Benzothiadiazole-containing pendant polymers prepared by RAFT and their electro-optical properties

Article abstract of DOI:10.1021/ma1008572

The synthesis of a series of novel, unsymmetrically substituted benzothiadiazole-containing vinyl monomers and their free radical polymerization with and without the control of a reversible addition-fragmentation chain transfer (RAFT) agent is reported. The resulting polymers with electroactive pendants show tunable absorption and emission spectra depending on their molecular architecture. Using RAFT allows the synthesis of block copolymers using a hole-transporting vinyl-triarylamine as a second monomer. Efficient energy transfer between the two pendants has been detected. Cyclic voltammetry and photoelectron spectroscopy in air measurements have been employed to reveal the location of the HOMO and LUMO of the block copolymers. The block copolymers also influence the morphology of spin-casted films and show rectifying behavior in organic photovoltaic devices.

Full text of DOI:10.1021/ma1008572

Macromolecules 2010, 43, 7101–7110 7101
DOI: 10.1021/ma1008572
Benzothiadiazole-Containing Pendant Polymers Prepared by RAFT and
Their Electro-Optical Properties
,†
Matthias Haussler,* Y. Phei Lok, Ming Chen, Jacek Jasieniak, Raju Adhikari,
†
†
†
†
€
Simon P. King,‡ Saif A. Haque,‡ Craig M. Forsyth,§ Kevin Winzenberg,† Scott E. Watkins,†
Ezio Rizzardo,† and Gerard J. Wilson†
†CSIRO Materials Science and Engineering, Bayview Avenue, Clayton, Vic 3168, Australia, ‡Department of
Chemistry, Imperial College London, London SW7 2AZ, United Kingdom, and §School of Chemistry, Monash
University, Clayton, Vic 3800, Australia
Received April 19, 2010; Revised Manuscript Received July 6, 2010
ABSTRACT: The synthesis of a series of novel, unsymmetrically substituted benzothiadiazole-containing
vinyl monomers and their free radical polymerization with and without the control of a reversible addition-
fragmentation chain transfer (RAFT) agent is reported. The resulting polymers with electroactive pendants
show tunable absorption and emission spectra depending on their molecular architecture. Using RAFT
allows the synthesis of block copolymers using a hole-transporting vinyl-triarylamine as a second monomer.
Efficient energy transfer between the two pendants has been detected. Cyclic voltammetry and photoelectron
spectroscopy in air measurements have been employed to reveal the location of the HOMO and LUMO of the
block copolymers. The block copolymers also influence the morphology of spin-casted films and show
rectifying behavior in organic photovoltaic devices.
Introduction
To date, only a few examples of side-chain-functionalized poly-
mers have been reported, mainly using perylene diimide as
Plastic electronics is an emerging field of intense research,
where high performing organic semiconductors have the poten-
tial to deliver low-cost solutions, particularly in areas such as
thin-film transistors, organic light-emitting diodes (OLEDs), and
organic photovoltaics.1 For most of these devices, control of the
morphology of the active layer is important in optimizing the
device performance. For example, organic solar cells consist of a
blend of donor and acceptor materials with the need for order on
the nanoscale to compensate for the low exciton diffusion length
of organic materials.2 Inmost cases, a thermal annealing step after
the blend deposition enables the donor and acceptor to assemble
in a bicontinuous morphology. However, this desired morpho-
logy is often only metastable and far away from the thermo-
dynamic minimum of complete phase-separation.3
Lamellar-type structures are postulated to be ideal for organic
photovoltaic devices.4 Block copolymers are known to self-
assemble into various morphologies and also to stabilize blends
between two immiscible materials by acting as a compatibilizer.3c,5
Block copolymers are typically made by living polymerization,
where the chain-ends of the first block can initiate or control the
addition of the second block. Among all methods available,
reversible addition-fragmentation chain transfer (RAFT) poly-
merization is one of the most versatile methodologies for con-
trolling the architecture of a polymer chain and can be applied to
a wide range of vinyl monomers.6 The RAFT process equips a
conventional radical polymerization with living characteristics
through the presence of chain transfer agents such as thiocarbo-
nylthio compounds SdC(Z)-SR. Such control enabled the
synthesis of well-defined light-harvesting polymers, as previously
demonstrated by our group.7 Because of their pendant character
and limited π-conjugation length, the polymers obtained in those
studies, however, mainly absorbed in the UV range and have
only limited use for applications such as organic photovoltaics.
chromophore.8 This calls for the design of new monomers with
lower bandgaps, allowing extended absorption into the visible
region. One methodology could be to increase the π-conjugation
length of the pendant monomer. Although very useful, this app-
roach has its limitations as the molecular weights (MWs) of the
monomers quickly become too large. Other methods include π-
conjugated length alterations, increased aromaticity, substi-
tuent effects, and intermolecular interactions, which are often
limited to fully conjugated polymer systems.9 Another approach
that provides a more feasible way of lowering the bandgap in
pendant systems is the use of donor-acceptor (D-A) or partial
charge-transfer chromophores. These donor-acceptor chromo-
phores are formed when strong electron-donating and strong
electron-withdrawing moieties interact through the high-lying
HOMO of the donor and the low-lying LUMO of the acceptor.10
The hybridization of these two orbitals results in new D-A
orbitalswith a smaller bandgap. Low bandgap materials reported
in the literature are often based on electron-rich aromatic building
blocks such as thiophenes or pyrroles sandwiching electron-poor o-
quinoid units like benzothiadiazole (BT) or benzobisthiadiazole.11
In this contribution, we report the design and synthesis of new
unsymmetrically substituted monomers containing the BT group
as well as their radical polymerization with and without the con-
trol of a RAFT agent. Furthermore, the use of RAFT polymer-
ization allows for the synthesis of block copolymers by using a
vinyltriarylamine as the second monomer. We studied the ther-
mal and optical properties of all of the new electroactive polymers
as well as the morphology of spin-casted films. Finally, initial
results for their use as the active layer in a bulk heterojunction
solar cell will be reported.
Experimental Section
Materials. 4-Vinylphenylboronic acid, 4-methoxyphenyl-
boronic acid, and 4-methylphenylboronic acid were purchased
*Corresponding author. E-mail: Matthias.Haeussler@csiro.au.
r 2010 American Chemical Society
Published on Web 08/06/2010
pubs.acs.org/Macromolecules

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Haussler et al.
7102 Macromolecules, Vol. 43, No. 17, 2010
from Boron Molecular. 4-Bromo-2,1,3-benzothiadiazole, 4,7-
dibromo-2,1,3-benzothiadiazole,12 potassium vinyltrifluorobo-
rate,13 and 2-cyanopropan-2-yl butyl trithiocarbonate14 were
prepared according to literature methods. 2,20-(Diazene-1,2-
diyl)bis(2-methylpropanenitrile) (AIBN), 1,10-(diazene-1,2-diyl)-
dicyclohexanecarbonitrile (Vazo-88), and 1,2-bis(2,4,4-trimethyl-
pentan-2-yl)diazene (V110) were obtained from TCI, Dupont,
and Wako, respectively. Silica gel (0.040 to 0.063 mm) and thin
layer chromatography plates (TLC silica gel 60 F254) used for
column chromatography were purchased from Merck. All other
starting materials were obtained from Aldrich and were used as
received. Solvents used for absorption and fluorescence measure-
ments were spectroscopic grade, and solutions were degassed by
nitrogen prior to spectroscopic measurements.
isopropanol. The substrates were then exposed to a UV-ozone
clean (at RT) for 10 min. The PEDOT/PSS (HC Starck, Baytron
P AI 4083) was filtered (0.2 μm RC filter) and deposited by spin
coating at 5000 rpm for 60 s to give a 38 nm layer. The PEDOT/
PSS layer was then annealed on a hot plate in the glovebox at
145 °C for 10 min. The polymers and PCBM (Nano-C) were
dissolved in o-dichlorobenzene (Aldrich, anhydrous) in indivi-
dual vials with stirring. The solutions were then combined,
filtered (0.2 μm RC filter), and deposited by spin coating. The
coated substrates were then transferred (without exposure to
air) to a vacuum evaporator in an adjacent glovebox. A layer of
Ca (20 nm) and then Al (100 nm) was deposited by thermal
evaporation at pressures below 2 ꢀ 10-6 mbar. The total active
device area was 0.2 cm2. We made a connection point for the
ITO electrode by manually scratching off a small area of the
active layers. A small amount of silver paint (Silver Print II, GC
electronics, part no. 22-023) was then deposited onto all of the
connection points, both ITO and Al. The completed devices
were then encapsulated with glass and a UV-cured epoxy
(Summers Optical, lens bond type J-91) by exposing to 365 nm
UV light inside a glovebox (H2O and O2 levels both <1 ppm) for
10 min. The encapsulated devices were then removed from the
glovebox and tested in air within 1 h. Electrical connections were
made using alligator clips. The cells were tested with an Oriel
solar simulator fitted with a 1000 W Xe lamp filtered to give an
output of 100 mW/cm2 at AM 1.5. The lamp was calibrated
using a standard, filtered Si cell from Peccell Limited, which was
subsequently cross-calibrated with a standard reference cell
traceable to the National Renewable Energy Laboratory. The
devices were tested using a Keithley 2400 Sourcemeter con-
trolled by Labview Software.
Synthesis of N,N-Di-p-tolyl-3-vinylaniline (2). A mixture of
3-bromostyrene (5 g, 27.3 mmol), di-p-tolylamine (1,5.9g,30mmol),
sodium tert-butoxide (3.9 g, 41 mmol), bis(dibenzylideneacetone)
palladium (0.157 g, 0.29 mmol), and tri-tert-butylphosphonium
tetrafluoroborate 0.063 g (0.22 mmol) in 20 mL of anhydrous toluene
was degassed and stirred under nitrogen overnight. Afterward,
50 mL of dichloromethane and 100 mL of 5% hydrochloric acid
were added, and the solution was vigorously stirred for 30 min.
The organic layer was separated, washed three times with 100 mL
of water, and dried over MgSO4. The product was further purified
by column chromatography using 5% DCM in hexane, giving
a white solid (7 g, 85%). 1H NMR (CDCl3, δ): 2.31 (s, 6H, CH3),
5.62 5.19 (d, 1H, dCHH), (d, 1H, dCHH), 6.61 (dd, 1H,
dCArH), 7.08-7.02 (m, 12H, ArH).
Methods. 1H and 13C NMR spectra were obtained on a
Bruker AC-400 spectrometer in deuterated chloroform. UV-
vis absorption spectra were recorded on a Cary 5E spectro-
photometer. Steady-state fluorescence spectra were recorded on
a Perkin-Elmer LS50 fluorimeter. The optical densities of the
solutions for fluorescence measurements were kept below 0.15 at
the excitation wavelength. We carried out time-correlated single
photon counting experiments by employing a Jobin Yvon IBH
fluorocube laser system. Excitation was performed at 467 nm
(1 MHz, 80 μW cm-2 average intensity, instrument response
250 ps fwhm), and the emission was detected at 600 and 650 nm.
Photoelectron spectroscopy in air (PESA) measurements were
recorded with a Riken Keiki AC-2 PESA spectrometer with a
power setting of 5-10 nW and a power number of 0.5. Samples
for PESA were prepared on glass substrates. The electrochem-
istry measurements were carried out using a Powerlab ML160
potentiostat interfaced via a Powerlab 4/20 controller to a PC
running Echem for Windows version 1.5.2. The measurements
were run in argon-purged acetonitrile with tetrabutylammo-
nium hexafluorophosphate (0.1 M) as the supporting electro-
lyte. The voltammograms were recorded using a standard three-
electrode configuration with a glassy carbon (2 mm diameter)
working electrode, a platinum wire counter electrode, and a silver
wire pseudoreference electrode. The silver wire was cleaned in
concentrated nitric acid and then in concentrated hydrochloric
acid to generate the Ag/Agþ reference. Voltammograms were
recorded with a sweep rate of 50-200 mV s-1. The sample con-
centration for monomers in solution was 1 mM. For the polymers,
we prepared films of the polymers on the working electrode by
drop-casting toluene solutions onto the working electrode. All
potentials were referenced to the E1/2 of the ferrocene/ferrocenium
couple. The highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) were estimated
from the onsets of the oxidation and reduction peak, respectively,
versus the midpoint potential of ferrocene, which was added as
internal reference. MW data were obtained by gel permeation
chromatography (GPC). Tetrahydrofuran (1.0 mL min-1) was
used as eluent. The GPC was calibrated with narrow polydisper-
sity polystyrene (PS) standards, and MWs are reported as PS
equivalents. Differential scanning calorimetry (DSC) analyses
were conducted on a Mettler Toledo 821 apparatus with “Star
Software” version 9. The module was calibrated with indium/
zinc total method. Samples were run in alternating DSC mode
with an underlying heating rate of 10 °C min-1. Samples were
encapsulated in lightweight aluminum pans. A sample size
between 5 and 10 mg was used. Thermogravimetric analyses
(TGA) were conducted on a Mettler Toledo TGA/STDA851
apparatus with “Star Software” version 9. Samples of ∼10 mg
were placed in a 70 μm alumina pan and run with a heating rate
of 10 °C min-1 under nitrogen. Atomic force microscopy
(AFM) images were recorded on a Pacific Nanotechnology
microscope (Nano-R2).
Synthesis of 4-(4-Bromo-phenyl)-2,1,3-benzothiadiazole. 4-
Bromophenyl boronic acid (2.82 g, 13.1 mmol) and 4-bromo-
2,1,3-benzothiadiazole (3.0 g, 14.9 mmol) were dissolved in
toluene (40 mL) and THF (30 mL). A solution of Na2CO3
(2.36 g, 25.0 mmol) in water (16 mL) was added, followed by
[Pd(PPh3)4] (1.6 g, 10 mol %). The reaction mixture was stirred
vigorously while heating at reflux at 85 °C for 18 h. The reaction
mixture was poured in iced water (400 mL), and this aqueous
mixture was extracted with ether (3 ꢀ 250 mL). The combined
ether extracts were dried (MgSO4), filtered, and evaporated to
dryness. A minimum volume of ethyl acetate was added to this
crude product, and insoluble impurities were filtered. The
filtrate was purified by column chromatography with petroleum
ether 40-60 (PETH40-60), followed by 20:80 DCM/PETH40-60).
Fractions were combined, evaporated to dryness, and recrystal-
lized from ethyl acetate to give the product as bright-yellow
needles. 2.56 g (63%). TLC (5% ethyl acetate/PETH40-60) Rf =
1
0.30. mp 106.4-107.9 °C. H NMR (CDCl3, δ): 7.54-7.71 (m,
4H), 7.81-7.83 (m, 2H), 8.00-8.04 (m, 1H). 13C NMR (CDCl3):
121.0, 122.8, 127.6, 129.6, 130.8, 131.8, 133.3, 136.2, 153.2, 155.6.
GC-MS (EI): m/z 292.0.
Device Fabrication. ITO-coated glass (Kintek, 15 Ω/cm2) was
cleaned by standing in a stirred solution of 5% (v/v) Deconex
12PA detergent at 90 °C for 20 min. The ITO was then succes-
sively sonicated for 10 min each in distilled water, acetone, and
Synthesis of 4-(4-Vinylphenyl)-2,1,3-benzothiadiazole (4). Mono-
mer 4 was prepared using the reaction conditions described by
Molander and Rivero.13 To a solution of 4-(4-Bromo-phenyl)-benzo-
[1,2,5]thiadiazole (0.68 g, 2.3 mmol), potassium vinyltrifluoroborate

Article
Macromolecules, Vol. 43, No. 17, 2010 7103
(0.447 g, 3.7 mmol), and [1,1-bis(diphenylphosphino)ferrocene]-
dichloropalladium(II) [PdCl2(dppf)] in THF (25 mL) was added a
solution of Cs2CO3 (2.3 g, 7.0 mmol) in water (4 mL). The reaction
mixture was heated at reflux for 4 h. After the reaction mixture was
quenched in iced water, the aqueous mixture was extracted with
ether (4 ꢀ 150 mL). The combined organic extracts were washed
with 20 wt % aqueous citric acid (3 ꢀ 200 mL), aqueous 1 M HCl
(1 ꢀ 200 mL), and saturated brine (1 ꢀ 100 mL). This ether solu-
tion was then dried (MgSO4), filtered, and the filtrate evaporated
to dryness. The resulting residue was dissolved in DCM and filtered
through a silica gel plug to give the product as a bright-yellow solid
(0.44 g, 79%). TLC (5% ethyl acetate/PETH40-60) Rf = 0.33. mp
56.9-59.6 °C. 1H NMR (CDCl3, δ): 5.32 (dd, 1H, dCHH), 5.85
(dd, 1H, dCHH), 7.57 (d, 2H, Ar-H), 7.67-7.68 (m, 2H, BT-H),
7.91 (d, 2H,, Ar-H), 7.98 (dd, 1H, BT-H). 13C NMR (CDCl3):
114.5, 120.5, 126.4, 127.4 129.4, 129.6, 134.1, 136.4, 136.7, 137.7,
153.4, 155.6. GC-MS (EI): m/z 238.
130 °C for 20 h. The mixture was cooled to room temperature,
poured in iced water (150 mL), and extracted with diethyl ether
(3 ꢀ 250 mL). The combined organic extracts were dried (MgSO4),
filtered, and evaporated to dryness. The crude product was purified
through a short silica gel plug (DCM) eluting with 2% ethyl
acetate/PETH40-60, followed by recrystallization from DCM/
PETH40-60 to give the product as yellow needles (0.64 g, 94%).
1H NMR (CDCl3, δ): 2.44 (s, 3H), 7.05 (d, 2H), 7.33 (d, 2H),
7.71-7.88 (m, 4H). 13C NMR (CDCl3): 115.6, 127.4, 127.8, 129.1,
129.3, 130.2, 130.6, 132.6, 132.8, 134.6, 138.2, 154.1, 154.2, 155.8.
MS (EI): m/z 318 [M]þ. HRMS calcd for C19H14N2OS, 318.0827;
found, 318.0823.
Synthesis of 4-(7-(5-Methylthiophen-2-yl)-2,1,3-benzothiadia-
zol-4-yl)phenol (7b) and 4-(7-(50-Hexyl-2,20-bithiophen-5-yl)-
2,1,3-benzothiadiazol-4-yl)phenol (7c). Intermediates 7b and 7c
were prepared similar to 7a from 6b and 6c, respectively.
Characterization of 7b. Yellow powder. 1H NMR (CDCl3, δ):
2.44 (br,1H, OH), 2.58 (s, 3H, TpCH3), 6.87 (d, 1H), 7.01 (d,
2H), 7.65 (d, 1H), 7.83 (d, 1H), 7.87 (d, 2H), 7.91 (d, 1H).
Characterization of 7c. Red powder. 1H NMR (CDCl3, δ): 0.91
(t, 3H), 1.27-1.45 (m, 6H), 1.71 (m, 2H), 2.83 (t, 2H, TpCH2),
4.99 (br, 1H, OH), 6.74 (d, 1H), 7.01 (d, 2H), 7.12 (d, 1H), 7.20 (d,
1H), 7.68 (d, 1H), 7.86-7.91 (m, 3H), 8.04 (d, 1H).
Synthesis of 4-p-Tolyl-7-[4-(4-vinyl-benzyloxy)-phenyl]-2,1,3-
benzothiadiazole (8a). To a solution of 7a (0.60 g, 1.9 mmol) in
dry acetonitrile (10 mL) and dry DMF (4 mL) was added
4-vinylbenzyl chloride (0.32 mL, 2.3 mmoL), followed by anhy-
drous K2CO3 (1.29 g, 9.4 mmol). The reaction mixture was
stirred vigorously at 65 °C for 4 h, cooled to room temperature,
poured in ice water (150 mL), and extracted with DCM (3 ꢀ
250 mL). The combined organic extracts were dried (MgSO4),
filtered, and evaporated to dryness. The crude product was
purified by column chromatography starting with a 5% ethyl
acetate/PETH40-60 and eluting the product with 20% MeOH/
ethyl acetate. The monomer is obtained as a bright-yellow
powder (0.79 g, 96%). 1H NMR (CDCl3, δ): 5.27 (d, 1H), 5.77
(d, 1H), 6.73 (dd, 1H), 7.14 (d, 2H), 7.35 (d, 2H), 7.45 (s,
4H), 7.73 (d, 2H), 7.85 (d, 2H), 7.93 (d, 2H). 13C NMR
(CDCl3) 154.1, 154.2, 159.0, 114.1, 115.0, 126.4, 127.7, 127.6,
127.8, 129.0, 129.3, 130.2, 130.4, 132.6, 132.8, 134.6, 136.4,
137.4, 138.2. HRMS (EI) calcd for C28H22N2OS, 434.1453;
found, 434.1446,.
Synthesis of 4-Bromo-7-(4-methoxy-phenyl)-2,1,3-benzothia-
diazole. 4,7-Dibromo-2,1,3-benzothiadiazole (1.56 g, 5.3 mmol),
4-methoxyphenyl boronic acid (0.81 g, 5.3 mmol), and [Pd-
(PPh3)4] (0.30 g, 10 mol %) were dissolved in toluene (25 mL)
and THF (25 mL). A solution of 2 M Na2CO3 (4 mL) was added,
and the reaction mixture was vigorously stirred while being
refluxed at 85 °C for 20 h. The reaction mixture was poured in
iced water (150 mL), and this aqueous mixture was extracted with
ethyl acetate (3 ꢀ 250 mL). The combined organic extracts were
dried (MgSO4), filtered, and evaporated to dryness. The crude
product were purified by column chromatography (5% ethyl
acetate: PETH40-60), followed by recrystalliztion from ethyl
1
acetate to give the product as yellow needles (0.99 g, 58%). H
NMR (CDCl3, δ): 3.88 (s, 3H), 7.05 (d, 2H), 7.52 (d, 1H), 7.83-
7.91 (m, 3H). 13C NMR (CDCl3): 55.4, 112.2, 114.2, 127.3, 129.0,
130.4, 132.3, 133.6, 153.2, 153.8, 160.0. MS (EI) m/z 322 [M]þ.
HRMS calcd for C13H9BrN2OS, 319.9619; found, 319.9614.
Synthesis of 4-(4-Methoxyphenyl)-7-p-tolyl-2,1,3-benzothia-
diazole (6a). 4-Bromo-7-(4-methoxyphenyl)-2,1,3-benzothia-
diazole (1.51 g, 4.6 mmol), p-tolyl boronic acid (0.77 g, 5.7
mmol), and [Pd(PPh3)4] (0.27 g, 10 mol %) were dissolved in
toluene (15 mL) and THF (15 mL). A solution of 2 M Na2CO3
(10 mL) was added, and the reaction mixture was vigorously
stirred while being heating at reflux at 85 °C for 24 h. The
reaction mixture was poured in ice water (150 mL) and
extracted with ethyl acetate (3 ꢀ 250 mL). The combined
organic extracts were dried (MgSO4), filtered, and evaporated
to dryness. The crude product was purified by column chro-
matography (5% ethyl acetate: PETH40-60), followed by re-
crystallization from DCM/PETH40-60 to give the product as
yellow needles (0.32 g, 70%). 1H NMR (CDCl3, δ): 2.45 (s, 3H,
ArCH3), 3.90 (s, 3H, OCH3), 7.09 (d, 2H), 7.35 (d, 2H), 7.74 (d,
2H), 7.84-7.96 (m, 4H). 13C NMR (CDCl3): 21.3, 55.4, 114.1,
127.3, 127.8, 129.1, 129.3, 129.2, 130.4, 132.7, 132.7, 134.6,
138.2, 154.2, 154.3, 159.8. MS (EI) m/z 332 [M]þ. HRMS calcd
for C20H16N2OS, 332.0983; found, 332.0974.
Synthesis of 4-(5-Methylthiophen-2-yl)-7-(4-(4-vinylbenzy-
loxy)phenyl)-2,1,3-benzothiadiazole (8b) and 4-(50-Hexyl-2,20-
bithiophen-5-yl)-7-(4-(4-vinylbenzyloxy)phenyl)-2,1,3-benzothia-
diazole (8c). Monomers 8b and 8c were prepared similarly to 8a
from 7b and 7c, respectively, by stirring at room temperature
overnight.
Characterization of 8b. Yellow powder after purification by
column chromatography (hexane/CHCl3 1:1 v/v). Red needles
suitable for XRD analysis obtained by slow diffusion of hexane
1
in DCM. H NMR (CDCl3, δ): 2.58 (s, 3H, TpCH3), 5.15 (s,
2H), 5.27 (d, 1H), 5.77 (d, 1H), 6.74 (dd, 1H), 6.87 (d, 1H), 7.14
(d, 2H), 7.43 (m, 4H), 7.65 (d, 1H), 7.83 (d, 1H), 7.89-7.93 (m,
3H). Data from single-crystal XRD analysis are shown in the
Supporting Information.
Synthesis of 4-(4-Methoxyphenyl)-7-(5-methylthiophen-2-yl)-
2,1,3-benzothiadiazole (6b) and 4-(50-Hexyl-2,20-bithiophen-5-
yl)-7-(4-methoxyphenyl)-2,1,3-benzothiadiazole (6c). Intermedi-
ates 6b and 6c were prepared similar to 6a from 4-bromo-7-(4-
methoxyphenyl)-2,1,3-benzothiadiazole and the respective
boronic acid pinacol esters.
1
Characterization of 8c. Red powder. H NMR (CDCl3, δ):
0.91 (t, 3H), 1.29-1.44 (m, 6H), 1.71 (m, 2H), 2.83 (t, 2H,
TpCH2), 5.15 (s, 2H), 5.27 (d, 1H), 5.78 (d, 1H), 6.73 (d, 1H),
6.74 (dd, 1H), 7.11 (d, 1H), 7.13 (d, 2H), 7.19 (d, 1H), 7.44 (m,
4H), 7.66 (d, 1H), 7.88 (d, 1H), 7.92 (d, 2H), 8.03 (d, 1H).
General Procedure for Free Radical Polymerization of Vinyl
Monomers. A mixture of the radical initiator, monomer, and
solvent was degassed with three freeze-pump-thaw cycles,
sealed under vacuum, and heated in a constant-temperature
oil bath for the stated time. The mixtures were then cooled and
precipitated twice in MeOH, filtered, and washed with MeOH to
give the polymers.
Characterization of 6b. Yellow needles. 1H NMR (CDCl3, δ):
2.60 (s, 3H, TpCH3), 3.90 (s, 3H, OCH3), 6.87 (d, 1H), 7.08 (d,
2H), 7.66 (d, 1H), 7.84 (d, 1H), 7.92 (m, 3H).
Characterization of 6c. Red solid. 1H NMR (CDCl3, δ): 0.91
(t, 3H), 1.27-1.45 (m, 6H), 1.71 (m, 2H), 2.83 (t, 2H, TpCH2),
3.91 (s, 3H, OCH3), 6.73 (d, 1H), 7.08 (d, 2H), 7.11 (d, 1H), 7.20
(d, 1H), 7.68 (d, 1H), 7.89 (d, 1H), 7.93 (d, 2H), 8.04 (d, 1H).
Synthesis of 4-(7-p-Tolyl-2,1,3-benzothiadiazol-4-yl)phenol
(7a). To a suspension of 6a (0.72 g, 2.2 mmol) in HBr (48%,
20 mL) was added hexadecyldibutylphosphonium bromide
(0.14 g, 0.22 mmol), and the reaction mixture was stirred at
Characterization of P4. Yellow powder. 1H NMR (CDCl3, δ):
1.27, 2.38, 6.71, 7.5-7.9 (broad).

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7104 Macromolecules, Vol. 43, No. 17, 2010
Characterization of P8a. Yellow powder. 1H NMR (CDCl3,
δ): 0.91, 1.32, 1.72, 2.08, 2.40, 4.95, 6.71, 6.95, 7.20, 7.5-8.0.
Characterization of P8b. Orange powder. 1H NMR (CDCl3,
δ): 1.44, 2.40, 4.77, 6.66, 7.14, 7.54, 7.6-8.0 (broad).
Scheme 1. Synthesis of Benzothiadiazole- and Triarylamine-Contain-
ing Monomers
General Procedure for RAFT-Controlled Free Radical Polym-
erization of Vinyl Monomers. Mixtures of the 2-cyanopropan-2-
yl butyl trithiocarbonate (RAFT agent), radical initiator, mono-
mer, and solvent were degassed with three freeze-pump-thaw
cycles, sealed under vacuum, and heated in a constant-tempera-
ture oil bath for the stated time. The polymers were precipitated
twice into MeOH, filtered, and washed with MeOH.
Characterization of P2-RAFT. Light-yellowpowder.1HNMR
(CDCl3, δ): 0.89, 0.99, 1.28, 1.79, 1.97, 2.18, 3.21, 6.08, 6.82.
Characterization of P8b-RAFT. Orange powder. H NMR
1
(CDCl3, δ): 0.93, 1.15, 1.35, 1.57, 1.73, 2.07, 2.54, 3.27, 4.97,
6.97, 7.30, 7.46, 7.71, 7.93.
Characterization of P2-b-8b-RAFT. Orange powder. 1HNMR
(CDCl3, δ): 0.92, 0.99, 1.31, 1.57, 1.78, 1.99, 2.19, 2.50, 3.23, 4.87,
6.08, 7.30, 7.45, 7.71, 7.93.
Characterization of P8c-RAFT. Red powder. 1H NMR
(CDCl3, δ): 0.92, 1.34, 1.62, 2.18, 2.79, 6.0-8.25 (very broad).
Characterization of P2-b-8c-RAFT. Red powder. H NMR
1
(CDCl3, δ): 0.92, 1.35, 1.65, 2.16, 2.79, 6.06, 6.78, 7.23, 7.51.
General Procedure for Removal of Thiocarbonylthio Group.
The thiocarbonylthio groups were removed by radical-induced
reduction using N-ethylpiperidine hypophosphite.15 A typical
procedure is described below.
A mixture of polymer P8b-RAFT (Mn = 11 230, Mw/Mn =
1.24, 280 mg, 0.022 mmol), N-ethylpiperidine hypophosphite
(77 mg, 0.43 mmol), and Vazo-88 (5.3 mg, 0.022 mmol) in
chlorobenzene (3.5 mL) was placed in an ampule, degassed with
three freeze-pump-thaw cycles, sealed under vacuum, and
heated to 110 °C for 4 h. The solution was precipitated in
80 mL of acetone, washed twice with 50 mL of acetone, and dried
under vacuum. To ensure the complete removal of the thiocarbo-
nylthio group, the polymer was afterward dissolved in 15 mL of
CHCl3 and 0.5 mL of n-butylamine and stirred overnight at room
temperature. Removal of the solvent under reduced pressure gave
a yellow polymer (265 mg, Mn= 13 050; Mw/Mn= 1.34).
of the product was problematic because of the instability of the
compound to the extensive chromatography necessitated by the
presence of the byproduct 4,40-divinyl-biphenyl. Monomer 4
was therefore obtained by reacting 4-(4-bromophenyl)-2,1,3-
benzothiadiazole with potassium vinyltrifluoroborate.13 This
reaction is an efficient way of introducing the vinyl functionality
and proceeded smoothly on a 1 g scale in 95% yield.
We prepared another three BT-containing monomers by
isolating the electroactive pendants from their polymerizable
styrene functionality. For this, we sandwiched the BT moiety
between a methoxyphenyl on one side and a tolyl, a thio-
phenyl, and a bithiophenyl chromophore on the other side.
Those electron-donating groups are expected to push elec-
trons to the electron-withdrawing BT moiety forming donor-
acceptor dyes 6a-c with low band gaps.20 The syntheses of the
monomers were completed by deprotection of the methoxy-
functionality with hydrogen bromide, followed by coupling
with 4-vinylbenzyl chloride.
Results and Discussion
Monomer Synthesis. To prepare functional polymers by
radical polymerization, it is necessary to synthesize unsym-
metrical electroactive pendant monomers. The design and
synthesis is outlined in Scheme 1. As a hole-transporting
pendant, we chose a triarylamine derivative, which is readily
prepared by the palladium-catalyzed Buchwald-Hartwig
cross coupling of di-p-tolylamine and 3-bromostyrene.16 The
methyl substituents in the para position have been chosen to
prevent the known irreversible oxidative coupling of tripheny-
lamine moieties.17 The meta-substituted vinyl bond has been
selected over its para-substitutedisomertobreakthesymmetry
and thus to give better solubility in the resulting polymer chain.
Another reason is the known improved stability of the meta-
linkage when used as hole-transporting material in OLEDs.18
BT is a well-known electron-deficient synthon utilized in
small molecular dyes and fully conjugated polymers.19 Thanks
to its low-lying LUMO, it imparts electron-transporting prop-
erties to the resulting electroactive material. The first monomer
prepared in this series was 4-vinyl-2,1,3-benzothiadiazole
(Scheme S1 of the Supporting Information), which sponta-
neously polymerized when isolated to give low-molecular-
weight oligomers. Given the instability of this monomer, no
further attempts were made to resynthesize this compound.
To obtain more stable monomers, the vinyl group was
separated by a phenyl group in the second monomer 4-(40-
vinylphenyl)-2,1,3-benzothiadiazole (4), which was initially pre-
pared from 4-vinylphenyl boronic acid. However, purification
As part of the final purification procedure before polym-
erization, monomers were usually recrystallized. Slow diffu-
sion of hexane into a concentrated dichloromethane solution
formed large needle-like red crystals of monomer 8b, which
were suitable for X-ray diffraction (XRD) analysis. Mole-
cular diagrams of the crystal structure from its top and side
view are shown in Figure 1A,B, respectively. The structure of
the BT pendant is very flat, with only the phenyl moiety
slightly out of plane, suggesting a very good π-conjugation
between the three aromatic rings. In contrast, the vinylbenzyl
group is twisted in the opposite direction to the phenyl group,
indicating only limited interaction between the two aromatic
systems due to the nonconjugated -CH2O spacer. The mole-
cular packing of the monomers occurs along the b axis in an
alternating up-down-up-down fashion of the BT chromo-
phore, with each second monomer rotated 180°. The overlapping
parts of each chromophore are always a BT with a thiophene
˚
ring with a distance between two units of 3.4 A. This suggests
a very tight packing and strong intermolecular interactions.

Article
Macromolecules, Vol. 43, No. 17, 2010 7105
Figure 1. Molecular diagram of monomer 8b shown with 50% thermal ellipsoids and hydrogen atoms as spheres of arbitrary size in (A) top view and
(B) side view. (C) Molecular packing arrangement (hydrogen atoms are omitted for clarity).
Scheme 2. Synthesis of Electroactive Homo and Block Copolymers
Polymerization. Homopolymers of the electroactive pen-
dant monomers 2, 4, 8a, and 8b (Scheme 1) were obtained by
free-radical polymerization in excellent yields (up to 95%
conversion, Scheme 2 and Table 1). The MWs varied accord-
ing to the monomer polymerized ranging from 7 to ∼100
kDa. This broad variation can be attributed to variations in
the solubility of the monomer and hence the molar concen-
tration of the vinyl bonds. Monomer 2 and 8b showed the
highest solubility among the four monomers; therefore, less
solvent was used for the polymerization, and as a result, the
highest MWs were obtained. Attempts to increase the con-
centration of the monomers were only of limited success, and
the data shown here represent the best results achieved to
date. PS control polymerizations, carried out under compar-
able monomer concentrations, confirmed the obtained con-
versions and MWs. Monomer 8c was available only in small
quantities and only used during the following described
controlled radical polymerizations.
(absorbing light in the visible region). All polymerizations
proceeded smoothly, and polymers with expected MWs and
very narrow MW distributions (Mw/Mn = 1.10 to 1.24) were
obtained. From each of the macro-RAFT agents, a second
block could be polymerized with slightly broader MW
distribution in the range of 1.29 to 1.64, as demonstrated
by the block copolymers P2-b-8b, P2-b-8c, and P8b-b-2.
In one of our previous studies, we found that the thiocar-
bonylthio group can interfere with the luminescence proper-
ties of the electroactive polymers and quench emitted
light.8a,21 We thus removed the thiocarbonylthio groups of
the polymers investigated in the Optical Properties section of
this article. We achieved removal of the thiocarbonylthio
groups by reacting the final polymer with N-ethylpiperidine
hypophosphite and AIBN, followed by treatment with
n-butylamine. (See the Experimental Section for details.)15
Structural and Thermal Analysis. All polymers were char-
acterized by spectroscopic methods, from which satisfactory
analysis data corresponding to their molecular structures
were obtained. (See the Experimental Section for details.)
Figure 2 shows the 1H NMR spectra of monomer 8b and its
RAFT-synthesized homopolymer P8b-RAFT. Whereas 8b
exhibits the characteristic resonance peaks of the vinyl group
from the styrene moiety between 5.3 and 6.8 ppm, those
To obtain well-defined block copolymers, monomers 2,
8b, and 8c (Scheme 1) were selected to be polymerized under
the control of a trithiocarbonate RAFT agent. These mono-
mers were chosen on the basis of their potential electroactive
properties because triarylamine is known as a hole-trans-
porting material16-18 and 8b and 8c are strongly colored

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Haussler et al.
7106 Macromolecules, Vol. 43, No. 17, 2010
Table 1. Molecular and Thermal Properties of Homo and Block Copolymersa
polymer
conv. (%)b
DP
Mn,theor (Da)c
Mn (Da)d
Mw (Da)d
Mw/Mn
Tg (°C)e
Td (°C) f
1
2
3
4
5
6
7
8
P2
P2-RAFT
P4g
95.0
71.0
79
80
80
59.1
27.9
52.0
78.9i
68.5i
64.4i
39.0i
48 000
6180
5990
97 440
7110
11 720
7570
34 990
13 930
11 800
15 400
52 130
44 600
13 960
51 730
2.03
1.15
1.96
1.60
1.60
1.24
1.17
1.15
1.64
1.29
1.33
1.42
134
119
150
117
130
127
128
122
133
117
127
134
399
388
400
35
10 700
P8ah
P8b
4730
21 860
11 230
10 100
13 420
31 860
34 540
10 490
36 540
353
326
339
P8b-RAFT
P8b-RAFTg
P8c-RAFT
P2-b-8b-RAFT
P2-b-8c-RAFT
P8b-b-2-RAFT
P8b-b-2-RAFT
29
28
26
12 920
12 550
15 620
28 060
30 980
22 120
70 860
9
35/39
35/34
28/32
28/195
333
404
339
349
10
11
12
a Polymerization conditions: AIBN, chlorobenzene, 70 °C, 16 h, for RAFT polymerizations: [monomer]/[RAFT]/[initiator] 50:1:0.1. b Conversion.
c The number-averagemolecularweight was calculatedaccording to: Mn,theor = DP ꢀ Mmono þ MRAFT þ Mini, where DP is the degree of polymerization
(obtained from 1H NMR), Mmono is the molecular weight of the monomer, MRAFT is the molecular weight of the RAFT agent, and Mini is the molecular
weight of the initiator fragment. d Molecular weight obtained from gel permeation chromatography in chloroform against polystyrene calibration.
e Glass-transition temperature obtained from DSC at a heating rate of 10 °C/min under nitrogen. f Decomposition temperature obtained from TGA at a
heating rate of 20 °C/min under nitrogen. g Initiator Vazo88 at 90 °C for 20 h, toluene. h Initiator V110 at 110 °C for 20 h, solvent: chlorobenzene/toluene
4:1. i Conversion of second block.
respectively, directly attached to the PS backbone and the lack
of long solubilizing alkyl groups. It is somewhat surprising that
P8a has a lower Tg than P8c because the latter polymer
contains long hexyl groups in each repeating unit. It is likely
that the presence of the two thiophene moieties in P8c (vs only
a phenyl ring in P8a) negates the lubricating effects of the alkyl
group. At the same time, the MW is known to influence the Tg
as well, and it is usual for the Tg to increase with increasing
MW.24 This can be shown by comparing the low-molecular-
weight P2-RAFT (Mn = 6180; Tg = 119 °C) with its structu-
rally identical high-molecular-weight analog P2 (Mn = 48 000;
Tg = 134 °C).
The block copolymers show thermal properties similar to
those of their homopolymer analogs, although only one Tg
can be detected from DSC instead of two. This can be
understood considering that the Tg values of all employed
building blocks are in the same range of 127-134 °C. The
only exception is P2-b-8c-RAFT, which exhibits a Tg of 117 °C,
suggesting that the P8c block has a bigger influence on the
thermal properties than the polytriarylamine block of P2.
TGA analysis shows all the pendant polymers to be very
Figure 2. 1H NMR spectra of monomer 8b and its homopolymer
P8b-RAFT (solvent is marked with asterisk).
stable, with weight loss beginning at temperatures well above
300 °C (Table 1). Figure 3 illustrates the thermogram of poly-
mer P2 as an example. The triarylamine pendant polymer
peaks completely disappear after the polymerization. At the
same time, new peaks emerge around 1.5 to 2.2 ppm asso-
ciated with the formation of the PS polymer backbone.22
Additionally, peaks at 0.8 to 1.4 and 3.2 ppm can be iden-
tified corresponding to the presence of the RAFT end group
of the polymer chain. All other peaks are readily assigned
to the electroactive pendant, confirming the proposed mole-
cular structure.
The thermal properties of the polymers were evaluated
using DSC and TGA, and the results are summarized in
Table 1. Despite their large aromatic chromophores, which
could potentially induce π-π stacking and thus some higher
order, all polymers are amorphous in nature (also confirmed
by powder XRD shown in Figure S2 in the Supporting In-
formation), and no transition peaks, such as melting points,
were observed in the heating scan of the DSC. The glass-
transition temperatures (Tg) of the pendant polymers are in
the range of 120-150 °C, which is considerably higher than
their atactic PS congener (Tg of ∼100 °C).23 As expected, the
Tg of the homopolymers depends on the molecular structure
and follows the trend P8a<P8c < P8b < P2 < P4. This order
is not very surprising because P2 and P4 should have the most
rigid backbone due to their bulky ditolyl amine and BT groups,
exhibits a constant weight until its final degradation tem-
perature of ∼400 °C. Its RAFT-synthesized analog P2-RAFT
shows a slightly different behavior by losing around 3% of its
original weight at ∼230 °C. The weight afterward remains
constant until ∼390 °C before it reaches its final degradation
temperature as well. The weight loss of the RAFT polymer can
be rationalized by a concerted thermolysis of the trithiocarbo-
nate end group.25 As depicted in the inset of Figure 3, it is
postulated that the high temperatures induce a 2 þ 2 þ 2
electron rearrangement resulting in an unsaturated polytriar-
ylamine chain end plus S-butyl trithiocarboxylic acid, which
readily decomposes into carbon disulfide and butyl thiol. The
measured ∼3% weight loss agrees very well with the amount
of RAFT agent in P2-RAFT. Considering a measured MW
of 6180 g/mol, the fragment of the RAFT agent weighs 165
g/mol, and this corresponds to 2.67 wt % in the polymer.
Optical Properties. One aim of this investigation was to
study the optical properties of nonconjugated polymers with
electroactive pendant. In contrast with conventional con-
jugated polymers, the electro-optical properties of the mono-
mer are reflected in the resulting polymer because the poly-
merization does not alter the attached chromophoric unit.
This is clearly illustrated in Figure 4. Chloroform solutions

Article
Macromolecules, Vol. 43, No. 17, 2010 7107
Figure 5. Normalized absorption spectra of benzothiadiazole-contain-
ing polymers in chloroform solutions.
Figure 3. Thermal gravimetric analysis of P2 and P2-RAFT. Inset shows
proposed concerted mechanism for thermolysis of RAFT end groups
occurring at ∼230 °C (TAA = triarylamine fragment of P2-RAFT).
maximum absorption by 45 nm. Although the conjugation
path length of P8b is similar to that of P8a, the electron-rich
thienyl group is a stronger donor than tolyl and as a conse-
quence shows an even further bathochromic-shifted UV ab-
sorption with its maximum at 440 nm. Polymer P8c exhibits
the largest π-conjugation among the four different pendants
and as a result shows the longest wavelength absorption peak
at 465 nm. Clearly, the UV absorption of the polymers is
strongly affected by the choice of the pendant chromophore.
The block copolymers also show similar absorption spec-
tra compared with their homopolymer components with an
additional peak at 305 nm from the triarylamine block
(Table 2).
We continued our spectroscopic study by evaluating the
photoluminescence properties of thin films of pendant homo
and block copolymers. All homopolymers are highly fluor-
escent and emit light depending on their pendant chromo-
phore in the broad range from 424 to 658 nm (Table 2).
The block copolymers show a somewhat different picture.
Exciting block copolymer P2-b-8b at 350 nm, a wavelength
that mainly excites the P2 block as P8b exhibits an absorp-
tion minimum (cf. Figure 4), results in an emission peak at
585 nm originating from the P8b block (Figure 6). The emis-
sion peak of the polytriarylamine block P2 at 424 nm is
completely quenched. Such behavior suggests efficient inter-
action between the different electroactive pendants mainly
in the form of energy transfer. More detailed photophysical
investigations are currently underway to understand the
electronic interactions between these two building blocks
better.
Another interesting finding can be observed by exciting
homopolymer P8b and its block copolymer P2-b-P8 at
450 nm, a wavelength outside the polytriarylamine absorp-
tion. Despite having comparable UV absorptions of the BT
chromophore at the excitation wavelength, which means
identical numbers of photons are absorbed in both films,
the block copolymer shows a higher luminescence than its
homopolymer counterpart. Initial fluorescence lifetime mea-
surements support these findings (Figures S5 and S6 in the
Supporting Information),26 which suggests that the BT chro-
mophore undergoes self-quenching due to aggregate forma-
tion. These aggregates get broken up in the copolymer by the
polytriarylamine blocks and as a result overcome the self-
quenching.
Figure 4. Normalized absorption spectra of monomer 8b and its
homopolymer P8b in chloroform solutions and as a thin film spin-
casted from CHCl3 solution.
of monomer 8b and its homopolymer P8b show almost iden-
tical UV absorption peaks with their maximum at 440 nm.
Even the solid state does not alter the absorption spectrum
much, and only a 6 nm bathochromic shift can be observed
when polymer P8b is spin-casted on a glass slide. All poly-
mers are strongly absorbing in the visible region with molar
extinction coefficients up to 35 000 M-1 cm-1 (cf. Figure S3
and S4 in the Supporting Information).
The retention of the absorption properties of the monomer
chromophore in the polymer allows for a molecular engi-
neering approach to variation of the electronic properties of
the corresponding polymers. BT is known to be a strong,
electron-withdrawing building block, and the attachment of
an electron donor at its four- and seven-positions alters the
bandgap of the resulting chromophore. This is demonstrated
by variations in the UV absorption spectra of the four BT-
containing homopolymers. As shown in Figure 5 and Table 2,
the absorption band shifts to lower energy, an indication of
increasing π-conjugation and increasing donor strength, as the
number of aryl rings on the electroactive pendant increases.
P4, with its BT moiety directly attached to the styrene back-
bone and only a proton as substituent at the seven-position,
has the smallest conjugation path length and consequently
exhibits the shortest absorption peak among all four polymers,
peaking at 360 nm in dilute chloroform solution. The BT unit
of polymers P8a, P8b, and P8c is sandwiched between an
oxyphenyl on one side and a tolyl, thienyl, and bithienyl group,
respectively, on the other. Substituting the proton of P4 with a
tolyl group in P8a extends the π-conjugation and red-shifts the
Electronic Properties. To characterize fully our electro-
active materials, we determined the HOMO and LUMO
levels by cyclic voltammetry and PESA in combination with
UV absorption and photoluminescence spectroscopy. Most
of the BT-containing polymers P4-P8 exhibit symmetrical,
quasi-reversible oxidation and reduction peaks, from which

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Haussler et al.
7108 Macromolecules, Vol. 43, No. 17, 2010
Table 2. Photophysical Properties of Thin Films of Homo and Block Copolymers
polymer
UVmax (nm)
PLmax (nm)
Eg,op (eV)a
IP (eV)b
HOMO (eV)c
LUMO (eV)c
Eg,el (eV)d
P2
P4
P8a
P8b
P8c
P2-b-8b
P2-b-8c
305
365
415
440
467
440
468
424
490
541
610
658
608
651
3.40
2.90
2.59
2.36
2.20
2.37
2.22
-5.59
-5.70
-5.89
-5.85
-5.44
-5.60
-5.44
-5.25
-5.73
-5.42
-5.40
-5.27
-5.17
-5.25
n.d.
-2.85
-2.94
-2.99
-3.10
-2.99
-3.10
2.88
2.48
2.41
2.17
2.18
2.15
a Optical bandgap, determined by the onset of the UV absorption peak. b Ionization potential, measured from photoelectron spectroscopy in air
(PESA). c HOMO and LUMO obtained from the onset of the peaks in the CV spectra against Fc/Fcþ redox couple, assuming a HOMO level of ferrocene
of -4.8 eV (n.d. = not detected). d Electronic bandgap, calculated from the differences between HOMO and LUMO level.
Figure 6. Photoluminescence spectra of thin films of pendant homo
and block copolymers spun-coated on glass. The films were excited at
350 and 450 nm. The photoluminescence is normalized by the UV
absorption of the films at the excitation wavelength.
Figure 7. Cyclic voltammetry of P2-b-P8b spin-coated on the glassy
carbon electrode.
be found by spin-casting solutions of homopolymers P2 and
P8b onto glass slides (Figure 8B). Very rough films are obtai-
ned with height differences up to 250 nm, suggesting the
formation of phase-separated domains of homopolymers. In
contrast, spin-casting solutions of block copolymer P2-b-8b
gives very smooth films, demonstrating that block copoly-
mers can be used to improve the morphology of films
(Figure 8C).
Application in Organic Photovoltaic Devices. Thanks to
their self-assembly behavior, donor-acceptor block copoly-
mers could potentially be used as single component materials
in bulk heterojunction solar cells. All attempts to utilize only
our block copolymers in an OPV device, however, were
unsuccessful, and no rectifying behavior was obtained. This
can be rationalized by the fact that the block copolymers
showed only limited self-quenching of their photolumines-
cence (cf. Figure 6), which translates into few charges being
separated upon illumination.
To improve the electron transfer and to enhance the charge
mobility in the devices, PCBM, a strong electron acceptor, was
added. Figure 9a shows the dark and light current-voltage
characteristics of a typical OPV device based on a P2-b-8b/
PCBM 1:4 active layer composition. The strong rectification
behavior in the dark highlights the favorable diode properties
of the device. An equivalent circuit analysis of the diode
suggests series and parallel resistance values of 6 Ω cm2 and
750 kΩ cm2, respectively. Although devices with varying ratios
of P2-b-8b/PCBM were tested, the best devices were obtained
from a blend ratio of P2-b-8b/PCBM of 1:4 with Voc, Jsc, FF,
and PCE of 0.77 V, 0.73 mA/cm2, 0.28, and 0.16%, respec-
tively. Whereas the Voc, which reflects the difference between
the ionization potential of the donor and the electron affinity
of the acceptor, in this device is high, the photocurrent is
relatively low. A comparison of the incident photon to current
efficiency (IPCE) to the absorptance spectrum of the active
film deposited on glass confirms that the photocurrent is
HOMO and LUMO levels can be readily calculated using the
ferrocenium/ferrocene redox couple as internal reference
and assuming that this is related to the ferrocene HOMO
level of -4.8 eV.27 Figure 7 shows the cyclic voltammogram
of P2-b-P8b spin-coated as thin film on top of a glassy car-
bon electrode. The plot shows two overlapping reversible
oxidation peaks at 0.42 and 0.55 V, which, by comparison
with CVs of the homopolymers, are assigned as originating
from the triphenylamine and BT pendants, respectively.
Additionally, a reversible reduction peak can be identified
at -1.88 V associated with the BT unit as well. This suggests
that the HOMO of P2-b-P8b is associated with the poly-
triarylamine block, whereas the LUMO is associated with
the BT-containing pendant polymer block. This conclusion
is supported by the measured IP of the block copolymer,
which gives a value of -5.60 eV, very close to that of P2
(-5.59 eV). The block copolymer P2-b-8c shows a slightly
different behavior. Here both HOMOs and LUMOs are
mainly associated with the BT-containing pendant polymer
block, which is supported by the IP of -5.44 eV, similar to
that of P8c (Table 2).
Film Morphology. A key parameter in device fabrication is
the morphology of solid-state films. Rough or highly phase-
segregated films are susceptible to electrical shorting, varia-
tion in current density, or both when used in devices. To
study the effect of incorporating the electroactive groups into
block copolymers on this key parameter, we prepared thin
films of model compound 6b and block copolymer P2-b-P8b
as well as a blend of the two homopolymers P2 and P8b.
Model compound 6b is similar to monomer 8b, a highly
crystalline material, and forms long needle-like crystals even
when spin-coating films from CHCl3 solution (Figure 8A).
Blending two different polymers together usually results in a
phase separation of the two materials. A similar behavior can

Article
Macromolecules, Vol. 43, No. 17, 2010 7109
Figure 8. AFM images of thin films of (A) model compound 6b, ( B) blends of homopolymers P2 and P8b, and (C) block copolymer P2-b-P8b spin-
coated from 2 wt % CHCl3-solutions.
Figure 9. (A) Current-voltage curve of a 147 nm organic solar cell device consisting of a blend of block copolymer P2-b-8b and PCBM (blend ratio
1:4) as active layer. (B) IPCE and absorption spectra of active layer.
generated from the active layer (Figure 9b). Unfortunately, as
indicated by the low FF, the excitons and carriers that are
generated within this active layer are recombining strongly.
Recent work on block copolymer systems suggests that their
major benefits to OPV are as modifiers for morphology and
as stabilizers of multicomponent mixtures.28 Indeed, the use
of our pendant polymers could be advantageously used in a
similar manner.
Acknowledgment. This work was funded through the Flex-
ible Electronics Theme of the CSIRO Future Manufacturing
Flagship as part of an Office of the Chief Executive Postdoctoral
Fellowship and was also supported by the Victorian Organic
Solar Cells Consortium (Victorian Department of Primary
Industries, Sustainable Energy Research and Development
Grant) and the Australian Department of Innovation, Indus-
try, Science and Research, International Science Linkage
grant CG100059. We thank Prof. Andrew Holmes from the
University of Melbourne for fruitful discussions and helpful
suggestions.
Conclusions
New electroactive pendant polymers based on BT-containing
chromophores have been synthesized by free radical polymeriza-
tion with and without the control of a RAFT agent. RAFT syn-
thesized polymers could be chain-extended to yield well-defined
block copolymers. The resulting pendant homo- and block
copolymers exhibit interesting photophysical properties such as
efficient energy transfer between the chromophores. The block
copolymers influenced the morphology of spin-casted films and
showed rectifying behavior in OPV devices, making them poten-
tial candidates for various applications in the field of organic
electronics.
Supporting Information Available: X-ray structural infor-
mation on 8b, powder XRD spectrum of P8b, absorption
spectrum of P8c, single photon counting measurements, and
an X-ray crystallographic file (CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
Note Added after ASAP Publication. This article posted
ASAP on August 6, 2010. The name of the seventh author of
this paper has been changed from Saif A. Hague to Saif A.
Haque. The correct version posted on August 17, 2010.

€
Haussler et al.
7110 Macromolecules, Vol. 43, No. 17, 2010
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