Synthesis and electrochemical characterization of donor-acceptor phenylazomethine dendrimers

Article abstract of DOI:10.1016/j.tetlet.2005.10.073

The synthesis of the first donor-acceptor phenylazomethine dendrimers (4 and 5) is described. A convergent method is used via the condensation of aromatic ketones with an appropriately functionalized tri(aminophenyl)-s- triazine promoted by titanium (IV)

Full text of DOI:10.1016/j.tetlet.2005.10.073

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 8861–8864
Synthesis and electrochemical characterization of donor–acceptor
phenylazomethine dendrimers
*
*
´
´
´
Rafael Juarez, Rafael Gomez, Jose L. Segura and Carlos Seoane
Departamento de Quı´mica Orga´nica, Facultad de Quı´mica, Universidad Complutense, E-28040 Madrid, Spain
Received 21 September 2005; revised 17 October 2005; accepted 18 October 2005
Available online 2 November 2005
Abstract—The synthesis of the first donor–acceptor phenylazomethine dendrimers (4 and 5) is described. A convergent method is
used via the condensation of aromatic ketones with an appropriately functionalized tri(aminophenyl)-s-triazine promoted by titan-
ium (IV) chloride. Cyclic voltammetry investigations show a donor–acceptor behaviour due to the presence of the electron acceptor
s-triazine core and the donor ability of the conjugated peripheral butoxybenzene units.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Synthetic macromolecules containing p-conjugated
systems1 have attracted a great interest owing to their
potential as photosynthetic antennas, molecular wires
for electron and energy transfer and also as materials
for optoelectronic devices. In this context, monodisperse
dendritic materials emerge as an attractive alternative to
conjugated polymers in optoelectronic applications, as
they combine a well-defined architecture and conjuga-
tion length with good processability.2 For instance, den-
dritic macromolecules have been incorporated into
organic light emitting diodes (OLEDs) as hole transport
components, electron transporting components or even
as single component emitters.3
In this communication we report a straightforward syn-
thesis for the first donor–acceptor dendrimeric ana-
logues of poly(azomethine)s 4 and 5 (Fig. 2), in which
electron donating butoxybenzene units are linked to a
s-triazine acceptor core through phenylazomethine
spacers.
Compounds containing s-triazine units have been the
subject of extensive research owing to their ability to
form mesophases9 and as NLO10 and two photon
absorption materials,11 among others. More interest-
ingly, its high electron affinity and structural symmetry
has enabled its use as electron injection and transport
materials in macromolecular systems.12
So far, dendritic analogues of the most important
conjugated polymeric materials, namely poly(phenyl-
eneethynylene) (1),4 poly(phenylenevinylene) (2)5 and
poly(phenylene) (3),6 derivatives (Fig. 1) have been
already proposed as monodisperse macromolecular
alternatives to the parent polymers.
Since poly(azomethines)s can be synthesized from
diamines and dialdehydes by solid-state or solution
polycondensation,7b we have selected the 2,4,6-tri-
(p-aminophenyl)-s-triazine (12) as the core for the
attachment of electron donor conjugated dendrons in
the convergent syntheses of dendrimers 4 and 5. Thus,
generation of p-nitrobenzilimidate by heating p-nitro-
benzonitrile (10) in the presence of sodium methoxide
followed by acid catalyzed cyclotrimerization affords
in a Ôone potÕ procedure, the corresponding symmetri-
cally substituted tri(p-nitrophenyl)-s-triazine (11),13
which was finally reduced to yield the target tri(p-amino-
phenyl)-s-triazine (12) in a 94% yield (Scheme 1).
Although poly(azomethine)s have attracted growing
attention during the last few years as advanced electron-
ics materials, owing to their semiconductive and photo-
conductive properties,7 the dendrimeric analogues of
poly(azomethine)s have received much less attention
from the materials point of view and only recently have
been used as metal complexing systems for sensors.8
Dendritic polyphenylazomethines 4 and 5 were synthe-
sized by following the convergent method depicted
in Scheme 1. 4,40-Dibutoxybenzophenone (6)14 was
allowed to react with tri(p-aminophenyl)-s-triazine (12)
*
Corresponding authors. Tel.: +34 91 394 5142; fax: +34 91 394
4103; e-mail: segura@quim.ucm.es
0040-4039/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.10.073

8862
R. Jua´rez et al. / Tetrahedron Letters 46 (2005) 8861–8864
R
R
R
R
R
R
R
n
R
R
n
n
R
R
R
n
n
n
R
R
n
n
n
3
2
1
R
R
R
R
Figure 1. Polyphenyleneethynylene (1), polyphenylenevinylene (2) and polyphenylene (3) type dendrimers.
OBu
OBu
N
BuO
OBu
OBu
N
N
N
N
N
OBu
N
N
OBu
N
OBu
OBu
N
N
BuO
N
N
N
N
N
BuO
N
BuO
OBu
N
4
BuO
OBu
OBu
BuO
OBu
5
Figure 2. Donor–acceptor dendritic polyphenylazomethines 4 and 5.
BuO
OBu
OBu
N
OBu
OBu
N
OBu
O
O
6
i
ii
+
N
N
BuO
OBu
BuO
OBu
9
8
H2N
NH2
7
NO2
NH2
6
NO2
4
5
i
N
N
N
iii
iv
N
N
N
9
i
CN
O2N
NO2
H2N
NH2
12
10
11
Scheme 1. Reagents and conditions: (i) DABCO, TiCl4, toluene, Ar, 110 ꢁC; (ii) KMnO4, Bu4PBr, benzene, reflux; (iii) (1) NaOMe, MeOH, 25 ꢁC,
(2) AcOH, 90 ꢁC; (iv) Fe, FeSO4 (7 H2O), H2O, DME, reflux.
in the presence of titanium (IV) chloride as dehydrating
agent and DABCO to yield the first generation dendri-
mer 4. The synthesis of the imino-containing dendron
9 to form the second generation dendrimer started with
a similar condensation between 4,40-dibutoxybenzophe-
none (6) and the commercially available 4,40-methylene-
dianiline (7) under analogous conditions to afford 8 in
75% yield. In our hands, the direct condensation of 6
with 4,40-diaminobenzophenone to give directly 9 tends
to be difficult to control and yields complex mixtures of
self-condensation products. In order to circumvent self-
condensation, an advantageous strategy to obtain 9

R. Jua´rez et al. / Tetrahedron Letters 46 (2005) 8861–8864
8863
involves the condensation of 4,40-methylene-dianiline
140000
with 6, followed by selective oxidation of 8 with potas-
sium permanganate in the presence of tetrabutylphos-
phonium bromide. This procedure allows the obtention
of the suitably functionalized dendron 9 in a 48% yield.
8
4
5
120000
100000
80000
60000
40000
20000
0
N
N
N
The second generation dendrimer 5 was obtained in 19%
yield by condensation of tri(p-aminophenyl)-s-triazine
(12) with dendron 9 under similar conditions to the syn-
thesis of the first generation dendrimer 4.15
13
In contrast to the characteristic low solubility of
poly(phenylazomethines) and also of some s-triazine
derivatives, dendrimers 4 and 5 show high solubility in
common organic solvents due to the presence of the alk-
oxy chains. This enhanced solubility has made the puri-
fication of the dendrons and dendrimers possible by
column chromatography and has allowed its full spec-
troscopical characterization.16
250
300
350
400
450
500
Wavelength (nm)
Figure 3. UV–vis absorption spectra of dendrimers 4 and 5, dendron 8
and triazine reference 13 in dichloromethane.
Thus, the FT-IR spectra of the dendrimers show the dis-
appearance of the typical absorption bands of the NH2
and CO groups in the precursors 12 and 6 and 9, respec-
tively, in favour of the appearance of the characteristic
However, although in 5 longer para-conjugated frag-
ments than in 4 can be observed, both dendrimers show
similar absorption maxima. This is probably due to dis-
tortion from planarity of the higher generation dendri-
mer,8a which restrains the extension of the conjugated
chromophore.
absorption band of the imine group around 1604 cmÀ1
.
The 13C NMR spectra of 4 and 5 displays the signal
corresponding to the s-triazine carbons at 171 ppm.
Whereas a single signal is observed for the equivalent
imino groups in the first generation dendrimer 4, two
signals can be observed for the second generation den-
drimer 5 corresponding to the two different types of
imine groups.
The redox behaviour of dendrimers 4 and 5 was investi-
gated by cyclic voltammetry. The reduction waves corre-
sponding to the triaryl-s-triazine moieties17 can be
observed at ca. À2.1 V. This value is cathodically shifted
in comparison with that of s-triazine (À2.59 V) due to
the extension of the conjugated system (Fig. 4).
UV–vis absorption spectra were recorded in dichloro-
methane solutions (Fig. 3 and Table 1) and show max-
ima at 298 and 341 nm for the first generation
dendrimer 4 and 287 and 354 nm for the second genera-
tion dendrimer 5 (Fig. 3). Higher molar absorption coef-
ficients can be observed for 5 as corresponds to the
higher number of chromophores per molecule in com-
parison with 4. In contrast to the conjugated dendrimers
1–3, in which meta linkages prevent the extension of the
conjugation, in dendrimers 4 and 5 long para-conju-
gated fragments can be observed. This is in agreement
with the red shift of the absorption maxima of dendri-
mers 4 and 5 with respect to triaryl-s-triazines such as
tri(m-xylyl)-s-triazine (13).
Further conjugation with phenylazomethine moieties
stands for the second reduction process observed for
the dendrimers at ca. À2.6 V, which is not observed nei-
ther in the triaryl-s-triazine (13) nor in the dendron 8
used as references. Oxidation waves corresponding to
the conjugated butoxybenzene fragments can also be ob-
served in the cyclic voltammograms. Similar oxidation
values are observed for dendrimer 4 and reference 8,
whereas more positive values are observed for the second
generation dendrimer 5. This is in agreement with a stron-
ger distortion of the peripheral part of the dendrimer in
the second generation one, which causes a decrease in
the effective conjugation, the other consequence of which
is the small red-shift observed in the absorption spectra.
Table 1. Electrochemical and optical properties of dendrimer 4 and 5 and reference compounds 8, s-triazine and tri(m-xylyl)-s-triazine
a
a
b
b
Entry
E11=2;red ðVÞ
E21=2;red ðVÞ
E1OX ðVÞ
E2OX ðVÞ
kmax (nm) [e (MÀ1 cmÀ1)]c
s-Triazine
Tri(m-xylyl)-s-triazine
8
4
5
À2.59
À2.14
—
—
—
—
—
—
0.87
0.75
1.06
—
—
1.07
1.07
1.26
269 [855]
280 [66,090]
338 [4413], 281 [17,753]
341 [91,704], 298 [87,713]
354 [93,623], 287 [125,873]
À2.11
À2.55
À2.12
À2.61
Values recorded using TBAHFP (0.1 M) as supporting electrolyte and platinum as counter and working electrodes versus FC/FC+ at l00 mV/s in
c = 5 · 10À3 M.
a N,N-Dimethylformamide.
b Dichloromethane.
c In dichloromethane.

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R. Jua´rez et al. / Tetrahedron Letters 46 (2005) 8861–8864
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-2
-4
-2
-4
-3.0
-2.5
-2.0
0.5
1.0
1.5
Voltage (V)
Figure 4. Cyclic voltammogram of 4 in dichloromethane (oxidation)
and dimethylformamide (reduction). TBAHPF (0.1 M) solutions,
20 ꢁC, scan rate of 100 mV/s, Pt disk working electrode, potentials
versus FC/FC+.
In conclusion, two novel donor–acceptor phenylazome-
thine dendrimers with a s-triazine core and bearing
peripheral butoxybenzene moieties have been synthe-
sized. These materials show an amphoteric redox beha-
viour due to the acceptor ability of the s-triazine moiety
and the donor ability of the conjugated butoxybenzene
systems. The synthetic strategy employed paves the
way for the incorporation of stronger donors at periph-
eral positions and the syntheses of higher generation
dendrimers. Work is in progress in order to conduct
the photophysical study of these donor–acceptor dendri-
mers to determine the existence of energy and/or photo-
induced electron transfer processes.
´
10. Cherioux, F.; Maillote, H.; Audebert, P.; Zyss, J. Chem.
Commun. 1999, 2083.
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14. Xuong, N. D.; Buu-Hoi, N. P. J. Chem. Soc. 1952, 3741.
15. Synthesis of dendrons. General procedure: to a solution
under argon atmosphere of 12 (0.42 mmol), 6 or 9
(1.35 mmol) and DABCO (3.78 mmol) in 10ml of dry
toluene, TiCl4 (1.45 mmol) is added dropwise and the
crude is heated at reflux. After 3 h, the mixture is filtered,
vacuum-evaporated and the remaining material is purified
by column chromatography over silica gel.
Acknowledgement
16. Selected spectroscopic data for 4: 1H NMR (CDCl3,
300 MHz): d = 8.55 (m, 6H), 7.73 (m, 6H), 7.10–6.73 (m,
24H), 4.02 (m, 6H), 3.90 (m, 6H), 1.86–1.70 (m, 12H),
1.54–1.45 (m, 12H), 1.03–0.92 (m, 18H). 13C NMR
(CDCl3, 75 MHz): d = 171.1, 168.6, 162.1, 160.0, 156.1,
132.6, 131.9, 131.6, 131.2, 131.0, 130.0, 128.2, 121.8, 114.7,
114.4, 114.2, 68.3, 68.0, 31.6, 19.6, 14.2. FT-IR (KBr): m
(cmÀ1): 3052, 2956, 2931, 1591, 1569, 1506. MS (FAB:
NBA): m/z = 1279.7 [M+H]+. Elem. Anal. Calcd for
C84H90N6O6: C: 78.84%, H: 7.09%, N: 6.57%. Found C:
78.97%, H: 6.95%, N: 6.31%. Mp: 186 ꢁC. Compound 5:
1H NMR (CDCl3, 300 MHz): d = 7.85–7.43 (m, 24H),
7.15–6.48 (m, 60H), 4.13–3.75 (m, 24H), 1.85–1.66 (m,
24H), 1.55–1.37 (m, 24H), 1.08–0.80 (m, 36H). 13C NMR
(CDCl3, 75 MHz): d = 171.2, 168.2, 168.1, 162.8, 161.6,
159.8, 156.1, 134.1, 132.6, 131.9, 131.5, 131.4, 130.6, 121.8,
121.3, 68.3, 68.1, 31.6, 19.6, 14.2. FT-IR (KBr): m (cmÀ1):
3037, 2958, 2933, 2871, 1604, 1587, 1569, 1508, 1247. MS
(ESI): m/z = 2788.1 [M+H]+. Elem. Anal. Calcd for
MCyT (Ref. CTQ2004-03760) and Comunidad de Mad-
rid (Ref. GR/MAT/0628/2004) for financial support.
´
R.G. is indebted to the ÔPrograma Ramon y CajalÕ.
References and notes
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C
186H192N12O12: C: 80.14%, H: 6.94%, N: 6.03%. Found:
C: 79.64%, H: 6.83%, N: 5.89%. Mp: 109 ꢁC.
17. Fink, R.; Frenz, C.; Thelakkat, M.; Schmidt, H.-W.
Macromolecules 1997, 30, 8177.