Facile synthesis of 3-(ω-acetoxyalkyl)thiophenes and derived copolythiophenes using Rieke zinc

Article abstract of DOI:10.1016/j.reactfunctpolym.2013.11.009

An optimized synthetic protocol toward highly reactive Rieke zinc is applied for the preparation of a series of ω-(2,5-dibromothiophene-3-yl) alkyl acetate monomers and statistical copolythiophenes derived thereof, with 10% of ester-functionalized side chains. The obtained conjugated polymers are attractive electron donor materials for organic solar cells, notably to increase the thermodynamic stability of the bulk heterojunction polymer:fullerene active layer blend, and are fully characterized by a combination of spectroscopic, thermal analysis and electrochemical techniques.

Full text of DOI:10.1016/j.reactfunctpolym.2013.11.009

Reactive & Functional Polymers 75 (2014) 22–30
Contents lists available at ScienceDirect
Reactive & Functional Polymers
journal homepage: www.elsevier.com/locate/react
Facile synthesis of 3-(x-acetoxyalkyl)thiophenes and derived
copolythiophenes using Rieke zinc
Suleyman Kudret a, Jurgen Kesters a,b,c, Sander Janssen a, Niko Van den Brande d, Maxime Defour d,
Bruno Van Mele d, Jean Manca b,c, Laurence Lutsen c, Dirk Vanderzande a,c, Wouter Maes a,c,
⇑
a Hasselt University, Institute for Materials Research (IMO-IMOMEC), Design & Synthesis of Organic Semiconductors (DSOS), Agoralaan 1 – Building D, B-3590 Diepenbeek, Belgium
b Hasselt University, Institute for Materials Research (IMO-IMOMEC), Organic and Nanostructured Electronics & Energy Conversion (ONE2), Universitaire Campus –
Wetenschapspark 1, B-3590 Diepenbeek, Belgium
c IMEC, IMOMEC Ass. Lab., Universitaire Campus – Wetenschapspark 1, B-3590 Diepenbeek, Belgium
d Physical Chemistry and Polymer Science (FYSC), Vrije Universiteit Brussel (VUB), Pleinlaan 2, B-1050 Brussels, Belgium
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 October 2013
Received in revised form 25 November 2013
Accepted 30 November 2013
Available online 7 December 2013
An optimized synthetic protocol toward highly reactive Rieke zinc is applied for the preparation of a ser-
ies of
x-(2,5-dibromothiophene-3-yl)alkyl acetate monomers and statistical copolythiophenes derived
thereof, with 10% of ester-functionalized side chains. The obtained conjugated polymers are attractive
electron donor materials for organic solar cells, notably to increase the thermodynamic stability of the
bulk heterojunction polymer:fullerene active layer blend, and are fully characterized by a combination
of spectroscopic, thermal analysis and electrochemical techniques.
Keywords:
Rieke zinc
Ó 2013 Elsevier Ltd. All rights reserved.
Optimized synthetic protocol
Ester side chains
Random copolythiophenes
Organic photovoltaics
1. Introduction
do not only improve solubility and hence processability, but also
alter the polymer’s electrical and optical properties [6–9]. It has
Polymer solar cells have a number of advantages over tradi-
tional silicon-based photovoltaics, such as versatility in form factor
(flexible, semi-transparent, colored) and better performance in dif-
fuse light, which makes them particularly attractive for portable or
wearable electronics and building-integrated photovoltaics [1].
Moreover, solution-processability allows low cost large-area thin
film fabrication by e.g. roll-to-roll printing. Hesitations by (photo-
voltaic) industry to make large investments in organic photovolta-
ics (OPV) are driven by the limited device/module efficiency and
lifetime, so far not at the required levels to successfully compete
with the existing PV technologies. Among the conjugated polymers
applied as electron donor materials in bulk heterojunction (BHJ)
OPV, polythiophenes have been the most studied and implemented
material class [2], with P3HT (poly(3-hexylthiophene)) as the most
famous representative [3]. Solution processability can be ensured
by the introduction of alkyl side chains at the b-position(s) of the
aromatic thiophene units [4,5]. It is known that these side chains
been reported that poly(3-alkylthiophenes) (P3ATs) with a chain
length of more than 10 carbon atoms show characteristics of liquid
crystals [7]. In another study by McCullough et al. it was stated
that alkyl chains up to 10–12 carbon atoms lead to a consistent in-
crease in mean conjugation length due to a higher planarity of the
main polymer chain [8]. On the other hand, Koeckelberghs and co-
workers have seen important substituent effects on the chiroptical
and magnetic properties of polythiophene materials [9]. The nature
of the side chains can also lead to important modifications in the
nanomorphology of the polymer:fullerene active layer blends ap-
plied in BHJ devices [1,10,11], a crucial aspect to achieve highly
efficient and stable organic solar cells. A large number of polythio-
phenes bearing a functional moiety at the end of (some of) the side
chains, such as esters [12], alcohols [12c], sulfonates [13], fluor-
oalkyls [14], mesogenic groups [15], and optically active units
[9,16], has been prepared and evaluated in OPV.
The building blocks for P3ATs, 3-substituted dibromothiophene
derivatives, are generally prepared via cross-coupling of organo-
magnesium compounds to 3-bromothiophene. On the other hand,
organozinc reagents are also useful synthetic tools as they can
readily be prepared by oxidative addition of Rieke zinc to alkyl
halides [17], and, unlike Grignard reagents and organolithium
⇑
Corresponding author at: Hasselt University, Institute for Materials Research
(IMO-IMOMEC), Design & Synthesis of Organic Semiconductors (DSOS), Agoralaan 1
– Building D, B-3590 Diepenbeek, Belgium. Tel.: +32 11268312; fax: +32 11268299.
E-mail address: wouter.maes@uhasselt.be (W. Maes).
1381-5148/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.reactfunctpolym.2013.11.009

S. Kudret et al. / Reactive & Functional Polymers 75 (2014) 22–30
23
anhydride (99+%) were obtained from Acros Organics. All chemi-
cals were used as received without any further purification. Mela-
mine trisulfonic acid was prepared according to the procedure
reported by Shirini et al. [25].
NMR chemical shifts (d, in ppm) were determined relative to the
residual CHCl3 absorption (7.26 ppm) or the 13C resonance shift of
CDCl3 (77.16 ppm). Gas chromatography–mass spectrometry (GC–
MS) analysis was carried out with a thermoquest TSQ7000 apply-
ing a DB-5MS 30 m  0.25 mm column and a temperature range
of 35–320 °C. Analysis of the molar masses and distributions of
the polymer samples was performed on a Tosoh EcoSEC System,
comprising of an autosampler, a PSS guard column SDV (50
 7.5 mm), followed by three PSS SDV analytical linear XL columns
Scheme 1. Optimized procedure for the preparation of Rieke zinc.
compounds, they tolerate the presence of a wide range of func-
tional groups, such as ketones, nitriles and esters [18]. The
polythiophenes themselves are commonly synthesized via the
Grignard metathesis (GRIM) protocol [2,3,19]. The alternative Rie-
ke method [20], which uses organozinc intermediates, is much less
popular [21], mainly due to difficulties in the (reproducible) prep-
aration of the reactive Rieke zinc [22]. Nevertheless, as for the
monomer synthesis, the wide functional group tolerance is an
important advantage. Recently, we have developed an optimized
method for the preparation of highly reactive Rieke zinc metal
(ZnÃ) under standard laboratory conditions [17]. This procedure in-
volves the preparation of Znà by reduction of the zinc chloride pre-
cursor with lithium naphthalenide prepared in situ in the presence
of a catalytic amount (3 mol%) of benzothiophene (Scheme 1). The
active zinc species readily undergoes oxidative addition to alkyl
halides and 2,5-dibromothiophenes under mild conditions to yield
the corresponding organozinc intermediates.
In this study, the optimized Rieke zinc method was applied to
synthesize ester-functionalized dibromothiophenes with different
alkyl chain length, as well as copolymers containing the functional
monomers, in a straightforward and efficient way. As recent work
has shown that ester-functionalized random copolythiophenes can
give similar solar cell efficiencies as regular P3HT [12], as far as the
functionalized side chains are introduced in a moderate ratio,
while improving the thermal stability of the BHJ active layer
[1g,23], the synthesized materials allow to analyze the effect of
functionalized side chains (length) on BHJ blend nanomorphology
formation and evolution, and thereby achieve a more profound
fundamental insight in molecular miscibility on a molecular scale.
Moreover, further post-polymerization reactions readily afford
alternative functionalized polythiophene derivatives [12c,24].
(5
lm, 300 Â 7.5 mm), a differential refractive index detector (Tos-
oh EcoSEC RI) and a UV-detector using THF as the eluent at 40 °C
with a flow rate of 1.0 mL/min. The SEC system was calibrated
using linear narrow polystyrene standards ranging from 474 to
7.5 Â 106 g/mol (K = 14.1 Â 10À5 dL/g and
a = 0.70). The energy lev-
els of the copolythiophenes were determined using cyclic voltam-
metry and optical band gap analyses. UV–visible spectra were
recorded on a Varian Cary 500 spectrometer. Electrochemical mea-
surements were performed with an Eco Chemie Autolab PGSTAT
30 potentiostat/galvanostat using a three-electrode microcell con-
taining a Ag/AgNO3 reference electrode (silver wire in 0.01 M
AgNO3 and 0.1 M NBu4PF6 in MeCN), a platinum counter electrode
and a platinum working electrode, and 0.1 M NBu4PF6 in anhy-
drous MeCN or CH2Cl2 was used as the electrolyte. To prevent air
from entering the system, the buffer solutions were degassed with
Ar prior to each measurement and all experiments were carried out
under a curtain of Ar. The respective copolymers were coated as
thin films on the working electrode by dipping the electrode in
the polymer solution. Cyclic voltammograms were recorded at a
scan rate of 100–300 mV sÀ1. For the conversion to eV, the onset
oxidation and reduction potential were used and referenced to
the known standard ferrocene/ferrocenium (0.05 V vs. Ag/AgNO3),
which in MeCN solution was estimated to have an oxidation poten-
tial of À4.98 eV vs. vacuum. To estimate the optical bandgap, the
intersection of the tangent line to the absorption spectrum on
the low energy side with the X-axis was used. FT-IR spectra were
obtained on a Perkin–Elmer spectrophotometer with a resolution
of 4 cmÀ1 (16 scans) using films drop-casted on a NaCl disk. DSC
measurements were performed at 20 K minÀ1 in aluminum cruci-
bles on a TA Instruments Q2000 Tzero DSC equipped with a refrig-
erated cooling system (RCS), using nitrogen (50 mL minÀ1) as
purge gas. For all DSC results the second heating was depicted to
avoid thermal history effects.
2. Experimental
2.1. Materials and methods
All manipulations were carried out on a dual manifold vacuum/
Ar system. Lithium (granular, Acros, 99+%) was stored in a Schlenk
tube under Ar. Lithium, naphthalene and benzothiophene were
weighed in air as needed and transferred to a Schlenk tube under
a stream of Ar. Naphthalene (Acros, 99+%) and benzothiophene
(Sigma–Aldrich, 98%) were stored in a desiccator over phosphorous
pentoxide. Zinc chloride (Acros, analysis grade 98.5%) was trans-
ferred into small vials in a glove box and stored in a desiccator over
phosphorous pentoxide. Zinc chloride was dried by treating it with
thionyl chloride and subsequent heating with a Bunsen burner, and
removed under a stream of Ar gas. THF was freshly distilled from
Na/benzophenone under a N2 atmosphere at atmospheric pressure
prior to use. Brass cannulas were stored in an oven at 110 °C and
cleaned immediately after use with acetic acid (in the case of ZnÃ
remnant), acetone (to clean non-polymeric residues), or hot chlo-
roform and/or chlorobenzene (for polymer-based contaminations).
3-Bromothiophene (99%) was purchased from Fluorchem. 1,6-Hex-
anediol (97%), 1,8-octanediol (98+%), 1,9-nonanediol (97%), 1,10-
decanediol (97%), and 1,12-dodecanediol (97%) were obtained from
Alfa Aesar. Chlorosulfonic acid (97%), melamine (99%), and acetic
2.2. Synthesis
2.2.1. Preparation of
x-bromoalkanols
All brominated alcohols were prepared in the same manner
from the corresponding diols following literature procedures.
Spectral data were in accordance with the literature [26,27].
Acetylation of the
sulfonic acid) [25] – General procedure 1.
6-Bromo-1-hexanol (1a) (29.3 g, 162 mmol) and MTSA (17.9 g,
49 mmol) were dissolved together in 400 mL of CH2Cl2. Acetic
anhydride (17 mL, 181 mmol) was added and the mixture was stir-
red at rt. After completion of the reaction, as monitored by thin-
layer chromatography, the mixture was filtered. The organic layer
was washed with a 5% NaHCO3 solution and water, and dried over
MgSO4. Concentration of the organic layer under reduced pressure
gave the crude acetylated product with minor impurities. Purifica-
tion was performed either by Kugelrohr distillation or by column
chromatography (silica, eluent hexanes-ethyl acetate, 80–20).
x-bromoalkanols with MTSA (melamine tri-

24
S. Kudret et al. / Reactive & Functional Polymers 75 (2014) 22–30
Kugelrohr distillation (110 °C, 0.079 mbar) afforded a pure color-
less liquid of 6-bromohexyl acetate (2a) (33.9 g, 94%).
(39.3 mmol) at rt. The reaction mixture was stirred for 3 h, after
which the solution was allowed to stand for a couple of h to allow
the excess of zinc to settle down from the dark brown organozinc
bromide solution.
6-Bromohexyl acetate (2a). 1H NMR (CDCl3, 300 MHz): d = 1.34–
1.44 (m, 4H), 1.61 (q, JH–H = 7.3 Hz, 2H), 1.83 (q, JH–H = 6.7 Hz, 2H),
2.01 (s, 3H), 3.37 (t, JH–H = 6.9 Hz, 2H), 4.02 (t, JH–H = 6.7 Hz, 2H);
13C NMR (CDCl3, 75 MHz): d = 171.9, 65.0, 34.4, 33.3, 29.1, 28.4,
In an 300 mL flame-dried Schlenk vessel, LiBr (3.41 g,
39.2 mmol), Ni(dppe)Cl2 (0.94 g, 1.8 mmol), and 3-bromothio-
phene (2.5 mL, 26.8 mmol) were dissolved dry THF (150 mL). The
organozinc bromide solution was then transferred via cannula over
a period of 1 h to this mixture under stirring at rt and the reaction
was continued overnight. The reaction was quenched with a satu-
rated NH4Cl solution, followed by extraction with diethyl ether,
affording 6-(thiophene-3-yl)hexyl acetate (5a) (4.54 g, 75%) in rel-
atively pure form (as analyzed by 1H NMR; more thorough purifi-
cation was only performed upon subsequent bromination).
6-(Thiophene-3-yl)hexyl acetate (5a). 1H NMR (CDCl3,
300 MHz): d = 1.32–1.65 (m, 8H), 2.02 (s, 3H), 2.61 (t, JH–H = 8.1 Hz,
2H), 4.03 (t, JH–H = 6.7 Hz, 2H), 6.89–6.92 (m, 2H), 7.22 (dd,
JH–H = 4.8/3.0 Hz, 1H).
25.8, 21.7; FT-IR (NaCl, cmÀ1):
tmax = 2938, 2860, 1740 (CO),
1462, 1438, 1388, 1366, 1244, 1046, 970, 889, 761, 729, 642.
8-Bromooctyl acetate (2b). Yield (13.57 g, 95%). 1H NMR (CDCl3,
300 MHz): d = 1.25–1.44 (m, 8H), 1.58 (q, JH–H = 7.0 Hz, 2H), 1.81
(q, JH–H = 6.7 Hz, 2H), 2.01 (s, 3H), 3.37 (t, JH–H = 6.9 Hz, 2H), 4.01
(t, JH–H = 6.7 Hz, 2H); 13C NMR (CDCl3, 75 MHz): d = 171.9, 65.2,
34.6, 33.4, 29.7, 29.3, 29.2, 28.7, 26.5, 21.7; FT-IR (NaCl, cmÀ1):
tmax = 2932, 2857, 1740 (CO), 1465, 1438, 1387, 1365, 1244,
1037, 975, 894, 724, 643.
9-Bromononyl acetate (2c). Yield (31.17 g, 95%). 1H NMR (CDCl3,
400 MHz): d = 1.27–1.41 (m, 10H), 1.58 (q, JH–H = 7.3 Hz, 2H), 1.82
(q, JH–H = 7.1 Hz, 2H), 2.01 (s, 3H), 3.37 (t, JH–H = 6.9 Hz, 2H), 4.01
(t, JH–H = 6.8 Hz, 2H); 13C NMR (CDCl3, 100 MHz): d = 171.6, 65.2,
34.7, 33.4, 29.7, 29.3, 29.2, 28.7, 26.5, 21.7; FT-IR (NaCl, cmÀ1):
8-(Thiophene-3-yl)octyl acetate (5b). Yield (27.7 g, 76%). 1H
NMR (CDCl3, 300 MHz): d = 1.27–1.35 (m, 8H), 1.57–1.60 (m, 4H),
2.03 (s, 3H), 2.60 (t, JH–H = 7.5 Hz, 2H), 4.03 (t, JH–H = 6.7 Hz, 2H),
6.89–6.92 (m, 2H), 7.22 (dd, JH–H = 4.8/3.0 Hz, 1H).
tmax = 2930, 2856, 1741 (CO), 1465, 1438, 1387, 1365, 1242,
1039, 971, 890, 757, 723.
10-Bromodecyl acetate (2d). Yield (22.73 g, 94%). 1H NMR
(CDCl3, 300 MHz): d = 1.24–1.39 (m, 12H), 1.56 (q, JH–H = 6.7 Hz,
2H), 1.79 (q, JH–H = 7.0 Hz, 2H), 1.99 (s, 3H), 3.35 (t, JH–H = 6.7 Hz,
2H), 3.99 (t, JH–H = 6.7 Hz, 2H); 13C NMR (CDCl3, 75 MHz):
d = 171.8, 65.2, 34.7, 33.4, 30.01, 29.97, 29.8, 29.4, 29.2, 28.8,
9-(Thiophene-3-yl)nonyl acetate (5c). Yield (27 g, 77%). 1H NMR
(CDCl3, 300 MHz): d = 1.24–1.34 (m, 6H), 1.55–1.64 (m, 6H), 1.83
(q, JH–H = 6.7 Hz, 2H), 2.02 (s, 3H), 2.60 (t, JH–H = 7.5 Hz, 2H), 4.03
(t, JH–H = 6.7 Hz, 2H), 6.89–6.92 (m, 2H), 7.22 (dd, JH–H = 4.8/
3.0 Hz, 1H).
26.5, 21.7. FT-IR (NaCl, cmÀ1):
t
max = 2929, 2855, 1741 (CO),
10-(Thiophene-3-yl)decyl acetate (5d). Yield (21.7 g, 75%). 1H
NMR (CDCl3, 300 MHz): d = 1.17–1.34 (m, 10H), 1.52–1.64 (m,
6H), 2.03 (s, 3H), 2.60 (t, JH–H = 7.3 Hz, 2H), 4.03 (t, JH–H = 6.7 Hz,
2H), 6.89–6.92 (m, 2H), 7.21 (dd, JH–H = 4.8/3.0 Hz, 1H).
12-(Thiophene-3-yl)dodecyl acetate (5e). Yield (20.2 g, 77%). 1H
NMR (CDCl3, 300 MHz): d = 1.17–1.34 (m, 14H), 1.56–1.63 (m, 6H),
2.03 (s, 3H), 2.60 (t, JH–H = 7.8 Hz, 2H), 4.03 (t, JH–H = 6.7 Hz, 2H),
6.89–6.92 (m, 2H), 7.21 (dd, JH–H = 4.8/3.0 Hz, 1H).
1466, 1438, 1387, 1365, 1244, 1038, 971, 891, 723.
12-Bromododecyl acetate (2e). Yield (33.85 g, 93%). 1H NMR
(CDCl3, 300 MHz): d = 1.24–1.41 (m, 16H), 1.58 (q, JH–H = 1.6 Hz,
2H), 1.82 (q, JH–H = 7.1 Hz, 2H), 2.01 (s, 3H), 3.37 (t, JH–H = 6.9 Hz,
2H), 3.99 (t, JH–H = 6.7 Hz, 2H); 13C NMR (CDCl3, 75 MHz):
d = 172.1, 65.3, 34.7, 33.5, 30.14 (3x), 30.07, 29.9, 29.4, 29.2, 28.8,
26.5, 21.7; FT-IR (NaCl, cmÀ1):
tmax = 2854, 1741 (CO), 1466,
1439, 1387, 1365, 1240, 1039, 972, 722.
2.2.2. Preparation of Rieke Zinc (ZnÃ) [17]
2.2.4. Bromination of the
General procedure 3
x-(thiophen-3-yl)alkyl acetates [29] –
Two 120 mL Schlenk vessels, A and B, were dried by heating with
a Bunsen burner under reduced pressure and cooled to rt under a
stream of Ar. Schlenk vessel A, filled with Ar, was weighed and then
reassembled to the Schlenk line. Under a stream of Ar, ZnCl2 was
charged to the vessel. After three Ar/vacuum sequences, ZnCl2 was
wetted with a small amount of SOCl2. The Schlenk vessel was heated
with a bunsen burner until the ZnCl2 salt melted and a white fume
was released, and the Schlenk was subsequently cooled down under
an Ar flow. Schlenk flask A was weighed again to determine the exact
amount of ZnCl2 and a stirring bar was added. Dried ZnCl2 (1.1 equiv)
was dissolved in freshly distilled THF (25 mL/g). Li pellets (2.2
equiv), naphthalene (2.25 equiv) and benzothiophene (0.04 equiv)
were weighted in air and charged into Schlenk B under an Ar stream.
Dry THF was added (the same amount as added to dissolve ZnCl2)
and the solution (which turned from colorless to dark green within
less than 2 min) was stirred for 2 additional h to dissolve the Li pel-
lets. The ZnCl2 solution was transferred dropwise via cannula to the
lithium naphthalenide solution over 10–15 min. (The resulting
black suspension might be stirred for 1 more h to consume the Li that
is not dissolved or stirring can be stopped right after the addition.)
The highly reactive zinc was allowed to settle down for a couple of
h. The supernatant was siphoned off via cannula leaving the reactive
Znà powder. Thus prepared Rieke zinc was ready to use.
6-(Thiophene-3-yl)hexyl acetate (5a) (3.84 g, 17.9 mmol) was
placed in a three-neck round bottom flask and dissolved in DMF
(75 mL). N-bromosuccinimide (7.80 g, 43.8 mmol) was added por-
tionwise and the resulting mixture was left to stir at rt overnight in
the absence of light. The reaction mixture was poured into an
ice-cooled 2.5 M NaOH solution and stirred for 5 min, and then
extracted with diethyl ether. The organic phase was washed
sequentially with 2.5 M NaOH, H2O, and NaClsat, dried over MgSO4,
and filtered. The solvent was discarded under reduced pressure
and the crude product was purified by column chromatography
(silica, eluent hexanes:ethyl acetate) to yield a pale yellow liquid
(6.40 g, 93%).
6-(2,5-Dibromothiophene-3-yl)hexyl acetate (6a). 1H NMR
(CDCl3, 300 MHz): d = 1.28–1.42 (m, 4H), 1.49–1.66 (m, 4H), 2.03
(s, 3H), 2.49 (t, JH–H = 7.8 Hz, 2H), 4.03 (t, JH–H = 6.7 Hz, 2H), 6.75
(s, 1H); 13C NMR (CDCl3, 100 MHz): d = 171.7, 143.3, 131.5, 111.1,
108.7, 65.1, 30.1, 30.0, 29.3, 29.2, 26.4, 21.7; GC–MS: m/z = 381/
383/385 [M+] (P98%); FT-IR (NaCl, cmÀ1):
2857, 1738, 1541, 1463, 1418, 1387, 1364, 1241.
tmax = 3091, 2934,
8-(2,5-Dibromothiophene-3-yl)octyl acetate (6b). Yield (5.21 g,
92%). 1H NMR (CDCl3, 300 MHz): d = 1.27–1.34 (m, 8H), 1.47–1.60
(m, 4H), 2.00 (s, 3H), 2.46 (t, JH–H = 7.5 Hz, 2H), 4.01 (t, JH–H = 6.7 -
Hz, 2H), 6.73 (s, 1H); 13C NMR (CDCl3, 100 MHz): d = 171.8, 143.5,
131.6, 111.0, 108.6, 65.2, 30.2, 30.1, 29.9, 29.8, 29.6, 29.3, 26.5,
21.7; GC–MS: m/z = 409/411/413 [M+] (P99%); FT-IR (NaCl,
2.2.3. Preparation of organozinc halides 3a–e and their Ni-mediated
coupling with 3-bromothiophene [28] – General procedure 2
6-Acetoxyhexyl bromide (2a) (7.96 g, 35.7 mmol) was dissolved
in dry THF (40 mL) and added via cannula to the active zinc powder
cmÀ1):
1364, 1241.
tmax = 3091, 2929, 2855, 1739, 1541, 1464, 1418, 1387,

S. Kudret et al. / Reactive & Functional Polymers 75 (2014) 22–30
25
9-(2,5-Dibromothiophene-3-yl)nonyl acetate (6c). Yield (4.49 g,
93%). 1H NMR (CDCl3, 300 MHz): d = 1.25–1.35 (m, 10H), 1.46–1.64
(m, 4H), 2.03 (s, 3H), 2.48 (t, JH–H = 7.5 Hz, 2H), 4.03 (t, JH–H = 6.7 -
Hz, 2H), 6.75 (s, 1H); 13C NMR (CDCl3, 100 MHz): d = 171.9, 143.3,
131.6, 110.0, 108.6, 65.3, 30.2, 30.1, 30.04, 29.93, 29.9, 29.7, 29.3,
26.6, 21.7; GC–MS: m/z = 423/425/427 [M+] (P99%); FT-IR (NaCl,
129.3, 65.3, 32.4, 31.2, 30.2, 30.0, 29.3, 26.6, 23.4, 21.7, 14.8; FT-
IR (NaCl, cmÀ1):
1377, 1237, 819.
tmax = 3055, 2955, 2926, 2856, 1744, 1509, 1455,
Poly{[3-hexylthiophene-2,5-diyl]-co-[3-(10-acetoxydecyl)thio-
phene-2,5-diyl]} 9/1 (P4). 1H NMR (CDCl3, 300 MHz): d = 6.97 (s),
4.03 (t), 2.79/2.55 (t), 2.02 (s), 1.80–1.60 (m), 1.50–1.25 (m), 0.90
(t); 13C NMR (100 MHz, CDCl3): d = 171.9, 140.6, 134.4, 131.2,
129.3, 65.2, 32.4, 31.2, 30.2, 30.0, 29.3, 26.6, 23.4, 21.7, 14.8; FT-
cmÀ1):
1364, 1239.
tmax = 3090, 2927, 2854, 1740, 1542, 1464, 1418, 1387,
10-(2,5-Dibromothiophene-3-yl)decyl acetate (6d). Yield
IR (NaCl, cmÀ1):
1378, 1237, 820.
tmax = 3055, 2955, 2926, 2856, 1744, 1510, 1455,
(3.83 g, 92%). 1H NMR (CDCl3, 300 MHz): d = 1.23–1.35 (m, 12H),
1.48–1.64 (m, 4H), 2.03 (s, 3H), 2.48 (t, JH–H = 7.5 Hz, 2H), 4.03 (t,
JH–H = 6.7 Hz, 2H), 6.75 (s, 1H); 13C NMR (CDCl3, 100 MHz):
d = 171.8, 143.5, 131.5, 111.0, 108.6, 65.3, 30.2, 30.12, 30.10,
30.07, 29.97, 29.9, 29.7, 29.2, 26.5, 21.7; GC–MS: m/z = 437/439/
Poly{[3-hexylthiophene-2,5-diyl]-co-[3-(12-acetoxydode-
cyl)thiophene-2,5-diyl]} 9/1 (P5). 1H NMR (300 MHz, CDCl3):
d = 6.97 (s), 4.02 (t), 2.80/2.56 (t), 2.02 (s), 1.77–1.55 (m), 1.50–
1.22 (m), 0.90 (t); 13C NMR (CDCl3, 100 MHz): d = 171.9, 140.6,
134.4, 131.2, 129.3, 65.5, 32.4, 31.2, 30.3, 30.2, 30.2, 30.0, 29.3,
441 [M+] (P97%); FT-IR (NaCl, cmÀ1):
1739, 1540, 1463, 1417, 1387, 1364, 1240.
tmax = 3091, 2927, 2855,
26.6, 23.4, 21.7, 14.8; FT-IR (NaCl, cmÀ1):
tmax = 3055, 2955,
12-(2,5-Dibromothiophene-3-yl)dodecyl acetate (6e). Yield
(4.76 g, 93%). 1H NMR (CDCl3, 300 MHz): d = 1.22–1.34 (m, 16H),
1.48–1.62 (m, 4H), 2.03 (s, 3H), 2.48 (t, JH–H = 7.9 Hz, 2H), 4.03 (t,
JH–H = 6.8 Hz, 2H), 6.75 (s, 1H); 13C NMR (CDCl3, 100 MHz):
d = 171.9, 144.8, 131.5, 110.6, 108.6, 65.3, 30.22, 30.20, 30.16
(2x), 30.1, 30.0, 29.9, 29.7, 29.3, 26.7, 21.7; GC–MS: m/z = 466/
2926, 2856, 1744, 1509, 1456, 1378, 1238, 820.
2.3. Solar cell device fabrication
The BHJ organic solar cells were constructed using the tradi-
tional glass/ITO/PEDOT-PSS/active layer/Ca/Ag architecture. Prior
to device processing, the indium tin oxide (ITO, Kintec, 100 nm,
20 Ohm/sq) containing substrates were cleaned using soap, demin-
eralized water, acetone, isopropanol and a UV/O3 treatment. Subse-
quently, the ITO substrates were covered by a ꢀ30 nm thick layer
of PEDOT-PSS [poly(3,4-ethylenedioxythiophene)-poly(styrene-
sulfonic acid), Heraeus Clevios] by spin-coating. Further processing
was performed under nitrogen atmosphere in a glove box, starting
off with an annealing step at 130 °C for 15 min to remove any
residual water. The active layer consisting of polymer:PC61BM
([6,6]-phenyl-C61 butyric acid methyl ester, Solenne) was spin-
coated with a thickness of ꢀ80 nm (as confirmed by DEKTAK).
Blend solutions were prepared in a 1:1 ratio, with polymer concen-
trations of 10 mg/mL, using chlorobenzene as a solvent. After-
wards, an annealing step at 140 °C for 10 min was introduced to
optimize the film morphology. Finally, the devices were finished
off with Ca and Ag as the top electrodes, with thicknesses of 20
and 100 nm, respectively. In the standard cell configuration, an ac-
tive area of 8 mm2 was obtained.
468/470 [M+] (P97%); FT-IR (NaCl, cmÀ1):
2856, 1740, 1541, 1464, 1418, 1387, 1354, 1239.
tmax = 3090, 2927,
2.2.5. Synthesis of the copolythiophenes – General procedure 4
The monomer mixture, 2,5-dibromo-3-hexylthiophene (4.52 g,
13.9 mmol) and 2,5-dibromo-3-(6-acetoxyhexyl)thiophene (6a)
(0.59 g, 1.5 mmol), was added via cannula to freshly prepared ZnÃ
(16.9 mmol), as made by the optimized Rieke method [17], at
À78 °C. The mixture was stirred for 1 h at this temperature and
then allowed to warm to 0 °C gradually. The unreacted Znà was al-
lowed to settle down overnight and the formed organozinc super-
natant was filtered via a 0.45 lm acrodisc filter into a flame-dried
Schlenk flask. Ni(dppe)Cl2 (0.016 g, 0.031 mmol, 0.2 mol%) was
added via a cannula to the ice-cooled organozinc solution and
the Schlenk vessel was immersed into a preheated oil bath at
60 °C. The mixture was stirred at this temperature overnight. It
was then poured into a MeOH–HCl (2 M) solution, and the result-
ing dark precipitate was filtered off and washed several times with
MeOH. The crude polymer was transferred into an extraction thim-
ble and purification was performed by sequential Soxhlet extrac-
tions using MeOH, acetone and n-hexane. The polymer was
finally collected with chloroform and the solvent was removed
under reduced pressure. The polymer was redissolved in chloro-
form and reprecipitation was performed upon addition of MeOH.
Filtration and drying under high vacuum afforded the pure poly-
mer material (in ꢀ60% yield for all copolymers).
3. Results and discussion
3-Alkylthiophenes bearing a pendant functional group in the
alkyl side chain are attractive building blocks for the preparation
of functionalized P3ATs [5,30]. Despite the fact that syntheses of
3-substituted alkylthiophenes with a terminal carboxylic acid,
amide or nitrile functional group have already been reported, the
synthetic routes leading to these materials are often tedious,
involving multiple steps. Carboxylic acid and sulfonate-functional-
ized thiophenes were synthesized starting from 3-methylthioph-
ene [31] or 3-(2-hydroxyethyl)thiophene [12,32] by laborious
consecutive chain extensions. Bäuerle et al. developed a method
for the synthesis of 3-substituted thiophenes bearing a terminal
halogen substituent on alkyl side chains of length n P 4 by
reaction of 3-bromothiophene with the Grignard reagents of
Poly{[3-hexylthiophene-2,5-diyl]-co-[3-(6-acetoxyhexyl)thio-
phene-2,5-diyl]} 9/1 (P1). 1H NMR (300 MHz, CDCl3) d = 6.97 (s),
4.06 (t), 2.79/2.55 (t), 2.02 (s), 1.77–1.60 (m), 1.51–1.25 (m), 0.90
(t); 13C NMR (CDCl3, 100 MHz): d = 171.9, 140.6, 134.4, 131.2,
129.3, 65.3, 30.2, 30.0, 29.3, 26.6, 23.4, 21.7, 14.8; FT-IR (NaCl,
cmÀ1):
1235, 820.
tmax = 3055, 2955, 2927, 2856, 1745, 1509, 1456, 1378,
Poly{[3-hexylthiophene-2,5-diyl]-co-[3-(8-acetoxyoctyl)thio-
phene-2,5-diyl]} 9/1 (P2). 1H NMR (400 MHz, CDCl3): d = 6.96 (s),
4.03 (t), 2.80/2.56 (t), 2.02 (s), 1.75–1.64 (m), 1.48–1.29 (m), 0.90
(t); 13C NMR (CDCl3, 100 MHz): d = 171.9, 140.6, 134.4, 131.2,
129.3, 65.2, 32.4, 31.2, 30.2, 30.0, 29.3, 26.7, 23.4, 21.7, 14.8; FT-
x
-(p-methoxyphenoxy)alkyl bromides, followed by removal of
the p-methoxyphenoxy group with HBr/acetic anhydride to afford
3-( -bromoalkyl)thiophenes [33]. Further functionalization was
x
IR (NaCl, cmÀ1):
1377, 1237, 820.
tmax = 3055, 2955, 2926, 2856, 1744, 1509, 1455,
achieved by reaction with KCN to yield the nitrile derivatives,
which were later hydrolyzed to the corresponding carboxylic acids.
A similar route for the preparation of carboxylic acid-functional-
ized thiophenes employed a Grignard coupling reaction with long
alkyl chain allylic bromides [34]. This method has, however, not
Poly{[3-hexylthiophene-2,5-diyl]-co-[3-(9-acetoxynonyl)thio-
phene-2,5-diyl]} 9/1 (P3). 1H NMR (300 MHz, CDCl3): d = 6.97 (s),
4.03 (t), 2.79/2.55 (t), 2.02 (s), 1.80–1.60 (m), 1.50–1.25 (m), 0.90
(t); 13C NMR (CDCl3, 100 MHz): d = 171.9, 140.6, 134.3, 131.2,
been used for
x-(3-thienyl)alkanecarboxylic acids with alkyl chain

26
S. Kudret et al. / Reactive & Functional Polymers 75 (2014) 22–30
heterogeneous nature of the reaction facilitates the work-up.
Afterwards, following our recently developed procedure to prepare
highly reactive Rieke zinc [17], -(thiophene-3-yl)alkyl acetates
5a–e were prepared from ( -acetoxyalkyl)zinc bromides 3 and
3-bromothiophene (4) via nickel-mediated cross coupling reac-
tions. Thus, the -bromoalkyl acetates 2a–e were treated with
x
x
x
an excess of freshly prepared Rieke zinc. Oxidative addition of
Znà to 2a–e at a molar ratio of 1.0–1.2 ZnÃ/2 proceeded at room
temperature within 2 h to afford organozinc reagents 3a–e in very
high conversions (estimated to be > 99% according to GC–MS anal-
ysis after quenching with iodine). The formed organozinc solutions
were allowed to settle for 3 h to separate the unreacted ZnÃ. Then,
organozinc reagents 3a–e were cannulated to another flask con-
taining 3-bromothiophene and 5 mol% of Ni(dppe)Cl2 ([1,2-
bis(triphenylphosphino)ethane] nickel(II) dichloride) catalyst was
added to initiate the coupling reaction [35]. Best results were ob-
tained when the reaction mixture was stirred at room temperature
for 2 days. When the coupling reaction was carried out in the pres-
ence of lithium bromide, the reaction time could be significantly
shortened (from 2 days to 18 h) and the reaction yield strongly en-
hanced [17]. In all cases, minor amounts of homo-coupled 3,3’-
bithiophene were observed as well. Bromination of the obtained
Scheme 2. Preparation of
6a–e employing Rieke zinc.
x-(2,5-dibromothiophen-3-yl)alkyl acetate monomers
lengths below n = 11. Alternatively, Dai et al. have prepared thio-
phene compounds possessing carboxylic acid, carboxyamide,
imide or amine (hydrochloride) functionalities in two steps utiliz-
ing a commercially available organozinc reagent, 4-cyanobutylzinc
bromide, in Kumada-type cross coupling reactions [35]. The meth-
od was reported to be remarkably more efficient, affording the de-
sired products in fewer synthetic steps.
We have chosen to synthesize dibromothiophene monomers
with different alkyl chain lengths, varying from hexyl to dodecyl,
and an acetate end group starting from 3-bromothiophene and
employing self-prepared Rieke reagents (Scheme 2). For this pur-
pose, the recently optimized Rieke zinc conditions were benefi-
cially applied [17]. The monomers were then used consecutively
in polymerization reactions (in combination with regular 2,5-
dibromothiophene), again employing the Rieke zinc protocol, to-
ward the final copolythiophene semiconducting polymers.
x
-(thiophene-3-yl)alkyl acetates 5a–e with an excess of N-bromo-
succinimide (NBS) under mild conditions yielded the final -(2,5-
x
dibromothiophen-3-yl)alkyl acetate monomers 6a–e [29]. Simple
distillations afforded the monomers in high purity grades.
Functionalized polythiophenes can be prepared by incorporat-
ing the desired functional entities in the reactive thiophene mono-
mers, but this may lead to low conversions and side reactions [37].
Therefore, post-polymerization functionalization is often pre-
ferred, as it enables the insertion of the functional moiety on a
pre-synthesized polymer precursor [38]. An important aspect of
the functional groups in substituted alkylthiophenes is their effect
on the behavior of the polymerization reaction. Previously, ester-
functionalized P3ATs have been prepared via oxidative polymeri-
zation with FeCl3 [39]. In another study it was emphasized that,
although the alkylester functionalities render the polymer soluble,
they nevertheless suffered from chemical degradation (acidolysis)
depending on the polymerization medium [40]. Pomerantz et al.
have shown that synthesis of poly[3-(6-bromoalkyl)thiophenes]
by chemical oxidation yielded regiorandom polymers [41]. How-
ever, for most applications in organic electronics, well-defined reg-
ioregular (rr) polythiophenes are required (the rr being defined as
the precentage of head-to-tail couplings). Both the GRIM [42] and
Rieke method [43] afford highly rr P3ATs.
The desired
mers 6a–e were derived from 3-bromothiophene (4) and the
respective ( -acetoxyalkyl)zinc bromides 3, and subsequent bro-
x-(2,5-dibromothiophen-3-yl)alkyl acetate mono-
x
mination. As the required organozinc reagents are not commer-
cially available, they had to be prepared and a simple procedure
has been developed for that cause. The synthetic sequence starts
from the
costly, they were synthesized by monobromination of the corre-
sponding -diols. Traditionally, this is done by heating with
x-bromoalcohols 1 [26]. Since these derivatives are quite
In our case, the optimized Rieke method was employed to pre-
pare rr poly{[3-hexylthiophene-2,5-diyl]-co-[3-(x-acetoxylalkyl)
a,x
aqueous HBr and petroleum ether in a continuous extraction appa-
ratus [36]. However, this method is not convenient for large scale
synthesis and showed low selectivity, as significant amounts of
the dibrominated compounds were formed. In 1985, it was re-
ported that azeotropic water removal with benzene allowed to
achieve monobromination with high selectivity and material pur-
ity (>99%) [27]. Nevertheless, Chong and co-workers observed var-
iable results under these conditions as well [26]. Therefore, they
have used different solvents and found out that toluene was the
best choice to obtain pure bromoalkanols. Five different bromoalk-
anols 1a–e were synthesized applying this procedure. The materi-
als were obtained in pure form by simple Kugelrohr distillation of
the crude reaction mixture. Alternatively, column chromatography
(silica) could be used as well. Acetylation was then performed with
acetic anhydride using melamine trisulfonic acid (MTSA) as a cat-
thiophene-2,5-diyl]} materials (Scheme 3). It was chosen to limit
the functional monomers to 10%, as recent results showed that lar-
ger amounts of functional entities lead to materials with deviating
crystallinity and inferior photovoltaic performance [12c,d]. Oxida-
tive addition of Rieke zinc to the mixture of dibromothiophenes
(regular 3-hexylthiophene and
x-(2,5-dibromothiophen-3-yl)alkyl
acetates 6a–e in a 9:1 feed ratio) afforded the respective 2-bromo-
5-(bromozincio)thiophenes. The reaction was completed in 2 h and
the regioselectivity was found to be greater than 99% (based on
GC–MS analysis of the crude reaction mixture, quenched with a
saturated ammonium chloride solution). Polymerization of these
organozinc reagents with the aid of Ni(dppe)Cl2 (0.2 mol%) and
quenching of the polymerization with a HCl/MeOH solution then
yielded the rr statistical copolythiophenes P1–P5. The crude poly-
mers were purified by repetitive Soxhlet extractions with metha-
nol, n-hexane, acetone, and chloroform. The molar masses of the
copolymers were analyzed by size exclusion chromatography
(SEC) (Table 1). All copolythiophenes showed reasonable average
alyst to yield
x-bromoalkyl acetates 2a–e [25]. MTSA was easily
prepared from melamine and neat chlorosulfonic acid at room
temperature. The acetylation reactions proceeded smoothly, with
very short reaction times (ꢀ10 min) and yields over 90%. The

S. Kudret et al. / Reactive & Functional Polymers 75 (2014) 22–30
27
Scheme 3. Regiocontrolled synthesis of ester-functionalized P3AT copolymers P1–P5 employing the optimized Rieke zinc protocol.
molar masses (Mn = 14.2–21.6 Â 103 g molÀ1) and rather narrow
polydispersities (PDI = 1.37–1.53).
of the hexylthiophene units at d = 0.9 ppm. The ester-functional-
ized monomers are incorporated in ꢀ10%, closely matching the
feed ratio (Fig. 1). The FT-IR spectra of the copolymers are almost
identical (see Supplementary material) with a strong C@O stretch-
1H NMR analysis provided information on the regioregularity
and built-in monomer ratio of the synthesized statistical copoly-
mers. As an example, the 1H NMR spectrum of poly{[3-hexylthi-
ophene-2,5-diyl]-co-[3-(6-acetoxyhexyl)thiophene-2,5-diyl]) 9/1
(P1) is given in Fig. 1. The regioregularity of all copolythiophenes
synthesized via this process was found to be at least 94% (as de-
ing vibration at ꢀ1745 cmÀ1
.
UV–Vis spectroscopic analysis was performed both in chloro-
form solution and in films spin-coated from chloroform on a glass
disk (Fig. 2). The absorption spectra from solution show typical
P3AT features with a broad band covering the 300–560 nm range
and maximum absorption (kmax) at ꢀ450 nm. The solid-state spec-
tra are red-shifted, suggesting molecular organization in the thin
films. All copolymers show a maximum absorption at ꢀ550 nm
in film, with a shoulder at ꢀ605 nm representative for the degree
of solid-state aggregation. The stacking features of the copolythi-
ophenes seem to increase in the order P1 < P3HT < P5 < P3 <
P2 < P4.
fined by the ratio of the
a-CH2 triplet signals at d = 2.80 and
2.55 ppm). The monomer content in the random copolymers is
easily defined by comparing the intensity of the acetoxy (CH3) sin-
glet at d = 2.0 ppm and the triplet due to the terminal methyl group
Table 1
Molar masses and polydispersities for ester-functionalized copolymers P1–P5.
The electrochemical characteristics of the copolymers were
determined by cyclic voltammetry (CV). Using the Eonset oxidation
and reduction values, the HOMO and LUMO energy levels were
estimated (Table 2). The side-chain functionalized copolymers
show comparable values with respect to standard P3HT, indicating
that the length of the functionalized alkyl side chains does not have
a major impact on the electrochemical characteristics.
Polymer
Mn (Â103 g molÀ1
)
PDI
P1
P2
P3
P4
P5
18.4
14.2
18.4
17.6
21.6
1.53
1.37
1.41
1.42
1.46
Fig. 1. 1H NMR spectrum of poly{[3-hexylthiophene-2,5-diyl]-co-[3-(6-acetoxyhexyl)thiophene-2,5-diyl]} 9/1 (P1).

28
S. Kudret et al. / Reactive & Functional Polymers 75 (2014) 22–30
P3HT
P1
P3
P5
P2
P4
P3HT
P3HT
P1
2.4
1.6
0.8
0.0
1.0
0.8
0.6
0.4
0.2
0.0
P1
P2
P2
P3
P3
P4
P4
P5
Exo ↓
P5
-70
-20
30
80
130
180
230
280
Temperature / °C
Fig. 3. DSC thermograms of the second heating of copolymers P1–P5. P3HT is
included for comparison. All curves are shifted vertically for clarity.
200
300
400
500
600
700
800
Wavelength (nm)
Fig. 2. Solution (solid lines) and solid-state (dashed lines) UV–Vis spectra of
copolymers P1–P5.
Table 3
Summary of the melting points and melting enthalpies determined for copolymers
P1–P5. Values for P3HT are mentioned for comparison.
Polymer
Tm (°C)
D
Hm (J gÀ1
)
Fig. 3 shows the DSC thermograms for the series of copolymers
prepared. Melting point values and melting enhalpies are summa-
rized in Table 3. All materials are semi-crystalline with a higher
melting enthalpy, and probably thus a higher crystallinity, than
P3HT (prepared by the same Rieke procedure). Their melting peak
temperatures, however, are lower than that of P3HT, except for P1.
Copolymer P1, with the shortest side chain and containing the
same number of methylene spacing groups as in P3HT, showed
the highest melting temperature and melting enthalpy. For copoly-
mers P2, P3, and P4 a similar melting point and comparable melt-
ing enthalpies were found. The lowest melting point is seen for P5,
which has the longest alkyl side chain among the copolymers pre-
pared. No straightforward relation is observed with the stacking
properties deduced from the solid-state UV–Vis spectra.
P1
P2
P3
P4
216
205
205
204
198
215
22.3
19.4
16.3
20.6
18.0
16.2
P5
P3HTa
Mn = 22.9 Â 103 g molÀ1, PDI = 1.6.
a
Table 4
Photovoltaic performance of P3HT and P1–P5 based polymer solar cells.a
Polymer
Voc (V)b
Jsc (mA/cm2)b
FF (%)b
Average
g
(%)
Best
g
(%)
P3HTc
P1
P2
P3
P4
0.55
0.58
0.58
0.57
0.58
0.58
6.29
6.02
5.63
5.03
5.81
5.17
0.71
0.69
0.70
0.70
0.68
0.70
2.47
2.42
2.25
2.00
2.29
2.09
2.55
2.72
2.47
2.23
2.41
2.25
The copolymers were applied in polymer:PC61BM ([6,6]-phe-
nyl-C61 butyric acid methyl ester) BHJ organic solar cells to observe
the effect of the side chain length on photovoltaic performance.
Devices were made based on the standard glass/ITO/PEDOT-PSS/
polymer:PC61BM/Ca/Ag stack. Processing conditions were identical
for all materials, and no further individual optimization was per-
formed so far. As can be observed in Table 4, the resulting device
parameters are in close proximity with respect to each other and
pristine P3HT. There is a slight increase in open-circuit voltage
(Voc) for P1-P5, combined with a slight decrease in short-circuit
current density (Jsc). The Jsc values seem to have a positive correla-
tion with the melting enthalpy (Table 3), and hence polymer crys-
tallinity, although it should be noted that devices were not
individually optimized and the exact crystallinity of the copoly-
mers in the (annealed) BHJ blends was not analyzed. The J–V
curves for the best performing cells can be found in Supplementary
material. These results lead to conclude that the alkyl chain length
P5
a
b
c
Device architecture: glass/ITO/PEDOT-PSS/polymer:PC61BM/Ca/Ag.
Averaged values.
Mn = 22.9 Â 103 g molÀ1, PDI = 1.6.
does not have a detrimental effect on device efficiency and the
materials are hence attractive candidates as electron donor mate-
rials for polymer solar cells. Future efforts will therefore be direc-
ted toward individual optimization of the photovoltaic parameters
and thorough analysis of the effect of chain length on the thermal
stability of the phase-separated BHJ blend [12c,e].
Table 2
Electrochemical and optical data for random copolythiophenes P1–P5.
Polymer
UV–Vis
Cyclic voltammetry
LUMO (eV)a
Eg (eV)
ec
kmax
(nm)
Egopt(eV)
HOMO (eV)
film
P3HTb
P1
P2
P3
P4
520
523
555
552
558
549
1.88
1.88
1.90
1.90
1.89
1.89
À5.25
À5.11
À5.06
À5.16
À5.11
À5.12
À3.37
À3.23
À3.16
À3.26
À3.22
À3.23
2.55
2.43
2.39
2.47
2.45
2.44
P5
a
Calculated from the HOMO level as derived from CV and the optical bandgap.
b
Mn = 22.9 Â 103 g molÀ1, PDI = 1.6.

S. Kudret et al. / Reactive & Functional Polymers 75 (2014) 22–30
29
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Terminal ester-functionalized alkylthiophenes with different
alkyl chain lengths were prepared starting from readily available
3-bromothiophene and a,x-diols employing Rieke zinc in the key
step, i.e. the attachment of the side chains via a Kumada-type cross
coupling reaction. The required organozinc reagents were synthe-
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Wurfel, R. Andriessen, R. Rösch, H. Hoppe, G. Teran-Escobar, M. Lira-Cantu, A.
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treatment of the
x-alkylesters with highly reactive Rieke zinc,
prepared in situ by reduction of zinc chloride with lithium napht-
halenide in the presence of benzothiophene, proven to be an essen-
tial additive for stabilization of the Znà particles. Polymerization of
the functionalized dibromothiophene monomers by the Rieke
procedure afforded regioregular poly{[3-hexylthiophene]-co-[3-
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We thank IMEC and Hasselt University for providing the PhD
grant of S.K. We acknowledge the OPV-Life project from the IWT
(Agentschap voor Innovatie door Wetenschap en Technologie;
O&O 080368), the POLYSTAR project from PV ERA-NET, the IWT-
SBO project POLYSPEC (Nanostructured POLYmer photovoltaic
devices for efficient solar SPECtrum harvesting), and Belspo for
supporting the IAP P6/27 and IAP 7/05 networks. We are also
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