Alternating copolymers of diketopyrrolopyrrole or benzothiadiazole and alkoxy-substituted oligothiophenes: Spectroscopic, electrochemical and spectroelectrochemical investigations

Article abstract of DOI:10.1016/j.electacta.2014.07.147

A series of solution processable semiconducting donor-acceptor (DA) copolymers consisting of either diketopyrrolopyrrole or benzothiadiazole A units and alkoxy- or alkyl-substituted oligothiophene D units were synthesized. For all prepared copolymers the measured XPS spectra (C1s, S2p, N1s and O1s) were in a very good agreement with the expected chemical constitution. Spectroscopic studies of the synthesized copolymers showed that their optical band gaps were governed by the presence of the alkoxy substituents whose electron donating properties led to additional gap narrowing yielding semiconductors with band gaps of below 1.3 eV in the case of the polymers with the diketopyrrolopyrrole A unit. The same trend was observed with the electrochemical band gaps, whose values were however found to be ca. 0.4 eV superior to the corresponding optical band gaps values. Vibrational model was calculated for two of the synthesized copolymers with the goal to unequivocally attribute the observed Raman modes and to support the interpretation of the spectral changes induced by the electrochemical oxidation. It was established that the electrochemical oxidative doping of the copolymer with the benzothiadiazole A unit is limited to the oligothiophene segment in which the charge of the formed polycation is localized. To the contrary, in the case of the polymer with the diketopyrrolopyrrole A segment the charge imposed on the oligothiophene segment delocalizes towards the diketopyrrolopyrrole unit. These findings are in perfect agreement with the UV-vis-NIR spectroelectrochemistry data which show strong localization of electrochemically created charge carriers in the benzothiadiazole - oligothiophene copolymer and their metallic-like delocalization in the diketopyrrolopyrrole one. The latter seems to be very interesting not only as a potential low band gap component of organic photovoltaic cells but also, in the doped state, as electronic conductor of metallic character.

Full text of DOI:10.1016/j.electacta.2014.07.147

Electrochimica Acta 144 (2014) 211–220
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Alternating copolymers of diketopyrrolopyrrole or benzothiadiazole
and alkoxy-substituted oligothiophenes: spectroscopic,
electrochemical and spectroelectrochemical investigations
M. Goraa, W. Krzywiecb, J. Mieczkowskia, E.C. Rodrigues Maiac,d, G. Louarnc,∗
,
M. Zagorskab,∗, A. Pronb,∗
a Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland
b Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland
c Université de Nantes, CNRS, Inst. Mat. Jean Rouxel IMN, 2 rue de la Houssinière, F-44322 Nantes, France
d Depto. de Quimica, Universidade Estadual de Londrina, 86057-970 Londrina, PR, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 June 2014
Received in revised form 25 July 2014
Accepted 27 July 2014
Available online 13 August 2014
A series of solution processable semiconducting donor-acceptor (DA) copolymers consisting of either
diketopyrrolopyrrole or benzothiadiazole A units and alkoxy- or alkyl-substituted oligothiophene D units
were synthesized. For all prepared copolymers the measured XPS spectra (C1s, S2p, N1s and O1s) were in
a very good agreement with the expected chemical constitution. Spectroscopic studies of the synthesized
copolymers showed that their optical band gaps were governed by the presence of the alkoxy substituents
whose electron donating properties led to additional gap narrowing yielding semiconductors with band
gaps of below 1.3 eV in the case of the polymers with the diketopyrrolopyrrole A unit. The same trend
was observed with the electrochemical band gaps, whose values were however found to be ca. 0.4 eV
superior to the corresponding optical band gaps values. Vibrational model was calculated for two of
the synthesized copolymers with the goal to unequivocally attribute the observed Raman modes and
to support the interpretation of the spectral changes induced by the electrochemical oxidation. It was
established that the electrochemical oxidative doping of the copolymer with the benzothiadiazole A unit
is limited to the oligothiophene segment in which the charge of the formed polycation is localized. To
the contrary, in the case of the polymer with the diketopyrrolopyrrole A segment the charge imposed
on the oligothiophene segment delocalizes towards the diketopyrrolopyrrole unit. These findings are in
perfect agreement with the UV-vis-NIR spectroelectrochemistry data which show strong localization of
electrochemically created charge carriers in the benzothiadiazole - oligothiophene copolymer and their
metallic-like delocalization in the diketopyrrolopyrrole one. The latter seems to be very interesting not
only as a potential low band gap component of organic photovoltaic cells but also, in the doped state, as
electronic conductor of metallic character.
Keywords:
organic semiconductors
diketopyrrolopyrrole-oligothiophene
copolymers
benzothiadiazole-oligothiophene
copolymers
UV-vis-NIR spectroelectrochemistry
Raman spectroelectrochemistry vibrational
model
© 2014 Elsevier Ltd. All rights reserved.
1. Introduction
processed can be controllably tuned and adapted to a given appli-
cation through the design of appropriate D and A building blocks.
Alternating donor-acceptor (DA) conjugated copolymers play
an important role in the development of organic electronics
[1–3]. They are also interesting from the point of view of applied
electrochemistry since they exhibit electrochromic and electroflu-
orochromic effects [4]. Vivid interest in this family of electroactive
materials is caused by the fact that their redox, electronic and opto-
electronic properties as well as their capability of being solution
Benzothiadiazoles and diketopyrrolopyrroles constitute pop-
ular electron accepting units in DA copolymers, especially with
non-functionalized or functionalized oligothiophene D units [3].
Many of these polymers were tested as active components of such
devices as field effect transistors (FETs) [5–11], photovoltaic cells
(PC) [1,2,6,7,12–15], light emitting diodes [16,17], and others.
Copolymers of oligothiophenes and benzothiadiazoles or dike-
topyrrolopyrroles are electrochemically active both in oxidative
and reductive regimes since the presence of an electron accept-
ing center in the polymer repeating unit increases its electron
affinity (|EA|) and diminishes the electrochemical gap [3]. This
∗
Corresponding authors.
E-mail address: zagorska@ch.pw.edu.pl (M. Zagorska).
http://dx.doi.org/10.1016/j.electacta.2014.07.147
0013-4686/© 2014 Elsevier Ltd. All rights reserved.

212
M. Gora et al. / Electrochimica Acta 144 (2014) 211–220
H17C8O
S
OC8H17
S
effect is also reflected in the value of the optical band gap which
makes some of these polymers interesting, solution processable
near infrared absorbers [14,18,19]. Moreover, a pronounced elec-
trochromic effect, associated with the oxidation or reduction of the
neutral polymers, is observed, which involves different spectral
regions than those reported for the large majority of conjugated
polymers [3].
n
S
N
N
S
P1
In this paper we present
a joint experimental - theo-
retical study on series of new diketopyrrolopyrrole (or
a
benzothiadiazole)–oligothiophene copolymers in which the elec-
tron donating effect of the oligothiophene unit is amplified by the
presence of alkoxy substituents. The obtained electrochemical and
spectroelectrochemical (UV-vis-NIR and Raman) results are sup-
ported by the calculation of the vibrational models for the studied
copolymers.
To clearly identify the effect of alkoxy substituents on the opti-
cal and redox properties of the alternating copolymers studied, a
copolymer with alkyl substituents is also studied for comparative
reasons.
N
N
O
H17C8O
S
OC8H17
S
O
n
S
P2
2. Experimental
2.1. Synthesis
The investigated alternating copolymers are depicted in
Scheme 1. Polymer P1 was obtained from 5,5ꢀꢀ-dibromo-
N
N
O
3ꢀ,4ꢀ-dioctyloxy-2,2ꢀ:5ꢀ,2”-terthiophene
and
2,1,3-benzot-
hiadiazole-4,7-bis(boronic acid pinacol ester) through Suzuki
coupling. The remaining three copolymers were synthesized
using the direct arylation method. In particular, polymer P2
and polymer P3 were prepared from 3,6-bis(thiophen-2-yl)-
2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione
and 2,5-dibromo-3,4-dioctyloxythiophene or 5,5ꢀꢀ-dibromo-3ꢀ,4ꢀ-
dioctyloxy-2,2ꢀ:5ꢀ,2”-terthiophene, respectively. In the preparation
of polymer P4 the bromination of the substrates was inversed
i.e. dibromo derivative of 3,6-bis(5-bromothiopen-2-yl)-2,5-
S
S
S
O
n
S
S
H17C8O
OC8H17
P3
bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione
was
reacted with 4,4ꢀ-dioctyl-2,2ꢀ-bithiophene. All four polymers
were precipitated with methanol, then repeatedly washed with
this solvent and finally dried. P1 was additionally washed with
acetone to remove the shortest oligomers as recommended in [20].
Its macromolecular parameters were as follows; Mw = 3499 Da,
PDI = 4.7. Sequential fractionation of P3 and P4 with CH2Cl2 and
CHCl3 yielded two fractions. In both cases more abundant CH2Cl2
fraction was taken for further investigations (for P3 Mw = 4432 Da,
PDI = 2.8; for P4 Mw = 5609 Da, PDI = 2.4. P2 could not be fraction-
ated in this manner and as prepared polymer after washing with
methanol and drying showed Mw = 5275 Da, PDI = 4.61. Molecular
weight was determined by size exclusion chromatography (SEC)
in CH2Cl2. The detailed description of the synthetic procedures for
monomers and polymers can be found in Supplementary data.
N
N
O
H17C8
S
S
O
n
S
S
C8H17
P4
Scheme 1. Chemical formulae of the investigated DA alternating copolymers.
2.2. X-ray photoelectron spectroscopy
XPS measurements were performed with an Axis Nova instru-
ment from Kratos Analytical spectrometer with Al K␣ line
(1486.6 eV) as an excitation source. Survey spectra were acquired
at pass energies of 80 eV. The core level spectra (C 1s, O 1s, N
1s and S 2p) were acquired with an energy step of 0.1 eV and
using a constant pass energy mode of 20 eV, to obtain data in
a reasonable experimental time (energy resolution of 0.48 eV).
The pressure in the analysis chamber was maintained lower than
10−7 Pa.
Shirley-type and curve fitting is accomplished with a mixture of
Gaussian-Lorentzian functions. Concerning the calibration, bind-
ing energy for the C1s hydrocarbons peak was set at 284.8 eV.
The error in defining the position of peaks is estimated at about
0.1 eV. No surface cleaning, using Ar sputtering for example was
applied. It is known that carbonaceous atmospheric contamina-
tion on the sample’s surface usually occurs, but taking into account
the soft matter nature of the polymers studied, ion sputtering can
be suspected of changing the chemical composition and inducing
structural damage.
Data analysis was carried out using CasaXPS software. In
the used approach, the background spectra are considered as

M. Gora et al. / Electrochimica Acta 144 (2014) 211–220
213
Table 1
2.3. Raman scattering and infrared absorption spectroscopy
Optical parameters of the studied copolymers (P1-P4).
Raman spectra of neutral and oxidized copolymers were
recorded on a Rénishaw InVia reflex spectrometer (␭exc = 514.5 nm,
laser power = 10 mW, 2 ␮m diameter spot, typical exposure times
60 sec) or on a FT Raman Brucker RFS 100 spectrometer with the
near-IR excitation line (1064 nm, laser power = 50 mW). Fourier
transform infrared spectroscopy (FTIR) was recorded on a Bruker
Vertex 70 spectrometer (4 cm−1).
polymer solution
max/nm
thin film
max/nm
thin film
␭onset/nm
Eg optical/eV
␭
␭
P1
P2
P3
P4
407, 595
390, 751
432, 686
309, 654
415, 628
416, 755
445 (shoulder), 692
432, 690
833
1000
908
1.49
1.23
1.37
1.39
892
2.4. UV-vis-NIR spectroscopy
constants expressed in terms of internal coordinates. It is neces-
sary to assume that certain force constants must be fairly local i.e.
depending on the position and the type of a few nearest neighbours.
For this reason they can be transferred from other compounds of
appropriate chemical structure. Hence, force constants correspond-
ing to common groups were expected to be transferable among
these molecules. The number of force constants utilized in the
calculations has to be minimized. This approach leads to a signif-
icant reduction of the number of independent parameters, which
becomes much lower than the number of observed frequencies.
The final parameters are adjusted from experimental and cal-
culated frequencies using a least-squares root procedure. The
vibrational normal-mode assignments were based on the best-fit
comparison of the calculated Raman spectrum with the observed
FT-Raman spectrum, after adjusting the calculated wavenumbers.
In these conditions the refinement yielded an average error of
7 cm−1 between the observed and calculated frequencies.
Solution spectra of P1 - P4 dissolved in chloroform and solid
state spectra of their thin layers were recorded using a Varian Cary
5000 spectrometer. Thin layers of polymers were deposited on a
quartz substrate by drop-casting.
2.5. Cyclic voltammetry, UV-vis-NIR and Raman
spectoelectrochemistry
For the cyclic voltammetry studies thin layers of polymers
P1–P4 were deposited on a platinum electrode. The measurement
were carried out in a three electrode cell in a dry argon atmosphere
on an Autolab potentiostat (Eco Chimie). The electrolytic medium
consisted of a 0.1 M solution of Bu4NBF4 in acetonitrile. A plat-
inum counter electrode, and a Ag/0.1 M AgNO3/CH3CN reference
electrode were used. The potential of the reference electrode with
respect to the ferrocene redox couple was always measured after
each experiment. The same working electrode, cell and electrolyte
were used for Raman spectroelectrochemistry. For UV-vis-NIR
spectroelectrochemical measurement thin layers of polymers were
deposited on an ITO electrode, the rest of the electrochemical
equipment remaining the same. Spectroelectrochemical experi-
ments were carried out in the oxidative mode. The potential of
the working electrode was being increased in small increments.
After each potential raise, a wait time was applied until the quasi-
equilibrium state was reached, then the UV-vis-NIR or Raman
spectrum was registered. It was assumed that the equilibrium state
was reached when the potential change induced current became
negligible.
3. Results and discussion
The main effect of the donor-acceptor interactions in alter-
nating DA copolymers is narrowing of the band gap, through
re-hybridization of bonds. This effect is manifested by a sig-
nificant bathochromic shift of their ␲ − ␲* transitions towards
lower energies (bathochromic shift) as compared to those recorded
for the corresponding homopolymers [25]. Being low band gap
semiconductors, these copolymers are very well suited for the fab-
rication of organic photovoltaic cells or ambipolar transistors [3].
UV-vis spectra registered for chloroform solution and thin films
of all polymers studied are shown in Fig. 1, whereas the corre-
sponding spectroscopic data, together with the optical gap values,
are collected in Table 1. In the solution spectrum of P1 two clear
absorption bands can be distinguished at 407 nm and at 595 nm
which undergo a bathochromic shift in the solid state spectrum to
415 nm and 628 nm, respectively. In similar polymers both bands
bathochromically shift with increasing strength of the donor [25].
The position of the lower energetic band in P1 clearly manifests
the electron donating effect of the alkoxy substituents since in
the spectra of benzothiadiazole-based copolymers in which the
D segments consist of either unsubstituted or alkyl-substituted
thiophene rings, this band appears in the 530 nm - 560 nm range
[26–28]. It is found at higher wavelengths than in the case of P1
only in DA copolymers where the rotation of the adjacent thio-
phene rings in the D segment is blocked by an appropriate bridging
group [29] or the D unit consists of fused heterocyclic rings [30–33].
Copolymers of diketopyrrolopyrrole with oligothiophenes also
exhibit two bands in their UV-vis spectra: a very weak band in the
vicinity of 400 - 450 nm and a strong one of lower energy whose
position is dependent on the nature of the donor segment [34]. In
the spectra of polymers in which the D unit consists of alkyl sub-
stituted oligothiophene this band is located at shorter wavelengths
than in alkoxy-substituted P3 [34]. If fused aromatic groups are
used as donor segments such as thienoacenes [35] or pyrroloinda-
cenodithiophene [36] than this band is bathochromically shifted as
compared to the corresponding band in the spectrum of P3.
2.6. Computational methods
Quantum chemical calculations were limited to P1 and P3
as representative examples of two types of DA polymers stud-
ied. Density functional theory calculations were performed with
Gaussian 09 [21] at the B3LYP level of theory and employing the
6-31 + G(d,p) basis set for the macromolecules structure optimiza-
tion. The geometry optimization resulted in a nonplanar geometry,
and no imaginary frequencies were observed in the calculated
spectrum. However, to interpret more precisely the vibrational
spectra and their experimental evolutions as a function of the oxi-
dation potential, normal coordinate calculations were carried out
using general valence force field. This calculations method has pre-
viously been successfully applied to other conjugated polymers
[22] and polymers consisting of alternating conjugated and non-
conjugated segments [23]. The used methodology is described in
detail in [24]. It can be briefly summarized as follows. The calcu-
lations of the force field and frequencies are carried out using the
Fourier’s dynamic matrix. Since the macromolecules of P1 and P3
possess translational symmetry, the calculations can be restricted
to one repeat unit. They require the knowledge of force constants,
bond lengths, and bond angles. An internal coordinate system is
used to compute the fundamental vibrations of the studies copoly-
mers. The local redundancies can be handled by suitable symmetry
coordinates. The calculation starts from a minimal set of force

214
M. Gora et al. / Electrochimica Acta 144 (2014) 211–220
work conditions, the accuracy is surprisingly good. A small excess
(A)
a) d) c) b)
a) P1
b) P2
c) P3
d) P4
of oxygen, with respect to the stoichiometries of the studied poly-
mers, consistently found for all samples, probably originates from
the fact that no surface cleaning by Ar sputtering was applied (see
Experimental part).
The following most striking features of the XPS spectra should be
noted: (i) a shift of the signals corresponding to N(1s) from 399.4 eV
in P1 (C = N-C) to 399.9 eV in P3 (C-N-C); (ii) two clearly identifi-
able S(2p1/2) peaks in the spectrum of P1 at 163.8 eV and 164.8 eV,
ascribed to thiophene and benzothiadiazole sulfur, respectively);
(iii) the O(1s) peak in the spectrum of P1 (C-O-C of alkoxy groups), at
532.7 eV splitted into two components in the corresponding spec-
trum of P3, at 532.7 and 530.8 eV, the latter being consistent with
the presence of the keto groups; (iv) a small atomic percentage
of bromine (< 0.9 0.2%) is recorded, originating from chain end
groups, but this contribution is minor, especially for P2 and P3.
Finally, it should be noticed the C(1s) signal can be deconvoluted
into two or three main lines, indicating the presence of at least 3
types of carbon bonds: sp2 carbons in the conjugated core and sp3
carbons in the substituents (∼ 284.8 eV), C–S (thiophene, thiadia-
zole), C–N (thiadiazole), C–O–C (alkoxy groups) (286.0 eV) and C = O
(diketopyrrolopyrrole) (>287 eV).
400
600
800
1000
1200
Wavelength (nm)
(B)
c)
a) d)
b)
a) P1
b) P2
c) P3
d) P4
Cyclic voltammetry is a convenient research tool for studying
the effect of the nature of A and D segments in a given alternating
copolymer on its redox properties. In Fig. 3a and b cyclic voltam-
mograms of thin layers of P1 and P3, deposited on a platinum
electrode by casting from a chloroform solution, are compared,
whereas those of P2 and P4 can be found in Fig. S1 of Supplementary
information. In the oxidative doping mode P1 yields two strongly
overlapping anodic peaks at 0.28 V and 0.54 V vs Fc/Fc+, which have
even broader cathodic counterparts. This part of the cyclic voltam-
mogram strongly resembles that of an alternating copolymer of
3,4-dialkoxythiophene and 2,2ꢀ-bithiophene [37], whose repeating
unit is the same as the D unit in P2. A small shift of the two anodic
peaks towards higher potentials is caused by the presence of ben-
zothiadiazole acceptors in the polymer main chain. In the negative
potentials range a reversible redox couple is observed correspond-
ing to the reduction of the neutral polymer chain to a polyanion
and its consecutive oxidation to the neutral state upon the scan
reversal.
In the oxidative mode, the electrochemical behavior of P3
resembles that of P1, its reduction at negative potentials is however
only partially reversible and the cathodic peak has two overlap-
ping anodic counterparts (Fig. 3b). The voltammogram of P4 in both
anodic and cathodic parts is similar to that registered for P3, how-
ever, the oxidation peak is shifted towards higher potentials. This
can be considered as a manifestation of weaker electron donat-
ing properties of alkyl substituents as compared to alkoxy ones.
The electrochemical parameters of P1–P4, together with their elec-
trochemically determined ionization potentials (IPs) and electron
affinities (EAs) as well as electrochemical band gaps, are listed in
Table 3.
400
600
800
1000
1200
Wavelength (nm)
Fig. 1. Normalized UV-vis absorption spectra of copolymers for solutions in CHCl3
(A) and for thin solid layers (B).
A comparison of P1 and P2 is instructive. Since in both polymers
the D units are identical, the observed difference in the ␲ − ␲* bands
position must originate from more pronounced electron accept-
ing character of diketopyrrolopyrrole which results in stronger
DA interactions in P2. The effect of electron donating alkoxy sub-
stituents is, in turn, clearly evidenced by comparing the spectrum
of P2 with that of P3. Shortening of the oligothiophene segment
from five rings in P3 to three rings in P2, while keeping the central
ring with two alkoxy substituents unchanged, results in an addi-
tional bathochromic shift of the ␲ − ␲* band and further band gap
narrowing. In P4, which contains alkyl substituents in the oligothio-
phene segment, the ␲ − ␲* transition is more energetic in solution
spectra than in the case of P2 and P3 giving rise to a clear band at
shorter wavelengths, again consistent with weaker DA interactions.
The XPS spectra of P1 and P3 are shown in Fig. 2 as represen-
tative examples whereas all XPS data are collected in Table 2. For
all copolymers the surfacial elemental composition, as measured
by XPS, is in good agreement with expected structures. In addition,
only residual quantities of bromine arising from the substrates can
be detected. However, it should be stressed that under the applied
It should be stated here that the listed–IP and EA values, deter-
mined from cyclic voltammetry experiments, are the same as the
values termed as “electrochemically determined HOMO and LUMO
levels”. We prefer–IP and EA instead of HOMO and LUMO levels
since the latter are not experimental observables but the results of
quantum chemical calculations, although their values being closely
related to–IP and EA. Moreover, electrochemical IP values very well
correlate with IPs determined by UPS which is a direct method of
this parameter determination [38].
Table 2
Atomic percentages of C, N, O, S and Br measured for thin films of P1 - P4.
polymer
C (%)
N (%)
O (%)
S (%)
Br (%)
To summarize this part of the research, the obtained cyclic
voltammetry results are consistent with the UV-vis ones, showing
in addition that the potentials of the oxidative doping in the studied
copolymers (and by consequence their ionization potentials (IP))
are governed mainly by the nature of the solubilizing substituents
P1
P2
P3
P4
83.7
83.8
84.1
88.1
3.3
2.8
2.5
2.5
5.1
10.0
9.2
7.0
3.4
4.1
4.2
0.9
< 0.1
< 0.1
0.2
4.9

M. Gora et al. / Electrochimica Acta 144 (2014) 211–220
215
C−S−C
(a) C1s
P3
C−(C,H)
(b) S2p
P3
S2p3/2
S2p1/2
C−N, C−S
C−O−C
C=O
C−S−C
C−(C,H)
C−N, C−S
C−O−C
N−S−N
P1
170
P1
290 289 288 287 286 285 284 283
Binding energy (eV)
168
166
164
162
Binding energy (eV)
C−O−C
(c) N1s
(d) O1s
C−N−C
C=O
P3
P1
P3
C=N−S
C−O−C
P1
402
400
398
396
538 536 534 532 530 528
Binding energy (eV)
Binding energy (eV)
Fig. 2. XPS spectra of P1 and P3: a) C1s, b) S2p, c) N1s and d) O1s core level spectra and curve-fitting results.
in the oligothiophene segment and to a lesser extent by the nature
of the acceptor group. The latter determines, in turn, their potential
of the reductive doping and the electron affinity (EA).
with increasing wavelength. This absorption tail is attributed to
delocalized bipolarons and is characteristic of the metallic state
[39]. By electron-hole symmetry, the same spectroelectrochemi-
cal behaviour is found in the process of electrochemical reduction
of the neutral conjugated polymers to polyanions (reductive or n-
type doping). The doping-induced bands correspond in this case
to either localized or delocalized negative bipolarons and polarons
[40].
In Fig. 4a UV-vis-NIR spectra of P1, registered for increasing
working electrode potentials, are presented. The obtained spec-
troelectrochemical data are in line with the voltammetric ones.
The first oxidation induced spectral changes appear at E = 0.06 V
vs Fc/Fc+ i.e. at a potential which is only 50 mV lower than the
potential of the oxidation onset in the cyclic voltammogram. This
small shift of the oxidative doping onset, as probed by the UV-vis-
NIR spectroelectrochemistry, can be rationalized by the fact that
In the UV-vis-NIR spectroelectrochemical investigations of elec-
troactive conjugated polymers two types of spectral evolution are
typically found. In the first type, oxidation of a neutral polymer
chain to a polycation, advancing with increasing working electrode
potential, results in bleaching of the absorption band character-
istic of the ␲ − ␲* transition with concomitant growth of two
oxidation-induced bands ascribed to specific charge storage con-
figurations, namely localized bipolarons and polarons. These two
bands persist after the complete ␲ − ␲* band bleaching. In the
second type of spectroelectrochemical response, at potentials cor-
responding to the complete or nearly complete bleaching of the
␲ − ␲* band, the two oxidation-induced bands merge, yielding
“an absorption tail” showing monotonically increasing absorbance
Table 3
Electrochemical parameters of the studied copolymers (P1 - P4). Potentials given vs Fc/Fc+.
polymer
Ered1/V
E
ox1/V
Ered1(onset)/V
Eox2/V
Ered2/V
Eox2(onset)/V
EA*/eV
IP*/eV
Eg electrochem/eV
P1
P2
P3
P4
-1.76
-1.56
-1.62
-1.69
-1.62
-1.47
-1.53-1.32
-1.57
-1.67
-1.45
-1.52
-1.52
0.28, 0.54
0.52
0.34
0.17
0.42
0.31
0.47
0.11
0.25
0.20
0.21
-3.43
-3.65
-3.58
-3.58
5.21
5.35
5.30
5.31
1.78
1.70
1.72
1.73
0.5
*Calculated using the following equations: EA(eV) = -|e|(Ered1(onset) + 5.1), IP(eV) = |e|(Eox2(onset) + 5.1)

216
M. Gora et al. / Electrochimica Acta 144 (2014) 211–220
(a)
40
20
(a)
0.56V
0.51V
0.46V
0.41V
0.36V
0.31V
0.26V
0.21V
0.16 V
0.11V
0.06V
-0.04V
0
-20
-40
-60
-80
-2.0
-1.5
-1.0
-0.5
E (V)
0.0
0.5
(b)
300 600 900 1200 1500 1800 2100 2400 2700
Wavelength (nm)
40
20
0
(b)
-20
-40
0.65V
0.60V
0.55V
0.50V
0.45V
0.40V
0.35V
0.30V
0.25V
0.20V
0.15V
0.10V
0.05V
-1.5
-1.0
-0.5
0.0
0.5
E (V)
Fig. 3. Cyclic voltammogram of P1 (a) and P3 (b). Electrolyte 0.1 M Bu4NBF4 in
acetonitrile, scan rate 50 mV/s, E vs Fc/Fc+.
the measurements presented here were performed in a quasi-static
mode: the electrode potential was raised in small increments and
after each potential raise a wait time was applied until the equilib-
rium state was reached in which the measured current dropped to
zero or to a negligible value. The UV-vis-NIR spectrum was reg-
istered after this wait time. This technique tends to detect the
first signs of oxidative doping at slightly lower potentials than
cyclic voltammetry which is dynamic in nature. At the end of the
oxidation (E = 0.56 V) two doping-induced broad bands at 790 nm
and 1770 nm dominate the spectrum. This means that the formed
charge storage configurations (positive bipolarons and polarons)
are localized and the metallic state has not been reached. Simi-
lar spectroelectrochemical behaviour was found for an alternating
copolymer of benzothiadiazole and unsubstituted bithiophene
[26]. There exist however reports on “metallic-like” UV-vis-NIR
spectra of fully doped alternating copolymers of alkoxysubstituted
bithiophenes and benzothiadiazole [41] as well as copolymers of
dithienopyrroles and benzothiadiazole [31].
300
600
900
1200 1500 1800 2100 2400
Wavelength (nm)
(c)
P1
P3
Spectral response of P3 to the working electrode potential
increase is shown in Fig. 4b. Again, the obtained results are fully
consistent with the voltammetric data. Similarly as in the case of P1,
oxidation-induced spectral changes start at a slightly lower poten-
tial (0.15 vs Fc/Fc+), than the onset of the anodic peak in the cyclic
voltammogram. One should notice that at the highest potential,
corresponding to the end of the anodic peak, the recorded spectrum
is nearly “metallic” in character.
The change of the appearance of the polymer films during
their UV-vis-NIR spectroelectrochemical investigation is shown in
Fig. 4c. The oxidation of P1 induces a variation of its color from
greenish to blue. The concomitant increase of the film transparency
neutr
ox
neutr
ox
Fig. 4. Evolution of the UV-vis-NIR spectra with increasing working electrode poten-
tial: (a) P1 (b) P3. Electrolyte 0.1 M Bu4NBF4 in acetonitrile, E vs Fc/Fc+. (c) Images
of colored neutral and oxidized films of P1 and P3.
is a result of bleaching of both absorption bands (415 and 628 nm)
present in the visible part of the neutral polymer spectrum. Similar
features are observed for P3, which changes the color from dark
blue, characteristic of neutral low band gap polymers, to highly
transparent light blue.

M. Gora et al. / Electrochimica Acta 144 (2014) 211–220
217
Raman spectroelectrochemistry is also a very powerful tool
for studying the redox properties of conjugated polymers. The
oxidation of these polymers chain to polycations (or reduction
to polyanions) affects the force constants of selected bonds and
frequently induces changes in the chain geometry. As a result pro-
found doping-induced changes in the Raman spectra are observed
which involve disappearance or shifts of some bands and appear-
ance of new ones. One should also be aware of the fact that in Raman
spectroelectrochemical studies of conjugated polymers resonance
effects are frequently observed and the resulting spectra strongly
depend on the position of the excitation wavelength with respect to
the observed electronic transitions. Moreover, since the oxidation
orreductionprocesses profoundly changethe UV-vis-NIRspectraof
these compounds the resonance conditions change with increasing
(decreasing) working electrode potential [23,42]. All these factors
must be taken into consideration in any spectroelectrochemical
study.
In Fig. 5 Raman and IR spectra of all four copolymers studied
are presented. To facilitate the attribution of bands and to better
elucidate structural changes induced by the oxidative doping we
have established vibrational models for P1 and P3. In developing
these models we have optimized the macromolecules geometries
using DFT calculations. The obtained bond lengths as well as angles
between bonds and the obtained force constants (FG matrix) are
shown in Chart S1 and S2 of Supplementary Information. The exper-
imentally measured and calculated vibrational modes are listed
in Table 4, where their attribution is additionally given (for more
complete description of Raman scattering and infrared absorption
bands of the studied copolymers see Supplementary Information).
Vibrational attributions listed in Table 4 were done with the sup-
port of previous publications and exploiting the potential energy
distributions (PED) [44,45]. Results presented in these tables are
largely self-explanatory, so the discussion is limited to the evolu-
tion of the modes during the spectroelectrochemical experiments.
In addition, it should be stressed here that infrared absorbtion data
were mainly dominated by bands from alkyl group. So, assignments
of infrared absorption bands was not fully performed.
Fig. 6 shows the evolution of the Raman spectra of P1 with
increasing working electrode potential. The observed spectral
changes start at E = 0.10 V vs. Fc/Fc+, fairly consistent with the
UV-vis-NIR spectroelectrochemical data. The most pronounced
changes can be described as follows. In poly(thiophene) homopoly-
mers with alkoxy substituents two bands in the spectral range
of 1420 -1480 cm−1 dominate the spectrum. They are ascribed
to C␣-C␤ stretchings in the nonsubstituted and alkoxysubstituted
thiophene rings [37,46]. These bands can be found in the spectrum
of neutral P1 at 1428 cm−1 and 1471 cm−1. Upon electrochem-
ical oxidation they shift to lower wave numbers. In particular,
at E= 0.6 V the former is located at 1417 cm−1, whereas the sec-
ond merges with the residual 1428 cm−1 band and is present as
a shoulder. The band at 1368 cm−1, originating from C␤-C␤ of the
Fig. 5. Vibrational spectra of the studied copolymers: (A) Raman spectra (514.5 nm)
of P1-P4; (B) Raman scattering and infrared absorption of P1; a) FTIR spectrum, b)
and c) Raman scattering (514.5 and 1064 nm excitation wavelength respectively);
(C) Raman scattering and infrared absorption of P3; a) FTIR spectrum, b) and c)
Raman scattering (514.5 and 1064 nm excitation wavelength respectively).
thiophene ring shifts towards higher wavenumbers (1378 cm−1
)
upon the electrochemical oxidation [47]. Concomitantly the band
at 1205 cm−1, ascribed to C␣-C␣ intering stretchings in the olig-
othiophene segment increases in intensity. These changes are
very similar to those observed in poly(thiophene) homopolymers
containing alkoxy substituents [37,46]. Thus, Raman spectroelec-
trochemical results seem to indicate that the oxidation of P1 takes
place on the donor units whose benzoid-type structure is being
transformed into the quinoid one as in the case of poly(thiophene)
homopolymers. Since the UV-vis-NIR spectra show that the formed
bipolarons are localized, it may be concluded that the benzoth-
iadiazole units constitute obstacles against the delocalization of
these charge storage configurations. To confirm this assumption,
we can stress here that the main characteristic Raman bands of
the benzothiadiazole moieties, the C–C stretch of benzothiadia-
zole aromatic ring observed at 1521/1532 cm−1 and 1269 cm−1, are
unaffected by the electrochemical oxidation.
Raman spectroelectrochemical response of P3 is shown in Fig. 7.
Up to E= 0.2 V vs. Fc/Fc+, apart from lowering of the peaks intensity
no change in their positions can be noticed. This means that only
non oxidized segments give rise to Raman bands, being however
progressively more and more out of resonance. No bands charac-
teristic of the oxidized (doped) form can be detected. Above this

218
M. Gora et al. / Electrochimica Acta 144 (2014) 211–220
Table 4
Calculated and experimentally measured frequencies (in cm−1) of the main vibrational modes of P1and P3 in their neutral form. Vibrational modes of regioregular poly(3-
hexylthiophene), P3HT, reported in [43] are given for comparison.
P1
P3
PTh/P3HT
Assignments
Exptl bands
Calcd
Exptl bands
Calcd
␯/cm−1
modes
modes
3062(1)
2950(1)
2920(1)
2850(1)
3063
3065(1)
2950(1)
2920(1)
2850(1)
1658(1)
1548(1)
1508
3063
-
-
3072
2950
2920
2850
C–H stretch (thiophene ring)
-
-
-
methyl symmetric CH stretching
methylene antisymmetric CH stretching
methylene antisymmetric CH stretching
C = O stretch (DPP group)
C = C asym stretch (DPP group)
C = C sym stretch (DPP group)
-
1655
1541
1521
1521
1471
1428
1368
1530
1465
1430
1370
C–C aromatic stretch (BTD group)
C = C asym stretch (thiophene group)
C = C sym stretch (thiophene group)
C–C stretch (thiophene group)
1467
1423
1370
1327
1465
1430
1370
1331
1515
1438
1385
C–C stretch (DPP group)
1314
1269
1205
1320
1270
1225
C = N stretch + C = N–S def (BTD group)
C–C stretch (benzothiadiazole aromatic ring)
C–C inter-ring stretch
C–N strech + ring deformation (DPP group)
C–H bend (thiophene ring)
1233
1066
1053
1023(1)
1225
1065
1045
1021
1208
1063
1023(1)
887
1045
1021
899
1045
1015
C–O-C deformation (alkoxythiophene group)
N-S-N + ring deformation (BTD group)
875
883
854
860
C–H out-of-plane bend (BTD group)
827
812
DPP and thiophene ring in-plane deformation
In-plane ring deformation of the BTD group
C-S-C stretch + ring defm (thiophene ring)
730
728
705
684
716
696
724
541
551
in-plane ring deformation of the BTD group
(1)
from infrared absorption spectra; BTD - benzothiadiazole; DPP - diketopyrrolopyrrole
at 1327 cm−1 remains essentially unchanged with the electrode
potential increase. This observation indicates that the ␲-system of
DPP is affected by the oxidation of the oligothiophene segment and
the positive charge imposed on the chain during electrochemical
oxidation can delocalize through the conjugated pathway of the
DPP moiety. The obtained results are fully consistent with the UV-
vis-NIR spectroelectrochemical data which clearly indicate charge
carriers delocalization in P3 (see Fig. 4). Additional support for this
conclusion comes from DFT calculations which indicate planarity of
the thienylene-DPP-thienylene segment in P3 whereas in the corre-
sponding thienylene-BTD-thienylene segment of P1 a torsion angle
of 27o is found. Thus, Raman spectroelectrochemical data clearly
show that BTD units in P1 constitute an obstacle for charge carri-
ers delocalization whereas in P3 this delocalization passes through
the DPP units. For the reader’s convenience all oxidation induced
changes in Raman modes of P1 and P3 are listed in Table 5.
potential the spectrum gains features characteristic of the oxidized
state. At E = 0.6 V vs. Fc/Fc+ all bands corresponding to C␣-C␤ sym-
metric (1420 cm−1) and asymmetric stretchings (1467 cm−1) of the
thiophene ring undergo a shift to lower wave numbers–1403 and
∼1439 cm−1 (shoulder), respectively. The band at 1371 cm−1 which
is ascribed to C␤-C␤ stretching in the thiophene ring, in the oxidized
sample is located at slightly higher wave numbers (1374 cm−1).
Finally the band attributed to C␣-C␣ interring stretchings grows
in intensity with increasing working electrode potential. All these
changes unequivocally prove the oxidation of the oligothiophene
segments. The next question to be answered is whether the
modes characteristic of diketopyrrolopyrrole (DPP) moiety are
affected by the chain oxidation or they remain unchanged as in
the case of benzothiadiazole (BTD) modes? The band at 1508 cm−1
,
assigned to the C = C stretching mode in the DPP moiety shifts
to 1478 cm−1 whereas the position of the C-C stretching band
Table 5
Raman frequencies (in cm−1) of the main vibrational modes of P1 and P3 in their neutral and highest oxidation state. Potentials given versus Fc/Fc+.
P1
P3
Assignments
Exptl bands
Exptl bands
Neutral
(-0.2 V)
Oxidized
(0.6 V)
Neutral
(-0.2 V)
Oxidized
(0.6 V)
1508
1478
C = C sym stretch (DPP group)
C–C aromatic stretch (BTD group)
C = C asym stretch (thiophene group)
C = C asym stretch (thiophene group)
C–C stretch (thiophene group)
C = N stretch + C = N−S def. (BTD group)
C–C stretch (DPP group)
1521
1471
1428
1368
1314
1516
shoulder
1417
1367
1339
1467
1420
1370
shoulder
1403
1374
1327
1326
1269
1262
C–C stretch (benzothiadiazole
aromatic ring)
1205
1063
1204
1105
1233
1066
1210
1093
C–C inter-ring stretch
C–H def. (semi-quinonoid thiophene
ring)

M. Gora et al. / Electrochimica Acta 144 (2014) 211–220
219
Acknowledgment
This work has been supported in part by the European Union
in the framework of Regional Development Fund through the
Joint UW and WUT International PhD Program of Foundation for
Polish Science–“Towards Advanced Functional Materials and Novel
Devices” (MPD/2010/4). W.K. and A.P acknowledge the financial
support of Foundation for the Polish Science Team Programme
co-financed by the EU European Regional Development “New solu-
tion processable organic and hybrid (organic/inorganic) functional
materials for electronics, optoelectronics and spintronics” (Con-
tract No. TEAM/2011-8/6). M.G. thanks for financial support from
National Science Centre (Poland) through a research project PRE-
LUDIUM 5, 2013/09/N/ST5/02987.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.electacta.
2014.07.147.
Fig. 6. Raman spectra (␭exc = 1064 nm) of P1 recorded at increasing working elec-
trode potentials. Electrolyte 0.1 M Bu4NBF4 in acetonitrile, E vs Fc/Fc+.
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