Deeply colored polymers containing 1,3,4,6-tetraarylpyrrolo[3,2- b ]pyrrole-2,5-dione (IsoDPP) units in the main chain

Article abstract of DOI:10.1021/ma300483u

Synthesis and characteristic properties of polymers P1-P4 containing 1,3,4,6-tetraarylated pyrrolo[3,2-b]pyrrole-2,5-dione (isoDPP) units in the main chain are described. P1 and P2 were prepared upon palladium-catalyzed polycondensation of 3,6-bis(4-bromophenyl)-1,4-bis(4-tert-butylphenyl)pyrrolo[3, 2-b]pyrrole-2,5-dione (M1) or 1,4-bis(4-bromophenyl)-3,6-bis(4-tert-butylphenyl) pyrrolo[3,2-b]pyrrole-2,5-dione (M2) and 2,2′-(9,9-dihexyl-9H-fluoren-2,7- diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (M4), while P3 and P4 were synthesized upon polycondensation of 3,6-bis(5-bromothien-2-yl)-1,4-bis(4- dodecylphenyl)pyrrolo[3,2-b]pyrrole-2,5-dione (M3) and 2,5-bis(4,4,5,5- tetramethyl-1,3,2-dioxaborolan-2-yl)thiophene (M5) or 9-(2-ethylhexyl)-2,7- bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)carbazole (M6). Deeply colored polymers with molecular weights between 3.5 and 22 kDa were obtained. The polymers were soluble in common organic solvents such as dichloromethane, chloroform, and tetrahydrofuran. All polymers exhibit broad absorption bands with high extinction coefficients (ε > 2 × 10⁴ L mol^-1 cm^-1) but weak fluorescence, the quantum yields being below 1%. Although P1 and P2 are isomers, their optical properties are rather different. P1 with polyconjugated backbone exhibits an absorption maximum at 409 nm, while P2 has a maximum at 360 nm due to interruption of χ-conjugation at the lactam N atoms. The presence of thienyl-isoDPP units in the backbone causes a red-shift of the absorption to 489 nm (P3) and 435 nm (P4). All polymers exhibit nearly irreversible oxidation and reduction behavior. Bandgaps of the polymers with phenyl-substituted isoDPP units (P1 and P2) are at about 2 eV, while those of polymers with thienyl-substituted isoDPP (P3 and P4) are at about 1.5 eV.

Full text of DOI:10.1021/ma300483u

Article
pubs.acs.org/Macromolecules
Deeply Colored Polymers Containing 1,3,4,6-Tetraarylpyrrolo[3,2-
b]pyrrole-2,5-dione (IsoDPP) Units in the Main Chain
Irina Welterlich, Olga Charov,† and Bernd Tieke*
Department Chemie, Universitat zu Koln, Luxemburger Str. 116, D-50939 Koln, Germany
̈
̈
̈
S
* Supporting Information
ABSTRACT: Synthesis and characteristic properties of
polymers P1−P4 containing 1,3,4,6-tetraarylated pyrrolo[3,2-
b]pyrrole-2,5-dione (isoDPP) units in the main chain are
described. P1 and P2 were prepared upon palladium-catalyzed
polycondensation of 3,6-bis(4-bromophenyl)-1,4-bis(4-tert-
butylphenyl)pyrrolo[3,2-b]pyrrole-2,5-dione (M1) or 1,4-bis-
(4-bromophenyl)-3,6-bis(4-tert-butylphenyl)pyrrolo[3,2-b]-
pyrrole-2,5-dione (M2) and 2,2′-(9,9-dihexyl-9H-fluoren-2,7-
diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (M4), while
P3 and P4 were synthesized upon polycondensation of 3,6-
bis(5-bromothien-2-yl)-1,4-bis(4-dodecylphenyl)pyrrolo[3,2-b]pyrrole-2,5-dione (M3) and 2,5-bis(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)thiophene (M5) or 9-(2-ethylhexyl)-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)carbazole (M6).
Deeply colored polymers with molecular weights between 3.5 and 22 kDa were obtained. The polymers were soluble in
common organic solvents such as dichloromethane, chloroform, and tetrahydrofuran. All polymers exhibit broad absorption
bands with high extinction coefficients (ϵ > 2 × 104 L mol−1 cm−1) but weak fluorescence, the quantum yields being below 1%.
Although P1 and P2 are isomers, their optical properties are rather different. P1 with polyconjugated backbone exhibits an
absorption maximum at 409 nm, while P2 has a maximum at 360 nm due to interruption of π-conjugation at the lactam N atoms.
The presence of thienyl-isoDPP units in the backbone causes a red-shift of the absorption to 489 nm (P3) and 435 nm (P4). All
polymers exhibit nearly irreversible oxidation and reduction behavior. Bandgaps of the polymers with phenyl-substituted isoDPP
units (P1 and P2) are at about 2 eV, while those of polymers with thienyl-substituted isoDPP (P3 and P4) are at about 1.5 eV.
1. INTRODUCTION
In recent years, the tailoring of specific π-conjugated polymer
structures led to new materials with interesting optoelectronic
properties.1−4 A useful method to prepare such polymers is
based on the incorporation of deeply colored and fluorescent
organic dyes and colorants in conjugated polymers. Recent
examples are related to conjugated polymers containing the
diketopyrrolopyrrole (DPP),5,6 isoindigo,7 quinacridone,8
benzodifuranone,9 and benzodipyrrolidone chromophores.10
Especially the DPP-based polymers have proven useful for
applications as active components in light-emitting11 and
photovoltaic devices.12
Previous studies on DPP-based polymers were always
concerned with polymers containing the diketopyrrolo[3,4-
c]pyrrole (or pyrrolo[3,4-c]pyrrole-2,5-dione) unit, which is
prepared upon either condensation of arylnitriles with succinic
acid diesters13 or a reaction of diketofuro[3,4-c]furans with
arylamines to yield the corresponding bislactams.14 Besides
diketopyrrolo[3,4-c]pyrrole derivatives, the regioisomeric
diketopyrrolo[3,2-b]pyrrole (or pyrrolo[3,2-b]pyrrole-2,5-
dione, isoDPP) derivatives (Figure 1) also represent useful
organic pigments. Previously, isoDPP derivatives were prepared
in three steps starting from (N-phenylacetyl)acetic acid amino
ester15 or in one step from pulvinic acid.16 A new and efficient
synthesis was recently described by Langer et al.17 Having
Figure 1. Molecular structures of diketopyrrolo[3,4-c]pyrrole (DPP)
and regioisomeric diketopyrrolo[3,2-b]pyrrole (isoDPP) derivatives.
studied conjugated polymers with the diketopyrrolo[3,4-
c]pyrrole chromophore, we recently turned to the isomer and
prepared and characterized a number of related polymers
containing the tetraarylated diketopyrrolo[3,2-b]pyrrole unit.
Up to now, only very little has been reported on these
polymers. Recently a patent was filed on polymers containing
N,N′-dialkylated diaryl diketopyrrolo[3,2-b]pyrrole units.18
The purpose of the present article is to describe synthesis as
well as optical and electrochemical properties of a collection of
tetraaryl-isoDPP-containing conjugated polymers. Dibromi-
nated tetraaryl-isoDPP monomers were prepared upon
condensation of 2 equiv of ethyl arylacetates with 1 equiv of
oxalic acid bis(arylimidoyl) dichlorides.17 The resulting isoDPP
Received: March 8, 2012
Revised: May 14, 2012
Published: May 30, 2012
© 2012 American Chemical Society
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1
yellow solid (4.5 g, 69%); mp 180−183 °C. H NMR (300 MHz,
DMSO) δ (ppm): 7.71−7.10 (dd, a,b-8H). 13C NMR (500 MHz, d1-
CHCl3) δ (ppm): 159.0, 137.4, 132.0, 126.11, 122.9, 117.0. HRMS
(ESI): m/z [M + Na]+ 455.02.
monomers were polymerized with aryl diboronester com-
pounds using palladium-catalyzed Suzuki coupling reactions.19
The polymers were characterized using size exclusion
chromatography, spectroscopic methods (NMR, UV, fluores-
cence), and cyclic voltammetry. Deeply colored polymers of
moderate to high molecular weight and in some cases rather
low bandgaps of 1.4 to 1.8 eV were obtained, which might be
useful for electronic applications.
Oxalic Acid N,N′-Bis(4-dodecylphenyl)imidoyl Dichloride, 2c. The
synthesis was carried out in analogy to 2a except that 1c (3.90 g, 6.8
mmol) and 5.63 g (27.0 mmol) of PCl5 were used. 2c was isolated as a
yellow oil. Since the oil was not stable under ambient conditions it was
used in the next reaction step without further purification.
Synthesis of M1−M3. 3,6-Bis(4-bromophenyl)-1,4-bis(4-tert-
butylphenyl)pyrrolo[3,2-b]pyrrole-2,5-dione, M1. To a 200 mL
THF solution of the ethyl 2-(4-bromophenyl)acetate 14.17 g, 40.0
mmol) was added a THF solution of sodium bis(trimethylsilyl)amide
solution (1 M, 42.0 mL, 146.0 mmol). After stirring for 1 h the
reaction mixture was cooled to −78 °C, and a solution of 2a (7.79 g,
20.0 mmol) in 120 mL of THF was slowly added under stirring (30
min). The temperature was allowed to rise to ambient, and the mixture
was stirred for 2 h. Then the mixture was transferred to an aqueous
solution of NH4Cl (5 M, 1 L). Addition of a mixture of ether and THF
(1:1) resulted in precipitation of the product, which was isolated upon
filtration, washed with ether, dried in vacuum, and precipitated in
2. EXPERIMENTAL SECTION
2.1. Materials. Reagents. The monomers 2,2′-(9,9-dihexyl-9H-
fluoren-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (M4)
and 9-(2-ethylhexyl)-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)carbazole (M6) were prepared according to literature methods.20,21
Monomer 2,5-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
thiophene (M5) was kindly provided by BASF Schweiz AG.
Tetrakis(triphenylphosphine)palladium(0), 4-tert-butylaniline, 4-bro-
moaniline, 4-dodecylaniline, oxalyl chloride, ethyl 2-(4-bromophenyl)-
acetate, ethyl 2-thienylacetate, sodium bis(trimethylsilyl)amide
solution (1 M), N-bromosuccinimide (NBS), phosphorus penta-
chloride, and potassium carbonate were obtained from Aldrich, Acros,
and Fluka and used without further purification. Methyl tert-
butylphenylacetate was obtained from SAFC and converted into the
ethyl ester.22 Solvents were of analytical grade and used without
further purification.
1
methanol. Yield: 5.12 g (36%) of a yellow powder; mp >300 °C. H
NMR (300 MHz, d1-CHCl3) δ (ppm): 7.32−7.20 (dd, d,e-8H), 7.05−
6.94 (dd, b,c-6H), 1.34 (s, a-18H). 13C NMR (500 MHz, d1-CHCl3) δ
(ppm): 148.3, 142.8, 142.7, 135.4, 133.6, 131.8, 125.6, 122.5, 120.5,
34.8, 31.6, 31.5. IR (KBr, cm−1): 3131 (vs, CarH), 2957−2867 (v,
CH3), 1730 (v, CO), 1689 (v, CN), 1511−1418 (v, CC), 562 (v,
CBr). UV (CHCl3): 360 nm; PL (CHCl3): 628 nm (excitation at 300
nm). HRMS (ESI): m/z [M + H]+ 711.26. Anal. Calcd for
C38H34Br2N2O2: C/H 13.32. Found: C/H 13.82.
Synthesis of N,N′-Bisaryloxalamides 1a−1c. N,N′-Bis(4-tert-
butylphenyl)oxalamide, 1a. 3.79 mL (44.8 mmol) of oxalyl chloride
was dropwisely added to a stirred solution of 4-tert-butylaniline (15.00
mL, 94.2 mmol) and triethylamine (14.70 mL, 112.1 mmol) in 360
mL of THF under N2 at 0 °C. After addition, the reaction was allowed
to warm up to room temperature while it was stirred for 2 h. Then the
reaction mixture was concentrated in vacuum and diluted with 200 mL
of water. The white precipitate was collected by filtration, washed with
dilute 1 M HCl (200 mL) and water (2 × 200 mL), and dried in
vacuum. 1a was isolated as a white solid (13.5 g, 85%); mp 218−220
1,4-Bis(4-bromophenyl)-3,6-bis(4-tert-butylphenyl)pyrrolo[3,2-b]-
pyrrole-2,5-dione, M2. The synthesis was carried out in analogy to
M1 except that ethyl 2-(4-tert-butylphenyl)acetate (52 g, 20.5 mmol),
sodium bis(trimethylsilyl)amide in THF solution (21.00 mL, 1 M,
76.0 mmol), and 2b (4.35 g, 10.0 mmol) were used. M2 was isolated
1
as a yellow solid (2.3 g, 32%); mp >300 °C. H NMR (300 MHz, d1-
1
CHCl3) δ (ppm): 7.36−7.21 (dd, a,b-8H), 7.03−7.01 (dd, c,d-8H),
1.33 (s, e-18H), 6.41−6.39 (d, g-2H), 2.68−2.63 (t, f-4H), 1.65−1.63
(m, e-4H), 1.26 (s, b,c,d-12H), 0.88 (t, a-6H). HRMS (ESI): m/z [M
+ Na]+ 733.02. Anal. Calcd for C38H34Br2N2O2: C/H 13.32. Found:
C/H 14.03. UV (CHCl3): 330 nm; PL (CHCl3): 613 nm (excitation
at 300 nm).
°C. H NMR (300 MHz, d1-CHCl3) δ (ppm): 9.31 (s, h-2H), 7.61−
7.58 (d, l-4H), 7.61−7.58 (d, l-4H), 1.33 (s, a-18H). 13C NMR (300
MHz, d1-CHCl3) δ (ppm): 157.54, 148.62, 133.69, 126.11, 119.63,
34.56, 31.36. HRMS (ESI): m/z [M + Na]+ 375.32.
N,N′-Bis(4-bromophenyl)oxalamide, 1b. The synthesis was carried
out in analogy to 1a except that oxalyl chloride (3.45 mL, 40.0 mmol),
4-bromoaniline (14.9 g, 84.0 mmol), and triethylamine (6.97 mL,
100.0 mmol) were used. 1b was isolated as a white solid (15.6 g, 98%);
mp 305−308 °C. 1H NMR (300 MHz, d1-CHCl3) δ (ppm): 9.32 (s, c-
2H), 7.63−7.55 (dd, a,b-8H), 7.61−7.58 (d, l-4H). 13C NMR (500
MHz, d1-CHCl3) δ (ppm): 130.0, 120.0. HRMS (ESI): m/z [M + H]+
400.42.
1,4-Bis(4-dodecylphenyl)-3,6-bis(thiophen-2-yl)-pyrrolo[3,2-b]-
pyrrole-2,5-dione, 3. The synthesis was carried out in analogy to M1
except that ethyl 2-thienylacetate (3.24 g, 19.1 mmol), sodium
bis(trimethylsilyl)amide solution in THF solution (15.47 mL, 1 M,
76.0 mmol), and 2c (9.82 g, 22.0 mmol) were used. 3 was isolated as a
1
yellow solid (1.2 g, 23%); mp >300 °C. H NMR (300 MHz, d1-
CHCl3) δ (ppm): 7.28−7.26 (d, d-4H), 7.23−7.20 (d, e,h-6H), 6.76−
6.73 (t, f-2H), 6.41−6.39 (d, g-2H), 2.68−2.63 (t, f-4H), 1.65−1.63
(m, e-4H), 1.26 (s, b,c,d-12H), 0.88 (t, a-6H). UV (CH2Cl2): 421 nm;
PL (CH2Cl2): 635 nm (excitation at 420 nm).
N,N′-Bis(4-dodecylphenyl)oxalamide, 1c. The synthesis was
carried out in analogy to 1a except that oxalyl chloride (0.73 mL,
8.6 mmol), 4-dodecylaniline (5.00 g, 19.1 mmol), and triethylamine
(3.18 mL, 23.0 mmol) were used. 1c was isolated as a white solid (3.9
g, 35%); mp 165−167 °C. 1H NMR (300 MHz, d2-CH2Cl2) δ (ppm):
9.33 (s, f-2H), 7.61−7.25 (dd, d,e-8H), 2.51 (m, c-4H), 1.69 (s, b-
40H), 1.29 (s, a-6H). 13C NMR (500 MHz, d1-CHCl3) δ (ppm):
142.9, 135.7, 128.6, 123.9, 119.5, 75.0, 36.0, 33.0, 30.2, 24.9, 14.9.
Synthesis of Oxalic Acid N,N′-Bisarylimidoyl Dichlorides 2a−2c.
Oxalic Acid N,N′-Bis(4-tert-butylphenyl)imidoyl Dichloride, 2a.
10.57 g (30.0 mmol) of 1a and 13.7 g (66.0 mmol) of PCl5 were
dissolved in 300 mL of toluene and refluxed for 24 h under N2. Then
the solution was filtered, and the solvent and POCl3 formed upon the
reaction were removed in vacuum. The residue was recrystallized from
n-heptane to give yellow crystals. Yield 10.1 g (86%), mp 163−165 °C.
1H NMR (300 MHz, d1-CHCl3) δ (ppm): 7.47−7.4 (d, l-4H), 7.13−
Synthesis of 3,6-Bis(5-bromothiophene-2-yl)-1,4-bis(4-
dodecylphenyl)pyrrolo[3,2-b]-pyrrole-2,5-dione, M3. Compound 3
(1.50 g, 2.7 mmol) was dissolved in 100 mL of anhydrous chloroform,
covered with aluminum foil, and stirred at room temperature under N2
for 15 min. Then N-bromsuccinimide (1.18 g, 6.6 mmol) was added,
and the reaction mixture was kept at room temperature for 40 h.
Subsequently the mixture was poured into 100 mL of methanol. The
solid was collected by filtration, washed with several portions of hot
methanol, and dried in vacuum. M3 was obtained as an orange solid
(1.5 g, 79%), mp >300 °C. 1H NMR (300 MHz, d1-CHCl3) δ (ppm):
7.48−7.44 (d, l-4H), 7.24−7.21 (d, k-4H), 6.67−6.65 (d, i-2H), 6.09−
6.07 (d, j-2H), 1.37 (s, a-18H). 13C NMR (500 MHz, d1-CHCl3) δ
(ppm): 159.9, 157.9, 143.9, 142.4, 141.9, 141.1, 129.8, 129.2, 127.1,
35.6, 31.9, 31.5, 29.7, 29.6, 29.5, 29.4, 29.1, 22.7, 14.1. Anal. Calcd for
C50H62Br2N2O2S2 (946.98): C, 63.42; H, 6.60; N, 2.96. Found: C,
63.09; H, 6.52; N, 2.92. UV (CH2Cl2): 429 nm; PL (CH2Cl2): 639 nm
(excitation at 400 nm); UV (film): 420 nm. ε (DCM): 40 229 L mol−1
cm−1.
7.10 (d, k-4H), 1.35 (s, a-18H). 13C NMR (500 MHz, d1-CHCl3) δ
(ppm): 150.0, 142.8, 126.12, 125.8, 120.7, 119.5, 34.6, 31.6, 31.5, 30.1.
HRMS (ESI): m/z [M + H]+ 389.31.
Oxalic Acid N,N′-Bis(4-bromophenyl)imidoyl Dichloride, 2b. The
synthesis was carried out in analogy to 2a except that 1b (5.97 g, 15.0
mmol) and 6.87 g (33.0 mmol) of PCl5 were used. 2b was isolated as a
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a
Scheme 1. Synthesis of isoDPP Monomers M1−M3
a
Reagents and conditions: (i) THF, 0 °C; (ii) PCl5, toluene, 110 °C; (iii) THF, −78 to 0 °C; (iv) NBS, chloroform, room temperature.
Synthesis of the isoDPP Polymers. Polymer P1. 50.0 mg (0.70
mmol) of M1, 41.0 mg (0.70 mmol) of M4, and 2.4 mg (0.002 mmol)
tetrakis(triphenylphosphino)palladium(0) were dissolved in a mixture
of toluene/dioxane (1.5 mL/0.75 mL) and purged with nitrogen for
10 min. Then the mixture was heated to 100 °C, a solution of 24 mg
(0.70 mmol) of K2CO3 in 0.75 mL of Milli-QPlus-water was added, and
the mixture was stirred under reflux for 24 h. After cooling to room
temperature, the reaction mixture was diluted with 20 mL of toluene
and successively extracted with 2 × 30 mL of brine and 30 mL of
water. The organic phase was dried over MgSO4. After concentration
in vacuum the residue was poured in 20 mL of methanol to precipitate
the polymer. The polymer was filtered off and dried in vacuum. 26 mg
(42%) of an orange powder was obtained; mp >300 °C. Molecular
weight (GPC, THF, 40 °C): MW = 7.6 kDa, MN = 5.9 kDa, PD = 1.3.
1H NMR (300 MHz, d1-CHCl3): δ = 7.75−7.45 (m, f, g, h-H), 7.37−
7.15 (m, b, c, d, e-H), 1.99 (m, i-H), 1.35 (s, a-H), 1.33−1.04 (m, j-H),
0.79 (t, k-H). Anal. Calcd for C63H66N2O2: C/H 11.04. Found: C/H
10.81. UV (CHCl3): 328, 409 nm, PL (CHCl3): 625 nm (excitation at
400 nm); PL quantum yield Φ: 0.7%.
CH2Cl2) δ (ppm): 7.24−6.92 (m, d,e,f,g,h,i −14H), 2.58 (m, c-4H),
1.73−0.90 (m, b-40H), 0.83 (m, a-6H). Anal. Calcd for
C54H66N2O2S23: C, 74.44; H, 7.63; N, 3.22. Found: C, 74.56; H,
8.09; N, 2.96. UV (CH2Cl2): 489 nm, PL (CH2Cl2): 610 (sh), 720 nm
(excitation at 400 nm); PL quantum yield Φ: 0.2%.
Polymer P4. The synthesis was carried out as described for P1
except that M3 and M6 were used as monomers. A red powder was
obtained (150 mg, 69%); mp >300 °C. Molecular weight (GPC, THF,
40 °C): Mw = 6.6 kDa, MN = 4.1 kDa, PD = 1.6. 1H NMR (300 MHz,
d2-CH2Cl2) δ (ppm): 8.04−6.05 (m, j-p-18H), 4.17 (m, i-2H), 2.58−
3.12 (m, h-4H), 1.23−1.06 (m, d,e,f,g-45H), 0.85 (m, a,b,c-12H).
Anal. Calcd for C70H87N3O2S2: C, 78.83; H, 8.22; N, 3.94. Found: C,
77.02; H, 8.34; N, 3.76. UV (CH2Cl2): 345, 435 nm, PL (CH2Cl2):
643 nm (excitation at 480 nm).
1
2.2. Methods. Instrumentation. H NMR spectra were recorded
using a Bruker DPX 300 spectrometer, which operates at 300 MHz.
UV/vis absorption spectra were recorded using a Perkin-Elmer
Lambda 14 spectrometer. Photoluminescence spectra were recorded
using a Perkin-Elmer LS50B spectrometer. Photoluminescence
quantum yields were measured in chloroform (P1, P2) and
dichloromethane solution (P3, P4). The excitation wavelength was
400 nm (P1−P3) and 480 nm (P4). The values were calculated by
comparing with Rhodamine 6G in ethanol (Φf = 0.95). For size
exclusion chromatography (SEC) a Waters/Millipore UV detector 481
and an SEC column combination (Latek/Styragel 50/1000 nm pore
size) were used. All measurements were carried out in tetrahydrofuran
at 45 °C. The columns were calibrated using commercially available
polystyrene standards. High resolution mass spectrometry (HRMS)
was carried out on a Thermo LTQ Orbitap XL-linear ion trap analyzer
using electrospray ionization (ESI); resolution: normal scan. Electro-
chemical experiments were performed using a PG390 potentiostat
from Heka Co. (model PG 390, Heka Electronic, Lambrecht,
Polymer P2. The synthesis was carried out as described for P1
except that M2 instead of M1 was used as monomer. An orange-yellow
powder was obtained (165 mg, 52%); mp >300 °C. Molecular weight
1
(GPC, THF, 40 °C): MW = 3.4 kDa, MN = 2.5 kDa, PD = 1.37. H
NMR (300 MHz, d1-CHCl3): δ = 7.88−7.47 (m, f, g, h-H), 7.37−7.20
(dd, b, c-H), 7.06−6.98 (dd, d, e-H), 2.05 (m, i-H), 1.32 (s, a-H),
1.43−1.07 (m, j-H), 0.79 (t, k-H). Anal. Calcd for C63H66N2O2: C/H
11.04. Found: C/H 11.67. UV (CHCl3): 331, 360 nm (sh), PL
(CHCl3): 611 nm (excitation at 400 nm); PL quantum yield Φ: 0.2%.
Polymer P3. The synthesis was carried out as described for P1
except that M3 and M5 were used as monomers. A purple powder was
obtained (120 mg, 70%); mp >300 °C. Molecular weight (GPC, THF,
40 °C): Mw = 21 kDa, MN = 4 kDa, PD = 5.6. 1H NMR (300 MHz, d2-
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a
Scheme 2. Synthesis of Polymers P1−P4
a
Reagents and conditions: (i) Pd(PPh3)4, K2CO3, toluene/dioxane/water, reflux, 24 h; (ii) Pd(PPh3)4, K2CO3, toluene/water, reflux, 24 h.
Table 1. Molecular Weight and Optical Properties of the Polymers
polymer
MW [Da]
PD
λmax [nm]
λem [nm]
Stokes shift
ΦF [%]
extinct coeff ε (λ) [L mol−1 cm−1
]
a
a
P1
P2
P3
P4
7600
3400
1.3
1.4
5.6
1.6
328, 409
625
216
251
231
208
0.7
0.4
0.5
0.3
30947 (409 nm)
a
a
331, 360
611
27609 (360 nm)
b
b
21000
6500
489
631, 720
643
21322 (489 nm)
b
b
345, 435
19960 (345 nm)
b
10400 (435 nm)
a
b
In chloroform. In dichloromethane.
Germany). A thin film of the polymer was cast on an ITO electrode
and cycled in acetonitrile (saturated with nitrogen) containing 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6) as electrolyte
salt. Reference and counter electrodes were platinum. The voltage data
were calculated for the standard calomel electrode (SCE). Scan rate:
50 mV s−1; temperature: 20 °C. Onset potentials were determined
from the intersection of two tangents drawn at the rising and
background currents of the cyclic voltammogram.
bis(alkylimidoyl) dichlorides failed. In addition to the synthesis
of the 1,3,4,6-tetraphenylated monomers M1 and M2 it was
also possible to prepare a 1,4-diphenyl-3,6-dithienyl-substituted
derivative 3, which was subsequently brominated with NBS to
yield monomer M3. Because of the donor (D) character of the
thiophene units and the acceptor (A) character of the
diketopyrrolopyrrole units, it was hoped that D−A interactions
are introduced in the molecule leading to a bathochromic shift
of the absorption maximum, which in fact turned out to be the
case.
M1 and M2 were synthesized first. M1 carries the
polymerizable 4-bromophenyl units in the 3- and 6-position
and M2 in the 1- and 4-position. From M1 the formation of π-
conjugated polymers with the carbon−carbon chain passing
through the isoDPP units can be expected, whereas M2 should
lead to polymers with interruption of π-conjugation at the N
atoms of the lactam units. Corresponding results were already
obtained for polymerization of 1,3,4,6-tetraarylated pyrrolo[3,4-
c]pyrrole-2,5-dione (DPP) monomers.5g
3. RESULTS AND DISCUSSION
We began our studies with the synthesis of dibromo-substituted
isoDPP monomers M1, M2, and M3. The method described by
Langer et al. was applied.17 According to this method, oxalic
acid bis(arylimidoyl) dichlorides 2a−2c were reacted with ethyl
arylacetates to yield monomers M1 and M2 and compound 3
(Scheme 1). The three-stage synthesis led to the tetraarylated
isoDPP derivatives in good yield. In order to improve the
solubility, derivatives containing 4-n-dodecylphenyl groups
instead of 4-tert-butylphenyl groups were prepared. However,
attempts to prepare N-alkylated derivatives via oxalic acid
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1
Figure 2. H NMR spectra of P1, P2 in deuterated chloroform and P3, P4 in deuterated dichloromethane.
M1 and M2 were obtained as orange solids of limited
solubility in a variety of organic solvents. M1 was readily
soluble in chlorinated solvents, whereas M2 was moderately
soluble in dipolar aprotic solvents such as N-methylpyrrolidone
and dimethylformamide at high temperature. The limited
solubility of the compounds is a consequence of their highly
unsaturated structure, and the lack of long alkyl chains, which
could not be introduced directly at the lactam N atoms using
our synthetic approach. The 1H NMR spectrum of M1
displayed all the expected resonances with no discernible peaks
corresponding to impurities (see Supporting Information). A
proton NMR spectrum of M2 in chloroform was monitored at
40 °C. It shows the expected resonances (see Supporting
Information).
Optical spectra of M1 (see Supporting Information) indicate
a strong absorption maximum at 360 nm, the extinction
coefficient being near 20 000 L mol−1 cm−1, and a tail at long
wavelengths reaching to 520 nm. The fluorescence is only
weak, and the maximum occurs at 635 nm. M2 exhibits an
absorption maximum at 330 nm, a shoulder at about 360 nm,
and a long wavelength tail reaching to 520 nm. The maximum
fluorescence of M2 is at 613 nm. For both compounds,
extremely large Stokes shifts of more than 260 nm were found,
but the quantum yields of the fluorescence were below 1%.
Compound 3 was prepared upon reaction of oxalic acid N,N′-
bis(4-dodecylphenylimidoyl) dichloride (2c) with ethyl 2-
thienylacetate. The subsequent bromination of 3 with NBS
led to monomer M3 in good yield. It was also possible to
prepare M3 from ethyl 5-bromo-2-thienyl acetate and 2c in a
one-step reaction. However, the yield from this reaction was
only a few percent. Proton NMR spectra of 3 and M3 are
shown in the Supporting Information. The spectrum of 3
shows the expected signals of the thiophene protons at 6.50−
6.70 ppm. For M3 the signals appear at 6.12−6.70 ppm due to
the bromination. Optical spectra of M3 in dichloromethane
(see Supporting Information) indicate an absorption maximum
at 429 nm and a tail at long wavelengths up to 570 nm. The
absorption coefficient at λmax is 40 229 L mol−1 cm−1. The
solution is weakly fluorescing with a maximum at 639 nm, the
Stokes shift being 210 nm.
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The polymers P1−P4 were prepared upon palladium-
catalyzed Suzuki cross-coupling reaction of M1−M3 with
aromatic diboronic acid esters M4−M6 as indicated in Scheme
2.
The yields were between 42 and 70%. The molecular weights
were between 3.4 and 21 kDa. Characteristic data are compiled
in Table 1. The low molecular weight of P2 was due to the
poor solubility of M2 in the reaction medium. P1 and P2 were
obtained as deep red and orange red powders, respectively,
while P3 and P4 were obtained as purple and red powders. P1
was only slightly soluble in common organic solvents such as
THF, dichloromethane, and chloroform, whereas P2 was
moderately soluble in various organic solvents. P3 and P4
were readily soluble in common organic solvents except
alcohols. The molecular structure was analyzed using 1H
1
NMR spectroscopy. In Figure 2, the H NMR spectra of the
four polymers are shown. P1 and P2 exhibit typical proton
signals from the alkyl groups between 0.7 and 1.5 ppm, the
strong signal at 1.33 ppm originating from the tert-butyl groups.
The signal at about 2 ppm originates from the α-CH2 protons
of 9,9′-dihexylfluorene. The signals of the aromatic protons
occur at 6.9−7.9 ppm. P3 and P4 exhibit strong signals of the
dodecyl protons of isoDPP at 0.7−2.4 ppm and signals of the
aromatic protons at 6.5−8.0 ppm. Additional signals of the 2-
ethylhexyl protons appear at 2.5 and 4.2 ppm.
Optical Properties. Optical absorption and fluorescence
spectra of the polymers are shown in Figure 3. All polymers are
deeply colored, the extinction coefficients of the strongest
bands being higher than 20 000 L cm−1 mol−1 (see also Table
1).
Figure 3. UV/vis absorption and emission spectra of P1, P2 in
chloroform and P3, P4 in dichloromethane.
Scheme 3. Molecular Structures of Four Isomeric Polymers Containing isoDPP Units (Upper Row) and DPP Units (Lower
a
Row) with Conjugated Main Chain (Left) and Nonconjugated Chain (Right)
a
The conjugated chain is indicated in blue, and the nonconjugated parts are indicated in red and green. Data of DPP polymers are taken from ref 5g.
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isomeric isoDPP- and DPP-containing polymers are displayed
in Scheme 3.
It is striking that the absorption maxima of the isoDPP-
containing polymers occur at much shorter wavelengths than
those of the related DPP polymers. It has its origin in a less
efficient coupling of the aryl groups with the adjacent
diketopyrrolopyrrole central unit in the isoDPP derivatives.23
The UV/vis spectra also indicate that the absorption maxima of
the thienyl-substituted polymers P3 and P4 are considerably
red-shifted compared with the phenyl-substituted DPP-
polymers P1 and P2 (see Table 1 and Figure 3). This can be
ascribed to donor−acceptor interactions between the thienyl
groups and the adjacent isoDPP core and between the
comonomer units along the backbone. The four polymers
exhibit a weak red fluorescence with maxima between 611 and
720 nm (Figure 3). The quantum yield is low and does not
exceed 0.7% (for details see Table 1). Photographic images of
the polymer solutions in different solvents are shown in Figure
4.
In order to determine the optical bandgaps of the polymers,
thin films were cast from solution and the onset wavelength
λabs,onset of the absorption bands were determined. UV/vis
absorption spectra of the films are shown in the Supporting
Information, and the optical data are compiled in Table 2. It
can be seen that the thienyl-substituted isoDPP polymers P3
and P4 exhibit bandgaps of about 1.5 eV, whereas the phenyl-
substituted polymers P1 and P2 exhibit larger bandgaps of
about 2 eV. The optical bandgap of P2 is about 0.15 eV larger
than for P1 due to the lack of π-conjugation in the main chain.
Electrochemical Properties. In order to study the
electrochemical properties, films of the four polymers were
cast on ITO-coated glass slides, and cyclic voltammograms (CV
diagrams) were recorded in the potential range from −2.5 to
+1.5 V vs FOC. The CV diagrams are shown in Figure 5.
It can be seen that anodic oxidation of P3 and P4 sets in at
low potentials of 0.38 and 0.48 V. Two anodic waves with
maxima at 0.5 and 0.8 V occur. Anodic oxidation of P1 and P2
sets in at 0.59 and 0.42 V. Only single peaks occur with maxima
at 0.8 and 0.7 V, respectively. Upon cathodic reduction, single
peaks with maxima at −1.6 V (P1, P2), and −1.3 V (P3, P4)
occur.
Figure 4. Colors of P1 (a), P2 (b), P3 (c), and P4 (d) in various
solvents.
P1 and P2 exhibit a strong absorption maximum at about
330 nm and a second maximum either at 409 nm (P1) or 360
nm (P2). Compared with monomer M1, the absorption of P1
is red-shifted by 64 nm, which can be ascribed to the π-
conjugation of the backbone. In contrast, the 360 nm maximum
of P2 correlates with a shoulder in the absorption band of M2
at the same wavelength. This indicates a missing red-shift for
P2, which can be explained with an interruption of π-
conjugation at the N atoms of the lactam units, the backbone
containing electronically isolated isoDPP and phenylene−
fluorene−phenylene units. A similar effect was reported for
regioisomeric DPP polymers, in which a tetraarylated
diketopyrrolo[3,4-c]pyrrole unit was incorporated in the
backbone via the 3- and 6-positions or the 1- and 4-positions.5g
The molecular structures and absorption maxima of the
The oxidation and reduction behavior of all polymers is
irreversible. From the onsets of the anodic and cathodic waves
HOMO and LUMO values as well as electrochemical bandgaps
were calculated. As shown in Table 2, the electrochemical
bandgap values are similar to the optical ones. The thienyl-
substituted isoDPP polymers exhibit the lowest bandgaps of
about 1.4−1.5 eV, whereas the values for the phenyl-substituted
isoDPP polymers are clearly higher (Table 2).
a
Table 2. Bandgap Data of P1−P4
polymer
λabs,onset of film [nm]
oxidation onset [V] {HOMO [eV]}
reduction onset [V] {LUMO [eV]}
opt/electrochem bandgap [eV]
P1
P2
P3
P4
640
599
746
698
0.59 { −5.39}
0.42 { −5.22}
0.38 {−5.18}
0.48{ −5.28}
−1.36 { −3.44}
−1.41 { −3.39}
−1.04 { −3.76}
−0.99 { −3.81}
1.94/1.95
2.07/1.83
1.66/1.42
1.78/1.47
a
Optical bandgap Eopt was measured at the onset of absorption of polymer film (Eopt = 1240/λonset eV). HOMO−LUMO gap was calculated
according to the equation −ELUMO = Eonset(red) + 4.8 eV and −EHOMO = Eonset(ox) + 4.8 eV, were Eonset(ox) and Eonset(red) are onset potentials for
oxidation and reduction processes vs Fc/Fc+ couple.
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Figure 5. Cyclic voltammograms of P1−P4 (a−d). Scan rate: 50 mV s−1; temperature: 20 °C; electrolyte solution: 0.1 M tetrabutylammonium
hexafluorophosphate in acetonitrile.
absorption spectra of films of P1−P4. This material is available
free of charge via the Internet at http://pubs.acs.org.
4. CONCLUSIONS
Our study shows that conjugated polymers containing 1,3,4,6-
tetraarylated diketopyrrolo[3,2-b]pyrrole (isoDPP) units can be
prepared in only few reaction steps. For the synthesis of the
monomers, the method described by Langer et al. can be
applied. The polymers are subsequently prepared in one step
using established palladium-catalyzed polycondensation proce-
dures such as Suzuki coupling. Deeply colored polymers with
absorption coefficients higher than 20 000 L mol−1 cm−1 are
obtained. Variation of the chromophore structure by use of
phenyl- or thienyl-substituted isoDPP units and incorporation
of isoDPP units in conjugated polymers via the 3- and 6-
position or 1- and 4-position lead to remarkable differences in
the optoelectronic and electrochemical properties. They
qualitatively resemble those observed for the DPP polymers.
However, less efficient coupling between aryl groups and the
adjacent central pyrrolopyrrole unit23 causes a blue-shift of the
absorption maxima, and the colors of the polymers are less
brilliant. Moreover, the quantum yields of fluorescence are not
higher than 0.7%. Despite of these drawbacks the polymers
exhibit a deep and broad absorption in the visible, which
renders the compounds attractive for applications as dyes and
colorants. They also might be useful as comonomers in
photovoltaic polymers increasing the light absorption in the
visible. Further studies will be concerned with the preparation
of DPP/isoDPP copolymers, from which a very broad
absorption in the visible can be expected.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: tieke@uni-koeln.de.
Present Address
†Universitat Bremen, FB 4 Produktionstechnik, D-28359
̈
Bremen, Germany.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support by BASF Schweiz AG, Basle, Switzerland, is
gratefully acknowledged. Drs. P. Hayoz, M. Duggeli, and R.
Lenz from BASF Schweiz AG are kindly thanked for helpful
discussions.
̈
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* Supporting Information
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