Mechanistic investigation of energy transfer in perylene-cored anthracene dendrimers

Article abstract of DOI:10.1039/b716908f

In this publication, we describe results of investigations focusing on detailed mechanisms of directed energy transfer in perylene-cored anthracene dendrimers. To obtain definitive statistical data for probing the energy transfer pathways, we synthesized four analogous dendrimers, which were designed to funnel the energy only from remote anthracene groups to the perylene cores. Static fluorescence studies with these dendrimers revealed that excitation of the anthracene groups led to the core emissions, indicating efficient energy transfer should be involved. Inspection of the energy transfer efficiencies obtained from all ten dendrimers demonstrated that single-step energy transfer should represent a key mechanism for the long-range energy transport in these dendrimers. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b716908f

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Mechanistic investigation of energy transfer in perylene-cored anthracene
dendrimersw
Masaki Takahashi,*a Hironao Morimoto,b Kentaro Miyake,a Hideki Kawai,c
Yoshihisa Sei,d Kentaro Yamaguchi,d Tetsuya Sengokua and Hidemi Yoda*a
Received (in Durham, UK) 1st November 2007, Accepted 6th December 2007
First published as an Advance Article on the web 2nd January 2008
DOI: 10.1039/b716908f
In this publication, we describe results of investigations focusing on detailed mechanisms of
directed energy transfer in perylene-cored anthracene dendrimers. To obtain definitive statistical
data for probing the energy transfer pathways, we synthesized four analogous dendrimers, which
were designed to funnel the energy only from remote anthracene groups to the perylene cores.
Static fluorescence studies with these dendrimers revealed that excitation of the anthracene groups
led to the core emissions, indicating efficient energy transfer should be involved. Inspection of the
energy transfer efficiencies obtained from all ten dendrimers demonstrated that single-step energy
transfer should represent a key mechanism for the long-range energy transport in these
dendrimers.
unaffected by relative orientation of the donor and acceptor
Introduction
chromophores within the dendrimer domains. In the present
Natural photosynthetic organisms have developed the most
publication, we describe the results of continuing investiga-
sophisticated light harvesting systems that contain large num-
tions, which delineate the mechanism of energy transfer in the
bers of porphyrins held in particular three-dimensional arrays.
multichromophore systems. A key to successful implementa-
The highly ordered structure of the antenna pigments in the
tion of the plan was a consideration of possible pathways
nanoscopic domains plays a key role in efficient transport of
available to long-range energy transfer from the remote an-
the solar energy to a reaction center that ultimately achieves
thracene donor groups to the perylene cores: direct (single-
solar energy conversion.1 In order to mimic the function of the
step) and indirect (multistep) pathways could be envisaged. To
natural light-harvesting antenna systems, considerable effort
resolve this mechanistic problem, we prepared and studied
has been directed towards designing and synthesizing artificial
four simplified dendrimer analogues 7–10 incorporating 1,4-
structures containing donor and acceptor chromophoric units
disubstituted benzene moieties as an interlinking interior unit.
in supramolecular and dendritic systems.2,3 Nevertheless,
As a result of detailed analyses of energy transfer efficiencies,
photophysical characteristics of the light-harvesting dendrimer
we clearly demonstrated that the single-step energy transfer
systems have remained a standing concern due to the fact that
was the major pathway in the dendrimer systems.
these nanoscopic-sized multichromophoric assemblies result in
mechanistic complexities of the excited-state behavior arising
Results and discussion
from many excitonic interactions between neighboring chro-
mophores.4 In previous publications, we have reported a series
The construction of new dendritic structures (7–10) was
of perylene-cored anthracene dendrimers 1–6 (Fig. 1), which
successfully achieved by direct coupling of peripheral (A1
exhibited light-harvesting functionalities from peripheral an-
thracene groups to the perylene cores.5,6 During the course of
and A2) and core fragments (P1 and P2), using our previous
strategy for preparations of anthracene-based dendrimers
(Scheme 1).5–7 Since we have established the synthetic ap-
our studies we found that energy transfer efficiencies were
proaches to P1 and P2, our initial focus was the synthesis of
a Department of Materials Science, Faculty of Engineering, Shizuoka
University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8561,
Japan. E-mail: tmtakah@ipc.shizuoka.ac.jp,
A1 and A2 following the previous synthetic routes with slight
structural modifications.6 The synthesis of A1 and A2 started
with free radical bromination of commercially available
methyl 4-methylbenzoate 11 (Scheme 2). Upon treatment of
11 with NBS/AIBN, the methyl group was converted to the
corresponding bromomethyl functionality, giving methyl 4-
(bromomethyl)benzoate 12 as a predominant product (87%).
This synthetic intermediate underwent nucleophilic substitu-
tion reactions with 3-benzoyloxyphenol and 3,5-dibenzoyl
oxyphenol to afford 13L and 13B in 91% and 92% yield,
respectively. The benzoyl groups of these substances were
chemoselectively removed with n-butylamine to give 14L
(85%) and 14B (83%) without affecting the ester endgroups.
tchyoda@ipc.shziuoka.ac.jp; Fax: +81-53-478-1621, +81-53-478-
1150; Tel: +81-53-478-1621, +81-53-478-1150
b Graduate School of Science and Engineering, Shizuoka University,
3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8561, Japan
c Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku,
Naka-ku, Hamamatsu, Shizuoka 432-8011, Japan
d Faculty of Pharmaceutical Sciences at Kagawa Campus, Tokushima
Bunri University, Shido, Sanuki, Kagawa 769-2193, Japan
w Electronic supplementary information (ESI) available: Table for
SEC results (Table S1); plot for SEC results (Fig. S1); synthetic details
and complete characterization data for the synthetic intermediates
(A1, A2 and 12–16); 1H and 13C NMR spectra (A1, A2, 7–10 and
12–16); SEC traces (7–10). See DOI: 10.1039/b716908f
ꢀc
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Fig. 1 Structures of 1–10.
These intermediates were then subjected to reaction with
9-(chloromethyl)-10-hexylanthracene under conditions used
for the other anthracenemethyl ethers5–7 to yield 15L (88%)
and 15B (87%). The methoxycarbonyl groups of these mole-
cules were readily reduced with lithium aluminum hydride,
rendering the corresponding benzylic alcohols 16L (98%) and
16B (91%). Finally, treatment of these compounds with
methanesulfonyl chloride and an excess of lithium chloride
led to efficient production of A1 (96%) and A2 (93%).
plots of molecular weight values vs. SEC retention volumes (V)
for the examined compounds, which are almost linear and
have an unified downward slope. On the basis of this correla-
tion, the molecular weight values for 1–10 were approximated
through calibration with the polystyrene standards. Appar-
ently, the estimated molecular weights (Mw) proved to be
sufficiently close to the nominal values for smaller and med-
ium-sized dendrimers (entries 1–5 and 7–9), whereas larger-
sized dendrimers gave considerably underestimated Mw values
(entries 6 and 10). It is well-established that dendritic struc-
tures give smaller Mw values than their actual molecular
weights particularly for higher-generation dendrimers.8 There-
fore, it is clear that the SEC experiments were consistent with
the formation of 7–10, providing a convincing evidence that
our synthetic strategy could be applied in the construction of
these covalently bound multichromophore systems.
With these dendritic fragments in hand, we synthesized 7–10
by combinatorial method employing direct coupling reactions
of the peripheral (A1 and A2) and core (P1 and P2) fragments
at the multiple reaction sites. These dendrimers were satisfac-
torily characterized by elemental analyses and a range of
spectroscopies. To obtain further structural information about
polydispersity indices and molecular masses for these macro-
molecular systems, size exclusion chromatography (SEC) was
examined (Table S1, see ESIw). From the SEC data obtained
from 10 individual dendrimers and 6 individual polystyrene
standards, it appears that all the dendrimers gave low poly-
dispersity values (Mw/Mn o 1.1), being in the range found for
monodisperse structures with a precisely controlled number of
functional groups.8 Fig. S1 (see ESIw) shows semilogarithmic
The presence of the precise number of chromophores in
7–10 was confirmed by UV-Vis spectroscopy, which exhibited
three absorption bands assigned to the anthracene groups in a
shorter-wavelength region (350–410 nm) and two absorption
bands assigned to the perylene groups in a longer-wavelength
region (430–500 nm) as we have previously observed for 1–6
(Fig. 2 and Table 1). In Fig. 3, representative absorption
coefficients of both chromophores were plotted as a function
of the total number of anthracene groups. Throughout the
series of 1–10, the absorption coefficients for anthracene at 359
nm exhibited a linear relationship passing through the origin,
while those for perylene at 477 nm were approximately con-
stant. These results indicated that a number of the chromo-
phores held in the dendritic arrays behaved as non-interacting
monomeric species that could be distinguished in the
Scheme 1
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Fig. 2 Absorption spectra of 7–10 in chloroform solutions.
Scheme 2 Reagents and conditions: (i) NBS, AIBN, CHCl3, reflux; (ii)
3-(benzoyloxy)phenol, 18-crown-6, K2CO3, DMF, 55 1C; (iii)
n-BuNH2, THF, reflux; (iv) 9-(chloromethyl)-10-hexylanthracene
(1 equiv.), 18-crown-6, K2CO3, DMF, 55 1C; (v) LAH, THF, rt;
(vi) LiCl, MsCl, Et3N, THF, rt; (vii) 3,5-di(benzoyloxy)phenol,
18-crown-6, K2CO3, DMF, 55 1C; (viii) 9-(chloromethyl)-10-hexylan-
thracene (2 equiv.), 18-crown-6, K2CO3, DMF, 55 1C.
normalized for the acceptor peaks, the fET values for 7, 8, 9
and 10 were estimated to be 0.63, 0.65, 0.58 and 0.55,
respectively,10 which were summarized together with those
previously reported for 1–6 in Table 1.6 In Fig. 5, we plotted
all these fET data as a function of the number of anthracene
chromophores incorporated at the periphery of the dendri-
mers. As can be seen from the figure, the fET values were
found to be fitted into two distribution patterns, where lower-
density dendrimers 1, 2, 3, 7 and 8 gave superior quantum
efficiencies to higher-density dendrimers 4, 5, 6, 9 and 10. In
this context, it has been recognized that dendritic architectures
functionalized with anthracene clusters gave rise to decreased
photoluminescence quantum efficiencies.7 This observation
may be connected with aggregated nature of the multichro-
mophore systems which promote radiationless deactivation
through excited state interactions between chromophores.
Based on this consideration, the observed difference in fET
values between the lower- and higher-density dendrimers
could be reasonably explained by the mechanism of energy
migration between branches of the dendrimers. Of particular
interest in the course of our investigations were molecular
structure–property relationships that would govern the energy
transfer efficiencies and provide mechanistic information
about the energy transfer processes. Remarkably, the fET
values for the dendrimers 2, 3, 5 and 6 were nearly equal to
those for the benzyl-ether counterparts 7, 8, 9 and 10, respec-
tively, indicating that the interior anthracene donor groups
present near the cores did not play a role in long-range energy
transfer from the exterior donors to the acceptors. An inter-
pretation of this result was that at any donor–acceptor dis-
tance the energy transfer occurred in a single-step manner
within the dendritic shells, because the multistep mechanism
was no longer efficient. Another remarkable finding was that
the fET values for 7 and 9 were almost the same as those for 1
and 4, respectively. In fact, averaged distances for the short-
and long-range energy transfer were estimated to be 1.4 and
2.3 nm, respectively, by means of molecular modeling studies
performed on fully extended structures of 1–10.6 The obvious
conclusion which could be drawn from these considerations
was that the energy transfer efficiencies at the two different
donor–acceptor distances were essentially equal. This conclu-
sion is intriguing when one considers the Forster relation that
scales energy transfer efficiency with the inverse sixth power of
absorption spectra. Thus, the precise stoichiometry of the
chromophoric groups was unambiguously verified through
these investigations, providing definitive support for all the
dendrimer structures.
Fig. 4 shows steady-state fluorescence and excitation spectra
of 7–10 in chloroform solutions. In the fluorescence emission
spectra (Fig. 4(A)), these dendrimers displayed significant
perylene emission bands in
a longer-wavelength region
(470–600 nm) and weak anthracene emission bands in a
shorter-wavelength region (400–450 nm) upon excitation of
the anthracene chromophores at 378 nm. Furthermore, the
excitation spectra obtained at lem of the core at 500 nm (Fig.
4(B)) were found to be similar in shape to their absorption
spectra, which exhibited three sharp peaks characteristic of the
anthracene groups in the shorter-wavelength region (350–410
nm) and two clearly structured peaks characteristic of the
perylene groups in the longer-wavelength region (430–
500 nm). These observations were consistent with a mechan-
ism which involved intramolecular energy transfer driven by
donor–acceptor interactions between the peripheral anthra-
cene groups and the perylene cores.5,6 It should be noted that
fluorescence emissions arising from the anthracene periphery
showed different degrees of relative intensity (Table 1), which
may distinguish two dendrimers 9 and 10 from the others. In
these systems, the dendritic backbones may serve as sterically
demanding spacers that keep the anthracene groups away
from the perylene core, restricting possibilities of energy
redistribution around the excited donors. On the contrary, it
can be deduced that suppression of the donor emissions
observed for 1–8 was indicative of increased exciton delocali-
zation processes, since the excited donor groups could be in
close contact with neighboring chromophores and have a
tendency to interact each other.
To understand the mechanistic scheme taking place in the
dendrimers, examination of energy transfer efficiencies (fET
)
would indicate fundamental aspects of the energy transfer
processes.9 By comparing the donor peaks in the excitation
spectra of the dendrimers with those in the absorption spectra
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Table 1 UV-Vis and fluorescence data and energy transfer efficiencies for 1–10
max/nm (e/104)a
l
em/nm (I)b
fET
c
Entry
l
1
2
3
4
5
6
7
8
9
10
a
342 (1.21), 359 (2.29), 378 (3.56), 399 (3.74), 446 (3.49), 475 (4.26)
340 (2.48), 357 (4.81), 375 (7.30), 396 (7.34), 448 (3.44), 477 (4.14)
342 (3.63), 358 (7.32), 377 (11.4), 398 (11.4), 448 (3.40), 476 (4.14)
343 (2.32), 360 (4.62), 378 (7.30), 399 (7.32), 448 (3.47), 477 (4.22)
342 (5.00), 358 (9.64), 376 (14.8), 397 (14.6), 449 (3.57), 477 (4.19)
342 (7.00), 359 (14.1), 378 (22.4), 399 (22.3), 449 (3.63), 478 (4.33)
342 (1.52), 359 (2.84), 378 (4.43), 399 (4.54), 447 (3.25), 477 (4.03)
342 (2.75), 359 (5.50), 378 (8.80), 399 (8.87), 447 (3.42), 477 (4.14)
342 (2.54), 359 (5.12), 378 (8.11), 399 (8.06), 447 (3.50), 477 (4.23)
342 (4.82), 359 (9.84), 378 (15.8), 399 (15.5), 447 (3.59), 477 (4.31)
409 (0.03), 431 (0.03), 493 (1.00), 524 (0.64)
409 (0.02), 432 (0.02), 493 (1.00), 525 (0.63)
410 (0.02), 432 (0.02), 494 (1.00), 524 (0.61)
409 (0.04), 432 (0.06), 493 (1.00), 525 (0.69)
409 (0.02), 433 (0.03), 495 (1.00), 527 (0.69)
411 (0.02), 432 (0.02), 495 (1.00), 526 (0.64)
408 (0.09), 431 (0.09), 493 (1.00), 525 (0.63)
408 (0.04), 431 (0.04), 493 (1.00), 525 (0.57)
409 (0.50), 431 (0.48), 493 (1.00), 525 (0.58)
409 (0.63), 431 (0.62), 493 (1.00), 525 (0.56)
0.69d
0.69d
0.65d
0.54d
0.54d
0.48d
0.63
0.65
0.58
0.55
Absorption wavelengths (lmax) and absorption coefficients (e) for the absorption maxima. The e values were determined by measurements
employing chloroform solutions of the samples, where all calibrations were linear in the range from 1 to 20 mmol LÀ1
.
Emission wavelengths
b
(lem) and signal intensities (I) for the fluorescence emission maxima. The I values represent fluorescence intensities relative to those for the highest
c
fluorescence peaks around 494 nm. Quantum efficiencies for energy transfer. The fET values were determined by comparison between the
absorption spectra and the excitation spectra, see ref. 10. See ref. 6.
d
the donor–acceptor distances.11 The observed distance inde-
pendence in energy transfer efficiency could be rationalized by
taking into account the Forster critical radius (R0) of 3.6 nm
that has been previously reported for the anthracene–perylene
pair.12 Using this parameter in the Forster relation, we could
make simple estimations of the fET values for the short- and
long-range energy transfer processes, which were found to be
almost equivalent.13 The results discussed above indicated that
the dendrimer domains confined within the Forster critical
radius served as a unique molecular environment that ensured
the efficient pathways for the single-step energy transfer.
nanoscopic energy transfer properties for the fabrication of
superior light-harvesting materials, which can facilitate future
device applications in the areas of photonics,14 sensors,15 and
solar energy conversion.16
Experimental
General:
All solvents and reagents were of reagent grade quality from
Wako Pure Chemicals used without further purification. The
Conclusions
We have investigated the mechanistic aspects of the energy
transfer in the perylene-cored anthracene dendrimers through
inspection of the energy transfer efficiencies. The results of
these studies led to the definitive conclusion that all the
competitive energy transfer processes occurred in the single-
step manner. Another remarkable finding was that variations
in the interchromophore distance between the donor and
acceptor groups had no influence on the efficiencies of energy
transfer processes occurring within the nanoscopic dimension
of dendritic architectures. Consequently, the mechanistic stu-
dies described here exemplify the fundamental importance of
Fig. 3 Plots of extinction coefficients (e) at 359 nm (m, solid line) and
at 477 nm (K, dotted line) vs. the total number of anthracene units (n)
for 1–10.
Fig. 4 (A) Fluorescence spectra (lex = 378 nm, normalized at
500 nm) and (B) excitation spectra of 7–10 (lem = 500 nm, normalized
at 470 nm) in chloroform solutions.
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reaction mixture was cooled to room temperature, and poured
into saturated ammonium chloride solution (100 cm3) to
precipitate the product. The precipitate was collected by
filtration, intensively washed with water, and dried in a vacuum.
Purification of the residue by the preparative HPLC (chloro-
form as eluent) gave 7 (0.056 g, 59%) as an orange-colored
powder (Found: C, 82.2; H, 6.1. C192H172O20 requires C, 82.4;
H, 6.2%); mp 92–93 1C (from chloroform–hexane); nmax(KBr)/
cmÀ1 1589 (CQC) and 1716 (CO); m/z (ESI) 2823 (1%, M +
Na+), 640 (43), 588 (33), 275 (88) and 191 (100); dH (300 MHz;
CDCl3; Me4Si) 0.91 (12H, t, J 7.0, (CH2)5CH3), 1.28–1.42 (16H,
m, (CH2)3(CH2)2CH3), 1.50–1.62 (8H, m, (CH2)2C-
H2(CH2)2CH3), 1.69–1.85 (8H, m, CH2CH2(CH2)3CH3), 3.55
(8H, t, J 8.1, CH2(CH2)4CH3), 4.94 (8H, s, OCH2), 4.97 (8H, s,
OCH2), 5.25 (8H, s, OCH2), 5.81 (8H, s, OCH2), 6.57–6.63
(4H, m, ArH), 6.70–6.77 (8H, m, ArH), 6.84–6.90 (4H, m,
ArH), 6.97–7.04 (4H, m, ArH), 7.05–7.09 (4H, m, ArH),
7.18–7.27 (8H, m, ArH), 7.34 (16H, s, ArH), 7.46–7.49 (16H,
m, ArH), 7.83 (4H, d, J 8.1, ArH), 7.90 (4H, d, J 8.1, ArH) and
8.18–8.30 (16H, m, ArH); dC (75 MHz; CDCl3) 14.0 (q), 22.6
(t), 28.3 (t), 30.0 (t), 31.3 (t), 31.7 (t), 62.8 (t), 66.8 (t), 69.56 (t),
69.59 (t), 102.1 (d), 107.2 (d), 107.6 (d), 114.7 (d), 114.8 (d),
120.9 (d), 121.4 (d), 124.7 (d), 125.1 (d), 125.2 (d), 126.0 (d),
127.7 (d), 128.6 (s), 128.9 (s), 129.3 (s), 129.7 (d), 130.1 (d),
130.7 (s), 130.9 (s), 132.9 (s), 136.6 (s), 136.7 (s), 137.2 (s),
138.1 (s), 159.0 (s), 160.1 (s), 160.6 (s) and 168.2 (s).
Fig. 5 Plots of energy transfer efficiencies (fET) vs. the total number
of anthracene units (n) in the dendrimer systems 1–6 (m) and
7–10 (K).
1H- and 13C-nuclear magnetic resonance (NMR) spectra
operating at the frequencies of 300 and 75 MHz, respectively,
were recorded on a JEOL JNM-AL300 spectrometer in
chloroform-d (CDCl3) or acetone-d6 ((CD3)2CO). Chemical
shifts are reported in parts per million (ppm) relative to
TMS and the solvent used as internal standards, and the
coupling constants are reported in hertz (Hz). Fourier
transform infrared (FT-IR) spectra were recorded on a
JASCO FT/IR-410 spectrometer as KBr disks. UV-Vis and
fluorescence spectra were recorded on a JASCO model V-570
Dendrimer 8
UV-VIS-NIR spectrophotometer and
a Hitachi F-4500
spectrofluorometer, respectively. These measurements were
carried out in sufficiently low concentrations (10À8–10À5 M)
of the analytes to exclude the possibilities of intermolecular
chromophoric interactions. Melting points were measured
with a Yanaco MP-S3 melting point apparatus. Fast atom
bombardment (FAB) mass spectra were obtained on a JASCO
JMS-HX110A using a 3-nitrobenzyl alcohol matrix. Electro-
spray ionization-time-of-flight (ESI-TOF) mass spectra were
obtained on a Micromass LCT mass spectrometer KB 201.
Cold-spray ionization mass spectrometry (CSI-MS) was per-
formed by two-sector (BE) mass spectrometer (JMS-700,
JEOL) equipped with a cold-spray ionization (CSI) source.
Elemental analyses were performed by Thermo Flash EA 1112
instrument. Size exclusion chromatography (SEC) was per-
formed by a system consisting of a JASCO model 880-PU
pump at a flow rate of 0.5 cm3 minÀ1 and JASCO 875-UV
absorbance detector (254 nm) equipped with a Shodex K-
802.5 column (chloroform as eluent). Preparative HPLC
purification was performed by a Japan Analytical Industry
LC-918 recycling system. Since singly charged molecular ion
peaks of 9 and 10 could not be detected by all available mass
spectral techniques (FAB, MALDI-TOF, ESI and CSI), the
SEC analyses were applied to the molecular weight determina-
tions for these compounds.
A solution containing A2 (0.12 g, 0.15 mmol), P1 (0.022 g,
0.026 mmol), potassium carbonate (0.021 g, 0.15 mmol) and
18-crown-6 (0.040 g, 0.15 mmol) in DMF (7 cm3) was heated
at 55 1C with stirring under argon atmosphere. After 4 h, the
reaction mixture was cooled to room temperature, and poured
into saturated ammonium chloride solution (100 cm3) to
precipitate the product. The precipitate was collected by
filtration, intensively washed with water, and dried in a
vacuum. Purification of the residue by the preparative HPLC
(chloroform as eluent) gave 8 (0.050 g, 49%) as an orange-
colored powder (Found: C, 83.5; H, 6.5. C276H260O24 requires
C, 83.7; H, 6.6%); mp 123–124 1C (from chloroform–hexane);
n
max(KBr)/cmÀ1 1591 (CQC) and 1718 (CO); m/z (CSI) 3997
(100%, M + ClÀ); dH (300 MHz; CDCl3; Me4Si) 0.89 (24H, t,
J 7.0, (CH2)5CH3), 1.25–1.41 (32H, m, (CH2)3(CH2)2CH3),
1.47–1.61 (16H, m, (CH2)2CH2(CH2)2CH3), 1.66–1.81 (16H,
m, CH2CH2(CH2)3CH3), 3.51 (16H, t, J 7.0, CH2(CH2)4CH3),
4.89 (8H, s, OCH2), 4.92 (8H, s, OCH2), 5.20 (8H, s, OCH2),
5.76 (16H, s, OCH2), 6.42–6.46 (8H, m, ArH), 6.55–6.59 (4H,
m, ArH), 6.80–6.87 (4H, m, ArH), 6.92–6.99 (4H, m, ArH),
7.00–7.06 (4H, m, ArH), 7.14–7.22 (4H, m, ArH), 7.31 (16H, s,
ArH), 7.38–7.47 (32H, m, ArH), 7.71–7.84 (8H, m, ArH) and
8.15–8.27 (32H, m, ArH); dC (75 MHz; CDCl3) 14.0 (q), 22.6
(t), 28.3 (t), 29.9 (t), 31.3 (t), 31.7 (t), 62.8 (t), 66.8 (t), 69.6 (t),
69.7 (t), 94.6 (d), 95.0 (d), 114.7 (d), 114.8 (d), 120.8 (d),
121.3 (d), 124.7 (d), 125.05 (d), 125.13 (d), 126.1 (d), 127.7 (d),
127.8 (d), 128.5 (s), 128.9 (s), 129.3 (d), 129.7 (s),
129.8 (s), 130.6 (s), 130.9 (d), 132.8 (s), 136.6 (s), 136.7 (s),
137.3 (s), 138.1 (s), 139.3 (s), 141.0 (s), 159.0 (s), 160.9 (s),
161.4 (s) and 168.2 (s).
Dendrimer 7
A solution containing A1 (0.11 g, 0.21 mmol), P1 (0.029 g,
0.034 mmol), potassium carbonate (0.028 g, 0.20 mmol) and
18-crown-6 (0.054 g, 0.20 mmol) in DMF (7 cm3) was heated
at 55 1C with stirring under argon atmosphere. After 4 h, the
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Dendrimer 9
Acknowledgements
A solution containing A1 (0.24 g, 0.46 mmol), P2 (0.043 g,
0.047 mmol), potassium carbonate (0.064 g, 0.46 mmol), and
18-crown-6 (0.12 g, 0.45 mmol) in DMF (7 cm3) was heated at
55 1C with stirring under argon atmosphere. After 4 h, the
reaction mixture was cooled to room temperature, and poured
into saturated ammonium chloride solution (100 cm3) to
precipitate the product. The precipitate was collected by
filtration, intensively washed with water, and dried in a
vacuum. Purification of the residue by the preparative HPLC
This research was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science, and Technology, Japan and Suzuki Foundation. We
thank Prof. Shuichi Hiraoka of the University of Tokyo,
Japan.
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9 For energy transfer dynamics, time-resolved fluorescence measure-
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of experimental setup (100 ps of response time).
(chloroform as eluent) gave
9 (0.099 g, 44%) as an
orange-colored powder (Found: C, 82.8; H, 6.3. C332H308O32
requires C, 82.9; H, 6.5%); mp 103–104 1C (from
chloroform–hexane);
n
max(KBr)/cmÀ1 1591 (CQC) and
1716 (CO); dH (300 MHz; CDCl3; Me4Si) 0.90 (24H, t, J 6.8,
(CH2)5CH3), 1.27–1.42 (32H, m, (CH2)3(CH2)2CH3),
1.50–1.61 (16H, m, (CH2)2CH2(CH2)2CH3), 1.67–1.83 (16H,
m, CH2CH2(CH2)3CH3), 3.53 (16H, t, J 7.0, CH2(CH2)4CH3),
4.90 (16H, s, OCH2), 4.91 (16H, s, OCH2), 5.19 (8H, s, OCH2),
5.78 (16H, s, OCH2), 6.46–6.51 (4H, m, ArH), 6.54–6.61 (8H,
m, ArH), 6.62–6.67 (8H, m, ArH), 6.67–6.75 (16H, m, ArH),
7.20 (8H, t, J 8.4, ArH), 7.30 (32H, s, ArH), 7.38–7.48 (32H,
m, ArH), 7.87 (4H, d, J 8.0, ArH), 8.03 (4H, d, J 8.0, ArH) and
8.14–8.31 (32H, m, ArH); dC (75 MHz; CDCl3) 14.0 (q),
22.6 (t), 28.3 (t), 30.0 (t), 31.3 (t), 31.7 (t), 62.8 (t), 66.9 (t),
69.6 (t), 69.7 (t), 102.1 (d), 107.2 (d), 107.6 (d), 121.5 (d),
122.0 (d), 124.7 (d), 125.1 (d), 125.2 (d), 126.0 (d), 127.3 (s),
127.4 (s), 127.8 (d), 129.3 (s), 130.1 (d), 130.9 (s), 136.5 (s),
136.7 (s), 138.1 (s), 139.3 (s), 141.0 (s), 160.1 (s), 160.6 (s) and
168.2 (s).
Dendrimer 10
A solution containing A2 (0.20 g, 0.25 mmol), P2 (0.023 g,
0.025 mmol), potassium carbonate (0.064 g, 0.46 mmol), and
18-crown-6 (0.12 g, 0.45 mmol) in DMF (7 cm3) was heated at
55 1C with stirring under argon atmosphere. After 4 h, the
reaction mixture was cooled to room temperature, and poured
into saturated ammonium chloride solution (100 cm3) to
precipitate the product. The precipitate was collected by
filtration, intensively washed with water, and dried in a
vacuum. Purification of the residue by the preparative HPLC
(chloroform as eluent) gave 10 (0.082 g, 46%) as an orange-
colored powder (Found: C, 84.1; H, 6.8. C500H484O40 requires
C, 84.2; H, 6.8%); mp 133–134 1C (from chloroform–hexane);
n
max(KBr)/cmÀ1 1593 (CQC) and 1716 (CO); dH (300 MHz;
CDCl3; Me4Si) 0.89 (48H, t, J 6.8, (CH2)5CH3), 1.25–1.40
(64H, m, (CH2)3(CH2)2CH3), 1.45–1.61 (32H, m, (CH2)2C-
H2(CH2)2CH3), 1.64–1.81 (32H, m, CH2CH2(CH2)3CH3),
3.38–3.58 (32H, m, CH2(CH2)4CH3), 4.81 (16H, s, OCH2),
4.85 (16H, s, OCH2), 5.13 (8H, s, OCH2), 5.71 (32H, s, OCH2),
6.36–6.47 (24H, m, ArH), 6.50–6.64 (12H, m, ArH), 7.18–7.31
(32H, m, ArH), 7.32–7.48 (64H, m, ArH), 7.74–8.00 (8H, m,
ArH) and 8.05–8.30 (64H, m, ArH); dC (75 MHz; CDCl3)
14.0 (q), 22.6 (t), 28.3 (t), 29.9 (t), 31.3 (t), 31.7 (t), 62.8 (t),
69.6 (t), 69.7 (t), 94.4 (d), 94.9 (d), 107.1 (d), 121.5 (d), 124.7
(d), 125.0 (d), 126.0 (d), 127.3 (d), 127.7 (d), 127.8 (d), 129.3
(s), 130.9 (s), 136.5 (s), 138.0 (s), 160.0 (s), 160.9 (s), 161.3 (s)
and 168.2 (s).
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13 The energy transfer efficiency was calculated from the relation, fET
= 1/[1 + (r/R0)6], where r is the interchromophore distance in the
structure and R0 is the Forster critical radius for the donor–
acceptor pair. Application of the above equation yielded the fET
values for the short- and long-range processes in a ratio of 1 : 0.94.
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New J. Chem., 2008, 32, 547–553 | 553