Synthesis of lipophilic bisanthracene fluorophores: Versatile building blocks toward the synthesis of new light-harvesting dendrimers

Article abstract of DOI:10.1016/j.tet.2011.10.021

Lipophilic bisanthracene-based fluorophore and its derivatives were synthesized by the Suzuki-Miyaura cross-coupling reaction of 9-anthrylboronic acid with a substituted dibromobenzene. In addition to desirable fluorescent properties, these molecular systems were demonstrated to serve as versatile building blocks toward the synthesis of two types of new light-harvesting dendrimers due to their chemical stability.

Full text of DOI:10.1016/j.tet.2011.10.021

Tetrahedron 67 (2011) 9484e9490
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Synthesis of lipophilic bisanthracene fluorophores: versatile building blocks
toward the synthesis of new light-harvesting dendrimers
,
a
b
a
a,
*
Masaki Takahashi a , Ayato Yamamoto , Toshiyasu Inuzuka , Tetsuya Sengoku , Hidemi Yoda
*
a Department of Materials Science, Faculty of Engineering, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8561, Japan
b Division of Instrumental Analysis, Life Science Research Center, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 July 2011
Received in revised form 6 October 2011
Accepted 8 October 2011
Available online 14 October 2011
Lipophilic bisanthracene-based fluorophore and its derivatives were synthesized by the SuzukieMiyaura
cross-coupling reaction of 9-anthrylboronic acid with a substituted dibromobenzene. In addition to
desirable fluorescent properties, these molecular systems were demonstrated to serve as versatile
building blocks toward the synthesis of two types of new light-harvesting dendrimers due to their
chemical stability.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Bisanthracene
Fluorophore
SuzukieMiyaura cross-coupling reaction
Light-harvesting dendrimer
Energy transfer
1. Introduction
anthracene-based systems and their application in molecular en-
gineering of new functional materials is substantially less advanced
Highly fluorescent molecules, so-called fluorophores, have
attracted a great deal of attention because of their versatility1 not
only as useful sensors for analytical2 and biological imaging3
studies, but also as efficient energy and/or electron transmitters
for fabrication of photoelectric devices4 such as light-harvesting
dendrimers5 and organogels.6 From this basis, considerable effort
has been devoted over the past decade to develop organic dye
fluorophores bringing advantageous functionality for many uses as
well as ideal performance with high extinction coefficient, high
fluorescence quantum yield, and good chemical stability.7 In this
context, a number of polyaromatic hydrocarbons exhibit favorable
and unique properties such as strongly fluorescent behavior and
intrinsically high intercalative binding affinity with DNA due to
due to their poor solubility in a range of solvent systems and low
synthetic flexibility in accessing diverse analogs. In this publication,
we disclose a new type of bisanthracene-based molecular system,10
which works as strongly emitting fluorophores and as versatile
building blocks toward the synthesis of new light-harvesting
dendrimers.
2. Results and discussion
For the construction of bisanthracene-based structures, we
utilized the SuzukieMiyaura cross-coupling reaction to connect
two anthracene rings covalently to the 1,3-positions of benzene
nucleus, where a substituted dibromobenzene 1 should be pre-
requisite for the synthesis (Scheme 1). Thus, our synthetic approach
started with the preparation of 1 from commercially available 3,5-
dibromo-4-benzaldehyde through a three-step sequence involving
Williamson methylation, NaBH4 reduction, and TBS protection (92%
for three steps). Subsequently, 1,8-bis(2-ethylhexyl)anthracene 2
prepared according to published procedures11 was brominated at
the C10-position upon treatment with NBS to give 3 in 77% yield.
After conversion of 3 to the corresponding boronic acid, this syn-
thetic intermediate was subjected to the coupling with 1 under the
typical conditions for this transformation,12 followed by removal of
TBS protective groups upon treatment with TBAF, to provide
bisanthracene-substituted benzyl alcohol 4 in 38% for three steps.
their extensively p-conjugated and planar structures, making them
multifunctional fluorescent probes for biological applications.8
Among a variety of polyaromatic compounds, anthracene and its
derivatives represent an important class of functional materials,
which show blue-light emitting characteristics with high thermal
stability, thus serving as excellent fluorophores suitable for the
opto-electronic device fabrication.9 Nevertheless, development of
* Corresponding authors. Tel./fax: þ81 53 478 1621 (M.T.), þ81 53 478 1150
(H.Y.); e-mail addresses: tmtakah@ipc.shizuoka.ac.jp (M. Takahashi), tchyoda@
ipc.shizuoka.ac.jp (H. Yoda).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.10.021

M. Takahashi et al. / Tetrahedron 67 (2011) 9484e9490
9485
A
B
Fig. 1. Normalized (A) absorption and (B) fluorescence spectra (lex 360 nm) of 2
(dotted line), 4 (solid line), and 7 (dashed line) in CHCl3.
Table 1
Fluorescence quantum yields (fF) in various solventsa
Scheme 1. Reagents and conditions: (a) (i) MeI, K2CO3, DMF, 55 ꢀC, 3 h; 97%; (ii)
NaBH4, EtOH, rt, 5 h; 97%; (iii) TBSCl, imidazole, DMF, rt, 3 h; 98%; (b) NBS, DMF, rt, 5 h;
77%; (c) (i) n-BuLi, B(OMe)3, THF, ꢁ78 ꢀC for 30 min then rt for 1.5 h; 54%; (ii) 1,
Pd(PPh3)4, Na2CO3aq, tolueneeEtOH (2:1), reflux, 4 h; (iii) TBAF, THF, 0 ꢀC, 3 h; 71%
(two steps); (d) CBr4, PPh3, THF, rt, 0.5 h; 95% for 5, 91% for 8; (e) (i) phthalimide,
K2CO3, DMF, rt, 24 h; 94% for 6, 73% for 9; (ii) (NH2)2$H2O, THF, reflux, 4 h; 98% for 6,
80% for 9; (f) (i) 5, K2CO3, 18-crown-6, acetone, reflux, 5 h; 78%; (ii) LAH, THF, 0 ꢀC,
12 h; 69%.
Fluorophores
CHCl3
THF
Hexane
MeOH
2
4
5
6
7
8
9
0.16
0.50
0.18
0.52
0.45
0.53
0.48
0.27
0.43
0.37
0.41
0.57
0.52
0.46
0.26
0.31
0.38
0.36
0.28
0.35
0.36
0.29
0.37
0.24
0.40
0.49
0.72
0.47
a
The success in the synthesis of this product was confirmed by
a range of spectroscopies. In this structural system, the 1H NMR
spectrum clearly indicated that the two anthracene rings were
oriented orthogonally to the benzene unit, as evidenced by the
large upfield shift in the resonance for the methoxy protons relative
to that of 1 (from dH 3.87 to 2.50 ppm) due to the aromatic ring
currents.13
As a matter of fact, good chemical and thermal stability of this
compound allowed facile derivatization with a variety of functional
groups.14 Appel bromination of 4 led to efficient production of 5 in
95% yield, which was converted to the corresponding amine 6 by
the Gabriel synthesis in 92% (for two steps). Then 5 could be uti-
lized to generate its dendronized derivative 7 via the Williamson
reaction with methyl 3,5-dihydroxy benzoate and subsequent re-
duction with LAH (54% for two steps). A similar set of functional
group transformations performed on 4 was also encountered with
this product to afford the corresponding bromide 8 and amine 9 in
91% and 58% (two steps from 8), respectively.
The bisanthracene-based fluorophores 4e9 showed good solu-
bilities in a wide range of common organic solvents such as CHCl3,
THF, hexane, and MeOH, due to the lipophilic nature of 2-ethylhexyl
moieties. Indeed, the absorption and fluorescence spectra of these
compounds represented characteristic monomeric features, whose
vibrational structures are similar but red-shifted by 10e15 nm
relative to their building component 2 (Fig. 1 and S1eS7 in
Supplementary data). Of particular interest in this context is that
almost all of the bisanthracene-based systems are highly fluores-
cent with greater efficiencies (fF) than for 2 (Table 1), even though
the covalent assemblage of chromophoric units on single molecular
platforms would invoke attenuation of fluorescence quantum yield
as observed in our earlier works.14aec Further consideration of the
high fF values for benzylamines 6 and 9 indicated that the presence
of terminal amino substituents had no influence on fluorescence
quantum yield, thereby excluding a possibility of electron-transfer
quenching of the excited chromophores.15
Fluorescence quantum yields
(fF) were determined by relative method
employing a dilute solution of anthracene in cyclohexane (fF¼0.36)16 as a standard.
The quantum yields for the fluorophores were averaged from the values obtained at
three different excitation wavelengths (lex 355, 360, and 365 nm).
The desirable combination of attractive photophysical char-
acteristics, good solvent solubility, and functional group tolerance
prompted us to undertake practical application of the
bisanthracene-based systems as efficient antennae for light-
harvesting materials. In our previous works, it has been dem-
onstrated that perylene dyes can serve as a suitable acceptor to
funnel energy from anthracene chromophores because of their
high degree of spectral overlap between the donor emission and
acceptor absorption profiles.11b,17 Thus, we designed new model
compounds 10 and 11, containing the dendritic substituents at
one side of the perylene nuclei (Scheme 2). The synthesis of
these dendrimers employed a sequential imidationereduction
protocol to couple the bisanthracene and perylene units,18 where
requisite substrate 12 was prepared from commercially available
Scheme 2. Reagents and conditions: (i) 6 or 9, NMP, 100 ꢀC, 35 h; 57% for 10, 28% for
11; (ii) AlH3, THF, 0 ꢀC to rt, 4 h, 99% for 10 and 99% for 11; (two steps).

9486
M. Takahashi et al. / Tetrahedron 67 (2011) 9484e9490
Table 2
3,4,9,10-peryl-enetetracarboxylic dianhydride according to pub-
lished procedures.19 Consequently, the dendrimers 10 and 11
could be readily obtained from 12 and two benzylamines 6 and 9
via the above synthetic sequences.
Averaged quantum yields for the core emissions (fF1) upon excitation of the donor
groups,a quantum yields for the core emissions (fF2) upon excitation of the acceptor
group,a and energy transfer efficiencies (fET
)
b
a
a
b
Dendrimer
Solvent
fF1
fF2
fET
The photophysical properties of these dendrimers were
characterized by steady-state absorption and fluorescence spec-
troscopic measurements (Fig. 2 and S8, S9 in Supplementary
data). In the absorption spectra, all the samples displayed two
series of vibronic fine-structures with spectrally resolved bands
in the regions of 350e415 and 415e480 nm, whose intensities
vary according to the number of anthracene and perylene groups
embedded within the dendritic frameworks, respectively, in-
dicative of independent behavior of these light absorbing chro-
mophores (Fig. 2A and S8, S9A). Remarkably, the fluorescence
measurements of these materials showed strong or moderate
fluorescence quenching of the excited anthracene and appear-
ance of significant fluorescence emissions arising from the per-
ylene chromophores upon excitation at 360 nm, where direct
excitation of the anthracene chromophores was most likely
(Fig. 2B and S8, S9B). As described previously for other analogous
systems,11b these observations could be adequately rationalized
in terms of energy transfer from the anthracene antennae to the
perylene cores, suggesting that the dendritic frameworks con-
taining the densely arrayed donor (anthracene) and acceptor
(perylene) units would give geometrical and electronic properties
suitable for creating the unique light-harvesting antenna
systems.
10
CHCl3
THF
Hexane
MeOH
0.63
0.69
0.47
0.69
0.69
0.79
0.63
0.81
0.92
0.88
0.74
0.86
11
CHCl3
THF
Hexane
MeOH
0.41
0.54
0.15
0.42
0.50
0.83
0.25
0.65
0.82
0.65
0.60
0.65
a
Quantum yields for the core emissions (fF1 and fF2) were determined by relative
method employing a dilute solution of perylene in cyclohexane (fF¼0.94)16 as
a standard. The fF1 values represent an average of measurements at three different
excitation wavelengths (lex 355, 360, and 365 nm) and the fF2 values were obtained
by measurements at lex 420 nm.
b
fET values were given by fF1/fF2.
3. Conclusions
In conclusion,
a new type of bisanthracene-based fluo-
rophores was synthesized by the SuzukieMiyaura cross-coupling
reaction of 9-anthrylboronic acid with the substituted dibro-
mobenzene. This aromatic system has been demonstrated to
exhibit the desirable fluorescent properties as well as the re-
markably high solubilities in a variety of organic solvents, serv-
ing as excellent fluorophores suitable for many applications.
Furthermore, the high chemical stability of the molecular system
allowed us to produce several synthetic intermediates, which
were consequently applied as versatile building blocks for de-
veloping the efficient light-harvesting dendrimers. In addition,
the present bisanthracene-based systems represent potentially
superior functional materials that can be used to create novel
sensors and opto-electronic devices. Further elaboration of the
bisanthracene and dendrimer systems for the design and de-
velopment of complex structures with advanced functionality
will be presented in future publications.
A
B
4. Experimental section
4.1. General
All solvents and reagents were of reagent grade quality from
Wako Pure Chemicals and Tokyo Chemical Industry (TCI) used
without further purification. The 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 spec-
trometer in chloroform-d (CDCl3). 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 (FTIR) spectra were recorded on
a JASCO FT/IR-4100 spectrometer. UVevis and fluorescence (exci-
tation) spectra were recorded on a JASCO V-630 spectrophotometer
and a JASCO FP-6200 spectrofluorometer, respectively. Melting
points were measured with a Stanford Research Systems MPA100
automatic melting point apparatus. Elemental analysis of 3 was
performed by JSL Model JM 10 instruments. Laser desorption ion-
ization (LDI) or matrix-assisted laser desorption ionization (MALDI)
mass spectra were obtained on a JEOL JMS-700/GI mass spec-
trometer, which performed low- and high-resolution mass spec-
Fig. 2. Normalized (A) absorption and (B) fluorescence spectra (lex 360 nm) of 10
(dotted line) and 11 (solid line) in CHCl3.
Table 2 summarizes quantum yields for energy transfer (fFT
)
occurring in 10 and 11, which were estimated by comparing
quantum yields for the acceptor emissions obtained upon excita-
tion of the donors (fF1) with those upon excitation of the acceptors
(
fF2).20 From these data, it appears that the energy transfer occurs
efficiently under all the solvent conditions with particularly high
quantum yields for the chloroform solutions, but differences in the
fET values were noticed between the dendrimers bearing the less
and more advanced antenna units. We believe that this can be
simply explained by differences in average distances between the
peripheral anthracene antennae and the perylene cores in-
corporated within individual dendrimer systems,21 which primar-
trometry. For measuring the MALDI mass spectra,
a
-cyano-4-
ily reflect the extent of their Coulombic interactions given by the
hydroxycinnamic acid (a-CHCA) matrix was used as a cationiza-
tion agent.
22
€
Forster theory.

M. Takahashi et al. / Tetrahedron 67 (2011) 9484e9490
9487
4.2. Preparation and characterization of new compounds
temperature. This mixture was stirred for 30 min at ꢁ78 ꢀC,
warmed to room temperature. After stirring for additional 1.5 h, it
was quenched by addition of 3% aqueous HCl (5.0 mL), extracted
with ethyl acetate (40 mL), washed with water (40 mL), saturated
aqueous NaHCO3 (30 mL), and brine (30 mL), dried over Na2SO4,
filtered, and concentrated in vacuo. The residue was purified by
column chromatography on silica gel (eluent: hexane/ethyl acetate,
30:1) to give the boronic acid intermediate (0.500 g, 1.12 mmol,
54%) as a dark yellow oil: IR (NaCl) 3401 cmꢁ1 (OeH), 2957 cmꢁ1
(CeH), 2927 cmꢁ1 (CeH), 2858 cmꢁ1 (CeH); 1H NMR (300 MHz,
4.2.1. Synthesis and characterization of 1. To a solution of 3,5-
dibromo-4-hydroxybenzaldehyde (1.00 g, 3.57 mmol) and methyl
iodide (MeI, 0.5 mL, 1.14 g, 8.03 mmol) in DMF (15 mL) was added
anhydrous potassium carbonate (K2CO3, 0.992 g, 7.18 mmol). The
reaction mixture was stirred at 55 ꢀC for 3 h, cooled to room tem-
perature, and quenched by addition of water (80 mL). The resultant
white precipitate was filtered under vacuum, washed with water
(40 mL), and dried to give the anisaldehyde intermediate (1.02 g,
3.47 mmol, 97%) as a white solid: mp 90e91 ꢀC; IR (NaCl)
1688 cmꢁ1 (C]O), 1546 cmꢁ1 (C]C); 1H NMR (300 MHz, CDCl3)
CDCl3)
d
8.79 (s, 1H, ArH), 7.69 (d, J¼8.7 Hz, 2H, ArH), 7.35 (dd, J¼8.7,
6.6 Hz, 2H, ArH), 7.30 (d, J¼6.6 Hz, 2H, ArH), 6.16 (s, 2H, OH), 3.12 (d,
J¼7.2 Hz, 4H, ArCH2), 1.94 (sept, J¼7.2 Hz, 2H, CH), 1.43e1.21 (m,
16H, CH2), 0.95e0.85 (m, 12H, CH3). To an argon-purged solution of
this material (0.239 g, 0.535 mmol) and 1 (0.0549 g, 0.134 mmol) in
a mixture of toluene/EtOH (2:1, 13 mL) were sequentially added
sodium carbonate (Na2CO3, 2.7 mL, 8.10 mmol, 3.0 M aqueous so-
lution) and tetrakis(triphenylphosphine)palladium (Pd(PPh3)4,
0.023 g, 0.020 mmol) at room temperature. The reaction mixture
was heated under reflux for 4 h, cooled to room temperature, fil-
trated through a pad of Celite followed by successive washings with
hexane (30 mL), and extracted with hexane (50 mL). The extract
was washed with brine (30 mL), dried over Na2SO4, filtered, and
concentrated in vacuo. The residue was purified by column chro-
matography on silica gel (eluent: hexane/ethyl acetate, 50:1) to give
d
9.86 (s, 1H, CHO), 8.03 (s, 2H, ArH), 3.97 (s, 3H, OCH3); 13C NMR
(75 MHz, CDCl3) 188.6 (CHO), 159.2 (C), 134.3 (C), 134.0 (CH), 119.3
d
(C), 60.8 (OCH3). To a solution of this material (0.100 g, 0.340 mmol)
in EtOH (3.5 mL) at 0 ꢀC was added sodium borohydride (NaBH4,
0.022 g, 0.592 mmol). The reaction mixture was stirred at room
temperature for 5 h, quenched by addition of 3% aqueous HCl
(5 mL), extracted with ethyl acetate (50 mL), washed with water
(50 mL), saturated aqueous NaHCO3 (30 mL), and brine (30 mL),
dried over Na2SO4, filtered, and concentrated in vacuo to give the
alcohol intermediate (0.098 g, 0.331 mmol, 97%) as a white solid:
mp 38e39 ꢀC; IR (NaCl) 3350 cmꢁ1 (OeH), 1548 cmꢁ1 (C]C); 1H
NMR (300 MHz, CDCl3)
d
7.51 (s, 2H, ArH), 4.62 (s, 2H, ArCH2), 3.88
139.6
(s, 3H, OCH3), 1.90 (br s, 1H, OH); 13C NMR (75 MHz, CDCl3)
d
(C), 130.9 (CH), 118.1 (C), 63.3 (CH2), 60.6 (OCH3). To a solution of
this material (0.387 g, 1.31 mmol) and imidazole (0.104 g,
1.53 mmol) in DMF (13 mL) at 0 ꢀC was added tert-butyldime-
thylsilyl chloride (TBSCl, 0.116 g, 0.770 mmol). The reaction mixture
was warmed to room temperature, stirred for 3 h, quenched by
addition of water (20 mL), extracted with hexane (50 mL), washed
with water (50 mL) and brine (20 mL), dried over Na2SO4, filtered,
and concentrated in vacuo. The residue was purified by column
chromatography on silica gel (eluent: hexane/ethyl acetate, 50:1) to
yield 1 (0.527 g, 1.28 mmol, 98%) as a colorless oil: IR (NaCl)
a mixture of TBS-protected and deprotected bisanthracenes
(0.151 g). To a solution of this material (0.151 g) in THF (5.0 mL) at
0 ꢀC was added tetra-n-butylammonium fluoride (TBAF, 0.29 mL,
0.290 mmol, 1.0 M THF solution). The reaction mixture was stirred
at 0 ꢀC for 3 h, quenched by addition of saturated aqueous NaHCO3
(5.0 mL), extracted with hexane (50 mL), washed with water
(50 mL) and brine (30 mL), dried over Na2SO4, filtered, and con-
centrated in vacuo. The residue was purified by column chroma-
tography on silica gel (eluent: hexane/ethyl acetate, 10:1) to yield 4
(0.0892 g, 0.0950 mmol, 71%, for twꢁo1steps) as a pale yellow solid:
1546 cmꢁ1 (C]C); 1H NMR (300 MHz, CDCl3)
4.64 (s, 2H, ArCH2), 3.87 (s, 3H, OCH3), 0.94 (s, 9H, CH3), 0.11 (s, 6H,
CH3); 13C NMR (75 MHz, CDCl3)
152.9 (C), 140.2 (C), 130.2 (CH),
d
7.45 (s, 2H, ArH),
mp 54e59 ꢀC; IR (NaCl) 3582 cm
(OeH), 2957 cmꢁ1 (CeH),
2926 cmꢁ1 (CeH), 2857 cmꢁ1 (CeH); MS (LDI) m/z 939 (Mþ), 940
(MHþ); HRMS (LDI) m/z calcd for C68H90O2: 938.6941, found
d
118.0 (C), 63.2 (CH2), 60.6 (OCH3), 25.8 (CH3), 18.3 (C), ꢁ5.5 (CH3).
938.6927; 1H NMR (300 MHz, CDCl3)
d 8.92 (s, 2H, ArH), 7.77 (d,
J¼8.4 Hz, 4H, ArH), 7.50 (s, 2H, ArH), 7.38 (dd, J¼8.4, 6.6 Hz, 4H,
ArH), 7.29 (d, J¼6.6 Hz, 4H, ArH), 4.83 (d, J¼5.9 Hz, 2H, ArCH2), 3.16
(br s, 8H, ArCH2), 2.50 (s, 3H, OCH3), 1.98 (br s, 4H, CH), 1.75 (t,
J¼5.9 Hz, 1H, OH), 1.41e1.27 (m, 32H, CH2), 0.96e0.82 (m, 24H,
4.2.2. Synthesis and characterization of 3. To a solution of 1,8-bis(2-
ethylhexyl)anthracene (1.69 g, 4.20 mmol) in DMF (21 mL) was
added N-bromosuccinimide (NBS, 0.895 g, 5.03 mmol) at room
temperature. The reaction mixture was stirred for 5 h, quenched by
addition of water (10 mL), extracted with hexane (50 mL), washed
with water (50 mL) and brine (30 mL), dried over Na2SO4, filtered,
and concentrated in vacuo. The residue was purified by column
chromatography on silica gel (eluent: hexane) to yield 3 (1.56 g,
3.24 mmol, 77%) as a yellow solid: mp 43e44 ꢀC; IR (NaCl)
2957 cmꢁ1 (CeH), 2926 cmꢁ1 (CeH), 2858 cmꢁ1 (CeH); 1H NMR
CH3); 13C NMR (75 MHz, CDCl3)
d 157.3 (C), 138.2 (C), 136.2 (C), 134.5
(C), 133.1 (C), 131.6 (C), 130.6 (C), 130.2 (CH), 126.1 (CH), 125.2 (CH),
125.1 (CH), 120.0 (CH), 64.9 (CH2), 60.5 (OCH3), 39.8 (CH), 38.4
(CH2), 32.7 (CH2), 32.6 (CH2), 28.7 (CH2), 25.8 (CH2), 25.7 (CH2), 23.1
(CH2), 14.0 (CH3), 10.6 (CH3).
4.2.4. Synthesis and characterization of 5. To a mixture of 4
(91.0 mg, 0.0969 mmol), carbon tetrabromide (CBr4, 80.3 mg,
0.242 mmol), and triphenylphosphine (PPh3, 63.5 mg, 0.242 mmol)
was added THF (0.54 mL) at room temperature. The reaction mix-
ture was stirred at this temperature for 12 h, quenched by addition
of water (50 mL), extracted with hexane (50 mL), washed with
MeOH (30 mL) and brine (30 mL), dried over Na2SO4, filtered, and
concentrated in vacuo. The residue was purified by column chro-
matography on silica gel (eluent: hexane/ethyl acetate, 30:1) to
yield 5 (92.0 mg, 0.0918 mmol, 95%) as a pale yellow solid: mp
85e90 ꢀC; IR (NaCl) 2957 cmꢁ1 (CeH), 2926 cmꢁ1 (CeH),
2857 cmꢁ1 (CeH); HRMS (LDI, Mþ) m/z calcd for C68H89BrO:
(300 MHz, CDCl3)
d
8.86 (s, 1H, ArH), 8.42 (d, J¼8.7 Hz, 2H, ArH),
7.49 (dd, J¼8.7, 6.6 Hz, 2H, ArH), 7.31 (d, J¼6.6 Hz, 2H, ArH), 3.13 (d,
J¼7.2 Hz, 4H, ArCH2), 1.92 (sept, J¼7.2 Hz, 2H, CH), 1.54e1.26 (m,
16H, CH2), 0.92 (t, J¼7.5 Hz, 6H, CH3), 0.86 (t, J¼7.2 Hz, 6H, CH3); 13
C
NMR (75 MHz, CDCl3)
d 138.4 (C), 131.0 (C), 130.8 (C), 126.7 (CH),
126.6 (C), 126.3 (CH), 124.0 (CH), 120.0 (CH), 39.9 (CH), 38.4 (CH2),
32.7 (CH2), 32.6 (CH2), 28.6 (CH2), 25.7 (CH2), 25.6 (CH2), 23.1 (CH2),
14.0 (CH3), 10.5 (CH3). Anal. Calcd for C30H41Br: C, 74.83; H, 8.58.
Found: C, 75.15; H, 8.23.
4.2.3. Synthesis and characterization of 4. An oven-dried round-
bottom flask was purged with nitrogen, charged with a solution of 3
(1.00 g, 2.08 mmol) in dry THF (14.0 mL), and cooled to ꢁ78 ꢀC. To
this solution were sequentially added n-butyl lithium (n-BuLi,
3.0 mL, 4.95 mmol, 1.65 M solution in hexane) and trimethyl borate
(B(OMe)3, 2.3 mL, 2.14 g, 20.6 mmol) dropwise via syringe at this
1000.6097, found 1000.6126; 1H NMR (300 MHz, CDCl3)
d 8.93 (s,
2H, ArH), 7.75 (d, J¼8.7 Hz, 4H, ArH), 7.55 (s, 2H, ArH), 7.40 (dd,
J¼8.7, 6.6 Hz, 4H, ArH), 7.30 (d, J¼6.6 Hz, 4H, ArH), 4.62 (s, 2H,
ArCH2), 3.17 (br s, 8H, ArCH2), 2.49 (s, 3H, OCH3), 1.98 (br s, 4H, CH),
1.41e1.27 (m, 32H, CH2), 0.97e0.82 (m, 24H, CH3); 13C NMR

9488
M. Takahashi et al. / Tetrahedron 67 (2011) 9484e9490
(75 MHz, CDCl3)
d
157.9 (C), 138.2 (C), 133.9 (C), 133.8 (C), 133.3 (C),
NaHCO3 (50 mL), and brine (50 mL), dried over Na2SO4, filtered, and
concentrated in vacuo. The residue was purified by column chro-
matography on silica gel (eluent: hexane/ethyl acetate, 10:1) to
yield 7 (17.0 mg, 0.00857 mmol, 69%) as a pale yellow solid: mp
48e53 ꢀC; IR (NaCl) 3444 cmꢁ1 (OeH), 2958 cmꢁ1 (CeH),
2927 cmꢁ1 (CeH), 2858 cmꢁ1 (CeH), 1159 cmꢁ1 (CeO); HRMS (LDI,
MNaþ) m/z calcd for C143H184NaO5: 2004.4041, found 2004.4024;
133.1 (C), 130.5 (C), 130.3 (CH), 126.1 (CH), 125.4 (CH), 124.9 (CH),
119.9 (CH), 60.4 (OCH3), 39.8 (CH), 38.5 (CH2), 38.4 (CH2), 33.1 (CH2),
32.7 (CH2), 28.7 (CH2), 25.8 (CH2), 23.1 (CH2), 14.0 (CH3), 10.6 (CH3).
4.2.5. Synthesis and characterization of 6. To a solution of 5
(28.0 mg, 0.0279 mmol) and phthalimide (16.4 mg, 0.112 mmol) in
DMF (0.56 mL) was added anhydrous potassium carbonate (K2CO3,
22.6 mg, 0.164 mmol) at room temperature. The reaction mixture
was stirred for 24 h, quenched by addition of saturated aqueous
NH4Cl (1.5 mL), filtrated through a pad of Celite followed by suc-
cessive washings with hexane (20 mL). The filtrate was extracted
with hexane (50 mL), washed with water (50 mL) and brine
(30 mL), dried over Na2SO4, filtered, and concentrated in vacuo. The
residue was purified by column chromatography on silica gel (el-
uent: hexane/ethyl acetate, 10:1) to give the phthalimide in-
termediate (28.0 mg, 0.0262 mmol, 94%) as a pale yellow solid: mp
55e60 ꢀC; IR (NaCl) 2959 cmꢁ1 (CeH), 2928 cmꢁ1 (CeH),
2858 cmꢁ1 (CeH), 1772 cmꢁ1 (C]O), 1714 cmꢁ1 (C]O); 1H NMR
1H NMR (300 MHz, CDCl3)
d
8.90 (s, 4H, ArH), 7.75 (d, J¼8.4 Hz, 8H,
ArH), 7.55 (s, 4H, ArH), 7.35 (dd, J¼8.4, 6.9 Hz, 8H, ArH), 7.24 (d,
J¼6.9 Hz, 8H, ArH), 6.60 (d, J¼2.1 Hz, 2H, ArH), 6.54 (t, J¼2.1 Hz, 1H,
ArH), 5.14 (s, 4H, OCH2), 4.55 (d, J¼6.0 Hz, 2H, CH2OH), 3.14 (d,
J¼6.9 Hz, 16H, ArCH2), 2.47 (s, 6H, OCH3), 1.97 (br s, 8H, CH),
1.46e1.26 (m, 65H, CH2 and OH), 0.95e0.81 (m, 48H, CH3); 13C NMR
(75 MHz, CDCl3)
d 160.2 (C), 157.5 (C), 143.4 (C), 138.1 (C), 134.2 (C),
133.1 (C),132.3 (C),130.6 (C),130.2 (CH),129.1 (CH),128.3 (CH),126.1
(CH), 125.2 (CH), 119.8 (CH), 106.5 (CH), 101.7 (CH), 69.8 (CH2), 65.2
(CH2), 60.3 (OCH3), 39.7 (CH), 38.4 (CH2), 32.7 (CH2), 28.7 (CH2), 25.7
(CH2), 23.1 (CH2), 14.0 (CH3), 10.6 (CH3).
(300 MHz, CDCl3)
d
8.90 (s, 2H, ArH), 7.80 (q, J¼2.7 Hz, 2H, ArH),
4.2.7. Synthesis and characterization of 8. To a mixture of 7
(17.0 mg, 0.00857 mmol), carbon tetrabromide (CBr4, 13.5 mg,
7.73 (d, J¼8.7 Hz, 4H, ArH), 7.66 (q, J¼2.7 Hz, 2H, ArH), 7.52 (s, 2H,
ArH), 7.36 (dd, J¼8.7, 6.6 Hz, 4H, ArH), 7.27 (d, J¼6.6 Hz, 4H, ArH),
5.01 (s, 2H, ArCH2), 3.15 (br s, 8H, ArCH2), 2.44 (s, 3H, OCH3), 1.97 (br
s, 4H, CH), 1.40e1.26 (m, 32H, CH2), 0.96e0.82 (m, 24H, CH3). To
a solution of this material (43.0 mg, 0.0402 mmol) in THF (3.0 mL)
was added hydrazine monohydrate ((NH2)2$H2O, 103 mg,
3.21 mmol). The reaction mixture was heated under reflux for 4 h,
cooled to room temperature, quenched by addition of water
(50 mL), extracted with hexane (50 mL), washed with brine
(30 mL), dried over Na2SO4, filtered, and concentrated in vacuo to
yield 6 (37.0 mg, 0.0394 mmol, 98%) as a pale yellow solid: mp
35e40 ꢀC; HRMS (LDI, Mþ) m/z calcd for C68H91NO: 937.7101, found
0.0407 mmol),
and
triphenylphosphine
(PPh3,
10.6 mg,
0.0404 mmol) was added THF (0.38 mL) at room temperature. The
reaction mixture was stirred at this temperature for 12 h, quenched
by addition of water (30 mL), extracted with hexane (50 mL),
washed with MeOH (10 mL) and brine (30 mL), dried over Na2SO4,
filtered, and concentrated in vacuo. The residue was purified by
column chromatography on silica gel (eluent: hexane/ethyl acetate,
50:1) to yield 8 (16.0 mg, 0.00782 mmol, 91%) as a pale yellow
solid: mp 64e69 ꢀC; IR (NaCl) 2957 cmꢁ1 (CeH), 2927 cmꢁ1 (CeH),
2858 cmꢁ1 (CeH), 1161 cmꢁ1 (CeO); HRMS (MALDI, MNaþ) m/z
calcd for C143H183O4NaBr: 2066.3198, found 2066.3205; 1H NMR
937.7105; 1H NMR (300 MHz, CDCl3)
d
8.92 (s, 2H, ArH), 7.78 (d,
(300 MHz, CDCl3)
d
8.90 (s, 4H, ArH), 7.74 (d, J¼8.7 Hz, 8H, ArH),
J¼8.7 Hz, 4H, ArH), 7.42 (s, 2H, ArH), 7.38 (dd, J¼8.7, 6.6 Hz, 4H,
ArH), 7.29 (d, J¼6.6 Hz, 4H, ArH), 4.00 (br s, 2H, ArCH2), 3.16 (br s,
8H, ArCH2), 2.50 (s, 3H, OCH3), 1.97 (br s, 4H, CH), 1.60 (br s, 2H,
NH2), 1.40e1.26 (m, 32H, CH2), 0.96e0.82 (m, 24H, CH3); 13C NMR
7.54 (s, 4H, ArH), 7.35 (dd, J¼8.7, 6.9 Hz, 8H, ArH), 7.24 (d, J¼6.9 Hz,
8H, ArH), 6.61 (d, J¼2.1 Hz, 2H, ArH), 6.54 (t, J¼2.1 Hz, 1H, ArH), 5.14
(s, 4H, OCH2), 4.35 (s, 2H, CH2Br), 3.14 (d, J¼7.2 Hz,16H, ArCH2), 2.47
(s, 6H, OCH3), 1.97 (br s, 8H, CH), 1.39e1.26 (m, 64H, CH2), 0.95e0.81
(75 MHz, CDCl3)
d
156.6 (C), 138.2 (C), 134.7 (C), 133.0 (C), 131.4 (C)
(m, 48H, CH3); 13C NMR (75 MHz, CDCl3)
d 160.1 (C), 157.6 (C), 139.7
130.6 (C), 130.3 (C), 130.3 (CH), 126.1 (CH), 125.1 (CH), 119.7 (CH),
60.5 (OCH3), 39.8 (CH), 39.7 (CH2), 38.4 (CH2), 32.7 (CH2), 28.7
(CH2), 25.8 (CH2), 23.1 (CH2), 23.1 (CH2), 14.0 (CH3), 10.6 (CH3).
(C), 138.1 (C), 134.3 (C), 133.2 (C), 132.2 (C), 130.6 (C), 130.3 (CH),
126.1 (CH), 125.2 (CH), 125.1 (CH), 119.8 (CH), 100.6 (CH), 93.0 (CH),
69.9 (CH2), 60.5 (CH2), 39.8 (CH), 38.4 (CH2), 32.7 (CH2), 31.5 (CH2),
28.7 (CH2), 25.7 (CH2), 23.1 (CH2), 14.0 (CH3), 10.6 (CH3).
4.2.6. Synthesis and characterization of 7. To a solution of 6
(99.0 mg, 0.105 mmol) and methyl 3,5-dihydroxybenzoate (7.4 mg,
0.0440 mmol) in acetone (0.73 mL) was added anhydrous potas-
sium carbonate (K2CO3, 30.0 mg, 0.217 mmol) and 18-crown-6
(10.0 mg, 0.0367 mmol) at room temperature. The reaction mixture
was heated under reflux for 5 h, quenched by addition of saturated
aqueous NH4Cl (2.0 mL), extracted with hexane (50 mL), washed
with water (100 mL) and brine (50 mL), dried over Na2SO4, filtered,
and concentrated in vacuo. The residue was purified by column
chromatography on silica gel (eluent: hexane/ethyl acetate, 30:1) to
give the benzoate intermediate (69.0 mg, 0.0343 mmol, 78%): mp
98e103 ꢀC; IR (NaCl) 2958 cmꢁ1 (CeH), 2927 cmꢁ1 (CeH),
2858 cmꢁ1 (CeH), 1725 cmꢁ1 (C]O), 1161 cmꢁ1 (CeO); 1H NMR
4.2.8. Synthesis and characterization of 9. To a solution of 8
(16.0 mg, 0.00782 mmol) and phthalimide (11.9 mg, 0.0809 mmol)
in DMF (0.5 mL) was added anhydrous potassium carbonate
(K2CO3, 17.6 mg, 0.127 mmol) at room temperature. The reaction
mixture was stirred at this temperature for 24 h, quenched by ad-
dition of saturated aqueous NH4Cl (1.0 mL), filtrated through a pad
of Celite followed by successive washings with hexane (20 mL). The
filtrate was extracted with hexane (50 mL), washed with water
(50 mL) and brine (20 mL), dried over Na2SO4, filtered, and con-
centrated in vacuo. The residue was purified by column chroma-
tography on silica gel (eluent: hexane/ethyl acetate, 15:1) to give
the phthalimide intermediate (12.0 mg, 0.00568 mmol, 73%) as
a pale yellow solid: mp 51e56 ꢀC; IR (NaCl) 2957 cmꢁ1 (CeH),
2927 cmꢁ1 (CeH), 2858 cmꢁ1 (CeH), 1718 cmꢁ1 (C]O), 1162 cmꢁ1
(300 MHz, CDCl3)
d
8.90 (s, 4H, ArH), 7.74 (d, J¼8.7 Hz, 8H, ArH), 7.56
(s, 4H, ArH), 7.37e7.24 (m,18H, ArH), 6.78 (t, J¼2.3 Hz,1H, ArH), 5.17
(s, 4H, OCH2), 3.79 (s, 3H, OCH3), 3.15 (d, J¼6.9 Hz, 16H, ArCH2), 2.47
(s, 6H, OCH3), 1.97 (br s, 8H, CH),1.39e1.26 (m, 64H, CH2), 0.95e0.81
(m, 48H, CH3). To a solution of this material (25.0 mg, 0.0124 mmol)
in THF (2.0 mL) at 0 ꢀC was added lithium aluminum hydride (LAH,
3.0 mg, 0.079 mmol) in small portions. The reaction mixture was
stirred at 0 ꢀC for 12 h, quenched by addition of water (0.5 mL) and
3% aqueous HCl (3.0 mL), filtrated through a pad of Celite followed
by successive washings with hexane (30 mL), extracted with hex-
ane (100 mL), washed with water (100 mL), saturated aqueous
(CeO); 1H NMR (300 MHz, CDCl3)
d 8.90 (s, 4H, ArH), 7.77e7.74 (m,
10H, ArH), 7.63 (q, J¼3.0 Hz, 2H, ArH), 7.54 (s, 4H, ArH), 7.34 (dd,
J¼8.7, 6.9 Hz, 8H, ArH), 7.23 (d, J¼6.9 Hz, 8H, ArH), 6.66 (d, J¼2.1 Hz,
2H, ArH), 6.52 (t, J¼2.1 Hz, 1H, ArH), 5.09 (s, 4H, OCH2), 4.71 (s, 2H,
ArCH2), 3.14 (d, J¼7.2 Hz, 16H, ArCH2), 2.46 (s, 6H, OCH3), 1.96 (br s,
8H, CH), 1.39e1.26 (m, 64H, CH2), 0.95e0.81 (m, 48H, CH3). To
a solution of this material (20.0 mg, 0.00947 mmol) in THF (2.0 mL)
was added hydrazine monohydrate ((NH2)2$H2O, 103 mg,
3.21 mmol). The reaction mixture was heated under reflux for 4 h,

M. Takahashi et al. / Tetrahedron 67 (2011) 9484e9490
9489
cooled to room temperature, quenched by addition of water
(50 mL), extracted with hexane (50 mL), washed with brine
(20 mL), dried over Na2SO4, filtered, and concentrated in vacuo to
yield 9 (15.0 mg, 0.00757 mmolþ, 80%) as a pale yellow solid: mp
31e36 ꢀC; HRMS (MALDI, MNa ) m/z calcd for C143H185NaNO4:
hexane (50 mL). The filtrate was extracted with hexane (50 mL),
washed with water (100 mL) and brine (50 mL), dried over Na2SO4,
filtered, and concentrated in vacuo to yield 10 (19þ.0 mg,
0.0146 mmol, 99%) as a yellow solid: mp 55 ꢀC; HRMS (LDI, M ) m/z
calcd for C94H110N2O2: 1298.8567, found 1298.8609; 1H NMR
2003.4213, found 2003.4202; 1H NMR (300 MHz, CDCl3)
d
8.90 (s,
(300 MHz, CDCl3)
d
8.90 (s, 2H, ArH), 8.00 (d, J¼6.6 Hz, 4H, ArH),
4H, ArH), 7.75 (d, J¼8.7 Hz, 8H, ArH), 7.55 (s, 4H, ArH), 7.34 (dd,
J¼8.7, 6.6 Hz, 8H, ArH), 7.24 (d, J¼6.6 Hz, 8H, ArH), 6.54 (d, J¼2.1 Hz,
2H, ArH), 6.50 (t, J¼2.1 Hz, 1H, ArH), 5.14 (s, 4H, OCH2), 3.73 (br s,
2H, ArCH2), 3.14 (d, J¼6.9 Hz, 16H, ArCH2), 2.47 (s, 6H, OCH3), 1.97
(br s, 8H, CH), 1.64 (br s, 2H, NH2), 1.39e1.26 (m, 64H, CH2),
0.95e0.81 (m, 48H, CH3).
7.81 (d, J¼8.4 Hz, 4H, ArH), 7.53 (s, 2H, ArH), 7.40 (dd, J¼8.4, 6.9 Hz,
4H, ArH), 7.28 (d, J¼6.9 Hz, 4H, ArH), 7.16 (d, J¼7.8 Hz, 4H, ArH), 4.05
(s, 2H, CH2), 3.99 (s, 4H, CH2), 3.97 (s, 4H, CH2), 3.78 (t, J¼4.8 Hz, 2H,
CH2), 3.16 (br s, 8H, ArCH2), 2.81 (t, J¼4.8 Hz, 2H, CH2), 2.53 (s, 3H,
OCH3), 1.97 (br s, 4H, CH), 1.40e1.26 (m, 33H, CH2 and OH),
0.94e0.81 (m, 24H, CH3). Following the above procedures with 9
(15.0 mg, 0.00757 mmol) and 12 (9.9 mg, 0.023 mmol) afforded the
corresponding perylene bisimide (5.0 mg, 0.0021 mmol, 28%) as
a red-purple solid: mp 175e180 ꢀC; 1H NMR (300 MHz, CDCl3)
4.2.9. Synthesis and characterization of 12. The synthesis of 12 was
achieved by the following modified procedure of the previously
reported methods.19,23,24 To
a
bulk solid of 3,4,9,10-
d 8.90 (s, 4H, ArH), 8.42e8.36 (m, 4H, ArH), 8.19 (br s, 4H, ArH), 7.80
perylenetetracarboxylic dianhydride (PTCA, 242 mg, 0.617 mmol)
was added an aqueous solution of KOH (5%, 7.0 mL). The reaction
mixture was heated at 90 ꢀC with stirring for 6 h, whereupon sat-
urated aqueous H3PO4 (10%, 5.1 mL) was added and it was heated
for additional 1 h to form a brown precipitate. After cooling to room
temperature, this precipitate was filtered under vacuum, washed
with water (50 mL) and MeOH (30 mL), and dried in vacuo to give
a solid material (244 mg). To an aqueous solution of this material
(244 mg, in 10 mL of water) was added 2-aminoethanol (316 mg,
5.17 mmol) at 0 ꢀC. After stirring at this temperature for 1 h, the
reaction mixture was heated under reflux for additional 4 h, cooled
to room temperature, quenched by addition of 10% aqueous HCl
(15 mL) to give a brown precipitate, whereupon aqueous sulfuric
acid (2 mL, 24.0 mmol, 12 M solution) was added. It was heated at
90 ꢀC for 30 min and cooled to room temperature to give a dark
brown precipitate. The precipitate was filtered under vacuum,
washed with water (50 mL) and MeOH (20 mL), and dried in vacuo
to yield 12 (26.9 mg, 0.0618 mmol, 10% for two steps): mp >400 ꢀC;
IR (KBr) 1763 cmꢁ1 (C]O), 1730 cmꢁ1 (C]O), 1692 cmꢁ1 (C]O),
1655 cmꢁ1 (C]O).
(d, J¼8.4 Hz, 8H, ArH), 7.60 (s, 4H, ArH), 7.35 (dd, J¼8.4, 6.9 Hz, 8H,
ArH), 7.24 (d, J¼6.9 Hz, 8H, ArH), 6.89 (br s, 2H, ArH), 6.63 (br s, 1H,
ArH), 5.55 (s, 2H, ArCH2), 5.26 (s, 4H, OCH2), 4.39 (br s, 2H, CH2),
4.04 (br s, 2H, CH2), 3.13 (br s, 16H, ArCH2), 2.44 (s, 6H, OCH3), 1.96
(br s, 8H, CH), 1.36e1.25 (m, 65H, CH2 and OH), 0.90e0.75 (m, 48H,
CH3). The AlH3 reduction of this material (3.5 mg, 0.00146 mmol)
provided 11 (3.4 mg, 0.00144 mmol, 99%) as a viscous yellow syrup:
HRMS (MALDI, MHþ) m/z calcd for C169H205N2O5: 2342.5849, found
2342.5817; 1H NMR (300 MHz, CDCl3)
d 8.90 (br s, 4H, ArH), 8.04 (br
s, 4H, ArH), 7.76 (br s, 8H, ArH), 7.55 (s, 4H, ArH), 7.36e7.30 (m, 16H,
ArH), 7.13 (br s, 4H, ArH), 6.69 (br s, 3H, ArH), 5.14 (br s, 4H, OCH2),
3.92 (br s, 10H, CH2), 3.64 (t, J¼6.0 Hz, 2H, CH2), 3.15 (br s, 16H,
ArCH2), 2.77 (br s, 2H, CH2), 2.46 (s, 6H, OCH3), 1.96 (br s, 8H, CH),
1.43e1.25 (m, 65H, CH2 and OH), 0.90e0.83 (m, 48H, CH3).
Acknowledgements
We thank Mr. Yuki Numata and Mr. Kazuhiro Kemmonchi for
analytical data. This research was supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
4.2.10. Synthesis and characterization of 10 and 11. The synthesis of
10 was achieved by the following typical procedure.18b To a solution
of 6 (24.4 mg, 0.0260 mmol) in N-methyl pyrrolidinone (NMP,
0.75 mL) was added 12 (17.0 mg, 0.0390 mmol). The reaction mix-
ture was heated at 100 ꢀC with stirring for 35 h, cooled to room
temperature, and quenched by addition of water (5 mL). The
resulting dark purple precipitate was extracted with hexane
(30 mL), washed with water (100 mL) and brine (50 mL), dried over
Na2SO4, filtered, and concentrated in vacuo. Purification of the
residue by flash column chromatography on silica gel (CHCl3/
MeOH¼40:1) gave the perylenebisimide intermediate (20.0 mg,
0.0148 mmol, 57%) as a red-purple solid: mp 130e135 ꢀC; IR (NaCl)
3495 cmꢁ1 (OeH), 2958 cmꢁ1 (CeH), 2926 cmꢁ1 (CeH), 2857 cmꢁ1
(CeH), 1694 cmꢁ1 (C]O), 1652 cmꢁ1 (C]O); 1H NMR (300 MHz,
Supplementary data
Absorption and fluorescence spectra for all the fluorophores and
dendrimers in all the organic solvents tried. Supplementary data
associated with this article can be found in the online version, at
doi:10.1016/j.tet.2011.10.021.
References and notes
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€
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potentially much more susceptible to hydrolysis and photooxidation; the dendritic
macromolecules composed of those building units decompose readily to complex
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15. In order to gain further insight into the details of chromophoric characteristics,
fluorescence lifetime studies on the fluorophores are needed. This will be the
subject of future work.
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21. Considering that significant difference in the fET values depending on the ge-
ometry of the dendritic architectures should be noticed, the energy transfer
would be understood in terms of
a non-radiative fluorescence resonance
mechanism, rather than a radiative emission-reabsorption mechanism. In fact,
averaged donoreacceptor distances for 10 and 11 were estimated to be 1.1 and
1.5 nm, respectively, by means of molecular modeling studies performed on the
fully extended structures. These geometrical parameters meet the desired cri-
teria for achieving the fluorescence resonance energy transfer with sufficiently
high efficiency.
€
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€
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