Variable-color poly(3,4-propylenedioxythiophene) electrochromics from precursor polymers

Article abstract of DOI:10.1016/j.polymer.2009.12.015

We have developed two approaches to processable precursors to conjugated polymers: main-chain and side-chain. Upon oxidative conversion to the conjugated polymer, a main-chain precursor yielded a nearly perfect spectral match, an identical λ_max, and an equivalent band gap to that of electrodeposited chromophore, resulting in a color match. This precursor method has the potential to incorporate any chromophore and achieve spectral and color matching with relative ease, as opposed to extensive synthetic monomer design. We describe the synthesis and characterization of a new side-chain and two new main-chain precursor polymers, each of which contains a derivative of 3,4-propylenedioxythiophene, ProDOT-Me₂. The side-chain precursor tethers one ProDOT-Me₂ molecule to a poly(norbornene) backbone; the main-chain systems are perfectly alternating copolymers of ProDOT-Me₂, one with dimethylsilane and one with tetramethyldisiloxane. Electronic and optical properties for these converted precursors were described and compared to electrodeposited PProDOT-Me₂.

Full text of DOI:10.1016/j.polymer.2009.12.015

Polymer 51 (2010) 378–382
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Variable-color poly(3,4-propylenedioxythiophene) electrochromics from
precursor polymers
Michael A. Invernale, Jayesh G. Bokria, Matthew Ombaba, Ki-Ryong Lee, Donna M.D. Mamangun,
*
Gregory A. Sotzing
University of Connecticut, Institute of Materials Science, Polymer Program and Department of Chemistry, 97 N. Eagleville Rd., Storrs, CT 06269-3136, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
We have developed two approaches to processable precursors to conjugated polymers: main-chain and
side-chain. Upon oxidative conversion to the conjugated polymer, a main-chain precursor yielded a nearly
perfect spectral match, an identical lmax, and an equivalent band gap to that of electrodeposited chro-
mophore, resulting in a color match. This precursor method has the potential to incorporate any chro-
mophore and achieve spectral and color matching with relative ease, as opposed to extensive synthetic
monomer design. We describe the synthesis and characterization of a new side-chain and two new main-
chain precursor polymers, each of which contains a derivative of 3,4-propylenedioxythiophene, ProDOT-
Me2. The side-chain precursor tethers one ProDOT-Me2 molecule to a poly(norbornene) backbone; the
main-chain systems are perfectly alternating copolymers of ProDOT-Me2, one with dimethylsilane and one
with tetramethyldisiloxane. Electronic and optical properties for these converted precursors were
described and compared to electrodeposited PProDOT-Me2.
Received 4 November 2009
Received in revised form
10 December 2009
Accepted 12 December 2009
Available online 21 December 2009
Keywords:
Conjugated polymers
Electrochromics
Precursors
Published by Elsevier Ltd.
1. Introduction
polymer can be patterned in any number of ways (spin coating,
spray coating, ink-jetting, melt-processing, drop-casting, e-spinning,
ProDOT-Me2 continues to be an exciting molecule for electro-
chromics, due in part to its high contrast and blue to (essentially)
colorless transition. Conjugated and conducting polymer process-
ing constraints have led to the search for methods other than
electrodeposition or bulk chemical polymerizations for creating
and patterning these materials. Various methods have been
explored which solve the processing issue [1–9], including
methods developed by our group such as the main-chain precursor
polymer approach [10] and the side-chain precursor polymer
approach [11,12]. Side-chain precursors have the advantage of
being made photopatternable, while main-chain precursors are
melt-processable, as we have demonstrated [13,14]. The precursor
polymer technique is an effective method for preparing films and
nanofiber mats of conducting polymers from processable precursor
polymers. Various pre- and post-derivatizations have been per-
formed in the hopes of imparting processability as well as color
variation, and hybrid materials have also been investigated [15–23].
These precursor-based processing methods involve an oxidative
conversion process that can be accomplished in the solid state via
chemical or electrochemical means. A soluble, insulating precursor
etc.); after which, oxidative conditions will cause a conversion to
occur wherein the electroactive species in the precursor polymers
couple and form an insoluble, conjugated polymer, which is
electrochromic.
These precursors are versatile materials, allowing for the incor-
poration of many different thiophenes, including 3,4-ethylene-
dioxythiophene (EDOT), as well as other aromatic heterocycles or
fused ring systems. The achievement herein is the spectral and color
match to PProDOT-Me2 that was accomplished by the conversion of
a precursor polymer. Polymer chain mobility, crosslinkingeffects, and
leeching into electrolyte during the conversion process are all factors
which dictate the effective conjugation length and
causing the observed variable-color systems.
p-orbital overlap,
The processing issue has led to many different approaches, each
of which has their own effect on the final color of the polymer
obtained. These effects make it difficult to predict the color of the
final electrochromic system. Our system eliminates the uncertainty
associated with synthetic monomer design. We are able to create
a simply prepared precursor polymer making use of any number of
aromatic, electroactive monomers. The monomers, whose colori-
metric properties are well known through electrochemical poly-
merization, can then be processed and subsequently converted. The
conjugated, electrochromic product of this process will yield
a material whose colorimetric properties are the same as that of the
* Corresponding author. Tel.: þ1 860 486 4619; fax: þ1 860 486 4745.
E-mail address: sotzing@mail.ims.uconn.edu (G.A. Sotzing).
0032-3861/$ – see front matter Published by Elsevier Ltd.
doi:10.1016/j.polymer.2009.12.015

M.A. Invernale et al. / Polymer 51 (2010) 378–382
379
electrodeposited polymer. This shows the remarkable versatility of
this method, lending itself to the development of a full library of
colors. Such development could be a necessary component for the
realization of full-color electrochromic displays.
via spray casting for imaging, except for the control material which
was electrodeposited. Films made on Pt button working electrodes
were drop cast.
2.3. Electrochemistry
2. Experimental
Switching speed and basic CV studies of the polymers are per-
formed on a platinum button electrode (0.2 cm diameter) using
a platinum flag counter electrode (1 cm2), a non-aqueous Ag/Agþ
reference electrode, calibrated against the ferrocene/ferrocenium
couple to be 0.455 V vs. NHE. The electrolyte bath was 0.1 M LITRIF
in acetonitrile. The buttons were exposed by drop-casting to
precursor polymer solutions in DCM of each polymer and allowed
to dry in air before submersion in the three-electrode cell. The scan
rate dependency of each converted polymer thin film adhered to
the electrode surface follows the modified Randles–Sevcik equation
(ip ¼ n2F2AGtn/4RT; where ip is the peak current, n is the number of
moles, F is Faraday’s constant, A is the electrode area, Gt is the total
2.1. Materials
Acetonitrile (ACN) and dichloromethane (DCM) were purchased
from Thermo-Fisher and distilled over calcium hydride to dry
before using. Xylene and toluene were purchased from Thermo-
Fisher and dried over sodium prior to use. Tetra-n-butylammonium
hexafluorophosphate (TBAPF6), lithium trifluoromethane sulfonate
(LITRIF), propylene carbonate (PC), polyethylene glycol diacrylate
(PEG-DA), and dimethoxyphenylacetophenone (DMPAP) were
purchased from Sigma–Aldrich and used as received. Indium-
doped Tin Oxide (ITO)/glass cuvette slides (0.7 ꢀ 7 ꢀ 55 mm,
1.500 ꢀ 200, and 3 ꢀ 3 inch, Rs ¼ 8–12
U, unpolished float glass SiO2
surface concentration of the film,
n is the scan rate, R is the gas
passivated) were purchased from Delta Technologies and cleaned by
sonication in acetone prior to use. Polyethylene terephthalate
(PET)–ITO substrates were purchased from CP Films, also cleaned by
sonication in acetone prior to use. Anhydrous magnesium sulfate, N-
bromo succinimide (NBS), 1-bromooctane, N,N-dimethyl form-
amide, thionyl chloride, tetrahydrofuran (THF), petroleum ether,
chloroform, hexane, dodecylbenzene sulfonic acid, 2-(hydroxy-
methyl)-2-methylpropane-1,3-diol, and triethylamine were
purchased from Thermo-Fisher and used as received. 2.5 M n-Butyl
lithium in hexanes was purchased from Sigma–Aldrich and titrated
for accuracy prior to use. 2,2-Dimethylpropane-1,3-diol, 5-norbor-
nene-2-carboxylic acid, and 5-norbornene-2-methanol (mixtures of
endo and exo for both) were each purchased from Sigma–Aldrich
and used as received. Grubbs’ Generation I Catalyst was purchased
from STREM Chemicals and stored in a glove box. Deuterated
chloroform was purchased from Cambridge Isotope Laboratories
and used as received. 3,4-Dimethoxythiophene was synthesized
according to literature procedure [24]. Silica gel was purchased from
Sorbent Technologies and was used as received. Dimethyldi-
chlorosilane and tetramethyldichlorodisiloxane were purchased
from Gelest, Inc. and distilled prior to use (stored in desiccator).
constant, and T is the temperature), such that the current response
grew linearly with a linear increase in scan rate in all cases.
Spectroelectrochemistry was carried out on films of the
precursors which were spray coated for the same amount of time
from DCM solutions of the same concentration onto ITO/glass
cuvette slides and annealed at 100 ꢁC for 20 min before conversion
(using the same electrochemical setup and reference as described
for the button electrode conversions).
3. Results and discussion
3.1. Polymer synthesis and properties
New monomers are constantly being developed and explored in
order to controllably tune the band gap, color transitions, and
polymer properties of the resultant materials. The precursor poly-
mers herein involve the use of ProDOT-Me2 moieties. ProDOT-Me2-
containing precursors are a step towards processability from
a monomer that is often polymerized via electro-oxidation to
generate a high contrast blue to colorless electrochromic. The
synthesis of a ProDOT derivative with an alcohol substituent
allowed for the attachment of a single electroactive fused hetero-
cycle to the norbornene, which undergoes ROMP to yield the
precursor abbreviated pN1P (Scheme 1). Further, the synthesis of
the silane and siloxane precursors, alternating copolymers of
a solubilizing spacer and ProDOT-Me2, appears in Scheme 2. The
2.2. Instrumental
Spectroelectrochemical measurements were taken using
a
Varian Cary 5000 Spectrophotometer with a 150 mm Integrating
Sphere DRA accessory. Colorimetric data were calculated using the
accompanying Varian software using CIE 1976 Lu0v0 color coordi-
nates, a 10ꢁ observer, and a D65 illuminant. Electrochemical
experiments were carried out with CH Instruments 660A, 400 and
400A potentiostats. Thermogravimetric analysis (TGA) was per-
formed on a TGA Q-500 using both inert and oxygen atmospheres
and Differential Scanning Calorimetry (DSC) was performed on
a DSC Q-100. The TGA was run by first heating to 120 ꢁC to drive
off solvent, cooling back to 50 ꢁC and then heating to 600 ꢁC at
20 ꢁC/min. DSC was carried out at 10 ꢁC/min. Gel Permeation
Chromatography was carried out on a Viscotek GPC MAX; using
THF, samples of these polymers were passed through 0.45
mm
filters and run through a standard GPC column with polystyrene
standard calibration. Nuclear Magnetic Resonance (NMR) spec-
troscopy was carried out on a Bruker DMX500 instrument analyzed
with XWINNMR software. Film thicknesses were obtained by using
a Veeco Dektak mechanical profilometer. Electron microscopy was
performed using an AMRAY 1810 SEM and IXRF Systems Inc 500
Digital Processing. Films with similar film thicknesses (within
25 nm of one another) were prepared on ITO/glass cuvette slides
Scheme 1. Synthetic scheme for the preparation of pN1P.

380
M.A. Invernale et al. / Polymer 51 (2010) 378–382
These precursors were used for in situ conversion, a process
recently developed for the preparation of solid state devices [35].
Under these conditions, where an electrolyte bath is not necessary,
we achieved the same color match to PProDOT-Me2 with the silane
as for the siloxane. The reason for this is the leeching of the
monomer during the conversion process, as well as the escape of
oligomeric precursor into solution prior to the application of
potential. This was spectroscopically determined by examining the
electrolyte bath both pre- and post-immersion and pre- and post-
conversion. The lower molecular weight system, PProDOT-Si, was
not as amenable to solution conversions as the other precursors
and thus resulted in a system with less conjugation.
Scheme 2. Synthetic scheme for the preparation of PProDOT-Si and PProDOT-SiOSi.
Chemical oxidants, such as ferric chloride (FeCl3), may be
applied to films on non-conductive substrates in order to achieve
conversion. However, for most characterization methods, including
optical studies, electrochemical conversion is preferred. Cyclic
voltammetry (CV) is employed to ascertain the electronic proper-
ties of the materials. This method showed the oxidation potential
necessary for the conversion of precursor to electrochromic, as well
as the redox processes of the resultant polymer. The thin film on the
electrode was also probed for its adherence to theoretical current
responses by cyclic voltammetric scan rate studies, which showed
a linear increase of current response to increases in scan rate.
For pN1P, the CVs show an onset for precursor polymer oxida-
tion, and thus conversion to conjugated polymer, at 0.88 V and
a sharp peak at 1.07 V. The polymer had observed redox-active
peaks at 0.45 V (oxidation) and 0.30 V (reduction). The redox
window for pN1P is from 0.0 V to 0.8 V. For PProDOT-Si the
conversion onset is at 0.8 V, with a pre-peak at 0.94 V and a main
peak at 1.3 V. The operating window for PProDOT-Si is from
ꢂ0.445 V to 0.656 V with redox peaks appearing at 0.242 V and
0.411 V for oxidation and at 0.225 V and ꢂ0.159 V for reduction. For
PProDOT-SiOSi the onset for conversion occurs at 0.953 V, with
a peak appearing at 1.330 V. The converted polymer had redox-
active peaks at ꢂ0.016 V and ꢂ0.325 V. The redox window is from
ꢂ0.65 V to 0.60 V. For ProDOT-Me2, the polymerization potential is
1.3 V with an onset for polymerization at 1.09 V. The operational
voltage window for the polymer, PProDOT-Me2 is ꢂ0.52 V to 0.50 V
with redox-active peaks at 0.08 V and ꢂ0.134 V. The higher voltages
necessary for redox switching of pN1P and PProDOT-Si indicate
materials which are harder to oxidize. In other words, their
respective conjugation lengths are shorter than that of PProDOT-
Me2 and PProDOT-SiOSi. The converted PProDOT-SiOSi has similar
electronic properties as compared to electrodeposited PProDOT-
Me2, offering further evidence that the resultant polymers match
each other. All CVs for growth, conversion, and polymer redox
appear in the Supporting information.
simplicity of the main-chain syntheses is evident. Synthetic details
are available in the Supporting information.
The precursor polymers were soluble in traditional organic
solvents, like toluene, THF, DMF, and DCM. Using polystyrene
calibration standards, the Mns for pN1P, PProDOT-Si, and PProDOT-
SiOSi were determined to be 65,413 (PDI ¼ 1.11; DP ¼ 195), 2100
(PDI ¼ 1.35; DP ¼ 9), and 8800 (PDI ¼ 1.22; DP ¼ 31) respectively.
DSC was carried out and it revealed the following glass transition
temperatures (Tgs) for the precursors: pN1P ¼ 65 ꢁC; PProDOT-
Si ¼ 61 ꢁC; PProDOT-SiOSi ¼ 26 ꢁC. TGA was performed on pN1P in
N2 and in O2 atmospheres, to observe decomposition; under both
atmospheres, the polymer was found to have an onset of degra-
dation occur at 375 ꢁC and 50% mass loss occurred at 415 ꢁC.
Complete degradation occurred only under O2 atmosphere, the
polymer having reached essentially no mass remaining by 475 ꢁC.
Degradation onset temperatures for the silane and the siloxane are
275 ꢁC and 347 ꢁC, respectively. Note that the temperatures for 50%
mass loss for the silane and siloxane are 375 ꢁC and 451 ꢁC,
respectively.
3.2. Electronic and optical properties
A myriad of new materials, varying in color from green to black,
have been developed and have been made processable [25–32].
Work with these chromophores in typical electrochromic device
architectures has also been important to our group with the recent
incorporation of poly(thieno[3,4-b]thiophene) as an ion storage
layer [33]. It is this desire to easily prepare processable electro-
chromic materials that makes the precursors herein of value. We
have the ability to simply incorporate a chromophore of the desired
color into a soluble, processable precursor and subsequently ach-
ieve the same color from conversion of this material as is achieved
with electrodeposition.
For the side-chain approach, oxidation causes crosslinking of the
pendant units. In the main-chain approach, cleavage of the silane
or siloxane unit (electrochemically or chemically) results in the
coupling of the aromatic units. It has been found that, though these
three precursors have the same chromophore, the siloxane
precursor yielded the closest color match to electrodeposited
ProDOT-Me2. The crosslinking in the pendant-approach, coupled
with the restrictions of the norbornene backbone itself, does not
lend itself towards achieving a saturation of properties for its
ProDOT-Me2 moiety. Indeed, as a control, electrochemical poly-
merizations of N1P from solution (the monomer prior to the
precursor polymer) result in a material with color transitions
similar to that of other PProDOTs with bulky substituents, which
are typically blue to colorless [34]. Further, bisTMS-ProDOT-Me2
monomer was also synthesized and electropolymerized from
solution, yielding a material that matched the properties of
PProDOT-Me2. This means that it is either the extent of crosslinking
or a constrained geometry of the backbone that limits linear
In order to obtain the color properties of these materials,
spectroelectrochemical measurements were taken for electro-
chemically converted films of each precursor, as well as electro-
chemically grown films of the pristine chromophore, ProDOT-Me2.
Fig. 1 illustrates the full spectroelectrochemistry for PProDOT-Me2
and converted PProDOT-SiOSi, at iterative voltages. The absorption
maximum is the same, as is the band gap (Eg ¼ 1.81 eV). This is
a clear illustration of the overlapping properties for PProDOT-SiOSi
as compared to PProDOT-Me2. The same experiments were per-
formed on PProDOT-Si and pN1P, however they did not yield
spectral matches to electrochemically generated PProDOT-Me2.
They exhibited reddish to bluish color transitions, consistent with
their different absorbance maxima (Fig. 1C).
Color coordinates were calculated for all films. Fig. 2 contains
the color coordinates for the siloxane precursor and for electro-
deposited PProDOT-Me2 in their oxidized and neutral states.
Notably, a spectral and colorimetric match to that of PProDOT-Me2
is obtained for converted films of PProDOT-SiOSi. The small
conjugation and p-orbital overlap.

M.A. Invernale et al. / Polymer 51 (2010) 378–382
381
Fig. 1. Spectroelectrochemistry in the UV–Vis–NIR for PProDOT-Me2 (A) and converted
PProDOT-SiOSi (B), showing a close spectral match of the converted precursor to the
pristine chromophore. Also, spectral overlay in the visible spectrum (C) for the neutral
states of PProDOT-Si (dashed line) and pN1P (solid line).
Fig. 2. CIE Lu0v0 color coordinates for the chromophores. PProDOT-Me2 (square),
PProDOT-SiOSi (circle), PProDOT-Si (triangle), and pN1P (diamond). Empty represents
the neutral states and filled represents the oxidized states of each polymer.
Fig. 3. SEM images of (A) PProDOT-Me2, (B) PProDOT-SiOSi, (C) PProDOT-Si, and (D) pN1P.

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M.A. Invernale et al. / Polymer 51 (2010) 378–382
difference in the color coordinate values between PProDOT-Me2
and converted PProDOT-SiOSi is attributed to the broadness of the
spectrum for the latter. Both materials display a deep purple-blue
color in their neutral state and go to a transmissive, sky-blue in the
oxidized state. The color transitions for converted pN1P and
PProDOT-Si are from reddish neutral states to light blue oxidized
states.
advantage over materials which require a ‘‘breaking in’’ period for
counterion shuttling.
Acknowledgement
We would like to thank the University of Connecticut Prototype
Fund, MC10 Inc., and the National Science Foundation (CAREER
CHE-0349121) for support of this work.
3.3. Film morphology and switching speed
Appendix. Supporting information
The nature of the conversion process and the crosslinking
process of the precursor polymers differs significantly from elec-
trodeposition, which is a nucleation and growth mechanism. Nor-
bornene precursors rely on the crosslinking of their pendant units
upon oxidation, whereas the silane and siloxane systems involve
the cleavage of the spacer units. However, the coupling of the
aromatic units can occur both intra-chain and inter-chain. For each
precursor, a film is made prior to conversion, whereas electrode-
position occurs from monomer solution. These differences in
the mechanism of film formation result in a multitude of film
morphologies and roughnesses. SEM images were taken for films of
each of the four systems, and these appear in Fig. 3. Notably, the
similarity in film morphology of PProDOT-Me2 with both PProDOT-
Si and PProDOT-SiOSi. PProDOT-Me2 is overall rough, with many
small features across its surface. PProDOT-SiOSi is similarly rough,
with somewhat larger spherical features. PProDOT-Si appears much
less uniform, however, and exhibits porosity and cracking. Finally,
pN1P exhibits an irregular topology as a result of its crosslinking
nature and norbornene backbone. Many nanoscale features appear
in the film and they are severely scattered across the surface.
The switching speeds for each material were evaluated, as well,
representing the same film area and approximately the same film
thicknesses. Thickness values averaged 300 ꢃ 25 nm. PProDOT-
Me2 switches completely within 100 ms, whereas PProDOT-SiOSi
switches in 200 ms, PProDOT-Si switches in 250 ms, and pN1P
switches in just over 700 ms. The similarity in switching properties
between electrodeposited and converted main-chain precursors is
expected. The relative slowness of the pendant system is also
expected as a result of the crosslinking nature of the conversion and
the presence of the norbornene backbone itself, causing diffusion
effects within the film. Optical memory for all of these systems in
solution persists for over 30 min, by absorbance measurements
(less than 1% loss of absorbance over this time period).
Cyclic voltammetry and UV–Vis spectra for each of the four
systems described above, as well as synthesis information for the
same, are available. The supplementary data associated with this
article can be found, in the on-line version, at doi:10.1016/j.
polymer.2009.12.015.
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