Donor-acceptor type random copolymers for full visible light absorption

Article abstract of DOI:10.1039/c0cc04934d

Copolymerization towards obtaining full visible light absorption is highlighted. Randomly distributed segments of different oligomers resulted in neutral state black copolymers. Solution processability and highly transmissive gray oxidized states make cop

Full text of DOI:10.1039/c0cc04934d

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COMMUNICATION
Donor–acceptor type random copolymers for full visible light absorptionw
a
a
a
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Gozde Oktem, Abidin Balan,z Derya Baran and Levent Toppare*
Received 12th November 2010, Accepted 8th February 2011
DOI: 10.1039/c0cc04934d
Copolymerization towards obtaining full visible light absorption
is highlighted. Randomly distributed segments of different
oligomers resulted in neutral state black copolymers. Solution
processability and highly transmissive gray oxidized states make
copolymers great candidates to be used in low cost flexible
organic electronics.
switching materials became available for low cost flexible
display devices.15,16 Having all primary additive colors in
different redox states together with a transmissive state made
these polymers unique and enormous candidates for ECDs.
However, for a possible application of electrochromic polymers,
smart windows, black colored conjugated polymers integrated
with a transmissive state should have been designed.11 There
are only a few examples for neutral state black colored
polymers and their improvements could be considered as
important since they provide easier processing through printing,
spraying and coating methods and lower the cost compared
with their inorganic counterparts.17,18 Besides, in the field of
polymeric solar cells these materials which absorb in a broad
region of the ultraviolet–visible (UV-vis) spectrum would yield
high efficiency since light harvesting is one of the most
important parameters for these devices.19
Conjugated polymers (CPs) are a fundamental research subject
both in academia and industry. Many different types and
derivatives of CPs have been successfully prepared over the
decades and their potentials as advanced materials have been
investigated in several optoelectronic applications.1,2 In order
to tune the band gap and thus optical and electronic properties,
structural design is the keystone.3,4 There are several methods
utilized for structural modifications.5–7 However, to obtain
desired CPs, recent research interest has mainly focused on
donor–acceptor (DA) type materials in which electron rich and
electron deficient groups are both present on the backbone.8 This
method allowed production of multipurpose smart materials
which can be used efficiently in many different optoelectronic
applications.9
Here, we report on the synthesis and preliminary opto-
electronic properties of a series of DA type polymers differing
by the acceptor units in the polymer backbone. Polymers
CoP1, CoP2 and CoP3 were designed to yield alternating
DA segments with randomly distributed different acceptor
units along the polymer backbone. Polymers were obtained
via the Stille cross coupling reaction between di-stannylated
thiophene and di-brominated acceptors (Fig. 1).
In terms of electrochromic devices, the use of DA type CPs
as active layers became more popular over the time due to
a variety of achievable colors via structural alternations.10
Electrochromic and non-emissive applications of CPs provide
a stimuli effect-free display in large and flexible devices and
they constitute the most important candidates of active materials
for future display technologies.11 During the investigation of
soluble, fast switching, high optical contrast and stable CPs for
electrochromic devices, neutral state blue,12 red13 and green14
polymers were successfully achieved. Furthermore, introducing
the benzotriazole (BTz) bearing CPs into the electrochromic
field, solution processable, multi-colored to transmissive
In order to obtain full visible absorption, different oligomers
absorbing in different regions of the spectrum were combined.
Benzoquinoxaline or benzoselenadiazole containing polymers
were shown to be green in their neutral states in previous
reports.20,21 It is well known that neutral state green conjugated
polymers reveal two distinct lmax at around 400 and 700 nm.14
On the other hand, BTz and thiophene bearing polymers are
red in their neutral states with lmax at ca. 500 nm.16 Thus,
combination of benzoquinoxaline or benzoselenadiazole and
benzotriazole with thiophene allows different segments on the
same polymer chain (Fig. 1) which ended up in the absorption
of all visible light.
a Department of Chemistry, Middle East Technical University,
06531 Ankara, Turkey. E-mail: toppare@metu.edu.tr;
Tel: +90 3122103251
The long alkyl chain substituent at the 2-N position of BTz
units served as a solubilizing agent for the polymers. Alternating
copolymers of benzoquinoxalines or benzoselenadiazole and
thiophene without BTz units were not soluble in common
organic solvents. CoP1, CoP2 and CoP3 were characterized by
1H-NMR and GPC (see ESIw) prior to the electrochemical and
spectral studies. There was no significant difference in solubility
of three copolymers in chloroform and they were spray coated
on indium tin oxide (ITO) coated glass slides from 5 mg mlꢀ1
solutions in chloroform under ambient conditions. On the
b Department of Biotechnology, Middle East Technical University,
06531 Ankara, Turkey
c Department of Polymer Science and Technology,
Middle East Technical University, 06531 Ankara, Turkey
w Electronic supplementary information (ESI) available: Synthetic
procedures and spectral details for copolymers. See DOI: 10.1039/
c0cc04934d
z Current address: Laboratory of Macromolecular and Organic
Chemistry, Department of Chemical Engineering and Chemistry,
Eindhoven University of Technology, P.O. Box 513, 5600 MB, The
Netherlands
c
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Chem. Commun., 2011, 47, 3933–3935 3933

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polymer chains were achieved with CoP1 which also showed
low redox potentials.
All polymers were shown to be n-type dopable at negative
potentials in 0.1 M TBAPF6/ACN solution under ambient
conditions. Strong electron accepting ability of benzoselena-
diazole in CoP3 resulted in the lowest n-doping potential
among all three which is ꢀ1.77 V. Ambipolar characteristics
of polymers provide estimation of HOMO–LUMO energy
levels electronically from the onset potentials of the oxidation
and/or reduction potentials for polymers. Inset table in Fig. 2
summarizes the redox potentials and HOMO–LUMO for
polymer films. Electronic band gaps of the polymers were
determined as 1.67, 1.76 and 1.82 eV for CoP1, CoP2 and
CoP3, respectively, from the HOMO–LUMO values determined
electrochemically.
To observe the spectral behavior of the polymers in situ
UV-vis-NIR spectra were taken accompanied with applied
potential. Initially, neutral copolymer films were spray coated
with 5 mg mLꢀ1 solution in chloroform. The absorbance
changes were noted as the potential was gradually increased
between 0.0 V and 1.5 V in 0.1 M TBAPF6/ACN (see ESIw for
full spectra). As seen in Fig. 3, all copolymers have broad
absorptions in the entire visible region. Fig. 3a shows the
normalized absorption spectra for copolymers only in the
visible region to have a better insight into their visible absorption
characteristics. Compared to CoP1 and CoP3, CoP2 revealed
a more homogenous absorption in the visible region. Thus the
color of the CoP2 film is black in its neutral state. Lower
energy absorption maxima of CoP2 yield the lowest optical
band gap as 1.48 eV (CoP1: 1.72 eV and CoP3: 1.55 eV). Due
to dominant absorbance at 600 nm, CoP3 is blue; whereas
strong and blue shifted absorption resulted in purple color in
the neutral state for CoP1. However, since visible absorption
of the films did not diminish completely upon applied potentials,
oxidized forms of copolymer films revealed transmissive
gray color.
Fig. 1 Synthetic pathway for copolymers (top) and cartoon repre-
sentation of different segments in the full visible absorbing copolymer
CoP2 (bottom).
other hand, the lowest polydispersity index (PDI) for CoP1
suggests either different acceptor units, No1 and No 3 (ESIw),
have similar solubility or they have similar reactivity throughout
the polycondensation reaction. All electrochemical studies
were performed in three electrode cells using ITO as working,
Pt wire as counter and Ag wire (0.3 V vs. Fc/Fc+) as pseudo
reference electrodes. The potentials were swept between 1.5 V
and ꢀ2.0 V vs. Ag wire. During cyclic potential scan, CoP1
showed the lowest oxidation potential whereas a stronger
electron acceptor benzoselenadiazole unit in CoP3 increased
the oxidation potential of the copolymer (Fig. 2). Oxidation
potentials of polymers are in correlation with increasing
electron deficiency for the additional acceptor unit on the
polymer backbone. Besides, GPC revealed that longest
Switching tests were performed to monitor the percent
transmittance changes as a function of time and to determine
the switching times of the polymers at the wavelength (600 nm)
where the human eye is highly sensitive, by stepping potentials
repeatedly between their fully neutral and oxidized states
within 5 s time intervals. Under these conditions, CoP2
showed an optical contrast of 23%. Time required for polymers
to switch between their neutral and oxidized states, switching
times, were defined at a 95% contrast value since the human
eye is sensitive up to 95% of the full contrast. Switching times
for polymers were found as 1.2 s for CoP1, 0.7 s for CoP2 and
1.0 s for CoP3. Among all, CoP2 exhibited shortest switching
times as well as highest optical contrast values.
DA type benzotriazole containing copolymers were synthe-
sized via the Stille polycondensation reaction. Their potentials
as full visible absorbing materials were investigated. Copolymers
were designed to reveal mixed colors of different oligomer
segments. To achieve full visible absorption, green (400 nm
and 700 nm) and red (500 nm) materials were combined.
2-Alkylbenzotriazoles were incorporated as both red colour
providing segments and the solubilizing units. Among all the
three, dithienylbenzoquinoxaline containing copolymer CoP2
absorbs almost all visible light and reveals a black color in its
Fig. 2 CVs, oxidation/reduction potentials and HOMO–LUMO
levels of polymer films vs. Fc/Fc+ in 0.1 M TBAPF6/ACN.
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electrochromic applications as well as different organic
electronic applications where neutral state visible absorbance
is of importance. Kinetic properties such as optical contrasts
(10%, 23% and 18% at 600 nm) and switching times (1.2 s, 0.7 s,
and 1 s) make them potential materials to be used in devices.
Nevertheless, these polymers need further investigation of their
different structural derivatives for applications in solar cells and
electrochromic display devices.
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