Synthesis and optical properties of conjugated dendrimers with unsymmetrical branching

Article abstract of DOI:10.1016/S0040-4020(03)00827-5

An improved synthetic approach to conjugated monodendrons with unsymmetrical branching structures is reported. Dendrimers containing two or three such conjugated monodendrons are synthesized and their optical properties are studied. Such dendrimers exhibit broad absorptions and very high fluorescence quantum yields, making them promising candidates for applications in molecular-based photonics.

Full text of DOI:10.1016/S0040-4020(03)00827-5

Tetrahedron 59 (2003) 5495–5506
Synthesis and optical properties of conjugated dendrimers with
unsymmetrical branching
b
Yongchun Pan,a Zhonghua Penga and Joseph S. Melinger
,
*
aDepartment of Chemistry, University of Missouri—Kansas City, 5100 Rockhill Road, Kansas City, MO 64110, USA
bNaval Research Laboratory, Electronics Science and Technology Division, Code 6812, Washington, DC 20375, USA
Received 9 April 2003; revised 27 May 2003; accepted 27 May 2003
Abstract—An improved synthetic approach to conjugated monodendrons with unsymmetrical branching structures is reported. Dendrimers
containing two or three such conjugated monodendrons are synthesized and their optical properties are studied. Such dendrimers exhibit
broad absorptions and very high fluorescence quantum yields, making them promising candidates for applications in molecular-based
photonics. q 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction
improved synthesis of such unsymmetrical PA mono-
dendrons. Further, by taking advantage of the phenol
group at the dendron locus, larger dendrimers with two or
three unsymmetrical PA monodendrons are prepared. The
detailed synthesis, structural characterizations, and optical
properties of such dendrimers are discussed.
Dendrimers are hyperbranched macromolecules with a
branching structure that may be described as tree-like.1
Dendrimers are characterized by unique structural features
such as a high density of peripheral sites, a convergent
periphery-to-core branching structure, and, for sufficiently
large generations, a globular-like three-dimensional shape.
These features suggest that dendrimers are good candidates
for light-harvesting applications.2–4 Indeed, numerous
light-harvesting dendrimers have been reported. One class
of light harvesting dendrimers utilize dendritic branches as
scaffolds on which to anchor light-absorbing chromo-
phores.2–3 Another class uses p-conjugated branches as
both the light-absorbing units and energy transport
medium.4,5 The p-conjugated branches are usually linked
through the meta positions of phenyl rings, which interrupts
inter-branch p-conjugations. As a result, the electronic
excitations are localized on individual branches.4–6 One
example is the compact phenylacetylene (PA) monodendron
synthesized and studied by Moore, Kopelman, and
co-workers.4,6 The PA monodendron is structurally sym-
metric, resulting from substitution entirely at the meta-
positions of the benzene ring. Recently, we reported a
light-harvesting monodendron based on an unsymmetrical
branching frame where conjugated branches are linked
through the meta and the para positions.7 Such a dendron
not only exhibits broad absorptions, but also possesses an
intrinsic energy gradient from the surface branches to the
core, resulting in efficient and ultrafast energy funneling
properties.8 In this paper, we report the detailed and
2. Results and discussion
2.1. Synthesis of monodendrons
Our previous synthetic approach utilizes compound I as the
building block molecule, and monodendrons up to the
fourth generation have been synthesized according to
Scheme 1.7
Since compound I is the building block molecule, its facile,
efficient and large-scale synthesis is desirable. Scheme 2
shows such an efficient synthetic pathway to compound I.
We have previously reported using BBr3 as the mono-
demethylation agent to convert compound 3 to 4,7 which
gave only about 40% yield. Although using B-bromo-9-
BBN, selective demethylation of one of the methoxy groups
could be achieved in 81% yield (Scheme 2),9 the reaction is
rather slow and the starting material never disappeared.9–10
About 5% of starting material was recovered after the
reaction mixture was refluxed for 3 days. It is advised not to
increase the ratio of B-Br-9-BBN to the substrate.
Additional B-Br-9-BBN gave noticeably more of the
dihydroxy side product. It was thus desirable to introduce
another protective group that is stable enough to withstand
the vigorous iodination conditions and yet can be easily
removed afterwards. We have chosen the isopropyl group
since it has been shown that isopropyl aryl ethers can be
Keywords: dendrimers; fluorescence; light-harvesting.
*
Corresponding author. Tel.: þ1-816-235-2288; fax: þ1-816-235-5502;
e-mail: pengz@umkc.edu
0040–4020/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4020(03)00827-5

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Y. Pan et al. / Tetrahedron 59 (2003) 5495–5506
Scheme 1. Synthesis of monodendrons: (i) (CF3SO2)2O, pyridine, 08C; (ii) PdCl2(PPh3)2, NEt3, DMF, 658C.
selectively cleaved by boron trichloride or aluminum
trichloride under mild conditions while methyl aryl ethers
were left intact.11 Thus, compound 2 was synthesized in 2
steps from 2-methoxylphenol (Scheme 2) in excellent
yields. Treatment of 2 with aluminum trichloride11b at
room temperature resulted in an unidentified mixture
including the unreacted starting material. However, the
reaction of 2 with 1.05 equiv. of B-Br-9-BBN (1 M in
methylene chloride) at reflux temperature went smoothly.
Most of the starting material was consumed in only 1 h.
After simple acid/base extraction, 4,5-diiodo-2-methoxy-
phenol was obtained in 92% yield. This procedure allows
simple and efficient synthesis of 4,5-diiodo-2-methoxy-
phenol on large scale (,100 g).
G1OH, as illustrated in Scheme 1, was initially synthesized
by a coupling reaction of compound I with 3,5-bis(tert-
butyl)phenol triflate (G0OTf). The reaction proceeded
without the protection of the hydroxy group, which greatly
simplified the synthesis, and the product can be easily
separated from the unreacted triflate G0OTf due to the
presence of the polar hydroxy group in the product. This
strategy using unprotected phenol as the monomer offers an
additional advantage when compared to the coupling
reaction using aryl halides. The diacetylene side product
resulting from the homocoupling reaction of terminal
alkynes, if ever formed, has significantly different polarity
than the desired product and can thus be easily separated.
Unfortunately, compound I, with an enediyne structure, is
Scheme 2. Improved synthesis of 4,5-diiodo-2-methoxyphenol 4.

Y. Pan et al. / Tetrahedron 59 (2003) 5495–5506
5497
temperature sensitive and appears not to be very stable
under the reaction conditions. One possible side reaction is
the Bergman cyclization reaction of enediyne.12 It is also
possible that compound I reacted with itself to form a
polymeric material, as evidenced by our attempt to
synthesize 4-ethynylphenol. Upon treatment of 4-trimethyl-
silylethynylphenol, a quite stable compound, with tert-
butylammonium fluoride, the product, 4-ethynylphenol,
decomposed quickly during chromatography and solvent
evaporation to give a black tar-like material. Because of the
side reactions, excess compound I has to be used in each
coupling step. Even so, there is still a significant amount of
triflate recovered (yields shown in Scheme 1 are calculated
based on triflates reacted).
the two ortho reaction sites. By comparing the two possible
routes, we conclude that the strategy using coupling
reactions of compound 4 and alkynes is superior only for
the preparation of G1OH and G1OTf. For higher
generations of monodendrons, the initial plan using
enediyne I and triflates works better, even though excess
of enediyne I had to be used and unreacted triflates were
recovered.
Since it appeared that the use of enediyne (compound I) is
unavoidable, we set out to optimize the reaction conditions.
It is observed that the solvent plays an important role. The
coupling reaction of aromatic iodides with alkynes took
place readily in pure dry triethylamine without any other
co-solvents. The heat released raised the reaction tempera-
ture rapidly. An external cooling bath had to be used to
dissipate the energy. The reaction normally is complete in
1–2 h. However, if DMF was added as a co-solvent to
enhance solubility, the reaction rate is significantly lower.
No temperature increase was observed and extended
reaction time was necessary to ensure complete transform-
ation. For the coupling reaction of aryltriflates with alkynes
such as G0OTf with trimethylsilylacetyelene, no reaction
was detected by TLC after 4 h of reaction at 558C when pure
triethylamine was used as the only solvent. However,
when DMF was injected into the mixture, the reaction
proceeded smoothly and the reaction finished in 5 h.
Clearly, DMF and triethylamine have distinctive effects
on the reactivity of triflates and iodides. Reducing the
amount of triethylamine used in the coupling reaction of
triflates appears to be beneficial. Best results were obtained
when a minimum amount of triethylamine (3–5 mol equiv.
relative to monomer) was used. The modified procedure
allows the use of considerably less thermally unstable
enediyne monomer I. Although the yields did not increase
considerably, the percentage of unreacted triflates decreased
dramatically, for example, from 21 to 3%, for the
preparation of G2OH.
To circumvent using enediyne I directly, one strategy is to
generate alkyne in situ.13 Thus, 4,5-bis[(trimethylsilyl)-
ethynyl]-2-methoxyphenol was heated with K2CO3,
G0OTf, and dichlorobis(triphenyl)palladium(II) in
methanol. However, the reaction was not complete and a
complicated mixture including the desired product was
obtained.
To avoid using enediyne I, we turned to an alternative route
using 4,5-diiodo-2-methoxyphenol 4 as the building block
molecule. As shown in Scheme 3, the Sonogashira Coupling
of compound 4 with slightly over 2 equiv. of 3,5-di-tert-
butylphenylacetylene gave G1OH in excellent yields.14 The
conversion of G1OH to G1OTf is efficient. Taking
advantage of the relative reactivity of iodo versus triflate,
G1OTf can be synthesized in a single step by the coupling
reaction of 4 with 4,5-diiodo-2-methoxyphenol triflate
(II).15 The disadvantage of this procedure is that it is
difficult to isolate the product G1OTf from the diacetylene
byproduct and the unreacted compound II. The conversion
of G1OTf to G1-acetylene compound G1A is very efficient.
However, the coupling of G1A with either 4 or II gives very
poor yields, presumably due to the severe steric hindrance at
Scheme 3. Synthesis of monodendrons by an alternative route: (a) (i) trimethlysilylacetylene, PdCl2(PPh3)2, Et3N, DMF, 658C, (ii) Bu4NF, THF.
(b) PdCl2(PPh3)2, CuI, Et3N, DMF, rt.

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Y. Pan et al. / Tetrahedron 59 (2003) 5495–5506
Scheme 4. Synthesis of didendron 2Gn.
3. Synthesis of didendrons and tridendrons
substitution and significant self-coupling side products. The
reaction of 1,3,5-triethynylbenzene with G1OTf gave no
desired product either, likely due to the sluggishness of the
coupling reaction and the instability of triethynylbenzene. It
is thus desirable to prepare 1,3,5-triiodobenzene.
Taking advantage of the triflate functional group at the core,
a series of didendrons were synthesized by the coupling
reaction of 1,4-diethynylbenzene with the GnOTf
(Scheme 4). 2G1, 2G2, and 2G3 are synthesized in 78, 59
and 48%, respectively.
There have been a few reports about the synthesis of 1,3,5-
triiodobenzene which all involve the halogen exchange
reaction of tribromobenzene with various iodination
agents.16–18 A mixture of homo and hetero-halogenated
compounds are usually obtained, from which pure
To synthesize tridendrons, trisubstituted benzene has to be
prepared. 1,3,5-tribromobenzene was not reactive enough in
the Sonogashira reaction, which would result in incomplete
Scheme 5. Synthesis of G1 dendrimer 3G1.

Y. Pan et al. / Tetrahedron 59 (2003) 5495–5506
5499
Scheme 6. Synthesis of 3G2.
1,3,5-triiodobenzene is difficult to isolate. We prepared
triiodobenzene from 3,5-diiodoaniline. By following a
reference procedure that was designed to prepare triazene,19
1,3,5-triiodobenzene was prepared in good yields from 3,5-
diiodoaniline. Thus, 3,5-diiodoaniline was treated with
hydrochloric acid and then sodium nitrite. The resulting
diazonium salt was decomposed with potassium iodide to
give a precipitate, which was separated easily by filtration. It
can be recrystallized from methylene chloride to give
analytically pure product in about 80% yield. 1H NMR gives
a singlet at 8.00 ppm. 13C NMR gives two carbon signals at
144.5 and 95.4 ppm. There were no impurities detectable by
NMR spectroscopy. Elemental analysis confirms the high
purity of the compound.
chloroform, THF and DMF. Their structures and purity are
confirmed by thin-layer chromatography, elemental anal-
1
ysis, H and 13C NMR spectroscopy, and matrix-assisted
G1 dendrimer 3G1 was easily prepared by the Sonogashira
coupling reaction of G1A and 1,3,5-triiodobenzene, as
shown in Scheme 5. To ensure the complete coupling
reaction, an excess of G1A was used, which resulted in a
homocoupling side product 2G1A. Even the strict exclusion
of oxygen did not reduce the amount of this side product. G2
dendrimer 3G2 was synthesized similarly (Scheme 6). The
separation of the desired product 3G2 and side product
2G2A required repetitive chromatography. For the synthesis
of 3G3 dendrimer, we envisioned that the bulkiness of the
acetylene terminated G3 monodendron would cause a
considerable drop in the yield of 3G3. We also anticipated
difficulties in separation of 3G3 and the homocoupling
byproduct. The synthesis of 3G3 dendrimer was thus not
attempted.
4. Structural characterizations
All generations of monodendrons, didendrons and tri-
dendrons are soluble in most organic solvents including
Figure 1. Stacked plot of 1H NMR spectra of GnOH in CDCl3.

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Y. Pan et al. / Tetrahedron 59 (2003) 5495–5506
(HMBC) experiments, are assigned to the H6 and H3 on
the core benzene ring, respectively.
By analogy, the aromatic hydrogen H6 (see Scheme 1 for
labeling) ortho to the hydroxy group at the focal point is
found to be a singlet at 7.12, 7.15, 7.16 and 7.18 ppm for
G1OH, G2OH, G3OH and G4OH, respectively (Fig. 2). It
is interesting to note that this singlet gradually shifts
downfield as the monodendron grows. This is most likely
due to the more extended electron delocalization. After the
hydroxy group is transformed to its triflate, this aromatic
hydrogen resonance signal shifts significantly downfield to
7.42, 7.45, 7.46 and 7.48 ppm for G1OTf, G2OTf, G3OTf
and G4OTf, respectively (Fig. 3). The aromatic hydrogen
H3 ortho to the methoxy group on the central benzene ring
is located as a singlet at 7.05, 7.06, 7.09 and 7.10 ppm for
G1OH, G2OH, G3OH and G4OH, respectively.
The aromatic hydrogens next to the methoxy groups in the
inner layers (c, c0 and c00 protons) are all located on the
upfield side ranging from 7.06 to 7.18 ppm. G1OH has only
a periphery and a core and does not have an inner layer, and
thus shows no such proton signals. On the downfield side of
the aromatic hydrogen region, with chemical shifts from
7.80 to 7.90 ppm, lies another group of hydrogens for all
monodendrons except G1OH. These hydrogen signals are
assigned to the aromatic hydro0g0 ens that are wedged between
two triple bonds (b, b0 and b protons). For G1-acetylene
compound G1A, where the hydroxy group of G1OH is
replaced by an acetylene moiety, this signal (H6) shows up
at d¼7.68 ppm. G2OH has two such hydrogens and we
observed two singlets at d¼7.81 and 7.80 ppm. For the same
Figure 2. Stacked plot of aromatic regions of 1H NMR spectra of GnOH in
CDCl3.
laser desorption/ionization time-of-flight (MALDI-TOF)
mass spectroscopy.
NMR spectra acquired in d-chloroform (CDCl3) of GnOH
give adequately dispersed signals. The signals are divided
into four regions (Fig. 1). The signals at around d¼1.26 ppm
are due to methyl groups on the periphery. The signals at
around d¼3.88–3.97 ppm are due to hydrogens on methoxy
groups at different layers of the monodendrons. The small
singlet at approximately d¼5.75–5.79 ppm is assigned to
the hydrogen on the hydroxy group at the focal point. Its
intensity rapidly diminishes compared to other parts of the
spectrum for higher generation monodendrons and dis-
appears for its corresponding triflate derivative. The
aromatic regions provide the most important information
since the aromatic rings are the building blocks in every
monodendron. Fortunately, signals in the aromatic region
are well dispersed (Fig. 2). The intense signals at around
d¼7.40–7.34 ppm are due to the aromatic hydrogens at the
peripheral 3,5-di-tert-butylphenyl groups. The aromatic
hydrogens on the benzene rings of inner layers are all
singlets and divided into three groups. To help understand
the spectrum in the aromatic region, it is useful to look at the
spectrum for G1OH. The highly overlapped and broad
signals at around d¼7.40–7.34 ppm are due to the aromatic
hydrogens at the peripheral phenyl rings. G1OH also gives
two well-separated singlets at d¼7.12, 7.05 ppm, which
based on heteronuclear multiple-quantum correlation
(HMQC) and heteronuclear multi-bond correlation
Figure 3. Stacked plot of aromatic regions of 1H NMR spectra of GnOTf in
CDCl3.

Y. Pan et al. / Tetrahedron 59 (2003) 5495–5506
5501
reason, G3OH and G4OH give 6 and 14 hydrogens,
respectively, in that region with certain peak overlaps.
After the hydroxy group is transformed to its triflate, the
aromatic hydrogens are better separated. Again, the two
hydrogens H3 and H6 on the phenyl ring at the core are well
separated from the rest of the signal and can be easily
identified. The layer structures are visible by inspecting the
locations and integrations of all the singlets in the upfield
region from 7.04 to 7.18 ppm (Fig. 3). As discussed above,
these signals are due to 0t0he hydrogens ortho to the methoxy
groups (Hc, Hc0 and Hc ). It is obvious that the closer these
hydrogens are to the core, the more deshielded they are. And
also, the hydrogens in the same layers shift downfield as the
sizes of the monodendrons increase.
The 1H NMR spectra of the didendrons show features
similar to those of GnOH and GnOTf. For example, the
signals corresponding to protons at the peripheral phenyl
rings (protons a/a0) appear at 7.35–7.40 ppm. The aromatic
protons ortho to the methoxyl group have chemical shift
between 7.07 and 7.15 ppm. The 00protons ortho to both
acetylene groups (protons c/c0/c ) appear as singlets
between 7.70 and 7.90 ppm. The protons belonging to the
center phenyl ring give a singlet at about 7.55 ppm for all
three didendrons. The 13C NMR spectra of 2Gn didendrons
provide important structural information as well. The
signals in the range of 70–100 ppm correspond to alkynyl
carbons. The 13C NMR spectra of 2G1, 2G2, and 2G3 show
6, 14, and 16 signals in that range respectively, consistent
with their respective number of alkynyl carbons. The t-butyl
carbons give two signals at 31.5 and 35.0 ppm, while the
methoxy carbon appears at 56.3 ppm.
Figure 4. Stacked plot of aromatic regions of 1H NMR spectra of 3Gn and
2GnA (n¼1, 2) in CDCl3.
1
The H NMR spectrum of 3G1 gives two singlets at 7.72
consistent with the calculated mass for C129H144O3. For
2G1A, its NMR spectrum is similar to that of 3G1 except it
lacks the signal (d) corresponding to the central phenyl ring
in 3G1. The 13C NMR spectra of 3G1 show six alkynyl
carbon signals (between 70 and 100 ppm) and 16 aryl
carbon signals, which matches perfectly with its structure.
The 13C NMR spectra of 2G1A, which give six alkynyl
and 7.73 ppm, as shown in Figure 4, which can be assigned
to protons d and b (see Scheme 5 for the labeling)
respectively. The fact that two singlets are present in this
region, and that the integration ratio is nearly 1:1, confirms
that the central phenyl ring was fully substituted with the
same groups. LRFAB gives a mass of 1741, which is
Table 1. Optical properties of dendritic compounds
Compound
l
edge
(nm)
l
max
(nm)
1max
(M21 cm21
)
tf (ns)
ab
a
ab
b
c
em d
l (nm)
max
e
ffl
G1OH
G2OH
G3OH
G4OH
3G1
3G2
2G1
2G2
2G3
G1A
G2A
2G1A
2G2A
356
416
440
453
396
432
418
442
454
376
422
427
454
283
299
310
321
310
324
382
304
308
276
304
304
304
48000
82500
360, 374 (sh)
417, 438 (sh)
440, 466 (sh)
452, 479 (sh)
401
433, 455 (sh)
415, 437 (sh)
439, 463 (sh)
452, 478 (sh)
392
0.40
0.81
0.70
0.65
0.80
0.84
0.80
0.73
0.71
0.66
0.84
0.71
0.65
1.7
2.0
1.9
1.7
1.7
1.8
0.79
1.1
1.2
2.7
2.4
0.84, 2.8
0.95
171000
330000
183000
328000
75000
160000
330000
48000
110000
71000
180000
425
422, 448 (sh)
450, 477 (sh)
a
Absorption band edge.
Maximum absorption wavelengths.
Molar extinction coefficients at the absorption maximum.
Fluorescence emission wavelengths.
Fluorescence quantum yields (ffl).
Fluorescence lifetimes.
b
c
d
e
f

5502
Y. Pan et al. / Tetrahedron 59 (2003) 5495–5506
carbon signals and 14 aryl carbon signals, are consistent
with its structure as well.
a 2Gn didendron and G(n11)OH monodendron have the
same number of phenylacetylene units, and, thus, the same
number of p-electrons. With increasing generation, the
absorption band edges for 2Gn and GnOH shift to longer
wavelengths and the molar extinction coefficient approxi-
mately doubles for each generation. It is worth noting that
the 2Gn dendrimers exhibit nearly identical band edges to
their G(n11)OH dendrons. An inspection of the structures
tells that these two series differ in the connectivity at the
center of the molecule, where lower generation mono-
dendrons join together. For 2Gn series, two Gn mono-
dendrons are connected to a benzene ring in the para
positions, whereas for G(n11)OH series, the Gn mono-
dendrons are joined at the ortho positions. Both para and
ortho connectivities enable extended conjugation between
monodendrons. The similar band edges indicate that the
effective conjugation lengths are comparable for the two
types of linkages.
The 1H NMR spectra of 3G2 and 2G2A show well resolved
signals as well. As shown in Figure 4, besides the two broad
signals at 7.36–7.40 ppm which are due to protons in the
peripheral phenyl rings, 3G2 gives seven well separated
singlets, corresponding to the seven different types of
aromatic protons in the inner-layer phenyl rings. 2G2A,
without the central phenyl ring, shows six singlets. The 13C
NMR spectra of 3G2 and 2G2A give 14 and 12 signals,
respectively, in the range of 70–100 nm, consistent with the
number of alkynyl carbons in their respective structure. The
structures of 3G2 and 2G2A are also confirmed by mass
spectroscopy.
5. Optical and photophysical properties
Although 2Gn dendrimers and G(n11)OH dendrons
possess similar band edges, the 2Gn dendrimers have
much stronger absorptions at wavelengths near the band
edge. This can be clearly seen by comparing the absorption
spectrum of 2G1 with that of G2OH. For G2OH, the
absorption maximum is at 299 nm and the absorption
declines gradually to its band edge. For 2G1, the strongest
absorption occurs at 382 nm and a much steeper absorption
edge is observed. Although the effect is not as significant,
when 2G2 and 2G3, are compared to G3OH and G4OH,
respectively, they show higher absorption strength at longer
wavelengths as well. The absorption features at wavelengths
near the band edge are likely associated with the central
phenyl ring and its two extended p-conjugated arms. The
linear (para) linkage may possess stronger transition
moment than the bent (ortho) linkage, which contributes
to the stronger absorption of 2Gn. This argument is
supported by the fluorescence lifetime measurements
where shorter fluorescence lifetimes are observed for 2Gn
dendrimers (see Table 1).
The optical properties of all monodendrons, didendrons and
tridendrons were studied by UV/Vis absorption, static-state
and time-resolved fluorescence measurements. The photo-
physical properties of these compounds are collected in
Table 1.
The absorption spectra of the 2Gn (didendrons) series are
shown in Figure 5. The absorption spectra of the GnOH
monodendrons are also shown for comparison. We note that
The steady-state fluorescence emission spectra of 2Gn and
GnOH are shown in Figure 6. The fluorescence spectra of
2Gn and G(n11)OH are nearly identical with respect to
Figure 6. Fluorescence spectra of 2Gn and GnOH in methylene chloride
solutions. The number above each curve in the bottom figure corresponds to
n in GnOH.
Figure 5. UV/Vis absorption spectra of 2Gn and GnOH in methylene
chloride solutions.

Y. Pan et al. / Tetrahedron 59 (2003) 5495–5506
5503
both the peak position and the emission profile. With
increasing generation, the emission wavelength red shifts.
For each member of the 2Gn and G(n11)OH series, the
emission profiles remain independent of the excitation
wavelengths, indicating that rapid and efficient energy
transfer occurs from shorter branches to the main linear
conjugated branch.
As shown in Table 1, both 2Gn and GnOH, in general,
show high fluorescence quantum yields. 2G1 exhibits the
highest quantum yield of 80%, comparable to that of G2OH
(81%) within the experimental error. The quantum yields
decrease slightly with increasing generations, with 2G3
exhibiting a quantum yield of 70%. The quantum yields for
2G2 and 2G3 are slightly higher than those of G3OH and
G4OH, respectively, presumably due to the shorter
radiative lifetime of the emitting state in the 2Gn series,
which enhances relaxation to the ground state via
fluorescence over competing non-radiative processes.
Figure 8. UV/Vis absorption and fluorescence spectra of 3G1 and 3G2.
The optical properties of tridendrons (3G1 and 3G2) are
shown in Figure 8. With dendrons linked at the meta-
positions, p-conjugation is not expected to significantly
extend across branches in different monodendrons. Indeed,
the absorption band edges for 3G1 and 3G2 are at shorter
wavelengths than that of 2G1 and 2G2, but at longer
wavelengths that that of G1OH and G2OH. The red shift of
3G1 with respect to G1OH (40 nm; 2837 cm21) is much
larger than the red shift of 3G2 with respect to G2OH
(16 nm; 890 cm21). For the 3Gn series, each monodendron
interacts with the central benzene ring, which provides an
additional pathway for p-electron delocalization. The
additional pathway has a larger effect in the smaller (3G1)
dendrimer. Both 3G1 and 3G2 exhibit high fluorescence
quantum yields of over 80%. Clearly, coupling the three
dendrons at the meta-position of the central benzene ring
provides enough spatial isolation so that interactions
between the dendrons that potentially reduce fluorescence
quantum efficiency are not significant.
Figure 7 shows the absorption and fluorescence emission
spectra of 2G1A and 2G2A. Structurally, for 2GnA,
monodendrons are connected by forming a single bond
between two triple bonds of GnA, while for the 2Gn series,
monodendrons are linked to a central benzene ring at para
positions. As shown in Figure 7 and Table 1, the absorption
spectrum of 2G1A displays two intense bands at wave-
lengths as high as 382 and 406 nm. Compared to the
absorption spectrum of 2G1, the spectrum of 2G1A exhibits
slightly higher intensities at short wavelengths and lower
intensities at longer wavelengths. The absorption band edge
for 2G1A (ledge) is red shifted by 9 nm with respect to 2G1,
indicating that the p-delocalization through a diacetylene
bridge is somewhat more efficient than a diethynylphenyl
bridge. This effect is more evident in the higher generation
2G2A. As shown in Figure 7, the absorption band edge for
2G2A extends to 454 nm, comparable to that of 2G3, which
is more than twice as large as 2G2A in terms of the
molecular weight. 2G1A and 2G2A are also highly
fluorescent materials with fluorescence quantum yields
around 70% (Table 1). While 2GnA have similar
fluorescence quantum yields to that of 2Gn, their
fluorescence life times are significantly shorter. This again
may reflect the fact that 2GnA have longer conjugation
lengths, resulting in stronger transition dipoles.
6. Conclusions
An improved synthetic approach to the building block
molecule and an alternative route to unsymmetrical PA
monodendrons are reported. A series of dendrimers contain-
ing two or three such PA monodendrons are synthesized.
These dendrimers exhibit broad absorptions and large
extinction coefficients, and show efficient energy transfer
from the shorter branches to the longest conjugated branch.
All these rigid dendrimers are highly fluorescent materials
with quantum yields over 70%, suggesting that they are
promising materials for application in molecular photonics.
7. Experimental
7.1. General
All reagents and solvents were obtained from either Aldrich
or Fisher and were used as received unless otherwise stated.
Anhydrous THF and acetonitrile were distilled prior to use
from sodium metal/benzophenone. Triethylamine was
distilled from calcium hydride prior to use. All air and
moisture-sensitive reactions were carried out under N2
atmosphere.
Figure 7. UV/Vis absorption and fluorescence spectra of 2G1A and 2G2A.

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Y. Pan et al. / Tetrahedron 59 (2003) 5495–5506
1H NMR spectra were recorded on Varian Unity 400 MHz.
UV–VIS absorption spectra were recorded using a
Hewlett–Packard 8452A diode array spectrophotometer.
The fluorescence emission spectra were measured using a
The synthetic procedures for G1A and G2A are similar to
those of G0A.
1
7.1.2. Compound G1A. Yield: 91%. Mp 187–1888C. H
Shimadzu
RF-5301PC
spectrofluorophotometer.
NMR (400 MHz, CDCl3) d 7.68 (s, 1H), 7.39–7.36 (m, 6H),
7.08 (s, 1H), 3.95 (s, 3H), 3.38 (s, 1H), 1.26 (m, 36H). 13C
NMR (100 MHz, CDCl3) d 159.8, 151.1, 151.0, 137.6,
128.1, 126.2, 126.0, 123.5, 123.1, 122.4, 122.0, 119.0,
113.8, 111.8, 96.7, 93.9, 87.0, 86.0, 82.9, 79.3, 56.3, 35.0,
31.5. Anal. calcd for C41H48O: C, 88.44; H, 8.69. Found: C,
88.32; H, 8.74.
Fluorescence quantum yields were determined using
quinine sulfate in 1N H2SO4 (ffl<0.55) as the standard.20
Time-dependent fluorescence measurements were
performed using the technique of time-correlated single
photon counting (TCSPC).
The preparations of compounds GnOH (n¼1–4) and
GnOTf (n¼0–4) were reported previously.7
1
7.1.3. Compound G2A. Yield: 94%. H NMR (400 MHz,
CDCl3) d 7.88 (s, 1H), 7.83 (s, 1H), 7.74 (s, 1H), 7.42–7.39
(m, 12H), 7.12–7.11 (m, 3H), 3.99–3.93 (m, 9H), 3.43 (s,
1H), 1.30–1.29 (m, 72H). 13C NMR (100 MHz, CDCl3) d
160.0, 159.5, 159.3, 151.0 (m), 137.9, 137.5, 137.2, 127.9,
127.6, 127.3, 126.1, 126.0, 123.4, 123.3, 123.0, 122.9,
122.6, 122.5, 122.2, 122.1, 119.0, 118.9, 118.6, 114.1,
113.9, 113.8, 113.4, 112.8, 112.2, 96.9, 96.5, 94.0, 93.8 (two
overlapped peaks), 93.0, 91.1, 88.5, 87.3, 87.2, 86.4, 86.3,
83.1, 79.2, 56.4, 35.0, 31.5. Anal. calcd for C91H100O3: C,
88.02; H, 8.12. Found: C, 87.76; H, 8.24.
G1OTf by the alternative route. Triethylamine (50 mL) and
DMF (30 mL) were added into a mixture containing
compound II (15.5 g, 30.5 mmol), (3,5-di-tert-butyl-
phenyl)acetylene GoA (14.5 g, 67.6 mmol), Pd(PPh3)2Cl2
(1.03 g, 1.47 mmol) and copper(I) iodide (0.119 g,
0.623 mmol). The resulting mixture was stirred at room
temperature for 1.5 h and then poured into 50 mL of 3 M
hydrochloric acid. After extraction with methylene
chloride (3£100 mL), the combined organic extracts
were washed with water (3£50 mL) and brine (50 mL),
dried over anhydrous sodium sulfate and evaporated in
vacuo to give a red brown oil, which was then purified by
column chromatography eluting with hexane, and then
hexane/ethyl acetate (12:1) to give 20.1 g of the title
compound as white crystals (97% yield). The structural
characterization data for G1OTf have previously been
reported.
7.1.4. Compound 2G1. The mixture containing 1,4-
0.370 mmol),
Pd(PPPh3)2Cl2
diethynylbenzene (0.0467 g,
(0.507 g, 0.745 mmol),
G1OTf
(0.0409 g,
0.0583 mmol), copper(I) iodide (0.0024 g, 0.0126 mmol),
triethylamine (5.0 mL) and DMF (5.0 mL) was stirred under
N2 at 638C for 14 h. The reaction mixture was slowly poured
into hydrochloric acid (3 M) and was then extracted with
methylene chloride. The organic extracts were washed with
water and brine, dried over anhydrous sodium sulfate and
the solvent was then evaporated. The resulting solid was
purified by chromatography eluting with hexane to yield the
7.1.1. 1,3-Bis(1,1-dimethylethyl)-5-ethynylbenzene
(G0A). A mixture of 3,5-di-tert-butylphenyl trifluoro-
methanesulfonate G0OTf (1.00 g, 2.96 mmol), trimethyl-
silylacetylene (0.50 mL, 3.54 mmol), Pd(PPh3)2Cl2
(0.119 g, 0.169 mmol), triethylamine (2.0 mL) and DMF
(5.0 mL) was stirred under nitrogen at 508C overnight. The
reaction mixture was slowly poured into cold hydrochloric
acid (3 M) and was then extracted with methylene chloride.
The organic extracts were washed with water and brine,
dried over anhydrous sodium sulfate and the solvent was
then evaporated. The resulting black solid was then purified
by passing through a silica gel column eluting with hexane
to yield 0.84 g of [[3,5-bis(1,1-dimethylethyl)phenyl]-
ethynyl]trimethylsilane (G0A-TMS) as white crystals
(99%, mp 133–1358C). 1H NMR (250 MHz, CDCl3) d
7.38 (s, 1H), 7.33 (s, 2H), 1.32 (s, 18H), 0.27 (s, 9H). G0A-
TMS was disilylated by adding a sample of Bu4NF (0.27 g,
0.857 mmol) to a THF solution of G0A-TMS (0.221 g,
0.772 mmol in 2.0 mL of THF). The solution was stirred at
room temperature for 30 min and poured into water. It was
extracted with methylene chloride. The organic extracts
were washed with water, dried over anhydrous sodium
sulfate and evaporated in vacuo to give a yellow solid. It
was then purified by passing through a short silica gel
column, eluting with hexane to afford 0.150 g of G0A as a
white solid (90%, mp 86–878C). 1H NMR (250 MHz,
CDCl3) d 7.42 (t, 1H, J¼2.5 Hz), 7.35 (d, 2H, J¼2.5 Hz),
3.03 (s, 1H), 1.31 (s, 18H). 13C NMR (62.5 MHz, CDCl3) d
151.1, 126.6, 123.5, 121.2, 85.1, 76.0, 35.0, 31.5. Anal.
calcd for C16H22: C, 89.65; H, 10.35. Found: C, 89.47; H,
10.56.
1
title compound as a white solid (0.350 g, 78% yield). H
NMR (400 MHz, CDCl3) d 7.72 (s, 2H), 7.54 (s, 4H), 7.40–
7.38 (m, 12H), 7.10 (s, 2H), 3.97 (s, 6H), 1.27 (s, 72H). 13C
NMR (62.5 MHz, CDCl3) d 159.2, 151.1, 150.9, 139.7,
137.0, 131.8, 127.5, 126.2, 126.1, 123.5, 123.0, 122.5,
122.1, 119.0, 113.8, 112.9, 96.7, 95.1, 93.8, 87.2, 87.1, 86.2,
56.3, 35.0, 31.5. Anal. calcd for C88H98O2: C, 88.99; H,
8.32. Found: C, 88.71; H, 8.40. MS (LRFAB) calcd for
C88H98O2 1187.7, found 1188.
The synthetic procedure for 2G2 and 2G3 is similar to that
of 2G1.
1
7.1.5. Compound 2G2. Yield: 59%. H NMR (400 MHz,
CDCl3) d 7.85 (s, 2H), 7.81 (s, 2H), 7.77 (s, 2H), 7.57 (s,
4H), 7.39–7.36 (m, 24H), 7.12 (s, 2H), 7.10 (s, 2H), 7.08 (s,
2H), 3.99 (s, 6H), 3.91 (s, 12H), 1.27–1.26 (m, 144 H). 13C
NMR (62.5 MHz, CDCl3) d 159.4, 159.3, 150.9, 137.4,
137.2, 132.6, 131.8, 127.8, 127.3, 127.1, 126.1, 126.0,
123.3, 123.0, 122.5, 122.4, 122.1, 122.0, 118.9, 118.8,
118.6, 114.0, 113.8, 113.4, 113.3, 112.9, 96.8, 96.5, 95.4,
94.0, 93.9, 93.8, 93.3, 91.0, 88.4, 87.3, 87.2, 87.0, 86.4,
86.3, 56.3, 34.9, 31.5. Anal. calcd for C188H202O6C, 88.28;
H, 7.96. Found: C, 88.05; H, 8.13. MS (LRFAB) calcd for
C188H202O6 2557.5, found 2557.
1
7.1.6. Compound 2G3. Yield: 48%. H NMR (400 MHz,
CDCl3) d 7.88–7.78 (m, 14H), 7.57 (s, 4H), 7.38–7.35 (m,

Y. Pan et al. / Tetrahedron 59 (2003) 5495–5506
5505
48H), 7.13–7.07 (m, 14H), 4.00–3.91 (m, 42H), 1.26–1.24
(m, 288 H). 13C NMR (62.5 MHz, CDCl3) d 159.7, 159.6,
159.4, 159.3, 150.9, 137.8, 137.5, 137.2, 132.6, 131.8,
129.0, 127.8, 127.7, 127.3, 127.2, 127.1, 126.8, 126.4,
126.1, 125.9, 123.3, 122.9, 122.5, 122.4, 122.1, 118.9,
118.8, 118.6, 118.4, 114.1, 113.8, 113.3, 96.7, 96.4, 94.3,
94.1, 94.0, 93.9, 93.8, 93.5 (m), 90.9, 88.5, 88.4, 87.2, 86.4,
86.3, 56.4, 34.9, 31.5. Anal. calcd for C388H410O14 C, 87.97;
H, 7.80. Found C, 87.43; H, 8.07. MS (LRFAB) calcd for
C388H410O14 5297.2, found 5298.
122.8, 122.7, 122.4, 122.3, 122.0, 121.9, 118.8, 118.7,
118.4, 113.9, 113.7, 113.6, 113.3, 113.0, 112.8, 96.6, 96.3,
93.9, 93.8, 93.7, 93.6, 93.1, 90.9, 88.2, 87.2, 87.1 86.2, 86.1,
86.0, 56.2 (m), 34.7, 31.3. MS (LRFAB) calcd for
C279H300O9 3797.2, found 3798.
7.1.11. 3G2 side product (2G2A). A yellow solid
(0.0343 g) was separated as a side product after flash
column chromatography of the above reaction. Yield: 16%.
1H NMR (400 MHz, CDCl3) d 7.84 (s, 2H), 7.81 (s, 2H),
7.74 (s, 2H), 7.39–7.36 (m, 24H), 7.09 (m, 6H), 3.98 (s,
6H), 3.91 (m, 12H), 1.27 (m, 144 H). 13C NMR (100 MHz,
CDCl3) d 160.8, 159.5, 159.4, 151.0, 138.1, 137.5, 137.2,
135.4, 135.3, 135.2, 131.5, 128.4, 128.0, 127.9, 127.4,
126.2, 126.0, 123.4, 123.3, 123.0, 122.9, 122.6, 122.5,
122.2, 122.1, 119.0, 118.9, 118.7, 114.1, 113.9, 113.8,
113.4, 112.8, 112.2, 94.0 (m, three overlapped peaks), 93.0,
91.7, 88.5, 87.3, 87.2, 86.4, 86.3, 79.9, 78.8, 56.4 (m), 35.0
(m), 31.5. MS (MALDI) calcd for C182H198O6 2479.5,
found 2479.7.
7.1.7. 1,3,5-Triiodobenzene. To a solution of 3,5-diiodo-
aniline (34.1 mmol) in acetonitrile (60 mL) was added
20 mL of water, 20 mL of concentrated hydrochloric acid.
The resulting milk like mixture was cooled to 08C. To this
suspension was added dropwise a cold solution of sodium
nitrite (2.82 g, 40.9 mmol) in 15 mL of water. A solution of
KI (7.20 g, 43.4 mmol) was then added and the resulting
mixture was stirred overnight. The precipitate was filtered
and then recrystallized from methylene chloride to give the
1
product as pink needles (79%. Mp 181–1828C). H NMR
(400 MHz, CDCl3) d 8.00 (s, 3H). 13C NMR (100 MHz,
CDCl3) d 144.5, 95.4. Anal. calcd for C6H3I3: C, 15.81; H,
0.66. Found: C, 15.83; H, 0.70.
Acknowledgements
This work is supported by the Office of Naval Research and
the Defense Advanced Research Project Agency.
7.1.8. Compound 3G1. The mixture containing 3,5-
triiodobenzene (0.0528 g, 0.116 mmol) and G1A (0.211 g,
0.380 mmol), Pd(PPh3)2Cl2 (0.0124 g, 0.0184 mmol),
copper(I) iodide (0.0011 g, 0.00578 mmol) and triethyl-
amine (10 mL) were stirred at room temperature for 8.5 h
and was then poured into hydrochloric acid (3 M). After
extraction with methylene chloride, the organic extracts
were washed with water and dried over anhydrous sodium
sulfate and the solvent was then evaporated. The resulting
crude product was purified by flash chromatography using
hexane as the eluent to give 0.161 g of 3G1 as a yellow
powder (74% yield). 1H NMR (400 MHz, CDCl3) d 7.73 (s,
3H), 7.72 (s, 3H), 7.40–7.38 (m, 18H), 7.11 (s, 3H), 3.99 (s,
9H), 1.27 (m, 108 H). 13C NMR (100 MHz, CDCl3) d 159.2,
150.9, 150.7, 136.8, 134.3, 127.5, 126.0, 125.8, 124.0,
123.2, 122.8, 122.3, 121.9, 118.8, 113.79, 112.5, 96.5, 93.7,
93.5, 87.0, 86.1, 86.0, 56.1, 34.8, 31.3 (two peaks). MS
(LRFAB) calcd for C129H144O3 1741.1, found 1741.
References
¨
1. (a) Newkome, G. R.; Moorefield, C. N.; Vogtle, F. Dendritic
Molecules: Concepts, Syntheses, Perspectives; VCH:
´
Weinheim, 1996. (b) Frechet, J. M. J.; Hawker, C. J. Synthesis
and Properties of Dendrimers and Hyperbranched Polymers.
Comprehensive Polymer Science; Aggarwal, S. L., Russo, S.,
Eds.; Pergamon: Oxford, 1996; 2nd Suppl. (c) Fischer, M.;
¨
Vogtle, 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. (g) Grayson, S. M.; Frechet, J. M. J. Chem.
Rev. 2001, 101, 3819–3867.
7.1.9. 3G1 side product (2G1A). A yellow solid (0.0115 g)
was separated as a side product after flash column
chromatography in the above reaction. Yield: 6%. 1H
NMR (400 MHz, CDCl3) d 7.69 (s, 2H), 7.40–7.38 (m,
12H), 7.07 (s, 2H), 4.00 (s, 6H), 1.27 (m, 72H). 13C NMR
(100 MHz, CDCl3) d 160.6, 151.1, 151.0, 137.7, 128.3,
126.2, 126.1, 123.5, 123.1, 122.4, 122.0, 119.1, 113.9,
111.8, 97.3, 94.0, 87.2, 86.0, 79.7, 78.8, 56.3, 35.0 (m, two
overlapped peaks), 31.5 (m, two overlapped peaks). MS
(LRFAB) calcd for C82H94O2 1111.6, found 1112.
2. (a) Denti, G.; Campagna, S.; Serroni, S.; Ciano, M.; Balzani,
V. J. Am. Chem. Soc. 1992, 114, 2944–2950. (b) Stewart,
G. M.; Fox, M. A. J. Am. Chem. Soc. 1996, 118, 4354–4360.
(c) Balzani, V.; Campagna, S.; Denti, G.; Juris, A.; Serroni, S.;
Venturi, M. Acc. Chem. Res. 1998, 31, 26–34. (d) Sato, T.;
Jiang, D.-L.; Aida, T. J. Am. Chem. Soc. 1999, 121,
10658–10659. (e) Schenning, A. R. H. J.; Peeters, E.; Meijer,
E. W. J. Am. Chem. Soc. 2000, 122, 4489–4495. (f) Gust, D.;
Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40–48.
3. (a) Adronov, A.; Frechet, J. M. J. Chem. Commun. 2000,
´
1701–1710. (b) Gilat, S. L.; Adronov, A.; Frechet, J. M. J.
Angew. Chem. Int. Ed. 1999, 38, 1422–1427. (c) Gilat, S. L.;
´
Adronov, A.; Frechet, J. M. J. J. Org. Chem. 1999, 64(20),
7.1.10. Compound 3G2. The synthetic procedure is similar
to that of 3G1. Yield: 61%. Yellow powder. 1H NMR
(400 MHz, CDCl3) d 7.86 (s, 3H), 7.82 (s, 3H), 7.78 (s, 3H),
7.75 (s, 3H), 7.40–7.36 (m, 36H), 7.13 (s, 3H), 7.10 (s, 3H),
7.08 (s, 3H), 4.01 (s, 9H), 3.92 (m, 18H), 1.27 (m, 216 H).
13C NMR (100 MHz, CDCl3) d 159.4, 159.3, 159.2, 150.8
(m, three overlapped peaks), 137.2, 137.1, 137.0, 134.4,
127.6, 127.1, 127.0, 125.9, 125.8, 124.0, 123.2, 123.1,
7474–7484. (d) Adronov, A.; Gilat, S. L.; Frechet, J. M. J.;
Ohta, K.; Neuwahl, F. V. R.; Fleming, G. R. J. Am. Chem. Soc.
2000, 122(6), 1175–1185.
4. Devadoss, C.; Bharathi, P.; Moore, J. S. J. Am. Chem. Soc.
1996, 118(40), 9635–9644.
5. (a) Moore, J. S. Acc. Chem. Res. 1997, 30, 402. (b) Berresheim,

5506
Y. Pan et al. / Tetrahedron 59 (2003) 5495–5506
A. J.; Mu¨llen, K. Chem. Rev. 1999, 99, 1747. (c) Miller, T. M.;
12. Bergman, R. G. Acc. Chem. Res. 1973, 6(1), 25–31.
13. Shultz, D. A.; Gwaltney, K. P.; Lee, H. J. Org. Chem. 1998,
63(12), 4034–4038.
Neenan, T. X.; Zayas, R.; Bair, H. E. J. Am. Chem. Soc. 1992,
114, 1018. (d) Deb, S.; Madux, T. M.; Yu, L. J. Am. Chem.
Soc. 1997, 119, 9079. (e) Meier, H.; Lehmann, M.; Yu, L.
Angew. Chem., Int. Ed. Engl. 1998, 37, 643.
14. Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett.
1975, 4467.
6. Shortreed, M. R.; Swallen, S. F.; Shi, Z.; Tan, W.; Xu, Z.;
Devadoss, C.; Moore, J.; Kopelman, S. R. J. Phys. Chem. B
1997, 101, 6318.
15. Lu, M.; Pan, Y.; Peng, Z. Tetrahedron Lett. 2002, 43/44, 7903.
16. Godt, A.; Franzen, C.; Veit, S.; Enkelmann, V.; Pannier, M.;
Jeschke, G. J. Org. Chem. 2000, 65(22), 7575–7582.
17. (a) Schoeberl, U.; Magnera, T. F.; Harrison, R. M.; Fleischer,
F.; Pflug, J. L.; Schwab, P. F. H.; Meng, X.; Lipiak, D.; Noll,
B. C.; Allured, V. S.; Rudalevige, T.; Lee, S.; Michl, J. J. Am.
Chem. Soc. 1997, 119(17), 3907–3917. (b) Yang, S. H.; Li,
C. S.; Cheng, C. H. J. Org. Chem. 1987, 52(4), 691–694.
18. Rot, N.; Bickelhaupt, F. Organometallics 1997, 16(23),
5027–5031.
7. Peng, Z.; Pan, Y.; Yu, B.; Zhang, J. J. Am. Chem. Soc. 2000,
122(28), 6619–6623.
8. Melinger, J. S.; Pan, Y.; Kleiman, V. D.; Peng, Z.; Davis, B. L.;
McMorrow, D.; Lu, M. J. Am. Chem. Soc. 2002, 124(40),
12002–12012.
9. Wang, J.; Pan, Y.; Lu, M.; Peng, Z. J. Org. Chem. 2002,
67(22), 7781.
10. Bhatt, M. V. J. Organomet. Chem. 1978, 156, 221–226.
11. (a) Sala, T.; Sargent, M. V. J. Chem. Soc., Perkin Trans. 1
1979, 10, 2593–2598. (b) Banwell, M. G.; Flynn, B. L.;
Stewart, S. G. J. Org. Chem. 1998, 63(24), 9139–9144.
19. Moore, J. S.; Xu, Z. Macromolecules 1991, 24(21),
5893–5894.
20. Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75,
991–1024.