Donor-acceptor-type polymers based on dithieno[2,3-b;7,6-b ]carbazole unit for photovoltaic applications

Article abstract of DOI:10.1021/ol300709u

Dithieno[2,3-b;7,6-b]carbazole (TCZT), a type of heteropentacene with nitrogen and sulfur atoms, was synthesized with a focus on the unique reactivity of carbazole via double intramolecular Friedel-Crafts acylation. A donor-acceptor-type polymer (PTCZTBT)

Full text of DOI:10.1021/ol300709u

ORGANIC
LETTERS
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Vol. XX, No. XX
000–000
Donorꢀacceptor-Type Polymers Based
on Dithieno[2,3-b;7,6-b]carbazole Unit for
Photovoltaic Applications
Atsushi Kimoto* and Yusuke Tajima
Center for Intellectual Property Strategies, RIKEN, 2-1 Hirosawa, Wako 351-0198, Japan
a_kimoto@riken.jp
Received March 20, 2012
ABSTRACT
Dithieno[2,3-b;7,6-b]carbazole (TCZT), a type of heteropentacene with nitrogen and sulfur atoms, was synthesized with a focus on the unique
reactivity of carbazole via double intramolecular FriedelꢀCrafts acylation. A donorꢀacceptor-type polymer (PTCZTBT) was also synthesized,
and its physicochemical properties are reported.
Solution-processed organic photovoltaic (OPV) devices
consist of a bulk heterojunction with a π-conjugated
polymer (electron donor) and a fullerene derivative
(electron acceptor). In the past decade, these devices have
attracted increasing research interest from the viewpoint
of their many applications. Many studies have attempted
to fabricate optimized photovoltaic devices based on poly-
(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric
acid methyl ester (PC61BM); however, these devices
showed a maximum photoconversion efficiency of only
∼5%.1 This is because P3HT shows an absorption band at
around 550 nm with a HOMO energy level of ꢀ4.89 eV. As
a result, the poor light absorbing property at longer
wavelengths prevents efficient exciton (excited state) for-
mation, which is the origin of the short circuit current (Jsc)
in photovoltaic devices. In order to improve the light
absorbing property, Janssen proposed a new class of
donorꢀacceptor-type low-bandgap conjugated polymers
that consist of an electron-sufficient backbone and an
electron-deficient one.2 Broad light absorption has been
achieved by exploiting the intramolecular charge transfer
(CT) transition between these two backbones. This design
strategy has driven research on the use of π-conjugated
polymers for producing photovoltaic devices having high
photoconversion efficiency.
Poly(2,7-carbazole-alt-dithienylbenzothiadiazole) (PCD-
TBT, Figure 1), which was first reported by Leclerc, is
considered one of the most promising donorꢀacceptor-type
low-bandgap polymers.3 In attempts to synthesize a new
(3) (a) Blouin, N.; Michaud, A.; Leclerc, M. Adv. Mater. 2007, 19,
2295. (b) Blouin, N.; Michaud, A.; Gendron, D.; Wakim, S.; Blair, E.;
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M.; Hsieh, C.-H.; Chang, C. Y.; Hsu, C. S. Chem. Commun. 2010, 46,
3259. (c) Chen, C. H.; Cheng, Y. J.; Dubosc, M.; Hsieh, C. H.; Chu,
C. C.; Hsu, C. S. Chem. Asian J. 2010, 5, 2483. (d) Zheng, Q.; Jung, B. J.;
Sun, J.; Katz, H. E. J. Am. Chem. Soc. 2010, 132, 5394. (e) Zhang, Y.;
Zou, J.; Yip, H. L.; Chen, K. S.; Zeigler, D. F.; Sun, Y.; Jen, A. K. Y.
Chem. Mater. 2011, 23, 2289. (f) Cheng, Y. J.; Wu, J. S.; Shih, P. I.;
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King, S.; Zhang, W.; Ashraf, R. S.; Heeney, M.; Anthopoulos, T. D.;
de Mello, J.; McCulloch, I. Macromolecules 2011, 44, 6649. (h)
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Kim, J. Y.; Yang, C. J. Mater. Chem. 2011, 21, 843.
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Mater. 2005, 15, 1617. (b) Dang, M. T.; Hirsch, L.; Wants, G. Adv.
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(2) (a) van Duren, J. K. J.; Dhanabalan, A.; van Hal, P. A.; Janssen,
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10.1021/ol300709u
XXXX American Chemical Society

Scheme 1. Synthesis of Dibromodithienocarbazole (5) via Intramolecular Acylation
annulation through the acid-mediated FriedelꢀCrafts reac-
tion. For example, Hsu reported the synthesis of copla-
narized PCDTBT by the sp3 carbon (poly(carbazole-
dicyclopentathiophene-alt-benzothiadiazole, PCDCTBT,
Figure S9, Supporting Information).4b,f In a recent study,
the presence of a quaternary carbon atom was found to
suppress the device stability against photoirradiation.6 As a
new polymer design, we designed a copolymer of dithieno-
[2,3-b;7,6-b]carbazole (TCZT), a type of heteropentacene,
and benzothiadiazole (PTCZTBT, Figure 1), a model
polymer of PCDTBT, by direct ring fusion using the sp3-
carbon-free aromatic backbone, which cannot be achieved
via the traditional strategy using FriedelꢀCrafts reaction.
Herein, we report the direct ring formation procedure of
TCZT via intramolecular acylation and the synthesis and
physicochemical properties of PTCZTBT.
The synthesis of the key building block is shown in
Scheme 1. Mohanakrishnan7 first reported the construc-
tion of the TCZT backbone via 1,5-sigmatropic rearrange-
ment followed by electrocyclization and subsequent
aromatization (Scheme S1, Supporting Information). An-
other synthetic approach, which serves as a fundamental
method to construct thiophene-fused aromatic backbones,
involves the ring-closure reaction of 3,6-diethynyl-2,7-
dibromocarbazole in the presence of sodium sulfide at very
high temperature (Scheme S2, Supporting Information).5d
We focused on the direct intramolecularring formation via
FriedelꢀCrafts acylation. An attempt to construct the
thiophene-fused aromatic system via an intramolecular
acylation approach was made to synthesize thieno[3,2-
b]thiophene, which was obtained from 3-(carboxymethyl-
sulfanyl)thiophene as a starting compound.8 This approach
is supported by the electronic feature of the thiophene ring,
Figure 1. Chemical structure of PCDTBT and PTCZTBT.
class of low-bandgap polymers, the incorporation of a
planar building block is potentially promising because (a)
the interchain interaction supported by the overlapping of
the π orbital between planar aromatic moieties would
facilitate the charge transport between the polymer chains
and (b) interchromophore interaction would allow wide
absorption as in the case of P3HT. Indeed, various
π-conjugated polymers containing ring-fused aromatic
compounds such as heteroacene have been synthesized for
developing organic electronic devices.4,5 However, few at-
tempts have been made to incorporate them into low-
bandgap polymers with the objective of developing OPV
devices.4 In these studies, the most popular approach for
constructing the fused aromatic system has been to bridge
two aromatic rings by the sp3 carbon using intramolecular
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A. K. Eur. J. Org. Chem. 2009, 531.
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M.; Sauvage, F. X.; Roncali, J.; Frere, P.; Skabara, P. J. J. Mater. Chem.
(5) (a) Youn, J.; Chen, M. C.; Liang, Y. J.; Huang, H.; Ortiz, R. P.;
Kim, C.; Stern, C.; Hu, T. S.; Chen, L. H.; Yan, J. Y.; Facchetti, A.;
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B
Org. Lett., Vol. XX, No. XX, XXXX

which can be easily functionalized by electrophilic aromatic
substitution at its 2,5-positions (ortho position from the
sulfur atom). We adopted this strategy to construct the
thiophene-fused aromatic system based on unique reactiv-
ity of the carbazole backbone and designed a synthetic
scheme that involves intramolecular FriedelꢀCrafts acyla-
tion. Compound 1 was prepared by palladium-catalyzed
carbonꢀsulfur bond formation9 of N-alkyl-2,7-dibromo-
carbazole3a using Xantphos (4,5-bis(diphenylphosphino)-
9,9-dimethylxanthene) as a ligand and N,N-diisopropyl-
ethylamine (DIPEA) as a base with 85% yield. After
hydrolysis, thioglycolic acid (2, 98% yield) was converted
into acid chloride by oxalyl chloride, followed by ring
closure to form dihydrothiophenone-fused carbazole 3
using double intramolecular FriedelꢀCrafts acylation. Be-
cause of the strong electron-rich property of the nitrogen
atom in the carbazole backbone, acylation is favored at the
3- and 6-position of the carbazole ring (para position from
the nitrogen atom) to result in regioselective cyclization.10
The reduction and dehydration of 3 without purification
allows the successful formation of N-90-heptadecanyl-
dithieno[2,3-b;7,6-b]carbazole 4 (52% yield, starting from
2). The characteristic signal around 3.8 ppm attributed to
the thiomethyl proton disappeared after the formation of
the fused thiophene ring (Figure 2). Compound 4 was
allowed to react with N-bromosuccinimide (NBS) to yield
5 as a colorless powder with 82% yield. The overall yield for
our method to obtain the sp3-carbon-free TCZT backbone
starting from N-alkyl-2,7-dibromocarbazole was 43%,
whereas that reported in the literature5d was 29%. The
peculiarity of our synthetic procedure is that the target
thiophene-fused aromatic system can be obtained under
mild reaction conditions in good overall yield.
Scheme 2. Synthesis of PTCZTBT (P1 and P2)
benzothiadiazole-4,7-bis(boronic acid pinacol ester) in
toluene (Scheme 2). The crude polymer was purified using
Soxhlet extraction to obtain a chloroform-soluble fraction
(P1) and a chlorobenzene-soluble one (P2) as black pow-
ders. Theyields of P1, P2, and the insolublepartwere35%,
11%, and 47%, respectively. The formation of a large
amount of insoluble fraction is due to the intrinsic low
solubility of 5 with a planar aromatic structure, despite the
introduction of branched long alkyl chains. The number-
(Mn) and weight-average (Mw) molecular weights of P1
and P2 as determined by gel permeation chromatography
(GPC) with polystyrene as the standard are summarized in
Table S1 (Supporting Information). P1 has Mw of 21 kDa,
with a polydispersity index (PDI) of 3.9; P2 has Mw of
28 kDa, with PDI of 4.2.
Figure 3. (a) UVꢀvis absorption spectra of P1 in chlorobenzene
and film prepared by spin-coating from chlorobenzene solution
(normalized at λmax in visible region. (b) Cyclic voltammogram
of the cast film of P1 measured in acetonitrile at scan rate of
100 mV/s with Bu4NPF6 (0.1 mol/L) as electrolyte. The film was
prepared by spin-coating chlorobenzene solution on the work-
ing electrode.
Figure 2. Magnified view of 1H NMR spectra of (a) 1, (b) 2, and
(c) 4 (400 MHz, 293 K CDCl3).
The target copolymer, PTCZTBT was synthesized
by a Suzuki coupling reaction between 5 and 2,1,3-
The normalized UVꢀvis absorption spectra of P1 in
chlorobenzene are shown in Figure 3a. A characteristic
absorption at around 600 nm is attributed to intramo-
lecular CT transition between the electron-sufficient
(9) Itoh, T.; Mase, T. Org. Lett. 2004, 6, 4587.
ꢀ
ꢀ
(10) Velasco, D.; Castellanos, S.; Lopez, M.; Lopez-Calahorra, F.;
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Brillas, E.; Julia, L. J. Org. Chem. 2007, 72, 7523.
Org. Lett., Vol. XX, No. XX, XXXX
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TCZT and the electron-deficient benzothiadiazole unit. P1
and P2 show quite similar absorption spectra with absorp-
tion maxima (λmax) at 578 and 586 nm, respectively (Figure
S10a, Supporting Information). The difference in λmax is
due to the degree of π-conjugation with different molecular
weights. On the other hand, a broader absorption of the
cast film of P1 and P2 was observed at around 600 nm,
whereas the absorption of chlorobenzene solution shows a
narrow spectral shape. The absorption spectrum of the
PCDTBT film, which has an analogous structure to P1
and P2, shows slight broadening of the spectral shape.
These results suggest that in the solid states thin film
crystallization and subsequent backbone planarization
occur due to strong πꢀπ interaction between the planar
TCZT units. The optical bandgap of P1 and P2 is 1.80 and
1.78 eV, respectively, as estimated from the absorption
onset (689 and 695 nm, respectively). Narrow bandgapand
broad absorption of PTCZTBT was observed by incor-
porating the planar electron-rich TCZT backbone instead
of the carbazole unit in PCDTBT (optical bandgap:
1.87 eV).3a On the other hand, the wider bandgap than
that of PCDCTBT (1.68 eV)5c is probably due to the
stronger electron-donating property of the heterohepta-
cene unit supported by the extended aromatic system.
The electrochemical properties of P1 and P2 were
studied by cyclic voltammetry (Figure 3b, Figures S10b
and S11, Supporting Information). The HOMO and
LUMO values were determined from the onset values of
the oxidation and reduction potential with respect to
ferrocene (Fc), respectively, as shown in Table S2 (Supporting
Information). Both polymers showed oxidation waves
attributed to the oxidation of TCZT at 0.6 and 1.3 V vs
Fc/Fcþ. We estimated the HOMO and LUMO values
to be ꢀ5.1 and ꢀ3.7 eV, respectively, for both P1 and
P2. Considering that the HOMO level of PCDTBT
is ꢀ5.5 eV,3a the increased HOMO of PTCZTBT is due
to the electron richness of the TCZT unit. We also
evaluated the ionization potential (IP), which corresponds
to the HOMO energy level of the polymers in thin film by
photoelectron spectroscopy in air. The IPs estimated from
the onset of the spectra are 5.1 eV for both P1 and P2
(Figure S12, Supporting Information), which agrees with
the HOMO energy levels estimated by cyclic voltammetry.
Preliminary bulk heterojunction-type OPV devices
were fabricated with a device structure of ITO/Poly(3,4-
ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:
PSS)/polymer:PC61BM/Al. The devices were measured
under the simulated AM 1.5 G illumination condition
(100 mW/cm2) (Figure S13 and Table S3, Supporting
Information). The photoconversion efficiency of the best
device was 0.35%. A current density (JSC) was obtained
(2.06 mA/cm2) along with a fill factor (FF; 0.30) and open-
circuit voltage (VOC = 0.60 V). The VOC value agrees with
the calculated VOC (0.5 V) determined from electrochemical
data.11 We believe that the device performance can be
further improved by optimizing the processing conditions
to form a suitable bulk heterojunction morphology.
In summary, a new direct strategy for synthesizing
TCZT, a heteropentacene, via double-intramolecular
FriedelꢀCrafts acylation has been proposed, and TCZT
has been developed as a new class of low-bandgap poly-
mers. We investigated the basic optoelectronic properties
of PTCZTBT, and we used it to produce an OPV device.
The optimization of OPV device fabrication is currently
under investigation.
Acknowledgment. This work was supported in part by
the Integrated Collaborative Research Program with In-
dustry from RIKEN.
Supporting Information Available. Detailed experimen-
tal procedures and characterization data for 1ꢀ5, P1, and
P2. This material is available free of charge via Internet at
http://pubs.acs.org.
€
(11) Scharber, M. C.; Muhlbacher, D.; Koppe, M.; Denk, P.;
Waldauf, C.; Heeger, A. J.; Brabec, C. K. Adv. Mater. 2006, 18, 789.
The authors declare no competing financial interest.
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