Design, synthesis, and properties of benzobisthiadiazole-based donor-π-acceptor-π-donor type of low-band-gap chromophores and polymers

Article abstract of DOI:10.1139/V09-157

A novel low-band-gap chromophore (5, 0.86 eV) having fluorene as a donor, benzobisthiadiazole (BBTD) as an acceptor, and pyrrole as a π-spacer was successfully designed and synthesized, to probe the effect of π-spacer on the band-gap level of the donor-π-acceptor-π-donor type of chromophores. Compared with the thiophene spacer analogue (in compound 3), the intramolecular hydrogen bonding between the pyrrole and the neighboring BBTD unit pushes the absorption maximum and fluorescence emission of chromophore 5 into the near-infrared spectral region with a red shift of 172 and 158 nm, respectively. The same red-shift phenomenon can also be realized by addition of Lewis acid (e.g., BF₃) to the BBTD-containing chromophores with other spacers. Attempt of using low-band-gap chromophore 5 in bulk heterojunction (BHJ) solar cells was made, showing a non-optimized photovoltaic device with the power conversion efficiency of 0.01%. A precursor approach to introduction of the alkaline-labile BBTD acceptor into the polymer backbone has been demonstrated by successful synthesis of low-band-gap polymer P2. The same strategy can be in principle applied to the synthesis of a series of low-band-gap chromophores or polymers with strong acceptors.

Full text of DOI:10.1139/V09-157

192
Design, synthesis, and properties of
benzobisthiadiazole-based donor–p–acceptor–
p–donor type of low-band-gap chromophores and
polymers
Gang Qian and Zhi Yuan Wang
Abstract: A novel low-band-gap chromophore (5, 0.86 eV) having fluorene as a donor, benzobisthiadiazole (BBTD) as an
acceptor, and pyrrole as a p-spacer was successfully designed and synthesized, to probe the effect of p-spacer on the
band-gap level of the donor–p–acceptor–p–donor type of chromophores. Compared with the thiophene spacer analogue
(in compound 3), the intramolecular hydrogen bonding between the pyrrole and the neighboring BBTD unit pushes the ab-
sorption maximum and fluorescence emission of chromophore 5 into the near-infrared spectral region with a red shift of
172 and 158 nm, respectively. The same red-shift phenomenon can also be realized by addition of Lewis acid (e.g., BF3)
to the BBTD-containing chromophores with other spacers. Attempt of using low-band-gap chromophore 5 in bulk hetero-
junction (BHJ) solar cells was made, showing a non-optimized photovoltaic device with the power conversion efficiency
of 0.01%. A precursor approach to introduction of the alkaline-labile BBTD acceptor into the polymer backbone has been
demonstrated by successful synthesis of low-band-gap polymer P2. The same strategy can be in principle applied to the
synthesis of a series of low-band-gap chromophores or polymers with strong acceptors.
Key words: donor–p–acceptor–p–donor, chromophore, low band gap, bulk heterojunction (BHJ), precursor polymer.
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Resume : Afin de pouvoir etudier l’effet d’un espaceur p sur le niveau d’energie interbande des chromophores de type
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donneur–p–accepteur–p–donneur, on a developpe et synthetise un nouveau chromophore de basse energie interbande (5,
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0,86 eV) comportant un fluorene comme donneur, un benzobisthiadiazole (BBTD) comme accepteur et un noyau pyrrole
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comme espaceur p. Par comparaison avec l’analogue comportant un espaceur thiophene (dans le compose 3), la liaison hy-
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drogene intramoleculaire entre le pyrrole et l’unite adjacente BBTD deplace le maximum d’absorption et l’emission de
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fluorescence du chromophore 5 dans la region spectrale du proche infrarouge avec des deplacements vers le rouge de res-
ˆ
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pectivement 172 et 158 nm. On peut observer le meme phenomene de deplacement vers le rouge par l’addition d’un acide
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de Lewis (tel le BF3) a des chromophores comportant du BBTD et d’autres espaceurs. On a essaye d’utiliser le chromo-
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phore 5 a faible energie interbande dans des cellules solaires a heterojonction globale (HJG); on a obtenu un dispositif non
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voltaıque non optimise ayant une efficacite de conversion de pouvoir de 0,01 %. On a demontre l’applicabilite d’une autre
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approche a l’introduction de l’accepteur BBTD labile aux produits alcalins dans le squelette d’un polymere en effectuant
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avec succes la synthese du polymere P2 de faible energie interbande. En theorie, la meme strategie peut etre appliquee a
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la synthese d’une serie de chromophores ou de polymeres de faible energie interbande avec des accepteurs forts.
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Mots-cles : donneur–p–accepteur–p–donneur, chromophore, faible energie interbande, heterojonction globale (HJG),
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polymere precurseur.
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[Traduit par la Redaction]
tant areas. Low-band-gap organic materials absorb light
with wavelengths longer than 700 nm or within the near-in-
frared (NIR) spectral region of 750–2000 nm.
In the area of solar-energy conversion, it is essential that
the absorption of the light-harvesting material matches the
spectral characteristics of the sun.1 For the polythiophene
(P3HT)-based bulk heterojunction (BHJ) solar cells, P3HT
has a band gap of ~1.90 eV and a rather narrow absorption
Introduction
Research on low-band-gap organic materials (e.g., chro-
mophores and polymers), whose band-gap levels, defined as
the difference between the highest occupied molecular orbi-
tal (HOMO) and the lowest unoccupied molecular orbital
(LUMO) energy levels, are typically below 1.8 eV, has
been the focal point in a number of technologically impor-
Received 30 August 2009. Accepted 13 October 2009. Published on the NRC Research Press Web site at canjchem.nrc.ca on 19 January
2010.
This article is part of a Special Issue dedicated to Professor M. A. Winnik.
G. Qian and Z.Y. Wang.1 Department of Chemistry, Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6, Canada.
1Corresponding author (e-mail: wayne_wang@carleton.ca).
Can. J. Chem. 88: 192–201 (2010)
doi:10.1139/V09-157
Published by NRC Research Press

Qian and Wang
193
band (FWHM & 150 nm) and can absorb only a limited
fraction of the solar photons (~30%). Thus, the NIR-absorb-
ing chromophores and polymers have the possibility to im-
prove the efficiency of organic BHJ solar cells owing to a
better overlap with the solar spectrum.2
Near-infrared fluorescent organic materials may find ap-
plications as NIR-fluorescent tags for bio-imaging3 and
chemical sensing4 or emitters in NIR light-emitting diodes
(LED) for information-secured display and background
lighting. To date, most of organic materials that have been
shown to emit the light around 700–1000 nm are mainly
with lanthanide complexes,5 transition-metal complexes,6
ionic dyes,7 and low-band-gap polymers.8 NIR organic pho-
tovoltaic materials have recently been demonstrated for use
in NIR photodetectors.9
Among various types of low-band-gap organic materials,
much effort has been focused on the design and synthesis
of the chromophores and polymers consisting of powerful
electron donor (D) and acceptor (A) units. Some of the
best-performing BHJ organic solar cells are fabricated with
the D–A type of conjugated polymers.10 Some of these
D–A polymers contain benzo-2,1,3-thiadiazole (BT) as the
acceptor unit.11 Recently, we have shown that the D–p–A–
p–D type of chromophores, where p is a spacer or linking
unit, can absorb light with the wavelength around 1000 nm
and fluoresce above 1000 nm, such as compounds 1–3 in
Fig. 1.12 The band-gap levels of this class of chromophores
depend on the strength of the donor and acceptor. Those
band-gap polymers having the D–p–A–p–D unit in the
backbone is demonstrated.
Experimental section
Materials
All chemicals and reagents were used as received from
commercial sources without purification. Solvents for chem-
ical synthesis were purified by distillation. All chemical
reactions were carried out under an argon atmosphere. 2-
Bromo-9,9-dioctylfluorene (6),15 N-(tert-butoxycarbonyl)-
2-(trimethylstannyl) pyrrole (7),16 4,8-dibromobenzo[1,2-
c:4,5-c’]bis([1,2,5]thiadiazole) (10),13d 4,7-bis[4-(N-phenyl-
N-(4-methylphenyl)amino)phenyl]-5,6-diamino-2,1,3-benzo-
thiadiazole (11),12b and 9,9-dioctyl-2,7-bis(trimethyleneboro-
nate)fluorene (13)17 were prepared according to literature
methods.
Methods
1H NMR spectra were recorded using a Bruker Avance
300 NMR spectrometer. The high-temperature NMR experi-
ments were performed on a Varian Unity-400 MHz spec-
trometer. High-resolution mass spectrometry (HRMS) was
obtained from 7.0 T Actively Shielded Fourier Transform
Ion Cyclotron Resonance Mass Spectrometers. The number-
and weight-average molecular weights of the polymers were
determined by gel-permeation chromatography (GPC) with a
Waters 410 instrument and polystyrene as a standard and
THF as eluent. Absorption and fluorescence spectra were
recorded with a Shimadzu UV-3600 or Lambda 900 Perkin-
Elmer spectrophotometer and a PTI fluorescence spectro-
photometer, respectively. IR spectra were recorded on a
Bio-Rad FTS-135 spectrophotometer. Cyclic voltammetry
(CV) was performed on a CHI660b electrochemical work-
station, in dry dichloromethane containing n-Bu4NPF6
(0.1 mol/L) with a scan rate of 50 mV/s at room temperature
under argon, using a Pt disk (2 mm diameter) as the working
electrode, a Pt wire as the counter electrode and a Ag/AgCl
electrode as the reference electrode.
containing
a
strong electron-withdrawing heterocyclic
quinoid, namely, benzo[1,2-c:4,5-c’]bis([1,2,5]thiadiazole)
(BBTD), tend to have a lower band gap than other
acceptor-containing chromophores, since the BBTD unit is
known to posses a substantial quinoidal character within a
conjugated backbone, allowing for greater electron delocali-
zation, and thus lowering the band gap.13 However, the
scope and limitations of the acceptor strength and the role
of the spacer in tuning the band gap are still not fully under-
stood. For example, a recent work shows that the band gap
of the D–A type of benzothiadiazole (BT)-containing chro-
mophores can be significantly lowered by the introduction
of Lewis acids as a result of changing the electronic proper-
ties of the BT fragment via interactions with Lewis acids
that bind nitrogen.14
Considering the D–p–A–p–D system such as compounds
1–3, by diminishing the electron density on the nitrogen of
the BBTD unit, a further decrease in band gap or red shift
in absorption and emission spectra of the chromophores
should be expected. Conceivably, the BBTD acceptor
strength can be increased through either the intermolecular
or intramolecular interaction by addition of Lewis acid or
other electron-accepting species. In this work, we intend to
show that by the introduction of pyrrole as a spacer, the
band gap of the D–p–A–p–D chromophores can be further
lowered due to the intramolecular hydrogen bonding with
the neighboring BBTD unit, which is similar to the effect
of adding a Lewis acid to the chromophores having other
spacers. Accordingly, a new D–p–A–p–D chromophore (5,
Fig. 1) is designed, synthesized, and characterized. Further-
more, for potential applications in thin-film devices, such as
polymer BHJ solar cells, a new general approach to low-
Photovoltaic device fabrication and characterization
The solar-cell devices were fabricated with the device
structure of [indium tin oxide (ITO)/poly(3,4-ethylene diox-
ythiophene):poly(styrenesulfonate) (PEDOT:PSS)/Compound
5:PCBM/Al]. The ITO glass was pre-cleaned and coated
with PEDOT:PSS. Then, the active layer was spin-coated
from the chlorobenzene solution of compound 5 and PCBM
(1:1, w/w) on the substrate. Finally, the Al cathode was de-
posited at a vacuum level of 4 Â 10–4 Pa. The effective area
of the unit cell is 16 mm2. The current–voltage (I–V) meas-
urement of devices was conducted on a computer controlled
Keithley 236 Source meter. A xenon lamp (500 W) with
AM 1.5 filter was used as the white light source, and the
optical power at the sample was 100 mW/cm2.
N-(tert-Butoxycarbonyl)-2-(9,9-dioctylfluoren-2-yl)pyrrole
(8)
2-Bromo-9,9-dioctylfluorene (6) (4.7 g, 10 mmol), N-
(tert-butoxycarbonyl)-2- (trimethylstannyl)pyrrole (7) (6.6 g,
12 mmol), and tetrakis(triphenylphosphane) palladium(0)
(115.5 mg, 0.1 mmol) were dissolved in a mixture of tol-
Published by NRC Research Press

194
Can. J. Chem. Vol. 88, 2010
Fig. 1. Chemical structures of D–p–A–p–D chromophores 1–5.
uene (20 mL) and a 1 mol/L aqueous solution of sodium
carbonate (20 mL). The mixture was stirred at 110 8C for
60 h. After cooling, the resulting solution was extracted
with dichloromethane. The combined organic layers were
washed with water and dried with anhydrous MgSO4. Evap-
oration of the solvent and subsequent column chromatogra-
phy (silica gel, dichloromethane/petroleum ether = 1/5)
afforded the product as pale yellow oil (1.1 g, 20%). H
NMR (300 MHz, CDCl3) d (ppm): 7.74–7.68 (m, 2H),
7.40–7.26 (m, 6H), 6.30–6.24 (m, 2H), 1.98 (t, 4H, J =
1
Published by NRC Research Press

Qian and Wang
195
8.37 Hz), 1.41 (s, 9H), 1.40–1.10 (m, 20H), 0.89–0.87 (m,
6H), 0.73–0.61 (m, 4H).
4,7-Bis[4-(N-(4-bromophenyl)-N-(4-
methylphenyl)amino)phenyl]-5,6-diamino-2,1,3-
benzothiadiazole (12)
A solution of tetrabutylammonium tribromide (0.96 g,
2.0 mmol) and 4,7-bis[4-(N-phenyl-N-(4-methylphenyl)ami-
no)phenyl]-5,6-diamino-2,1,3-benzothiadiazole (11) (0.61 g,
9.0 mmol) in freshly distilled dichloromethane (45 mL) was
stirred at room temperature for 3 h. The precipitates were
filtrated and washed with dichloromethane. The solid was
dried under vacuum to give the product as orange solid
Tributyl[N-(tert-butoxycarbonyl)-5-(9,9-dioctylfluoren-2-
yl)pyrrole-2-yl]stannane (9)
A 100 mL three-necked flask was charged with dry THF
(20 mL) and 2,2,6,6-tetramethylpiperidine (1.85 mL,
11 mmol). The mixture was cooled to –78 8C and n-BuLi
(2.5 mol/L in hexane, 4.8 mL, 12 mmol) was added drop-
wise. The mixture was stirred at –78 8C for 10 min, then
warmed to 0 8C and stirred for additional 10 min. At this
point, the mixture was cooled again to –78 8C, a solution of
N-(tert-butoxycarbonyl)-2-(9,9-dioctylfluoren-2-yl)pyrrole
(8) (5.5 g, 10 mmol) in dry THF (10 mL) was added, and
the mixture was stirred for 1.5 h while keeping the temper-
ature below –65 8C. Bu3SnCl (3.7 mL, 13 mmol) was added
dropwise. The mixture was stirred for 40 min at –75 8C and
for additional 40 min at 0 8C, and then for 12 h at room
temperature. The mixture was poured into water and ex-
tracted with diethyl ether. The organic extracts were dried
over anhydrous MgSO4. Upon evaporation of the solvent,
the crude product was obtained as pale yellow oil and used
for the next step without further purification. 1H NMR
(300 MHz, CDCl3) d (ppm): 7.75–7.67 (m, 2H), 7.40–7.24
(m, 5H), 6.44 (d, 1H, J = 3.75 Hz), 6.33 (d, 1H, J =
3.00 Hz), 1.99 (t, 4H, J = 8.10 Hz), 1.65–1.62 (m, 6H),
1.40–1.37 (m, 6H), 1.24 (s, 9H), 1.18–1.08 (m, 26H), 0.97–
0.92 (m, 9H), 0.89–0.83 (m, 6H), 0.73–0.61 (m, 4H).
1
(0.55 g, 73%). H NMR (300 MHz, CDCl3) d (ppm): 7.43
(d, 4H, J = 8.3 Hz), 7.34 (d, 4H, J = 8.7 Hz), 7.19 (d, 4H,
J = 8.5 Hz), 7.12 (d, 4H, J = 8.6 Hz), 7.08 (d, 4H, J =
9.1 Hz), 7.04 (d, 4H, J = 8.7 Hz), 2.33 (s, 6H).
Polymer P1
To a mixture of 9,9-dioctyl-2,7-bis(trimethyleneboronate)-
fluorene (13) (0.279 g, 0.500 mmol) and 4,7-bis[4-(N-(4-
bromophenyl)-N-(4-methylphenyl)amino)phenyl]- 5,6-dia-
mino-2,1,3-benzothiadiazole (12) (0.419 g, 0.500 mmol)
was added Aliquat 336 (0.120 g), Pd(PPh3)4 (7.9 mg,
0.007 mmol), degassed toluene (7 mL), and aqueous
2 mol/L potassium carbonate (3 mL) under a dry argon at-
mosphere. The mixture was heated to 95 8C and stirred in
the dark for 48 h. After cooling, the mixture was poured
into methanol. The precipitate was collected by filtration
and then dissolved in dichloromethane. The solution was
washed with water, dried with anhydrous MgSO4, and
then concentrated to an appropriate volume. The fiber-like
brown polymer (0.500 g, 94%) was obtained by pouring
the concentrated solution into methanol. 1H NMR
(300 MHz, CDCl3) d (ppm): 7.74 (d, 2H, J = 7.9 Hz),
7.61–7.55 (m, 8H), 7.47 (d, 4H, J = 8.7 Hz), 7.29–7.25
(m, 8H), 7.20–7.13 (m, 8H), 2.36 (s, 6H), 2.00 (br, 4H),
1.20–1.00 (m, 20H), 0.80–0.75 (m, 10H).
4,8-Bis[N-(tert-butoxycarbonyl)-5-(9,9-dioctylfluoren-2-
yl)-2-pyrrolyl]benzo[1,2-c:4,5-c’]bis([1,2,5]thiadiazole) (4)
To a solution of compound 9 (1.0 g, 1.2 mmol) and 4,8-
dibromobenzo[1,2-c:4,5-c’]bis([1,2,5]thiadiazole)
(10)
(0.18 g, 0.5 mmol) in toluene (40 mL) was added Pd(PPh3)4
(71 mg, 0.061 mmol). The mixture was stirred for 36 h at
110 8C. After cooling, the mixture was poured into water
and extracted with dichloromethane. The organic layer was
washed with saturated aqueous potassium fluoride and brine
before being dried over anhydrous MgSO4. After evapora-
tion of the solvent, the residue was purified by column chro-
matography on silica gel with dichloromethane/petroleum
ether (1:1) as the eluent to afford the product (0.13 g, 20%)
Polymer P2
To a solution of polymer P1 (0.107 g, 0.1 mmol) in dry
pyridine (10 mL) was added N-sulfinylaniline (0.5 mL,
4 mmol) and chlorotrimethylsilane (0.5 mL, 3.6 mmol) in
argon atmosphere. The mixture was heated to 80 8C and
stirred overnight. After workup, excess chlorotrimethylsilane
was distilled off. The solution was concentrated to 5 mL and
poured into methanol. The fiber-like dark blue polymer P2
(98.0 mg, 89%) was collected by filtration, washed with
ethanol, and dried under vacuum. 1H NMR (300 MHz,
CDCl3) d (ppm): 8.20 (d, 4H, J = 8.5 Hz), 7.75 (d, 2H, J =
7.4 Hz), 7.63–7.56 (m, 8H), 7.34–7.16 (m, 16H), 2.37 (s,
6H), 2.06 (br, 4H), 1.06 (br, 20H), 0.80–0.75 (m, 10H).
1
as black solid. H NMR (400 MHz, o-C6D4Cl2, 403 K) d
(ppm): 7.78–7.72 (m, 4H), 7.54–7.46 (m, 4H), 7.41–7.35
(m, 6H), 7.05 (d, 2H, J = 3.5 Hz), 6.56 (d, 2H, J = 3.5 Hz),
2.05–2.00 (m, 8H), 1.59 (s, 18H), 1.29–1.00 (m, 40H), 0.86
(t, 12H, J = 6.7 Hz), 0.70 (br, 8H).
Results and discussion
4,8-Bis[5-(9,9-dioctylfluoren-2-yl)-2-pyrrolyl]benzo[1,2-
c:4,5-c’]bis([1,2,5]thiadiazole) (5)
Design and synthesis of low-band-gap chromophores
Thermolysis of compound 4 (98 mg) at 200 8C under re-
duced pressure (10–5 torr, 30 min) (1 torr = 133.322 Pa) af-
forded 92 mg (94% yield) of product as black solid. 1H
NMR (400 MHz, o-C6D4Cl2, 403 K) d (ppm): 11.72 (br,
2H), 8.04 (br, 2H), 7.78–7.67 (m, 8H), 7.55–7.37 (m, 6H),
6.96 (br, 2H), 2.15–2.11 (m, 8H), 1.23–1.15 (m, 40H),
0.85–0.80 (m, 20H). HRMS (for [M]+) calcd.: 1100.65119;
found: 1100.64015.
Design concept and synthesis
The concept for designing low-band-gap chromophores is
based on the use of a strong electron acceptor, namely, ben-
zobisthiadiazole, and electron donors that are linked through
a p-conjugated spacer. In particular, the D–p–A–p–D type
of chromophores, such as compounds 1–3 (Fig. 1), previ-
ously reported by our group,12a is the focal point of design
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196
Can. J. Chem. Vol. 88, 2010
Scheme 1. Synthetic route to intermediates and chromophores 4 and 5. Reagents and conditions: (i) Pd(PPh3)4, Na2CO3 (aq., 1 mol/L),
toluene, 110 8C, 60 h; (ii) N-lithium-2,2,6,6-tetramethylpiperidine, –70 8C, (Bu)3SnCl, THF, –70 8C to room temperature; (iii) Pd(PPh3)4,
toluene, 110 8C, 36 h; (iv) 200 8C, 30 min, 10–5 torr.
Fig. 2. Normalized absorption (10–5 mol/L) and fluorescence emis-
sion (10–4 mol/L) spectra of compounds 4 and 5 in toluene.
and study. With the same acceptor, the absorption wave-
length of chromophores 1–3 varies according to the strength
of donors and the nature of spacer. Between compounds 2
and 3, the difference in band-gap levels is attributed to the
different donors. However, a larger difference in the band-
gap levels or absorption (Dlmax = 157 nm) between chro-
mophores 1 and 2 is clearly due to the presence of different
p-spacers (thiophene vs. benzene or nil). Therefore, the
p-spacer or linking moiety in the D–p–A–p–D type of
chromophores can play an important role in band-gap tun-
ing, which may lead to further band-gap lowering or spectral
red shift relative to chromophores 1–3. Accordingly, in this
work, pyrrole was introduced as p-spacer in a new chromo-
phore 5 (Fig. 1) to study the scope and limitations of the
p-spacer regarding the band-gap tuning. Any additional con-
tribution to a better D–A interaction or lowering the LUMO
level of the acceptor unit that may be brought by the use of
a new p-spacer is likely to result in a further red shift in ab-
sorption in reference to the structurally analogous chromo-
phore 3 (lmax = 848 nm).
Scheme 1 outlines the synthetic route to compounds 4 and
5. Compound 4 was readily synthesized by the Stille cou-
pling reaction of dibromo-BBTD 10 with tributyltin com-
pounds 9 derived from the donor. It should be noted that
aqueous sodium carbonate solution was not added, as re-
quired to suppress the C–Sn bond cleavage in the Stille cou-
pling reaction,18 due to instability of the BBTD group in
alkaline solution, which caused a rather poor yield (20%)
for compound 4. Removal of the tert-butoxycarbonyl
(t-Boc) protecting group in 4 by heating under reduced pres-
sure afforded the target compound 5 with high yield.
shift is mainly due to non-planarity between the BBTD and
the pyrrole induced by t-Boc group. After removal of the
t-Boc group, both the maximum absorption and emission
wavelengths were red-shifted dramatically (about 183 and
121 nm, respectively; Fig. 2) relative to its precursor 4. The
pyrrole spacer and the neighboring BBTD core in compound
5 are expected to be completely coplanar by virtue of hydro-
gen bonding (Fig. 3), which should facilitate the charge
transfer from donor to acceptor, and thus lowers the band-
gap level. This intramolecular hydrogen bonding also con-
tributes to altering the energy level of the BBTD acceptor
by sharing or removing partial electrons on the nitrogen in
1
BBTD. In the H NMR spectra of compound 5, the pyrrole
Optical and electrochemical properties
N–H signal is found at the low field (d = 11.68, Fig. 4),
which is an indication for strong intramolecular hydrogen
bonding.19
Similar to the effect of hydrogen-bonding interaction, ad-
dition of Lewis acids should also alter the band gap as a re-
sult of binding with the nitrogen in BBTD. Figure 5 shows
the absorption spectra of compound 3 and a mixture of 3
with excess BF3. The spectrum of compound 3 exhibits an
The absorption and photoluminescence of compounds 4
and 5 in toluene are shown in Fig. 2, and the data are pre-
sented in Table 1. As for compounds 1–3, compound 5 has
mainly two absorption bands. The peaks from 300 to
500 nm are attributed to the p–p* transition and the possi-
ble n–p* transition of the conjugated aromatic segments,
and those at longer wavelengths are due to the intramolecu-
lar charge transfer (ICT) transitions between the donor and
acceptor. Compared with compound 3, the maximum ab-
sorption of compound 4 is blue-shifted. Since pyrrole is
more electron-donating than thiophene, the observed blue
absorption maximum (lmax) at 834 nm with an onset (lonset
)
at 958 nm. Upon addition of an excess of BF3, a color
change took place immediately, going from yellowish green
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Qian and Wang
197
Table 1. Optical and electrochemical data of chromophores.
abs
max
PL
max
Compound
l
(nm)a
Log 3a
l
(nm)b
Stokes shift (nm)
Ff (%)c
HOMO (eV)d
LUMO (eV)d
Energy gap (eV)
1e
2e
3e
4
763
920
848
837
1020
4.38
4.86
4.71
4.53
4.57
1065
1125
1055
1092
1213
302
205
207
255
193
7.1
5.3
18.5
NA
0.3
4.95
4.77
5.14
4.96
4.75
3.76
3.94
3.99
3.85
3.89
1.19
0.83
1.15
1.11
0.86
5
aMeasured in toluene with a concentration of 10–5 mol/L.
bMeasured in toluene with a concentration of 10–4 mol/L. Excitation wavelengths for 1–4 are at their maximum absorption wavelengths, and at
980 nm for 5.
cFluorescence quantum yield measured relative to IR-125 (Ff = 0.13 in DMSO).
dCalculated from the formula, E(HOMO)= –(Eox + 4.34) (eV), E(LUMO) = –(Ered + 4.34) (eV).
eSee ref. 12a.
Fig. 3. Hydrogen bonding in compound 5 and the 3-BF3 adduct.
Fig. 5. Normalized absorption spectra of compound 3 (10–4 mol/L
in THF) and a mixture of 3 with excess BF3.
1
Fig. 4. H NMR spectra (400 MHz, o-C6D4Cl2, 403 K) of com-
pounds 4 and 5.
diazole rings (Fig. 3), effectively pulling the electron density
away from the BBTD unit, enhancing the ICT transition be-
tween peripheral donor and acceptor core, and thus leading
to a significant bathochromic shift in absorption. Addition-
ally, there is a synergistic lowering of the absolute energies
of the two frontier molecular orbitals, thereby giving rise to
the narrower band gap.
Considering the potential application of the D–p–A–p–D
type of low-band-gap chromophores in photovoltaic cells,
compound 5 was blended with PCBM and cast into films.
Figure 6 shows the absorption spectrum of the blend film
(1:1, w/w; solid line) and displays mainly three peaks cen-
tered at 333, 428, and 1046 nm, respectively. Compared
with the absorption spectrum of 5, the peak at 333 nm in-
creases, which was due to the absorption of PCBM. Addi-
tionally, the ICT transition peak becomes broad and is
red-shifted (about 26 nm). Thus, there is additional charge
transfer interaction in the blend, which is likely due to the
one between the acceptor and PCBM and similar to the ef-
fect of adding a Lewis acid to the chromophores.
The electrochemical properties of compounds 4 and 5
were investigated by CV (Fig. 7), and the data are summar-
ized in Table 1. They are all electrochemically active, hav-
ing two reversible oxidation and two reduction waves in the
cyclic voltammogram, respectively. The first reduction po-
tential is from the reduction of the BBTD core, and the first
to dark blue visually. Consequently, the lmax and lonset of
compound 3-BF3 adduct appeared at 1260 nm and
1580 nm, being red-shifted by 426 nm and 622 nm, respec-
tively. According to the confirmed structure of the complex
of benzothiadiazole derivatives with Lewis acid,14 in the
3-BF3 adduct, BF3 should bind to the nitrogen of each thia-
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198
Can. J. Chem. Vol. 88, 2010
Table 2. Performance of photovoltaic device under illumination of
Fig. 6. Absorption spectra of the blend film of compound 5 with
PCBM (1:1, w/w; solid line) and compound 5.
100 mW/cm2 white light.
Material
Voc (V)
Jsc (mA/cm2)
FF
PCE (%)
5:PCBM (1:1, w/w)
0.27
0.13
0.28 0.01
Fig. 8. Current density–voltage curves of solar-cell devices in dark
and under illumination of 100 mW/cm2 white light. Inset: Proposed
energy-level scheme of compound 5 and PCBM.
Fig. 7. Cyclic voltammograms of compounds 4 and 5 (1 mmol/L
concentration).
current (Jsc) of 0.13 mA/cm2 under AM 1.5 solar simulator
(100 mW/cm2). Accordingly, a fill factor (FF) was calcu-
lated to be 0.28. For BHJ devices, Voc is linearly correlated
with the energy difference of the HOMO of the donor and
the LUMO of the acceptor. Our result (0.27 V) coincides
well with the empirical calculation.20 The relatively low Voc
is likely due to the high HOMO level of the donor (Fig. 8,
inset). The Jsc is determined by the amount of absorbed light
and the internal conversion.21 The absorption profile of the
active layer shows a quiet mismatch to the solar photon
flux, thus leading to the quite low Jsc. Nevertheless, the de-
vice based on low-band-gap chromophore 5 is still compara-
ble well to some other solution-processed molecular BHJ
solar cells.
oxidation wave is attributed to the oxidation of the donors
and p-spacers. For chromophore 5, the first oxidation poten-
tial increases significantly (about 0.2 eV); however, the re-
duction potentials keep nearly unchanged. The HOMO and
LUMO levels of compounds 4 and 5 were calculated ac-
cording to empirical equations of E(HOMO)= –(Eox + 4.34)
(eV) and E(LUMO) = –(Ered + 4.34) (eV), respectively. As
shown, compound 5 has the band gap of 0.86 eV and ab-
sorbs at the longest wavelength (1020 nm) among chromo-
phores 1–5.
Design and synthesis of low-band-gap polymer
The band gap of polymers can be reduced by incorporat-
ing alternating donor and acceptor moieties in the polymer
main chain. BBTD is a strong electron acceptor and can be
used by design to construct many different kinds of low-
band-gap polymers. However, because the BBTD unit is
unstable in alkaline solution and readily attacked by nucleo-
philes (e.g., OH?), the polymers containing the BBTD moi-
ety are often difficult to be synthesized by the Suzuki
coupling reaction or other methods that need to use a base.
Up to now, several polymers containing BBTD have been
reported,22 and are mainly synthesized by the Stille coupling
reaction or electrochemical polymerization. However, the re-
quired tributylstannane monomers for the Stille polymeriza-
tion are toxic and difficult to purify, and electrochemical
polymerization is difficult to produce high-molecular-weight
soluble polymers. Therefore, it remains a challenge to syn-
thesize high-molecular-weight low-band-gap polymers con-
taining BBTD unit.
Photovoltaic property
Organic BHJ photovoltaic cells were fabricated using
compound 5 as electron donor and PCBM as electron ac-
ceptor, with a structure configuration of ITO/PEDOT:PSS/
5:PCBM/Al. Typical performance data of the devices are
listed in Table 2, and the corresponding current–voltage
curves are illustrated in Fig. 8. Without optimization, the de-
vice gave power conversion efficiency (PCE) of 0.01% with
an open-circuit voltage (Voc) of 0.27 V and a short-circuit
Published by NRC Research Press

Qian and Wang
199
Scheme 2. Synthetic routes to polymers P1 and P2. Reagents and conditions: (i) Bu4NBr3, CH2Cl2, 25 8C, 3 h; (ii) Pd(PPh3)4, K2CO3 (aq.,
2 mol/L), toluene, Aliquat 336, 95 8C, 24 h; (iii) PhNSO, TMSCl, pyridine, 80 8C, overnight.
1
Fig. 9. H NMR spectra (300 MHz, CDCl3) of polymers P1 and
P2.
Fig. 10. IR spectra of polymers P1 and P2.
In this work, we intend to explore a precursor approach to
introduction of the BBTD unit in polymers. The feasibility
of such a precursor approach is based on the fact that the
monomers containing the 1,2-diaminophenylene group can
be utilized in the Suzuki coupling reaction to produce high-
molecular-weight polymers,23 and the 1,2-diaminophenylene
group can be converted into various electron-deficient
groups (e.g., quinoxaline and benzothiadiazole).24 Therefore,
we demonstrated this precursor strategy by making the pre-
cursor polymer P1 and subsequently converting to the
BBTD-containing polymer P2 (Scheme 2). The monomer
12 was prepared by bromination of compound 11 with tetra-
butylammonium tribromide. Because of the presence of the
methyl group on one of the two phenyl groups, bromination
only takes place at one of the phenyl rings of the diphenyla-
mino moiety in 11. Using the Suzuki cross-coupling reac-
tion, polymerization of monomers 12 and 13 gave the
precursor polymer P1. By simple treatment of P1 with
N-sulfinylaniline and chlorotrimethylsilane in dry pyridine,
the target low-band-gap polymer P2 was readily obtained
with high yield (89%).
Structural characterization
The chemical structures of polymers P1 and P2 were fully
characterized by NMR and IR spectroscopic methods. Fig-
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200
Can. J. Chem. Vol. 88, 2010
Table 3. Characterization of Polymers P1 and P2.
abs
max
PL
max
Mna (Â103)
Mw (Â103)
PDI
3.2
2.7
l
(nm)b
Log 3
NA
4.39
l
(nm)b
Stokes shifts (nm)b
Band gap (eV)c
a
Polymer
P1
P2
56
28
179
75
371, 440 (sh)d
749
558
980
187
231
2.67
1.41
aDetermined by GPC.
bMeasured in chlorobenzene, excitation wavelengths for P1 and P2 are at their maximum absorption wavelengths.
cOptical band gap, estimated from the onset wavelength of optical absorption spectra.
dShoulder peak.
1
ure 9a shows the H NMR spectrum of polymer P1. A peak
at d 4.18 ppm is assigned to the amine hydrogen and the ali-
phatic and aromatic hydrogens are observed at d 0.6–2.5 and
d 7.1–7.7 ppm, respectively. After conversion of the diamine
into the thiadiazole group, the peak at d 4.18 disappeared
(Fig. 9b), indicating that the transformation reaction was
successful and the amino group was converted completely.
A new peak at d 8.20 is assigned to the hydrogens in the
phenylene that is adjacent to the BBTD core (Ha shown in
Scheme 2). Because of the strong electron-withdrawing
BBTD unit, the signal of Ha shifted to a lower field. The ob-
served chemical shift (d 8.20) coincides well with that of a
small compound with similar structure (d 8.18),12b further
demonstrating the formation of the BBTD unit in P2. The
IR spectrum of polymer P1 (Fig. 10) shows two distinct
n(NH) bands at 3442 and 3357 cm–1, which, as expected,
are absent in the spectrum of P2 (Fig. 10).
Fig. 11. Absorption and fluorescence emission of polymer P2 in
chlorobenzene.
The molecular weights were estimated by gel-permeation
chromatography (THF as eluent) relative to polystyrene
standards. Polymer P1 shows high molecular weight with
the number-average molecular weight (Mn) of 56 000 g/mol
and a polydispersity index of 3.2 (Table 3). The apparent Mn
of P2 was found to be only 28 000 g/mol, although it should
be the same or similar to that of P1, which was mainly due
to poor solubility of the high-molecular-weight fraction of
P2 in THF.
tive and beneficial in further lowering the band-gap level of
the D–p–A–p–D chromophores. Compared with the thio-
phene spacer (in compound 3), the pyrrole spacer provides
the intramolecular hydrogen bonding to the BBTD acceptor,
which pushes the absorption maximum and fluorescence
emission of chromophore 5 into the near-infrared spectral
region with a red shift of 172 and 158 nm, respectively. A
precursor approach to the introduction of the alkaline-labile
BBTD acceptor into the polymer backbone has been demon-
strated by successful synthesis of low-band-gap polymer P2.
The same strategy can be in principle applied to the synthe-
sis of a series of low-band-gap polymers with the structures
and properties similar to chromophores 1–5.
Optical property
The absorption and photoluminescence of polymers P1
and P2 were recorded in chlorobenzene. Without a strong
acceptor or strong intramolecular charge transfer, P1 absorbs
in the visible spectral region with a maximum (lmax) at
440 nm and emits at 558 nm. In comparison, owing to the
presence of the BBTD acceptor, the absorption and emission
maxima of P2 were red-shifted dramatically to 749 and
980 nm, respectively (Table 3 and Fig. 11). Similar to chro-
mophores 1–5, the absorption bands of polymer P2 mainly
have two parts. The peaks in the higher energy region are
attributed to the p–p* and also possible n–p* transitions of
the conjugated aromatic segments. The high molar extinc-
tion coefficient (Log 3 = 4.39) of the ICT band of P2 is
ideal for photovoltaic application. The optical band-gap lev-
els of P1 and P2 were estimated from the onset wavelength
(lonset) of the optical absorption spectra to be 2.7 and
1.4 eV, respectively.
Acknowledgement
This work was supported by the Natural Sciences and En-
gineering Research Council of Canada (NSERC).
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