Synthesis and optical properties of novel unsymmetrical conjugated dendrimers

Article abstract of DOI:10.1021/ja0006907

The first unsymmetrical conjugated dendrimer was synthesized using a rather simple approach that involves only one synthon and two sets of reaction conditions. This dendrimer shows broad absorption in the UV and visible range and possesses an intrinsic en

Full text of DOI:10.1021/ja0006907

J. Am. Chem. Soc. 2000, 122, 6619-6623
6619
Synthesis and Optical Properties of Novel Unsymmetrical Conjugated
Dendrimers
Zhonghua Peng,* Yongchun Pan, Bubin Xu, and Jianheng Zhang
Contribution from the Department of Chemistry, UniVersity of Missouri-Kansas City, 5100 Rockhill Road,
Kansas City, Missouri 64110
ReceiVed February 25, 2000
Abstract: The first unsymmetrical conjugated dendrimer was synthesized using a rather simple approach that
involves only one synthon and two sets of reaction conditions. This dendrimer shows broad absorption in the
UV and visible range and possesses an intrinsic energy gradient from the outside branches to the core, naturally
suggesting itself as a potentially efficient light harvesting material.
Dendrimers possess a multibranched structure that radiates
out from a central core.1 Such a dendritic structure not only
represents a fundamentally new molecular architecture, it also
offers an enormous opportunity for creating new functional
materials.1 One characteristic of dendritic molecules is the
presence of numerous peripheral end groups that all converge
to a single core. Such a structure naturally suggests itself as a
molecular antenna suitable for transferring energy or electrons
from the surface to the core. Indeed, several elegant light-
harvesting dendrimers have been reported by Fre´chet,2 Moore,3
and others.4 Fre´chet’s dye-labeled dendrimer utilizes flexible
poly(aryl ether)s as the dendritic skeleton. The energy transfer
from the peripheral dye to the center dye takes place through a
dipole-dipole interaction (Fo¨rster Energy Transfer), which is
distance dependent. Efficient energy transfer is achieved with
dendrimers up to the fourth generation.2 Moore reported a series
of rigid phenylacetylene-based dendrimers with a fluorescent
perylene chromophore at the locus.3 One particularly novel
design is the so-called “extended” dendrimer that possesses
consecutively increasing conjugation length toward the center
of the molecule, thus creating an energy gradient from the
outside branches to the inside branches. Such a dendrimer could
act as an energy funnel.
However, all these light-harvesting systems utilize sym-
metrical dendrimers: dendrimers with equivalent branches. As
a matter of fact, almost all the dendrimers (not just those
developed for light harvesting applications) studied so far have
symmetrical structures.1 This is due partly to the chemists’
Figure 1. Models of dendrimers with symmetrical branches (A) and
fascination with molecular regularity and partly to the ease in
tree-like branches (B).
(1) (a) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendritic
Molecules: Concepts, Syntheses, PerspectiVes; VCH: Weinheim, 1996. (b)
Fre´chet, J. M. J.; Hawker, C. J. In Synthesis and Properties of Dendrimers
and Hyperbranched Polymers, ComprehensiVe Polymer Science, 2nd Suppl.;
Aggarwal, S. L., Russo, S., Eds.; Pergamon: Oxgord, 1996. (c) Fischer,
M.; Vo¨gtle, F. Angew. Chem., Int. Ed. Engl. 1999, 38, 884-905. (d) Smith,
D. K.; Diederich, F. Chem. Eur. J. 1998, 4 (8), 1353-1361. (e) Tomalia,
D. A.; Naylor, A. M.; Goddart, W. A., III Angew. Chem., Int. Ed. Engl.
1990, 29, 138-175. (f) Zeng, F.; Zimmerman, S. C. Chem. ReV. 1997, 97,
1681-1712.
synthesis. A model of such dendrimers is shown in Figure 1 as
model A. Trees in nature, however, prefer unsymmetrical
branches, which presumably allow better transportation of
nutrients among leaves and roots, as well as sub-branches. This
can be easily understood by inspecting the two models shown
in Figure 1. For symmetrical dendrimers, communication
between peripheral groups and the core has to go through each
sub-branch. For tree-like dendrimers (Model B in Figure 1),
however, shortcuts such as the trunk or main branches exist so
that the core can reach the end groups directly. We thus believe
that dendrimers with unsymmetrical branches may be better
materials if signal communication between dendrimer periphery
segments and the core is required. In other words, tree-like
(2) Gilat S. L.; Adronov, A.; Fre´chet, J. M. J. Angew. Chem., Int. Ed.
Engl. 1999, 38, 1422-1427.
(3) (a) Devadoss, C.; Bharathi, P.; Moore, J. S. J. Am. Chem. Soc. 1996,
118, 9635. (b) Shortreed, M. R.; Swallen, S. F.; Shi, Z.; Tan, W.; Xu, Z.;
Devadoss, C.; Moore, J. S.; Kopelman, R. J. Phys. Chem. B 1997, 101,
6318.
(4) (a) Balzani, V.; Campagna, S.; Denti, G.; Juris, A.; Serroni, S.;
Venturi, M. Acc. Chem. Res. 1998, 31, 26-34. (b) Stewart, G. M.; Fox,
M. A. J. Am. Chem. Soc. 1996, 118, 4354-4360.
10.1021/ja0006907 CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/23/2000

6620 J. Am. Chem. Soc., Vol. 122, No. 28, 2000
Peng et al.
Scheme 1. Synthetic Sequence to Conjugated Unsymmetrical Dendrimers
dendrimers may be better candidates as light-harvesting antennae
and energy-transferring funnels.
hydroxyl group at the core which should allow the attachment
of a variety of structural units to the dendritic core, resulting in
an array of novel new functional dendrimers. This paper reports
the detailed synthesis and optical properties of these dendrimers.
To materialize and demonstrate the above concept, we
recently synthesized a new type of conjugated dendrimer based
on meta- and para-linked phenylacetylenes (taking the core as
the reference point for describing the branching). Two features
are unique to this new dendritic structure: different branches
exhibit different conjugation lengths, which results in a broad
absorption spectral range, and conjugation lengths increase from
the outside branches to the core, which generates an intrinsic
energy gradient pointing inward. Thus, such a dendrimer could
act as an extremely efficient light-harvesting material by
funneling a broad wavelength range of light to the energy trap
at the locus. In addition, this dendrimer possesses a phenol
Experimental Section
Scheme 1 shows the synthetic approach to various generation
dendrimers. The building block molecule, compound I, was synthesized
according to Scheme 2. All chemicals were purchased from Aldrich
Chemical Co. and used as received unless otherwise stated.
4,5-Diiodo-2-methoxylphenol (2): A 17.1 mL sample of BBr3 (1
M solution in CH2Cl2) was added dropwise to the solution of compound
1 (20 g, 51.3 mmol) in CH2Cl2 (25 mL) at -15 °C (ice-salt bath).
After being stirred for 1 h at room temperature, the mixture was poured
into water. The organic layer was separated, washed with water, and
then dried over sodium sulfate. The solvent was removed and the

NoVel Unsymmetrical Conjugated Dendrimers
J. Am. Chem. Soc., Vol. 122, No. 28, 2000 6621
Scheme 2. Synthesis of 4,5-Diethynyl-2-methoxylphenol
resulting residue was subjected to chromatographic separation. The
1.26 (s, 36H, t-Bu). 13C NMR (CDCl3, ppm): δ 150.9, 146.6, 145.8,
126.0, 122.5, 120.0, 118.7, 117.8, 113.8, 93.5, 93.1, 87.3, 86.9, 56.2,
34.9, 31.5.
desired product (compound 2) was obtained in 43% yield (8.5 g, mp
1
80-82 °C). H NMR (CDCl3, ppm): δ 7.39 (s, 1H, Ar-H), 7.24 (s,
1H, Ar-H), 5.58 (s, 1H, OH), 3.86 (s, 3H, OCH3). 13C NMR (CDCl3,
ppm): δ 147.4, 146.4, 125.0, 121.1, 96.9, 94.9, 56.4.
4,5-Di(3′,5′-di-tert-butylphenylethynyl)-2-methoxyphenyl trifluo-
romethanesulfonate (G1-OTf): A 0.472 g sample of trifluoromethane-
sulfonic anhydride (1.67 mmol) was added to a solution containing
G1 (0.765 g, 1.40 mmol) and 10 mL of pyridine at 0 °C. The resulting
mixture was stirred for 3 h. The workup procedure was similar to that
of 3,5-di-tert-butylphenyl trifluoromethanesulfonate. G1-OTf (0.82 g)
2-Methoxy-4,5-di(trimethylsilylethynyl)phenol: A 2.36 g (24
mmol) sample of trimethylsilylacetylene was added to a mixture
containing compound 2 (3.76 g, 10 mmol), Pd(PPh3)2Cl2 (0.281 g, 0.4
mmol), CuI (0.152 g, 0.8 mmol), and trimethylamine (30 mL) at room
temperature. The resulting mixture was stirred for 5 h and then the
solids were separated from the solution by filtration. The filtrate was
concentrated and then dissolved in CH2Cl2. The resulting solution was
washed with an aqueous HCl solution and then with water. The organic
layer was separated and dried over Na2SO4. The crude product was
purified by chromatography eluting with 5:1 hexane/ethyl acetate to
give the title compound as a white powder (2.6 g, 82%). Mp: 60-62
°C. 1H NMR (CDCl3, ppm): δ 6.99 (s, 1H, Ar-H), 6.91 (s, 1H, Ar-
H), 5.68 (s, 1H, OH), 3.88 (s, 3H, OCH3), 0.26 (s, 18H, SiMe3).
4,5-Diethynyl-2-methoxyphenol (Compound I): A 20 mL sample
of aqueous KOH solution (1 N) was added to the solution containing
2.6 g of 2-methoxy-4,5-di(trimethylsilylacetylene)phenol and 20 mL
of methanol at room temperature. The mixture was stirred at room
temperature for 1 h. After removal of the majority of the solvent, the
residue was treated with an aqueous HCl solution. The resulting solution
was extracted with methylene chloride. The organic layer was collected
and the solvent was evaporated. The resulting mixture was purified by
chromatography on silica gel using 3:1 hexane/ethyl acetate as the eluent
to yield compound I as a brownish yellow crystal (1.1 g, 80%). Mp:
1
was obtained (86%). Mp: 157-159 °C). H NMR (CDCl3, ppm): δ
7.41 (s, 2H, Ar-H), 7.40 (s, 1H, Ar-H), 7.38 (s, 2H, Ar-H), 7.36 (s,
2H, Ar-H), 7.20 (s, 1H, Ar-H), 3.97 (s, 3H, OCH3), 1.26 (s, 36H,
t-Bu). 13C NMR (CDCl3, ppm): δ 151.2, 138.0, 127.7, 126.2, 126.1,
125.8, 123.7, 123.4, 122.0, 121.7, 121.5, 119.5, 116.4, 116.1, 96.6,
95.1, 85.9, 85.4, 56.7, 35.0, 31.5.
The synthetic procedure for G2, G3, and G4 is similar to that of
G1, and the procedure for the synthesis of G2-OTf and G3-OTf is
similar to that of G1-OTf.
1
G2. Yield: 74%. Mp: 150-152 °C. H NMR (CDCl3, ppm, see
Scheme 1 for the labeling): δ 7.81 (s, 1H, Hb), 7.80 (s, 1H, Hb′), 7.38
(s, 6H, Ha′), 7.36 (s, 6H, Ha), 7.15 (s, 1H, Hd), 7.07 (s, 3H, Hd′ and Hc),
5.74 (s, 1H, OH), 3.96 (s, 3H, OCH3), 3.89 (s, 3H, OCH3), 3.88 (s,
3H, OCH3), 1.25 (m, 72H, t-Bu). 13C NMR (CDCl3, ppm): δ 159.3,
151.0, 146.9, 146.1, 137.2, 127.1, 126.1, 126.0, 123.3, 122.9, 122.5,
122.1, 119.7, 118.8, 118.3, 118.1, 114.1, 113.8, 113.5 (Ar-C); 96.4,
94.4, 94.0, 93.8, 88.0, 87.6, 87.3, 86.4 (alkyne); 56.3 (OCH3), 34.9,
31.5 (t-Bu). Anal. Calcd for C89H100O4: C, 86.64, H, 8.17. Found: C,
86.57, H, 8.08. MALDI-TOF for C89H100O4 calcd 1232.76, found
1232.24.
1
91-93 °C. H NMR (CDCl3, ppm): δ 7.04 (s, 1H, Ar-H), 6.95 (s,
1H, Ar-H), 5.79 (s, 1H, OH), 3.88 (s, 3H, OCH3), 3.26 (s, 2H). 13C
NMR (CDCl3, ppm): δ 146.9, 146.3, 118.9, 118.5, 117.6, 114.5, 82.2,
81.9, 79.9, 79.5, 56.3.
G2-OTf: Yield: 86%. Mp: 140-142 °C. 1H NMR (CDCl3, ppm):
δ 7.81 (s, 1H, Ar-H), 7.79 (s, 1H, Ar-H), 7.44 (s, 1H, Ar-H), 7.38
(s, 6H, Ar-H), 7.35 (s, 6H, Ar-H), 7.21 (s, 1H, Ar-H), 7.08 (s, 1H,
Ar-H), 7.07 (s, 1H, Ar-H), 3.98 (s, 3H, OCH3), 3.88 (s, 3H, OCH3),
1.25 (m, 72H, t-Bu). 13C NMR (CDCl3, ppm): δ 159.5, 159.4, 151.3,
151.0, 138.2, 137.4, 137.2, 128.1, 127.7, 127.2, 126.1, 125.9, 123.4,
123.0, 122.5, 122.1, 119.1, 118.9, 116.2, 113.8, 112.8 (Ar-C); 97.0,
96.8, 94.0, 93.9, 92.6, 92.2, 90.5, 89.6, 87.2, 86.3, 86.2 (alkyne); 56.7,
56.4 (OCH3), 34.9, 31.5 (t-Bu).
3,5-Di-tert-butylphenyl trifluoromethanesulfonate: A 16.4 g sample
of trifluoromethanesulfonic anhydride (58.2 mmol) was added slowly
to the solution of 3,5-di-tert-butylphenol (10 g, 48.5 mmol) in 20 mL
of pyridine at 0 °C. The resulting mixture was warmed to room
temperature and stirred for 3 h, then poured into water. The aqueous
solution was extracted with CH2Cl2. The organic layer was washed
with an aqueous HCl solution and then dried over Na2SO4. After
stripping off the solvent, the residue was purified through a short silica
gel column eluting with 8:1 hexane/ethyl acetate to yield the title
compound as a colorless oil (15.3 g, 93%). 1H NMR (CDCl3, ppm): δ
7.44 (s, 1H, Ar-H), 7.09 (s, 2H, Ar-H), 1.33 (s, 18H, t-Bu).13C NMR
(CDCl3, ppm): δ 153.9, 150.0, 122.4, 121.6, 116.6, 115.7, 35.4, 31.5.
4,5-Di(3′,5′-di-tert-butylphenylethynyl)-2-methoxyphenol (G1): A
2.4 g sample of triethylamine (24 mmol) was added to the mixture of
compound I (1.03 g, 6 mmol), 3,5-di-tert-butylphenyl trifluoromethane-
sulfonate (4.08 g, 12 mmol), Pd(PPh3)2Cl2 (0.250 g, 0.36 mmol), and
20 mL of DMF at room temperature. The resulting mixture was stirred
at 70 °C for 4 h under nitrogen. After cooling to room temperature,
the mixture was poured into water and extracted with CH2Cl2. The
organic layer was washed with an aqueous HCl solution and then dried
over MgSO4. The crude product was purified by chromatography using
silica gel (230-400 mesh) eluting with 5:1 hexane/ethyl acetate to yield
G1 as a pale powder (1.1 g, 68% yield according to the amount of
triflate that reacted; triflate (2.0 g) was recovered after the reaction).
G3: Yield: 78%. Mp: 180 °C dec. 1H NMR (CDCl3, ppm, see
Scheme 1 for the labeling): δ 7.83 (s, 4H, Hb), 7.80 (s, 2H, Hb′), 7.37
(s, 12H, Ha′), 7.34 (s, 12H, Ha), 7.16 (s, 1H, Hd), 7.10 (s, 2H, Hc′),
7.06 (s, 5H, Hd′ and Hc), 5.97 (s, 1H, OH), 3.97 (s, 3H, OCH3), 3.91
(s, 6H, OCH3), 3.89 (s, 12H, OCH3), 1.25 (m, 144H, t-Bu). 13C NMR
(CDCl3, ppm): δ 159.4, 150.9, 146.9, 146.2, 137.5, 137.2, 128.2, 127.7,
127.2, 126.7, 126.1, 125.9, 123.3, 122.9, 122.5, 122.1, 119.7, 118.8,
118.3, 113.9, 113.8, 113.4, 112.9 (Ar-C), 96.7, 96.4, 94.6, 94.3, 94.1,
93.8, 93.7, 93.4, 90.9, 88.7, 88.3, 87.9, 87.6, 87.2, 86.4, 86.3 (alkyne),
56.4, 56.3 (OCH3), 34.9, 31.7 (t-Bu). Anal. Calcd for C189H204O8: C,
87.19, H, 7.90. Found: C, 87.22, H, 7.87. MALDI-TOF for G3, calcd
for C189H204O8 2601.56, found 2600.42.
G3-OTf: Yield: 76%. Mp: 174-176 °C. 1H NMR (CDCl3, ppm):
δ 7.85 (s, 4H, Ar-H), 7.80 (s, 2H, Ar-H), 7.46 (s, 1H, Ar-H), 7.37
(s, 12H, Ar-H), 7.34 (s, 12H, Ar-H), 7.07-7.12 (m, 7H, Ar-H),
4.00 (s, 3H, OCH3), 3.91 (s, 6H, OCH3), 3.89 (s, 12H, OCH3), 1.25
(m, 144H, t-Bu). 13C NMR (CDCl3, ppm): δ 159.6, 159.4, 159.3, 150.9,
138.2, 137.5, 137.2, 127.9, 127.6, 127.3, 126.1, 125.9, 123.3, 123.0,
122.5, 122.1, 118.9, 118.8, 118.5, 114.1, 113.8, 113.3, 113.2, 113.1,
112.8, 112.7 (Ar-C); 96.8, 96.5, 94.0, 93.9, 93.8, 93.2, 93.1, 92.8,
1
Mp: 138-140 °C. H NMR (CDCl3, ppm): δ 7.37 (s, 6H, Ha), 7.12
(s, 1H, Hd), 7.05 (s, 1H, Hd′), 5.74 (s, 1H, OH), 3.93 (s, 3H, OCH3),

6622 J. Am. Chem. Soc., Vol. 122, No. 28, 2000
Peng et al.
92.4, 91.4, 91.2, 90.9, 89.5, 88.6, 88.5, 87.3, 87.2, 86.4, 86.2 (alkyne);
56.7, 56.4 (OCH3), 34.9, 31.5 (t-Bu).
1
G4: Yield: 49%. Mp: 230 °C dec. H NMR (CDCl3, ppm, see
Scheme 1 for the labeling): δ 7.79-7.88 (m, 14H, Hb, Hb′ and Hb′′),
7.36 (s, 24H, Ha′), 7.33 (s, 24H, Ha), 7.16 (s, 1H, Hd), 7.06-7.11 (m,
15H, Hd′, Hc, Hc′, and Hc′′), 5.75 (s, 1H, OH), 3.89 (br, 45H, OCH3),
1.25 (m, 288H, t-Bu).13C NMR (CDCl3, ppm): δ 159.7, 159.6, 159.4,
159.3, 150.9, 146.9, 146.1, 137.8, 137.5, 137.2, 127.7, 127.2, 126.8,
126.1, 126.0, 123.3, 122.9, 122.5, 122.2, 118.8, 118.8, 118.4, 114.1,
113.9, 113.8, 113.5, 113.4, 113.3, 112.9 (Ar-C); 96.7, 96.4, 94.6, 94.4,
94.4, 94.3, 94.2, 94.1, 93.9, 93.8, 93.7, 93.5, 93.3, 91.2, 90.9, 88.5,
88.3, 87.4, 87.3, 86.5, 86.3 (alkyne); 56.4 (OCH3), 34.9, 31.5 (t-Bu).
Anal. Calcd for C389H412O16: C, 87.44, H, 7.77. Found: C, 87.56, H,
7.79. MALDI-TOF for G4, calcd for C389H412O16 5339.14, found
5339.10.
Characterization. The 1H NMR spectra were collected on a Bruker
250 MHz FT NMR spectrometer. A Hewlett-Packard 8452A diode array
spectrophotometer was used to record the UV/vis absorption spectra.
Photoluminescence properties were measured using a Shimadzu RF-
5301PC spectrofluorophotometer.
Results and Discussion
The synthesis of compound I is shown in Scheme 2. 4,5-
Diiodo-1,2-dimethoxybenzene was synthesized from 1,2-
dimethoxylbenzene.5 The monodemethylation of compound 1
was accomplished by slow addition of 0.33 equiv of BBr3 and
the product was separated by chromatography. The yield is 43%.
Direct iodination of 2-methoxyphenol to generate compound 2
with a variety of common iodination reagents6 such as I2/HIO3,
I2/Bu4N+-SO3OOSO3-+NBu4, I2/HgCl2, ICl, and (Me3N+CH2-
Ph)(ICl2)-/CaCO3 has not been successful. Palladium-catalyzed
coupling (Sonogashira reaction7) of compound 2 with trimeth-
ylsilylacetylene, followed by desilylation with potasium hy-
droxide, generates compound I in 61% yield.
The conversion of phenol to triflate is trivial, and high yield
is obtained in each case (over 75%). The cross-coupling of
triflate with acetylene could be carried out on compound I
without the protection of the hydroxy group,8 greatly simplifying
the synthetic process. Good yields (over 65%) are obtained for
the syntheses of G1, G2, and G3. Even for G4, a reasonable
yield of 49% is obtained.
Figure 2. 1H NMR spectra of G1-G4.
ppm is composed of two strongly overlapped but still distin-
guishable peaks at 3.89 and 3.88 ppm, indicating the inequiva-
lency of the two sets of methoxy groups in the two center phenyl
rings. For G3, three peaks are observed at 3.97, 3.91, and 3.88
ppm, with the later two again severely overlapped. For G4, all
the methoxyl protons are merged to one broad peak at 3.75 ppm.
The aromatic region of the 1H NMR spectra of G1-G4 can be
separated into three regions. Chemical shifts at 7.75-7.95 ppm
are due to protons at the ortho position of both ethynyl
substituents in the central phenyl rings (protons labeled b, b′,
and b′′ in Scheme 1). G1 shows no signal in this region, while
G2 shows two close signals. For G3, two well-separated sets
of peaks at 7.83 and 7.80 ppm are observed with an integration
ratio of 2:1. The two sets of signals are due to protons in the
two layers of phenyl rings (protons b and b′, respectively). For
G4, broad and multiple peaks are observed in this region. The
second region shows chemical shifts at 7.30-7.40 ppm, which
are the signals of the peripheral aryl protons. G1 shows one
peak, while G2, G3, and G4 all show two peaks with equal
intensity in this region. The third region shows chemical shifts
at 7.00-7.20 ppm, which are due to aromatic protons in the
core and aromatic protons at the ortho position of the methoxy
group in the central phenyl rings (protons labeled c, c′, and c′′
in Scheme 1).
These dendritic molecules show drastically different optical
properties from meta-linked conjugated dendrimers. As shown
in Figure 3, the lowest excitation energy shifts to the red with
increasing generations. The absorption edges for G1, G2, G3,
and G4 are 360, 420, 450, and 460 nm, respectively. Apparently,
the effective conjugation length continues to increase with
increasing dendrimer size, albeit the steric congestion resulting
from the ortho linkage prevents the phenyl rings from adopting
a planar geometry. Another important feature of this series of
dendrimers is that the core electronically communicates with
the periphery through two extended conjugated arms. Other
branches maintain cross-conjugation with these two extended
conjugation arms. We anticipate that this type of dendrimer may
be a better energy transfer funnel than meta-linked phenylacety-
lene dendrimers, due to the fact that this system provides a
G1 to G4 are soluble in common organic solvents such as
CHCl3, CH2Cl2, THF, etc. Their structures are confirmed by
NMR, elemental analysis, and MALDI-TOF experiments (see
1
Experimental Section). As shown in Figure 2, the H NMR
spectra of all four dendrons show clearly a single peak in the
range of 5.70-6.00 ppm, which can be assigned to the hydroxyl
proton in the core. No signals corresponding to the acetylene
protons are observed, which indicates that both terminal alkynes
coupled with the aryl triflate. The peaks of methoxy protons
appear between 3.88 and 3.97 ppm. For G1, only one single
peak corresponding to methoxy protons appears at 3.93 ppm.
For G2, there are two well-separated peaks at 3.96 and 3.89
ppm with an integration ration of 1 to 2. These two peaks can
be assigned to the methoxy protons on the core phenyl ring
and on the branch phenyl ring, respectively. The peak at 3.89
(5) Xu,B.; Zhang, J.; Peng, Z. Synth. Met. 1999, 107, 47-51.
(6) (a) Bao, Z.; Chen, Y.; Cai, R. B.; Yu, L. Macromolecules 1993, 26,
5281. (b) Bachki, A.; Foubelo, F.; Yus, M. Tetrahedron 1994, 50 (17),
5139-46. (c) Orito, K.; Hatakeyame, T.; Takeo, M.; Suginome, H. Synthesis
1995, 10, 1273. (d) Yusubov, M. S.; Filimonov, V. D.; Jin, H.-W.; Chi,
K.-W. Bull. Korean Chem. Soc. 1998, 19 (4), 400-401. (e) Yang, S. G.;
Kim, Y. H. Tetrahedron Lett. 1999, 40 (33), 6051-6054.
(7) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N. Synthesis
1980, 8, 627.
(8) (a) Chen, Q. Y.; Yang, Z. Y. Tetrahedron Lett. 1986, 27, 1171. (b)
Arcadi, A.; Cacchi, S.; Marinelli, F. Tetrahedron Lett. 1989, 30, 2581. (c)
Evans, K. L.; Prince, P.; Huang, E. T.; Boss, K. R.; Gandour, R. D.
Tetrahedron Lett. 1990, 31, 6753.

NoVel Unsymmetrical Conjugated Dendrimers
J. Am. Chem. Soc., Vol. 122, No. 28, 2000 6623
Figure 4. Fluorescence spectra (B) of G1-G4 in dilute chloroform
solutions. The top figure (A) shows the fluorescence spectra of G4
under different excitation wavelengths.
Figure 3. UV/vis absorption (bottom) and excitation (top) spectra of
G1-G4 in chloroform solutions.
energy acceptors with absorption wavelengths well separated
from the absorptions of the dendrimers to further explore the
excitation energy transfer process.
It is worth noting that, besides the application as efficient
light-harvesting antennae, many other functional materials may
be envisioned for our unsymmetrical dendrimers. Taking
advantage of the phenol functional group at the dendritic locus,
a variety of structural units with interesting properties can be
anchored to the dendritic core.9 Work in this area is underway
in our laboratory.
through-bond charge-transfer mechanism. Even where the
energy transfer occurs through space via a multistep hopping
mechanism,3 notably fewer hopping steps might be required for
the energy to transfer from the periphery to the center core. In
addition, this type of dendrimer covers a much broader
absorption range than meta-linked dendrimers. G4, for example,
absorbs strongly from 240 nm all the way to 430 nm. Thus, the
energy utilization efficiency in a light-harvesting application
will be significantly improved in our dendrimers.
Due to the meta and para linkages, different branches in our
dendrimers have varied conjugation lengths, which accounts for
the broad absorption spectra. The energy transfer from the
shorter conjugation-length branches to the most extended
conjugation arm is expected. As shown in Figure 4, the
fluorescence emission is red-shifted with higher generation
dendrimers. The maximum emission wavelengths for G2, G3,
and G4 are 418, 441, and 455 nm, respectively. For a given
dendrimer, only the emissions from the longest conjugated
segment are observed. As shown in Figure 4 (top), whether the
excitation wavelength is 250, 300, or 430 nm, the same emission
maximum and the same emission edge are obtained. The
emissions from G4 are clearly independent of the excitation
wavelengths. The excitation spectra of the dendrimers are also
similar to their absorption spectra, as shown in Figure 3 (top).
These results indicate an efficient energy transfer from the higher
band-gap branches to the lowest band-gap branches. Due to the
severe overlap of the absorption of the lowest band-gap branch
with that of other branches, quantitative energy transfer ef-
ficiency cannot be obtained. We are currently attaching other
Conclusion
In conclusion, we have synthesized a new type of conjugated
dendrimer using a rather simple approach that involves only
one synthon and two sets of reaction conditions. This dendrimer
possesses extended conjugation beyond its repeating units and
shows efficient energy transfer from higher band-gap branches
to the longest conjugated segment.
Acknowledgment. This work is supported by the Defense
Advanced Research Projects Agency, Research Corporation, and
the University of Missouri Research Board. We thank Ms.
Kenna K. Hyatt and Ms. Florence Middleton for their assistance
in preparing this manuscript.
JA0006907
(9) Our preliminary studies have shown that the core phenolic oxygen
is accessible as a nucleophile for SN2 reactions. For example, G3 or G4
can react with 1-pyrenemethanol, resulting in pyrene-anchored dendrimers.