One-pot synthesis of star-shaped conjugated oligomers based on 3-hexylthiophene, pyrene and triphenylamine as TNT chemosensors

Article abstract of DOI:10.1016/j.jphotochem.2020.112496

Star-shaped conjugated oligomer T3HTP containing pyrene, 3-hexylthiophene and triphenylamine, has been successfully synthesized via a simple one-pot sequential direct arylation process. The chemical structure and optical properties of T3HTP oligomer were

Full text of DOI:10.1016/j.jphotochem.2020.112496

Journal Pre-proof
One-Pot Synthesis of Star-Shaped Conjugated Oligomers Based on
3-Hexylthiophene, Pyrene and Triphenylamine as TNT Chemosensors
Bao Kim Doan, Tung Viet Tuan Tran, Tam Huu Nguyen, Tam Hoang
Luu, Le-Thu T. Nguyen, Toai Pham Van, Vuong Mau Nguyen, Ha
Tran Nguyen
PII:
S1010-6030(20)30295-1
DOI:
https://doi.org/10.1016/j.jphotochem.2020.112496
JPC 112496
Reference:
To appear in:
Journal of Photochemistry & Photobiology, A: Chemistry
Received Date:
Revised Date:
Accepted Date:
10 November 2019
29 February 2020
8 March 2020
Please cite this article as: Doan BK, Tran TVT, Nguyen TH, Luu TH, Nguyen L-ThuT, Van TP,
Nguyen VM, Nguyen HT, One-Pot Synthesis of Star-Shaped Conjugated Oligomers Based on
3-Hexylthiophene, Pyrene and Triphenylamine as TNT Chemosensors, Journal of
Photochemistry and amp; Photobiology, A: Chemistry (2020),
doi: https://doi.org/10.1016/j.jphotochem.2020.112496
This is a PDF file of an article that has undergone enhancements after acceptance, such as
the addition of a cover page and metadata, and formatting for readability, but it is not yet the
definitive version of record. This version will undergo additional copyediting, typesetting and
review before it is published in its final form, but we are providing this version to give early
visibility of the article. Please note that, during the production process, errors may be
discovered which could affect the content, and all legal disclaimers that apply to the journal
pertain.
© 2019 Published by Elsevier.

One-Pot Synthesis of Star-Shaped Conjugated Oligomers Based on 3-
Hexylthiophene, Pyrene and Triphenylamine as TNT Chemosensors
Bao Kim Doan1, Tung Viet Tuan Tran1, Tam Huu Nguyen1, Tam Hoang Luu2, Le-Thu T.Nguyen2, Toai
Pham Van3, Vuong Mau Nguyen3, Ha Tran Nguyen*1,2
1National Key Laboratory of Polymer and Composite Materials, Ho Chi Minh City University of
Technology, Vietnam National University–Ho Chi Minh City (VNU–HCM), 268 Ly Thuong Kiet,
District 10, Ho Chi Minh City, Vietnam.
2Faculty of Materials Technology, Ho Chi Minh City University of Technology, Vietnam National
University, 268 Ly Thuong Kiet, District 10, Ho Chi Minh City, Vietnam.
3Institute of Propellants and Explosives, General Department of Defense Industry, 192 Duc Giang,
Long Bien District, Ha Noi City, Viet Nam.
Graphical abstract
Highlights
1

 Star-shaped conjugated oligomer T3HTP based on pyrene, 3-hexylthiophene and
triphenylamine moieties have been synthesized via simply one-pot sequential direct
arylation.
 The molar absorption coefficient of T3HTP was 80890 M-1.cm-1 indicated high
fluorescence efficiency
 The synthesized T3HTP oligomers have been applied for tracing of explosive TNT
moieties through FRET mechanism
Abstract:
Star-shaped conjugated oligomer T3HTP containing pyrene, 3-hexylthiophene and
triphenylamine, has been successfully synthesized via a simple one-pot sequential direct arylation
process. The chemical structure and optical properties of T3HTP oligomer were characterized via
proton nuclear magnetic resonance (1H-NMR), UV-Vis and fluorescence spectroscopies. The results
indicated that T3HTP oligomer showed good fluorescence quenching toward TNT, which is useful
for identifying explosive nitro aromatic compounds.
Keyword: Conjugated oligomer, Dluorescent polymers, 2,4,6-Trinitrotoluene, Direct arylation
reaction, Forster resonance energy transfer (FRET) mechanism.
1. Introduction
In the last few years, the detection of explosive traces such as 2,4,6-trinitrotoluene (TNT),
is an issue of global concern which is crucial for homeland security, anti-terrorism, military issues
and environment cleaning [1, 2]. In addition, the detection of explosives is also a foundation for
forensic examination, land mine detection and in the study of environment safety. Explosives have
caused enormous property damage and great public health threat, killed tens of thousands of
people per year and also commonly used by terrorists.
The methods for explosive detection include trained canine teams [3], surface-enhanced
Raman spectroscopy [4], mass spectroscopy [5], X-ray imaging [6], ion mobility spectroscopy [7, 8],
gas chromatography coupled with mass spectrometry [9] and electrochemical technique [10].
These methods have been used for the advantages of detection of low contamination of explosive
compounds. However, certain features of these methods are lack of portability. Moreover, the
2

spectroscopy methods are complicated and much expensive due to complicated equipments as
well as requirement of well trained technicians for equipment operation.
Recently, optical detection approach has attracted a great deal of attention due to low-
cost, good portability, high sensitivity and selectivity. These promising techniques are based on
colorimetric and fluorimetric responses which are mainly focused on by vast optical sensors for
explosive detection. Several reviews have extensively described fluorescence-based explosive
sensors from different perspectives including polymer sensors, optical chemosensors, and various
fluorescent materials such as conjugated oligomers/polymers, luminescent metal-organic
frameworks and nanomaterials [11-15]. In particular, recent research progress aims at the
methodology of fluorescence-based explosive sensors [16-20]. In generally, fluorescence-based
explosive sensors have been designed based on the Forster resonance energy transfer (FRET)
mechanism which can dramatically enhance the fluorescence-quenching efficiency and improve
sensitivity. In this mechanism, an initially excited molecule (donor) returns to the ground state
orbital, while the transferred energy simultaneously provides an electron on the acceptor to the
excited state [17, 21]. Poly(p-phenylene vinylene)s (PPVs) is one of the excellent conjugated
polymers exhibiting photo-and electro-luminescence which can act as fluorescent sensory
materials [14, 19, 22-27]. Chang et. al [22] synthesized dialkoxy and diphenyl substituted PPVs
towards fluorescence sensing of TNT and 2,6-DNT vapors. Swager and coworkers prepared
alternating conjugated polymers based on pentiptycene molecule with dialkoxy phenylene
moieties which exhibited fluorescence quenching in the presence of TNT and 2,4-DNT vapors [14,
19]. Accordingly, the intensity of emission quenching was influenced by the exposure time and by
thin film thickness. The best results were obtained for thin films of 25 Å and exposure time of 60 s
with quenching efficiencies of 100% and 75% for 2,4-DNT and TNT, respectively. The group of
Swager also reported conjugated polymers containing polyacetylene and pentiptycene, which
exhibited a moderate quenching of emission intensities by 16%‒28% in the presence of 2,4-DNT
vapor [28]. Feng et. al [29] has designed and synthesized PPV-based mesoporous silica hybrid
nanoparticles (PPV@MSNs-NH2), which could selectively detect TNT through FRET with
a
detection limit of TNT concentration of 1 x 10-7 M. Liu and coworkers reported the synthesis of
dibenz[a,h]-anthracene-containing polymer exhibiting efficient quenching in the presence of 2,4-
DNT at the bands of 427 and 453 nm in chloroform [30]. Water soluble poly(2,5-methoxy-
propyloxysulfonate phenylenevinylene) was also reported, with its fluorescence quenching
modifiable by surfactants [31]. Wu et. al also reported water‐dispersible hyperbranched
conjugated polymer nanoparticles (HCPN‐S) based on 9,9‐dihexyl‐2,7‐dibromofluorene and 1,3,5‐
3

tris‐[4‐(4,4,5,5‐tetramethyl‐[1,3,2]dioxaborolane)phenyl]‐benzene bearing the sulfonate terminal
groups which was obtained via Suzuki coupling polymerization in an oil‐in‐water miniemulsion
followed by post‐functionalization. HCPN‐S can sense TNT or other nitro compounds with a
detection limit at the nanomolar level [32]. Niamnont et. al [33] has reported the synthesis of star-
shaped conjugated molecules based on triphenyl amine linked with pyrene or corannulene for
visual detection of TNT. Lu and co-worker reported branched benzothiadiazole-cored conjugated
oligomers with terminal carbazoles [34]. Films based on these oligomers could emit strong red
light and their fluorescence emission intensities decreased obviously when exposed to gaseous
TNT and DNT [34]. More recently, Das and co-workers have reported conjugated triptycene-based
azobenzene-functionalized polymers (TBAFPs) towards chemsensors for picric acid detection, with
the azo linkage capable of switching from trans to cis isomerization resulting in enhancement of
the fluorescent emission intensity [35]. However, most of conjugated oligomers and polymers
used as explosive sensors have usually been synthesized via multiple complicated steps, leading to
decrease in the quality of the products.
It should be noted that conjugated molecules tend to aggregate in the solid state, causing
quenching in fluorescence. In order to prevent the aggregation of chromophores, star-shaped or
branched architecture structures have been proved to be useful for suppression of aggregation-
caused-quenching phenomenon and obtaining high solid fluorescence quantum yields [36-38].
Due to branched topological structures and numerous terminal groups in their, hyperbranched
and star-shaped structures are expected to enhance host–guest interactions, as, well as the
possibility of capturing and sensing.
It is well-known that pyrene with a large delocalized  system and well-matched orbital
energy levels shows good selectivity and sensitivity for TNT detection [39]. On the other hand,
sensors derived from electron-rich triphenyl amine have been developed for detection of
nitroaromatic explosives [40-42]. In addition, while both pyrene and triphenylamine have good
fluorescent properties, 3-hexylthiophene acts as a strong donor moiety. Inspired by these
fascinating characteristics, we hypothesized that a fluorophore molecule involving triphenylamine,
pyrene and 3-hexylthiophene maybe act as a promising candidate for visual detection of TNT. Due
to the incorporation of multiple fluorescent units, amplification of fluorescence quenching could
be achieved.
To the best our knowledge, there have not been any reports on the synthesis of sensors
based on these molecules for explosive detection. In this research, we focus on the synthesis of
star-shaped molecules containing triphenyl amine as a core with branches containing 3-
4

hexylthiophene and pyrene moieties via one-pot direct arylation coupling reaction. The obtained
1
conjugated molecules have been characterized via FTIR, H NMR, UV-vis and photoluminescence
(PL) spectroscopies for TNT sensoring application.
2. Experimental
2.1. Materials
Triphenylamine (98%), N-bomosuccinimide (99%) and 1-bromopyrene were purchased
from Acros Organic. [1,1′-Bis(diphenylphosphino)ferrocene]dichloropalladium(II) complex with
dichloromethane (Pd(dppf)Cl2 • CH2Cl2), bis(pinacolato)diboron, tetrakis(triphenylphosphine)
palladium(0) (Pd(PPh3)4) (99%), palladium(II) acetate (Pd(OAc)2, 98%), tricyclohexylphosphine
tetrafluoroborate (P(Cy)3.HBF4, 97%), cesium carbonate (Cs2O3, 99%), pivalic acid (PivOH, 99%)
were purchased from Sigma-Aldrich (St. Louis, MO, USA). Potassium acetate (KOAc, 99%) and
potassium carbonate (K2CO3, 99%) were purchased from Acros and used as received. Chloroform
(CHCl3, 99.5%), toluene (99.5%) and tetrahydrofuran (THF, 99%) were purchased from
Fisher/Acros and dried using molecular sieves under N2. Dicloromethane (DCM, 99.8%), hexane
(99%), absolute ethanol (99%), ethyl acetate (EA, 99%) and 1,4-dioxane were purchased from
Fisher/Acros and used as received.
2.2 Characterization
FTIR spectra, collected as the average of 264 scans with a resolution of 4 cm-1, were
1
recorded from KBr disk on the FTIR Bruker Tensor 27. H NMR spectra were recorded in
deuterated chloroform (CDCl3) with TMS as an internal reference, on a Bruker Avance 500 MHz.
Elemental analyses were performed by the Dumas combustion method, using a Costech ECS 4010
Elemental Analyzer. UV–vis absorption spectra of polymers in solution and polymer thin films were
recorded on a Shimadzu UV-2450 spectrometer over a wavelength range of 300 nm – 900 nm.
Solution UV-Vis absorption spectra were taken in 10-6 M concentration in chloroform solution.
Fluorescence spectra were measured on a HORIBA IHR 325 spectrometer.
2.3 Synthesis of Tris(4-bromophenyl)amine
Triphenylamine (1.00 g, 4.08 mmol) was added to 20 ml of anhydrous THF in a 50 ml two
o
neck round bottom flask. After cooling the solution to 0 C, N-bromosuccinimide (NBS) (2.61 g,
14.69 mmol) was added slowly to the reaction mixture. Then, the reaction mixture was warmed to
room temperature and stirred for overnight. After completion, the reaction was quenched by
5

addition of ice water and extracted with chloroform (60 ml). The organic layer was washed with
brine (150 ml), dried over anhydrous K2CO3, and finally isolated by filtration. The solvent was
rotary evaporated, and the product was precipitated with cold hexane to obtain the pure white
powder. The product was subsequently dried in a vacuum oven at RT to obtain the white powder
(Yield = 1.671 mg, 85%). 1H-NMR (500 MHz, CDCl3), δ (ppm): 7.35 (d, J = 8.5 Hz, 6H), 6.95 (d, J = 8.5
Hz, 6H).
2.4 Synthesis of Tris(4-(3-hexyl-2-(pyren-1-yl)thiophen-5-yl)phenyl)amine (Scheme 1a)
To a solution of Pd(OAc)2 (33.68 mg, 0.15 mmol), P(Cy)3.HBF4 (110.47 mg, 0.30 mmol),
PivOH (91.92 mg, 0.9 mmol), and Cs2CO3 (1.47 g, 4.5 mmol) in toluene (8 mL) in a two neck round
bottom flask were added tris(4-bromophenyl)amine (144.60 mg, 0.3 mmol) and 3-hexylthiophene
(302.94 mg, 1.8 mmol) under nitrogen. The reaction mixture was degassed by three free-pump-
thaw cycles and purged with nitrogen. Then the reaction was carried out at 110°C under N2 for
48h. After the oil bath had been removed, the corresponding 1-bromopyrene (421.73 mg, 1.5
mmol) and additional Cs2CO3 (1.47 g, 4.5 mmol) were added to the reaction mixture which was
then heated again to 110°C and stirred at the same temperature under N2 for another 24h. After
the reaction mixture had cooled to room temperature, the mixture was extracted with chloroform
three times (3 x 30 ml), and the combined organic layers were washed with 10% NaCl (100 ml),
dried over K2CO3 and isolated by filtration. The solvent was rotary evaporated, and the product
was purification by column chromatography on a silica gel column using a mixture of Hexane and
DCM (V/V=4:1). Finally, the product was precipitated with cold hexane to obtain the pure bright
yellow powder of T3HTP. The product was dried in a vacuum oven at RT. Yellow powder (Yield ~
1
5%). H-NMR (500 MHz, CDCl3), δ (ppm): 8.64 (d, J = 8.0 Hz, 3H), 8.22-7.86 (m, 30H), 7.53 (d, J =
8.0 Hz, 6H), 7.38 (s, 3H), 2.74 (t, J = 7.5 Hz, 6H), 1.73 (m, 6H), 1.28 (m, 18H), 0.87 (t, J = 6.5 Hz, 9H).
2.5 Synthesis of Tris(4-(3-hexyl-5-(pyren-1-yl)thiophen-2-yl)phenyl)amine (Scheme 1b)
To a solution of Pd(OAc)2 (33.68 mg, 0.15 mmol), P(Cy)3.HBF4 (110.47 mg, 0.30 mmol),
PivOH (91.92 mg, 0.9 mmol), and Cs2CO3 (1.47 g, 4.5 mmol) in toluene (8 mL) in a two neck round
bottom flask were added 1-bromopyrene (421.73 mg, 1.5 mmol) and 3-hexylthiophene (302.94
mg, 1.8 mmol) under nitrogen. The reaction mixture was degassed by three free-pump-thaw
cycles and purged with nitrogen. Then the reaction was carried out at 110°C under N2 for 6 h. After
the oil bath had been removed, the corresponding tris(4-bromophenyl)amine (144.60 mg, 0.3
mmol) and additional Cs2CO3 (1.47 g, 4.5 mmol) were added to the reaction mixture which was
then heated again to 110°C and stirred at the same temperature under N2 for another 48h. After
6

the reaction mixture had cooled to room temperature, the mixture was extracted with chloroform
three times (3 x 30 ml), and the combined organic layers were washed with 10% NaCl (100 ml),
dried over K2CO3 and isolated by filtration. The solvent was rotary evaporated, and the product
was purification by column chromatography on a silica gel column using a mixture of Hexane and
DCM (V/V=4:1). Finally, the product was precipitated with cold hexane to obtain the pure bright
yellow powder of T3HTP. The product was dried in a vacuum oven at RT. Yield = 51% (205.77 mg).
1H-NMR (500 MHz, CDCl3), δ (ppm): 8.64 (d, J = 8.0 Hz, 3H), 8.10-8.22(m, 30H), 7.53 (d, J = 8.0 Hz,
6H), 7.28 (s, 3H), 2.83 (t, J = 7.5 Hz, 6H), 1.77 (m, 6H), 1.28 (m, 18H), 0.87 (t, J = 6.5 Hz, 9H).
Analysis calculated for C96H81NS3: C, 85.74; H, 6.07; N, 1.04; S, 7.15. Found: C, 86.02; H, 6.14; N,
0.96, S, 6.88.
3. Results and Discussion
Synthesis and oligomer structure
The traditional coupling methods have been widely used to synthesize a broad range of π-
conjugated systems. However, we have noticed that they often require tedious preactivation of C-
H bonds using organometallic reagents which are costly, nonstable and with highly toxic
intermediate products. Therefore, in this report, we report a simple and straightforward synthesis
pathway where star-shaped conjugated oligomers can easily be obtained via sequential direct
arylation one-pot synthesis approach. The synthesis of the star-shaped conjugated oligomer
containing triphenyl amine, 3-hexylthiophene and pyrene is described in Scheme 1. To investigate
the most effective way to synthesize the oligomer via one-pot approach, we conducted a survey
on two synthetic routes for the star-shaped conjugated oligomer with the starting reactants tris(4-
bromophenyl)amine and 1-bromopyrene.
In the first synthetic route, as shown in Scheme 1a, compound 1 was generated through
direct arylation of tris(4-bromophenyl)amine (A) with 3-hexylthiophene (compound B). Without
isolation and purification, the intermediate compound 1 subsequently reacted with 1-
bromopyrene (compound C) via the second direct arylation process leading to the formation of
T3HTP-1. However, for the synthesis of T3HTP-1, we obtained a very low yield of around 5%. This
result indicated that the coupling reaction between 1-bromopyrene with the intermediate
compound 1 was not efficient. Thin-layer chromatography (TLC) results showed that the
intermediate compound 1 occupied as the main product in the final reaction mixture. Even though
the coupling reaction between 3-hexylthiophene and tris(4-bromophenyl)amine is well-known to
be efficient, this result confirmed that 1-bromopyrene does not have a preference for reacting
7

with position “5” of 3-hexylthiophene since 1-bromopyrene has electron-withdrawing property.
On the other side, we tried the second route (Scheme 1b) to synthesize T3HTP-2. In this route, the
starting reactant was 1-bromopyrene (compound C), which reacted with 3-hexylthiophene
(compound B) to synthesize compound (2). The reaction was performed in the presence of the
Pd(OAc)2 as a catalyst in anhydrous toluene at 110 °C under N2 for 6 h. Without isolation and
purification, the intermediate compound (2) subsequently reacted with tris(4-bromophenyl)amine
via direct arylation coupling reaction. The mixture was then heated again to 110 °C and stirred
under N2 for 48 h. The T3HTP-2 oligomer product has been obtained with a yield of 51% after
purification via column chromatography. To get more insight into the one-pot direct arylation
coupling reaction, we also performed a model direct coupling reaction between compound C and
compound B to investigate the effect of the reaction. The result showed a high yield of
intermediate product (compound 2) of about 85.7%. This result confirmed that 1-bromopyrene
has a preference for reacting with position “2” rather than position “5” of 3-hexylthiophene.
In the 1H-NMR spectrum of the obtained oligomer T3HTP-2 (Figure 1), the signals observed
at 7.53 and around 8.05 ppm (peaks r and q, respectively) were assignable to the aromatic protons
of triphenylamine ring, which are consistent with those at 6.95 ppm and 7.35 ppm for tris(4-
1
bromophenyl)amine. Moreover, the H-NMR spectrum of T3HTP-2 showed all the characteristic
peaks of 3-hexylthiophene and pyrene. Peak j at 7.28 ppm was attributed to the proton of the
thiophene ring, which was originally at 7.08 ppm for 4-hexyl-2-(pyren-1-yl)thiophene. In the
aromatic region, the broad multiplet peaks in the ranges from 8.10 to 8.22 ppm and that at 8.64
ppm were assigned to the protons of the pyrene ring. The result indicated that the star-shaped
oligomer T3HTP-2 was successfully synthesized.
Optical properties
The normalized optical absorption spectra of T3HTP-2 conjugated oligomer were
investigated in dilute solutions and in solid state film. Fig. 2A and B shows the UV-Vis spectra of
T3HTP-2 measured in THF at different concentrations (10 M, 20M, 30M, 40M) and in solid
state film, respectively. In THF, the T3HTP-2 solutions showed absorption maxima at 374 nm
attributed to triphenylamine, 3-hexylthiophene, and pyrene moieties. Based on this result, the
molar absorption coefficient “” of T3HTP-2was estimated according to Lambert-Beer law with
the value of 80890 M-1.cm-1. This high “” indicates high absorption for UVA light of T3HTP-2 due
8

to efficient - interactions of building units including triphenylamine, 3-hexylthiophene, and
pyrene moieties.
In addition, we compared the absorption spectra of T3HTP-2 in solution and in solid state
film (Fig. 2B). The T3HTP-2 solution was coated on a transparent glass substrate via the spin-
coating method. Interestingly, the solid-state film of T3HTP-2 exhibited an absorption maximum at
451 nm broadening to 530 nm. Compared with the absorption spectrum of the T3HTP-2 solution,
the absorption peak was red-shifted, which was ascribed to the formation of aggregation of
oligomer chains in the solid state as a result of the intermolecular π-π stacking of conjugated
moieties [43-45].
Fluorescence quenching studies with TNT in T3HTP-2 solution
In order to explore the sensory properties of the synthesized T3HTP-2 oligomer for
detecting explosive TNT nitroaromatic compound caused by Forster resonance energy transfer
(FRET) mechanism, we investigated the photoluminescent emission spectral changes of T3HTP-2 in
THF upon adding different amounts of TNT (Fig. 3A). A solution of T3HTP-2 in THF (CM = 10 M)
was prepared as the host stock solution, followed by addition of different amounts (0.2 mM – 60
mM) of TNT. The result showed that the fluorescence intensities decreased gradually with
successively increasing the concentration of TNT, as a result of the binding between TNT and
T3HTP-2 through efficient energy and electron transfer [17, 26, 46]. Fig. 4A presents the
fluorescence ratiometric responses of T3HTP-2 to trace TNT analyte. Clearly the fluorescence of
T3HTP-2 was quenched completely at 60 mM of TNT. It should be noted that the fluorescence
quenching behavior of the separated pyrene did not make sense for detecting TNT (Fig. 3B)
because of the lack of expanded  conjugated systems. The fluorescence spectrum of T3HTP-2 in
solution excited at the absorption maximum at 380 nm is shown in Fig. 4A. The fluorescence
quantum yield (ΦF) of the oligomer in dilute THF was measured in comparison to 9,10-
diphenylanthracence as a standard (in cyclohexane, ΦF = 0.9). The fluorescence quantum yield of
the oligomer was determined following the equation: ΦF (oligomers) = ΦF (Ref) . η2/η2
. I/A .
(Ref)
A(Ref)/I(Ref), where ΦF (Ref) is the quantum yield of the reference compound, η is the refractive index of
the solvent, I is the integrated fluorescence intensity and A is the absorbance at the excitation
wavelength. Accordingly, the quantum yield was estimated to be about 0.88 for T3HTP-2.
The fluorescence quenching property of T3HTP-2 was further investigated using THF as the
dispersing medium and TNT as the quencher. Fig. 4A showed the PL emission spectrum of T3HTP-2
solutions upon the incremental addition of TNT (with TNT concentrations from 0 mM to 60 mM).
9

The excitation wavelength for the T3HTP-2 solution samples was at 380 nm, leading to the high
emission at 530 nm assigned as excimer emission. This result confirms the expanded  conjugated
systems between pyrene, 3-hexyl thiophene and triphenyamine which causes the high
fluorescence property of T3HTP-2. Upon addition of TNT, the fluorescence intensity of the T3HTP-
2 solution decreased gradually. It was shown that 1 equivalent (eq.) of T3HTP-2 could detect
minimum 100 eq. of TNT, corresponding to a decrease in the PL intensity by 10% (Fig. 4A, cT3HTP-2
=
10 µM, cTNT = 1 mM) (Fig. 4A). The fluorescence emission of T3HTP-2 solution was efficiently
quenched with TNT concentration of 2 mM corresponding to a decrease by 15% of the PL
intensity. The fluorescence emission has been quenched completely at a TNT concentration of 60
mM.
In order to inspect the validity of Stern-Volmer quenching mechanism, the quenching
response was analyzed by fitting the data to the Stern–Volmer plot as shown in Fig. 4B. The Stern-
Volmer equation is given as:
퐼표
= 1 + 퐾푆푉퐶퐴
퐼
where I0 and I are the fluorescence intensities in the absence and presence of the analyte
(quencher, A), respectively, CA represents the molar concentration of the analyte, and KSV denotes
the Stern–Volmer quenching constant. The fluorescence quenching of T3HTP-2 induced by TNT
moieties had a distinct linear response to this equation in the concentration range of 0.2 – 60 mM
with a correlation coefficient of 0.992. The linear Stern-Volmer relation indicates that the probable
quenching mechanism of fluorescence of T3HTP-2 by TNT is a dynamic quenching procedure [47,
48]. T3HTP-2 exhibited a Stern‐Volmer quenching constant, Ksv, of 1.3 x 103 M-1 with a limit of
detection down to 0.2 mM based on the standard method reported [33]. This result showed that
T3HTP-2 exhibited relatively high sensitivity toward TNT, as compared to the star-shaped triphenyl
amine-based fluorophores reported by Niamnont et al. [33]. Therein, the fluorescence quenching
of T3HTP-2 was comparable to those of the fluorophores bearing corannulene and pyrene
moieties with triazole bridge (Ksv = 1.7 x 104 ‒ 6.4 x 103 M-1) and was much higher than than those
of the analogous fluorophores without triazole bridge Ksv = 8.4 x 102 ‒ 7.2 x 102 M-1) [33]. Based
on these results, it is suggested that fluorescence quenching was caused by interactions between
T3HTP-2 and TNT, as demonstrated in Fig. 5. As such, fluorescence quenching was initiated by the
dynamic quenching mechanism, where deactivation process occurred as the excited T3HTP-2
fluorophore bound with the TNT quencher in solution.
10

To further confirm the effect of fluorescence quenching of T3HTP-2 induced by TNT
moieties, we have compared the UV-vis spectra of T3HTP-2 in solution with and without the
presence of TNT. As seen in Fig. 6, the UV-Vis spectrum of T3HTP-2 + TNT solution exhibited the
absorption maximum peak at 335 nm, which was blue-shifted as compared with that of T3HTP-2.
This result can be explained by that the π-π conjugated intermolecular stacking was interrupted in
the presence of TNT moieties with the formation of the linkage between T3HTP-2 and the nitrogen
group of TNT. This phenomenon agrees with the well-known phenomenon where the absorption
maxima of conjugated polymers/oligomers is blue-shifted in the presence of strong acids.
It is well-known from the literature that the interactions of TNT with TNT-adsorbents can be
hydrogen bonding, π-π stacking, hydrophobic interaction, hydrogen bonding, donor-acceptor (D-
A) interaction (electron transfer) and π-dipole interaction [26, 33, 49-51].
Thus, based on the characteristic structures of T3HTP-2 and TNT, we proposed that the
binding of TNT to T3HTP-2 could occur via donor-acceptor interactions between electron-
poor nitro groups and electron-rich thiophene moieties with cooperative π-π interactions
between the pyrene and phenyl units with nitroaromatic ring, forming supramolecular
complexation (Fig. 5).
1
In addition, interactions between T3HTP-2 and TNT was confirmed by H NMR
spectroscopy of T3HTP-2 , which showed changes in positions and shapes of peaks attributed to
the proton signals of the pyrene, phenyl and thiophene units upon addition of 200 eq. of TNT
(Figure 7). The observation was consistent with those previously reported using the same analysis
methodology [33, 52]. It is noted that the downfield shift of thiophene proton singlet signal (Hj,
from 7.28 to 7.29 ppm, Fig. 7) and its change to overlapped mutiple peaks upon addition of TNT
suggests an interaction between Hj and the nitro group. The doublet signals attributed to phenyl
protons (peak r, Fig. 7) changed to multiple signals, indicating the π-π interaction between the
phenyl and nitroaromatic ring. The change in shape and upfield shifts of pyrene signals (peak a‒i),
Figure 7) are consistent with the interaction between the pyrene and nitroaromatic ring [52].
Normally, TNT powders have been mixed with starch for transportation by smugglers. For
practical application of T3HTP-2 for tracing TNT, we tesed the fluorescence properties of T3HTP-2
in the presence of starch and starch/TNT compound. The T3HTP-2 solution was prepared in a
concentration of 10 M for detecting TNT moieties that was mixed with starch. Fig. 8 shows
clearly that the fluorescence of T3HTP-2 was not impacted by starch while it was quenched in the
11

presence of starch/TNT. This result affirms that T3HTP-2 is promising for tracing TNT powders at
the customs boundary where smuggling situation takes place frequently. Last but not least, the
fluorescence property of the T3HTP-2 solution was not changed after 6 months, which could be
useful for practical application.
It is also worth mentioned that the detection of a couple of other nitro compounds besides
TNT, such as 2,6-dinitrotoluene (DNT) and 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) by T3HTP-2
using fluorescence quenching has also been preliminarily tested. It was interesting that efficient
quenching could only be achieved for DNT, whereas RDX was hardly detected by T3HTP-2. The
results suggested that T3HTP-2 is only suitable for sensing nitroaromatic explosive compounds.
4. Conclusion
In this research, we have successfully synthesized a novel three-armed star-shaped
conjugated oligomer T3HTP-2 (tris(4-(3-hexyl-5-(pyren-1-yl)thiophen-2-yl)phenyl)amine) in 51%
yield via one-pot sequential direct arylation approach. The molar absorption coefficient of T3HTP-
2 was 80890 M-1.cm-1. The obtained T3HTP-2 oligomer has been tested for tracing explosive TNT
moieties through FRET mechanism with high sensitivity (KSV = 1.3 x 103 M-1). The materials based
on the T3HTP-2 oligomer could be promising candidates for fundamental studies and technical
application for explosive-detecting chemosensors.
AUTHORS STATEMENT FOR PUBLICATION
Bao Kim Doan synthesized T3HTP-2 and characterized the structure and properties of this
compound. Tung Viet Tuan Tran synthesized T3HTP-1 and conducted tests for TNT detections in
solution. Tam Huu Nguyen synthesized monomers, conducted the model reacion and performed
UV-vis experiments. Tam Hoang Luu performed the PL experiments. Le-Thu T. Nguyen performed
the FTIR analysis. Toai Pham Van and Vuong Mau Nguyen tested the sensing of TNT mixed with
starch. Ha Tran Nguyen conceptualized the research, designed methodology, supervised the
experiments, wrote and reviewed the manuscript, and administrated the project.
Declaration of interests
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
12

Acknowledgment
This research was supported by The Department of Science and Technology (DOST) – Ho
Chi Minh City [grant number 23/2018/HĐ-QKHCN, signed on 04 December 2018].
13

References
[1] Y. Salinas, R. Martínez-Máñez, M.D. Marcos, F. Sancenón, A.M. Costero, M. Parra, S. Gil, Optical
chemosensors and reagents to detect explosives, Chem. Soc. Rev., 41 (2012) 1261-1296.
[2] S.W. Thomas, G.D. Joly, T.M. Swager, Chemical sensors based on amplifying fluorescent
conjugated polymers, Chem. Rev., 107 (2007) 1339-1386.
[3] K.G. Furton, L.J. Myers, The scientific foundation and efficacy of the use of canines as chemical
detectors for explosives, Talanta, 54 (2001) 487-500.
[4] J.M. Sylvia, J.A. Janni, J. Klein, K.M. Spencer, Surface-enhanced Raman detection of 2, 4-
dinitrotoluene impurity vapor as a marker to locate landmines, Anal. Chem., 72 (2000) 5834-5840.
[5] J. Yinon, Mass spectrometry of explosives: Nitro compounds, nitrate esters, and nitramines,
Mass Spectrom. Rev., 1 (1982) 257-307.
[6] S.F. Hallowell, Screening people for illicit substances: a survey of current portal technology,
Talanta, 54 (2001) 447-458.
[7] T.M. Griffin, N. Popkie Jr, M.A. Eagan, R.F. McAtee, D. Vrazel, J. McKinly, Instrument response
measurements of ion mobility spectrometers in situ: Maintaining optimal system performance of
fielded systems, in: Chemical and Biological Sensing VI, International Society for Optics and
Photonics, 2005, pp. 54-64.
[8] G. Eiceman, J. Stone, Peer reviewed: ion mobility spectrometers in national defense, in, ACS
Publications, 2004.
[9] K. Håkansson, R.V. Coorey, R.A. Zubarev, V.L. Talrose, P. Håkansson, Low‐mass ions observed in
plasma desorption mass spectrometry of high explosives, J. Mass Spectrom., 35 (2000) 337-346.
[10] M. Krausa, K. Schorb, Trace detection of 2, 4, 6-trinitrotoluene in the gaseous phase by cyclic
voltammetry, J. Electroanal. Chem., 461 (1999) 10-13.
[11] C.J. Cumming, C. Aker, M. Fisher, M. Fok, M.J. La Grone, D. Reust, M.G. Rockley, T.M. Swager,
E. Towers, V. Williams, Using novel fluorescent polymers as sensory materials for above-ground
sensing of chemical signature compounds emanating from buried landmines, IEEE transactions on
geoscience and remote sensing, 39 (2001) 1119-1128.
[12] S.J. Toal, W.C. Trogler, Polymer sensors for nitroaromatic explosives detection, J. Mater.
Chem., 16 (2006) 2871-2883.
[13] T.M. Swager, The molecular wire approach to sensory signal amplification, Acc. Chem. Res.,
31 (1998) 201-207.
[14] J.-S. Yang, T.M. Swager, Porous shape persistent fluorescent polymer films: an approach to
TNT sensory materials, J. Am. Chem. Soc., 120 (1998) 5321-5322.
[15] D. Gao, Z. Wang, B. Liu, L. Ni, M. Wu, Z. Zhang, Resonance energy transfer-amplifying
fluorescence quenching at the surface of silica nanoparticles toward ultrasensitive detection of
TNT, Anal. Chem., 80 (2008) 8545-8553.
[16] D.T. McQuade, A.E. Pullen, T.M. Swager, Conjugated polymer-based chemical sensors, Chem.
Rev., 100 (2000) 2537-2574.
[17] D.A. Olley, E.J. Wren, G. Vamvounis, M.J. Fernee, X. Wang, P.L. Burn, P. Meredith, P.E. Shaw,
Explosive sensing with fluorescent dendrimers: the role of collisional quenching, Chem. Mater., 23
(2010) 789-794.
14

[18] P.M. Cotts, T.M. Swager, Q. Zhou, Equilibrium flexibility of a rigid linear conjugated polymer,
Macromolecules, 29 (1996) 7323-7328.
[19] J.-S. Yang, T.M. Swager, Fluorescent porous polymer films as TNT chemosensors: electronic
and structural effects, J. Am. Chem. Soc., 120 (1998) 11864-11873.
[20] S. Yamaguchi, T.M. Swager, Oxidative cyclization of bis (biaryl) acetylenes: synthesis and
photophysics of dibenzo [g, p] chrysene-based fluorescent polymers, J. Am. Chem. Soc., 123 (2001)
12087-12088.
[21] X. Sun, Y. Liu, G. Shaw, A. Carrier, S. Dey, J. Zhao, Y. Lei, Fundamental study of electrospun
pyrene–polyethersulfone nanofibers using mixed solvents for sensitive and selective explosives
detection in aqueous solution, ACS Appl. Mater. Interfaces, 7 (2015) 13189-13197.
[22] C.-P. Chang, C.-Y. Chao, J.H. Huang, A.-K. Li, C.-S. Hsu, M.-S. Lin, B.R. Hsieh, A.-C. Su,
Fluorescent conjugated polymer films as TNT chemosensors, Synth. Met., 144 (2004) 297-301.
[23] I.A. Levitsky, W.B. Euler, N. Tokranova, A. Rose, Fluorescent polymer-porous silicon
microcavity devices for explosive detection, Appl. Phys. Lett., 90 (2007) 041904.
[24] S. Zahn, T.M. Swager, Three‐dimensional electronic delocalization in chiral conjugated
polymers, Angew. Chem. Int. Ed., 41 (2002) 4225-4230.
[25] D. Zhao, T.M. Swager, Sensory responses in solution vs solid state: a fluorescence quenching
study of poly (iptycenebutadiynylene) s, Macromolecules, 38 (2005) 9377-9384.
[26] G. He, N. Yan, J. Yang, H. Wang, L. Ding, S. Yin, Y. Fang, Pyrene-containing conjugated
polymer-based fluorescent films for highly sensitive and selective sensing of TNT in aqueous
medium, Macromolecules, 44 (2011) 4759-4766.
[27] S. Chen, Q. Zhang, Q. Liu, J. Gu, L. Zhang, J. Zhou, X. Fan, L. Fang, Optical bandgaps and
fluorescence resonance energy transfer studies of a series of poly (phenyleneethynylene)
derivatives, React. Funct. Polym., 71 (2011) 1008-1015.
[28] J.P. Amara, T.M. Swager, Synthesis and properties of poly (phenylene ethynylene) s with
pendant hexafluoro-2-propanol groups, Macromolecules, 38 (2005) 9091-9094.
[29] L. Feng, H. Li, Y. Qu, C. Lü, Detection of TNT based on conjugated polymer encapsulated in
mesoporous silica nanoparticles through FRET, Chem. Commun., 48 (2012) 4633-4635.
[30] Y. Long, H. Chen, Y. Yang, H. Wang, Y. Yang, N. Li, K. Li, J. Pei, F. Liu, Electrospun nanofibrous
film doped with a conjugated polymer for DNT fluorescence sensor, Macromolecules, 42 (2009)
6501-6509.
[31] L. Chen, D. McBranch, R. Wang, D. Whitten, Surfactant-induced modification of quenching of
conjugated polymer fluorescence by electron acceptors: applications for chemical sensing, Chem.
Phys. Lett., 330 (2000) 27-33.
[32] X. Wu, H. Hang, H. Li, Y. Chen, H. Tong, L. Wang, Water-dispersible hyperbranched conjugated
polymer nanoparticles with sulfonate terminal groups for amplified fluorescence sensing of trace
TNT in aqueous solution, Materials Chemistry Frontiers, 1 (2017) 1875-1880.
[33] N. Niamnont, N. Kimpitak, K. Wongravee, P. Rashatasakhon, K.K. Baldridge, J.S. Siegel, M.
Sukwattanasinitt, Tunable star-shaped triphenylamine fluorophores for fluorescence quenching
detection and identification of nitro-aromatic explosives, Chem. Commun., 49 (2013) 780-782.
15

[34] J. Peng, K. Ye, J. Sun, Y. Zhan, J. Jia, P. Xue, G. Zhang, Z. Zhang, R. Lu, Branched
benzothiadiazole-cored oligomers with terminal carbazoles: Synthesis and fluorescence probing
nitroaromatics, Dyes Pigm., 116 (2015) 36-45.
[35] M. Ansari, R. Bera, S. Mondal, N. Das, Triptycene-Derived Photoresponsive Fluorescent Azo-
Polymer as Chemosensor for Picric Acid Detection, ACS Omega, 4 (2019) 9383-9392.
[36] T. Xu, R. Lu, X. Liu, X. Zheng, X. Qiu, Y. Zhao, Phosphorus(V) Porphyrins with Axial Carbazole-
Based Dendritic Substituents, Org. Lett., 9 (2007) 797-800.
[37] Z. Fei, B. Li, Z. Bo, R. Lu, Synthesis and Properties of Hyperbranched Conjugated Porphyrins,
Org. Lett., 6 (2004) 4703-4706.
[38] T.H. Xu, R. Lu, X.P. Qiu, X.L. Liu, P.C. Xue, C.H. Tan, C.Y. Bao, Y.Y. Zhao, Synthesis and
Characterization of Carbazole-Based Dendrimers with Porphyrin Cores, Eur. J. Org. Chem., 2006
(2006) 4014-4020.
[39] Y.H. Lee, H. Liu, J.Y. Lee, S.H. Kim, S.K. Kim, J.L. Sessler, Y. Kim, J.S. Kim, Dipyrenylcalix [4]
arene—a fluorescence‐based chemosensor for trinitroaromatic explosives, Chemistry–A European
Journal, 16 (2010) 5895-5901.
[40] A. Chowdhury, P.S. Mukherjee, Electron-Rich Triphenylamine-Based Sensors for Picric Acid
Detection, J. Org. Chem., 80 (2015) 4064-4075.
[41] H. Wu, F. Tao, Y. Cui, L. Guo, Two triphenylamine-Based AIE materials for the detection of PA
in aqueous medium, Mater. Chem. Phys., 240 (2020) 122141.
[42] Q. Wang, H. Liu, C. Jiang, H. Liu, Silsesquioxane-based triphenylamine functionalized porous
polymer for CO2, I2 capture and nitro-aromatics detection, Polymer, 186 (2020) 122004.
[43] N. DiCésare, M. Belletête, M. Leclerc, G. Durocher, Intermolecular Interactions in conjugated
oligothiophenes. 2. quantum chemical calculations performed on crystalline structures of
terthiophene and substituted terthiophenes, J. Phys. Chem. A, 103 (1999) 803-811.
[44] A. Ruini, M.J. Caldas, G. Bussi, E. Molinari, Solid state effects on exciton states and optical
properties of PPV, Phys. Rev. Lett., 88 (2002) 206403.
[45] A. Ferretti, A. Ruini, E. Molinari, M.J. Caldas, Electronic properties of polymer crystals: the
effect of interchain interactions, Phys. Rev. Lett., 90 (2003) 086401.
[46] H. Cavaye, P.E. Shaw, X. Wang, P.L. Burn, S.-C. Lo, P. Meredith, Effect of dimensionality in
dendrimeric and polymeric fluorescent materials for detecting explosives, Macromolecules, 43
(2010) 10253-10261.
[47] H.S. Geethanjali, D. Nagaraja, R.M. Melavanki, Exploring the mechanism of fluorescence
quenching in two biologically active boronic acid derivatives using Stern-Volmer kinetics, J. Mol.
Liq., 209 (2015) 669-675.
[48] M. Shaikh, Y.M. Swamy, H. Pal, Supramolecular host–guest interaction of acridine dye with
cyclodextrin macrocycles: Photophysical, pKa shift and quenching study, Journal of
Photochemistry and Photobiology A: Chemistry, 258 (2013) 41-50.
[49] Z. Ye, Q. Meng, S. Lu, Adsorption of 2,4,6-trinitrotoluene on carboxylated porous polystyrene
microspheres, Appl. Surf. Sci., 258 (2012) 3624-3628.
[50] P. Das, S.K. Mandal, Strategic Design and Functionalization of an Amine-Decorated
Luminescent Metal Organic Framework for Selective Gas/Vapor Sorption and Nanomolar Sensing
of 2,4,6-Trinitrophenol in Water, ACS Appl. Mater. Interfaces, 10 (2018) 25360-25371.
16

[51] Y. Wang, L. Zhang, L. Yang, G. Chang, An indole-based smart aerogel for simultaneous visual
detection and removal of trinitrotoluene in water via synergistic effect of dipole-π and donor-
acceptor interactions, Chem. Eng. J., 384 (2020) 123358.
[52] Y.H. Lee, H. Liu, J.Y. Lee, S.H. Kim, S.K. Kim, J.L. Sessler, Y. Kim, J.S. Kim,
Dipyrenylcalix[4]arene—A Fluorescence-Based Chemosensor for Trinitroaromatic Explosives,
Chemistry – A European Journal, 16 (2010) 5895-5901.
17

Figure 1: 1H-NMR spectrum of T3HTP-2 synthesized via one-pot approach.
Figure 2. UV-vis absorption spectra of T3HTP-2 at different concentrations in THF with a path
length of 1 cm (A); UV − visible spectra of T3HTP-2 in THF and in solid state film (B).
18

Figure 3. Fluorescence quenching of T3HTP-2 in THF (10 M) (A) and pyrene (B) upon addition of
TNT.
70000
80
70
60
50
40
30
20
10
0
0 mM
0.2 mM
1 mM
2 mM
5 mM
10 mM
15 mM
30 mM
60 mM
A
60000
50000
40000
30000
20000
10000
0
B
0
20
40
60
80
TNT Concentration (mM)
350
400
450
500
550
600
650
700
750
800
Wavelength (nm)
Figure 4. Fluorescence emission spectra of T3HTP-2 (10 M in THF) in the presence of TNT at
various concentrations (A); Stern-Volmer plot for quenching of T3HTP-2 by TNT (B).
19

Figure 5. Proposed schematic diagram of T3HTP-2–TNT supramolecular complexation.
Figure 6. UV-vis absorption spectra of T3HTP-2 and T3HTP-2 + TNT in THF with a path length of 1
cm.
20

Figure 7. 1H NMR spectra of T3HTP-2 (in CDCl3) in the absence and presence of TNT.
Figure 8. Fluorescence emission spectra of T3HTP-2 (10 M in THF) in the presence of starch and
starch/TNT mixture.
21

Scheme 1: Synthetic routes for the star-shaped conjugated oligomers T3HTP-1 and T3HTP-2.
22