Construction of rod-like diketopyrrolopyrrole oligomers with well-defined length

Article abstract of DOI:10.1016/S0040-4039(01)01305-3

Oligomers of well-defined length were prepared by a stepwise sequence of Suzuki couplings using brominated 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP) derivatives and 1,4-dibromo-2,5-di-n-hexylbenzene as the monomers. The resulting oligomers contained three, five and seven DPP units, respectively, and could be characterized by ¹H NMR spectroscopy. These compounds could be of potential use as new electroluminescent materials.

Full text of DOI:10.1016/S0040-4039(01)01305-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 6527–6530
Construction of rod-like diketopyrrolopyrrole oligomers with
well-defined length
Mario Smet, Bert Metten and Wim Dehaen*
Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3001 Heverlee (Leuven), Belgium
Received 18 June 2001; revised 16 July 2001; accepted 19 July 2001
Abstract—Oligomers of well-defined length were prepared by a stepwise sequence of Suzuki couplings using brominated
1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP) derivatives and 1,4-dibromo-2,5-di-n-hexylbenzene as the monomers. The
1
resulting oligomers contained three, five and seven DPP units, respectively, and could be characterized by H NMR spectroscopy.
These compounds could be of potential use as new electroluminescent materials. © 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction
Br
1,4-Dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP) and
variously substituted analogues are a relatively new
O
R
N
N
R
class of red pigments that usually display excellent
photostability and a high quantum yield for fluores-
cence.1–6 These properties give them a potential interest
for the construction of new luminescent materials.
Recently we used this molecule as a core reagent in
dendrimers, which could thus be detected by single
molecule spectroscopy.7 A few examples of DPP-con-
taining aliphatic copolyesters and copolyurethanes have
been reported by other authors.8 Beyerlein and co-
workers used the Suzuki coupling to synthesize p-conju-
gated polymers consisting of various ratios of the
building blocks 1, 2 and 3 depicted in Fig. 1.9 The
p-conjugated polymers were found to be strongly
luminescent and more photostable than the correspond-
ing saturated polymers containing isolated DPP units in
the backbone. These compounds were anticipated to be
suitable for electronic applications such as the construc-
tion of organic light emitting devices.
2 R = Br
3 R = B(OH)2
O
1
O
Br
B
6 =
O
Br
O
N
N
O
4 R = Br
5 R = H
R
Figure 1.
obtain highly soluble oligomers, we used building block
4 instead of 1 (Fig. 1). The preparation of 4 (as well as
the terminating unit 5), by N-alkylation of the dibromo
(or monobromo) DPP1 with 3,5-bis-t-butylbenzyl bro-
mide instead of n-hexyl bromide, was found to be far
more efficient than the described alkylation yielding 1.
Dibromide 4 could readily be isolated in good yield
(72%) by simple precipitation with n-pentane. In order
to guarantee an easy separation by column chromato-
graphy we esterified boronic acid 3,10 analogously to a
literature procedure using pinacol in dioxane at 60°C,11
to obtain the diester 6. This reduces the polarity of the
boron containing intermediates. Moreover, the use of
pinacol ester provides a convenient handle to character-
2. Results and discussion
We wished to prepare oligomers of controlled length
using a similar approach based on Suzuki coupling.
These oligomers might allow even better processability
into thin films and display superior electronic properties
compared to the analogous polymers. In order to
1
ize these intermediates by H NMR spectroscopy due
to the presence of an intense singlet arising from the
methyl protons of the pinacolates.
* Corresponding author. Tel.: +32 16 32 74 39; fax: +32 16 32 79 90;
e-mail: wim.dehaen@chem.kuleuven.ac.be
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01305-3

6528
M. Smet et al. / Tetrahedron Letters 42 (2001) 6527–6530
starting material and the desired product. An overview
of the synthesized oligomers can be found in Table 1.
The construction of the desired oligomers proceeded
readily using a combination of both convergent and
divergent coupling strategies. The convergent strategy
starts from the peripheral unit 5 which was allowed to
react with a 5-fold excess of diboronic ester 6 under
standard Suzuki-coupling conditions (Pd(PPh3)4, tolu-
ene/Na2CO3 1 M, reflux, overnight) giving 7 in 91%
isolated yield after column chromatography (Scheme
1). This boronic ester 7 could then be coupled to
dibromide 4 yielding the DPP trimer 8.
The synthesized DPP oligomers 8–14 turned out, as
anticipated, to be highly soluble in common solvents
such as dichloromethane, chloroform and toluene. The
solubility of dibromide 4 was significantly lower in the
latter solvent, allowing us to remove a large part of the
excess 4 from the reaction mixtures leading to 10 and
12 by simple precipitation from toluene. The excess of
bisboronic ester 6 could be recuperated by column
chromatography from the reaction leading to 9 and 11.
This is of interest because of the poor yields of the
literature procedure to obtain the precursor 3.10
For the construction of longer oligomers, basically a
divergent strategy was used. Thus, we started from
dibromide 4, which was allowed to react with a 10-fold
excess of diboronic ester 6 giving species 9 in 76%
isolated yield (Scheme 2). Successive reactions with a
10-fold excess of 4, a 10-fold excess of 6 and finally
again a 10-fold excess of 4 gave rise, via intermediates
10 and 11 (Table 1), to the dibromo oligomer 12,
consisting of five DPP units, in acceptable yield.
Analogously, pentamer 13 could be prepared by reac-
tion of bisboronate 11 with the terminating unit 5. For
the preparation of heptamer 14 we decided to couple
monoboronate 7 with bisbromide 12 as we expected the
chromatographic purification to be easier because of
the more pronounced difference in polarity between the
The oligomers described could be readily characterized
by 1H NMR spectroscopy. For instance, in the 1H
NMR spectrum of bisboronic ester 9, triplets (each 4H)
at l 2.85 and 2.55 ppm correspond to the benzylic CH2
of the hexyl group, respectively, ortho to the boronic
ester or the phenyl substituent on the heterocycle. Sin-
glet resonances at l 7.69 and 7.00 ppm (each 2H) can
be, respectively attributed to the corresponding aryl
protons. The intensities of the signals have the expected
ratios towards the pinacolate singlet (24H).
1
The H NMR spectrum of the tris DPP dibromide 10
shows three singlets (each 4H) between l 4.9 and 5.2
ppm, reflecting the symmetry of the molecule, which
has three different 3,5-bis-t-butylbenzyl groups. The
displacement of the benzylic triplet around 2.85 ppm to
a signal of higher field (2.58 ppm) indicates that the
boronic ester has reacted. The two doublets at 7.64 and
7.56 ppm can be assigned to the peripheral 4-bromo-
phenyl substituent. Moreover, the integration ratios
confirm that there are indeed three DPP residues
1
present. The other H NMR spectra of 11–14 can be
assigned in the same way.
Finally we wanted to show that dendritic oligomers
could be prepared as functionalization with dendrons
might allow changing solubility and crystallization
properties. Thus, we prepared the dendritic monobro-
mide 15 by a procedure analogous to the synthesis of 5
namely N-alkylation with the second generation den-
dritic benzyl bromide developed by Fre´chet (Scheme
3).12 Dendron 15 was then reacted with bisboronic ester
6 affording boronic ester 16. This compound was cou-
pled to bisbromide 4 affording the dendritic trimer 17,
which was also found to be highly soluble in
dichloromethane, chloroform and toluene.
Acknowledgements
Dr. O. Wallquist (Ciba Specialty Chem.) is gratefully
acknowledged for kindly supplying us with samples of
brominated DPP derivatives. The Katholieke Univer-
siteit Leuven, the FWO-Vlaanderen and the Ministerie
voor Wetenschapsbeleid are thanked for their continu-
ing financial support. M.S. thanks the FWO-Vlaan-
deren for a postdoctoral fellowship.
Scheme 1.

M. Smet et al. / Tetrahedron Letters 42 (2001) 6527–6530
6529
10-fold excess
O
O
B
O
B
O
O
B
O
Pd(PPh3)4
O
6
Toluene
N
N
O
Br
Na2CO3 1M
reflux
O
N
N
B
O
O
O
9
Br
4
See text
O
N
O
N
R
O
N
O
N
O
N
N
O
N
O
N
O
R
N
O
N
O
See text
O
N
O
N
O
N
O
N
O
N
N
O
O
N
N
O
O
N
N
O
N
O
N
O
N
O
N
O
Scheme 2.
Table 1. Overview of the synthesized rod-like DPP oligomers
Entry
End groups
Isolated yield (%)
Mol. weight (g/mol)
No. of DPP units
8
9
H
69
76
55
69
51
46
58
2563
1434
2722
3304
4592
4434
6305
3
1
3
3
5
5
7
Boronate
Br
Boronate
Br
H
H
10
11
12
13
14

6530
M. Smet et al. / Tetrahedron Letters 42 (2001) 6527–6530
BnO
OBn
BnO
O
BnO
OBn
O
N
O
N
O
OBn
OBn
BnO
O
BnO
BnO
O
O
O
16 (3 eq.), 4, Pd(PPh3)4
Toluene / Na2CO3 1M
O
N
R
R
BnO
OBn
R
N
O
OBn
OBn
O
O
N
N
O
O
BnO
OBn
R
BnO
OBn
R
15 R = Br
6
BnO
BnO
Pd(PPh3)4
Toluene
Na2CO3 1M
O
C6H13
O
N
O
B
O
N
16 R =
OBn
OBn
O
O
O
C6H13
O
BnO
OBn
17
Scheme 3.
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