Synthesis and structure-property relationship of carbazole-alt-benzothiadiazole copolymers

Article abstract of DOI:10.1002/pola.27660

A series of four π-conjugated carbazole-alt-benzothiadiazole copolymers (PCBT) were prepared by Suzuki cross-coupling reaction between synthesized dibromocarbazoles as electron-rich subunits and 4,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,1,3-benzothiadiazole as electron-deficient subunits. The subunits were directly linked through 2,7- or 3,6- positions of the carbazole. In addition, the carbazole monomers have been N-substituted by a branched or a linear side-chain. The chemical structure of the copolymers and their precursors was confirmed by NMR and IR spectroscopies, and their molar masses were estimated by SEC. Thermal analysis under N₂ atmosphere showed no weight loss below 329°C, and no glass transition was observed in between 0 and 250°C. The band gaps of all PCBTs evaluated by optical spectroscopies and by cyclic voltammetry analysis were consistent with expectations and ranged between 2.2 and 2.3 eV. Finally, 2,7 and 3,6 linkages were shown to influence optical properties of PCBTs.

Full text of DOI:10.1002/pola.27660

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Synthesis and Structure–Property Relationship of Carbazole-
alt-Benzothiadiazole Copolymers
1
,2,3
1,2,3
1,2,3
1,2,3
1,2,3
Jules Oriou,
Feifei Ng,
Georges Hadziioannou,
Cyril Brochon,
Eric Cloutet
1
CNRS, Laboratoire de Chimie des Polym e` res Organiques, UMR 5629, 16 avenue Pey Berland, Pessac Cedex, F-33607, France
2
Universit eꢀ de Bordeaux, Laboratoire de Chimie des Polym e` res Organiques, 16 avenue Pey Berland, Pessac Cedex, F-33607,
France
3
IPB, Laboratoire de Chimie des Polym e` res Organiques, UMR 5629, 16 avenue Pey Berland, Pessac Cedex, F-33607, France
Correspondence to: C. Brochon (E-mail: cbrochon@enscbp.fr) or E. Cloutet (E-mail: cloutet@enscbp.fr)
Received 6 February 2015; accepted 26 March 2015; published online 00 Month 2015
DOI: 10.1002/pola.27660
ꢀ
and no glass transition was observed in between 0 and 250 C.
ABSTRACT: A series of four p-conjugated carbazole-alt-benzo-
thiadiazole copolymers (PCBT) were prepared by Suzuki cross-
coupling reaction between synthesized dibromocarbazoles as
electron-rich subunits and 4,7-bis(4,4,5,5-tetramethyl-1,3,2-diox-
aborolan-2-yl)22,1,3-benzothiadiazole as electron-deficient sub-
units. The subunits were directly linked through 2,7- or 3,6-
positions of the carbazole. In addition, the carbazole mono-
mers have been N-substituted by a branched or a linear side-
chain. The chemical structure of the copolymers and their pre-
cursors was confirmed by NMR and IR spectroscopies, and
The band gaps of all PCBTs evaluated by optical spectroscop-
ies and by cyclic voltammetry analysis were consistent with
expectations and ranged between 2.2 and 2.3 eV. Finally, 2,7
and 3,6 linkages were shown to influence optical properties of
PCBTs. V
2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A:
C
Polym. Chem. 2015, 00, 000–000
KEYWORDS: Carbazole; conjugated polymers; benzothiadiazole;
p-conjugated alternated copolymers; step-growth polymeriza-
tion; structure-property relation; Suzuki polycondensation
their molar masses were estimated by SEC. Thermal analysis
ꢀ
under N
2
atmosphere showed no weight loss below 329 C,
2
3–27
INTRODUCTION Over the last decades, organic semiconduc-
tors have been widely considered in the composition of
numerous electrical and optical devices, as an alternative to
phene)s
that have been widely studied over the years,
2
8
29
polyfluorenes and polycarbazoles have attracted specific
attention for application in Organic Light Emitting Diodes
OLEDs) or in Organic Photovoltaic (OPV). Nevertheless, car-
1
–3
(
their inorganic analogous.
These materials have been used
4
bazole derivatives are somehow preferred as they show bet-
ter stability (both chemically and environmentally), the 9-
position of carbazole being less sensitive than in the case of
as active layers in molecular electronics, nonlinear optical
5
,6
7,8
9–11
devices,
transistors,
sensors,
light-emitting diodes,
field-effect
1
2–14
15
16–19
memories,
and organic solar cells.
3
0,31
fluorene,
and still allowing the grafting of a solubilizing
Their organic nature offers a great potential for low cost, light-
weight, easy processability, and large-area flexible devices.
3
2
group. Homopolymers of 2,7- and 3,6-carbazoles have been
3
3,34
widely investigated as blue light emitters in OLED
and
The synthetic progresses over decades have allowed a great
versatility of methods and tailoring of p-conjugated polymers
with specific properties. For instance, alternating electron-
rich and electron-deficient moieties in polymeric systems
as semiconducting layer in organic field-effect transistors
3
5
(OFET). Due to the electron-donating character of the car-
bazole unit, its coupling with other aromatic electron-
deficient moieties allowed an enhanced conjugation in push-
3
6
(
pull-push copolymer) have shown promising properties
pull type copolymers. In this context, electron-deficient
3
7–40
41
especially for photovoltaic applications, since they favour the
formation of an intramolecular charge transfer (ICT), which,
moieties such as benzothiadiazole,
quinoxaline,
benzoxadiazole,
4
2,43
were considered as comonomers and intro-
duced at various ratios in combination with 2,7- and 3,6-
linked carbazole subunits. For instance, carbazole-alt-benzo-
thiadiazole copolymers have attracted particular attention
since Witker and Reynolds demonstrated that this combina-
among other parameters, allows
band-gap.
a narrowing of the
2
0
Among all p-conjugated polymers, such as poly(1,4-phenyl-
2
1
22
37
ene)s,
poly(phenylene vinylene)s,
and poly(thio-
tion allows a significant decrease of the band gap. In the
Additional Supporting Information may be found in the online version of this article.
2015 Wiley Periodicals, Inc.
V
C
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literature, the band gaps of carbazole-alt-benzothiadiazole
1314, 1239, 1223, 1129, 1052, 997, 911, 832, 818, 792,
721, 593 cm . HRMS (EI1, m/z) [M]1 calculated for
2
1
copolymers using thiophene units as spacers are weakly
3
9,44
dependent of the molar masses
and the linkage
C H Br N: 491.0823, found 491.0815.
2
4
31
2
4
5
position (2,7- or 3,6-) on the carbazole.
This can be
N-Dodecyl-3,6-Dibromocarbazole (3).
White crystals (5.503 g, 55%). m.p. 68 C. H NMR (400
explained by an equivalent effective intramolecular charge
transfer. The influence of the linkage position had been stud-
ꢀ
1
4
6
MHz, CDCl ): d (ppm) 8.15 (d, J 5 1.9 Hz, 2H), 7.57 (dd,
ied by Park et al. on the latter materials. The main differ-
ence was that (3,6-carbazole) derivatives exhibited higher
hole mobility.
3
J 5 8.7 Hz, 2.0 Hz, 2H), 7.32 (d, J 5 8.7 Hz, 2H), 4.25 (t,
J 5 7.2 Hz, 2H), 1.83 (m, 2H), 1.28 (m, 18H), 0.88 (t, J 5 6.9
1
3
Hz, 3H).
C NMR (101 MHz, CDCl ): d (ppm) 139.46,
3
In this work, we investigated the alternation between carba-
zole and benzothiadiazole in the absence of any spacer in
order to observe the direct influence of each subunit onto
optoelectronic properties taking into account 2,7- and 3,6-
linked carbazole moieties. We thus designed four copolymers
which differ by their linkage position (2,7- and 3,6-) and the
nature of their substituents. The properties of each polymer,
such as thermal, optical, and electrochemical will be
discussed.
129.14, 123.59, 123.40, 112.07, 110.53, 43.50, 32.05, 29.72,
29.66, 29.59, 29.47, 28.98, 22.83, 14.27. FTIR (ATR):
m 5 2919, 2847, 1469, 1434, 1288, 1224, 1148, 1057,
21
1017, 870, 831, 802, 785, 724, 648, 564 cm . HRMS
(EI1, m/z) [M]1 calculated for C H Br N: 491.0823,
2
4
31
2
found 491.0810.
General Procedure for N-9’-
Heptadecanyldibromocarbazole Derivatives Synthesis
Dibromocarbazole derivative (1 eq) and powdered KOH
EXPERIMENTAL
(
excess) were stirred in DMSO under argon, and then
Materials
solution of 9-heptadecanyl-p-toluenesulfonate (1.5 eq) dis-
solved in DMSO was added dropwise. The reaction
medium was stirred at room temperature for 20 h. The
mixture was extracted with cyclohexane and water and
Ethyl formate and 4-4’-Dibromobiphenyl were purchased
from Alfa Aesar and used without further purification. Tetra-
hydrofuran (THF), methylene chloride (CH Cl ), toluene and
2
2
dimethylsulfoxide (DMSO) were purified from a solvent puri-
fication system (MBraun MB-SPS-800) prior to use. Dime-
thylformamide (DMF) was purchased from Scharlau and
used as received. Poly(3,4-ethylenedioxy-thiophene):poly(s-
tyrene sulfonate) (PEDOT:PSS) was purchased from Baytron
P and passed successively through a 0.8 mm and a 0.45 mm
PVDF syringe filter before spin-coating. [6,6]-Phenyl-C61-
butyric acid methyl ester (PCBM) was obtained from Solaris.
All other reagents were purchased from Alfa Aesar, Sigma-
Aldrich, Acros Organics or Strem Chemicals and used as
received.
dried over MgSO . After removing the solvent, the crude
product was purified by flash chromatography (cyclohex-
ane) to give:
4
N-9-Heptadecanyl-2,7-Dibromocarbazole (2).
ꢀ
1
White crystals (0.519 g, 58%). m.p. 73 C. H NMR (400 MHz,
CDCl ): d (ppm) 7.90 (t, J 5 9.4 Hz, 2H), 7.69 (s, 1H), 7.54 (s,
3
1
2
2
H), 7.33 (d, J 5 3.7 Hz, 2H), 4.42 (tt, J 5 10.2, 5.1 Hz, 1H),
.26 – 2.13 (m, 2H), 1.96 – 1.84 (m, 2H), 1.31 – 1.07 (m,
2H), 1.04 – 0.91 (m, 2H), 0.83 (t, J 5 7.0 Hz, 6H). Multiple
1
and broad H peaks are due to the phenomenon of atropiso-
merism.
4
5,47 13
C NMR (101 MHz, CDCl ): d (ppm) 143.06,
3
Synthesis of Precursors and Polymers
General Procedure for N-Dodecyldibromocarbazole
Derivatives Synthesis
139.59, 122.47, 121.62, 121.36, 120.98, 119.91, 114.68,
112.31, 57.10, 33.62, 31.88, 29.42, 29.40, 29.25, 26.87,
1
3
22.74, 14.20. Multiple C peaks are due to the phenomenon
4
5,47
Dibromocarbazole derivative (1 eq), powdered
K
2
CO
3
of atropisomerism.
FTIR (ATR): m 5 2918, 2850, 1585,
ꢀ
(
excess) and DMF were stirred at 80 C under argon, and
1450, 1421, 1331, 1235, 1218, 1131, 1058, 999, 922, 790,
21
then 1-bromododecane (1.5 eq) was added dropwise. The
721, 669, 607 cm . HRMS (EI1, m/z) [M]1 calculated for
ꢀ
reaction medium was heated at 80 C for 16 h. The mixture
C H Br N: 561.1606, found 561.1601.
2
9
41
2
was extracted with diethyl ether and water and dried over
N-9-Heptadecanyl-3,6-Dibromocarbazole (4).
MgSO . After removing the solvent, the crude product was
4
ꢀ
1
White crystals (4.0 g, 77%). m.p. 55 C. H NMR (400 MHz,
CDCl ): d (ppm) 8.15 (d, J 5 8.6 Hz, 2H), 7.57 – 7.27 (m, 4H),
.47 (tt, J 5 10.1, 4.9 Hz, 1H), 2.27 – 2.15 (m, 2H), 1.94 –
purified by flash chromatography (cyclohexane/ethyl acetate
3
9/1) to give:
4
N-Dodecyl-2,7-Dibromocarbazole (1).
White crystals (0.352 g, 47%). m.p. 70 C. H NMR (400
1.82 (m, 2H), 1.38 – 1.03 (m, 22H), 0.99 – 0.86 (m, 2H), 0.83
ꢀ
1
1
(t, J 5 7.1 Hz, 6H). Multiple and broad H peaks are due to
4
5,47 13
MHz, CDCl ): d (ppm) 7.88 (d, J 5 8.3 Hz, 2H), 7.52 (d,
the phenomenon of atropisomerism.
C NMR (101 MHz,
3
J 5 1.5 Hz, 2H), 7.34 (dd, J 5 8.3, 1.7 Hz, 2H), 4.18 (t,
J 5 7.4 Hz, 2H), 1.89 – 1.77 (m, 2H), 1.42 – 1.17 (m, 18H),
CDCl ) d (ppm) 141.07, 137.53, 130.06, 129.14, 128.71,
3
124.65, 123.48, 123.14, 113.18, 112.05, 111.76, 110.66,
56.97, 33.77, 31.86, 29.41, 29.39, 29.23, 26.80, 22.72, 14.19.
1
3
0.88 (t, J 5 6.9 Hz, 3H). C NMR (101 MHz, CDCl
3
): d
1
3
(ppm) 141.51, 122.66, 121.61, 121.42, 119.83, 112.16,
Multiple C peaks are due to the phenomenon of atropiso-
4
5,47
43.50, 32.05, 29.74, 29.70, 29.63, 29.47, 28.91, 27.31,
merism.
FTIR (ATR): m 5 2921, 2849, 1469, 1438, 1284,
14.27. FTIR (ATR): m 5 2917, 2847, 1586, 1447, 1423,
1220, 1061, 1016, 863, 824, 801, 794, 723, 635, 552,
2
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2
1
5
C
38 cm
.
HRMS (EI1, m/z) [M]1 calculated for
Poly(N-9-Heptadecanyl-3,6-Carbazole-alt-
Benzothiadiazole) PCBT4.
Orange powder (234 mg, 87%). H NMR (400 MHz, CDCl ):
29 2
H41Br N: 561.1606, found 561.1599.
1
3
General Procedure for Suzuki Cross-Coupling
Polymerization
Dibromocarbazole derivative (0.5 mmol), 2,1,3-benzothiadia-
zole-4,7-bis(boronic acid pinacol ester) (194 mg, 0.5 mmol)
and tetrakis(triphenylphosphine)palladium(0) (15 mg, 0.013
mmol) were dissolved in 10 mL of a degassed THF/Toluene
d (ppm) 8.97 – 7.37 (m, 8H), 4.68 (br s, 1H), 2.39 (br s, 2H),
2
6
1
3
.11 – 1.94 (m, 2H), 1.48 – 0.99 (m, 24H), 0.95 – 0.68 (m,
13
H). C NMR (101 MHz, CDCl ): d (ppm) 155.10, 142.77,
3
33.57, 130.04, 129.47, 128.80, 128.25, 127.61, 57.13, 34.06,
1.91, 29.62, 29.50, 29.35, 27.09, 25.74, 22.73, 14.20. FTIR
(
ATR): m 5 2921, 2851, 1626, 1600, 1473, 1403, 1341, 1283,
(
1/1) mixture. Then, 10 mL of a degassed 2M K CO3 aque-
2
1224, 1147, 1099, 1027, 885, 848, 799, 754, 721, 694, 670,
ous solution was added. The reaction medium was heated at
reflux under argon for 24 h. Afterwards, bromobenzene (6
mL, 0.05 mmol) in 1 mL of a degassed THF/toluene (1/1)
mixture was added. The reaction was allowed to stir at
reflux for 6 h, and benzeneboronic acid (12 mg, 0.1 mmol)
in 1 mL of a degassed THF/toluene mixture (1:1 vol) was
added, followed by stirring under reflux for 16 h. The reac-
tion mixture was then poured into 200 mL of methanol. The
orange precipitate was collected by filtration, washed with
water and methanol, followed by purification on Soxhlet
apparatus (methanol, pentane, and THF). No insoluble frac-
tion remained. The THF fraction was precipitated in metha-
nol. The resulting solid was recovered by filtration and dried
21
6
05 cm , Mn 5 2500 g/mol ( D- 5 1.7).
Characterizations
The precursors and polymers were characterized by H, C,
1
13
1
13
and H- C HSQC NMR in order to determine the chemical
structures of compounds. NMR spectra were recorded on a
Bruker 400 MHz spectrometer from deuterated chloroform
(
CDCl ) solutions. IR spectra were recorded with Bruker Ten-
3
sor 27 spectrometer using a 0.6 mm-diameter beam and a
21
4
cm resolution. Samples were analyzed with the attenu-
ated total reflexion (ATR) method. A Kofler bench (Wagner
and Munz Heizbank system Kofler type wme) was used to
ꢀ
obtain melting points with a 62 C precision. High resolution
ꢀ
under vacuum at 40 C overnight.
mass spectroscopy analyses were performed on an
AutoSpec-Waters spectrometer (EI).
Poly(N-Dodecyl-2,7-Carbazole-alt-Benzothiadiazole)
PCBT1.
Molar masses were estimated by size exclusion chromatogra-
phy with a refractive index detector (PL-GPC 50 plus Inte-
grated GPC system from Polymer laboratories-Varian) from
the soluble part of copolymer in THF which was eluted by
THF at 40 C. Standard polystyrene samples were used for
calibration.
1
Orange powder (75 mg, 32%). H NMR (400 MHz, CDCl ): d
3
(ppm) 8.48 – 7.35 (m, 8H), 4.49 (br, 2H), 2.13 – 1.78 (br,
2H), 1.73 – 0.95 (br, 18H), 0.92 – 0.62 (br, 3H). Due to a too
1
3
small quantity of material available, a satisfying C NMR
spectrum could not be obtained. FTIR (ATR): m 5 2920,
ꢀ
2851, 1454, 1434, 1335, 1258, 1094, 1020, 874, 843, 800,
21
7
54, 724, 695 cm , Mn 5 1800 g/mol ( D- 5 1.3).
Absorption and photoluminescence spectra were recorded
for a polymer solution (2.5 g/L in o-dichlorobenzene) and a
polymer film casted from an o-dichlorobenzene solution
Poly(N-9-Heptadecanyl-2,7-Carbazole-alt-
Benzothiadiazole) PCBT2.
Orange powder (145 mg, 50%). H NMR (400 MHz, CDCl ):
d (ppm) 8.48 – 7.38 (m, 8H), 4.81 (br s, 1H), 2.45 (br, 2H),
(
10 g/L) using the spectrophotometers UV-3600, Shimadzu
1
3
and Fluoromax-4, Horiba Scientific, respectively.
2
.12 (br, 2H), 1.47 – 0.95 (m, 24H), 0.81 (t, J 5 5.6 Hz, 6H).
Energetic levels were evaluated by cyclic voltammetry in a
glovebox using a potentiostat in the presence of electrolyte
solutions of 0.1M TBAPF6 in dry CH Cl . Platinum was used
for both working and counter electrodes and a silver wire
was used as reference electrode. The obtained potentials
were recalibrated outside the glovebox using ferrocene and
saturated calomel electrode as reference.
1
3
C NMR (101 MHz, CDCl ): d (ppm) 154.76, 143.19, 139.87,
3
135.52, 134.94, 134.38, 132.43, 132.33, 129.43, 128.81,
128.54, 120.85, 120.61, 120.46, 113.44, 110.63, 34.11, 31.95,
29.65, 29.55, 29.41, 27.09, 22.75, 14.20. FTIR (ATR):
2
2
m 5 2922, 2851, 1594, 1557, 1455, 1434, 1334, 1220, 1112,
1
Mn 5 8100 g/mol ( D- 5 2.2).
21
059, 999, 881, 842, 800, 754, 724, 657, 611, 557 cm
,
RESULTS AND DISCUSSION
Poly(N-Dodecyl-3,6-Carbazole-alt-Benzothiadiazole)
PCBT3.
Synthesis of Monomers and Polymers
1
Orange powder (173 mg, 74%). H NMR (400 MHz, CDCl ):
d (ppm) 8.92 – 7.29 (m, 8H), 4.50 – 3.72 (m, 2H), 2.07 –
3
Four carbazole monomers have been synthesized bearing
bromo functions at different positions for further chain cate-
nation (i.e., at 2,7 and 3,6) and N-substituted by two differ-
ent alkyl chains (i.e., n-dodecyl and 9-heptadecanyl). The
general synthesis procedures are shown in Scheme 1.
1
3
1.57 (m, 2H), 1.55 – 0.99 (m, 2H), 0.87 (br, 3H). C NMR
(101 MHz, CDCl ): d (ppm) 154.95, 141.31, 133.67, 128.38,
3
1
3
27.66, 127.46, 123.76, 123.36, 121.69, 121.39, 109.20,
2.05, 29.76, 29.65, 29.49, 29.24, 27.49, 22.82, 14.26. FTIR
(
1
ATR): m 5 2919, 2849, 1627, 1600, 1473, 1384, 1343, 1278,
To improve the solubility of the final polymers, side chains
can be incorporated on the nitrogen atom of the carbazole
unit. Linear (n-dodecyl) and branched (9-heptadecanyl) alkyl
21
231, 1143, 1025, 886, 846, 798, 720, 694, 646, 561 cm
,
Mn 5 4800 g/mol ( D- 5 1.5).
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SCHEME 1 Synthesis of carbazole monomers and subsequent PCBTs by Suzuki cross-coupling.
groups were chosen as substituents on the nitrogen atom to
study their influence on the materials properties. Precursors
were prepared from well-established synthetic proce-
distinguished, corresponding to the branched 9-heptadecanyl
chain moiety [Fig. 1(b)]. We observed similar features when
comparing the linear and branched 3,6-dibromocarbazoles
derivatives 3 and 4 [Fig. 1(c,d)]. The multiple and broad
peaks displayed in the case of carbazole derivative bearing
the 9-heptadecanyl chain (2 and 4) were due to the phenom-
enon of atropisomerism, as already reported for similar
4
5,48–50
dure
(details available in Supporting Information).
The N-alkylation reaction was carried out under nucleophilic
substitution conditions in the presence of strong base
4
5,47
(
K CO or KOH) in polar solvent such as DMSO or DMF. The
2 3
compounds.
final dibromocarbazoles were obtained in good yields (47–
1
All four dibromocarbazoles (1–4) were reacted separately
with 4,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
7
1
7 %). Their chemical structures were confirmed by H (Fig.
1
3
1
13
), C and H- C HSQC NMR, as well as FTIR and HRMS
yl)22,1,3-benzothiadiazole under Suzuki cross-coupling reac-
tion conditions, leading to four different alternated p-
conjugated polymers (Scheme 1). The polymerizations were
carried out using tetrakis(triphenylphosphine)palladium(0)
as catalyst and K CO as base in a THF/Toluene (1/1) mix-
(
see Experimental Section and Supporting Information, Figs.
S2–S9, for details).
The achievement of alkylated dibromocarbazoles was
assessed with H NMR through the disappearance of the sig-
1
2
3
nal at 8.04 ppm, assigned to the proton of the amine group
ture for 48 h at reflux. An end-capping reaction was per-
formed using benzeneboronic acid and bromobenzene, in
order to ensure a better stability of the resulting materials
of the carbazole and the appearance of signals at higher
1
magnetic fields corresponding to the side chains. The
H
5
1
NMR spectrum of 1 [Fig 1(a)] showed one triplet at 4.18
ppm, two multiplets in the 1.89–1.77 and 1.42–1.17 ppm
regions and one triplet at 0.88 ppm, assigned to the linear
dodecyl chain. For the branched derivative 2 one triplet of
triplet at 4.42 ppm, four multiplets ranging between 2.26
and 0.91 ppm, and one triplet at 0.83 ppm could be
by replacing reactive chain-ends with phenyl groups. The
polymers (PCBT1–4) were obtained as strong orange col-
ored powders soluble in common organic solvents such as
toluene, THF, dichloromethane, chloroform or ortho-dichloro-
benzene (ODCB). The different polymers were subjected to
Soxhlet extractions using selective solvents (i.e., methanol,
4
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1
3
FIGURE 1 H NMR (400 MHz) spectra of 2,7-dibromocarbazole (a) 1, (b) 2 and 3,6-dibromocarbazole (c) 3, (d) 4 in CDCl .
pentane and tetrahydrofuran) before characterizations. Their
chemical structures were confirmed by FTIR and NMR analy-
sis (see Supporting Information Figs. S10–S16) and their
molar masses as well as dispersities were determined by
SEC in THF (Table 1, traces in Supporting Information Figs.
S17–S20).
monomers along with more twisted reactants could be
incriminated. While some of the materials (PCBT1 and 4)
showed short chain lengths, such molar masses could be suf-
ficient to show interesting opto-electronic features, since it
was already reported that even oligomers with a suitable
4
4
ICT could lead to small band-gaps. Most of the measured
dispersities were below 2, which is the theoretically
The number-average molar masses of the polymers meas-
ured by SEC ranged from 1800 to 8100 g/mol. The differ-
ence between PCBT1 and 2 could be attributed to a better
solubility and thus reactivity of the formed chains bearing 9-
heptadecanyl groups, explaining the higher molar masses in
the case of PCBT2. However, this trend was not observed
expected for a step-growth polymerization mechanism.
Herein, dispersities values lower than 2 can be assigned to
the Soxhlet extraction procedures performed on the different
copolymers. Several sharp peaks could be observed on the
SEC traces of PCBT1 and 4. This is due to the relatively high
molecular weights of the monomeric units, which makes the
different chain lengths more noticeable in SEC for low
when comparing PCBT3 and
4 for which purity of
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TABLE 1 Molar Masses and Thermal Properties of PCBT Copolymers
Total Weight
21 a
)
21 a
)
a
ꢀ
b
c
Copolymer
Mn (g.mol
Mw (g.mol
Ð (Mw/Mn)
T
d,5% ( C)
Loss (%)
PCBT1
PCBT2
PCBT3
PCBT4
1,800
8,100
4,800
2,500
2,300
17,600
7,000
4,200
1.3
2.2
1.5
1.7
400
329
400
423
99
50
33
41
a
c
Determined by gel permeation chromatography (GPC) relative to poly-
Total weight loss after TGA, evaluated under N
2
at a heating rate of
ꢀ
ꢀ
styrene standards from the soluble part of PCBT in THF at 40 C.
b
10 C/min.
Decomposition temperature at 5% weight loss, evaluated under N
2
at
ꢀ
a heating rate of 10 C/min.
molecular weights sample (such as PCBT1 and 4). In order
to prepare materials of higher molar masses we need to fur-
ther investigate other type of catalytic systems.
weight of alkyl chain (calculated values of 36 and 45 %w for
dodecyl and for 9-heptadecanyl chains, respectively). This
indicated that the first degradation mechanism under inert
atmosphere was related to the elimination of side-chains
from the backbone and this is consistent with previous
1
The H NMR spectra of the synthesized PCBTs showed a
general broadening compared to those of the monomers, as
depicted for instance in the spectrum of PCBT1 (Fig. 2). The
peaks corresponding to the alkyl protons in close vicinity to
the nitrogen atom of the carbazole units (b) can be clearly
identified in the 4–5 ppm region and were thus used as ref-
erence for signals integration. No residual peak correspond-
ing to the boronic pinacol ester functions (sharp singlet at
46
report from the literature. The highest weight loss was
observed for PCBT1 (99 %w) and could be due to its lower
molar mass. Second, differential scanning calorimetry analy-
ꢀ
ses between 0 and 250 C did not show any clear glass tran-
sition nor crystallinity behavior, indicating the amorphous
nature of these materials (see Supporting Information Fig.
S22).
1.44 ppm) could be observed, which could be assigned to a
successful end-capping reaction or the occurrence of proto-
deborylation in the process of Suzuki coupling reaction. Fur-
1
3
1
13
thermore, the C and H- C HSQC NMR spectra were con-
sistent with the expected structures (see Supporting
Information Figs. S11, S13, and S15).
Thermal Characterizations
Thermal properties of the polymers were first determined
by thermogravimetric analysis under N atmosphere at a
2
2
1
heating rate of 10 C min (see Supporting Information Fig.
S21). All PCBTs showed a good thermal stability, with an
ꢀ
onset temperature for decomposition (T5%) above 329 C
(
Table 1). It has been noticed that for PCBT2 – 4, the weight
loss at T5% is proportional within experimental errors to the
FIGURE 3 Absorption spectra of PCBT copolymers a) in ODCB
solution (0.12 g/L) and b) in spin-coated film from ODCB
solution.
1
FIGURE 2 H NMR spectra (400 MHz) of PCBT1 in CDCl
3
.
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observed. They showed red-shifted absorbance in solid state,
which reveals an intermolecular arrangement between poly-
mer main chains (p-stacking). However, the optical band gap
determined by the onset of the high-wavelength absorption
band achieved up to 2.2–2.3 eV and is similar for all
polymers.
Photoluminescence spectra of neat polymers were recorded
in ODCB solution and in films from the same samples as for
the light absorption analyses (Fig. 4). The kexcitation of each
sample was selected according to the previously observed
kmax. In solution, the four polymers showed two-peaks emis-
sion, which indicates that the light emission comes from two
different sources. On the one hand, the lower-wavelength
emission peak is probably due to the localization of the
excited state on one unit (carbazole or benzodiathiazole). On
the other hand, the higher-wavelength emission peak is
probably due to an ICT excited state, resulting in a structure
having a smaller band-gap (e.g., quinoid structure). It can be
noticed that the lower-wavelength emission peak of 3,6-
PCBTs is more red-shifted compared to the 2,7-PCBTs. This
can be explained by the more stabilized excited state of the
3,6-configuration due to the participation of the nitrogen
atom (positive charges are more stabilized when localized on
nitrogen).
In the solid state, the two emission peaks shifted towards
each other, and even merged in the case of polymers bearing
linear alkyl chains (PCBT1 and PCBT3), due to stronger
intermolecular interactions of polymer chains.
FIGURE 4 Photoluminescence spectra of PCBTs (a) in ODCB
solution and (b) in spin-coated films, excited at respective kmax
measured in absorption spectra (461–481 nm).
Optical Characterizations
Electrochemical Characterizations
Absorption spectra of polymers PCBT1 – 4 in ODCB solution
and in films are shown in Figure 3. As frequently observed
for alternated copolymers, all prepared PCBTs showed two
broad absorption bands in the range of 300–350 nm and
Electrochemical properties were estimated by cyclic voltam-
metry under inert atmosphere (N ) using dichloromethane
2
solutions of PCBT1 – 4, tetrabutylammonium hexafluoro-
phosphate (TBAPF ) as electrolyte, silver for the reference
6
5
2
440–500 nm in both solution and film states. On the one
electrode and the working electrode and counter-electrode
hand, the high-wavelength band corresponds to the ICT from
the electron-rich subunit (carbazole) to the electron-deficient
one (benzothiadiazole) and its maximum intensity is similar
were made of platinum (see Supporting Information Figs.
S23–S26). The calculated values of the energy levels are
summarized in Table 2. We noticed slight differences
between the electrochemical and optical band-gaps, which
can be explained by the fact that the redox peaks result from
3
2,53,54
for the four polymers.
On the other hand, the low-
wavelength differs depending on the linkage, that is,. 2,7-
PCBT versus 3,6-PCBT. Indeed, PCBT3 – 4 showed a first
absorbance around 305 nm, whereas it was observed around
5
6
localized sites rather than from the conjugated backbone.
The LUMO energy level is similar for all PCBTs independ-
3
30 nm for PCBT1 – 2. This indicates that the low-
ently from the linkage and the side chain nature, and slightly
higher than that of the P3HT. Except for the PCBT3, the
5
7
wavelength can be attributed to the absorption of the carba-
4
6
zole moiety. The 3,6-dibromocarbazoles showed red-shifted
bands compared to those of the 2,7-dibromocarbazoles,
whereas in the case of polymers, the 2,7-PCBTs bands are
red-shifted, especially in film state [Fig. 3(b)]. This might be
explained by a better conjugation along the 2,7-PCBTs
chain. Indeed, the 3,6-PCBTs induce conjugation breaks on
the nitrogen atom, which limits the conjugation length and
thus lower the absorption wavelengths.
HOMO energy level is lower than that of the P3HT, which is
5
8
below the air oxidation threshold and made PCBTs good
candidates for bulk heterojunction (BHJ) solar cells applica-
tions by having a positive impact on open-circuit voltage
(VOC).
5
5
Optical Characterizations of PCBT: PCBM Blends
Due to their low-lying HOMO levels, the prepared PCBTs
might be considered as donor material in the active layer of
BHJ solar cells. A suitable morphology of the active layer,
which avoids loss of energy through charge transport and
maximizes the efficiency of the device, is the key to achieve
high performances of a BHJ solar cell.
4
6
The side chains have a weak influence in the PCBT absorb-
ance in both solution and film states. All polymers showed
good solubility in common solvents (in part due to quite low
molar masses polymers) and no solvatochromic effect was
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TABLE 2 Optical and Electrochemical Properties of PCBT Copolymers
sol
max
a
film
max
b
opt
c
d
d
elec
e
Copolymer
k
(nm)
k
(nm)
E
g
(eV)
HOMO (eV)
LUMO (eV)
E
g
(eV)
PCBT1
PCBT2
PCBT3
PCBT4
456
465
462
447
481
481
477
461
2.2
2.3
2.2
2.3
25.5
25.3
25.0
25.3
23.0
23.0
22.8
23.0
2.2
2.3
2.2
2.3
a
d
Determined at high wavelength in o-DCB solution
Determined at high wavelength in film casted from
Estimated from the oxidation and reduction potentials measured
solution with the following equation:
b
a
o-DCB
by cyclic voltammetry in CHCl
3
solution.
EHOMOðeVÞ52ðE
1
ðVÞ14:8Þ; ELUMOðeVÞ52ðE
1
ðVÞ14:8Þ.
ox vs ðFe=Fe
Þ
red vs ðFe=Fe Þ
c
e
Estimated from the onset of the absorption edge in a film.
Calculated by difference between oxidation and reduction potentials.
Thus, preliminary studies were carried out on the blend of
other PCBTs. Photoluminescence spectra of the same sam-
ples were taken at 481 nm (kmax) and are shown in Figure
5(b). The presence of PCBM induced a drastic decrease of
the polymer fluorescence whatever the amount, which
revealed a possible dissociation of photogenerated excitons
in the semiconducting polymers by electron transfer to
PCBM. The same behavior was observed in the case of all
other PCBTs, whatever the linkage position and the substitu-
ent nature. Although this is an encouraging result for BHJ
solar cells application, this effect could also originate from
an energy transfer between the polymer and PCBM.
PCBT1 – 4 as donor and with phenyl-C -butyric acid methyl
6
1
ester (PCBM) as acceptor in order to evidence interactions
between these two components. Both materials were solubi-
lised at different ratios in ODCB, and subsequently spin-
casted onto a glass substrate coated with a PEDOT-PSS layer,
simulating conditions to make BHJ devices.
The absorption spectra of PCBT2 blended with different
ratio of PCBM in solid state are shown in Figure 5(a). At
constant amount of PCBT2, the increasing amount of PCBM
led to a slight blue shift of absorption bands, which could be
attributed to a decrease of the intermolecular interactions
between polymer chains. Additionally, the light absorption of
PCBM induces an intensity increase of the high energy
absorption peak. Similar behavior was observed for the three
In addition, the same blends were investigated in atomic
force microscopy (AFM), which highlighted the presence of
nanoscale domains (see Supporting Information Fig. S27).
These preliminary studies on PCBT/PCBM interactions are
promising as for the use of prepared PCBT polymers as
donor material in the active layer of BHJ solar cells.
CONCLUSIONS
Four alternated p-conjugated copolymers (named PCBT)
based on carbazole and benzothiadiazole subunits were suc-
cessfully synthesized by Suzuki cross-coupling polycondensa-
tion. The carbazole and benzothiadiazole subunits were
directly linked through 2,7- or 3,6- positions of a N-substi-
tuted carbazole bearing either linear or branched side-chain.
Their structures were confirmed by NMR, FTIR and their
molar masses were estimated by SEC. In the case of 2,7 con-
figuration (PCBT1 and 2) a direct effect of the improved sol-
ubility with branched side-chain could be observed since
higher molar masses were achieved. This effect was not con-
firmed for the 3,6 configuration (PCBT3 and 4), probably
due to purification issues, catalytic system and/or twisted
catenation. All prepared PCBTs were shown amorphous and
ꢀ
found stable up to 329 C. Spectroscopic investigations high-
lighted the existence of a donor–acceptor ICT in the main
chain, thus the band-gap of the PCBTs is lower than that of
homopolymers based on carbazole. Although a red-shift
effect was observed as a function of the position of the link-
age (2,7-PCBT or 3,6-PCBT) for the low-wavelength absorp-
tion band, the optical band gap of the four PCBTs is similar
and consistent to the measured electrochemical one. This
indicates the predominant role of the ICT over the electrons
delocalization along the backbone. The side chains proved to
FIGURE 5 (above) Light absorption and (below) photolumines-
cence (excited at 481 nm) spectra of PCBT2:PCBM blends at
different ratios in solid state.
8
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have a significant effect on the photoluminescence properties
by enhancing intermolecular interactions in the case of linear
alkyl groups. Preliminary studies showed that PCBT:PCBM
blends formed small-scale domains and that PCBM quenched
the polymers fluorescence, which indicates the exciton disso-
ciation and is favourable for photovoltaic application. Further
characterizations, such as measurements of the charge car-
rier mobility, are in progress.
18 H. Hoppe, N. S. Sariciftci, J. Mater. Res. 2004, 19,
1
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ACKNOWLEDGMENTS
2
5 Chen, T.-A.; X. Wu, R. D. Rieke, J. Am. Chem. Soc. 1995,
The authors acknowledge financial support from the Centre
National de la Recherche Scientifique, University of Bordeaux,
Institut Polytechnique de Bordeaux and the Aquitaine region.
They also thank Ms A.-L. Wirotius for her aid with the NMR
analyses, Dr. L. Vignau for her assistance with the electrochemi-
cal measurements and Dr G. Pecastaing for the AFM character-
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1
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