fact that it has significantly
donor–acceptor ProDOT–BTD. It is evident that each of the poly- more donor character than acceptor, containing four times the
mers, save for ProDOT–BTD, switch to the bleached state in a donor amount of ProDOT–BTD.
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13 H. Sirringhaus, T. Kawase, R. H. Friend, T. Shimoda,
M. Inbasekaran, W. Wu and E. P. Woo, Science, 2000, 290,
2123–2126.
14 S. Barik, D. Navarathne, M. LeBorgne and W. G. Skene,
J. Mater. Chem. C, 2013, 1, 5508–5519.
Conclusions
Through modulating the strain along the backbone of all
donating dioxythiophene ECPs the colour of the neutral state
was controlled, making new, desired shades of blues and
magentas with high electrochromic contrast. Blue hues (low
band gaps) could be attained by utilizing relaxation in the form
of EDOT content while magenta hues (mid gaps) could be
acquired through increasing steric hindrance with AcDOT. This
steric approach was used to achieve a polymer with a magenta
neutral state as defined by Munsell, allowing more accurate
subtractive CMY colour mixing. These polymers exhibit signifi-
cant changes in spectra between the pristine and electrochemi-
cally broken-in neutral state, which can be minimized through
increased steric strain. Compared to the donor–acceptor method
of colour control, sterically tuned all donors can achieve similar
vibrantly coloured neutral states but with the advantage of
greater contrast when oxidized. Based on this revelation it would
be advantageous to minimize the use of donor–acceptor archi-
tectures in ECPs where possible or to develop new acceptors that
would minimize the tailing from fully oxidized states.
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Acknowledgements
We thank BASF for funding and scale-up support, Mike Craig
for preliminary systems that inspired the work shown here,
Natasha Teran for synthetic & purification advice, Ray Bulloch
for electrochemical input and support. We also appreciate the
collaboration from Dow Chemical, Rongjuan Cong, and James
Ponder in obtaining GPC results.
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