Synthesis and single aggregate spectroscopy of a novel fluorescent dendrimer with highly efficient energy harvesting

Article abstract of DOI:10.1246/cl.2002.394

A novel fluorescent dendrimer with a diphenylanthracene core and pyrenyl groups in the dendrons was synthesized. Fluorescence spectra and its decay curves proved spontaneous energy transfer from the pyrene units to the cote diphenylanthracene, which was ascertained both in solution and for a single particle of the dendrimer aggregate.

Full text of DOI:10.1246/cl.2002.394

394
Chemistry Letters 2002
Synthesis and Single Aggregate Spectroscopy of a Novel Fluorescent
Dendrimer with Highly Efficient Energy Harvesting
Minoru Kobayashi,# Shinjiro Machida,## Satoshi Takahashi,### Kazuyuki Horie,ꢀ#### Hiroyuki Yoshikawa,y and Hiroshi Masuharay
Department of Chemistry and Biotechnology, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8654
yDepartment of Applied Physics, Osaka University, Yamadaoka, Suita, Osaka 565-0871
(Received December 14, 2001; CL-011259)
A novel fluorescent dendrimer with a diphenylanthracene
core and pyrenyl groups in the dendrons was synthesized.
Fluorescence spectra and its decay curves proved spontaneous
energy transfer from the pyrene units to the core diphenylan-
thracene, which was ascertained both in solution and for a single
particle of the dendrimer aggregate.
Dendrimers are hyperbranched macromolecules with well-
predictable three-dimensional architecture.1{4 They are com-
posed of core and dendron units which include building blocks
and functional groups. Recently, dendrimers have attracted great
attention as synthetic light-harvesting antennae,5{10 which means
that some dendrimers can emit strong fluorescence from their core
unit owing to energy transfer from dendron units to the core unit
even when the core unit does not absorb light directly. Low
Figure 1. Synthetic route of the L2dendrimer with diphenylanthra-
cene core and terminal pyrenyl groups.
temperature structural relaxation in porphyrin-cored dendrimers
was elucidated in our previous study.11 In the present study, we
synthesized a novel fluorescent dendrimer which contains
and MALDI TOF MS confirming its preparation.
pyrenyl groups in dendron units and a diphenylanthracene group
The measurements of fluorescence and fluorescence excita-
tion spectra were carried out for PyL2OH (3), 9,10-bis(4-
in a core unit so as to show high-efficient energy transfer12 to the
core unit. The reasons why we chose pyrene and diphenylan-
methoxyphenyl)anthracene (4), and L2dendrimer (6) to know
thracene groups are as follows: the absorption wavelength of
information on energy transfer from the dendron units to the core
diphenylanthracene is almost overlapped by the emission
unit. The concentrations of the dendrimer and reference samples
wavelength of pyrene, the fluorescence lifetime of pyrene is
long, and the fluorescence quantum yield of diphenylanthracene
is almost unity. We report here synthesis of the dendrimer and the
result of high efficient energy transfer from pyrene units to the
core unit by measuring fluorescence spectra and lifetime. This
dendrimer might be applied to a point light source. For example,
when we attach it to the top of near-field scanning optical
microscope (NSOM) probe as a point light source,13 it may give a
possibility of watching with NSOM a small object whose size is as
big as one molecule.
were set to be 1 ꢁ 10ꢂ5 M in dichloromethane so that we could
ignore the influence of intermolecular interaction. The results are
shown in Figure 2. The fluorescence spectrum of 6 shows almost
the same emission profile as that of 4 when excited at 337 nm. On
the other hand, the excitation spectrum of 6 shows almost the
same profile as that of 3 when monitored at 426 nm where the
strongest fluorescence intensity of 6 is observed. These results are
The synthesis of the dendrimer with a diphenylanthracene
core and dendrons with pyrenyl terminal groups (L2dendrimer)
was carried out with a convergent method3;14 as is shown in
Figure 1. PyL1ester (1) was prepared by the reaction of 1-
hydroxymethylpyrene and methyl 4-hydroxybenzoate in dry THF
in the presence of triphenylphosphine and by adding diethylazo-
dicarboxylate (DEAD) solution.15 The reduction of 1 to PyL1OH
(2) was performed by using lithium aluminum hydride (LAH).
The reaction of 2 with methyl 3,5-dihydroxybenzoate in the
presence of triphenylphosphine and DEAD resulted in PyL2ester,
which was reduced with LAH to PyL2OH (3), the dendron with
pyrenyl terminal groups. Then the dendron 3 was bonded with the
aid of triphenylphosphine and DEAD to 9,10-bis(4-hydroxyphe-
nyl)anthracene (5), which was prepared in advance from 9,10-
Figure 2. Fluorescence and its excitation spectra of core diphenylan-
thracene 4, L2dendimer 6, and L2dndron 3 in 1:0 ꢁ 10ꢂ5 M CH2Cl2
bis(4-methoxyphenyl)anthracene (4) by using borontribromide.
The resulting L2dendrimer (6) was characterized with 1H NMR
solution.
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
395
explained as follows. When we irradiate ultraviolet light at
337 nm onto 6, the dendrimer 6 absorbs almost all of the irradiated
light at the pyrene units and shows strong fluorescence only from
the core unit due to energy transfer from the pyrene units to the
core unit. Still more interesting to say is that though we could see a
broad fluorescence peak attributed to pyrene excimer at 480 nm in
the fluorescence spectra of 3, we could not observe that kind of
fluorescence peak in the spectra of 6. This result suggests that
energy transfer from pyrene units to the core unit takes place
much faster than the excimer-forming motion between adjacent
pyrene units in 6.
30 ps. The difference of its decay lifetime from that in solution
suggests the existence of various deactivation processes from the
excited state of diphenylanthracene unit due to their interaction in
solid state.
Transient fluorescence decay curves for 1 ꢁ 10ꢂ6 M solu-
tions of L2dendrimer 6, and its reference samples, 3 and 4 were
measured by using a sub-nano-sec pulsed N2 laser and streak
camera detection system. Details of the apparatus are given
elsewhere.16 The results for 3 and 6 are shown in Figure 3. First,
we irradiated UV pulse at 337 nm onto 6 and carried out a
transient decay measurement monitored at 406–477 nm. We
could see the peaks of the irradiation laser light and the
fluorescence from the core of 6 at almost the same time. Second,
we irradiated UV pulse at 337 nm onto 3 and observed a time-
decay monitored at 452–561 nm to see the fluorescence only from
the excimer. We could see the peak of the fluorescence after 16 ns
compared to the irradiation laser light. Therefore, we can say that
excimer formation between adjacent pyrene units takes place
within 16 ns, and this order of magnitude is in agreement with
previous studies on excimer formation dynamics of oligomeric
chains with chromophores at both chain ends in solution.17;18
However, the rate of energy transfer in the L2dendrimer 6 was
very fast and its time range was beyond the time resolution of the
apparatus (100 ps). The average decay lifetime of 6 is 4.7 ns,
which is almost the same as that of 4 (4.6 ns). The risetime of 3
(excimer) is 16 ns and the decay time of 3 (excimer) is about 22 ns.
Figure 4. Fluorescence image of a single L2dendrimer
aggregate observed with a confocal fluorescence microscope.
In conclusion, we synthesized an L2dendrimer which
contains pyrene in dendrons and diphenylanthracene in a core.
When UV light was irradiated at 337 nm onto this dendrimer, it
emitted fluorescence almost only from the core unit due to the
spontaneous energy transfer from the pyrene units to the core
diphenylanthracene. Fluorescence decay lifetimes were mea-
sured both in solution and for a single particle of its aggregate.
References
#
Presentaddress:Procter and GambleFar East, Inc., 17, Koyo-choNaka
1-chome, Higashinada-ku, Kobe 658-0032
## Present address: Department of Polymer Science and Engineering,
Kyoto Institute of Technology, Goshokaido-cho, Sakyo-ku, Kyoto
606-8585
### Present address: Department of Pure and Applied Physics, School of
Mathematics and Physics, The Queen’s University of Belfast, Belfast
BT7 1NN, Northern Ireland, UK
#### Present address: Department of Organic and Polymer Materials
Chemistry, Tokyo University of Agriculture and Technology, 4-24-
16 Naka-cho, Koganei, Tokyo 184-8588
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plate was prepared by spreading 1 ꢁ 10ꢂ8 M solution of 6 in
CH2Cl2 onto the silica plate, and was observed with a confocal
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