?-Conjugated Dendrimers Based on Bis(enediynyl)benzene Units

Full text of DOI:10.1021/ol049116g

ORGANIC
LETTERS
2004
Vol. 6, No. 16
2669-2672
π-Conjugated Dendrimers Based on
Bis(enediynyl)benzene Units
Gil Tae Hwang and Byeang Hyean Kim*
National Research Laboratory, Department of Chemistry, DiVision of Molecular and
Life Sciences, Pohang UniVersity of Science and Technology, Pohang 790-784, Korea
bhkim@postech.ac.kr
Received May 14, 2004
ABSTRACT
We have synthesized a new family of π-conjugated dendrimers that are based on bis(enediynyl)benzene units by using both divergent and
convergent approaches. The compounds at all three generations have strong bluish-green fluorescence, especially the third-generation dendrimer,
which has the highest extinction coefficient and quantum efficiency in this series.
The past several decades have been witness to dramatic
developments in the field of organic and polymer light-
emitting diodes (LEDs).1 Whereas low-molecular-weight
organic materials can be processed into device structures by
thermal evaporation under high vacuum, polymers can be
deposited simply by spin-coating or even printing.2 Excellent
progress has been made in the efficiencies and lifetimes of
devices and in the tuning of colors using both low-molecular-
weight organic materials and polymers. Dendrimers are
organic macromolecules that have a regular array of highly
branched units surrounding a central core.3 A wide range of
synthetic methodologies have been applied to the efficient
synthesis of dendrimers. Two general approaches are used
commonly to synthesize these molecules, namely the diver-
gent growth approach and the convergent approach.3 Along
with flexible backbone dendrimers, π-conjugated dendritic
molecules have received considerable attention as a new class
of LED materials.4 These shape-persistent macromolecules,
which have well-defined nanometer sizes and intrinsic
rigidity, exhibit interesting physical and chemical properties.
Curiously, there have been few examples of π-conjugated
dendrimers based on bis(enediynyl)benzene (Figure 1).
(1) (a) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.;
Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bre´das, J.
L.; Lo¨gdlund, M.; Salaneck, W. R. Nature 1999, 397, 121-128. (b) Kraft,
A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. 1998, 37,
402-428.
Figure 1. Bis(enediynyl)benzene.
Recently, we and Neckers’ group reported that bis(enediy-
nyl)aryl compounds possess photophysical and photochemi-
cal properties that are appropriate for various applications
as materials.5,6 In this paper, we present our design, synthesis,
and photophysical studies of bis(enediynyl)benzene-based
(2) Hebner, T. R.; Wu, C. C.; Marcy, D.; Lu, M. H.; Strum, J. C. Appl.
Phys. Lett. 1998, 72, 519-521.
(3) (a) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendritic
Molecules: Concepts, Synthesis, PerspectiVes; Wiley-VCH: Weinheim,
Germany, 2002. (b) Fre´chet, J. M. J.; Tomalia, D. A. Dendrimers and Other
Dendritic Polymers; Wiley: Chichester, U.K., 2001.
10.1021/ol049116g CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/16/2004

Scheme 1. Synthesis of Dendrimers Using a Divergent Method
dendrimers that we prepared using both divergent and
convergent synthetic methodologies.
only a few side products were formed in the Sonogashira
reactions in addition to the desired products.
The divergent approach we followed to prepare our den-
drimers is illustrated in Scheme 1. The synthetic strategy
we used to construct the bis-enediyne linkages involved
alternating Sonogashira reactions7 and Corey-Fuchs dibro-
moolefinations8 and began with 1,4-bis(2,2-dibromoethenyl)-
benzene 2. The Sonogashira reactions between aryl bromides
(2, 4, and 5) and 4-ethynylbenzaldehyde9 (3) proceeded
smoothly to afford dendrimers G1, G2, and G3, respectively,
in moderate yields (33-76%). Considering the rigidity and
steric hindrance in G3, the yield of 36% is quite remarkable.
The dibromoolefination reactions were relatively clean and
A convenient method for dendrimer synthesis is the
assembly of dendrons to a core unit (i.e., the convergent
approach), which is used widely to form functional den-
drimers or simply to enlarge a compound.10 We adopted this
method to synthesize G2 and G3, as indicated in Scheme 2,
using the readily available ethynyldibenzaldehyde11 6 as a
dendron. Pd/Cu-catalyzed cross-coupling of 2 and 4 with
dendron 6 afforded the dendritic molecules G2 and G3,
respectively, which bear peripheral aldehyde units.
All of these dendritic molecules are quite stable toward
air and commonly used organic solvents; they are slightly
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Org. Lett., Vol. 6, No. 16, 2004

lecular ions in the FAB mass of G1 and G2 were found at
m/z 643.3 and 1659.1, which are consistent with the calcu-
lated values of m/z 643.2 and 1659.5, respectively. MALDI-
TOF analysis verified the monodisperse nature of G3; the
mass of its molecular ion, m/z 3691.2, is consistent with the
calculated value of m/z 3691.1. The values of Mn of G1-G3
obtained by gel permeation chromatography (GPC) measure-
ments using polystyrene standardss540, 860, and 1500,
respectivelyswere lower than the mass spectrometry-derived
molecular weights.12 These significant deviations between
GPC and MS results, especially for G3, may reflect the
dissimilar shapes of these dendrimers, which are compact
and spherical in solution, and the polystyrene standards.4f,13
Table 1 summarizes the photophysical properties of den-
dritic molecules G1-G3, 4, and 5 in CHCl3 solutions. We
determined the fluorescence quantum yields (φF) of the
fluorophores in CHCl3, using a 0.1 N aqueous NaOH solution
of fluorescein as a standard.14
Scheme 2. Synthesis of Dendrimers Using a Convergent
Method
The UV-vis absorption spectrum of the G1 dendrimer
displays an absorption maximum at 433 nm. The relative
blue shifts observed for G2 and G3 probably are due to the
substitution effect of the aldehyde group: As the generations
increase, the formyl groups are located further from the core
(Figure 2a). The emission maxima of the dendrimers are
soluble in CHCl3, THF, DMF, and DMSO. We confirmed
1
the structures of all these dendrimers by H and 13C NMR
spectroscopy, mass spectrometry, and elemental analysis.12
1
The observed chemical shifts in the H NMR spectra are
very similar within this series of compounds. All dendrimers
G1-G3 possess proton chemical shifts at ca. 10 ppm that
are due to their formyl groups. We confirmed the structures
of the dendrimers clearly by mass spectrometry. The mo-
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Figure 2. Normalized (a) absorbance and (b) fluorescence spectra
of dendrimers G1-G3 recorded in CHCl3 at 20 °C. Emission
spectra were obtained upon excitation at the absorption maxima.
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(12) See the Supporting Information.
Org. Lett., Vol. 6, No. 16, 2004
2671

divergent or convergent methods. This approach is also very
versatile for the syntheses of the dendrimers that possess
different photophysical properties, which can be achieved
through variations in the nature of the building molecules
and the functional groups presented at the peripheries. The
second point is that this synthetic approach, which is based
on bis(enediynyl)benzene units, can be utilized in the design
of functional materials because of their excellent quantum
efficiencies.
We have prepared a new family of π-conjugated dendrim-
ers, which are based on bis(enediynyl)benzene units, as
potential candidates for materials applications by applying
two synthetic approaches: a divergent method using alternat-
ing dibromoolefination and Pd/Cu-catalyzed cross-coupling
and a convergent method using Pd/Cu-catalyzed cross-
coupling between bromoaryl compounds and ethynyldiben-
zaldehyde as a dendron. The resulting dendrimers maintain
uniform structures in terms of the linking units involved
between the generations. All three generations of dendrimers
exhibit strong bluish-green fluorescence, especially the third-
generation one, G3, which has the highest extinction coef-
ficient and quantum efficiency in the whole series of
compounds. Hence, we believe that this approach will be
applicable for the construction of other structurally uniform
and well-defined π-conjugated functional dendrimers.
Table 1. Photophysical Data of the Dendrimers in CHCl3
d
compd
λ
abs/nma
ꢀ/mol-1 cm-1
λ
em/nmb,c
φF
G1
4
G2
5
433
411
415
415
418
22 500
45 300
61 500
57 300
86 000
497
497
468
465
469
0.38
0.35
0.79
0.52
0.80
G3
a Only the largest absorption maximum is listed in each case. b Wave-
length of emission maximum when excited at the absorption maximum.
c Peaks contain a shoulder having a longer wavelength. d Quantum efficien-
cies were measured using fluorescein in 0.1 N NaOH as a standard, λex
436 nm.
)
affected by this shift pattern in the absorption spectra. The
fluorescence spectra of all these dendritic molecules show
two strong emission bands in the visible region, which results
in their bluish-green colors (Figure 2b). Although dendrimer
G3 has a longer conjugation length than G2, its absorbance
and emission maxima are not red-shifted significantly, which
presumably arises because of the torsion between the benzene
rings that is caused by the increased steric hindrance as the
number of generations increases. Interestingly, of this series
of dendrimers, G2 and G3 possess the highest quantum
efficiencies (φF ) 0.79 and 0.80, respectively).
Acknowledgment. We thank KISTEP for financial sup-
port through the National Research Laboratory Program.
There are two important points worth emphasizing. First,
the synthetic approachsconsecutive dibromoolefination and
Sonogashira couplingsthat we have developed here is very
efficient for preparing the conjugated dendrimers via either
Supporting Information Available: Complete experi-
1
mental details (including H and 13C NMR spectroscopic,
MS, GPC, and elemental analysis data). This material is
available free of charge via the Internet at http://pubs.acs.org.
(13) Yu, D.: Vladimirov, N.; Fre´chet, J. M. J. Macromolecules 1999,
32, 5186-5192
(14) Parker, C. A. In Photoluminescence of Solutions; Elsevier: Am-
sterdam, 1968.
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