Red emitting N-functionalized poly(1,4-diketo-3,6-diphenylpyrrolo[3,4-c] pyrrole) (poly-DPP): A deeply colored polymer with unusually large stokes shift

Article abstract of DOI:10.1021/ma061024e

New π-conjugated polymers are described, which entirely consist of aryl-aryl-coupled 1,4-diketo-2,5-dialkyl-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP) units. The alkyl groups are n-hexyl (4a) or 2-ethylhexyl (4b) groups. The polymers are synthesized using ni

Full text of DOI:10.1021/ma061024e

8250
Macromolecules 2006, 39, 8250-8256
Red Emitting N-Functionalized
Poly(1,4-diketo-3,6-diphenylpyrrolo[3,4-c]pyrrole) (Poly-DPP): A
Deeply Colored Polymer with Unusually Large Stokes Shift
A. R. Rabindranath, Y. Zhu, I. Heim, and B. Tieke*
Institut fu¨r Physikalische Chemie der UniVersita¨t zu Ko¨ln, Luxemburgerstrasse 116,
D-50939 Ko¨ln, Germany
ReceiVed May 8, 2006; ReVised Manuscript ReceiVed September 15, 2006
ABSTRACT: New π-conjugated polymers are described, which entirely consist of aryl-aryl-coupled 1,4-diketo-
2,5-dialkyl-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP) units. The alkyl groups are n-hexyl (4a) or 2-ethylhexyl (4b)
groups. The polymers are synthesized using nickel-promoted and palladium-catalyzed one-pot coupling reactions
starting from 1,4-diketo-2,5-dialkyl-3,6-di(4-bromophenyl)pyrrolo[3,4-c]pyrrole (dibromo-DPP) as monomer, or
conventional palladium-catalyzed coupling of dibromo-DPP and the 3,6-bis(pinacolato)boronester derivative of
DPP. Polymers exhibit molecular weights up to 33 000 and are excellently soluble in chloroform and toluene.
Deep red solutions (λmax of 4b: 525 nm) with intense red photoemission (λmax of 4b: 632 nm) and unusually
large Stokes shift up to 107 nm are obtained. Cyclic voltammetry indicates quasi-reversible oxidation and reduction
behavior, the onset potentials of 4b are +1.03 and -0.74 V vs SCE, respectively. From these potentials, HOMO/
LUMO levels of -5.47 and -3.70 eV, respectively, can be calculated. Good solubility and processability into
thin films render the compounds suitable for electronic applications.
1. Introduction
routes basically follow the Pd-catalyzed arene coupling de-
scribed by Suzuki and Miyaura.14 The coupling reactions are
either carried out as one-pot, two-step reaction using bis-
(pinacolato)diboron and the corresponding dibromo-DPP deriva-
tive,15,16 or they follow the conventional palladium-catalyzed
polycondensation of the 1:1 molar mixture of the dibromo-DPP
and the diboronester of DPP.17 In our contribution, the polymers
are characterized with regard to molecular structure, molecular
weight and weight distribution, optical and electrochemical
properties using size exclusion chromatography (SEC), 1H
NMR, UV/vis absorption, and fluorescence spectroscopy, and
cyclic voltammetry.
In our contribution, new red emitting polymers based on the
N-alkylated 1,4-diketo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP)
unit are reported, the common structure of the polymer being
The alkyl substituents groups R are either n-hexyl or 2-ethyl-
hexyl.
2. Experimental Section
Materials. 1-Bromohexane, 1-bromo-2-ethylhexane, tetrakis-
(triphenylphosphine)palladium(0), potassium tert-butoxide, 2,2-
bipyridine, bis(1,5-cyclooctadiene)nickel, 1,5-cyclooctadiene (cod),
palladium(II) acetate, diphenylphosphinoferrocene (dppf), palla-
dium(II) chloride, and acetonitrile (HPLC grade) were obtained from
Acros, Fluka, and Aldrich and used without further purification.
Bis(pinacolato)diboron (bpdb) was purchased from Combi-Blocks
Inc. Tetrahydrofuran (THF) was distilled over sodium hydride under
nitrogen. N-Methyl-2-pyrrolidone (NMP) and N,N-dimethylforma-
mide (DMF) were distilled over CaH2. Chro-matographic separa-
tions were carried out using Acros silica gel 60 (0.060-0.200 mm).
1,4-Diketo-3,6-bis(4-bromophenyl)pyrrolo[3,4-c]pyrrole was kindly
supplied by Dr. M. Dueggeli and Dr. R. Lenz from Ciba Specialty
Chemicals, Basle, Switzerland.
DPP and some of its derivatives are commercialized as high
performance pigments with exceptional light, weather and heat
stability.1,2 Recent studies have shown that the incorporation
of DPP units into polymers,3-9 dendrimers,10 polymer-surfac-
tant complexes,11 and oligomers12 results in deeply colored,
highly photoluminescent4-12 and electroluminescent8,9 materials.
In all DPP-containing polymers reported so far, the DPP units
were copolymerized with other monomer units, either in
polyester polycondensation,6 polyaddition,6 or palladium-
catalyzed Stille3 and Suzuki7-9,12 polycondensation reactions.
In the present article we report on new DPP-based homopoly-
mers, which only contain the N-alkylated DPP-chromophore as
para-linked repeating unit (the so-called “poly-DPP”). The
polymer is deep red and shows an intense bordeaux-red
photoemission. Several preparation routes are described. They
always start from the dialkylated 1,4-diketo-3,6-bis(4-bromo-
phenyl)pyrrole[3,4-c]pyrrole7 and lead to the polymer in either
one or two reaction steps. The first route follows the well-
established Ni-promoted coupling of dihaloarenes.13 The other
1
Instrumentation. H NMR spectra were recorded on a Bruker
1
AC 300 spectrometer operating at 300 MHz for H experiments.
UV/vis absorption spectra were recorded on a Perkin-Elmer Lambda
14 spectrometer. Photoluminescence spectra were recorded on a
Perkin-Elmer LS50B spectrometer. Molecular weights were deter-
mined upon size exclusion chromatography (SEC) using a Waters/
Millipore UV detector 481 and an SEC column combination (Latek/
Styragel 50/1000) nm pore size). All measurements were carried
out in tetrahydrofuran at 45 °C. The columns were calibrated vs
commercially available polystyrene standards. Cyclic voltammo-
grams were recorded using a potentiostat PG390 from Heka
* To whom correspondence should be addressed, tieke@uni-koeln.de
(coauthors: Aravinda.Rabindranath@uni-koeln.de, yzhu@uni-koeln.de, in-
goheim@ hotmail.com).
10.1021/ma061024e CCC: $33.50 © 2006 American Chemical Society
Published on Web 10/25/2006

Macromolecules, Vol. 39, No. 24, 2006
New π-Conjugated Polymers 8251
electronic, Lambrecht, Germany. For data acquisition and poten-
tiostat control, the POTPULSE software (Heka), version 8.4 was
used. A thin film of the polymer was cast on a Pt electrode and
cycled in CH3CN containing 0.1 M tetrabutylammoniumhexafluoro-
phosphate. Counter electrode: Au. Reference: Pt. Voltage data were
calculated for standard calomel electrode (SCE). Scan rate: 40 mV
s-1. T: 20 °C.
Monomer Synthesis. 1,4-Diketo-2,5-dihexyl-3,6-bis(4-bro-
mophenyl)pyrrolo[3,4-c]pyrrole (2a). First, 8.92 g (20 mmol) of
1,4-diketo-3,6-bis(4-bromophenyl)pyrrolo[3,4-c]pyrrole (1), 4.94 g
(44 mmol) of potassium tert-butoxide, and 150 mL of dry NMP
were heated to 60 °C. 16.9 mL (120 mmol) 1-bromohexane was
slowly added and the mixture was stirred at 60 °C for 18 h. After
cooling to room temperature, 250 mL of toluene was added and
the reaction mixture was washed with water to remove the NMP.
The organic solution was concentrated using a rotary evaporator.
The raw material was purified by column chromatography on silica
using dichloromethane as the solvent. 5.1 g (42%) of an orange,
polycrystalline powder were obtained. The melting point was 183
°C. 1H NMR (CDCl3, 300 MHz): δ (ppm) 0.70-1.0 (m, 6H,
methyl), 1.00-1.25 (m, 12H, methylene), 1.58 (m, 4H, â-CH2),
3.70 (t, 4H, N-CH2), 7.65 (d, 4H), 7.66 (d, 4H). UV(CHCl3): 476
nm; photoluminescence (PL): 533 nm.
orange solid was filtered off, rinsed with water, and dried under
ambient conditions. The pure product was obtained as orange
powder. Upon heating, the solid product decomposed at 204 °C.
1H NMR (CDCl3, 300 MHz) δ (ppm): 0.64-1.00 (m, 12H, methyl),
1.00-1.25 (m, 16H, methylene), 1.36 (s, 24H, B-O-C-CH3), 1.56
(m, 2H, â-CH), 3.72 (d, 4H, N-CH2), 7.74 (d, 4H), 7.94 (d, 4H).
Polymer Synthesis. Synthesis of P-4a-1. A 2.0 g (7.27 mmol)
sample of bis(1,5-cyclooctadiene)nickel(0), 1.136 g (7.27 mmol)
of 2,2-bipyridine, and 0.7 mL (6.06 mmol) of 1,5-cyclooctadiene
were dissolved in 65 mL of DMF under nitrogen. To this mixture,
3.722 g (6.06 mmol) of solid 1,4-diketo-2,5-dihexyl-3,6-bis(4-
bromophenyl)pyrrolo[3,4-c]pyrrole (2a) were added in small por-
tions within 90 min. Then the mixture was heated to 60 °C and
stirred for 48 h. After cooling to room temperature, it was poured
into 500 mL of methanol acidified with aqueous HCl. Then the
precipitate was filtered and washed with another 500 mL of the
acidified methanol. Finally the precipitate was collected, succes-
sively washed with 500 mL of methanol, 250 mL of EDTA solution
(pH ) 3.8), 250 mL of EDTA solution (pH ) 9) and 300 mL of
water, and dried in a vacuum. A 2.714 g (99%) yield of a dark red
powder were obtained. The solid polymer decomposed at a
temperature of approximately 295 °C. Molecular weight: 3100.
Polydispersity: 1.6. 1H NMR (CDCl3, 300 MHz): δ (ppm) 0.74-
1.20 (alkyl-H); 3.63 (N-CH2); 7.29 (aromatic CH); 7.67 (aromatic
CH).
1,4-Diketo-2,5-di(2-ethylhexyl)-3,6-bis(4-bromophenyl)pyrrolo-
[3,4-c]pyrrole (2b). First, 8 g (18 mmol) of 1,4-diketo-3,6-bis(4-
bromophenyl)pyrrolo[3,4-c]pyrrole (1) and 4.0255 g (36 mmol) of
potassium tert-butoxide were suspended in dry NMP and vigorously
stirred at 60 °C for 30 min, and then 20.8 g (108 mmol) of 1-bromo-
2-ethylhexane was added dropwise over a period of 90 min. After
20 h, the reaction mixture was diluted with 100 mL of toluene and
washed with water and brine several times. The organic phase was
dried over magnesium sulfate and concentrated using a rotary
evaporator. The crude product was purified by column chroma-
tography on silica using toluene as the solvent. Thus, 2.7 g (23%)
of bright orange needles were obtained. The melting point was 128
Synthesis of P-4a-2. Under nitrogen atmosphere, 0.1224 g (0.2
mmol) of 1,4-diketo-2,5-dihexyl-3,6-bis(4-bromophenyl)pyrrolo-
[3,4-c]pyrrole (2a), 0.122 g (0.48 mmol) of bis(pinacolato)diboron,
0.180 g (0.6 mmol) of potassium acetate, 1.5 mg (3 mol %) of
palladium(II) chloride and 9 mg of dppf were dissolved in 30 mL
of DMF. The mixture was heated to 80 °C for 100 min until no
starting material (2a) could be detected anymore. After that, 5 mg
(2 mol %) of tetrakis(triphenylphosphine)palladium, 0.1224 g (0.2
mmol) of 2a and 0.089 g (0.6 mmol) of potassium carbonate in
aqueous solution were added to the mixture. Simultaneously 30
mL of toluene was added to dissolve all solids. The reaction mixture
was heated to 95 °C for 12 h. After the mixture was cooled, 200
mL of toluene was added, and the mixture was extracted with water
to remove DMF and salts. The organic solution was dried in a
vacuum, redissolved in a minimum amount of chloroform, and
precipitated in ethanol. The whole procedure was carried out twice.
The polymer was collected and dried in a vacuum to give 0.147 g
(82%) of a black powder. The polymer did not melt or decompose
1
°C. H NMR (CDCl3, 300 MHz) δ (ppm): 0.67-1.00 (m, 12H,
methyl), 1.00-1.25 (m, 16H, methylene), 1.55 (m, 2H, â-CH), 3.69
(d, 4H, N-CH2), 7.63 (s, 8H).
1,4-Diketo-2,5-dihexylpyrrolo[3,4-c]pyrrole-3,6-diphenyl-4-pi-
nacolato Boron Ester (3a). First, 1 g (1.63 mmol) of 1,4-diketo-
2,5-dihexyl-3,6-bis(4-bromophenyl)pyrrolo[3,4-c]pyrrole (2a) and
0.91 g (3.59 mmol) of bis(pinacolato)diboron were dissolved in
60 mL of degassed DMF followed by the addition of 0.022 g (6
mol %) of palladium(II) acetate and 0.9593 g of (9.80 mmol)
potassium acetate. The reaction mixture was vigorously stirred under
nitrogen at 80 °C for 2 h. During this period, the progress of the
reaction was monitored by thin-layer chromatography (TLC) (silica,
dichlormethane/MeOH, 10:1). After completion, the reaction
mixture was poured in distilled water to induce the precipitation
of the crude product. A red solid formed, which was filtered off,
rinsed with water, and dried under ambient conditions. The crude
product was dissolved in a minimum amount of dichloromethane
and poured into ethanol to precipitate polymeric side products. After
filtration, the mother liquor was concentrated using a rotary
evaporator. Upon cooling, 0.7 g (61%) of the pure product were
1
up to 295 °C. Molecular weight: 8700. Polydispersity: 1.6. H
NMR (CDCl3, 300 MHz): δ (ppm) 0.70-1.80 (alkyl-H); 3.73
(N-CH2); 7.38 (aromatic CH); 7.76 (aromatic CH).
Synthesis of P-4a-3. In a 20 mL Schlenk tube, 0.061 g (0.1
mmol) of 2a and 0.070 g (0.1 mmol) of 3a were dissolved in
degassed toluene at room temperature. After addition of 5.7 mg (5
mol %) of tetrakis(triphenylphosphine)palladium(0) and finally 0.3
mL of 2 M aqueous sodium carbonate solution, the reaction mixture
was stirred under nitrogen at 90 °C for 24 h. The mixture was then
diluted with an appropriate amount of toluene, cooled to room
temperature, and washed with water and brine several times. The
aqueous phase was separated and extracted with chloroform. The
combined organic phases were dried over magnesium sulfate and
concentrated in a vacuum. An excess of methanol was used to
precipitate the polymer which was filtered off, washed with
methanol, and dried under ambient conditions. Finally 0.060 g
(66%) of a dark red solid were obtained. The polymer did not melt
or decompose up to 295 °C. Molecular weight: 6500. Polydisper-
sity: 1.6. 1H NMR (CDCl3, 300 MHz): δ (ppm) 0.74-1.70 (alkyl-
H); 3.85 (N-CH2); 7.35 (aromatic CH); 7.75 (aromatic CH).
Synthesis of P-4b-4. In a 100 mL round-bottom flask, 0.5 g
(0.75 mmol) of 1,4-diketo-2,5-di(2-ethylhexyl)-3,6-bis(4-bromophe-
nyl)pyrrolo[3,4-c]pyrrole (2b) and 0.417 g (1.64 mmol) of bis-
(pinacolato)diboron were dissolved in 30 mL of degassed DMF
followed by the addition of 5 mg (3 mol %) of palladium(II) acetate
and 0.22 g (2.24 mmol) potassium acetate. The reaction mixture
was stirred under nitrogen at 80 °C. The progress of the reaction
was monitored by TLC on silica with dichloromethane as the
1
obtained as bright red flakes. The melting point was 235 °C. H
NMR (CDCl3, 300 MHz): δ (ppm) 0.70-1.0 (m, 6H, methyl),
1.00-1.25 (m, 12H, methylene), 1.41 (s, 24H, B-O-C-CH3), 1.58
(m, 4H, â-CH2), 3.75 (t, 4H, N-CH2), 7.80 (d, 4H), 7.95 (d, 4H).
1,4-Diketo-2,5-di(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-3,6-di-
phenyl-4-pinacolato Boron Ester (3b). In a 100 mL round-bottom
flask, 0.5 g (0.75 mmol) of 1,4-diketo-2,5-di(2-ethylhexyl)-3,6-bis-
(4-bromophenyl)pyrrolo[3,4-c]pyrrole (2b) and 0.417 g (1.64 mmol)
of bis(pinacolato)diboron were dissolved in 30 mL of degassed
DMF followed by the addition of 5 mg (3 mol %) of palladium(II)
acetate and 0.22 g (2.24 mmol) of potassium acetate. The reaction
mixture was stirred under nitrogen at 80 °C. The progress of the
reaction was monitored by TLC on silica using dichloromethane
as solvent. After 150 min, the starting material was almost
quantitatively converted and the reaction mixture was poured into
distilled water to induce precipitation of the crude product. The

8252 Rabindranath et al.
Macromolecules, Vol. 39, No. 24, 2006
Scheme 1. Preparation Route to Monomers 2a, 2b and 3a, 3b
solvent. After 140 min, it was indicated that the starting material
had been completely reacted. Then the reaction was turned to Suzuki
conditions by adding 20 mL of toluene, another 0.5 g (0.75 mmol)
of 2b, 0.043 g (5 mol %) of tetrakis(triphenylphosphine)palladium-
(0), and 2.5 mL of 2 M aqueous potassium carbonate solution. The
temperature was raised to 90 °C, and the reaction mixture was
stirred for 12 h under nitrogen. Then it was diluted with ca. 20 mL
of toluene, cooled to room temperature, and washed with water
several times. The organic phase was dried over magnesium sulfate
and concentrated using a rotary evaporator. The polymer was
precipitated in a large excess of ethanol, filtered off, and washed
with ethanol. After drying, 0.69 g (90%) of a brilliant red powder
was obtained. The polymer did not melt or decompose up to 295
°C. Molecular weight: 33 000. Polydispersity: 3.1. 1H NMR
(CDCl3, 300 MHz): δ (ppm) 0.74-1.70 (alkyl-H); 3.85 (N-CH2);
7.35 (aromatic CH); 7.75 (aromatic CH).
coupling of dihaloarenes, which is promoted by Ni(0) complexes
such as Ni(cod)2.18 In a first approach to prepare poly-DPP we
have chosen this well-known method. The reaction was carried
out using Ni(cod)2, cod and 2,2′-bpy as coupling reagents. The
resulting polymer is denoted as 4a-1 (“a” for the hexyl
substituent groups, and “1” for the first approach).
The Ni(0) coupling has some disadvantages, which favored
the search for more suitable methods of dehalogenative cou-
pling: (i) Ni(cod)2 is highly sensitive to moisture so that the
reaction has to be carried out under completely dry conditions,
(ii) large amounts of nickel are needed (1.0-1.2 equiv to the
monomers), so that (iii) the workup procedure is tedious because
large nickel residues have to be removed. As an alternative, we
used a recently reported one-pot variant16 of the well-known
Suzuki-Miyaura cross-coupling. According to this route, 2a is
converted into 3a upon addition of bis(pinacolato)diboron as
condensation reagent and PdCl2(dppf) as catalyst. 3a is not
worked up, but immediately polymerized upon further addition
of 2a, potassium carbonate and tetrakis(triphenylphosphine)-
palladium as the catalyst. The polymer obtained from this
procedure is denoted as 4a-2. The two one-pot polycondensation
routes to poly-DPP are outlined in Scheme 2.
3. Results and Discussion
3.1. Synthesis of Monomers 2a and 2b. The starting
compound for the synthesis of the polymer is the dibrominated
DPP derivative 1. Because of the hydrogen-bonding of the
lactam units, 1 is poorly soluble in common organic solvents
and cannot directly be used as monomer. To increase the
solubility, the lactam groups were either alkylated upon reaction
with 1-bromohexane or with 1-bromo-2-ethylhexane (Scheme
1). The resulting alkylated DPP derivatives 2a and 2b were
purified using column chromatography. After purification, bright
red polycrystalline materials with melting points of 183 and
128 °C, respectively, and high solubility in common organic
solvents was obtained. The UV spectrum of 2a indicates two
peaks at 476 and 305 nm (Figure 1). The corresponding
spectrum of 2b is similar: a long wavelength peak occurs at
476 nm and a shoulder at 300 nm (Figure 4). The two monomers
2a and 2b are highly fluorescent with maximum emission at
533 and 525 nm, respectively.
Besides the one-pot routes we also applied the classical Pd-
catalyzed Suzuki polycondensation starting from equimolar
amounts of 2a and 3a outlined in Scheme 3. This method is
generally applicable for the coupling of DPP derivatives with a
variety of aromatic units. First, the DPP bis(pinacolato)-
diboronester 3a has to be prepared from 2a (Scheme 1). After
purification of 3a, a 1:1 molar mixture of 2a and 3a is
polymerized as described in the experimental part. The polymer
obtained upon the Suzuki coupling is denoted as polymer 4a-3
(Scheme 3).
3.3. Characteristic Properties of Polymer 4a-1 to 4a-3.
Molecular Weight. In this chapter, the characteristic properties
of dihexylated poly-DPP obtained upon the three different
preparation methods are compared. The polymer is obtained as
a dark red (in a single case almost black) powder, and is well
soluble in organic solvents such as chloroform and toluene, but
less soluble in THF. In polymer 4a-1, most of the nickel could
be removed after careful cleaning, the residual nickel content
was 14 mg/kg. In Table 1, the weight-average molecular weight,
the polydispersity, and the optical properties are listed. Size
exclusion chromatography indicated that the Ni-mediated po-
lymerization only leads to oligomers with molecular weight of
3100, which corresponds to an average degree of polymerization
of about 7. Different from this, the Pd-catalyzed one-pot
coupling route leads to a polymer with molecular weight of 8700
corresponding to a degree of polymerization of about 20. The
molecular weight obtained upon the classical Suzuki route is
somewhat lower. For all polymers, the polydispersity is 1.6.
Our study clearly indicates that the Pd-catalyzed one-pot reaction
is superior to the Ni-promoted reaction. It proceeds more rapidly,
and the yield and molecular weight of the polymer are higher.
3.2. Synthetic Routes to Polymer 4a. One of the classical
methods for preparation of polyarylenes is the dehalogenating
Figure 1. UV/visible and photoluminescence spectra of monomer 2a
and polymer 4a-2 in chloroform.

Macromolecules, Vol. 39, No. 24, 2006
New π-Conjugated Polymers 8253
Figure 2. 1H NMR spectra of monomers 2a, 3a, and polymers 4a-2 and 4a-1 (partial).
Figure 4. UV/visible and photoluminescence spectra of monomer 2b
and polymer 4b in chloroform.
Figure 3. Cyclic voltammogram of polymer 4a-2 as thin film on Pt
electrode (reference SCE). The CV was carried out in 0.1 M (TBA)-
PF6 in acetonitrile, scan rate: 40 mV/s, T ) 20 °C.
the quantum yield of fluorescence was determined to be 13%.
This value was much lower than for the corresponding monomer
2a (95%). The difference might be due to the partial O-
alkylation during polymerization, and to traces of phosphor and
palladium in the polymer, which still might be present after
applying standard cleaning procedures as described in the
experimental part. From the fluorescence behavior a Stokes shift
of 103 nm can be derived, the largest value ever reported for a
DPP-containing polymer. UV and fluorescence spectra of
polymer films cast from chloroform solution are very similar
to the solution spectra with the exception that the absorption
and emission maxima are red-shifted by 7 to 8 nm (not shown;
for data, see Table 1).
UV Absorption and Fluorescence Spectra. The optical
properties of the dihexylated poly-DPPs 4a-1 to 4a-3 are very
similar. As the UV absorption spectrum of 4a-2 in chloroform
indicates, the polymer exhibits three maxima at 528, 340, and
296 nm (Figure 1). The absorption coefficient (ꢀ value) of the
528 nm peak is 1.36 × 104 L monomol-1 cm-1. The polym-
erization is accompanied by a large bathochromic shift from
476 to 528 nm, which originates from the extension of the
π-conjugated system. Although 4a-1 is only an oligomer, its
spectrum is quite similar to 4a-2 or 4a-3 (not shown). The reason
is that the individual polymer chains of 4a-1 contain about 14
phenylene and 7 pyrrolopyrrol units on average, which is
obviously enough to cause that the influence of the chain length
on λmax becomes negligible.
Proton NMR Spectroscopy. The 1H NMR spectrum of
polymer 4a-2 is shown in Figure 2. The spectrum indicates
strong signals in the δ-region from 0.7 to 1.8 ppm originating
from the alkyl protons. The signal of the R-methylene unit
directly attached to the N atom of the lactam unit appears at
3.72 ppm. The origin of the small signal at 4.03 ppm is unclear.
It might be due to R-methylene units attached to the O atom of
the lactam carbonyl group. Since this signal is absent in the
The fluorescence spectra of the three polymers are also very
similar. The spectrum of polymer 4a-2 is shown in Figure 1.
The polymer exhibits a bordeaux-red emission color, the
fluorescence maximum appears at 631 nm (which is red-shifted
by 87 nm compared with the monomer 2a). For polymer 4a-2,

8254 Rabindranath et al.
Macromolecules, Vol. 39, No. 24, 2006
Scheme 2. One Pot Preparation Route to Polymers 4a-1, 4a-2, and 4b
Scheme 3. Preparation of Polymer 4a-3 upon Polycondensation of 2a and 3a
Table 1. Molecular Weights and Optical Properties of Different Polymers
UV (nm)
PL (nm)
Stokes shift
Mw
polydispersity
solution/film
solution/film
solution/film
4a-1
4a-2
4b
3070
8700
33 000
6500
1.6
1.6
3.1
1.6
530/-
528/535
525/531
523/533
633/-
631/640
632/640
627/643
103/-
103/105
107/109
104/110
4a-3
Table 2. Band Gap Data of Different Polymersa
monomer 2a, the change from N-alkylation to O-alkylation must
have taken place during the polymerization, perhaps under the
influence of the aqueous basic environment present during the
coupling reaction. The small size of the peak indicates that less
than 10 percent of the hexyl groups have changed their position.
The signals of the phenylene protons occur between 7.29 and
7.67 ppm (the strong peak at 7.27 ppm originates from protons
in the solvent CDCl3). The spectrum of polymer 4a-3 is almost
identical with 4a-2 (not shown), whereas polymer 4a-1 exhibits
additional small signals in the region from 3.5 to 5.5 ppm. The
signals can be ascribed to side products or end groups due to
the low molecular weight. A section of the proton NMR
spectrum of 4a-1 ranging from 2.5 to 8.5 ppm is shown in Figure
5. In addition, the spectrum of monomer 3a is indicated. It is a
very well resolved spectrum, showing the signals of the protons
of the hexyl group and the methyl groups of the bis(pinacolato)-
boronester at 0.7 to 1.7 ppm. The triplet at about 3.75 ppm
originates from the R-methylene unit bound to the N atom of
absorption
λonset
oxidation
onset (V)
{HOMO
(eV)}
reduction
onset (V)
{LUMO
(eV)}
(nm) in
opt/electrochem
band gap (eV)
polymer solution
4a-1
4a-2
4b
592
587
586
581
2.09/-
- {-}
- {-}
2.11/1.93
2.12/1.77
2.13/-
0.95 {-5.39} -0.98 {-3.46}
1.03 {-5.47} -0.74 {-3.70}
4a-3
- {-}
- {-}
a Band gap (Eopt) measured at the onset of electronic absorption of the
polymer solution (Eopt ) 1240/λonset eV). HOMO-LUMO gap according
to the equation: -ELUMO ) Eonset(red) + 4.44 eV and -EHOMO ) Eonset(ox)
+ 4.44 eV, where Eonset(ox) and Eonset(red) are the onset potentials for the
oxidation and reduction processes of polymer thin films vs SCE.
the lactam unit. The four symmetric signals at 7.8 and 7.98
originate from the phenylene groups of DPP.
Electrochemistry. Cyclic voltammetric (CV) data for the
polymers 4a-1 to 4a-3 are given in Table 2, and a representative
cyclic voltammogram of polymer 4a-2 is shown in Figure 3.
Thin polymer films cast from chloroform solution onto a Pt
electrode were investigated. As depicted in the Figure, the
polymer shows quasi-reversible oxidation processes, the onset
on
ox
potential for oxidation (E ) was observed at 0.95 V (vs SCE),
from which a HOMO energy level of -5.39 eV was calculated
on
using the equation -EHOMO ) E + 4.44 eV.19 The onset
ox
potential for reduction (Eorend) was found at -0.98 eV. From,
this value, a LUMO level of -3.76 eV was calculated. On the
on
ox
on
red
basis of E and E , the electrochemical band gap of 4a-2 is
1.93 eV. Taking into account an onset of the optical absorption
at λonset ) 587 nm, the optical band gap Eopt ) 1240/λonset [eV]
can be determined to be 2.11 eV (see also Table 2).
Synthesis and Characteristic Properties of Polymer 4b.
Polymer 4b was synthesized from 2a in a one-pot, two-step
route similar to polymer 4a-2 (Scheme 2). Instead of using the
PdCl2(dppf) catalyst, we used palladium(II) acetate, free of
ligand, as suggested by Zhang et al.20 The molecular weight of
Figure 5. UV/visible and photoluminescence spectra of polymer 4b
as solid film.

Macromolecules, Vol. 39, No. 24, 2006
New π-Conjugated Polymers 8255
Figure 6. 1H NMR spectra of monomer 2b and polymer 4b.
the polymer was 33 000, by far the highest one obtained upon
the various polymerization routes. The polymer is excellently
soluble in common organic solvents such as chloroform, toluene
and THF. The optical spectra of 4b are similar to 4a-2 with
absorption maxima at 525, 342, and 295 nm and a fluorescence
maximum at 632 nm (Figure 4). The quantum yield of
fluorescence of polymer 4b was 20%. Compared with the
monomer 2b, the absorption is bathochromically shifted by
about 50 nm due to the extension of the π-conjugated chro-
mophore. Solid films of 4b cast from chloroform solution exhibit
an absorption maximum at 531 nm and a fluorescence maximum
at 640 nm, the Stokes shift is 107 nm (Figure 5).
1H NMR spectra of monomer 2b and polymer 4b are shown
in Figure 6. The monomer exhibits strong signals in the region
from 0.67 to 1.55 ppm originating from the (2-ethyl)hexyl unit.
The doublet at 3.69 ppm can be ascribed to the R-methylene
unit of the alkyl group directly attached to the N atom of the
lactam group. The strong signal at 7.63 ppm originates from
the protons of the phenyl group of 2b. The polymer peaks are
generally broader. The main difference is the occurrence of a
new peak at about 4.02 ppm, which also occurred for polymer
4a-2 (Figure 2), and might indicate a side reaction during the
coupling process, in which the N-alkylation is partially changed
into O-alkylation catalyzed upon the presence of aqueous
potassium carbonate. In the region of the aromatic protons, two
signals occur at 7.35 and 7.75 ppm. The signals indicate the
presence of two nonequivalent protons at the phenyl groups,
which is the result of the arene coupling reaction.
Figure 7. Cyclic voltammogram of polymer 4b as thin film on Pt
electrode (reference SCE). The CV was carried out in 0.1 M (TBA)-
PF6 in acetonitrile, scan rate: 40 mV/s. T ) 20 °C.
4. Summary and Conclusions
In our contribution, it is demonstrated that dialkylated poly-
DPPs can be prepared from dihalo-DPP derivatives using Ni-
promoted and Pd-catalyzed arene coupling reactions. Best results
concerning molecular weight and structural unity are obtained,
if a modified version of the Miyaura arylboronate synthesis is
applied, in which the PdCl2(dppf) catalyst is substituted against
palladium(II) acetate, free of any ligands. For the first time,
this reaction step was successfully integrated in the one-pot
synthesis of a fully conjugated polyarylene. The resulting poly-
DPP exhibits interesting optical and electrochemical proper-
ties: Deep red color, intense red fluorescence with unusually
large Stokes shift of more than a hundred nanometer, and quasi-
reversible oxidation and reduction behavior render the polymers
attractive for electronic applications.
CV data of 4b are given in Table 2, a representative cyclic
voltammogram is shown in Figure 7. It is very similar to the
CV diagram of 4a-2 in Figure 3 and shows quasi-reversible
oxidation and reduction processes. The onset potential for
on
ox
oxidation, E , occurs at 1.03 eV, from which a HOMO energy
level of -5.47 eV could be calculated. The onset potential for
reduction, Eorend, is found at 0.74 eV. From this value a LUMO
level of -3.70 eV could be calculated (Table 2). On the basis
of these measurements, the electrochemical band gap could be
determinated to be 1.77 eV. Taking into account a λonset value
of 586 nm, the optical band gap could be calculated to be 2.12
eV, which is quite similar to 4a-2.
Acknowledgment. Financial support by Ciba Specialty
Chemicals, Basle, Switzerland, is gratefully acknowledged. Drs.
Roman Lenz and Matthias Dueggeli from Ciba are thanked for
helpful discussions and generously supplying dibromo-DPP 1.

8256 Rabindranath et al.
Macromolecules, Vol. 39, No. 24, 2006
(11) Behnke, M.; Tieke, B. Langmuir 2002, 18, 3815.
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