Conjugated alternating copolymers containing both donor and acceptor moieties in the main chain

Article abstract of DOI:10.1039/b617931b

A straightforward synthesis toward two conjugated alternating copolymers consisting of 2,7-linked carbazole donor (2,7-Cz) and ladderized pentaphenylene with diketone bridge (LPPK) acceptor chromophores is reported: the copolymers differ by the repeat uni

Full text of DOI:10.1039/b617931b

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Conjugated alternating copolymers containing both donor and acceptor
moieties in the main chain{
Ming Zhang, Changduk Yang, Ashok K. Mishra, Wojciech Pisula,{ Gang Zhou, Bruno Schmaltz,
Martin Baumgarten and Klaus Mu¨llen*
Received (in Cambridge, UK) 8th December 2006, Accepted 13th March 2007
First published as an Advance Article on the web 23rd March 2007
DOI: 10.1039/b617931b
Recently our group has reported the photovoltaic device based
on a poly(2,7-carbazole) derivative as an electron-donor material
in combination with PCBM acceptor yielding a good efficiency of
0.6%.22 Furthermore, we have previously synthesized a ladder-type
pentaphenylene with two ketone groups at the bridges as an
excellent electron-accepting (n-type) material.23 In these regards,
herein is presented the synthesis and characterization of a new class
of donor–acceptor (D–A) copolymers, represented by P1 and P2
(Chart 1), which consist of alternating 2,7-linked carbazole (2,7-
Cz) and ladderized pentaphenylene with diketone bridge (LPPK)
segments. P1 comprises 2,7-Cz and LPPK chromophores in a 1 : 1
ratio whereas P2 contains a 2 : 1 ratio in backbone. The basic
photophysics and supramolecular organization of the copolymers
(P1 and P2) in solid state have been studied.
A straightforward synthesis toward two conjugated alternating
copolymers consisting of 2,7-linked carbazole donor (2,7-Cz)
and ladderized pentaphenylene with diketone bridge (LPPK)
acceptor chromophores is reported: the copolymers differ by
the repeat unit ratio between the 2,7-Cz and LPPK units within
the backbone; energy and charge transfer properties and
supramolecular organizations of donor–acceptor moieties in
these copolymers have been studied via optical spectroscopy
and two-dimensional wide-angle X-ray scattering (2D-WAXS);
preliminary results such as the efficient energy and charge
transfer and p-stacking character in the solid state suggest that
the copolymers are potentially useful for photovoltaic devices.
Polymer solar cells attract considerable attention due to their
unique advantages, such as low cost, light weight, and potential
use in flexible devices.1–5 They are often based on polymer donors
and molecular acceptors in blends, so-called bulk heterojunctions.
A general drawback of the bulk heterojunction design is the fact
that the transport and collection of charges in a disordered
nanoscale blend can be hindered by phase boundaries and
discontinuities. One way to overcome this drawback is to
covalently link the donor and acceptor in a single polymer chain.6
This method has important advantages because the intrinsic
tendency of each segment in copolymers to aggregate in an
individual phase provides a means to create a well-ordered
nanoscale morphology (e.g., spheres, cylinders, lamellar), governed
by the relative volume fractions. Moreover, the built-in intramo-
lecular charge transfer can facilitate ready manipulation of the
electronic structure, and lead to low band gap semiconducting
polymers.7–13 In particular, through design and synthesis of new
polymer structures, donor–acceptor conjugated polymers can
extend to systems with efficient photoinduced charge-transfer,
-separation, and -transport processes for photovoltaic devices.14–16
While this principle has been utilized in engineering various
materials to create fascinating architectures, conjugated polymers
containing donor–acceptor units in the main chain have received
limited attention.17–21
The covalently linked D–A system 2,7-Cz–LPPK has been
synthesized according to the route depicted in Scheme 1 utilizing
the synthetic approach previously developed for 1 and 3.22–24 The
diboronic ester 2 as an acceptor unit was prepared by the
palladium-catalyzed Suzuki coupling reaction of 1 with bis(pina-
colato)diboron under Pd(dppf)Cl2–KOAc–dioxane conditions.
Since poly(2,7-carbazole) derivatives have been known to have
limited solubility in organic solvents, the introduction of the bulky
solubilizing group (4-[tris-(4-octyloxyphenyl)methyl]phenyl) at the
nitrogen on the carbazole unit was carried out. In addition, the
solubilizers with alkoxy-side chains can improve the electron-
donating characteristics. The coupling of 3a and p-fluorobenzoi-
cacidmethylester gave N-arylcarbazole derivative 4a under sodium
hydride as a strong base and subsequent addition of 4-octyloxy-
bromobenzene generated the carbazole alcohol compound 5a. The
corresponding phenol 6a was obtained by the treatment with
acetyl chloride followed by phenol, which then underwent
nucleophilic substitution of 6a with octylbromide to produce the
desired N-alkoxyaryl-2,7-dibromocarbarzole derivative 7a as a
Max-Planck-Institute for Polymer Research, Ackermannweg 10,
D-55128, Mainz, Germany. E-mail: muellen@mpip-mainz.mpg.de;
Fax: +49 6131 379100; Tel: +49 6131 3790
{ Electronic supplementary information (ESI) available: Synthesis, UV-vis
absorption and PL spectra of P1, the concentration dependent PL spectra
of the LPPK monomer and the measurement of the 2D-WAXS. See DOI:
10.1039/b617931b
{ Present address: Degussa AG, Process Technology & Engineering,
Process Technology – New Processes, Rodenbacher Chaussee 4, D-63457
Hanau-Wolfgang, Germany
Chart 1 Structures of 2,7-Cz and LPPK alternating copolymers (P1
and P2).
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Fig. 1 (a) UV-vis absorption of P2 in a dilute chloroform solution and
drop-casted film on quartz glass; (b) absorption spectra of P2 in different
solvents.
560 nm corresponding to the n–p* transition in the carbonyl of
LPPK was weak.23,25 The effect of the solvent polarity on P2
absorption was displayed in Fig. 1(b). By changing the solvent
from toluene to chloroform, the peak at around 550 nm shifted to
560 nm. Comparing the UV-vis spectra of P2 thin film with that of
the dilute solution, it can be clearly seen that the absorption
spectra of thin films were apparently broader, the position of the
main absorption peaks were red-shifted and the absorption
intensity at 560 nm was stronger, which may be due to the
stronger p–p stacking of the LPPK units in the main chain. It is
postulated that this feature is associated with the formation of
close-packed self-organized layers by a strong interchain interac-
tion in this p-conjugated system in the solid state.26 Comparing P1
with P2, it could be found that the absorption maximum at 560 nm
of P1 was stronger than that of P2 relative the intensity of the
copolymer backbone, which can be due to the higher 2,7-Cz
content in P2.
Scheme 1 Synthesis of the D–A copolymers P1 and P2. (i) bis(pinaco-
lato)diboron, KOAc, Pd(dppf)Cl2, dioxane, 80 uC, overnight; (ii) NaH,
p-fluorobenzoicacidmethylester, DMF, 70 uC, 3 d; (iii) 4-octyloxybromo-
benzene, n-BuLi, THF, RT, overnight; (iv) acetyl chloride, 80 uC , 1 day,
then phenol, 120 uC, 3 d; (v) KOH, octylbromide, RT, 1 day; (vi) 7a, 2,
K2CO3, tetrakis(triphenylphosphine)palladium, Toluene, H2O, Argon, 85
uC, 3 d; (vii) 7b, 2, K2CO3, tetrakis(triphenylphosphine)palladium, THF,
H2O, Argon, 80 uC, overnight; (viii) Bis(cyclooctadiene)nickel, cycloocta-
diene, 2,29-bipyridine, dry toluene and dry DMF, 60 uC, 30 min, then
monomer 8, 75 uC, 2 d.
The PL of P2 in solution was very weak. The emission spectra
of P2 showed only one maximum at 696 nm, which was attributed
to LPPK units in these copolymers. This was supported by
concentration dependent photoluminescence spectroscopy of
pentaphenylene diketone monomer, which demonstrated a red-
shift from 620 nm to 633 nm with increasing concentration, and
finally the emission in solid state was at around 720 nm
(supporting information{). There was no emission from the 2,7-
Cz subunits, implying complete energy transfer from 2,7-Cz donor
to LPPK acceptor moieties,27,28 while the luminescence quenching
of the acceptor moiety in P2 compared to pure acceptor LPPK
monomer indicated the charge transfer involved: when the P2 has
the same concentration as the pure monomer LPPK, stronger
emission quenching of P2 was found (supporting information{),
which supports the existence of a charge separated state upon
photoexcitation in the D–A polymer.
donor building unit. The copolymer P1 was synthesized by Suzuki
polycondensation of 2 and 7a under palladium(0) catalysed
condition.
The same procedures as described above were adopted for the
synthesis of the 2-bromo-7-chloro-9-{4-[tris-(4-octyloxyphenyl)-
methyl]phenyl}carbazole (7b) starting from carbazole 3b
(Scheme 1). Suzuki coupling of 2 and 7b under palladium catalysis
generated the target dichloride monomer 8. The analogous
copolymer P2 was prepared via a nickel(0)-mediated Yamamoto
polycondensation. Both copolymers (P1 and P2) were readily
soluble in common organic solvents, tetrahydrofuran (THF),
toluene, dichloromethane (DCM), etc.. Gel-permeation chromato-
graphy (GPC) analysis with PPP standard exhibited a number-
average molecular mass (Mn) of 1.0 6 104 g mol21 and 4.4 6
104 g mol21, for P1 and P2, respectively with a polydispersity of
1.38 and 1.91.
The supramolecular structures of both polymers P1 and P2
have been investigated by two-dimensional wide-angle X-ray
scattering (2D-WAXS) on extruded filaments.29 Fig. 2 shows a
characteristic 2D X-ray pattern of P2. The distinct and strong
reflections were related to a well-aligned macroscopic organization
in the extruded sample, which was also obvious for P1, confirming
no influence of the chemical structure difference on the ordering of
both copolymers. The equatorial small-angle reflections indicated
a lateral distance between the conjugated polymer chains of 1.8 nm
for P1 and 1.92 nm for P2. Since this value was determined by the
length of the substitutions, these distances were in a good
agreement with the difference in the attached units of P1 and
P2. The equatorial wide-angle reflections corresponded to the
The optical properties of P1 and P2 were similar. Here, we took
P2 as an example (P1 is shown in the supporting information{).
Fig. 1 shows the UV-vis absorption spectra of P2 in a dilute
chloroform solution and a drop-casted film on quartz glass at
room temperature, together with the absorption spectra in
different solvents. P2 showed two absorption maxima at around
345 nm and 408 nm in solution, which correspond to the p–p*
transition of the copolymer’s backbone. An absorption band at
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Fig. 2 (a) 2D-WAXS of P2 at room temperature, schematic illustration of the (b) polymer conformation and (c) supramolecular organization.
5 C. J. Brabec, N. S. Sariciftci and J. C. Hummelen, Adv. Funct. Mater.,
2001, 11, 15.
6 E. H. A. Beckers, S. C. J. Meskers, A. P. H. J. Schenning, Z. Chen,
F. Wu¨rthner, P. Marsal, D. Beljonne, J. Cornil and R. A. J. Janssen,
J. Am. Chem. Soc., 2006, 128, 649.
7 A. K. Agrawal and S. A. Jenekhe, Chem. Mater., 1996, 8, 579.
8 J. P. Ferraris, A. Bravo, W. Kim and D. C. Hrncir, J. Chem. Soc.,
Chem. Commun., 1994, 991.
9 C. Hay, D. Le Vilain, V. Deborde, L. Toupet and R. Reau, Chem.
Commun., 1999, 345.
10 B. L. Lee and T. Yamamoto, Macromolecules, 1999, 32, 1375.
11 T. Yamamoto, Z. H. Zhou, T. Kanbara, M. Shimura, K. Kizu,
T. Maruyama, Y. Nakamura, T. Fukuda, B. L. Lee, N. Ooba,
S. Tomaru, T. Kurihara, T. Kaino, K. Kubota and S. Sasaki, J. Am.
Chem. Soc., 1996, 118, 10389.
12 Q. T. Zhang and J. M. Tour, J. Am. Chem. Soc., 1997, 119, 5065.
13 Q. T. Zhang and J. M. Tour, J. Am. Chem. Soc., 1998, 120, 5355.
14 S. A. Jenekhe and S. J. Yi, Appl. Phys. Lett., 2000, 77, 2635.
15 S. A. Jenekhe and S. J. Yi, Adv. Mater., 2000, 12, 1274.
16 A. Kohler, D. A. dos Santos, D. Beljonne, Z. Shuai, J. L. Bredas,
A. B. Holmes, A. Kraus, K. Mu¨llen and R. H. Friend, Nature, 1998,
392, 903.
17 H. B. Wang, M. K. Ng, L. M. Wang, L. P. Yu, B. H. Lin, M. Meron
and Y. N. Xiao, Chem.–Eur. J., 2002, 8, 3246.
18 J. S. Liu, E. Sheina, T. Kowalewski and R. D. McCullough, Angew.
Chem., Int. Ed., 2002, 41, 329.
19 H. B. Wang, H. H. Wang, V. S. Urban, K. C. Littrell, P. Thiyagarajan
and L. P. Yu, J. Am. Chem. Soc., 2000, 122, 6855.
20 P. R. L. Malenfant, L. Groenendaal and J. M. J. Frechet, J. Am. Chem.
Soc., 1998, 120, 10990.
21 M. A. Hempenius, B. M. W. Langeveld-Voss, J. van Haare,
R. A. J. Janssen, S. S. Sheiko, J. P. Spatz, M. Moller and
E. W. Meijer, J. Am. Chem. Soc., 1998, 120, 2798.
22 J. L. Li, F. Dierschke, J. S. Wu, A. C. Grimsdale and K. Mu¨llen,
J. Mater. Chem., 2006, 16, 96.
23 J. Jacob, S. Sax, T. Piok, E. J. W. List, A. C. Grimsdale and K. Mu¨llen,
J. Am. Chem. Soc., 2004, 126, 6987.
24 F. Dierschke, A. C. Grimsdale and K. Mu¨llen, Synthesis, 2003, 2470.
25 R. Demadrille, M. Firon, J. Leroy, P. Rannou and A. Pron, Adv. Funct.
Mater., 2005, 15, 1547.
p-stacking period of 0.45 nm and were in a typical range for
conjugated polymers. On the other hand, the appearance of a
strong scattering intensity indicating p-stacking interactions
between the macromolecules in both the case of P1 and P2 is
quite surprising. One might assume initially that the 4-[tris-(4-
octyloxyphenyl)methyl]phenyl substituents hindered a stacking.
However, the packing during self-assembly takes place due to a
most probable flipping of the polymers. Additionally, the back-
bones of P1 and P2 were not very rigid and planar allowing a
spontaneous twisting during stacking and thus diminishing in this
way the sterically demanding influence of the 4-[tris-(4-octyloxy-
phenyl)methyl]phenyl. Furthermore, the relatively weak intensity
scattering correlated to 0.73 nm, which was in accordance to the
monomer repeating distance along the conjugated backbone,
suggested that the polymer chains were shifted towards each other,
supporting the self-assembly.
In summary, we have described the synthesis and characteriza-
tion of soluble conjugated D–A copolymers (P1 and P2)
containing electron-donating 2,7-Cz and electron-accepting
LPPK subunits. The resulting copolymers exhibited efficient
energy and charge transfer between D and A moieties in the solid
state, as proved by optical spectroscopy. Furthermore, both the
copolymers revealed an alignment well due to a pronounced
p-stacking interaction between the conjugated polymer chains.
These attractive properties such as energy and charge transfer and
macroscopic organization establish them as good candidates for
photovoltaic devices.
The financial support of the European Commission Project
NAIMO (NMP4-CT-2004-500355) and the Alexander von
Humboldt foundation are gratefully acknowledged.
Notes and references
26 A. Yassar, G. Horowitz, P. Valat, V. Wintgens, M. Hmyene,
F. Deloffre, P. Srivastava, P. Lang and F. Garnier, J. Phys. Chem.,
1995, 99, 9155.
1 J. M. Halls, C. Walsh, N. Greenham, E. Marseglla, R. Friend,
S. Moratti and A. Holmes, Nature, 1995, 376, 498.
27 F. Uckert, S. Setayesh and K. Mu¨llen, Macromolecules, 1999, 32,
4519.
2 G. Yu, J. Cao, J. C. Hummelen, F. Wudl and A. J. Heeger, Science,
1995, 270, 1789.
28 L. Oldridge, M. Kastler and K. Mu¨llen, Chem. Commun., 2006, 885.
29 W. Pisula, Z. Tomovic, C. Simpson, M. Kastler, T. Pakula and
K. Mu¨llen, Chem. Mater., 2005, 17, 4296–4303.
3 L. Schmidt-Mende, A. Fechtenko¨tter, K. Mu¨llen, E. Moons,
R. H. Friend and J. D. MacKenzie, Science, 2001, 293, 1119.
4 E. W. McFarland and J. Tang, Nature, 2003, 421, 616.
1706 | Chem. Commun., 2007, 1704–1706
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