Luminescent aggregates of a starburst silole-triphenylamine adduct for sensitive explosive detection

Article abstract of DOI:10.1016/j.dyepig.2011.03.006

A starburst luminogen (THPSTPA) consisting of a triphenylamine core and 1,1,2,3,4,5-hexaphenylsilole peripheries is designed and synthesized. Whereas it is weakly luminescent when molecularly dissolved in good solvent, it becomes highly emissive when aggregated in poor solvent, exhibiting a novel phenomenon of aggregation-induced emission (AIE). THPSTPA is morphologically and thermally stable, showing high glass transition and thermal degradation temperatures at 150 and 303 °C, respectively. The emission of its aggregates can be quenched exponentially by picric acid with a quenching constant up to ～7.0 × 10⁴ L mol^-1, suggesting that it can work as a sensitive chemosensor for explosive detection.

Full text of DOI:10.1016/j.dyepig.2011.03.006

Dyes and Pigments 91 (2011) 258e263
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Short communication
Luminescent aggregates of a starburst silole-triphenylamine adduct
for sensitive explosive detection
,
b
b
b
a
b,
*
Zujin Zhao a , Jianzhao Liu , Jacky Wing Yip Lam , Carrie Y.K. Chan , Huayu Qiu , Ben Zhong Tang
*
a College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou 310036, China
b Department of Chemistry, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A starburst luminogen (THPSTPA) consisting of a triphenylamine core and 1,1,2,3,4,5-hexaphenylsilole
peripheries is designed and synthesized. Whereas it is weakly luminescent when molecularly dissolved
in good solvent, it becomes highly emissive when aggregated in poor solvent, exhibiting a novel
phenomenon of aggregation-induced emission (AIE). THPSTPA is morphologically and thermally stable,
showing high glass transition and thermal degradation temperatures at 150 and 303 ꢀC, respectively. The
emission of its aggregates can be quenched exponentially by picric acid with a quenching constant up to
w7.0 ꢁ 104 L molꢂ1, suggesting that it can work as a sensitive chemosensor for explosive detection.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 24 January 2011
Received in revised form
28 February 2011
Accepted 4 March 2011
Available online 15 March 2011
Keywords:
Aggregation-induced emission
Silole
Triphenylamine
Aggregates
Explosive detection
Picric acid
1. Introduction
and its derivatives [10e12]. Whereas they are practically non-
luminescent when molecular dissolved in the solutions, they are
Explosive detection has attracted great attention due to the
unprecedented severe terrorism menace in the globe range.
Nitroaromatic compounds such as 2,4-dinitrotoluene, 2,4,6-trini-
trotoluene, and picric acid (PA) are warfare explosives, sensitive
detection of which is of antiterrorism implications. Sensing of these
explosives in groundwater or seawater is very important in the
detection and location of buried unexploded ordnance and
underwater mines as well as in the monitoring of groundwater and
soil for assessing contamination levels [1e4]. However, most
methods for the detection of explosives are only applicable to air
samples as a consequence of interference problems encountered in
complex aqueous media.
induced to emit intensely when aggregated as nanoparticles in
poor solvents or fabricated as thin films in solid state. Restriction of
intramolecular rotations (IMR) is rationalized to be the main cause
for the AIE effect [13,14]. Thanks to such effect, silole-containing
small molecules [15], oligomers [16], and linear and hyperbranched
polymers [17e19] have been utilized for the construction of effi-
cient chemosensors for explosive detection. The AIE aggregate-
based sensors exhibit strong light emissions, showing high
sensitivity and suffering no false-positive effect. They also contain
many cavities for the explosive molecules to enter and to interact
with the chromophores. All these advantageous structural features
endow them with a high sensing performance, making them more
applicable to real-world detection of trace explosives dissolved in
water. To further enhance the sensitivity of the detection requires
the synthesis of new materials. In this letter, we designed and
synthesized a novel starburst fluorophore by decorating triphe-
nylamine (TPA) with multiple 1,1,2,3,4,5-hexaphenylsilole (HPS)
[13,14] units as peripheries (Fig. 1). HPS is a typical AIE luminogen,
showing strong emission in the aggregate state and high electron
affinity. TPA, on the other hand, is electron rich but it, similar to
most conventional chromophores, suffers from aggregation-caused
emission quenching (ACQ) in the condensed phase. Adduct gener-
ated from these two units is anticipated to possess novel optical
Siloles (silacyclopentadienes) have been studied extensively due
to their unique electronic properties and potential high-techno-
logical applications. They are considered as novel
s p
*e * conju-
gated materials with a low-lying lowest unoccupied molecular
orbital energy level [5e7], and thus exhibiting high electron affinity
and fast electron mobility [8,9]. Recently, a novel phenomenon of
“aggregation-induced emission (AIE)” has been observed in silole
* Corresponding authors.
E-mail addresses: zujinzhao@gmail.com (Z. Zhao), tangbenz@ust.hk (B.Z. Tang).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.03.006

Z. Zhao et al. / Dyes and Pigments 91 (2011) 258e263
259
Fig. 1. Molecular structures of HPS, TPA and their adduct THPSTPA, and molecular model of THPSTPA optimized by the MMþ method in Hyperchem7.5 program.
properties and hence find potential high-technological applica-
tions. We herein describe how such hybrid is synthesized and
present its optical and sensory properties.
silica-gel column chromatography using hexane as eluent. Without
characterization, the product was transferred to a flask containing
40 mL THF, 60 mL methanol, and 0.5 g potassium carbonate. After
stirring for 8 h at room temperature, the mixture was poured into
water and extracted with dichloromethane. The organic layer was
washed with water and brine, and dried over magnesium sulphate.
After filtration and solvent evaporation under reduced pressure,
the residue was purified by silica-gel column chromatography
using hexane as eluent. Green solid of HPSA was obtained in 45%
2. Experimental
2.1. General
THF was distilled from sodium benzophenone ketyl under dry
nitrogen immediately prior to use. All other chemicals and reagents
were purchased from Aldrich (USA) and used as received without
further purification. 1H and 13C NMR spectra were measured on
a Bruker AV 300 spectrometer in deuterated chloroform using
(0.51 g). 1H NMR (300 MHz, CDCl3),
4H), 7.50e7.35 (m, 5H), 7.04e6.96 (m, 12H), 6.89e6.83 (m, 8H), 3.13
(s, 1H). 13C NMR (75 MHz, CDCl3),
(TMS, ppm): 157.7, 140.0, 139.8,
d (TMS, ppm): 7.65e7.62 (m,
d
139.3, 136.8, 136.6, 133.7, 132.4, 131.7, 131.0, 130.6, 129.9, 129.0,
128.5, 128.2, 127.2, 126.5, 124.5, 84.2, 79.1. HRMS (MALDI-TOF): m/z
562.2134 (Mþ, calcd 562.2117).
tetramethylsilane (TMS;
d
¼ 0) as internal reference. UV spectra
were measured on a Milton Roy Spectronic 3000 Array spectro-
photometer. PL spectra were recorded on a PerkineElmer LS 55
spectrofluorometer. The high resolution mass spectra were recor-
ded on a GCT premier CAB048 mass spectrometer. Thermogravi-
metric analysis (TGA) was carried on a TA TGA Q5000 under dry
nitrogen at a heating rate of 10 ꢀC/min. Thermal transitions were
investigated by differential scanning calorimetry (DSC) using a TA
DSC Q1000 under dry nitrogen at a heating rate of 10 ꢀC/min.
2.2.2. Synthesis of tris(4-{[4-(1,2,3,4,5-pentaphenylsilolyl)phenyl]
ethynyl}phenyl)amine (THPSTPA)
A mixture of HPSA (0.56 g, 1.0 mmol), tris(4-iodophenyl)amine
(0.18 g, 0.3 mmol), Pd(PPh3)4 (0.012 g, 0.01 mmol), CuI (3.8 mg,
0.02 mmol), and PPh3 (5.0 mg, 0.02 mmol) in 100 mL triethylamine
was refluxed for 24 h under nitrogen. After filtration and solvent
evaporation, the residue was purified by silica-gel column chro-
matography using hexane/dichloromethane mixture as eluent.
Yellow solid was obtained in 71% (0.42 g). 1H NMR (300 MHz,
2.2. Synthesis
CDCl3),
d
(TMS, ppm): 7.67e7.28 (m, 63H), 7.09e6.98 (m, 24H),
(TMS, ppm): 157.6,
2.2.1. Synthesis of 1-(4-ethynylphenyl)-1,2,3,4,5-pentaphenylsilole
(HPSA)
6.90e6.85 (m, 12H). 13C NMR (75 MHz, CDCl3),
d
147.4, 140.0, 139.9, 139.3, 136.8, 136.6, 135.0, 134.9, 133.6, 131.8,
130.9, 130.6, 130.2, 129.9, 129.1, 129.0, 128.5, 128.1, 127.1, 126.4,
124.7, 118.5, 91.3, 89.9. HRMS (MALDI-TOF): m/z 1926.7118 (Mþ,
calcd 1926.7119).
Under dry nitrogen, freshly cut lithium shavings (0.07 g,
10 mmol) were added to a solution of tolan 4 (2 g, 11 mmol) in dry
THF (20 mL). After stirring for 3 h at room temperature, a solution of
trichlorophenylsilane (0.64 mL, 4 mmol) in dry THF (20 mL) was
added dropwise into the reaction mixture. The mixture was
refluxed for 6 h to yield intermediate 6 and was then cooled to
room temperature. In the second experiment, n-butyllithium
solution (5.5 mmol, 2.5 M in hexane) was added dropwise to
a solution of 2 (1.3 g, 5 mmol) in dry THF (30 mL) under nitrogen
at ꢂ78 ꢀC. The mixture was stirred for 1 h to generate intermediate
3, which was then transferred to the previous reaction mixture
containing 6 by a syringe. After heating under reflux for 8 h, the
mixture was poured into water and extracted with dichloro-
methane. The organic layer was washed with water and brine, and
dried over magnesium sulphate. After filtration, the solvent was
evaporated under reduced pressure and the residue was purified by
2.3. Preparation of aggregates
Stock THF solution of THPSTPA with a concentration of 10ꢂ5
M
was prepared. Aliquots of the stock solution were transferred to
10 mL volumetric flasks. After appropriate amounts of THF were
added, water was added dropwise under vigorous stirring to
furnish 10ꢂ6 M solutions with different water contents (0e90 vol%).
The PL measurements of the resultant solutions were then per-
formed immediately.

260
Z. Zhao et al. / Dyes and Pigments 91 (2011) 258e263
3. Results and discussion
amount of water is added into its THF solution, intense PL is,
however, recorded at 495 nm under the same experimental condi-
tions. The higher the water fraction (fw), the stonger is the PL
intensity (Fig. 2B). Since water is a nonsolvent for THPSTPA, its
molecules must have been aggregated in the solvent mixtures with
high water contents, which restricts IMR process and blocks non-
radiative pathways, thus making the silole aggregates highly lumi-
nescent. Clearly, THPSTPA, similar to HPS, is AIE-active. It should be
pointed out that various chemical, physical, and engineering
approaches have been taken to frustrate aggregation of conventional
fluorophors but the attempts have met, in most cases, limited
success [20e28]. The AIE feature of THPSTPA demonstrates that
attachment of AIE units to ACQ dyes is a effective strategy for solving
their thorny ACQ problem and generating efficient luminophors in
the aggregate state [29e32].
THPSTPA has a 3D topological structure and its aggregates
contain numerous molecular cavities for capturing analytes and
have many pathways for exciton transport. Thus, it is expected that
it may function as an efficient chemosensor. With this in mind, we
chosen PA as a model explosive because of its commercial avail-
ability and monitored the PL change of THPSTPA aggregates in THF/
water mixture (fw ¼ 90%) upon PA addition. As shown in Fig. 3A, the
emission of THPSTPA is quenched upon PA addition. The fluores-
cence quenching can be clearly discerned at a PA concentration
THPSTPA was synthesized according to the multi-step reaction
route depicted in Scheme 1. HPSA was the key intermediate and
was prepared by the reaction of 3 with 6 followed by base-cata-
lyzed hydrolysis of the resultant product 7. The acetylene unit in
HPSA underwent Sonogashira coupling with 8 in the presence of
Pd(PPh3)2Cl2, CuI, and PPh3 under nitrogen, furnishing the desir-
able luminogen THPSTPA in satisfactory yield after column chro-
matography. The molecular structure of THPSTPA was fully
characterized by spectroscopic methods with satisfactory results.
The adduct is soluble in common organic solvents, such as THF,
dichloromethane, chloroform, and toluene but insoluble in meth-
anol and water. The thermal properties of THPSTPA are investigated
by differential scanning calorimetry and thermogravimetric anal-
ysis. The glass transition and thermal decomposition temperatures
of THPSTPA are observed at 150 and 303 ꢀC, respectively, suggesting
that it enjoys high thermal and morphological stabilities. The
structure of THPSTPA was optimized by the MMþ method in the
Hyperchem7.5 program package. The molecule shows a starburst
3D conformation, in which the silole peripheries arrange in
a “propeller-like” shape (Fig. 1). This hampers the close
pꢂp
stacking of the molecules and lengthens the intermolecular
distance, which creates many cavities in the aggregate state capable
of capturing small molecules.
([PA]) as low as 1 ppm. At a [PA] of 78.5 mM, virtually no light is
THPSTPA shows an absorption maximum at 376 nm associated
emitted from the solvent mixture. The SterneVolmer plot of rela-
tive PL intensity (I0/I) versus the PA concentration gives a curve
bending upward, instead of a linear line (Fig. 3B). Similar PL
quenching behaviours have been observed in our previous work
[17e19]. A possible reason for the THPSTPA aggregates to give
nonlinear SterneVolmer plots is that the PL annihilation is caused
by static quenching. Effective quenching sphere model [eqn. (1)]
can be used to describe the quenching process by the static
mechanism [17e19,33e35]
with the
absorption wavelength, an energy gap of 2.91 eV is determined. The
dilute THF solution (10 M) of THPSTPA is almost nonemissive and
exhibits only a noisy photoluminsecence (PL) spectrum without
a discernable peak (Fig. 2A). The fluorescence quantum yield (FF
pꢂ
p
* transitions of the silole units. From the onset
m
)
determined using 9,10-diphenylanthracene (FF ¼ 90% in cyclo-
hexane) as standard is 0.81%, revealing that THPSTPA is a genuinely
weak emitter when molecularly dissolved in solutions. When a large
Si
n-BuLi
Si
Li
Si
Br
Br
I
THF
Pd(PPh3)2Cl2
3
2
PPh3, CuI, Et3N
1
Li
THF
PhSiCl3
3
Si
Li Li
Cl
5
6
4
N
I
8
3
K2CO3
MeOH/THF
THPSTPA
Pd(PPh3)2Cl2, PPh3
CuI, Et3N/toluene
Si
Si
7
HPSA
Si
Scheme 1. Synthetic route to THPSTPA.

Z. Zhao et al. / Dyes and Pigments 91 (2011) 258e263
261
Fig. 2. (A) PL spectra of THPSTPA in THF/water mixtures with different water fractions (fw). (B) Plots of (I/I0 ꢂ 1) values versus water fractions in THF/water mixtures, where I0 was
the PL intensity in pure THF solution.
THPSTPA is highly emissive not only in suspension but also in
solid state. The powders of THPSTPA fluoresce intense greenish
blue light and the emission is so strong that has made the powders
look like white in the photo taken under 365 nm illumination from
a UV lamp (Fig. 4B). Therefore, the explosive detection can also be
carried out in the solid state using a THPSTPA-coated strip, thanks
to the AIE characteristic of THPSTPA. A prototype of a solid-state
chemosensor is prepared by simply dipping a filter paper into
a THPSTPA solution. After solvent evaporation, a highly emissive
test paper is obtained. The detection process is fast and the assay
procedure is simple. When a PA solution is dropped onto the
THPSTPA-coated filter paper, the PA-covered spot become non-
emissive, which is vividly discernable even by the naked eye
(Fig. 4C). These results encourage us to develop ultrasensitive and
ultrafast chemosensor devices with portability for on-site detec-
tions of explosive vapours and particulates [17].
The fluorescence quenching is probably due to the energy
transfer from the excited state of THPSTPA to the ground state of PA
via the spectral overlap of the absorption of PA and emission of the
THPSTPA as illustrated in Fig. 3D [1,36e38]. Moreover, instead
of the 3D topological structure of THPSTPA, which promotes
numerous internal voids to bind with quencher molecules and
diffusion pathways for excitons to migrate, the dipoleedipole and/
I0
I
¼ Aek½PAꢃ þ B
(1)
where A and B are constants and k denotes the static quenching
constant. By fitting the SterneVolmer plot shown in Fig. 3B, eqn. (2)
is obtained
I0
I
¼ 2:1e33333½PAꢃ ꢂ 1:1
(2)
When the [PA] is very low, eqn. (1) can be transformed to eqn.
(3) using a mathematical treatment of Taylor expansion:
I0
I
¼ Að1 þ k½PAꢃÞ þ B ¼ Ak½PAꢃ þ A þ B ¼ K½PAꢃ þ C
(3)
where K ¼ kA and C ¼ A þ B ¼ 1. Therefore, the static quenching
constant or the K value at the initial stage of the SterneVolmer plot
for THPSTPA aggregate in THF/water mixture (fw ¼ 90%) is
w7 ꢁ 104 L molꢂ1, which is about 3.5-fold higher than those of
linear conjugated polysiloles and 3,4-bis(4-isopropylphenyl)-
1,1,2,5-tetraphenylsilole (<20000 L molꢂ1) [15,35]. By plotting the
relative PL intensity (I0/I) versus ek[PA], a linear line was obtained
(Fig. 3C), which enables quantitative analysis.
Fig. 3. (A) Change in the PL spectrum of THPSTPA with the addition of different amounts of PA in THF/water mixture (fw ¼ 90%). Inset in panel A: fluorescent images of THPSTPA in
THF/water mixture (fw ¼ 90%) with [PA] of 0 and 78.5
m
M. Excitation wavelength ¼ 350 nm; concentration of THPSTPA ¼ 1
mM. (B) SterneVolmer plots of I0/I value versus [PA] in
THF/water mixture (fw ¼ 90%). I0 ¼ PL intensity in the absence of PA. (C) Plots of I0/I value versus ek[PA] in THF/water mixture (fw ¼ 90%). (D) Normalized absorption spectrum of PA
and PL spectrum of THPSTPA in THF/water (fw ¼ 90%).

262
Z. Zhao et al. / Dyes and Pigments 91 (2011) 258e263
Fig. 4. Photos of THPSTPA powders taken under (A) daylight and (B) UV illumination. (C) Fluorescence image of THPSTPA deposited on a filter paper with a spot of PA solution.
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or Lewis acid-base interactions between THPSTPA and the electron-
deficient PA may also play as an assistant in the quenching process
copolymers: selective turn-off/turn-on detection of explosives. Journal of Materials
Chemistry 2008;18(21):3143e56.
[4,20,39,40]. All these factors may have worked cooperatively to
achieve high sensing performance. Thus, THPSTPA is a promising
fluorescent chemosensor for explosive detection with high sensi-
tivity and low detection limit.
[4] Sanchez JC, DiPasquale AG, Mrse AA, Trogler WC. Lewis acidebase interactions
enhance explosives sensing in silacycle polymers. Analytical and Bioanalytical
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4. Conclusions
Journal of the Chemical Society, Dalton Transitions; 1998:3693e702.
[7] Liu J, Lam JWY, Tang BZ. Aggregation-induced emission of silole molecules and
polymers: fundamental and applications. Journal of Inorganic and Organo-
metallic Polymers 2009;19(3):249e85.
In summary,
a starburst HPS-TPA adduct (THPSTPA) was
designed and synthesized. Whereas THPSTPA is practically non-
emissive in the solution state, it is induced to emit intensely by
aggregate formation, demonstrating a novel AIE phenomenon. The
emission of THPSTPA aggregates can be quenched exponentially by
picric acid with large quenching constant, revealing that it is
[8] Murata H, Malliaras GG, Uchida M, Shen Y, Kafafi ZH. Non-dispersive and air-
stable electron transport in an amorphous organic semiconductor. Chemical
Physics Letters 2001;339(3ꢂ4):161e6.
[9] Murata H, Kafafi ZH, Uchida M. Efficient organic light-emitting diodes with
undoped active layers based on silole derivatives. Applied Physics Letters
2002;80(2):189e91.
a
promising chemosensor for sensitive explosive sensing.
[10] Luo J, Xie Z, Lam JWY, Cheng L, Chen H, Qiu C, et al. Aggregation-induced
emission of 1-methyl-1,2,3,4,5-pentaphenylsilole. Chemical Communications;
2001:1740e1.
[11] Zhao Z, Wang Z, Lu P, Chan CYK, Liu D, Lam JWY, et al. Structural modulation
of solid-state emission of 2,5-bis(trialkylsilylethynyl)-3,4-diphenylsiloles.
Angewandte Chemie International Edition 2009;48(41):7608e11.
[12] Yu G, Yin SW, Liu YQ, Chen JS, Xu XJ, Sun XB, et al. Structures, electronic states,
photoluminescence, and carrier transport properties of 1,1-disubstituted
2,3,4,5-tetraphenylsiloles. Journal of the American Chemical Society
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[13] Hong Y, Lam JWY, Tang BZ. Aggregation-induced emission: phenomenon,
mechanism and applications. Chemical Communications; 2009:4332e53.
[14] Li Z, Dong Y, Mi B, Tang Y, Häussler M, Tong H, et al. Structural control of the
photoluminescence of silole regioisomers and their utility as sensitive
regiodiscriminating chemosensors and efficient electroluminescent materials.
The Journal of Physical Chemistry B 2005;109(20):10061e6.
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Construction of efficient light-emitting diodes using THPSTPA as
active layer is currently underway in our laboratory.
Acknowledgments
We thank the financial supports from the Innovation and
Technology Commission (ITP/008/09NP), the University Grants
Committee of Hong Kong (AoE/P-03/08), the Initial Funding of
Hangzhou Normal University (HSQK0085), and National Natural
Science Foundation of China (21074028 and 20974031).
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