Phenylacetylene dendrimers containing a fluorene core: Synthesis and spectroscopic studies

Article abstract of DOI:10.1002/cjoc.201090134

Phenylacetylene dendrimers 9-11 containing fluorene as the core with a larger HOMO-LUMO energy gap were synthesized and characterized. Their structure and properties were studied by UV, FL, ¹H NMR, MS etc. These novel phenylacetylene dendrimers exhibit unique photophysical properties. They exist a new absorption band around 340 nm whose molar coefficient decreases with increasing generation. The band-gaps of 9-11 are 3.54, 3.43 and 3.02 respectively. The fluorescence quantum yield of 10 is as high as 0.61.

Full text of DOI:10.1002/cjoc.201090134

FULL PAPER
Phenylacetylene Dendrimers Containing a Fluorene Core:
Synthesis and Spectroscopic Studies
Wang, Chengyun*(王成云)
Zhong, Hanbin(钟汉斌)
Li, Shanshan(李珊珊)
Shu, Weifu(束伟夫)
Shen, Yongjia(沈永嘉)
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and
Technology, Shanghai 200237, China
Phenylacetylene dendrimers 9— 11 containing fluorene as the core with a larger HOMO-LUMO energy gap
1
were synthesized and characterized. Their structure and properties were studied by UV, FL, H NMR, MS etc.
These novel phenylacetylene dendrimers exhibit unique photophysical properties. They exist a new absorption band
around 340 nm whose molar coefficient decreases with increasing generation. The band-gaps of 9— 11 are 3.54,
3.43 and 3.02 respectively. The fluorescence quantum yield of 10 is as high as 0.61.
Keywords dendrimers, fluorescence, fluorene, band-gap, quantum yield
Introduction
have adopted phenyl as the core in order to compare
with their perylene-terminated dendrimers. These
phenyl-terminated dendrimers exhibit large band gaps
and moderate quantum yields. The highest quantum
yield is 0.31 at the second and third generation. Fluo-
rene is an excellent compound exhibiting unique pho-
tophysical properties,18,19 which has been investigated in
various areas such as OLEDs20,21 and phosphorescent
OLEDs.22,23 Also, fluorene possesses a slightly larger
HOMO-LUMO energy gap (the absorption edge is
about 315 nm) than phenylacetylene monodendrons (for
phenyl-terminated dendrimers, the absorption edge is
from 320 to 350 nm). In this paper, we choose fluorene
as the core of dendrimers and expect that these den-
drimers will have large energy gaps and intense photo-
luminescence, hence have potential applications in
functional materials such as host material for phospho-
rescent OLEDs.
Dendrimers are
a
class of highly ordered,
three-dimensional, and tree-like macromolecules and
potential building blocks for the construction of func-
tional materials. Among these dendrimers, stiff dendritic
macromolecules with antennas, such as poly(phenyl-
etylene) dendrimers,1 were the ideal photon-harvesting
systems2-5 with great energy transfer efficiency and
‘shape-persistent’ dendrimers6-9 which have potential
applications in fabricating OLEDs,10,11 sensors,12,13 ca-
talysis14 and membranes.15-17 Compared with den-
drimers, linear-chain macromolecules may not have the
most ideal architecture in the design of artificial pho-
ton-harvesting systems for efficient energy transfer.
Firstly, it is difficult to make polymeric systems with an
energy gradient so that there can be a vectorial trans-
duction of excitation energy. Secondly, most of the
polymeric chains are flexible and therefore may form
excimers which will act as an energy trap.2 Most
phenylacetylene dendrimers which have been reported
contain a chromophore with a smaller HOMO-LUMO
energy gap such as perylene,2 fullerene5 and anthra-
cene.10,11 In these cases, the dendritic antennas provide
efficient energy transfer from the multitopic surface to
the dendron focus and the energy transfer efficiency
could be estimated conveniently by a comparison of the
absorption spectrum and the excitation spectrum of the
dendrimer by monitoring the emission of the core.
Experimental
Instruments and materials
1H NMR spectra were obtained on a Bruker 500
spectrometer. Mass spectra were recorded using an LCQ
Advantage mass spectrometer. Absorption spectra were
measured with a CARY 100 Conc UV-Vis spectropho-
tometer. Fluorescence spectra were recorded with a
CARY Eclipse Fluorescence spectrophotometer in a 1
cm quartz cell. All reagents were purchased from com-
mercial sources and distilled or dried when necessary
using the standard procedures. All solvents and reagents
were of reagent quality, purchased from commercial
However, there are few reports on phenylacetylene
dendrimers holding a chromophore with a same or lar-
ger HOMO-LUMO energy gap. Moore and co-workers2
*
E-mail: cywang@ecust.edu.cn; Tel.: 0086-021-64252967; Fax: 0086-021-64252967
Received September 28, 2009; revised November 21, 2009; accepted December 30, 2009.
Project supported by the National Natural Science Foundation of China (Nos. 20872035, 20676036) and Specialized Research Fund for the Doctoral
Program of Higher Education (No. 20070251018).
Chin. J. Chem. 2010, 28, 699— 704
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699

Wang et al.
FULL PAPER
sources and used without further purification unless
mentioned below. Triethylamine was distilled from cal-
cium hydride. Toluene was distilled from Na. Reactions
were carried out under an inert atmosphere (dry argon)
when necessary.
℃; 1H NMR (CDCl3, 500 MHz) δ: 3.87 (s, 2H), 7.30—
7.56 (m, 4H), 7.66—7.90 (m, 3H).
4,4'-(5-Bromo-1,3-phenylene)bis(ethyne-2,1-diyl)-
bis(tert-butylbenzene) (3) A flask with 30 mL of de-
gassed Et3N was charged with 1,3,5-tribromobenzene
(1.5 g, 4.8 mmol, 1 equiv.), CuI (18 mg, 0.095 mmol,
0.02 equiv.) and Pd(PPh3)4 (110 mg, 0.095 mmol, 0.02
equiv.) under a stream of Ar. 2 (1.65 g, 10.5 mmol, 2.2
equiv.) dissolved in 10 mL degassed Et3N was drop-
wsied more than 1 h at 80 ℃. After that, the solution
Synthesis
The synthetic route is shown in Scheme 1 and
Scheme 2.
2-Iodofluorene (1) Compound 1 was synthesized
according to the literature.24 Yield 70%; m.p. 122—123
Scheme 1 Synthetic route to 2-iodofluorene and phenylacetylene monodendrons
Reagents and conditions: (a) I2, H5IO6, AcOH, H2SO4, H2O, reflux, 4 h; (b) Pd(PPh3)4, CuI, Et3N, Ar, 80 ℃, 5 h; (c) 2-methylbut-3-yn-2-ol, Pd(PPh3)4,
CuI, Et3N, Ar, 80 ℃, 5 h; (d) KOH, toluene, Ar, reflux, 3 h; (e) 1,3,5-tribromobenzene, Pd(PPh3)4, CuI, Et3N, toluene, Ar, 90 ℃, 8 h; (f)
2-methylbut-3-yn-2-ol, Pd(PPh3)4, CuI, Et3N, toluene, Ar, 90 ℃, 6 h.
700
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Chin. J. Chem. 2010, 28, 699— 704

Phenylacetylene Dendrimers Containing a Fluorene Core
Scheme 2 Synthetic route to fluorene dendrimers
Reagents and conditions: (a) Pd(PPh3)4, CuI, Et3N, Ar, 60 ℃, 5 h; (b) Pd(PPh3)4, CuI, Et3N, toluene, Ar, 80 ℃, 6 h.
was stirred for 4 h. Then the mixture was diluted with
dichloromethane, washed with water, dried over MgSO4,
and filtered. The concentrated product was purified by
flash chromatography, eluting with petroleum ether to
stream of Ar. The solution was stirred for about 5 h at
80 ℃. Then the mixture was diluted with dichloro-
methane, washed with water, dried over MgSO4, and
filtered. The concentrated product was purified by flash
1
give 3 as white powder (1.452 g, 64.4%). H NMR
chromatography, eluting with 15% EtOAc/petroleum
1
(CDCl3, 500 MHz) δ: 1.35 (s, 18 H), 7.39 (d, J＝8.3 Hz,
ether to give 4 as white powder (274 mg, 76.3%). H
4H), 7.46 ＋(d, J＝8.3 Hz, 4H), 7.61 (s, 3H); MS (EI) m/z:
468.1 [M] (calcd for C30H29Br 468.1).
NMR (CDCl3, 500 MHz) δ: 1.36 (s, 18 H), 1.64 (s, 6H),
7.39 (d, J＝8.4 Hz, 4H), 7.46 (d, J＝8.4 Hz, 4＋H), 7.53
(s, 2H), 7.64 (s, 1H);. MS (EI) m/z: 472.3 [M] (calcd
for C35H36O 472.3).
4-{3,5-Bis[(4-tert-butylphenyl)ethynyl]phenyl}-2-
methylbut-3-yn-2-ol (4) A flask with 20 mL of de-
gassed Et3N was charged with 3 (357 mg, 0.76 mmol, 1
equiv.), 2-methylbut-3-yn-2-ol (128 mg, 1.52 mmol, 2
equiv.), CuI (3 mg, 0.015 mmol, 0.02 equiv.) and
Pd(PPh3)4 (18 mg, 0.015 mmol, 0.02 equiv.) under a
4,4'-(5-Ethynyl-1,3-phenylene)bis(ethyne-2,1-diyl)-
bis(tert-butylbenzene) (5) 4 (274 mg), KOH (0.1 g,
1.8 mmol), and 15 mL of toluene were added into a
flask and protected by Ar. The solution was stirred for
Chin. J. Chem. 2010, 28, 699— 704
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Wang et al.
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about 3 h at reflux. Then the mixture was diluted with
dichloromethane, washed with water, dried over MgSO4,
and filtered. The concentrated product was purified by
flash chromatography, eluting with petroleum ether to
give 5 as colorless oil (225 mg, 93.6%). H NMR
(CDCl3, 500 MHz) δ: 1.36 (s, 18H), 3.10 (s, 1H), 7.38
ethynyl)-9H-fluorene (10) Compound 10 was syn-
thesized analogously to compound 9 except that toluene
was added as co-solvent. The product was purified by
flash
chromatography,
eluting
with
15%
1
CH2Cl2/petroleum ether to give 10 as white powder
1
(222 mg, 66.6%). H NMR (CDCl3, 500 MHz) δ: 1.40
(d, J＝8.2 Hz, 4H), 7.46 (d, J＝8.2 Hz, 4＋H), 7.58 (s,
(s, 18H), 3.90 (s, 2H), 7.30—7.34 (t, J＝7.4 Hz, 1H),
2H), 7.66 (s, 1H); MS(EI) m/z: 414.2 [M] (calcd for
C32H30 414.2).
4,4',4'',4'''-[5,5'-(5-Bromo-1,3-phenylene)bis-
(ethyne-2,1-diyl)bis(benzene-5,3,1-triyl)]tetrakis(eth-
7.37—7.41 (m, 5 H), 7.47 (d, J＝8.3 Hz, 4H), 7.56 (d,
J＝7.6 Hz, 2H), 7.64—7.66 (m, 3H), 7.71 (s, 1H),
7.76—7.81 (m, 2H); MS (EI) m/z: 578.3 [M]＋ (calcd
for C45H38 578.3).
yne-2,1-diyl)tetrakis(tert-butylbenzene) (6)
Com-
2-((3,5-Bis((3,5-bis((4-tert-butylphenyl)ethynyl)-
pound 6 was synthesized analogously to compound 3
except that toluene was added as co-solvent. The prod-
uct was purified by flash chromatography, eluting with
10% CH2Cl2/petroleum ether to give 6 as white powder
phenyl)ethynyl)phenyl)ethynyl)-9H-fluorene
(11)
Compound 11 was synthesized analogously to com-
pound 9 except that toluene was added as co-solvent.
The product was purified by flash chromatography,
eluting with 20% CH2Cl2/petroleum ether to give 11 as
1
(256 mg, 40.8%). H NMR (CDCl3, 500 MHz) δ: 1.35
(s, 36H), 7.39 (d, J＝8.3 Hz, 8H), 7.47 (d, J＝8.3 Hz,
slight yellow powder (21 mg, 38.5%). 1H NMR (CDCl3,
8H), 7.62—7.68 (m, 9H); MS (ESI) m/z: 981.4 [M]＋
500 MHz) δ: 1.33 (s, 36H), 3.90 (s, 2H), 7.25—7.30 (m,
(calcd for C70H61Br 980.4).
9H), 7.34 (d, J＝8.2 Hz, 8H), 7.38—7.43 (m, 9H), 7.50
—7.60 (m,＋3H), 7.62—7.75 (m, 3H); MS (ESI) m/z:
1090.5 [M] (calcd for C85H70 1090.6).
4-(3,5-Bis((3,5-bis((4-tert-butylphenyl)ethynyl)-
phenyl)ethynyl)phenyl)-2-methylbut-3-yn-2-ol
(7)
Compound 7 was synthesized analogously to compound
4 except that toluene was added as co-solvent. The
product was purified by flash chromatography, eluting
with 50% CH2Cl2/petroleum ether to give 7 as white
powder (106 mg, 54%). 1H NMR (CDCl3, 500 MHz) δ:
1.36 (s, 36H), 1.64 (s, 6H), 7.40 (d, J＝8.4 Hz, 8H),
7.47 (d, J＝8.4 Hz, 8H), 7.56—7.58 (m, 2H), 7.62—
7.64 (m, 5H), 7.66—7.68 (m, 2H); MS (ESI) m/z: 984.6
[M]＋ (calcd for C75H68O 984.5).
Results and discussion
Synthesis
The synthetic route to 2-iodofluorene has been re-
ported by Tsutsui and co-wokers24 by electrophilic
monoiodination of fluorene. Although almost all com-
pounds containing fluorene moiety possess two hexyls
groups on the 9,9 sites to improve solubility, here
2-iodofluorene reacts directly with phenylacetylene
monodendrons and even the third generation exhibits
good solubility in alkane solvents with small amounts of
polar solvent such as CH2Cl2. Phenylacetylene
monodendrons were synthesized through repeating
Pd/Cu-catalyzed cross-coupling. It seems that our syn-
thetic method is inferior to the route reported by Moore
and co-workers25,26 for lower yields and selectivities.
However, our strategy avoids expensive reagents such
as trimethylsilylacetylene and complex procedures [the
key monomer 1-(3,5-diethynylphenyl)-3,3-diethyltri-
azene needs five steps]. Also, we can improve overall
yield through controlling the ratio between terminal
acetylene and 1,3,5-tribromobenzene and reacting main
byproduct—monosubstitute tribromobenzene with one
equivalent terminal acetylene. In Pd/Cu-catalyzed
cross-coupling procedures, there always exists diacety-
lene byproduct because of Glaser coupling. The amount
of diacetylene byproduct increases with increasing gen-
eration. These byproducts are hardly removed through
normal column chromatography because the Rf values
of diacetylenes and target compounds are always almost
the same. Fortunately, for procedures preparing
phenylacetylene monodendrons we simply carry the
unreactive diacetylene to the next step because it can be
removed easily after reaction and for steps preparing
fluorene dendrimers the amount of diacetylene is small
4,4',4'',4'''-[5,5'-(5-Ethynyl-1,3-phenylene)bis(eth-
yne-2,1-diyl)bis(benzene-5,3,1-triyl)]tetrakis(ethyne-
2,1-diyl)tetrakis(tert-butylbenzene) (8) Compound 8
was synthesized analogously to compound 5. The con-
centrated product was purified by flash chromatography,
eluting with 10% CH2Cl2/petroleum ether to give 8 as
white powder (76 mg, 82%).1H NMR (CDCl3, 500 MHz)
δ: 1.34 (s, 36H), 3.15 (s, 1H), 7.39 (d, J＝8.4 Hz, 8H),
7.48 (d, J＝8.4 Hz, 8H), 7.62—7.65 (m, ＋6H), 7.66—
7.68 (m, 3H); MS (ESI) m/z: 926.4 [M] (calcd for
C72H62 926.5).
2-((4-Tert-butylphenyl)ethynyl)-9H-fluorene (9)
A flask with 20 mL of degassed Et3N was charged with
1 (150 mg, 0.52 mmol, 1 equiv.), 2 (91 mg, 0.57 mmol,
1.1 equiv.), CuI (1.9 mg, 0.01 mmol, 0.02 equiv.) and
Pd(PPh3)4 (11.6 mg, 0.01 mmol, 0.02 equiv.) under a
stream of Ar. The solution was stirred for about 5 h at
60 ℃. Then the mixture was diluted with dichloro-
methane, washed with water, dried by MgSO4, and fil-
tered. The concentrated product was recrystallized from
10% CH2Cl2/petroleum to give a white powder (120 mg,
1
72.0%). H NMR (CDCl3, 500 MHz) δ: 1.35 (s, 9H),
3.90 (s, 2H), 7.30—7.34 (t, J＝8.4 Hz, 1H), 7.36—7.41
(m, 3H), 7.49 (d, J＝8.3 Hz, 2H), 7.55 (d, J＝7.7 Hz,
2H), 7.71＋(s, 1H), 7.74—7.80 (m, 2H); MS (EI) m/z:
322.2 [M] (calcd for C25H22 322.2).
2-((3,5-Bis((4-tert-butylphenyl)ethynyl)phenyl)-
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Phenylacetylene Dendrimers Containing a Fluorene Core
since iodine atom has more activities than bromine atom.
For the problem of diacetylene byproduct we faced here,
Moore and co-workers25 have reported and fully dis-
cussed. However, we do not try to synthesize the fourth
generation because the higher generation dendrimers
normally have lower quantum yields2 and the byproce-
dure, Glaser coupling, will become more significant for
higher generations.25,26 Also, the poor solubility of the
next generation is considered.
tem. As a result, with increasing generation, the absorp-
tion band around 340 nm is suppressed and phenylace-
tylene monodendron dominates the absorption. The
band-gaps of 9—11 estimated from absorption edge are
3.54, 3.43 and 3.02 eV, respectively. The decreasing
energy gaps with increasing generation are attributed to
the extending conjugation. The characteristic of large
bang-gap exhibits that these compounds perhaps can be
used as host material in phosphorescent OLEDs.
Photophysical properties
The photophysical properties of fluorene dendrimers
10, 11 are interesting. The absorption spectra of fluo-
rene and 9—11 are shown in Figure 1 and the fluores-
cence spectra of 10, 11 are shown in Figure 2. The pho-
tophysical data in dichloromethane solution, along with
those of the parent compound 9 and phenyl-terminated
dendrimers of the same generation, are summarized in
Table 1.
From the absorption spectra of 9—11, we can not
find distinct absorption bands of fluorene. We deduce
that the absorption bands of fluorene probably are cov-
ered by the absorption bands of phenylacetylene
monodendrons because they mostly absorb in the same
region (250—325 nm). The intense absorption bands
around 290 and 310 nm of 10, 11 are due to the
phenylacetylene monodendrons. These bands are
analogous to their perylene-terminated and phenyl-
terminated dendrimers. The molar extinction coefficient
(ε) of compound 11 is somewhat larger than compound
10. This trend is consistent with their perylene-
terminated and phenyl-terminated dendrimers that the
molar extinction coefficient increases with increasing
generation. What is interesting is the absorption band
around 340 nm. This band did not appear in perylene-
terminated and phenyl-terminated dendrimers. Besides,
the molar extinction coefficient around 340 nm de-
creases with increasing generation. To explain this
phenomenon, we attribute this absorption band to the
conjugation between phenylacetylene monodendron and
fluorene. For the higher generation, the fluorene seg-
ment plays a less important role in the conjugated sys-
Figure 1 Absorption spectra of 9— 11 and fluorene in CH2Cl2.
The spectra are normalized to 1× 10－ 5 mol/L concentration.
Figure 2 Fluorescence spectra of 10, 11 in CH2Cl2 excited at
310 nm. The spectra are normalized to a constant absorbance at
the excitation wavelength 310 nm.
Table 1 Photophysical properties of fluorene dendrimers 9— 11 and phenyl-terminated dendrimers
Absorptiona
Bandgap
∆Eb/eV
Fluorescence
λFLmaxc/nm
343, 358
Compound
－
－
1
1
d
λmax/nm
320, 337
ε/(105 L•mol •cm )
φFL
9
0.60, 0.61
3.54
—
10
Phenyl-8e
293, 310, 340
290, 309
0.85, 1.03, 0.52
0.92, 0.95
3.43
362
0.61
0.26
0.33
0.31
—
358, 371 (sh)
370
11
Phenyl-9e
293, 311, 341
1.05, 1.15, 0.27
3.02
292, 310
2.04, 2.28
—
359, (372)
a In CH2Cl2. b Estimated by absorption edge. c Excited at 310 nm. d Determined with 2-aminopyridine as a standard, when excited at 310
e
nm. Phenyl-8 and phenyl-9 are phenyl-terminated dendrimers which are the same generation with 10 and 11 respectively. Their photo-
physical data are cited from reference.2
Chin. J. Chem. 2010, 28, 699— 704
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The fluorescence spectra of fluorene-terminated
monodendrons 10, 11 are excited at 310 nm where most
of the radiation is absorbed by the phenylacetylene
monodendrons in dichloromethane. The fluorescence
maximum of 10, 11 is around 370 nm which is similar
to phenyl-terminated dendrimers. We can not find the
fluorescence of fluorene at 300— 350 nm.27 This is con-
sistent with the absorption spectra where the distinct
absorption bands of fluorene do not appear. However,
distinct absorption bands and fluorescence bands can be
found in perylene-terminated dendrimers because of
large HOMO-LUMO energy gap difference between
phenylacetylene monodendron and perylene. Here we
find phenylacetylene dendrimers containing a core with
a larger HOMO-LUMO energy gap exhibit extraordi-
nary photophysical properties. For phenylacetylene
monodendrons are the main skeleton in 10, 11, the
fluorescence maximum does not shift significantly
compared with phenyl-terminated dendrimers (the fluo-
rescence maximum is around 360 nm). With extending
conjugation from 10 to 11, the fluorescence maximum
only shifts 8 nm. For higher generation, the fluorescence
intensity decreases significantly. This is analogous to
phenyl and perylene-terminated dendrimers. The fluo-
rescence quantum yields are determined with
2-aminopyridine as a standard when excited at 310 nm.
Compound 10 exhibits extremely high quantum yield of
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We have synthesized fluorene-core phenylacetylene
dendrimers 9— 11 by Pd/Cu-catalyzed cross-coupling.
These dendrimers contain a core with a larger
HOMO-LUMO energy gap. Their photophysical prop-
erties are extraordinary. In the absorption spectra, the
molar coefficient around 310 nm which belongs to
phenylacetylene monodendrons increases with increas-
ing generation. However, there exists a new absorption
band around 340 nm whose molar coefficient decreases
with increasing generation. The band-gaps of 9— 11 are
3.54, 3.43 and 3.02 eV, respectively. When 10— 11
were excited at 310 nm, they displayed an emission at
about 370 nm. In addition, the fluorescence quantum
yield of 10 is as high as 0.61. The fluorescence quantum
yield decreases with increasing generation. The findings
herein reported would be important in phenylacetylene
dendrimers. Further study along the line using 10— 11
as host material for phosphorescent OLEDs is currently
in progress.
(E0909282 Cheng, F.; Lu, Z.)
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Chin. J. Chem. 2010, 28, 699— 704