Photoluminescence properties of discrete conjugated wires wrapped within dendrimeric envelopes: "Dendrimer effects" on π-electronic conjugation

Article abstract of DOI:10.1002/anie.200353519

Third-generation dendrimers have been used to enable the synthesis and isolation of a series of spatially isolated, discrete conjugated wires G ₃-n (where n is the number of repeating monomer units, see schematic representation), including 147-

Full text of DOI:10.1002/anie.200353519

Angewandte
Chemie
Dendrimeric Structures
Photoluminescence Properties of Discrete
Conjugated Wires Wrapped within Dendrimeric
Envelopes: “Dendrimer Effects” on p-Electronic
Conjugation**
Wei-Shi Li, Dong-Lin Jiang,* and Takuzo Aida*
Conjugated molecular wires with discrete lengths are impor-
tant for the exploration of physical properties related to the
delocalization of p electrons, and have also attracted atten-
tion for their potential application in molecular electronics
and photonics.[1] Examples of discrete conjugated wires so far
reported include derivatives of polyphenylene, polyacetylene,
poly(phenylenevinylene), poly(phenyleneethynylene), poly-
thiophene, and polyporphyrin.[2] However, with the exception
of only one example,[2c] those oligomers are limited in length
to tens of nanometers, because of their low solubility and
strong tendency to aggregate. From a photochemical point of
view, such a strong tendency to aggregate is a major drawback
of “naked” nanowires, which results in collisional deactiva-
tion of photoexcited states and hinders their potential utilities.
A promising approach to solving this problem is to design
“isolated” nanowires bearing “insulating” shells. However,
such insulated nanowires with discrete molecular lengths are
unprecedented.[3,4]
We report herein the first example of discrete conjugated
wires wrapped in dendrimeric envelopes (Scheme 1a, Gm-n;
m = generation number of dendrimeric wedges, n = number
of repeating monomer units). Incorporation of large G3
poly(benzyl ether) dendrimeric wedges into the repeating
units[5] allowed us to overcome the solubility problem and to
synthesize conjugated wires with a molecular length of up to
147 nm. This dendrimeric core–shell strategy[6a] guarantees
that only a single conjugated chain is integrated into the focal
core, thereby representing a clear contrast to reported
strategies with other nanoscopic architectures, such as zeolite
channels, that incorporate bundles of conjugated poly-
mers.[6b,c] Herein, we highlight “dendrimer effects” on the
photoluminescence properties of the conjugated focal core,
with an emphasis on a possible effect of intramolecular
interactions between the large dendrimeric wedges on the p-
electronic conjugation of the backbone.
[*] Dr. W.-S. Li, Dr. D.-L. Jiang, Prof. T. Aida
Aida Nanospace Project
Exploratory Research for Advanced Technology (ERATO)
Japan Science and Technology Agency (JST)
2-41 Aomi, Koto-ku, Tokyo 135-0064 (Japan)
Fax: (+81)3-5841-7310
Fax: (+81)3-3570-9183
E-mail: jiang@nanospace.miraikan.jst.go.jp
aida@macro.t.u-tokyo.ac.jp
[**] We thank Y. Suna for technicalassistance and the Japan Anayltical
Industry Co., Ltd (JAI), for recycling preparative GPC.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200353519
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oligomerized to give higher oligomers (Scheme 1a). In
contrast to the case with short-chain G3-1–G3-8, the
coupling reaction of higher oligomers such as G3-16
and G3-32 proceeded rather sluggishly to afford only
their dimerized products G3-32 (26%) and G3-64
(15%), respectively. Likewise, oligomers G1[sBu]-n
were synthesized as lower-generation reference com-
pounds (Scheme 1a; n = 2–6, 8, 10, 12, and 16) from
G1[sBu]-1, which bears CO2sBu groups on its external
surface, since the G1 monomer (G1-1) with CO2Me
surface groups was barely soluble in common organic
solvents. G3-n and G1[sBu]-n, thus prepared, were
unambiguously characterized by analytical methods.[7]
A computer-aided molecular modeling study sug-
gested that higher oligomers G3-n (n ꢀ 4) adopt a
rodlike morphology with a diameter of roughly 4 nm
(Scheme 1b; G3-16). G3-64, which contains 192 aro-
matic rings and 256 triple bonds at its focal core, was
estimated to be 147 nm long,[9] which is the longest
1
discrete wire reported to date.[2] The H NMR spec-
trum of a solution of G3-1 in CDCl3 at 308C displayed a
set of signals at d = 6.98, 7.19, and 3.04 ppm corre-
sponding to the aromatic (Ha and Hb) and acetylenic
(Hc) protons, respectively, at the focal core. In
contrast, the lower-generation G1[sBu]-1 showed the
corresponding signals at slightly lower magnetic fields
with d values of 7.08 (Ha), 7.30 (Hb), and 3.13 ppm
(Hc), respectively. Interestingly, the spin–spin relaxa-
tion times (T2) of Ha and Hb, located in the proximity
of the dendrimeric wedges, are dependent on the
generation number m, whereas that of Hc is virtually
unaffected (Figure 1):[10] The T2 values of Ha and Hb in
G3-1 were 0.39 and 0.59 s, respectively, which were
clearly smaller than those of G1[sBu]-1 (0.78 [Ha] and
1.20 s [Hb]). The relatively short T2 values observed for
the backbone of G3-1 indicate there are constrained
conformational motions of the focal aromatic rings
attached to the large G3 dendrimeric wedges.
Solutions of G3-n in THF at 258C exhibited
absorption bands in the visible region as a conse-
quence of the conjugated backbone, as well as two
absorption bands at 231.0 and 276.0 nm arising from
the dendrimeric wedges (Figure 2a). As the number of
repeating units n increased, this absorption band was
red-shifted from 379.8 nm for G3-1 to 428.7 nm for G3-
64, with saturation of the spectral change observed at
around n = 8. Lower-generation G1[sBu]-n, under
identical conditions to the above, showed a similar
spectral change profile upon increment of n, again with
a saturation point around n = 8.[7] However, one may
Scheme 1. a) Synthesis of G1-1, G1[sBu]-n, and G3-n. Conditions: 1) [Pd(PPh3)4], CuI,
iPr2NH, THF, reflux; 2) Bu4NF, THF, RT; 3) Cu(OAc)2, TMEDA, THF, 558C. THF=tetrahy-
drofuran, TMEDA=tetramethylethylene diamine. b) Computer-generated images of the
molecular structures of G1[sBu]-16 and G3-16.
Monomer G3-1 was synthesized by a Pd0/CuI-catalyzed
coupling of dendrimeric 1,4-diethynylbenzene with 1-iodo-4-
trimethylsilylethynylbenzene in THF, followed by treatment
with Bu4NF.[7] Coupling of G3-1 in the presence of a mixture of
Cu(OAc)2 and TMEDA in THF at 558C for 10 minutes[8]
afforded a mixture of dimer G3-2 (23%), trimer G3-3 (15%),
tetramer G3-4 (10%), and pentamer G3-5 (7%), which were
separated by recycling preparative gel permeation chroma-
tography (GPC) with CHCl3 as the eluent, and then further
also note that the absorption bands of higher-generation G3-n
are located at a longer wavelength than those of G1[sBu]-n.
For example, decamers G3-10 and G1[sBu]-10 showed
absorption bands at 427.0 and 416.2 nm, respectively, thus
the energy difference is as large as 608 cmꢁ1 (Table 1). It is
unlikely that the observed spectral differences between G3-n
and G1[sBu]-n are caused by their surface groups, since the
transformation of the CO2Me groups on the exterior surface
of G3-10 into CO2sBu (G3[sBu]-10) resulted in no substantial
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change in the absorption spectral profile. Furthermore, G3-10
and G1[sBu]-10 showed only slight spectral changes when the
THF solvent was replaced with 1,3-dimethoxybenzene, an
analogue of the dendrimeric wedge building block. The
shorter-chain oligomers also displayed smaller energy differ-
ences, as shown in Table 1. In relation to this observation, 1,4-
diethynylbenzene derivatives bearing G1 and G3 dendrimeric
wedges (G1-DEB, G1[sBu]-DEB, and G3-DEB; Scheme 1a),
which are devoid of any conformational diversity at the focal
cores, all showed an absorption maximum at 335.0 nm,
irrespective of the surface group and generation number of
the poly(benzyl ether) dendrimeric wedges.[7] Thus, the
spectral differences between G3-n and G1[sBu]-n are most
likely related to conformational aspects of their conjugated
backbones; namely, the conformation of the backbone of G3-
n allows better conjugation between the chromophore units
than that in G1[sBu]-n, although the effective conjugation
lengths of G3-n and G1[sBu]-n are almost identical to one
another.
Dendrimeric compounds G3-n and G1[sBu]-n emitted a
blue fluorescence upon excitation of their conjugated back-
bones in THF at 258C (Figure 2b). As we have already
reported for a nondiscrete poly(phenyleneethynylene) with
G3 dendrimeric wedges,[3a] the fluorescence quantum yields
(FFL) of G3-n were all high (80–90%), irrespective of the
number of the repeating units n (Figure 3).[11] In sharp
Figure 1. 1H NMR spin–spin relaxation times T2 of Ha, Hb, and Hc in
G1[sBu]-1 and G3-1 recorded in CDCl3 at 308C.
Figure 2. a) Electronic absorption and b) emission spectra (normal-
ized) of G3-n (n=1–6, 8, 10, 12, 16, 24, 32, and 64; from left to right)
in THF at 258C.
Figure 3. Fluorescence quantum yields (FFL) of G3-n (red bars) and
G1[sBu]-n (blue bars) upon excitation at their absorption maxima
(A=0.1) in THF at 258C.
Table 1: Energy differences between G3-n and G1[sBu]-n at the absorption
and emission maxima of their conjugated backbones in THF at 258C.
contrast, the FFL values of lower-generation G1[sBu]-n dis-
played a tendency to drop when the n value was ꢀ 8, possibly
because of an enhanced probability of collisional quenching
of the singlet excited state. Higher-generation G3-n are less-
sensitive to concentration than G1[sBu]-n. For example, when
the absorbance of the solution was increased from 0.01 up to
0.24, the FFL value of G3-16 was preserved in a range of 80–
85%, whereas a notable decrease in the FFL value from 65%
to 45% was observed for G1[sBu]-16.[7] Thus, the large
dendrimeric envelope of G3-n wraps around the conjugated
backbone and prevents collisional deactivation of the excited
n
Energy differences [cmꢁ1] at
absorption maxima
Energy differences [cmꢁ1] at
emission maxima
1
2
4
6
8
266.1
353.0
501.1
558.6
608.3
155.6
182.5
201.5
200.2
200.0
209.9
214.4
209.6
10 607.7
12 594.8
16 602.8
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states.[3a] A closer look at the luminescence properties of G3-n
dendrimeric side groups have shown that the exciton migra-
tion subsides within several nanometers.[13,14]
and G1[sBu]-n showed a dependence of the luminescence
maximum on the “dendrimer size” (Table 1),[7] analogous to
that observed for the absorption spectral profiles. For
example, a solution of G3-10 in THF at 258C emitted a
luminescence centered at 449.4 nm, which is red-shifted by
4.2 nm from that of lower-generation G1[sBu]-10 (445.2 nm),
Taking all the above results into account, it is likely that
G3-n bearing the large G3 dendrimeric wedges prefers a
planar conformation of the conjugated backbone, which is
good for electronic conjugation (Table 1)[15] and may also
allow preservation of fluorescence anisotropy in a long-range
exciton migration (Figure 4). There are several examples of
attractive van der Waals interactions between poly(benzyl
ether) dendrimers in their self-organized structures.[16] We
assume that the dendrimeric wedges in G3-n could similarly
interact with one another intramolecularly. To support this
hypothesis we investigated the dynamics of the conforma-
tional change of the dendrimeric wedges of G3-n. The
1H NMR spectrum of a solution of G3-1 in CDCl3 at 308C
exhibited a doublet at d = 7.97 ppm which is attributed to the
ortho-H of the outermost aromatic rings in the dendrimeric
wedges.[7] Dimer G3-2 showed, in addition to this signal, a new
ortho-H doublet at a slightly higher magnetic field (d =
7.93 ppm). Furthermore, trimer G3-3 showed another new
doublet at d = 7.92 ppm. When the degree of polymerization
(n) of G3-n was larger, the signal at d = 7.92 ppm was more
intense. These characteristic signals were assigned as shown in
Figure 5, where the signal at d = 7.97 ppm originates from the
and the energy difference is calculated to be 210 cmꢁ1 [12]
In
.
contrast, 1,4-diethynylbenzene derivatives G1-DEB, G1[sBu]-
DEB, and G3-DEB, which had no conformational diversity at
the focal cores, displayed virtually identical fluorescence
spectra to one another. On the other hand, not only G3-10 but
also lower-generation G1[sBu]-10 showed only very small
changes in the fluorescence spectra when the THF solvent
was replaced by 1,3-dimethoxybenzene.
Since the conjugated backbone in G3-n is spatially isolated
by the thick G3 dendrimeric envelope, we investigated the
fluorescence depolarization profiles of G3-n (n = 4, 8, 12, 16,
24, 32, and 64), which are considered to reflect the photo-
chemical events in the isolated wires. Suppression of Brown-
ian motion in a viscous medium should result in the
fluorescence depolarization occurring predominantly by exci-
ton migration along the conjugated backbone.[13] Here the
degree of fluorescence depolarization (p) is defined as p =
(IkꢁGI?)/(Ik + GI?), where Ik and I? are the fluorescence
intensities of parallel and perpendicular components relative
to the polarity of the excitation light, respectively, while G is
an instrumental correction factor. Excitation of the absorp-
tion maxima of viscous solutions of G3-n in THF/polystyrene
at 258C gave fluorescence depolarization profiles (Figure 4)
Figure 4. Fluorescence depolarization p of G3-n upon excitation with a
polarized light at their absorption maxima (A=0.1) in THF/polystyrene
(degree of polymerization (DP)=1000–1400; 0.2 gmLꢁ1) as a viscous
solvent at 258C.
where the value of p gradually became smaller as the number
of the repeating units n increased. For example, the p value
for short-chain G3-4 was 0.38, which dropped to 0.21 and
further to 0.11 when the n value was increased to 16 (G3-16)
and then to 64 (G3-64). The absence of a saturation tendency
up to a molecular length of 147 nm (G3-64) is quite interest-
ing, since previous studies on conjugated polymers without
Figure 5. 1H NMR spin–spin relaxation times T2 of ortho-H in the out-
ermost aromatic rings of the dendrimeric wedges of G3-n in CDCl3 at
308C.
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miya, Y. Aso, T. Otsubo, J. Am. Chem. Soc. 2003, 125, 5286 –
5287.
dendrimeric wedges located at both ends of the backbone
(red), while the signals at d = 7.93 and 7.92 ppm are from the
other dendrimeric substituents (blue). We measured spin–
spin relaxation times (T2) of these characteristic signals
(Figure 5), and found that the T2 value of the signal at d =
7.92 ppm is smaller when the degree of polymerization n is
larger, and reaches a plateau at n = 10 (blue bars; the T2 value
of the signal at d = 7.93 ppm is shown for G3-2). In sharp
contrast, the T2 value of the signal at d = 7.97 ppm, which
arises from the dendron units at the edges of the backbone
(red), is virtually unchanged by n (red bars). These contrast-
ing results suggest that the molecular motions of the inner
dendrimeric wedges, which are densely aligned along the
rigid, conjugated backbone, are highly constrained as a
consequence of intramolecular van der Waals interactions.
In contrast, the T2 values of the corresponding signals in
lower-generation G1[sBu]-n were only slightly dependent on
n.[7,17]
In summary, we have reported the first example of
discrete conjugated wires G3-n bearing large dendrimeric
substituents. A great advantage of the dendrimeric architec-
ture is that it allows for the synthesis and isolation of a 147-nm
long discrete wire (G3-64), in which the conjugated backbone
consisting of 192 aromatic rings and 256 triple bonds is
wrapped in a thick dendrimeric envelope. Comparative
photochemical studies with lower-generation G1[sBu]-n as
reference compounds indicate that the conjugated backbone
in G3-n tends to adopt a planar conformation, most probably
because of intramolecular van der Waals interactions
between the large, densely aligned G3 dendrimeric wedges.
The planar conformation of the backbone allows efficient
electronic conjugation of the chromophores and low fluores-
cence depolarization in an exciton migration event. Applica-
tion of such insulated nanowires to molecular electronics and
photonics is one of the interesting subjects worthy of further
investigations.
[3] a) T. Sato, D.-L. Jiang, T. Aida, J. Am. Chem. Soc. 1999, 121,
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[5] The coupling reaction of G3-DEB took place very sluggishly and
gave only very small amounts of higher oligomers.
[6] a) S. Hecht, J. M. J. FrØchet, Angew. Chem. 2001, 113, 76 – 94;
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[9] The molecular lengths of Gm-n, as estimated by an MM2 module
implemented in the Chem. 3D software package, were 9 (n = 4),
18 (n = 8), 27 (n = 12), 36 (n = 16), 55 (n = 24), 74 (n = 32), and
147 nm (n = 64).
[10] F. A. Bovey, Nuclear Magnetic Resonance Spectroscopy, 2nd ed.,
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[11] By reference to a quinine sulfate solution (1.0n, A350nm = 0.06,
FFL = 55% upon 350 nm excitation) according to a method
reported in: R. P. Haugland, J. Yguerabide, L. Stryer, Proc. Natl.
Acad. Sci. USA 1969, 63, 23 – 30.
[12] I. Sluch, A. Godt, U. H. F. Bunz, M. A. Berg, J. Am. Chem. Soc.
2001, 123, 6447 – 6448.
[13] A. Rose, C. G. Lugmair, T. M. Swager, J. Am. Chem. Soc. 2001,
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[14] C. F. Wang, J. D. White, T. L. Lim, J. H. Hsu, S. C. Yang, W. S.
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[15] T. Miteva, L. Palmer, L. Kloppenburg, D. Neher, U. H. F. Bunz,
Macromolecules 2000, 33, 652 – 654.
[16] a) W.-D. Jang, D.-L. Jiang, T. Aida, J. Am. Chem. Soc. 2000, 122,
3232 – 3233; b) M. Enomoto, A. Kishimura, T. Aida, J. Am.
Chem. Soc. 2001, 123, 5608 – 5609; c) V. Percec, M. Glodde, T. K.
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Hudson, H. Duan, Nature 2002, 417, 384 – 387; d) T. Fujigaya, D.-
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[17] The slow spin–spin relaxation of the focal aromatic rings in G3-1
(Figure 1) suggests the large G3 dendrimeric wedges could also
affect the conformation of such a short-chain conjugated back-
bone on steric grounds.
Received: December 12, 2003 [Z53519]
Keywords: conjugation · dendrimers · exciton migration ·
.
luminescence · molecular wires
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