Dithienylbenzodipyrrolidone: New acceptor for donor-acceptor low band gap polymers

Article abstract of DOI:10.1021/ma400008h

An analogue of DPP, dithienylbenzodipyrrolidone (DTBDPD), was successfully synthesized. Several D-A copolymers, with different electron donating capability donor unit (fluorene, difluorene, dithienosilole, and dithienopyrrole) and DTBDPD acceptor, were prepared by means of Suzuki and Stille coupling reaction. The optical and electrochemical properties were demonstrated through UV-vis-NIR absorption and cyclic voltammetry (CV). All polymers showed near-infrared or even infrared absorption and deeper LUMO levels, as low as -3.80 eV.

Full text of DOI:10.1021/ma400008h

Article
pubs.acs.org/Macromolecules
Dithienylbenzodipyrrolidone: New Acceptor for Donor−Acceptor
Low Band Gap Polymers
Weibin Cui†,‡ and Fred Wudl*,†,‡
†Mitsubishi Chemical Center for Advanced Materials, University of California, Santa Barbara, Santa Barbara, California 93106, United
States
‡Department of Materials, University of California, Santa Barbara, Santa Barbara, California 93106, United States
S
* Supporting Information
ABSTRACT: An analogue of DPP, dithienylbenzodipyrroli-
done (DTBDPD), was successfully synthesized. Several D−A
copolymers, with different electron donating capability donor
unit (fluorene, difluorene, dithienosilole, and dithienopyrrole)
and DTBDPD acceptor, were prepared by means of Suzuki
and Stille coupling reaction. The optical and electrochemical
properties were demonstrated through UV−vis−NIR absorp-
tion and cyclic voltammetry (CV). All polymers showed near-
infrared or even infrared absorption and deeper LUMO levels,
as low as −3.80 eV.
the additional twist of being a quinodimethane derivative.
Benzodipyrrolidone can also be envisioned as an “extended”
DPP. It was expected that introduction of alkyl chains to
enhance the solubility and adjust the performance of devices
would be facile. Moreover, the quinodimethane moiety, in
analogy to tetracyanoquinodimethane (TCNQ)21 and DPP,
was expected to exhibit enhanced π−π stacking. Other similar
colorants with bis-lactam or quinodimethane molecular
architecture, dihydropyrroloindoledione (DPID)22 and benzo-
difuranone (BDF),23 have already been successfully used for
constructing low band gap polymers. Benzodipyrrolidone and
its derivatives should be considered as another interesting
building block for donor−acceptor conjugated polymers with
promising optoelectronic applications.
In our previous work, several benzodipyrrolidone-based small
molecules and polymers were synthesized.24 Benzodipyrroli-
done showed promising electron accepting properties, which
made it attractive for constructing new series of n-type and
donor−acceptor materials. While the benzodipyrrolidones we
described were phenyl, instead of thienyl, interspersed, it was
necessary to prepare monomers that were closer to the much-
studied dithienyl DPP. Herein, we report that we successfully
developed a method to prepare dithienylbenzodipyrrolidone
(DTBDPD). Several D−A copolymers, with different electron
donating capability donor unit (fluorene, difluorene, dithieno-
silole, and dithienopyrrole) and DTBDPD acceptor were
prepared by means of Suzuki and Stille coupling reaction
(Chart 1). The polymers showed very attractive properties, i.e.,
near-infrared (NIR) or even infrared (IR) absorption that was
1. INTRODUCTION
Conjugated polymers have received increasing attention due to
their intriguing electronic and optoelectronic properties for
application in a variety of optoelectronic devices, such as
polymeric light-emitting diodes (PLEDs),1 organic thin film
transistors (OTFTs),2 and organic photovoltaics (OPVs).3
Such applications, inter alia, encourage the continued
exploration of conjugated polymers with new structures.
Donor−acceptor architecture copolymers are of much interest
because of the facile tailoring of electronic structure. The
independent selection of electron donating and accepting
moieties facilitates the tuning of the HOMO and LUMO
energy level, energy band gap, photon absorption, and charge
carrier transport (p-type, n-type, or ambipolar).4−7 D−A
copolymers contain acceptors with electron withdrawing
moieties such as imides or lactams, e.g., rylene bisimides8 and
diketopyrrolopyrrole (DPP). The latter have been extensively
explored and were found to exhibit large electron affinity (EA >
3.4 eV), near-infrared absorption, and good charge carrier
transport. During the past decade, incorporation of DPP in
polymers,9−16 oligomers,17,18 and dendrimers19 resulted in
materials for optoelectronic applications, especially in organic
thin film transistors (OTFTs)10−14 and organic photovoltaics
(OPVs).15−19 Hole mobility as high as 10.5 cm2 V−1 s−1 under
ambient conditions has been achieved by developing a very
high molecular weight DPP−DTT (dithienylthieno[3,2-b]-
thiophene) polymer,14g and power conversion efficiency
(PCE) up to 6.71% for a DPP and bithiophene copolymer16c
have been reported.
Benzodipyrrolidone (BDPD) is a molecule originally
prepared as a colorant20 and is similar in structure to DPP;
i.e., a planar, highly conjugated, lactam structure, resulting in
strong π−π interactions and electron-withdrawing effects with
Received: January 2, 2013
Revised: August 5, 2013
© XXXX American Chemical Society
A
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(m, CH3, 6H). 13C NMR (CDCl3, 600 MHz) δ ppm: 165.64 (2(C
O)), 141.51 (2 × ArC), 129.65 (4 × ArCH), 50.21 (2(CO)CH2),
41.73 (2 × NCH2), 31.41, 27.51, 26.31, 22.49 (8 × CH2), 13.95 (4 ×
CH3). TOF MS (EI+): m/z (%): 428.21[M]+.
Chart 1. Structures of Dithienylbenzodipyrrolidone
(DTBDPD) D−A Polymers
1,5-Dihexyl-5,7-dihydro-1H,3H-pyrrolo[2,3-f ]indole-2,6-dione (3).
A mixture of 2 (5.00 g, 11.6 mmol) and anhydrous aluminum chloride
(10.8 g, 81.5 mmol) was heated at 190 °C for 20 min. After cooling,
cracked ice was added to quench the reaction. The precipitate formed
was collected by filtration. After being washed with water and
methanol, the solid was dried and further purified by column
chromatography on silica gel with DCM:ethyl acetate = 10:1 as eluent
yield 3 (2.88 g, 70%) as a white solid. IR (ATR): υmax (cm−1): 3054,
2958, 2929, 2870, 1711, 1677, 1501, 1476, 1350, 1242, 1182, 1127,
948, 843. TOF MS (FI+): m/z (%): 356.25 [M]+. Anal. Calcd for
(C22H32N2O2): C, 74.12; H, 9.05; N, 7.86. Found: C, 74.6; H, 10.1; N,
7.78.
3,3,7,7-Tetrabromo-1,5-dihexyl-5,7-dihydro-1H,3H-pyrrolo[2,3-
f ]indole-2,6-dione (4). A mixture of 3 (2.88 g, 8.08 mmol), AIBN
(0.264 g, 1.62 mmol), and NBS (8.63 g, 48.5 mmol) was added carbon
tetrachloride (20 mL). The resulting mixture was heated at reflux for
12 h. After cooling, the mixture was washed with brine and dried over
anhydrous MgSO4. On removal of solvent, the residue was purified by
column chromatography on silica gel with hexane:DCM = 1:2 as
eluent to yield 4 (3.12 g, 57%) as a yellow solid. IR (ATR): υmax
(cm−1): 3048, 2953, 2926, 2860, 1722, 1475, 1341, 1185, 1161, 1132,
879, 802, 702. TOF MS (FI+): m/z (%): 667.88 [M]+. Anal. Calcd for
(C22H28Br4N2O2): C, 39.32; H, 4.20; N, 4.17. Found: C, 39.4; H, 4.48;
N, 4.05.
3,7-Dibromo-1,5-dihexyl-1H,5H-pyrrolo[2,3-f ]indole-2,6-dione
(5). To a suspension of 4 (3.00 g, 4.46 mmol) in CH3CN (100 mL)
was added sodium iodine (2.00 g, 13.4 mmol). The resulting mixture
was stirred at room temperature for 30 min. After the solvent being
removed under reduced pressure, the residue was dissolved in DCM.
The solution was washed with sodium thiosulfate(aq) and water and
dried over anhydrous MgSO4. On removal the solvent, the residue was
purified by column chromatography on silica gel with hexane:DCM =
1:2 as eluent to yield 5 (2.00 g, 87.7%) as a dark purple solid. IR
(ATR): υmax (cm−1): 3067, 2955, 2927, 2862, 1727, 1692, 1599, 1578,
1473, 1376, 1326, 1279, 1232, 1155, 1115, 913, 865, 805. TOF MS (FI
+): m/z (%): 510.06 [M]+. Unstable during heat, elemental analysis is
not available.
1,5-Dihexyl-3,7-dithiophen-2-yl-1H,5H-pyrrolo[2,3-f ]indole-2,6-
dione (6). A mixture of 5 (1.00 g, 1.95 mmol), 2-(tributylstannyl)-
thiophene (1.60 g, 4.29 mmol), Pd(PPh3)4 (60 mg), and toluene (20
mL) was heated at reflux for 24 h. After cooling to room temperature,
the reaction mixture was poured into water and extracted with toluene.
The organic layer was washed with aqueous potassium fluoride and
brine and then dried over anhydrous MgSO4. On removal of the
solvent, the residue was purified by column chromatography on silica
gel with hexane:DCM = 1:1 as eluent to yield 6 (0.70 g, 70%) as a dark
purple solid with golden reflection. IR (ATR): υmax (cm−1): 3108,
3073, 2957, 292, 2859, 1676, 1584, 1563, 1439, 1415, 1376, 1348,
1216, 1192, 1077, 1067, 852, 821, 688. TOF MS (EI+): m/z (%):
518.21 [M]+. Anal. Calcd for (C30H34N2O2S2): C, 66.46; H, 6.61; N,
5.40. Found: C, 69.4; H, 6.88; N, 5.35.
1,5-Dihexyl-3,7-dithiophen-2-yl-1H,5H-pyrrolo[2,3-f ]indole-2,6-
dione (7). To a solution of 6 (0.275 g, 0.530 mmol) in THF (50 mL)
was added NBS (0.250 g, 1.40 mmol) portionwise. The resulting
mixture was stirred at room temperature overnight. To the resulting
mixture methanol was added. The precipitate was collected, washed
with methanol, and dried in vacuo to yield 7 as a dark purple solid
(0.300 g, 84.0%) with golden reflection. IR (ATR): υmax (cm−1): 3112,
2956, 2924, 2856, 1675, 1582, 1560, 1414, 1376, 1191, 1073, 1054,
962, 916, 618. TOF MS (FD+): m/z (%): 674.0 [M]+. Anal. Calcd for
(C30H32Br2N2O2S2): C, 53.26; H, 4.77; N, 4.14. Found: C, 53.2; H,
4.85; N, 4.13.
determined directly by the electron donating ability of the
donors as well as deeper LUMO levels, as low as −3.80 eV.
2. EXPERIMENTAL SECTION
Materials. Toluene and tetrahydrofuran (THF) were purified
through a solvent purification system. Commercially available reagents
were used without further purification unless otherwise stated.
1
General Measurements. H NMR and 13C NMR were recorded
on Varian-NMR 600 spectrometers in CDCl3 or d4-ODCB with
tetramethylsilane (TMS) as internal standard. Mass spectrometry was
performed by the UC Santa Barbara Mass Spectrometry Laboratory.
GPC measurements were conducted on a Waters 510 system using
polystyrene as standard. Electronic absorption spectra were obtained
on an Agilent Technologies G1103A UV/vis spectrometer.
Thermogravimetric analysis (TGA) was carried out on a TGA Q50
(TA Instruments) at a heating rate of 10 °C min−1 at a nitrogen flow.
Differential scanning calorimetry (DSC) was run on a DSC Q10 (TA
Instruments) at a heating/cooling rate of 10/−10 °C min−1 at a
nitrogen flow. Cyclic voltammetry (CV) were performed on a
Princeton Applied Research Model 263A potentiostat/galvanostat
with a three-electrode cell in a solution 0.1 M tetrabutylammonium
hexafluorophosphate (Bu4NPF6) as supporting electrolyte dissolved in
methylene chloride or o-dichlorobenzene at a scan rate of 50 mV/s. A
Pt disk was used as working electrode, a Pt wire as the counter
electrode, and an Ag electrode as the reference electrode. The
potential was calibrated by the ferrocene/ferrocenium standard.
HOMO and LUMO energy levels were estimated by the equations
onset
HOMO = −(4.80 + ΔEooxndset) and LUMO = −(4.80 − ΔE ).
red
N,N′-Dihexylbenzene-1,4-diamine (1). To a solution of cyclo-
hexane-1,4-dione (5.60 g, 50.0 mmol) in ethanol (500 mL) was added
hexylamine (10.1 g, 100 mmol). The resulting mixture was stirred
under bobbling air for 4 h. Solvent was removed under reduced
pressure. The residue was purified by column chromatography on
silica gel with hexane:DCM:ethyl acetate = 10:10:1 as eluent yield 1
(8.50 g, 61.5%) as a light yellow solid. 1H NMR (CDCl3, 600 MHz) δ
ppm: 6.54 (s, ArH, 4H), 3.15 (br, NH, 2H), 3.03 (tr, J = 7.5 Hz,
NCH2, 4H), 1.55−1.60 (m, CH2, 4H), 1.24−1.40 (m, CH2, 12H),
0.83−0.89 (m, CH3, 6H). 13C NMR (CDCl3, 600 MHz) δ ppm
140.96 (2 × ArC), 114.74 (4 × ArCH), 45.42 (2 × NCH2), 31.70,
29.78, 26.91, 22.63 (8 × CH2), 14.03 (4 × CH3). TOF MS (EI+): m/z
(%): 276.26 [M]+.
N,N′-1,4-Phenylenebis[2-chloro-N-hexyl]acetamide (2). To a
solution of chloroacetyl chloride (0.402 g, 3.56 mmol) in THF (5
mL) at 0 °C was added a solution of 1 (0.300 g, 1.19 mmol) and 4-
(dimethylamino)pyridine (DMAP) (0.135 g, 1.11 mmol) in THF (10
mL) dropwise. After 1 h, the reaction mixture was quenched with
water. The precipitate was collected by filtration. After being washed
with water and methanol, the solid was dried in vacuo to yield 2 (0.37
Stille Polymerization. PDTBDP-DTSi. A mixture of 7 (0.115 g,
0.148 mmol), 4,4′-bis(2-ethylhexyl)-5,5′-bis(trimethyltin)dithieno-
[3,2-b:2′,3′-d]silole (0.127 g, 0.170 mmol), Pd2(dba)3 (5 mg), (o-
tol)3P (3.0 mg), and toluene (20 mL) was heated at reflux for 24 h. 2-
1
g, 72.5%) as a white solid. H NMR (CDCl3, 600 MHz) δ ppm: 7.32
(s, ArH, 4H), 3.82 (s, (CO) CH2, 4H), 3.27 (t, J = 7.8 Hz, NCH2,
4H), 1.53−1.55 (m, CH2, 4H), 1.26−1.32 (m, CH2, 12H), 0.87−0.88
B
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Scheme 1. Synthetic Route for Dibromobenzodipyrrolidone
Bromothiophene (0.070 g, 0.44 mmol) in toluene (1 mL) was added,
and the resulting mixture was stirred for another 12 h. Then after
cooling to room temperature, the mixture was dropped into methanol
to precipitate. After being filtered and dried, the precipitation was
Soxhlet extracted with methanol, acetone, hexane, dichloromethane,
and chloroform in succession. The chloroform solution was
concentrated and reprecipitated in methanol. The solid was filtered
and dried to yield polymer (0.110 g, 79.7%) as a black solid. Anal.
Calcd for (C54H70N2O2S4Si)n: C, 69.33; H, 7.54; N, 2.99. Found: C,
69.2; H, 7.34; N, 3.05.
dihexylbenzene-1,4-diamine (1). Acetamide 2 was synthesized
by treating 1 with chloroacetyl chloride. The ring-closure
reaction was performed in molten AlCl3 at 190 °C to afford
dioxindole 3. The most important intermediate, dibromoben-
zodipyrrolidone (5), was obtained by bromination with NBS in
the presence of AIBN in CCl4, followed by reductive
debromination with sodium iodide in acetonitrile.
The structures of these compounds were determined by
1
means of H NMR, 13C NMR, MS elemental analysis, and
PDTBDPD-DTPy. The same procedure as preparing DTBDPD-
DTSi was followed. 2,6-Bis(trimethylstannyl)-4-[3,4,5-tris-
(dodecyloxy)phenyl]-4H-dithieno[3,2-b:2′,3′-d]pyrrole was used. Pol-
ymer DTBDPD-DTPy was obtained as a black solid with a yield of
85.9%. Anal. Calcd for (C80H113N3O5S4)n: C, 72.52; H, 8.60; N, 3.17.
Found: C, 72.15; H, 8.06; N, 3.20.
Suzuki Polymerization. PDTBDPD-F. A mixture of 7 (0.150 g,
0.221 mmol), 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-
diethylhexylfluorene (0.149 g, 0232 mmol), Pd2(dba)3 (5.0 mg), (o-
tol)3P (3.0 mg), K3PO4 (0.187 g), and toluene/H2O (20 mL/1 mL)
was heated at reflux for 24 h. 2-Bromothiophene (0.035 g, 0.22 mmol)
in toluene (1 mL) was added, and the resulting mixture was stirred for
another 12 h. Then after cooling to room temperature, the mixture
was dropped into methanol to precipitate. After being filtered and
dried, the precipitation was Soxhlet extracted with methanol, acetone,
hexane, dichloromethane, and chloroform in succession. The chloro-
form solution was concentrated and reprecipitated in methanol. The
solid was filtered and dried to yield polymer (0.165 g, 82.5%) as a
blue-green solid. Anal. Calcd for (C59H74N2O2S2)n: C, 78.10; H, 8.22;
N, 3.09. Found: C, 78.20; H, 8.05; N, 3.22.
single crystal X-ray diffraction. To our knowledge, due to the
symmetry of compounds 3, 4, and 5, proton a (Scheme 1)
should be a single peak in the H NMR spectrum. But two
1
peaks were observed at 6.775 and 6.756 ppm (ratio 1:0.5) in
the spectrum of 3, 7.068 and 7.048 ppm (ratio 1:0.5) in 4, and
5.896 and 5.885 ppm (ratio 1:0.5) in 5, respectively. In the 13
C
NMR spectra, at each position, there is also more than one
peak with very small difference in chemical shift (about 0.1
ppm). The complex NMR spectra indicated that there should
be some other component with very similar structure to the
target molecules, and the mass spectra and elemental analyses
showed the desired exact molecular weight and C, H content. A
possible explanation could be the formation of linear and bent
isomers during the ring-closure reaction that were similar to the
isomers obtained from a two-directional Pummerer-type
cyclization.22,25 However, this is not a plausible rationalization
because the X-ray structure analysis shows only one isomer.
Though the R factor was 10% (due to side chain disorder),26 it
is certainly good enough to have detected a “bent” isomer. The
crystal structure afforded bond lengths from C1 to C2 to C3 to
C4 to C5 to be 1.356, 1.441, 1.366, and 1.485 Å, respectively.
The alternating bond length pattern firmly verified the
quinodimethane structure of the benzodipyrrolidone core.
With two active bromine atoms in 5, any units that can react
with them can be easily introduced, in principle. Dithienylben-
zodipyrrolidone (6) can be readily synthesized by means of a
Stille coupling reaction with 2-(tributylstannyl)thiophene.
Bromination of 6 with NBS afforded dibromodithienylbenzo-
dipyrrolidone (7).
PDTBDPD−DF. The same procedure as preparing DTBDPD-F
was followed. Polymer DTBDPD-DF was obtained as a green solid
with a yield of 85%. Anal. Calcd for (C88H114N2O2S2)n: C, 81.56; H,
8.87; N, 2.16. Found: C, 81.30; H, 8.50; N, 2.35.
3. RESULTS AND DISCUSSION
Synthesis. For preparation of DTBDPD, attempts following
the same procedures as the phenyl analogues turned out to be
unsuccessful.24 The condensation reaction of p-phenylenedi-
amine with α-hydroxy-2-thiopheneacetic acid gave an insoluble,
dark solid complex mixture whose structure was not possible to
determine. We therefore developed another method to prepare
dibromobenzodipyrrolidone first and then synthesize dithie-
nylbenzodipyrrolidone by means of coupling reaction. Scheme
1 shows the synthetic route for the preparation of
dibromobenzodipyrrolidone. Condensation of 1,4-cyclohexane-
dione with hexylamine in the presence of air afforded N,N′-
DTBDPD copolymers were synthesized by means of
palladium-catalyzed Suzuki (fluorene and difluorene) and Stille
(dithienosilole and dithienopyrrole) polymerization. Diboronic
ester derivatives of fluorene27 and difluorene28 and distannyl
reagent of dithienosilole29 and dithienopyrrole7a were prepared
by literature procedures. Suzuki coupling reactions were
C
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Scheme 2. Preparation of Dithienylbenzodipyrrolidone (DTBDPD) Copolymers
performed using tris(dibenzylideneacetone)dipalladium as
catalyst, potassium phosphate as base, and triphenylphosphine
as ligand in toluene/water. Stille cross-coupling polymerizations
were performed in the presence of tris(dibenzylideneacetone)-
dipalladium in toluene. The reaction time of Suzuki and Stille
coupling was 24 h. The polymers were easily soluble in
common solvents such as chloroform and chlorobenzene and
1
were characterized by H NMR (d4-ODCB, 100 °C) and gel
permeation chromatography (GPC) with PS (polystyrene) as
standard and chloroform as eluent. Polymers showed strong
aggregation as featured by rather broad and featureless signals.
PDTBDPD-F and PDTBDPD-DF signals can be detected at
room temperature. But those of PDTBDPD-DTSi and
PDTBDPD-DTPy can only been observed after being heated
to 100 °C. The number-average molecular weight and (PDI)
were found to be 7 kDa (2.42) of PDTBDPD-F, 12 kDa (2.86)
of PDTBDPD-DF, 38 kDa (2.80) of PDTBDPD-DTSi (3.00),
and 20 kDa (3.20) of PDTBDPD-DTPy, respectively.
Solid State Structure of DTBDPP. A single crystal of
Figure 1. Single crystal structure of DTBDPD.
dithienylbenzodipyrrolidone (6) was prepared from chloroform
solution (Figure 1). The dihedral angle between the thiophene
and benzodipyrrolidone core was decreased to 12.7° compared
to 38.0° of the phenyl analogue.24 The packing structure of
BDPD molecules can be modified by linking with different
units. The solid state structure was found to be monoclinic for
dibromobenzodipyrrolidone, orthorhombic for diphenylbenzo-
dipyrrolidone, and triclinic for dithienylbenzodipyrrolidone.
Moreover, the nearest distance in DTBDPD is 3.29 Å between
thiophene and the middle six-member ring, a distance that is
much smaller than the 3.56 Å observed in the DPBDPD,
indicating much stronger π−π interactions in the thiophene
analogue.
Figure 2, all the polymers showed good thermal stability. The
5% weight loss of the polymers happened at 395.2 °C for
PDTBDPD-F, 399.0 °C for PDTBDPD-DF, 405.6 °C for
PDTBDPD-DTSi, and 396.9 °C for PDTBDPD-DTPy. No
transition was detected from the DSC characterization, during
the scan from 30 to 350 °C at a rate of 10 °C/min under
nitrogen flow.
Optical Properties. The UV−vis absorption spectra of
DTBDPD monomer and D−A polymers were recorded in
chloroform at a concentration of 10−5 M and in spin-coated
thin film. The maximum absorption wavelength of DTBDPD in
solution was 559 nm (Figure S2, top), which was 101 nm red-
shifted relative to DPBDPD (458 nm). The overwhelming red-
shift of absorption can be ascribed to the more electron-rich
Thermal Properties. The thermal behavior of the polymers
was evaluated by means of thermogravimetric analysis (TGA)
and differential scanning calorimetry (DSC). As depicted in
D
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Figure 2. TGA curves of DTBDPD polymers (10 °C/min under
nitrogen flow).
thiophene and the increased conjugation between thiophene
and BDPD. Moreover, due to the extended conjugation of the
central six-membered ring, the absorption of DTBDPD was
also at longer wavelength than DPP (548 nm). In the solid
state, the absorption spectrum of DTBDPD became broadened
and red-shifted (593 nm) and exhibited a distinguishable
structure. All these observations indicated a strong intermo-
lecular interaction of DTBDPD in the solid state, usually
associated with possible improved charge transport.
Figure 3. Absorption spectra of the DTBDPD copolymers in
chloroform solution (top) and film (bottom).
All the DTBDPD polymers absorbed in the NIR or even the
IR range. The fluorene and difluorene polymers showed two
absorptions: from 300 to 500 nm and from 500 nm to around
900 nm. The maximum absorption wavelengths were 706 nm
of PDTBDPD-F and 687 nm for PDTBDPD-DF. The slightly
(19 nm) blue-shifted absorption of PDTBDPD-DF, compared
to fluorene, could be ascribed to the twist between the fluorene
units. Polymers PDTBDPD-DTSi and PDTBDPD-DTPy
showed a main absorption band at long wavelength. The
maxima were 849 nm for PDTBDPD-DTSi and 1021 nm for
PDTBDPD-DTPy. In the solid state, absorption edges at long
wavelength for all the polymers were red-shifted (Table 1).
Comparing all the DTBDPD polymers together, we can see
red-shifted absorption wavelengths with increasing electron
donating capability of the donor moiety (Figure 1 and Table 1).
The absorption and band gaps of the DTBDPD polymers can
be tailored easily by choosing different donor strength.
Electrochemical Properties. The electrochemical proper-
ties of the monomer (Figure S3) and polymers (Figure 4) were
investigated by CV in DCM or ODCB. The dithienylbenzo-
dipyrrolidone monomer showed three reversible redox
processes during negative scan and no reversible redox under
positive potential. The energy levels were estimated to be
−3.76 eV for the LUMO and −5.43 eV for the HOMO. The
LUMO level was deeper than that of DPBDPD (−3.53 eV) and
DPP (−3.25 eV). Because of the more electron-rich thiophene
units, the HOMO level of DTBDPD increased compared to
Figure 4. CV curves of DTBDPD polymers in ODCB with Fc/Fc+ as
reference.
that of DPBDPD. As to DTBDPD copolymers, the oxidation
and reduction behavior varied with different donor unit.
PDTBDPD-F showed quasi-reversible reduction and oxidation
during negative and positive scans, while two couples of
reversible redox processes were observed for PDTBDPD-DF.
For PDTBDPD-DTSi and PDTBPD-DTPy, no obvious quasi-
Table 1. Photophysical and Electrochemical Data of DTBDPD Polymers
a
sol
max
polymer
Mn (kDa)/PDI
λ
/λonset (nm)
λfmilmax/λonset (nm)
ΔEopt (eV)
HOMO (eV)
LUMO (eV)
ΔEg (eV)
PDTBDPD-F
7.0/2.42
12.0/2.86
38/3.00
20/3.20
706/954
682/973
1.27
1.20
1.03
0.79
−5.13
−5.11
−4.78
−3.78
−3.81
−3.75
−3.78
1.35
1.30
PDTBDPD-DF
PDTBDPD-DTSi
PDTBDPD-DTPy
687/947
640/1008
878/1200
1006/1562
b
849/1118
1021/1508
b
−4.57
a
b
film
onset
Calculated from ΔEopt (eV) = 1240/λ . Calculated from the difference of LUMO level and optical band gap ΔEopt
.
E
dx.doi.org/10.1021/ma400008h | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
reversible or reversible redox was observed during positive
scans. Only two reversible redox processes were detected
during catholic scans. The LUMO levels of the polymers,
estimated from the reduction onset, were −3.78 eV for
PDTBDPD-F, −3.81 eV for PDTBDPD-DF, −3.75 eV for
PDTBDPD-DTSi, and −3.78 eV for PDTBDPD-DTPy. The
LUMO levels of the polymers did not show a substantial
difference with variation of the electron donating ability of the
donor units. They remained essentially constant and similar to
that of acceptor unit (DTBDPD) itself. The HOMO levels of
the polymers increased with the donor strength, which caused a
decrease in band gap (Table 1 and Figure 3).
REFERENCES
■
(1) (a) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J.
H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.;
Bredas, J. L.; Logdlund, M.; Salaneck, W. R. Nature 1999, 397, 121−
́
̈
128. (b) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int.
Ed. 1998, 37, 402−428. (c) Grimsdale, A. C.; Chan, K. L.; Martin, R.
E.; Jokisz, P. G.; Homlmes, A. B. Chem. Rev. 2009, 109, 897−1091.
(2) (a) Dimitrakopoulos, C. D.; Malenfant, P. R. L. Adv. Mater. 2002,
14, 99−117.
(3) (a) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007,
̈
́
107, 1324−1338. (b) Thompson, B. C.; Frechet, J. M. J. Angew. Chem.,
Int. Ed. 2008, 47, 58−77. (c) Cheng, Y.-J.; Yang, S.-H.; Hsu, C.-S.
Chem. Rev. 2009, 109, 5868−5923. (d) Martin, H.; Roar, S.; Frederik,
C. K. J. Mater. Chem. 2010, 20, 36−60. (e) Duan, C.; Huang, F.; Cao,
Y. J. Mater. Chem. 2012, 22, 10416−10434. (f) Zhang, Z.-G.; Wang, J.
J. Mater. Chem. 2012, 22, 4178−4187.
4. CONCLUSIONS
In summary, an analogue of DPP, dithienylbenzodipyrrolidone
(DTBDPD), was successfully synthesized. Comparing with
DPP, a longer wavelength absorption and even deeper LUMO
level were observed. A LUMO level as low as −3.75 eV makes
these molecules behave as strong electron withdrawing units.
Several D−A copolymers with different electron donating units
(fluorene, difluorene, dithienosilole, and dithienopyrrole) as
donors and DTBDPD as acceptor were prepared by means of
Suzuki or Stille coupling. Optical and electrochemical proper-
ties were demonstrated through UV−vis−NIR absorption and
CV. All polymers showed near-infrared or even infrared
absorption, directly related to the electron donating ability of
the donor units. A broad and long wavelength absorption
suggested good light harvesting, auguring well for the
application to organic photovoltaics. The LUMO levels were
estimated to be as low as −3.80 eV. Such low LUMO level
indicates a strong electron withdrawing capability of the
polymers, making them useful as polymer acceptors in organic
photovoltaics or other electron transporting applications.
Device performance evaluation by using DTBDPD copolymers
as donor, acceptor in solar cells, and n-type or ambipolar
transistors is on the way.
(4) (a) van Mullekom, H. A. M.; Vekemans, J. A. J. M.; Havinga, E.
E.; Meijer, E. W. Mater. Sci. Eng. R 2001, 32, 1−40. (b) Kroon, R.;
Lenes, M.; Hummelen, J. C.; Blom, P. W.; de Boer, B. Polym. Rev.
2008, 48, 531−582.
(5) (a) Wu, P.-T.; Bull, T.; Kim, F. S.; Luscombe, C. K.; Jenekhe, S.
A. Macromolecules 2009, 42, 671−681. (b) Yu, C.-Y.; Chen, C.-P.;
Chan, S.-H.; Hwang, G.-W.; Ting, C. Chem. Mater. 2009, 21, 3262−
3269. (c) 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, 732−742.
(6) (a) Kim, B.; Yeom, H. R.; Yun, M. H.; Kim, J. Y.; Yang, C.
Macromolecules 2012, 45, 8658−8664. (b) Hwang, Y.-J.; Kim, F. S.;
Xin, H.; Jenekhe, S. A. Macromolecules 2012, 45, 3732−3739.
(c) Stalder, R.; Mei, J.; Reynolds, J. R. Macromolecules 2010, 43,
8348−8352. (d) Ahmed, E.; Subramaniyan, S.; Kim, F. S.; Xin, H.;
Jenekhe, S. A. Macromolecules 2011, 44, 7207−7219.
(7) (a) Yuen, J. D.; Kumar, R.; Zakhidov, D.; Seifter, J.; Lim, B.;
Heeger, A. J.; Wudl, F. Adv. Mater. 2011, 23, 3780−3785. (b) Fan, J.;
Yuen, J. D.; Cui, W.; Seifter, J.; Mohebbi, A. R.; Wang, M.; Zhou, H.;
Heeger, A.; Wudl, F. Adv. Mater. 2012, DOI: 10.1002/
adma.201202195. (c) Fan, J.; Yuen, J. D.; Wang, M.; Seifter, J.; Seo,
J.-H.; Mohebb, A. R.; ZaKhidov, D.; Heeger, A.; Wudl, F. Adv. Mater.
2012, 24, 2186−2190.
(8) (a) Guo, X.; Watson, M. D. Org. Lett. 2008, 10, 5333−5336.
(b) Kim, F. S.; Guo, X.; Watson, M. D.; Jenekhe, S. A. Adv. Mater.
2010, 22, 478−482. (c) Guo, X.; Kim, F. S.; Jenekhe, S. A.; Watson,
M. D. J. Am. Chem. Soc. 2009, 131, 7206−7207. (d) Guo, X.; Kim, F.
S.; Seger, M. J.; Jenekhe, S. A.; Watson, M. D. Chem. Mater. 2012, 24,
1434−1442. (e) Ortiz, R. P.; Herrera, H.; Blanco, R.; Huang, H.;
Facchetti, A.; Marks, T. J.; Zheng, Y.; Segura, J. L. J. Am. Chem. Soc.
2010, 132, 8440−8452. (f) 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, 7246−7247. (g) Zhan, X.; Facchetti, A.;
Barlow, S.; Marks, T. J.; Ratner, M. A.; Wasielewski, M. R.; Marder, S.
R. Adv. Mater. 2011, 23, 268−284.
ASSOCIATED CONTENT
■
S
* Supporting Information
Information regarding single crystal X-ray diffraction data of 5
(image and CIF) and 6 (CIF), absorption and CV spectra of 6,
and NMR spectra of compounds 2, 3, 4, 5, 6, 7, polymer
PDTBDPD-F, PDTBDPD-DF, PDTBDPD-DTSi, and
PDTBDPD-DTPy. This material is available free of charge
via the Internet at http://pubs.acs.org.
(9) (a) Chan, W. K.; Chen, Y.; Peng, Z.; Yu, L. J. Am. Chem. Soc.
1993, 115, 11735−11743. (b) Beyerlein, T.; Tieke, B. Macromol. Rapid
Commun. 2000, 21, 182−189. (c) Beyerlein, T.; Tieke, B.; Foreno-
AUTHOR INFORMATION
■
Lenger, S.; Brutting, W. Synth. Met. 2002, 130, 115−119.
̈
Corresponding Author
(d) Rabindranath, A. R.; Zhu, Y.; Heim, I.; Tieke, B. Macromolecules
2006, 39, 8250−8256. (e) Cao, D.; Liu, Q. L.; Zeng, W.; Han, S.;
Peng, J.; Liu, S. Macromolecules 2006, 39, 8347−8355. (f) Zhu, Y.;
Rabindranath, A. R.; Beyerlein, T.; Tieke, B. Macromolecules 2007, 40,
6981−6989. (f) Nielsen, C. B.; Turbiez, M.; McCulloch, I. Adv. Mater.
2012, DOI: 10.1002/adma.201201795.
*E-mail: wudl@chem.ucsb.edu (F.W.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(10) (a) Burgi, L.; Turbiez, M.; Pfeiffer, R.; Bienewald, F.; Kirner, H.;
̈
■
Winnewisser, C. Adv. Mater. 2008, 20, 2217−2224. (b) Bijleveld, J. C.;
Zoombelt, A. P.; Mathijssen, S. G. J.; Wienk, M. M.; Turbiez, M.; De
Leeuw, D. M.; Janssen, R. A. J. J. Am. Chem. Soc. 2009, 131, 16616−
16617.
This work was supported by the Mitsubishi Center for
Advanced Materials, through Grant IRP-7. Profitable dis-
cussions with Prof. Daniel Little are deeply acknowledged.
Technical assistance from Dr. Guang Wu for the crystal
structure and from Dr. James Pavlovich for mass spectra is also
acknowledged.
(11) Turbiez, M.; Janssen, R.; Wienk, M.; Kirner, H.; Duggeli, M.;
̈
Tieke, B.; Zhu, Y. W.O. Patent Application 2008/00064 A1, 2008.
(12) Li, Y. U. S. Patent Application 2009/65766 A1, 2009.
F
dx.doi.org/10.1021/ma400008h | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
(13) (a) Nelson, T. L.; Young, T. M.; Liu, J.; Mishra, S. P.; Belot, J.
A.; Balliet, C. L.; Javier, A. E.; Kowalewski, T.; McCullough, R. D. Adv.
Mater. 2010, 22, 4617−4621. (b) Liu, Y.; Singh, S. P.; Sonar, P. Adv.
Mater. 2010, 22, 4862−4866. (c) Sonar, P.; Singh, S. P.; Li, Y.; Soh, M.
S.; Dodabalapur, A. Adv. Mater. 2010, 22, 5409−5413.
high, should have shown the other isomer. We thought there could be
an enol−carbonyl equilibrium related to the enol form of the lactam,
but temperature- and pH-dependent NMR spectroscopy did not
support this hypothesis. Closer examination of the alkyl region in the
NMR spectra as well as in the X-ray results indicated a deep-seated
rearrangement of the alkyl group with the possible migration of a
methyl group. We therefore examined simulated NMR spectra of
lactam N-alkylindolin-2-one with R varying n-hexyl and 3-methyl-
pentyl (Figure S20).
(14) (a) Li, Y.; Sonar, P.; Singh, S. P.; Soh, M. S.; Meurs, M.; Tan, J.
J. Am. Chem. Soc. 2011, 133, 2198−2204. (b) Ha, J. S.; Kim, K. H.;
Choi, D. H. J. Am. Chem. Soc. 2011, 133, 10364−10367. (c) Bronstein,
H.; Chen, Z.; Ashraf, R. S.; Zhang, W.; Du, J.; Durrant, J. R.; Tuladhar,
P. S.; Song, K.; Watkins, S. E.; Geerts, Y.; Wienk, M. M.; Janssen, R. A.
J.; Anthopoulos, T.; Sirringhaus, H.; Heeney, M.; McCulloch, I. J. Am.
Chem. Soc. 2011, 133, 3272−3275. (d) Kanimozhi, C.; Yaacobi-Gross,
N.; Chou, K. W.; Amassian, A.; Anthopoulos, T. D.; Patil, S. J. Am.
Chem. Soc. 2012, 134, 16532−16535. (e) Yi, Z.; Sun, X.; Zhao, Y.;
Guo, Y.; Chen, X.; Qin, K.; Yu, G.; Liu, Y. Chem. Mater. 2012, 24,
4350−4356. (f) Subramaniyan, S.; Kim, F. S.; Ren, G.; Li, H.; Jenekhe,
S. A. Macromolecules 2012, 45, 9029−9037. (g) Li, J.; Zhao, Y.; Tan,
H. S.; Guo, Y.; Di, C. A.; Yu, G.; Liu, Y.; Lin, M.; Lim, S. H.; Zhou, Y.;
Su, H.; Ong, B. S. Sci. Rep. 2012, DOI: 10.1038/srep00754.
The simulation containing N-n-hexylindolin-2-one with 3-
pentylindolin-2-one came closest in appearance to the alkyl
region of the NMR spectra of compounds 3−7; however, the
aromatic region did not show the additional peaks. Hence, this
part of the magnetic resonance spectroscopy still remains a
mystery.
(15) (a) Wienk, M. M.; Turbiez, M.; Gilot, J.; Janssen, R. A. J. Adv.
Mater. 2008, 20, 2556−2560. (b) Zou, Y.; Gendron, D.; Badrou-Aïch,
R.; Najari, A.; Tao, Y.; Leclerc, M. Macromolecules 2009, 42, 2891−
2894. (c) Huo, L.; Hou, J.; Chen, H.-Y.; Zhang, S.; Jiang, Y.; Chen, T.
L.; Yang, Y. Macromolecules 2009, 42, 6564−6571. (d) Zou, Y.;
Gendron, D.; Neagu-Plesu, R.; Leclerc, M. Macromolecules 2009, 42,
6361−6365. (e) Kanimozhi, C.; Balraju, P.; Sharma, G. D.; Patil, S. J.
Phys. Chem. B 2010, 114, 3095−3103.
(27) Geng, Y.; Culligan, S. W.; Trajkovska, A.; Wallace, J. U.; Chen,
S. H. Chem. Mater. 2003, 15, 542−549.
(28) Grisorio, R.; Dell’Aquila, A.; Romanazzi, G.; Suranna, G. P.;
Mastrorilli, P.; Cosma, P.; Acierno, D.; Amendola, E.; Ciccarellaf, G.;
Nobilea, C. F. Tetrahedron 2006, 62, 627−634.
(29) Hou, J.; Che, H.-Y.; Zhang, S.; Li, G.; Yang, Y. J. Am. Chem. Soc.
2008, 130, 16144−16145.
(16) (a) Bijleveld, J. C.; Gevaerts, V. S.; Nuzzo, D. D.; Turbiez, M.;
Mathijssen, S. G. J.; de Leeuw, D. M.; Wienk, M. M.; Janssen, R. A. J.
Adv. Mater. 2010, 22, E242−E246. (b) Woo, C. H.; Beaujuge, P. M.;
́
Holcombe, T. W.; Lee, O. P.; Frechet, J. M. J. J. Am. Chem. Soc. 2010,
132, 15547−15549. (c) Ye, L.; Zhang, S.; Ma, W.; Fan, B.; Guo, X.;
Huang, Y.; Ade, H.; Hou, J. Adv. Mater. 2012, DOI: 10.1002/
adma.201202855. (d) Dou, L.; Gao, J.; Richard, E.; You, J.; Chen, C.-
C.; Cha, K. C.; He, Y.; Li, G.; Yang, Y. J. Am. Chem. Soc. 2012, 134,
10071−10079. (f) Dou, L.; You, J.; Yang, J.; Chen, C.-C.; He, Y.;
Murase, S.; Moriarty, T.; Emery, K.; Li, G.; Yang, Y. Nat. Photonics
2012, 6, 180−185.
(17) (a) Tamayo, A. B.; Tantiwiwat, M.; Walker, B.; Nguyen, T.-Q. J.
Phys. Chem. C 2008, 112, 15543−15552. (b) Walker, B.; Tamayo, A.
B.; Dang, X.-D.; Zalar, P.; Seo, H.; Garcia, A.; Tantiwiwat, M.; Nguyen,
T.-Q. Adv. Funct. Mater. 2009, 19, 3063−3069. (c) Loser, S.; Bruns, C.
J.; Miyauchi, H.; Ortiz, R. P.; Facchetti, A.; Stupp, S. I.; Marks, T. J. J.
Am. Chem. Soc. 2011, 133, 8142−8145.
(18) Qu, S.; Wu, W.; Hua, J.; Kong, C.; Long, Y.; Tian, H. J. Phys.
Chem. C 2010, 114, 1343−1349.
(19) (a) Hofkens, J.; Verheijen, W.; Shukla, R.; Dehaen, W.; De
Schryver, F. C. Macromolecules 1998, 31, 4493−4497. (b) Verheijen,
W.; Hofkens, J.; Metten, B.; Vercammen, J.; Shukla, R.; Smet, M.;
Dehaen, W.; Engelborghs, Y.; De Schryver, F. Macromol. Chem. Phys.
2005, 206, 25−32.
(20) Greenhalgh, C. W.; Carey, J. L.; Newton, D. F. Dyes Pigm. 1980,
1, 103−120.
(21) Acker, D. S.; Hertler, W. R. J. Am. Chem. Soc. 1962, 84, 3370−
3374.
(22) Rumer, J. W.; Dai, S.-Y.; Levick, M.; Biniek, L.; Procter, D. J.;
McCulloch, I. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 1285−
1291.
(23) (a) Zhang, K.; Tieke, B. Macromolecules 2011, 44, 4596−4599.
(b) Zhang, K.; Tieke, B.; Forgie, J. C.; Vilela, F.; Skabara, P. J.
Macromolecules 2012, 45, 743−750.
(24) Cui, W.; Yuen, J.; Wudl, F. Macromolecules 2011, 44, 7869−
7873.
(25) (a) Smith, L. H. S.; Coote, S. C.; Sneddon, H. F.; Procter, D. J.
Angew. Chem., Int. Ed. 2010, 49, 5832−5844. (b) Miller, M.; Tsang,
W.; Merritt, A.; Procter, D. J. Chem. Commun. 2007, 498−500.
(26) Because the geometry of the bent structure is so dramatically
different and according to the NMR results, the “other isomer” is at
least 30% of the signal. An X-ray structure with a 10% R factor, while
G
dx.doi.org/10.1021/ma400008h | Macromolecules XXXX, XXX, XXX−XXX