Synthesis of photo- and electroactive stilbenoid dendrimers carrying dibutylamino peripheral groups

Article abstract of DOI:10.1021/ol016083l

(Equation presented) A novel convergent synthetic route for the preparation of functionalized and fluorescent stilbenoid dendrons built on the 1,3,5-benzene core and endowed with a periphery of dibutylamino groups has been developed. Long alkyl chains hav

Full text of DOI:10.1021/ol016083l

ORGANIC
LETTERS
2001
Vol. 3, No. 17
2645-2648
Synthesis of Photo- and Electroactive
Stilbenoid Dendrimers Carrying
Dibutylamino Peripheral Groups
,†
,‡
Jose´ L. Segura,† Rafael Go´mez,† Nazario Mart´ın,* and Dirk M. Guldi*
Departamento de Qu´ımica Orga´nica, Facultad de Qu´ımica, UniVersidad Complutense,
E-28040 Madrid, Spain, and Radiation Laboratory, UniVersity of Notre Dame,
Notre Dame, Indiana 46556
nazmar@eucmax.sim.ucm.es
Received May 9, 2001
ABSTRACT
A novel convergent synthetic route for the preparation of functionalized and fluorescent stilbenoid dendrons built on the 1,3,5-benzene core
and endowed with a periphery of dibutylamino groups has been developed. Long alkyl chains have been incorporated on the peripheral amino
moieties to increase the solubility of the final products. Good donor ability of the new dendrimers has been observed by cyclic voltammetry
measurements as a result of the presence of the peripheral dibutylaniline moieties.
On the basis of their unique properties, monodisperse
dendritic materials emerged as attractive candidates for
photonic applications. For example, dendritic macromol-
ecules have been blended into organic light emitting diodes
(OLEDs) as either (i) hole1 transporting components or (ii)
electron2 transporting components or even as (iii) single-
component emitters.3 Shirota and co-workers developed a
concept of π-electron starburst molecules such as triaryl-
amine-based dendritic molecules. The latter have been
successfully employed as hole transport layers in OLEDs.4
On the other hand, the interesting photophysical and photo-
chemical5 properties of stilbenoids evoked the incorporation
of stilbenoid chromophores into dendrimers. Only very
* Email for additional corresponding author: guldi.1@nd.edu.
† Universidad Complutense.
‡ University of Notre Dame.
(3) Wang, P. W.; Liu, Y. J.; Devadoss, C.; Bharathi, P.; Moore, J. S.
AdV. Mater. 1996, 8, 237.
(4) Shirota, Y. J. Mater. Chem. 2000, 10, 1.
(1) Kuwabara, Y.; Ogawa, H.; Inada, H.; Noma, N.; Shirota, Y. AdV.
Mater. 1994, 6, 677.
(2) Bettenhausen, Greczmiel, M.; Jandke, M.; Strohriegl, P. Synth. Met.
1997, 41, 223.
(5) (a) Meier, H. Angew. Chem., Int. Ed. Engl. 1992, 31, 1399. (b) Lewis,
F. D.; Kalgutkar, R. S.; Yang, J. S. J. Am. Chem. Soc. 1999, 121, 12045.
10.1021/ol016083l CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/02/2001

recently, several synthetic routes have been developed to
achieve this target.6
Scheme 2. Synthesis of Stilbenoid AB2 Building Block 6
In this Letter we wish to report a new convergent synthetic
route toward dendrimeric compounds that combines a
stilbenoid skeleton and an array of electron-donating dibutyl-
amine groups bearing solubilizing alkyl chains on the
peripheral positions.
The first generation dendrimer (3) was obtained in 30%
yield following the Wittig-Horner reaction between tri-
phosphonate (1) and 4-(N,N-dibutylaminobenzaldehyde (2)
in dry tetrahydrofuran, using potassium tert-butoxide as base
(Scheme 1).
Scheme 1. Synthesis of First-Generation Dendrimer 3
An alternative route involves the preparation of intermedi-
ates 8 and 11 as precursors to synthesize higher generation
dendrimers. Bromination of 3,5-dimethylbenzonitrile9 (9)
under radical conditions gave the 3,5-bis(bromomethyl)-
benzonitrile10 (10) as a white solid. Subsequent treatment
of 10 with triphenylphosphine in dimethylformamide allowed
us to obtain the 3,5-bis(triphenylphosphoniummethyl)-
benzonitrile (7) in 70% yield. On the other hand, reaction
Scheme 3. Alternative Syntheses of Stilbenoid AB2 Building
Block 6
To grow subsequent generations of dendrimeric ensembles,
through a convergent synthetic route, necessitated the design
of stilbenoid AB2 building block 6 by following three
different strategies (Schemes 2 and 3).
The first strategy involves the Wittig reaction between
1,3,5-triformylbenzene7 (4) and 4-(N,N,-dibutylaminobenzyl)-
triphenylphosphonium iodide8 (5) in dry ethanol, using
lithium ethoxide as base, under stoichiometric control. This
afforded 6 as a mixture of three possible configurational
isomers. In a second step, iodine-catalyzed thermal isomer-
ization of the mixture led to the all-trans isomer 6 in 65%
yield (Scheme 2).
(6) (a) Deb, S. K.; Maddux, T. M.; Yu, L. J. Am. Chem. Soc. 1997, 119,
9079. (b) Meier, H.; Lehmann, M. Angew. Chem., Int. Ed. Engl. 1998, 37,
643. (c) Meier, H.; Lehmann, M.; Kolb, U. Chem. Eur. J. 2000, 6, 2462.
(d) Pillow, J. N. G.; Halim, M.; Lupton, J. M.; Burn, P. L.; Samuel, I. D.
W. Macromolecules 1999, 32, 5985. (e) D´ıez-Barra, E.; Garc´ıa-Mart´ınez,
J. C.; Rodr´ıguez Lo´pez, J.; Go´mez, R.; Segura, J. L.; Mart´ın, N. Org. Lett.
2000, 2, 3651. (f) Lupton, J. M.; Samuel, I. D. W.; Beavington, R.; Ba¨ssler,
H. AdV. Mater. 2001, 13, 258.
(7) Formigue´, M.; Johannsen, I.; Boubekeur, K.; Nelson, C.; Batail, P.
J. Am. Chem. Soc. 1993, 115, 3752.
(8) Bredereck, H.; Simchen, G.; Griebenow, W. Chem. Ber. 1973, 106,
3732.
2646
Org. Lett., Vol. 3, No. 17, 2001

of 10 with excess trimethyl phosphite led to the correspond-
ing bis(phosphonate) 11 in 84% yield as a white solid. Wittig
reaction between 7 and 4-(N,N-dibutylamino)benzaldehyde
(2) in dry ethanol, using lithium ethoxide as base, afforded
8 as a mixture of isomers. Again, iodine-catalyzed thermal
isomerization of the mixture afforded 8 as an all-trans isomer.
In the third strategy the direct synthesis of the all-trans 8
was pursued, and 67% yield was accomplished by the
Wittig-Horner reaction between bis(phosphonate) 11 and
aldehyde 2 in dry THF at 0 °C, using potassium tert-butoxide
as base. Treatment of a dichloromethane solution of 8 with
a stoichiometric amount of DIBAL-H (1.0 M) afforded the
pure all-trans isomer 6 in 73% yield.
the reactive aldehyde group. It is worth to point out that, in
our hands, previously reported synthetic routes toward
stilbenoid dendrimers12 have failed to obtain higher genera-
tions of dendrimers containing peripheral dibutylamino
groups. This is due to the basic requirement of oxidizing a
benzylic alcohol to the corresponding aldehyde as a key step
to obtain the intermediate dendrons. However, the strong
electron-donor character of the dibutylaniline groups limits
the stability of these compounds under oxidation conditions
as a result of the possibility of electron transfer processes.
In the present approach, this intrinsic problem has been
efficiently overcome by using nitrile functionalities. The latter
can be readily converted into the corresponding aldehydes
upon selective reduction conditions, thus avoiding the
oxidation process.
Wittig-Horner reaction between tris(phosphonate) 1 and
aldehyde 6 in dry THF at 0 °C, using potassium tert-butoxide
as base, afforded the second-generation dendrimer 12 as a
yellow compound in a 40% yield (Scheme 4). The presence
The new dendrimers show in dichloromethane ground-
state absorption maxima at 373 and 379 nm for the second
and first generation, respectively (see Figure 1). A marked
Scheme 4. Syntheses of Second-Generation Dendrimer 12
Figure 1. Absorption (solid line) and emission (dashed line) spectra
(λexc 380 nm) of dendrimer 12.
red shift is noted in these transitions, relative to the absorption
of distyrylbenzene (λmax ) 360 nm),13 caused by the
dialkylamino substituted stilbene units. Moreover, the second-
generation dendrimer shows an additional 337 nm shoulder,
stemming from the core stilbenoid system, which lacks the
(11) Dendrimers were characterized on the basis of the FTIR and 1H
and 13C NMR analyses for which satisfactory results were obtained.
Synthetic procedure for 12: Under argon atmosphere, 1 mmol of trispho-
sphonate 1 and 3 mmol of aldehyde 6 were disolved in dry THF at 0 °C.
Afterwards, 3 mmol of potassium tert-butoxide was added portionswise,
and the reaction was allowed to stand for 10 min. After this time, first
methanol and then water were added. The layers were separated and
extracted with chloroform. The combined organic layers were washed with
water, dried over magnesium sulfate, and then evaporated to yield a solid
that was purified by chromatography. Yield: 40%. Selected data for 12:
of the peripheral dibutylamino groups provides enhanced
solubility of this compound, and therefore appropriate
spectroscopic11 and electrochemical characterization could
be carried out.
An overall advantage of the convergent route presented
in this communication is the use of a nitrile group in the
readily available AB2 starting material 9. Importantly, this
functionality remains unaffected during the subsequent
workup and could be easily converted, in the final step, to
FT-IR (KBr) ν 2955, 2926, 2854, 1607, 1585, 1518, 1367, 1221, 959 cm-1
;
1H NMR (CDCl3, 200 MHz) δ (ppm) 7.60 (s, 3H), 7.43 (s, 3H), 7.37 (d,
12H, J ) 8.6 Hz), 7.21 (s, 3H), 7.09 (d, 9H, Jtrans ) 16.2 Hz), 6.87 (d, 9H,
Jtrans ) 16.2 Hz), 6.59 (d, 12H, J ) 8.6 Hz), 3.23 (t, 24H), 1.53 (m, 24H),
1.39-1.21 (m, 24H), 0.90 (t, 36H); 13C NMR (CDCl3, 75 MHz) δ (ppm)
144.76, 135.83, 135.09, 134.62, 134.60, 126.39, 126.18, 121.42, 120.95,
120.46, 120.44, 120.38, 119.50, 108.55, 47.73, 26.62, 17.29, 13.87.
(12) D´ıez-Barra, E.; Garc´ıa-Mart´ınez, J.; Rodr´ıguez-Lo´pez, J. Tetrahedron
Lett. 1999, 40, 8181. See also: D´ıez-Barra, E.; Garc´ıa-Mart´ınez, J. C.;
Merino, S.; Del Rey, R.; Rodriguez-Lo´pez, J.; Sa´nchez-Verdu´, P.; Tejada,
J. J. Org. Chem., in press.
(9) Gryszkiewicz-Trochimowski, M. E.; Schimidt, W.; Gryszkiewicz-
Trochimowski, O. Bull. Soc. Chim. Fr. 1948, 593.
(10) Bodwell, J. G.; Bridsom, J. N.; Houghton, T. J.; Yarlagadda, B.
Tetrahedron Lett. 1997, 38, 7475.
(13) Halim, H.; Pillow, J. N. G.; Samuel, I. D. W.; Burn, P. L. AdV.
Mater. 1999, 11, 371.
Org. Lett., Vol. 3, No. 17, 2001
2647

conjugation with the peripheral p-dialkylaminostyryl groups
because of the chosen meta linkage.
Cyclic voltammetry measurements, conducted with 3 and
12 in dichloromethane, reveal an irreversible oxidation wave
at +0.59 V for both compounds. For the second-generation
dendrimer a second irreversible oxidation wave is observed
at +1.46 V. This suggests that, in contrast to stilbenoid
dendrimers containing a nitrogen core,6f where the first
oxidation step is directed to the core, in these new dendrim-
ers, the first oxidation occurs at the periphery, that is, the
dialkylaminostilbene units. The second oxidation, which was
solely seen for 12, is then assigned to the core (i.e., stilbenoid
unit). In agreement with these findings, the parent p-(N,N-
dibutylamino)stilbene used as reference exhibits an irrevers-
ible oxidation wave at +0.65 V under the same experimental
conditions.
Figure 2. Fluorescence decay profiles for dendrimer 12 (∼2.0 ×
10-5 M) in deoxygenated toluene (solid line). In addition, the lamp
scatter is shown (dashed line).
In analogy to the basic constituent of 3 and 12, namely, a
conjugated stilbene unit, both dendrimers are strong fluo-
rophores. Their singlet-excited states exhibit, mirror-imaged
to the ground-state absorption, a long-lived emission (3, τ
) 2.19 ns; 12, τ ) 2.1 ns) at 430 nm, produced in high
quantum yields (3, Φ ) 0.55; 12, Φ ) 0.55), measured
relative to a 9,10 diphenyl anthracene (Φ ) 1) (see Figures
1 and 2). It should be noted that these values differ,
nevertheless, from those seen for stilbenes (τ ) 0.075 ns; Φ
) 0.036), in which “cis-trans” isomerization dominates
(>95%) the deactivation of the singlet excited state. There-
fore, with the scope to suppress this contribution we added
p-(N,N-dibutylamino)stilbene to the emission assay, in which
one of the two arenes is substituted at the para position with
a bulky dibutylamino substituent. Indeed, the high fluores-
cence quantum yield (Φ ) 0.22) and the fluorescence
lifetime (τ ) 0.11 ns), as measured at the 425 maximum,
further support the features associated with the fluorescence
of dendrimers 3 and 12.
shown). The singlet lifetimes, which were determined by
fitting the singlet-singlet decay to a monoexponential rate
law, are 2.25 ns (3) and 2.06 ns (12). In other words they
are in excellent agreement with the emissive component of
the singlet excited state.
In conclusion, we have presented a new convergent
synthetic strategy en route to stilbenoid dendrimers carrying
a periphery of dibutylamino substituents. The synthesis of
the stilbenoiod AB2 building blocks 6 and 11 paves the way
for the convergent synthesis of higher generation dendrimeric
systems. Furthermore, as a result of the good donor ability
exhibited by these systems and the enhanced solubility
provided by the presence of peripheral dibutylamino groups,
the AB2 system 6 can be an appealing molecule to study the
charge-transport and processability properties of materials
for OLEDs and photovoltaic devices.
Acknowledgment. This work has been supported by the
DGESIC of Spain (Project PB98-0818) and the Office of
Basic Energy Sciences of the U.S. Department of Energy.
This is contribution NDRL-4324 from the Notre Dame
Radiation Laboratory.
As far as transient absorption characteristics of this new
class of dendrimers are concerned, short laser excitation (20
ps; 355 nm) generates a broadly absorbing singlet-singlet
excited-state absorption, which centers around 650 nm (not
OL016083L
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Org. Lett., Vol. 3, No. 17, 2001