Silole-core phenylacetylene dendrimers and their application in detecting picric acid

Article abstract of DOI:10.1002/jhet.796

Silole-core phenylacetylene dendrimers were designed and synthesized, among them, the model compound (n = 0) and the first generation of the dendrimer (n = 1) were obtained by the reaction of 2,5- dibromosilole with corresponding terminal alkynes, the sec

Full text of DOI:10.1002/jhet.796

64
Silole-Core Phenylacetylene Dendrimers and Their Application in
Detecting Picric Acid
Vol 49
Shanshan Li,a Taohua Leng,b Hanbin Zhong,a Chengyun Wang,a*
and Yongjia Shena
aKey Laboratory for Advanced Materials and Institute of fine chemicals, East China University of
Science and Technology, Shanghai 200237, People’s Republic of China
bNational Food Quality Supervision and Inspection Center (Shanghai)/Shanghai Institute of Quality
Inspection and Technical Research, Shanghai 200233, People’s Republic of China
*E-mail: cywang@ecust.edu.cn
Received September 21, 2010
DOI 10.1002/jhet.796
Published online 23 August 2011 in Wiley Online Library (wileyonlinelibrary.com).
Silole-core phenylacetylene dendrimers were designed and synthesized, among them, the model com-
pound (n ¼ 0) and the first generation of the dendrimer (n ¼ 1) were obtained by the reaction of 2,5-
dibromosilole with corresponding terminal alkynes, the second generation of the dendrimers (n ¼ 2)
was synthesized from 2,5-diiodosilole. These compounds indicated the absorptions of both phenylacety-
lene dendrons (250–350 nm) and silole core (400–500 nm). The first generation displayed efficient
energy transfer from phenylacetylene dendrons to silole core, whose energy transfer efficiency was as
high as 80%. These compounds were used as chemical sensors to probe explosive, for picric acid (PA),
the Stern–Volmer constants of model compound and the first generation are 7120 and 5490Mꢀ1
,
respectively.
J. Heterocyclic Chem., 49, 64 (2012).
INTRODUCTION
[9–2]. To our knowledge, phenylacetylene dendrimer
with silole as the core, has not been reported.
Siloles possess a unique silacyclopentadiene core
structure and exhibit excellent photophysical and elec-
tronic properties. Some siloles have special and useful
aggregation-induced enhanced emission (AIE) effect [1–
2]. The characteristics of siloles allow them to function
as organic electroluminescent devices, organic solar
cells, pH sensors, ion monitors, and explosive detectors.
Siloles may be used as efficient chemical sensors for
nitroaromatic explosives because of the Lewis acid/base
interactions between siloles and nitroaromatic explo-
sives. And reliable detection of trace explosive materials
becomes a focal point in the fields of humanitarian
demining and security. On the other hand, phenylacety-
lene dendrimer is one of the ideal photon-harvesting
systems. The stiff dendrimers exhibit high-energy trans-
fer efficiency, which will be of potential applications in
fabricating OLEDs [4,5], sensors [6–2], and membranes
In this work, we designed and synthesized a kind of
silole-core dendrimers with rigid phenylacetylene dendrons
and found that the energy transfer from the dendrons to
the silole core occurred within the dendrimers. Their appli-
cation in detecting picric acid was investigated as well.
RESULTS AND DISCUSSION
Synthesis. Phenylacetylene monodendrons 3c and 4c
were synthesized analogous to the ref. 12 by repeating
Sonogashira coupling (Scheme 1).
We selected 1,3,5-tribromobenzene as a key monomer
for the synthesis of phenylacetylene monodendrons
instead of 30,50-dibromo-2-phenyl-1-(trimethylsilyl)acety-
lene. To improve the yields of 3a and 4a, we dropwised
2 equivmolar terminal alkyne to 1,3,5-tribromobenzene.
C
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Silole-Core Phenylacetylene Dendrimers and Their Application in Detecting Picric Acid
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Scheme 1. Synthesis of phenylacetylene monodendrons. Reagents and conditions: (i) Pd(PPh3)4, CuI, Et3N, Ar, 80ꢁC, 5 h; (ii) 2-methylbut-3-yn-2-
ol, Pd(PPh3)4, CuI, Et3N, Ar, 80ꢁC, 5 h; (iii) KOH, toluene, Ar, reflux, 3h; (iv) 2, Pd(PPh3)4, CuI, Et3N, toluene, Ar, 90ꢁC, 8 h; (v) 2-methylbut-3-
yn-2-ol, Pd(PPh3)4, CuI, Et3N, toluene, Ar, 90ꢁC, 6 h.
During the reaction, some monosubstituted products
were formed. However, these monosubstituted products
could further react with 1 equivmolar terminal alkyne.
During the reaction of 3c with 2, diacetylene would
form due to Glaser coupling reaction. Rigorous de-oxy-
gen was necessary to suppress the formation of such
diacetylene byproduct. Because the solubility of 4a and
4b was poor in Et3N, toluene was added as cosolvent in
Sonogashira coupling reaction. However, 6–8 are solu-
ble fairly in common solvents such as CH2Cl2.
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S. Li, T. Leng, H. Zhong, C. Wang, and Y. Shen
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Scheme 2. Synthesis of silole-core phenylacetylene dendrimers.
2,5-Dibromo-1,1-dimethyl-3,4-diphenylsilole 5a was
cation. We speculate that 2,5-diiodosilole is also unstable,
therefore, we synthesized 2,5-diiodosilole analogous to
dibromosilole except using I2 instead of NBS, and the
resulting mixture was reacted with alkyne 4c directly
because of the instability of 2,5-diodosilole. In the begin-
ning, the reaction was conducted at 40ꢁC for 12 h; it was
found that no products formed during the reaction. So the
reaction was carried out at 65ꢁC. After reaction for 12 h,
some products appeared. Dendrimer 8 was obtained after
separation from the reaction mixture. However, 8 was not
so stable that decomposed slowly even keeping in dark.
When exposed to ultraviolet light, it would be decomposed
rapidly. The yield of 8 was extremely low (<10%). Per-
haps 8 would decompose once it formed in the reaction,
and this was confirmed by TLC experiments in the later.
Absorption and fluorescence spectra. The photo-
physical properties of silole-core phenylacetylene
dendrimers 7–8 and their parent compound 6 were stud-
ied by UV–vis and fluorescence spectroscopies. Their
photophysical data in dichloromethane are summarized in
synthesized according to the literature [13]. The parent
compound 6 and silole-core phenylacetylene dendrimers
7–8 were synthesized by Negishi coupling (Scheme 2).
Comparing to Sonogashira coupling system, diacetylene
byproduct was not formed because of the absence of
CuI. Dendrimers 6 and 7 were obtained conveniently
with moderate yields (53 and 67%, respectively).
We tried to synthesize dendrimer 8 by using the same
synthetic procedure as 6 and 7; however, no products
formed even if the reaction time was extended to longer
than 48 h. It might imply that molecule 4c was so large
that its reactivity not enough to react with 2,5-dibromosi-
lole. Therefore, we used 2,5-diiodosilole instead of 2,5-
dibromosilole. Yamaguchi et al. [14] had synthesized 2,5-
diiodosilole by the reaction of corresponding dilithiosilole
with 1,2-diiodoethane. Pagenkopf et al. [13] had synthe-
sized chloroiodosilole by the reaction of 2-chloro-5-metal-
losilole with I2, they found that chloroiodosilole was very
unstable and should be used in the next step without purifi-
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Silole-Core Phenylacetylene Dendrimers and Their Application in Detecting Picric Acid
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Table 1
Photophysical properties of silole-core dendrimers 6–8.
Absorptiona
Fluorescencea
Compounds
kmax (nm)
e (104 cmꢀ1 Mꢀ1
)
kFLmax (nm)b
UET (%)
c
6
7
8
269, 308, 435
292, 308, 431
293, 310, 434
2.40, 1.36, 0.98
3.94, 3.27, 1.10
13.83, 13.75, 1.54
362, 521
361, 520
370, 501
–
80
c
–
a In CH2Cl2.
b Excited at 310 nm.
c Not available.
Table 1, the absorption and fluorescence spectra of 7–8
are shown in Figures 1 and 2, respectively. In Figure 1,
the absorptions between 250 and 350 nm are due to the
phenylacetylene monodendrons, the absorptions between
350 and 500 nm are due to the silole core. From the
absorption spectra, we find that dendrimer 8 possesses the
absorption of phenylacetylene monodendrons and silole
core. The molar extinction coefficient of phenylacetylene
monodendrons increases rapidly from 6 to 8 because of
the enlarging phenylacetylene monodendrons. The change
of the molar extinction coefficient of silole group is very
small, which is coincide with their structures.
The energy transfer efficiency was estimated by a com-
parison of the absorption spectra and the excitation
spectra of 7 by monitoring the emission of silole core
(520 nm). The UET of 7 was as high as 80%, which
was in the range of UET values (54–95%) of perylene-
terminated phenylacetylene dendrimers reported by Moore
et al. [16] However, in the emission spectra of 8 excited
at 310 nm, we find intense emission around 370 nm and
moderate emission of silole core (ꢂ500 nm). This clearly
indicates that the energy transfer occurred within den-
drimer 8. However, due to the poor stability of 8, the
energy transfer efficiency was not estimated. Moore et al.
have reported that the emission around 370 nm was due
to the phenylacetylene dendrons [16]. Hence, from the
absorption and emission spectra of dendrimer 8, we sup-
pose that the emission around 370 nm is due to the prod-
uct formed by the decomposition of silole ring in
dendrimer 8. The huge rigid phenylacetylene dendrons at
2,5-positions of silole ring probably is the main cause
of the instability of 8. The steric effect between the two
phenylacetylene dendrons or phenylacetylene dendrons
and phenyls at 3,4-positions probably was too strong to
sustain the silole ring.
The emission spectra of 6 and 7 excited at 310 nm
are almost the same. Both compounds display an intense
emission around 520 nm, which could be attributed to
the silole core and negligible emission around 360 nm,
which could be attributed to the phenylacetylene den-
drons. For the intensity, these compounds have no appa-
rent AIE effect. Obviously, there exists efficient energy
transfer from the phenylacetylene dendrons to the silole
core in dendrimer 7 and the parent compound 6. The
energy transfer efficiency (UET) of the model compound
6 was not estimated analogous to the literature [15,16].
Figure 1. UV–vis absorption spectra of 6–8 in CH2Cl2, the spectra are
normalized to 1.0 ꢃ 10ꢀ5 M concentration.
Figure 2. Fluorescence spectra of 6–8 in CH2Cl2 excited at 310 nm, the
fluorescence intensities were normalized in the molar concentration.
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S. Li, T. Leng, H. Zhong, C. Wang, and Y. Shen
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Figure 5. Stern–Volmer plots for fluorescence quenching of 6 and 7
by PA.
Figure 3. Fluorescence quenching of 6 in CH2Cl2 by picric acid (PA);
[6] ¼ 1 ꢃ 10ꢀ5M, kex ¼ 310 nm.
[17]. The quenching constants are determined by the
Stern–Volmer equation:
Explosives detection. Because of the commercial
unavailability of explosives such as 2,4-dinitrotoluene
(DNT) and 2,4,6-trinitrotoluene (TNT), we used picric
acid (PA; a derivative of TNT). We investigated the
photoluminescence (PL) responses of 6 and 7 to PA by
adding PA solutions successively. Figures 3 and 4
showed fluorescence quenching of 6 and 7 in CH2Cl2
solution by PA, respectively. The fluorescence quench-
ing effect by PA is significant. For both 6 and 7, the
detection limit reaches as low as 1 lM. Hence, the
larger phenylacetylene dendrons at 2,5-positions have no
effect on the detection limit, because that they have lit-
tle block effect on the detection sites (silicon atoms).
Trogler et al. have reported that less steric encumbrace
surrounding silicon atoms, which caused by the substitu-
ent at 1,1-postions could lead to better detection limits
I0=I ꢀ 1 ¼ Ksv½Aꢄ
where I is the PL intensity at a specific analyte concen-
tration ([A]), I0 is the intensity at [A] ¼ 0, and Ksv is
the Stern–Volmer quenching constant.
Figure 5 shows Stern–Volmer plots for fluorescence
quenching of 6 and 7 by PA. The Stern–Volmer constants
of 6 and 7 are 7120 and 5490Mꢀ1, respectively. Dendrimer
8 was not investigated because of the poor stability.
CONCLUSIONS
Silole-core phenylacetylene dendrimers were synthe-
sized. The parent compound 6 and dendrimer 7 were
synthesized by the reaction of 2,5-dibromosilole with
corresponding terminal alkynes, the second generation 8
was obtained by the reaction of more reactive 2,5-diio-
dosilole with corresponding alkyne. Dendrimer 8 was
unstable. Their photophysical properties were investi-
gated. All of the compounds possess the absorption of
phenylacetylene dendrons (250–350 nm) and silole core
(400–500 nm). When excited at 310 nm, compounds 6
and 7 mainly display an emission at about 520 nm from
the silole core. The energy transfer efficiency of 7 is as
high as 80%. Dendrimer 8 displays an intense emission
at about 370 nm, which probably caused by the decom-
posed product. Their use in explosives detection was
investigated. When quenched by PA, the Stern–Volmer
constants of 6 and 7 are 7120 and 5490Mꢀ1, respectively.
EXPERIMENTAL
1
Instruments and materials. H-NMR spectra were obtained
on a Bruker 500 or 400 MHz spectrometer. Mass spectra were
Figure 4. Fluorescence quenching of 7 in CH2Cl2 by picric acid (PA);
[7] ¼ 1 ꢃ 10ꢀ5M, kex ¼ 310 nm
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Silole-Core Phenylacetylene Dendrimers and Their Application in Detecting Picric Acid
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recorded using an LCQ Advantage mass spectrometer. Absorp-
tion spectra were measured with a CARY 100 Conc UV–vis
spectrophotometer. Fluorescence spectra were recorded with a
CARY Eclipse fluorescence spectrophotometer in a 1-cm
quartz cell. All reagents were purchased from commercial
sources and distilled or dried when necessary using the stand-
ard procedures. All solvents and reagents were of reagent
quality, purchased from commercial sources and used without
further purification unless mentioned below. Triethylamine
was distilled from calcium hydride. Reactions were carried out
under an inert atmosphere (dry argon) when necessary.
Synthesis. The synthetic route is shown in Schemes 1 and 2.
4,40-(5-Bromo-1,3-phenylene)bis(ethyne-2,1-diyl)bis(tert-
butylbenzene) (3a). A flask with 30-mL degassed Et3N was
charged with compound 2 (1.5 g, 4.8 mmol, 1 eq.), CuI
(18 mg, 0.095 mmol, 0.02 eq.) and Pd(PPh3)4 (110 mg,
0.095 mmol, 0.02 eq.) under a stream of Ar. Compound 1
(1.65 g, 10.5 mmol, 2.2 eq.) dissolved in 10-mL degassed Et3N
was dropwised more than 1 h at 80ꢁC. After that, the solution
was stirred for 4 h. Then, the mixture was diluted with dichloro-
methane, washed with water, dried by MgSO4, and filtered. The
concentrated product was purified by flash chromatography, elut-
ing with petroleum ether to give 3a as white powder (1.452 g,
64.4%). 1H NMR (CDCl3, 500 MHz) d: 1.35 (s, 18 H), 7.39
(d, J ¼ 8.3 Hz, 4 H), 7.46 (d, J ¼ 8.3 Hz, 4 H), 7.61 (s, 3H).
MS(EI): m/z 468.1 [M]þ (calcd for C30H29Br 468.1).
4-(3,5-Bis((3,5-bis((4-tert-butylphenyl)ethynyl)phenyl)ethynyl)-
phenyl)-2-methylbut-3-yn-2-ol (4b). Compound 4b was synthe-
sized analogous to compound 3b except that toluene was
added as co-solvent. The product was purified by flash chro-
matography, eluting with 50% CH2Cl2/petroleum ether to give
4b as white powder (106 mg, 54%). 1H NMR (CDCl3, 500
MHz) d: 1.36 (s, 36 H), 1.64 (s, 6H), 7.40 (d, J ¼ 8.4 Hz, 8
H), 7.47 (d, J ¼ 8.4 Hz, 8 H), 7.56–7.58 (m, 2 H), 7.62–7.64
(m, 5 H), 7.66–7.68 (m, 2 H). MS(ESI): m/z 985.5 [MþH]þ
(calcd for C75H68OþH 985.5).
4,40,400,4000-(5,50-(5-Ethynyl-1,3-phenylene)bis(ethyne-2,1-diyl)-
bis(benzene-5,3,1-triyl))tetrakis(ethyne-2,1-diyl)tetrakis(tert-
butylbenzene) (4c). Compound 4c was synthesized analogous
to compound 3c. The concentrated product was purified by
flash chromatography, eluting with 10% CH2Cl2/petroleum
ether to give 4c as white powder (76 mg, 82%). 1H NMR
(CDCl3, 500 MHz) d: 1.34 (s, 36 H), 3.15 (s, 1 H), 7.39 (d,
J ¼ 8.4 Hz, 8 H), 7.48 (d, J ¼ 8.4 Hz, 8 H), 7.62–7.65 (m, 6
H), 7.66–7.68 (m, 3 H). MS(ESI): m/z 927.5 [MþH]þ (calcd
for C72H62þH 927.5).
2,5-Dibromo-1,1-dimethyl-3,4-diphenylsilole (5a). Compound
5a was synthesized according to the literature þ[13]. yield:
64.0%. m.p. 162–163ꢁC. MS(EI): m/z 419.9 [M] (calcd for
C18H16Br2Si 420.2).
2,5-Bis((4-tert-butylphenyl)ethynyl)-1,1-dimethyl-3,4-diphe-
nylsilole (6). A flask was charged with 5a (200 mg, 0.478
mmol, 1 eq.), compound 1 (107 mg, 1.05 mmol, 2.2 eq.),
ZnCl2 (40.8 g, 0.3 mmol), Pd(PPh3)4 (12 mg, 0.01 mmol),
5-mL Et3N and 20-mL THF, then degassed by Ar. The result-
ing mixture was placed in a 65ꢁC oil bath for 7 h. Upon com-
pletion, the mixture was diluted with petroleum ether and fil-
tered through a thin pad of silica (50% CH2Cl2/petroleum
ether). The concentrated product was purified by flash chroma-
tography, eluting with 10% CH2Cl2/petroleum ether to give 6
as yellow powder (186 mg, 67.5%).1H NMR (CDCl3, 400
MHz) d: 0.55 (s, 6 H), 1.30 (s, 18 H), 7.20 (s, 10 H), 7.24 (d,
J ¼ 8.8 Hz, 4 H), 7.29 (d, J ¼ 8.8 Hz, 4 H). MS(EI): m/z
574.3 [M]þ (calcd for C42H42Si 574.3).
4-(3,5-Bis((4-tert-butylphenyl)ethynyl)phenyl)-2-methylbut-3-
yn-2-ol (3b). A flask with 20-mL degassed Et3N was charged
with compound 3a (357 mg, 0.76 mmol, 1 eq.), 2-methylbut-
3-yn-2-ol (128 mg, 1.52 mmol, 2 eq.), CuI (3 mg, 0.015
mmol, 0.02 eq.), and Pd(PPh3)4 (18 mg, 0.015 mmol, 0.02 eq.)
under a stream of Ar. The solution was stirred about 5 h at
80ꢁC. Then the mixture was diluted with dichloromethane,
washed with water, dried by MgSO4, and filtered. The concen-
trated product was purified by flash chromatography, eluting
with 15% EtOAc/petroleum ether to give 3b as white powder
1
(274 mg, 76.3%). H NMR (CDCl3, 500 MHz) d: 1.36 (s, 18
H), 1.64 (s, 6H), 7.39 (d, J ¼ 8.4 Hz, 4 H), 7.46 (d, J ¼ 8.4
Hz, 4 H), 7.53 (s, 2H), 7.64 (s, 1 H). MS(EI): m/z 472.3 [M]þ
(calcd for C35H36O 472.3).
2,5-Bis((3,5-bis((4-tert-butylphenyl)ethynyl)phenyl)ethynyl)-
1,1-dimethyl-3,4-diphenylsilole (7). Compound 7 was synthe-
sized analogous to compound 6. The concentrated product was
purified by flash chromatography, eluting with 15% CH2Cl2/
petroleum ether to give 7 as yellow powder (84 mg, 53.2%).
1H NMR (CDCl3, 400 MHz) d: 0.52 (s, 6 H), 1.26 (s, 36 H),
7.11–7.18 (m, 10 H), 7.31 (d, J ¼ 8.8 Hz, 12 H), 7.39 (d, J ¼
8.4 Hz, 8 H), 7.49 (t, J ¼ 1.6 Hz, 2H). MS(ESI): m/z 1125.6
[MþK]þ (calcd for C82H74SiþK 1125.6).
4,40-(5-Ethynyl-1,3-phenylene)bis(ethyne-2,1-diyl)bis(tert-
butylbenzene) (3c). Compound 3b (274 mg, 0.58 mmol), KOH
(0.1 g, 1.8 mmol), and 15-mL toluene were added into a flask and
protected by Ar. The solution was stirred about 3 h at reflux. Then,
the mixture was diluted with dichloromethane, washed with water,
dried by MgSO4, and filtered. The concentrated product was puri-
fied by flash chromatography, eluting with petroleum ether to give
1
3c as colorless oil (225 mg, 93.6%). H NMR (CDCl3, 500 MHz)
2,5-Bis((3,5-bis((3,5-bis((4-tert-butylphenyl)ethynyl)phenyl)-
ethynyl)phenyl)ethynyl)-1,1-dimethyl-3,4-diphenylsilole (8).
Compound 5b was synthesized analogous to compound 5a
except using I2 instead of NBS. After concentration, the mix-
ture was used in the next step directly without purification.
Compound 4c (260 mg, 0.28 mmol), mixture with 5b, ZnCl2
(10 mg, 0.07 mmol), Pd(PPh3)4 (24 mg, 0.02 mmol), 40-mL
THF and 8-mL Et3N were charged in a flask, then degassed by
Ar. The resulting mixture was placed in a 40ꢁC oil bath for 12
h. However, when checked by TLC, there were lots of
unreacted 4c and no products and temperature was raised to
65ꢁC. After 12 h, the mixture was diluted with petroleum ether
and filtered through a thin pad of silica (50% CH2Cl2/petro-
leum ether). The concentrated product was purified by flash
d: 1.36 (s, 18 H), 3.10 (s, 1 H), 7.38 (d, J ¼ 8.2 Hz, 4 H), 7.46 (d,
J ¼ 8.2 Hz, 4 H), 7.58 (s, 2 H), 7.66 (s, 1 H). MS(EI): m/z 414.2
[M]þ (calcd for C32H30 414.2).
4,40,400,4000-(5,50-(5-Bromo-1,3-phenylene)bis(ethyne-2,1-diyl)-
bis(benzene-5,3,1-triyl))tetrakis(ethyne-2,1-diyl)tetrakis(tert-
butylbenzene) (4a). Compound 4a was synthesized analogous
to compound 3a except that toluene was added as cosolvent.
The product was purified by flash chromatography, eluting
with 10% CH2Cl2/petroleum ether to give 4a as white powder
1
(256 mg, 40.8%). H NMR (CDCl3, 500 MHz) d: 1.35 (s, 36
H), 7.39 (d, J ¼ 8.3 Hz, 8 H), 7.47 (d, J ¼ 8.3 Hz, 8 H),
7.62–7.68 (m, 9 H). MS(ESI): m/z 981.4 [MþH]þ (calcd for
C70H61Br þ H 981.4).
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chromatography, eluting with 10% CH2Cl2/petroleum ether to
give 8 as yellow powder (15 mg). When based on the starting
material dimethylbis(phenylethynyl)silane, the yield was below
1
1%. H NMR (CDCl3, 400 MHz) d: 0.53 (s, 6 H), 1.27 (s, 72
H), 7.15 (s, 10 H), 7.32 (d, J ¼ 8.4 Hz, 16 H), 7.41 (d, J ¼
8.4 Hz, 16 H), 7.52–7.60 (m, 18 H). MS(ESI): m/z 2113.9
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Acknowledgments. This work was supported by National Natu-
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21076078), Specialized Research Fund for the Doctoral Program
of Higher Education (No.20070251018).
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