Conjugated polymers from naphthalene bisimide

Article abstract of DOI:10.1021/ol801918y

(Chemical Equation Presented) Stille coupling of regioisomerically pure dibromonaphthalene bisimides (NBI) with various stannylated thiophene-based monomers yields (very) high molecular weight donor-acceptor conjugated polymers. Electrochemical and optica

Full text of DOI:10.1021/ol801918y

ORGANIC
LETTERS
2
008
Vol. 10, No. 23
333-5336
Conjugated Polymers from Naphthalene
Bisimide
5
Xugang Guo and Mark D. Watson*
Department of Chemistry, UniVersity of Kentucky, Lexington, Kentucky, 40506-0055
mdwatson@uky.edu
Received August 16, 2008
ABSTRACT
Stille coupling of regioisomerically pure dibromonaphthalene bisimides (NBI) with various stannylated thiophene-based monomers yields
very) high molecular weight donor-acceptor conjugated polymers. Electrochemical and optical absorption measurements reveal that LUMO
(
energies are essentially invariant and dictated by the NBI units, while HOMO energies are dictated by the thienyl comonomers. Optical energy
gaps ranging from 1.7 to 1.1 eV are thus obtained. The polymers are also characterized by differential scanning calorimetry and fiber WAXD.
4
Due to their strong electron-withdrawing ability, imide
substituents have been employed to impart n-type charac-
teristics to small-molecule organic semiconductors. While
there are some noteworthy exceptions, e.g., imide-function-
evaluated in PVDs. A 2,2′-bithiophene fused at its 3,3′-
positions via an imide linkage yielded a homo- and copoly-
mer with electron or hole mobilities g0.01 cm /V s.
2
5
6
However, there is only a single report of a copolymer
containing rylene bisimides electronically conjugated along
the polymer backbone (PBIs bridged via their “bay” positions
by dithienothiophene). This polymer yielded an n-type field-
effect transistor and an all-polymer photovoltaic device with
average power conversion efficiency slightly over 1%.
We report here a series of copolymers with NBI units
electronically conjugated along the backbone. As we show
here, relevant intrinsic electronic properties (e.g., LUMO)
imparted to polymers by NBI are very similar to those
imparted by PBI. However, NBI is an attractive alternative
1
alized anthracenes, the most studied imide-functionalized
small molecules for organic n-type materials are based on
the rylenes: naphthalene bisimide (NBI) and perylene bi-
2
simide (PBI). There are, however, only a few published
examples of conjugated polymers with imide-functionalized
π-systems entrained such that they can be in conjugation
along the backbone. Examples published without device
3
a
studies include polyanilines
and poly(phenylene
3
b
ethynylene)s with backbone phthalimide units and homo-
or copolymers of thiophene derivatives with imide groups
3
c-e
fused at their 3,4-positions.
An imide-functionalized
(
3) (a) Dierschke, F.; Jacob, J.; Muellen, K. Synth. Met. 2006, 156, 433–
43. (b) Witzel, S.; Ott, C.; Klemm, E. Macromol. Rapid Commun. 2005,
isothianaphthene gave a very low band gap homopolymer,
and its copolymers with 3,4-ethylenedioxythiophene (EDOT)
have been implicated as promising charge-storage media and
2
5
6, 889–94. (c) Zhang, Q. T.; Tour, J. M. J. Am. Chem. Soc. 1998, 120,
355–62. (d) Nielson, C. B.; Bjornholm, T. Org. Lett. 2004, 6, 3381–84.
(
e) Pomerantz, M.; Amarasekara, A Synth. Met. 2003, 135-136, 246–8.
(
4) (a) Meng, H; Wudl, F Macromolecules 2001, 34, 1810–16. (b)
Sonmez, G.; Meng, H.; Wudl, F. Chem. Mater. 2003, 15, 4923–9. (c)
Scharber, M. C.; Muehlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.;
Heeger, J.; Brabec, C. J. AdV. Mater. 2006, 18, 789–94.
(5) Letizia, J. A.; Salata, M.; Tribout, C.; Facchetti, A.; Ratner, M. A.;
Marks, T. J. J. Am. Chem. Soc. 2008, 130, 9679–94.
(
1) Wang, Z.; Kim, C.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc.
007, 129, 13362–3.
2) (a) See, K. C.; Landis, C.; Sarjeant, A.; Katz, H. E. Chem. Mater.
008, 20, 2609–16. (b) Jones, B. A.; Facchetti, A.; Wasielewski, M. R.;
2
2
(
Marks, T. J. J. Am. Chem. Soc. 2007, 129, 15259–78. (c) Ling, M. M.;
Erk, P.; Gomez, M.; Koenemann, M.; Locklin, J.; Bao, Z. AdV. Mater. 2007,
9, 1123–27.
(6) Zhan, X.; Tan, Z.; Domercq, B.; An, Z.; Zhang, X.; Barlow, S.; Li,
Y.; Zhu, D.; Kippelen, B.; Marder, S. R. J. Am. Chem. Soc. 2007, 129,
7246–7.
1
10.1021/ol801918y CCC: $40.75
Published on Web 11/04/2008
 2008 American Chemical Society

Table 1. Yields and properties of polymers
a
b
c
d
Egopt
(eV)
e
E1/21red
(V)
f
E1/22red
(V)
f
g
h
yield Mn [PDI]
%) (kDa)
Tm
soln λmax abs
film λmax abs
LUMO
(eV)
HOMO
(eV)
(
(°C)
(nm)
(nm)
∆ λmaxsoln-film
P1
94
98
99
91
95
23.6 [2.2]
252 [2.5]
73.0 [1.6]
78.7 [1.6]
133 [3.5]
235
285
222
269
252
568
693
539
534
985
381
618
707
626
588
966
50
14
87
54
-19
1.66
1.48
1.56
1.65
1.08
3.07
b
-0.97
-1.03
-1.04
-1.03
-1.04
-1.01
-1.40
-1.38
-1.39
-1.40
-1.40
-1.47
-3.83
-3.77
-3.76
-3.77
-3.76
-3.79
-5.49
-5.25
-5.32
-5.42
-4.84
-6.86
P2a
P2b
P2c
P2d
NBI
a
Number-average molecular weight and polydispersity (GPC vs polystyrene standards in THF). Peak melting point, differential scanning calorimetry
c
-6
d
e
(
10 °C/min) Solution absorption spectra (5 × 10 M THF). Thin film absorption spectra (annealed at Tm - 40 °C). Optical energy gap estimated from
f
the absorption edge of annealed films except parent NBI measured in solution. CV measurements of thin films except parent NBI measured in solution, vs
+
g
1
h
opt
Fc/Fc . Estimated from LUMO ) -4.8 - E1/2 red) Estimated from HOMO ) LUMO - Eg
.
to PBI as the requisite dibromo-monomers are much more
readily obtained in regioisomerically pure form, the smaller
naphthalene unit will likely generally lead to polymers with
greater solubility, and bridging aryl units connected to the
Scheme 1. Synthesis of Polymers
2
,6-positions of NBI should allow for a more coplanarized
backbone (extended conjugation) than bridging via the
sterically congested bay positions of PBI.
The NBI monomers were prepared in regioisomerically
pure form by standard procedures in two laboratory steps
7
(Scheme 1 and Supporting Information). It has been reported
that commercially available naphthalene bisanhydride (NBA)
can be selectively and quantitatively brominated at its 2,6-
positions with dibromoisocyanuric acid (DBI) in concentrated
sulfuric acid at 130 °C. We could not reproduce these results
8
but found the published adaptation using 1 equiv of DBI in
fuming sulfuric acid to be effective. The resulting mixture
of products with varying levels of bromination can be
enriched in the 2,6-dibromo NBA by recrystallization from
various polar aprotic solvents (not shown), but this effort is
unnecessary since separation is easily effected after subse-
quent imidization. The crude mixture of brominated NBAs
was thus converted to a mixture of brominated NBIs by
reaction with amines in glacial acetic acid. The desired 2,6-
dibromo NBIs 3 are easily separated from the coproducts of
nucleophilic aromatic substitution in moderate yields after
simple flash chromatography and/or recrystallization. Race-
mic “swallow-tail” side chains were employed to ensure
polymer solubility. 2-Ethylhexylamine is commercially avail-
able, while 2-decyltetradecylamine was prepared in three
9
steps from 2-decyltetradecanol via Gabriel synthesis (Sup-
porting Information).
All of the thiophene-based monomers were prepared by
standard procedures (Supporting Information). The stanny-
lated thiophenes 1 and 2 were characteristically susceptible
1
13
to protiodestannylation but gave satisfactory H/ C NMR
spectra and elemental analysis after chromatography using
alumina treated with triethylamine. Due to its electron-
donating alkoxy substituents, monomer 2d is particularly
unstable but can be stored under inert atmosphere at low
temperature.
(
7) Chaignon, F.; Falkenstroem, M.; Karlsson, S.; Blart, E.; Odobel, F.;
Hammarstroem, L. Chem. Commun. 2007, 64–66.
8) Thalacker, C.; Roeger, C.; Wuerthner, F. J. Org. Chem. 2006, 71,
098–8105.
9) Koeckelberghs, G.; De Cremer, L.; Vanormelingen, W.; Dehaen,
W.; Verbiest, T.; Persoons, A.; Samyn, C. Tetrahedron 2005, 61, 687–91.
(
Polymer molecular weight is an important factor affecting
8
10
performance of organic FETs and PVDs. The electron-
(
poor NBI monomers 3 are ideal coupling partners for
5334
Org. Lett., Vol. 10, No. 23, 2008

1
1
electron-rich stannylated thiophenes, leading to high mo-
lecular weights and isolated yields indicating excellent
conversion in the Stille coupling reaction using the catalyst
mer. This is essentially the same value for the parent NBI
1
3
measured by us and reported by others (after using a
1
4
+
standard conversion factor to convert from SCE to Fc/Fc ).
In fact, this is within 0.1 V of the value for the PBI
system of Pd
Cl Pd(PPh
molecular weights here, which has been previously at-
2
[dba]
3
and tri(o-tolyl)phosphine (Table 1).
6
2
3
)
2
as catalyst (not shown) gave slightly lower
copolymer reported by Marder, the parent PBI, and even
the higher rylenes: terrylene bisimide and quaterylene
1
1
15
tributed to stoichiometric imbalance upon reduction of
Pd(II) by stannyl monomers. Typical overestimation of
relative molecular weights of semirigid rod polymers by GPC
versus polystyrene standards is assumed here, and the
exceptionally high relative molecular weight for polymer P2a
probably further reflects aggregation in solution. All of the
polymers are soluble in common organic solvents including
THF, toluene, and chloroform. P2c is even highly soluble
in hexanes due to the branched side chains on its bithiophene
units, while P2a with unsubstituted bithiophene units requires
mild heating in chloroform.
The identity and purity of the polymers is supported by
elemental analysis and solution NMR measurements (Sup-
porting Information). Resolved H NMR spectra of higher
molecular weight polymers P2a and P2d could only be
obtained at elevated temperatures (130 °C) due to aggrega-
tion. No C signals arising from their polymer backbones
could be observed even after collection for 12 h at this
elevated temperature.
bisimide. The LUMO levels of our polymers (∼-3.8 eV)
are dictated entirely by the rylene and may be similar for all
thienyl copolymers of the rylene bisimides. No oxidation
waves were observed within our experimentally accessible
window (( 2 V), regardless of scan rate.
Room-temperature solution and thin film absorption
spectra of all new polymers are shown in Figure 2, and
max
relevant data are collected in Table 1. Absorption λ and
opt
optical energy gaps (Eg ), estimated from the low-energy
absorption edge, vary across the visible spectrum into the
near IR. Changing from thiophene to bithiophene comonomer
(P1 vs P2a) causes a red-shift of 125 nm in solution and 89
nm in annealed thin films. However, alkylation of the
bithiophene units (P2b,c vs P2a) leads to a solution blue-
shift of 154-159 nm, while alkoxylation (P2d vs P2a) leads
to a very large red shift of nearly 300 nm.
1
1
3
Reaction mixtures of the most electron-rich thienyl
monomer 2d and NBI 3b exhibited a green color upon
dissolution in THF, suggesting a charge-transfer (CT)
complex. Swager and Iverson reported CT complexes
between NBIs and 1,5-dialkoxynaphthalenes leading to
alternating donor-acceptor columnar mesophases with sub-
1
2
stantial thermal stability. Combination of nonstannylated
,3′-dialkoxy-2,2′-bithiohene (absorption λmax ) 326 nm) and
nonbrominated N,N′-2-decyltetradecylnaphthalene bisiimide
absorption λmax ) 381 nm) gives a blue solution with a CT
3
(
absorption band at λmax ) 588 nm (Figure 1). It is possible
that inter- and intramolecular CT states contribute to the
particularly low optical energy gap for polymer P2d de-
scribed below.
Figure 2. Absorption spectra of polymers in solution (dashed lines,
× 10- M THF) and thin film (solid lines).
6
5
opt
With LUMO levels constant, variation in E
g
is essentially
entirely a function of the nature of the thienyl comonomers.
(10) (a) Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Liu, J.; Frechet,
J. M. J.; Toney, M. F. Macromolecules 2005, 38, 3312–19. (b) Ma, W.;
Kim, J. Y.; Lee, K.; Heeger, A. J. Macromol. Rapid Commun. 2007, 28,
1776–80.
Figure 1. Charge-transfer complex (λmax ) 588 nm, 0.01 M toluene)
from 3,3′-dialkoxybithiophene and NBI.
(
11) Bao, Z.; Chan, W. K.; Yu, L. J. Am. Chem. Soc. 1995, 117, 12426–
3
5.
(
12) Reczek, J. J.; Villazor, K. R.; Lynch, V.; Swager, T. M.; Iverson,
B. L. J. Am. Chem. Soc. 2006, 128, 7995–8002.
Thin film electrochemical measurements reveal two re-
versible reduction waves for each polymer (Table 1 and
Supporting Information). The E1/2 values for the first reduc-
tion waves are invariably near -1 V versus the ferrocene/
(
13) Chopin, S.; Chaignon, F.; Blart, E.; Odobel, F. J. Mater. Chem.
2
007, 17, 4139–46.
(14) Pavlishchuk, V. V.; Addison, A. W. Inorg. Chim. Acta 2000, 298,
9
7–102.
(
15) Lee, S. K.; Zu, Y.; Herrmann, A.; Geerts, Y.; Muellen, K.; Bard,
+
ferrocenium redox couple (Fc/Fc ) regardless of the comono-
A. J. J. Am. Chem. Soc. 1999, 121, 3513–20.
Org. Lett., Vol. 10, No. 23, 2008
5335

Such dependence of LUMO on the acceptor and HOMO on
Two-dimensional wide-angle X-ray diffraction patterns
were collected with an area detector from extruded fibers
(Supporting Information). Series of equatorial intensity
maxima out to the fourth order show that polymers P1, P2a,
and P2d assemble to ordered lamella. Meridianal reflections
out to the third order also indicate relatively long-range
correlation of repeating elements along the backbone. Their
d-spacings reasonably agree with the repeating unit lengths
(P1: 9.9 Å, P2d: 13.1 Å). π-Stacking distances of 4.0, 3.8,
and 4.0 Å, respectively, were estimated from additional
equatorial reflections. It must be understood that these values
are upper limits, exceeding the actual π-stacking distance if
the polymer backbone rings tilt away from normal to the
the donor monomer is not unusual for donor-acceptor
16
opt
copolymers. A decrease in E
g
on changing from thiophene
to bithiophene (and further to terthiophene) was also reported
1
7a
for copolymers with fluorene. The HOMOs of oligoth-
iophenes are destabilized asymptotically with increasing
1
7b
oligomer length;
i.e., bithiophene is a “stronger donor”
than thiophene. The longer footprint of bithiophene bridges
may also diminish intramolecular steric interaction of NBI
side chains in solution and afford more commensurate space-
filling in the solid state.
1
8
We have shown that 3,3′-dialkylbithiophene (head-to-
head, HH) linkages do not preclude backbone planarity in
alternating donor-acceptor perfluorobenzene-bithiophene
copolymers. However, 2-ethylhexyl side chains proved too
bulky in that case and led to amorphous twisted backbones
in the solid state. Here, the large solution blue-shift upon
alkylation of the bithiophene units is due to a greater barrier
to rotation/planarization of the polymer backbones. P2b with
less sterically encumbering n-dodecyl chains undergoes a
larger red-shift (87 nm) on going from solution to the solid
2
1
π-stacking axes. This is equivalent to slipped π-stacks
formed by most large π-systems. P2b forms an ordered
superstructure but gives no clear diffractions attributable to
π-stacking. Either these diffractions are extinguished or the
red-shift (87 nm) of P2b upon going from solution to
annealed solid film should be attributed to backbone pla-
narization alone. Polymer P2c bearing bulky head-to-head
2
-ethylhexyl chains borders on amorphous.
state compared to P2c carrying 2-ethylhexyl chains (54 nm).
In summary, brominated NBIs are excellent coupling
opt
Increasing E
g
along the series P2a f P2b f P2c points
partners for Stille polymerization with electron-rich stanny-
lated monomers, leading to high-molecular-weight donor-
acceptor conjugated polymers. Optical energy gaps of the
polymers are easily tuned across the visible spectrum and
into the near IR by variation of comonomers. The polymers
are readily soluble and therefore can be solution processed
for device fabrication. The polymers reported here, as well
as related ones from comonomers based on fused thiophenes,
are currently being evaluated in transistors and as both the
donor and acceptor component in PVDs. We have so far
observed FETs with ambipolar transport characteristics in
ambient atmosphere, which will be reported in due course.
to attenuated conjugation (stabilize HOMO) and/or π-stack-
ing as a function of side-chain steric bulk.
Electrostatic attraction between ether oxygens and thienyl
1
9
sulfur atoms of P2d likely enhance planarization, while
mesomeric effects from pendant ether oxygens destabilize
opt
the HOMO, together causing the lowest E
g
. Fine structure
even in the solution spectrum of P2d is attributed to greater
backbone rigidity. The absorption profiles for all the other
polymers undergo small red-shifts (5-17 nm) upon thermal
annealing at 40 °C below their peak melting temperatures.
The thin-film absorption profile for P2a develops fine
structure upon annealing, indicating an increase in backbone
rigidification and order (Supporting Information). Although
Acknowledgment. We thank Dr. Genay Jones (University
of Kentucky) for valuable assistance with thin-film electro-
chemical measurements and the National Science Foundation
for partial funding.
2
0
highly fluorescent NBI derivatives have been reported,
polymers 1 and 2 barely visibly emit under illumination with
UV/Vis light.
(
16) (a) Thompson, B. C.; Kim, Y. G.; McCarley, T. D.; Reynolds, J. R.
Supporting Information Available: Synthesis and char-
acterization details, abs spectra, and fiber WAXD diffrac-
tograms. This material is available free of charge via the
Internet at http://pubs.acs.org.
J. Am. Chem. Soc. 2006, 128, 12714–25. (b) Blouin, N.; Michaud, A.;
Gendron, D.; Wakim, S.; Blair, E.; Neagu-Plesu, R.; Belletete, M.; Durocher,
G.; Tao, Y.; Leclerc, M. J. Am. Chem. Soc. 2008, 130, 732–42.
(
17) (a) Asawapirom, U.; Guentner, R.; Forster, M.; Farrell, T.; Scherf,
U. Synthesis 2002, 1136–43. (b) Bouzzine, S. M.; Bouzakraoui, S.;
Bouachrine, M.; Hamidi, M. THEOCHEM 2005, 726, 271–76.
(
18) Wang, Y.; Watson, M. D. Macromolecules, 2008, 41, 8643-47.
OL801918Y
(19) (a) Irvin, J. A.; Schwendeman, I.; Lee, Y.; Abboud, K. A.; Reynolds,
J. R. J. Polym. Sci., A Poly. Chem. 2001, 39, 2164–78. (b) Pomerantz, M.
Tetrahedron Lett. 2003, 44, 1563–65.
(21) Curtis, M. D.; Cao, J.; Kampf, J. W. J. Am. Chem. Soc. 2004, 126,
4318–4328.
(
20) Roeger, C.; Wuerthner, F. J. Org. Chem. 2007, 72, 8070–75.
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