Bi-directional synthesis of a series of 2,5-dodecanoxy-phenyleneethynylene oligomers

Article abstract of DOI:10.1055/s-2005-868474

A family of rigid rod like 2,5-dodecanoxy-phenyleneethynylene oligomers having 3, 5, 7, and 9 repeating units was selectively synthesized by a bi-directional iterative divergent-convergent approach. Starting from a central 1,4-bis(dodecanoxy)-2,5-diiodobe

Full text of DOI:10.1055/s-2005-868474

LETTER
1259
Bi-Directional Synthesis of a Series of 2,5-Dodecanoxy-phenyleneethynylene
Oligomers
S
ynthesis of 2,5-D
o
odecanoxy-p
r
henyle
n
m
eethynylene
O
ligom
a
ers González-Rojano,a Eduardo Arias-Marín,*a Dámaso Navarro-Rodríguez,a Steffen Weidnerb
a
Centro de Investigación en Química Aplicada (CIQA), Boulevard Enrique Reyna 140, 25100 Saltillo, Coahuila, México
Fax +52(844)4389839; E-mail: earias@ciqa.mx
b
Bundesanstalt für Materialforschung und -prüfung (BAM), Unter den Eichen 87, 12205 Berlin, Germany
Received 9 February 2005
this route, was the presence of TMSA as the terminal
group. After desilylation of the oligomers the acetylene
group becomes very sensitive to oxidation. The tendency
to oxidize becomes greater as the length of the oligomers
Abstract: A family of rigid rod like 2,5-dodecanoxy-phenylene-
ethynylene oligomers having 3, 5, 7, and 9 repeating units was se-
lectively synthesized by a bi-directional iterative divergent-
convergent approach. Starting from a central 1,4-bis(dodecanoxy)-
2,5-diiodobenzene monomer, only two repetitive reactions were in- increase, and in the next Pd/Cu cross-coupling reaction,
volved with each cycle of oligomerization: a Pd/Cu cross-coupling
diacetylenes are always obtained in larger amounts. On
with the bifunctional monomer 1-(3,3-diethyltriazene)-2,5-
the other side, Godt et al. utilized the fact that arylbro-
bis(dodecanoxy)-4-(ethynyl)benzene, generating oligomers with
mides react much less readily than the corresponding
terminal triazene groups, followed by a further nucleophilic substi-
aryliodides and generally require solvents at reflux in or-
tution with iodine.
der to effect the reaction. This energy difference allows
Key words: conjugated oligomers, phenyleneethynylenes, tri-
for the selective coupling of acetylenes with aryliodides at
azenes, aryl iodides, Sonogashira–Heck reaction
0 °C. In this work, we present the synthesis step by step of
a series of oligo(2,5-PEOC12)s, applying a synthetic
strategy that involves both methods developed by Moore,
Conjugated macromolecules represent an attractive class
Tour, and Godt. The growing oligomers never present
of semiconductor materials for a large number of applica-
acetylene as an end group, thus diacetylenes are only
tions in the optoelectronic area which are so far restricted
formed as dimers, which can easily be removed from the
to inorganic and single crystal semiconductors. In this
mixture.
respect, phenyleneethynylene polymers and oligomers
which exhibit an extended p-electron delocalization in
The synthetic route to obtain the bifunctional monomers
is outlined in Scheme 1. The chemical pathway was
their conjugated backbone can be modulated by either
adapted and modified from different procedures reported
changing the oligomer length or inserting electroactive
in the literature.5 The starting and commercially available
1,4-hydroquinone (1) was alkylated with 1-bromodode-
substituents into the molecular structure, thus allowing the
possibility to tune the emission to a wide range of colors
of the visible spectra.1 We recently demonstrated that the
optical properties of benzoate ethynylene oligomers hav-
ing at least seven repeating units compete with those of
their homologue polymer.2 The oligomers have the advan-
tage that their monodispersity makes them useful models
to better understand polymer properties such as electro-
luminiscence, linear, and non-linear optical properties,
among others.3
cane by the Williamson reaction, followed by the addition
of bromine to 2. The resulting product 3 was nitrated to
obtain 4 in 84% yield. Subsequently, the reduction of the
nitro to amine group was carried out with Sn/HCl, in THF.
The optimum time to avoid dealkylations was found to be
90 minutes after adding 4 and the internal temperature
should not exceed 50 °C. Due to the exothermicity of the
reaction; our attempts to reduce more than 6 g of 4 pro-
duced uncontrollable reactions with lower yields. The
product 5 is obtained in 60 5% yield. The conversion of
the aniline group from 5 to obtain 7 involves the prepara-
tion of the intermediary salt 6, which is obtained in high
yields (>95%). The salt 6 is stable and can be isolated by
adding cold hexanes to the reaction medium and then fil-
tering by suction. After its dissolution in a mixture of cold
DMF and diethylamine, the triazene 7 is obtained in 86%
yield. The next step was the Pd/Cu cross-coupling of
TMSA with 7 in triethylamine. The product 8 was ob-
tained in 80% yield and after desilylation with tetrabutyl-
ammonium fluoride (TBAF), the desired product 9
required for the iterative oligomerization was obtained in
high yields (90%).
In relation to their synthesis, undoubtedly Moore, Tour,
and Godt,4 have been making important contributions for
the preparation of phenyleneethyneylene oligomers with
greater than seven repeating units. On one side, the itera-
tive divergent-convergent pathway through the synthesis
of the bifunctional triazene-aryl-trimethylsilylacetylene
(TMSA) monomer proposed by the groups of Moore and
Tour appears to be an innovative method to double the
molecule length in each step, nevertheless the main draw-
back that we found trying to synthesize the 2,5-dodecan-
oxyphenyleneethynylene (2,5-PEOC12) series following
SYNLETT 2005, No. 8, pp 1259–1262
1
7.
0
5.
2
0
0
5
Advanced online publication: 21.04.2005
DOI: 10.1055/s-2005-868474; Art ID: S01505ST
© Georg Thieme Verlag Stuttgart · New York

1260
N. González-Rojano et al.
LETTER
OC12H25
Br
OC12H25
OC12H25
Br
OC12H25
Br
OC12H25
Br
Br
b
e
d
c
f
N
OH
OC12H25
–O
N
N
F4B-N2
+
H2N
N+
O
OC12H25
OC12H25
a
OC12H25
OC12H25
OC12H25
3
5
6
7
4
OC12H25
I
OH
OC12H25
i
g
1
2
I
OC12H25
OC12H25
Si
OC12H25
H
10
h
N
N
N
N
N
N
OC12H25
OC12H25
8
9
Scheme 1 (a) Br-C12, NaOH, DMF, 120 °C, (87%); (b) Br2, CCl4, 70 °C, (85%); (c) H2SO4, HNO3, CH2Cl2, 5 °C, (84%), (d) Sn, HCl, THF,
5 °C, (65%); (e) BF3O·Et2O, t-BuNO2, THF, –15 °C, (95%); (f) Et2NH, K2CO3, DMF, –10 °C, (86%); (g) [(C6H5)3P]2PdCl2, CuI, TMSA, Et3N,
80 °C, (80%); (h) TBAF, THF, r.t., (90%); (i) KIO3, I2, AcOH, H2SO4, H2O, 120 °C, (92%).
The central monomer was synthesized in only one step. A triazene terminated trimer 11. Then, 11 is submitted to a
portion of 1,4-bis(dodecanoxy) benzene (2) was subjected iodine nucleophilic substitution reaction to generate the
to iodination to obtain 1,4-bis(dodecanoxy)-2,5-diiodo- iodide terminated trimer 12. In another cycle, two equiva-
benzene (10) in 92% yield.
lents of 9 are further subjected to Pd/Cu cross-coupling
with 12 to generate the triazene terminated pentamer 13.
It is worth noting that triazene terminated molecules are
difficult to purify by silica gel chromatography and a
portion of triazene is always cleaved if CH2Cl2 is used as
solvent; isolation of all triazene oligomers could be
The strategy for 2,5-PEOC12 oligomer series preparation
involving only two sets of reaction is depicted in
Scheme 2. Starting from 10, the oligomers can grow in
two directions simultaneously. In the first step, two equiv-
alents of 9 undergo Pd/Cu cross-coupling to obtain the
C12H25
O
C12H25
O
C12H25O
C12H25
O
C12H25
O
C12H25
O
N
k
j
N
N
N N
I
I
(2X)
9 + 10
N
(87%)
(41%)
OC12H25
OC12H25
OC12H25
OC12H25
+
OC12H25
OC12H25
12
11
(2X)
9
C12H25
O
C12H25
O
C12H25
O
C12H25
O
C12H25O
j
k
13
(2X)
I
I
+
9
(41 %)
OC12H25
OC12h25
OC12H25
OC12H25
OC12H25
14
j
(51%)
15
k
C12H25
O
C12H25
O
C12H25
O
C12H25
O
C12H25
O
C12H25
O
C12H25O
j
(2X)
9
17
I
I
+
(50%)
OC12H25
OC12H25
OC12H25
OC12H25
OC12H25
OC12H25
OC12H25
k
16
C12H25
O
C12H25
O
C12H25
O
C12H25
O
C12H25
O
C12H25
O
C12H25
O
C12H25
O
C12H25O
I
I
OC12H25
OC12H25
OC12H25
OC12H25
OC12H25
OC12H25
OC12H25
OC12H25
OC12H25
18
Scheme 2 (j) [(C6H5)3P]2PdCl2, CuI, Et3N,THF, r.t.; (k) CH3I, 120 °C.
Synlett 2005, No. 8, 1259–1262 © Thieme Stuttgart · New York

LETTER
Synthesis of 2,5-Dodecanoxy-phenyleneethynylene Oligomers
1261
successfully carried out by using aluminum oxide chro-
matography and CHCl3, in addition there is a marked re-
tention difference between the unreacted iodide oligomers
and the triazene oligomers making their isolation easier.
The unreacted trimer 12 is fluorescent, while 13 does not
fluoresce, thus the two products can be followed and dis-
tinguished by irradiating the chromatography column
with a portable UV lamp (l exc. 365 nm). After iodination
of 13, pure pentamer 14 is obtained by passing the residue
through a silica gel column and a preparative gel perme-
ation chromatography column (Biorads, Bio-Beads SX1,
toluene). As 14 is terminated by iodides, the next Pd/Cu
cross-coupling reaction with two equivalents of 9 gener-
ates the triazene terminated heptamer 15, which after con-
version to iodide gives the heptamer 16. By alternating 9
at each step of oligomerization, oligomers up to nonamer
18 could be obtained, since the dodecanoxy chains substi-
tuted on the phenyls impart good solubility. It should be
pointed out, that at each oligomerization, the diacetylene
dimer formed as by-product from 9 can be successfully
isolated either by silica gel or preparative GPC chroma-
tography.
Figure 1 Normalized optical absorption spectra in CHCl3 of mono-
mer 10, trimer 12, pentamer 14, heptamer 16, nonamer 18, and the
corresponding polymer n. The insert shows bandgap vs. 1/n (n is the
number of phenyl groups)
The chemical structure of the oligomers is determined by
1H, 13C NMR, and UV-Vis spectroscopy.6 GPC is also a
useful tool to determine their purity by correlation with
the dispersity index, which is always <1.07. In addition,
the matrix (dithranol)-assisted laser desorption-ionization
time of flight (MALDI-TOF) mass spectra confirm the
structure of the oligomers 12, 14, 16, and 18. Figure 1
shows the absorption spectra for the complete series of
oligomers in chloroform. The band between 334 and 479
nm is due to the p-p* transition, which shifts to higher
wavelengths (l) as the conjugation increases, the maxi-
mum being at lmax = 308, 384, 414, 423, 429, and 436 nm
for 10, 12, 14, 16, 18, and n the homologue polymer,7 re-
spectively. It is evident from these values that the satura-
tion of conjugation is almost gained with nine repeat units.
This behavior is also described in Figure 1, which shows
the band-gap energy versus 1/n (n is number of phenyls).
These oligomers are promising materials for the construc-
tion of light emitting diodes, which is currently under
investigation.
References
(1) Arias-Marin, E.; Le Moigne, J.; Maillou, T.; Guillon, D.;
Moggio, I.; Geffroy, B. Macromolecules 2003, 36, 3570.
(2) Arias-Marin, E.; Arnault, J. C.; Guillon, D.; Maillou, T.; Le
Moigne, J.; Geffroy, B.; Nunzi, J. M. Langmuir 2000, 16,
4309.
(3) (a) Gu, T.; Tsamouras, D.; Melzer, C.; Krasnikov, V.;
Gisselbrecht, J. P.; Gross, M.; Hadziioannou, G.;
Nierengarten, J. F. ChemPhysChem 2002, 1, 124.
(b) Maillou, T.; Le Moigne, J.; Geffroy, B.; Lorin, A.;
Rosilio, A.; Dumacher, V.; Rocha, L.; Denis, C.; Fiorini, C.;
Nunzi, J. M. Synth. Met. 2001, 24, 87.
(4) (a) Zhang, J.; Moore, J. S.; Xu, Z.; Aguirre, R. A. J. Am.
Chem. Soc. 1992, 114, 2273. (b) Young, J. K.; Nelson, J. C.;
Moore, J. S. J. Am. Chem. Soc. 1994, 116, 10841.
(c) Pearson, D. L.; Schumm, J. S.; Tour, J. M.
Macromolecules 1994, 27, 2348. (d) Wu, R.; Schumm, J. S.;
Pearson, D. L.; Tour, J. M. J. Org. Chem. 1996, 61, 6906.
(e) Pearson, D. L.; Tour, J. M. J. Org. Chem. 1997, 62,
1376. (f) Ziener, U.; Godt, A. J. Org. Chem. 1996, 62, 6137.
(g) Kukula, H.; Veit, S.; Godt, A. Eur. J. Org. Chem. 1999,
277.
In conclusion, a series of (2,5-PEOC12) oligomers having
3, 5, 7, and 9 repeat units were bi-directionally synthe-
sized, starting from the 1,4-bis(dodecanoxy)-2,5-diiodo-
benzene 10. Two reaction sets are needed to grow the
oligomer’s length at each cycle: a Pd/Cu cross-coupling
with the 1-(3,3-diethyltriazene)-2,5-bis(dodecanoxy)-4-
(ethynyl)benzene bifunctional monomer, and the con-
version of the triazene to iodide to generate oligomers
terminated with iodide groups.
(5) (a) Wautelet, P.; Moroni, M.; Oswald, L.; Le Moigne, J.;
Pham, A.; Bigot, J. Y. Macromolecules 1996, 29, 446.
(b) Weder, C.; Wrighton, M. S. Macromolecules 1996, 29,
5157. (c) Bharathi, P.; Patel, U.; Kawaguchi, T.; Pesak, D.
J.; Moore, J. S. Macromolecules 1995, 28, 5955.
(6) Trimer 12: Mp 84 °C. 1H NMR (300 MHz, CDCl3): d = 0.87
(br s, 18 H, CH3), 1.25 (br s, 96 H, CH2), 1.47 (br s, 12 H,
CH2-g-O), 1.82 (m, 12 H, CH2-b-O), 3.99 (m, 12 H, CH2-a-
O), 6.90 (s, 2 H, Ar-H), 6.99 (s, 2 H, Ar-H), 7.30 (s, 2 H, Ar-
H). 13C NMR (75 MHz, CDCl3): d = 14.20 (CH3), 22.78
(CH2), 25.90 and 26.10 (C-g-O), 29.37 (CH2), 29.45 (C-b-
O), 29.58 (CH2), 29.70 (CH2), 29.76 (CH2), 32.01 (CH2),
69.75 and 70.13 (C-a-O), 87.62 (ArCI), 90.88 (C≡C), 91.15
(C≡C), 114.06 (ArC≡), 114.32 (ArC≡), 116.19 (Ar-H),
117.32 (Ar-H), 124.20 (Ar-H), 151.94 (ArCO), 153.56
(ArCO), 154.27 (ArCO). MALDI-TOF: m/z calcd for
C94H156I2O6, 1635.54; found, 1636.05.
Acknowledgment
We wish to acknowledge CONACyT for the financial support of the
project J38753-U and for the PhD scholarship 60571 awarded to
N.G.R. N.G.R also thanks CENAM for a scholarship through the
SIDEPRO program.
Synlett 2005, No. 8, 1259–1262 © Thieme Stuttgart · New York

1262
N. González-Rojano et al.
LETTER
Pentamer 14: Mp 105 °C. 1H NMR (300 MHz, CDCl3): d =
0.88 (br s, 30 H, CH3), 1.24 (br s, 160 H, CH2), 1.50 (br s, 20
H, CH2-g-O), 1.83 (br s, 20 H, CH2-b-O), 4.02 (m, 20 H,
CH2-a-O), 6.91 (s, 2 H, ArH), 7.0 (s, 6 H, ArH), 7.30 (s, 2 H,
ArH). 13C NMR (75 MHz, CDCl3): d = 14.20 (CH3), 22.78
(CH2), 26.03 and 26.11 (C-g-O), 29.37 (CH2), 29.46 (C-b-
O), 29.59 (CH2), 29.70 (CH2), 29.77 (CH2), 29.79 (CH2),
32.01 (CH2), 69.76 and 70.12 (C-a-O), 87.61 (ArCI), 90.93
(C≡C), 91.16 (C≡C), 91.68 (C≡C), 114.08 (ArC≡), 114.28
(ArC≡), 114.44 (ArC≡), 116.19 (Ar-H), 117.35 (Ar-H),
124.20 (Ar-H), 151.94 (ArCO), 153.57 (ArCO), 154.27
(ArCO). MALDI-TOF: m/z calcd for C158H260I2O10,
2572.34; found, 2573.56.
(CH2), 29.80 (CH2), 32.01 (CH2), 69.76 and 70.13 (C-a-O),
87.60 (ArCI), 91.0 (C≡C), 91.14 (C≡C), 91.69 (C≡C),
114.09 (ArC≡), 114.45 (ArC≡), 116.20 (Ar-H), 117.35 (Ar-
H), 124.21 (Ar-H), 151.94 (ArCO), 153.58 (ArCO), 154.28
(ArCO). MALDI-TOF: m/z calcd for C222H364I2O14,
3508.75; found: 3511.07.
Nonamer 18: Mp 125 °C. 1H NMR (300 MHz, CDCl3): d =
0.87 (br s, 54 H, CH3), 1.24 (br s, 288 H, CH2), 1.50 (br s, 36
H, CH2-g-O), 1.83 (br s, 36 H, CH2-b-O), 4.03 (br s, 36 H,
CH2-a-O), 6.91 (s, 2 H, Ar-H), 7.0 (s, 14 H, Ar-H), 7.30 (s,
2 H, Ar-H). 13C NMR (75 MHz, CDCl3): d = 14.19 (CH3),
22.78 (CH2), 26.12 (C-g-O), 29.46 (C-b-O), 29.58 (CH2),
29.80 (CH2), 32.02 (CH2), 69.76 and 70.13 (C-a-O), 91.69
(C≡C), 114.0 (ArC-≡), 114.45 (ArC-≡), 117.36 (Ar-H),
153.57 (ArCO). MALDI-TOF: m/z calcd for C286H468I2O18
4448.57; found, 4445.56.
Heptamer 16: Mp 116 °C. 1H NMR (300 MHz, CDCl3): d =
0.87 (br s, 42 H, CH3), 1.24 (br s, 224 H, CH2), 1.50 (br s, 28
H, CH2-g-O), 1.84 (br s, 28 H, CH2-b-O), 4.03 (m, 28 H,
CH2-a-O), 6.90 (s, 2 H, Ar-H), 7.0 (s, 10 H, Ar-H), 7.31 (s,
2 H, Ar-H). 13C NMR (75 MHz, CDCl3): d = 14.19 (CH3),
22.78 (CH2), 26.03 and 26.12 (C-g-O), 29.46 (C-b-O), 29.58
(7) Moroni, M.; Le Moigne, J.; Luzzati, S. Macromolecules
1994, 27, 562.
Synlett 2005, No. 8, 1259–1262 © Thieme Stuttgart · New York