1,2,3-Triazolyl functionalized thiophene, carbazole and fluorene based A-: Alt -B type π-conjugated copolymers for the sensitive and selective detection of aqueous and vapor phase nitroaromatics (NACs)

Article abstract of DOI:10.1039/d0tc03310c

A series of highly emissive π-conjugated A-alt-B type copolymers (P1-P3) appended with a 1,2,3-triazole moiety was synthesized via Suzuki polymerization. The well-defined and soluble π-conjugated copolymers were characterized via multinuclear NMR spectroscopy and tetradetector GPC studies, showing a molecular weight (Mn) in the range of 16.4-20.1 kDa with a polydispersity index in the range of 1.25-1.42. The synthesized emissive π-conjugated polymer probes were explored as fluorescent chemosensors for nitroaromatic compounds (NACs) in solution, vapor and contact mode. Detailed photophysical and sensing studies were performed to understand the polymer-NAC interaction, inducing the selective fluorescence quenching of the π-conjugated polymer probes through the photoinduced electron transfer (PET) mechanism. All the polymeric probes (P1-P3) were highly reversible in nature with NACs, and thus could be reused multiple times. The limit of detection of the probes towards nitroaromatics was found to be in the range of 120-200 ppb with a high association constant in the order of 104 M-1. Furthermore, test paper kits were also fabricated, which allowed the trace detection of picric acid by the naked eye, making it a practical means for the quick, easy and inexpensive on-site detection of NAC-based explosives.

Full text of DOI:10.1039/d0tc03310c

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ISSN 2050-7526
PAPER
Nosang V. Myung, Yong-Ho Choa et al.
A
noble gas sensor platform: linear dense assemblies of
a multi-layered
single-walled carbon nanotubes (LACNTs) in
ceramic/metal electrode system (MLES)
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DOI: 10.1039/D0TC03310C
ARTICLE
1,2,3-Triazolyl functionalized thiophene, carbazole and fluorene
based A-alt-B type π-conjugated copolymers for sensitive and
selective detection of aqueous and vapor phase nitroaromatics
(NACs)
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Dipanjan Giri a and Sanjib K. Patra* a
A series of highly emissive π-conjugated A-alt-B type copolymers (P1-P3), appended with 1,2,3-triazole moiety, have been
synthesized via Suzuki polymerization. The well-defined and soluble π-conjugated copolymers have been characterized by
multinuclear NMR spectra as well as tetradetector GPC studies, showing molecular weight (Mn) in the range of 16.4-20.1
kDa with polydispersity indices of 1.25-1.42. The synthesized emissive π-conjugated polymer probes have been explored as
fluorescent chemosensor for nitroaromatic compounds (NACs) in solution, vapor and contact mode. The detailed
photophysical and sensing studies have been studied to understand the insight of polymer-NACs interaction, inducing
selective fluorescence quenching of the π-conjugated polymer probes, through photoinduced electron transfers (PET)
mechanism. All the polymeric probes (P1-P3) are highly reversible in nature with NACs, and can be reusable for multiple
times. The limit of detection of the probes towards nitroaromatics is found to be in the range of 120-200 ppb with high
degree of association constant in the order of 104 M-1. Furthermore, test paper kits have also been fabricated demonstrating
trace detection of picric acid by naked eye, making it as a practical means for a quick, easy and inexpensive way of on-site
detection of NAC based explosives.
(HPLC),14-15 ion mobility spectrometry (IMS),16-17 Raman or
surface enhanced Raman scattering, surface Plasmon
resonance,18-19 energy dispersive X-ray diffraction,20-21 neutron
Introduction
Detection of explosives has gained enormous interest for
national security. Most of the explosive materials are also
recognized as toxic contaminants for environmental pollution.1-
activation analysis,22 electron capture detection,23 and cyclic
voltammetry24-26 are being used for the detection of trace NAC
explosives for long times. However, each approach has its own
drawbacks. Bulky instrumentation, operational inconvenience,
analysis cost and test speed of detection restrict its broad
applications. In the recent years, design and development of
novel and simple fluorescent chemosensors for selective
detection of NACs have been acknowledged much more
attention, due to its quick response to the analytes, operational
simplicity, high sensitivity and selectivity, cost-effectiveness and
synthetic ease. Therefore, the interest in developing simple and
highly sensitive optical fluorescent chemosensory probes for
the selective and easy recognition of NACs is high in demand.
Electron-withdrawing nature of the nitro groups on the
aromatic ring in NACs lower down the energy of the empty π*
orbitals, and thereby making nitroaromatic compounds as good
electron acceptors. This property of the nitroaromatics has
been exploited for its detection by the p-type fluorescent
chemosensory probes.27-28 A number of fluorescent probes for
detection of nitroaromatics have been developed by various
groups. Most of the fluorescent chemosensors for detection of
NACs are based on small molecule chemosensors, gels,
nanoparticle, nano-fibers and MOFs.29-45 However, compared to
the small molecular organic chemosensor, p-type π-conjugated
polymer based probe possesses several important advantages.
5
Use of explosives in a large scale in the recent years has
prompted the scientific community to develop novel sensing
materials for rapid, sensitive and selective detection of
explosives both in air and solution for the homeland security
and environmental concern.6-8 Mostly used explosive materials
are nitro-rich compounds such as 2,4,6-trinitrotoluene (TNT),
picric acid (PA), 2,4-dinitrotoluene (DNT) and 3,5-
trinitroperhydro-1,3,5-triazine (commonly known as RDX).9-11
Thus the development of new materials for the easy, rapid,
sensitive, selective, economic and trace detection of NACs for
the purpose of preventing environmental pollution is in high
demand. Nitroaromatic vapor is also hazardous and cause
headache, weakness, anaemia, respiratory disorders,
carcinogenicity, skin irritation and liver injury.1-5 There are
numbers of commercial sophisticated instrumental methods
such as gas chromatography(GC) coupled with different
detectors,12-13 high performance liquid chromatography
a. Department of Chemistry, IIT Kharagpur, Kharagpur 721302, WB, INDIA
E-mail: skpatra@chem.iitkgp.ac.in; Tel: +91 3222 283338.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: Materials and methods,
Synthesis, Spectroscopic characterization and data, Photophysical studies,
Electrochemical and FESEM data etc. See DOI: 10.1039/x0xx00000x
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The electron rich nature of the π-conjugated polymer, makes
them a potential candidate for the detection of NACs explosive
with high efficiency, and has been received much attention in
the recent years. The high sensitivity of the electron rich π-
conjugated polymers originates from the fast and facile exciton
migration along the entire polymer chain as a result of
molecular wire effect as demonstrated by Swager and
coworkers.46-47 Taking the advantage of molecular wire effect,
various conjugated polymer based detection probes have been
developed by different group for NACs detection. In the recent
past, significant efforts have been focused on the design and
construction of electron rich p-type π-conjugated polymers to
detect electron-deficient nitroaromatic compounds.48-61
In a previous work, we have demonstrated the role of
directly attached 1,2,3-triazole to the thiophene unit towards
NACs detection.62 Herein, we report facile and easy synthesis of
highly luminescent A-alt-B type π-conjugated copolymers with
appended 1,2,3-triazole moiety through Suzuki polymerization.
The 1,2,3-triazole functional moiety attached via -CH2-O-CMe2-
to the thiophene unit plays a crucial role in enhancing the
detection ability towards NACs. The detail photophysial and
spectroscopic studies of the synthesized polymer robes have
been carried out showing remarkable trace detection of NACs
with high sensitivity and efficiency through fluorescence
quenching.
View Article Online
DOI: 10.1039/D0TC03310C
Scheme
thiophene monomer.
1
Synthesis of 1,2,3-triazole functionalized
characterization and purity of the monomer (4) were
determined by multinuclear NMR and mass spectrometric
1
analyses (ESI†). H NMR of
4 in CDCl3 shows two multiplets at
7.39-7.37 and 7.30-7.28 ppm for the phenyl protons. The thienyl
protons resonate at 6.88 ppm as singlet. The signal for triazolyl
proton overlaps with multiplets of the aromatic protons. The
(Th)CH2-O- (Th = thienyl) and -CH2-Ph (Ph = phenyl) protons
resonate at 5.53 and 4.15 ppm respectively, whereas the six
methyl protons resonate as singlet at 1.46 ppm. 13C{1H}NMR
confirms the formation of triazole unit by showing resonance at
119.7 ppm for the ′′C-H′′ of the triazole ring. The other aromatic
carbons resonate in the range of 152.7-128.1. Formation of 4
was further confirmed by HRMS study by showing the molecular
ion peak (M+H)+ at 471.9539. Another key precursors, the
diboronic ester functionalized carbazole, fluorene and
thiophene monomers
(M1, M2 and M3) for Suzuki
polymerization were obtained by borylation of the dibromo-
derivatives (
addition
6
,
8
and 10) on treatment with n-BuLi followed by
of
2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (iPrO-Bpin) in dry THF at -78 °C with 68-78% yield
(Scheme S2 in ESI†).68-69 The dibromo-derivatives (
and 10)
Results and discussion
6
,
8
were synthesized from the commercially available carbazole,
fluorene and 3-bromothiophene respectively following the
previously reported methods (ESI†). Characterization and purity
of the monomers (M1-M3) were determined by multinuclear
NMR and mass spectrometry analyses (ESI†).
Synthesis and Characterization
The π-conjugated polymer having thiophene, fluorene,
carbazole units are one of the most studied π-conjugated
emissive polymers in the recent time as polymer based
fluorescence sensing probes due to their strong and tunable
emission with high quantum yield. The use of thiophene,
fluorene, carbazole units itself remains very limited due to its
tendency to from aggregates under high concentration or in the
solid state leading to fluorescence quenching. There is always a
challenge to prevent such aggregation. However, by introducing
long chain alkyl groups at suitable position on the polymer
backbone, one can restrict such aggregation. Electron-donating
groups are always preferred because of their ability to increase
the electron density on the conjugated polymer backbone and
thereby enhancing the electron-transfer ability towards the
electron-deficient NACs. Another way to increase the sensing
behavior is by incorporating the dipolar units such as 1,2,3-
triazole unit (5 Debye)63-64 as a pendant group. The triazole unit
was favored to take the advantage of facile supramolecular
interaction with the nitroaromatics providing a straight forward
route to enhance the sensory response of π-conjugated
polymer towards NACs by tuning electronic property of the
polymer backbone.65-67
To access the corresponding A-alt-B type copolymers (P1
-
P3), the poly-condensation of analytically pure monomers
4 and
M1 M2 M3 was carried out in degassed THF using aqueous
/
/
potassium carbonate as base and Pd(PPh3)4(2 mol%) as catalyst
following Suzuki coupling polymerization protocol as outlined in
Scheme 2. The polymerization reaction was carried out under
refluxing condition with vigorous stirring for 48 h under inert
atmosphere. The crude polymers were isolated by precipitation
in methanol from their concentrated CHCl3 solution. It was
further purified through Soxhlet extractor using MeOH,
followed by hexanes to remove the oligomers and catalyst
residue. Finally, the well-defined copolymers were obtained
from subsequent extraction in distilled chloroform. The
synthesized π-conjugated A-alt-B type copolymers with long
alkyl groups are readily soluble in common organic solvents
such as THF, toluene and chlorinated solvents and exhibits
strong blue, cyan and yellow fluorescence emission respectively
in solution. Interestingly, all the synthesized A-alt-B type
copolymers exhibit appreciable amount of fluorescence even in
solid state (as thin film).
The key starting precursor, 2,5-dibromo functionalized
1,2,3-triazole appended thiophene monomer
synthesized from 3-methylthiophene involving stepwise
bromonation, cycloaddition and hydration respectively as
(
4
)
was
The
synthesized
π-conjugated
polymers
were
unambiguously characterized by multinuclear NMR, FTIR and
1
tetradetector Gel Permeation Chromatography (GPC). H NMR
outlined in scheme
1 (and scheme S1 in ESI†). The
spectrum (in CDCl3) of the polymer P1 exhibits multiplets in the
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Scheme 2 Synthesis of A-alt-B type copolymers (P1
-P3).
region of 8.40-8.32 and 7.76-7.62 ppm for the carbazole properties of the polymers, P1
-
P3. TGA was carried out in
protons. The triazole proton resonates as broad singlet at 7.45 nitrogen atmosphere at heating rate of 10 °C/min from RT to
ppm. The signal for phenyl and thienyl proton overlaps and
resonate as multiplets in the range of 7.19-7.05 ppm. The
(Th)CH2-O- (Th = thienyl) protons resonate at 5.28 ppm as broad
singlet, whereas the -N-CH2(heptyl) protons of the heptyl
groups attached to 9-position of carbazole and -CH2Ph protons
resonate at 4.40-4.25 ppm as multiplet. The remaining heptyl
protons appear in the region of 1.95 to 0.86 ppm. The aromatic
carbons of P1 resonate at 143.2-120.0 ppm, whereas the alkyl
carbons resonate in the range of 59.3-14.1 ppm. The
characteristic resonance for “C-H” in triazolyl ring is observed at
119.6 ppm in 13C{1H} spectra. The FTIR spectroscopy further
confirms the existence of 1,2,3-triazole unit by showing bands
at 1635 and 1458 cm-1, characteristic for triazole moiety. Similar
1
characteristic observation was noted in H and 13C {1H} NMR
spectra for the polymers P2 and P3. The formation of the
polymers was further confirmed by tetradetector Gel
Permeation Chromatography as shown in Fig. 1a. The number-
Fig. 1 (a) Tetradetector GPC trace (RI response) of the
synthesized polymers (P1-P3), using THF as eluent.
average molecular weights (Mn) of the well-defined π-
conjugated polymers (P1-P3) are found to be in the range of
16.4-20.1 kDa with the very good polydispersity indices of 1.25-
1.42 respectively (Table 1). The molecular weight of the
synthesized p-type π-conjugated copolymers (P1-P3) is high
Table 1 Characterization data for P1 - P3
Polymer
P1
Mn (kDa)
16.4
Mw (kDa)a
21.9
PDIa
1.34
1.42
1.25
Tdc (oC)b
340
342
P2
18.4
26.2
P3
20.1
25.1
360
aObtained from GPC (gel permission chromatography), bCalculated
from TGA studies.cPolymer decomposition temperature.
enough to explore efficient sensing properties towards
nitroaromatics.
Thermal properties of the polymers are essential to
investigate the stability of the polymers. Thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC)
experiments were performed to determine the thermal
700 °C to find out the thermal stability of the polymers. The
onset point of decomposition (Td) was determined from the
temperature at 5% weight loss. The decomposition
Table 2 Photophysical data of the polymers P1, P2 and P3.
Polymer
Absorbance (nm)
Solution
Emission (nm)
Solution
Stoke’s shift (cm-1)
Solid
Solid
Solution
Solid
λmax (ε x 10-4)
λmax
λem
Φ (%)a
λem
P1
325 (2.11)
333
458
18
467
8935
8616
P2
P3
391 (2.15)
406 (2.03)
397
419
464, 489
551
20
19
488
566
5125
6481
4697
6198
aMeasured by relative method with quinine sulphate (in 0.1 M sulphuric acid) as the reference (Φ = 0.54).
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temperatures of the polymers (P1-P3) was found to be 340 °C,
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DOI: 10.1039/D0TC03310C
342 °C and 360 °C respectively as shown in Figure S17. All the
new polymers exhibit good thermal stability, suitable for
applications in explosive detections. Noticeable glass transition
temperature was not observed for the polymers during DSC
analysis.
(a)
Photophysical properties
Excellent solubility of the synthesized A-alt-B type
copolymers allowed us to evaluate the photophysical properties
of the polymers with UV-vis and fluorescence spectra. The
photophysical properties of the synthesized π-conjugated
copolymers were conducted in CHCl3 in 2×10-5 M concentration
as depicted in Fig. 2. In solution phase, polymer P1 exhibits a
well resolved intense absorption at λmax of 325 nm with ɛ values
of 2.11 x 104 M-1cm-1, which is attributed to the π-π* transition
through the π-conjugated backbone. The λmax for P2 and P3 are
red shifted by 66 and 81 nm respectively compared to that of
polymer P1 suggesting higher degree of π-conjugation in
fluorene-alt-thiophene and thiophene-alt-thiophene based
copolymers with relatively lower optical band gap. In thin film
(made by spin coating on quartz substrate), the absorption
(b)
maxima of the polymers P1, P2 and P3 were observed at 333,
397 and 419 nm respectively. The π-conjugated polymers P1
and P2 exhibit strong blue to cyan emission centered at 458 nm
(λex=325 nm) and 464 nm (λex=391 nm) for P1 and P2
respectively. For P2, a shoulder at 489 nm along with strong
emission at 464 nm was also observed.70-71 Interestingly,
polymer P3 shows a strong yellow emission at λem of 551 nm.
The photoluminiscent quantum yields (Ф) of the polymers are
in the range of 18 - 20% in THF as measured by relative method
with quinine sulphate (in 0.1M sulphuric acid) as reference.72
The solid state (as film) photophysical studies of the polymers
were also inspected. More interestingly, all the A-alt-B type
copolymers are highly emissive in blue to yellow spectral region
even as thin film showing λem at 467, 488 and 566 nm
respectively with Stoke's shift in the range of 4697-8935 cm-1. A
red shift by ca. 6-13 nm (in absorption spectra) and 9-24 nm (in
PL spectra), relative to that in solution state, was observed
presumably due to the enlarged π-conjugation of the polymer
backbone in solid state. The relevant photophysical data of the
polymers are tabulated in Table 2.
(c)
P1
P2
P3
Fig. 2 (a) Solution state (solid line) and solid state (dotted
line) UV-vis spectra for the polymers P1 P3 in CHCl3 with a
concentration of
10-5 M. (b) Solution and solid state (spin
coated on quartz plate from CHCl3 solution of polymers) PL
spectra for the polymers at 28 °C. (c) Photograph of
-
∼
Detection of nitroaromatics
The excellent and intense emission properties and favorable
HOMO-LUMO energy levels of the highly soluble copolymers
polymer P1, P2 and P3 in chloroform under 365 nm light)
P1, P2 and P3 prompted us to explore the potential applicability
as a chemosensors towards the detection of nitroaromatic
(NAC) compounds utilizing the fluorescence emission as a
measure of polymer-nitroaromatics interaction. Nitroaromatics
are electron deficient in nature and can undergo strong π-π
interaction with the electron rich p-type aromatic rings. The
high stability and the electron rich π-conjugated backbone of
the synthesized A-alt-B type copolymers make a supreme
candidate for sensing of nitroaromatic explosives through the
fluorescence quenching taking the advantage of molecular wire
effect of the π-conjugated polymers.
To investigate the optimum concentration in which
fluorescence quenching through self-aggregation of the π-
conjugated polymeric probes through π-π stacking is minimal,
the concentration dependent emission spectra as well as
dynamic light scattering (DLS) studies have been carried out.
Polymer concentrations are reported as moles of repeating unit
per litre. With increasing concentration from 10-5 to 10-3 M in
CHCl3, the emission intensity for P1-P3 decreased significantly
(Figure S19). The emission intensity of the probes also started
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to decrease when the solution was diluted to 10-6 M. DLS studies than the other nitroaromatics. Hence, the polymVeierwpArroticbleeOcnlainne
DOI: 10.1039/D0TC03310C
of the polymer probes were inspected by varying the selectively recognize the picric acid among the other
concentration, ranging from 10-3 to 10-5 M. The average nitroaromatics. The relative emission quenching ability of
aggregate size (DH) of 8-15 nm for the three polymer probes various electron deficient nitroaromatics towards P1-P3 probes
confirmed the negligible self-aggregation and the presence of are summarized as bar diagram in Fig. 4. It is interesting to
mainly unimers of the polymeric probes in relatively dilute observe that the percentage of quenching for the NACs is
solution of ca. 10-5 M in CHCl3 (Figure S20). Henceforth, the significantly higher for the fluorene-alt-thiophene based
sensing application was explored in the concentration of 10-5 M copolymer
(
P2
)
than the carbazole-alt-thiophene and
to avoid the self-aggregation behaviour of the probes.
thiophene-alt-thiophene based copolymers (P1 and P3). This
clearly demonstrates that the π-conjugated polymer backbone
of fluorene-thiophene unit and the dipolar triazole unit
facilitates supramolecular interaction between the polymer
probe and the nitroaromatics inducing fluorescence quenching
(vide supra).
Fig. 3 Emission quenching of polymer P2 towards different
nitroaromatics and non-nitroaromatics in CHCl3. Toluene:
TOL, 1,2-dichlorobenzene: DCB, Nitrobenzene: NB,
Nitrotoluene: NT, Nitrobenzoic acid: NBA, p-Hydroxy
nitrobenzene: HNB, 2,4-Dinitrophenol: DNP, 2.6-
Dinitrobenzene:DNT,
Picric
acid:
PA.
Analyte
concentration: ~10-5 M, polymer concentration: ~1 x 10-5 M
in CHCl3.
Fig. 4 Percentage of emission quenching for P1–P3 towards
different nitroaromatics (NACs) and non-nitroaromatic
compounds in CHCl3.
Polymers probes P1-P3 were treated with various
nitroaromatic compounds such as nitrobenzene, nitrotoluene,
nitrobenzoic acid, p-hydroxynitrobenzene, 2,4-dinitrophenol,
2,6-dinitrotoluene and picric acid. Solution state fluorescence
quenching of the polymer probes were witnessed by treating of
polymer probe in chloroform with known concentration of
nitroaromatic compounds. However, there is no effect on
change of fluorescence intensity upon addition of other
aromatic compounds, such as toluene and dichlorobenzene
even in excess equivalent, demonstrating selectivity of the
polymer probes as chemosensing probes towards
nitroaromatics. Due to the electron deficient nature of the
nitroaromatics, the electron transfer process between the
electron rich fluorophore and the electron deficient NAC
molecule becomes facile and the fluorescence intensity is
readily quenched upon gradual addition of nitroaromatics. Fig.
3 represents the fluorescence quenching profile of polymers P2
upon the addition of various analyte solution of ca. 10-5
,
M
Fig. 5 Fluorescence titration upon incremental addition of
PA (concentration range of ~1.1 × 10-5 M to ~1.2 ×10-4 M)
to P2 polymer probe (~1 x 10-5 M in CHCl3) with visual
change in fluorescence.
concentration to the solution of polymer probes (~1 x 10-5 M).
The different percentage of quenching was observed with
different nitroaromatics for all the three copolymers.
Interestingly, the percentage of quenching was significantly
higher for picric acid, having the highest number of nitro group
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(a)
Fig. 6 Fluorescence lifetime studies of the polymer probe
P2 in presence of picric acid.
(b)
To investigate the insight of quenching phenomenon and to
analyze the mechanistic interaction between the chemosensory
probes and nitroaromatics analyte, the fluorescence titration
has been carried out towards various nitroaromatics in ca. 10-5
M concentration in CHCl3. The intensity of the fluorescence
band centred at 458, 464 and 551 nm were dramatically
decreased for the polymers P1, P2 and P3 respectively, upon
the incremental addition of picric acid (Fig. 5 for P2). The
fluorescence quenching was clearly visible with naked eye
under the illumination of 365 nm light The fluorescence lifetime
studies have also been investigated for both the probes at
different concentrations of picric acid to study the mode of the
interaction. The fluorescence lifetime values of P2 (Table S1, Fig.
6) was found to be invariant at different concentration of picric
acid, suggesting static mechanism of fluorescence quenching.
Similar observation was also witnessed for P1 and P3 probes.
Fig. 7 a) The Stern-Volmer plot for P2. b) Comparison of
association constants of P1-P3 for different nitroaromatics
(1: NB, 2: NT, 3: NBA, 4: HNB, 5: DNP, 6: DNT, 7: PA).
The quenching behaviour due to the polymer-
nitroaromatics supramolecular interaction was also analyzed in
detail by the Stern-Volmer (SV) equation I0/I =1 + KSV[Q],73
where I and I0 are the fluorescence intensities in the presence
and absence of the quencher [Q] respectively and KSV is the
Stern-Volmer rate constant. Upon treatment with different
concentration of picric acid, a linear SV plot was obtained for
the polymer probe (P2) having fluorene-alt-thiophene unit as
shown in Fig. 7a. The decrease in the fluorescence intensity with
the linear SV-plot suggests for static fluorescence quenching of
the conjugated polymer probes in the presence of NACs. The KSV
value was found to be in the order of nitrobenzene (NB) <
nitrotoluene (NT) < nitrobenzoic acid (NBA) < p-hydroxy
nitrobenzene (HNB)
< 2,4-dinitrophenol (DNP) < 2,6-
dinitrotoluene (DNT) <picric acid (PA). The higher KSV value for
PA (6.40 × 104 M-1) and thus the higher percentage of quenching
compared to other NACs could be attributed to the higher
sensitivity of the fluorophore towards PA (Fig. 7b). It is
noteworthy to mention that the quenching rate constant
towards the picric acid is higher for P2 than that for the other
polymer probes P1 and P3 (Figure S27 and S28). As expected,
this result also indicates that the sensory responses towards the
Fig. 8 Plot (change in fluorescence intensity for the
emission at λem of 464 nm against the concentration of
PA) for evaluation of LOD for sensing PA by P2.
different nitroaromatics by the polymer P2 is highly sensitive
than the probes P1 and P3, as also observed from the relative
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percentage of quenching described in Fig. 4 (vide infra). absorption spectra), and were found to be -2.81, -2.58 and -2.26
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Moreover, the limit of detection (LOD)74 of polymer P2 for for P1 P2, and P3 respectively. The eleDctOrIo: c10h.e10m39ic/aDl0TdCa0t3a31a0rCe
,
nitroaromatics is found to be in the range of 900 to 525 nM summarized in Table 3. The LUMO of the analyte (PA) lies at -
(206-120 ppb) with the lowest value for picric acid (Fig. 8). While 3.89 eV. The higher energy levels of LUMO of the polymer
under the similar set of experimental condition, the polymer P1 probes than that of NACs facilitates electron transfer from the
and P3 shows ca.10-100 times less sensory responses towards polymer probes (P1-P3) to the electron deficient NACs and
the different nitroaromatic compounds.
induces photo-induced electron transfer (PET) followed by
reverse electron transfer to the HOMO of the probe in a non-
radiative process, resulting in fluorescence quenching as
depicted in Fig. 9.7-8 To understand further, the HOMO-LUMO
energy levels were also calculated by Density Functional Theory
(DFT) using B3LYP functional and polarized 6-31 G⁄ basis set for
the model monomer systems (Figure S48-50).82-86 The extent of
fluorescence quenching is relatively less for the other
nitroaromatics (such as NT, DNP, and DNT) having the LUMO
energy levels relatively higher in the range of -3.48 to -2.91 eV.31
Thus, the probes show higher sensitivity in recognizing picric
acid over other nitroromatics.
Table 3 Electrochemical data and HOMO and LUMO energy levels of
P1, P2 and P3.
Polymer
Eox, V
EHOMO
,
ELUMO, (eV)b
Egopt(eV)c
(eV)a
P1
P2
P3
0.67
0.90
0.74
-4.96
-5.19
-5.03
-2.15
-2.61
-2.77
2.81
2.58
2.26
aFrom the onset of the oxidation potential, and using the equation,
EHOMO (eV) = [Eoxonset - Eox(ferrocene)] - 4.80. b(ELUMO = EHOMO + Egopt). cEg
from solid state absorption spectra.
opt
Fig. 9 Proposed electronic transfer through PET leading to
PL quenching of fluorophore (P2) in the presence of PA. (ext.
= excitation, r = radiative, nr = non-radiative).
It is assumed that the favorable HOMO-LUMO energy levels
of the polymer probes, facile dipole-dipole and π-π interactions
with the electro deficient nitroaromatic analytes play a
synergistic effect inducing fluorescence quenching. The cyclic
voltammetric experiments of these π-conjugated copolymers
were conducted to investigate the electrochemical properties
of the polymers and to evaluate the HOMO and LUMO energy
levels. The cyclic voltammograms of thin film, drop-casted onto
glassy carbon disc electrode from the DCM solutions of the
polymers were shown in Figure S44. The cyclic voltammetric
measurement was carried out in dry CH3CN using nBu4NPF6 (0.1
M) as supporting electrolytes, glassy carbon disc working
electrode, Pt wire counter electrode and Ag/AgCl reference
electrode under argon atmosphere. The onset oxidation
Fig. 10 Change in chemical shift of the protons in P2 in 1H
NMR titration in CDCl3 on incremental addition of PA (* =
residual CHCl3).
The static fluorescence quenching, as confirmed by
fluorescence life time measurements and Stern-Volmer plot
(vide infra), allowed us to examine the supramolecular
interaction between the π-conjugated polymeric probe and the
1
electron deficient NACs by H NMR spectroscopic titration
1
studies. The H NMR titration for the polymer probe P2 has
been investigated with incremental addition of picric acid (PA)
(Fig. 10). The thienyl, fluorenyl and triazolyl protons
experienced a gradual shift upon incremental addition of picric
acid due to supramolecular complexation. Most interestingly,
the triazolyl proton (He) experienced down field shift by 0.02
ppm presumably due to hydrogen bonding with nitro groups of
NAC molecules, whereas the thienyl (Hd) and fluorenyl aromatic
protons (Ha, Ha′, Hb, Hb′, Hc, Hc′) were slightly up fielded
suggesting π-π intermolecular interactions between the
fluorene-thiophene unit of P2 probe and the aromatic ring of
PA.31,34,39 Similar results were obtained for other polymer
potentials of P1
respectively. HOMO energy levels of P1
-
P3 were located at 0.67, 0.90 and 0.74 V
P3 were calculated as -
-
4.96, -5.19 and -5.03 eV respectively from the onset oxidation
potentials considering the known reference level for ferrocene
(4.80 eV below the vacuum level) according to the equation;
EHOMO (eV) = [Eoxonset - Eox(ferrocene)] - 4.80.75-78 Whereas, the ELUMO
energy levels of P1-P3 were evaluated as -2.15, -2.61 and -2.77
eV respectively using the relation, ELUMO = EHOMO + Eg .
opt 79-81 The
opt
optical band gaps (Eg were estimated by considering the
equation, Egopt =1240/λcut-off (λcut-off is the edge of the solid state
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probes (P1 and P3) during the NMR titration with PA as demonstrate the reusability of the probe, the filmVwiewasAreticxlpe oOnsleinde
DOI: 10.1039/D0TC03310C
expected (Fig. S42-S43).
to vapor of picric acid for 200 s followed by repeated washing
with ethanol:water mixture and drying under hot air. The
emission spectra were recorded for the polymer film before and
after washing out the picric acid analyte. The whole process was
repeated for multiple times. The initial fluorescence intensity
was significantly regained and the quenching efficiency of 85%
was observed even after multiple cycles of exposure to the
picric acid, indicating its reusability and stability of the polymer
film (Fig. 12, Figure S35-S36).
The successful repeatability studies of the polymer film
towards picric acid vapor, encouraged us to explore the ability
of the sensing probes for detecting trace picric acid present as
contamination in water. Nitroaromatics are well known as toxic
and hazardous contaminants to environment results polluting
soil and drinking water.1-5 The percentage of fluorescence
quenching was determined after dipping the polymer (P2) film
into the aqueous samples of picric acid varying concentration
(Figure S34). The study reveals that the fluorescence emission
intensity is significantly quenched by ca. 64% when the
concentration of the picric acid is reached to 100 μM.
Moreover, similar results were observed for the other polymer
films as depicted in Figure S32-S33, with slightly less detection
efficiency. The scanning electron microscopic (SEM) image
analyses of the polymer probe samples (prepared by drop
casting of 5 x 10-4 M chloroform solution on aluminum
substrate) were conducted in presence and absence of picric
acid and its derivative to understand the change in surface
(b)
morphology. The sensing probes (free P1, P2 and P3) showed
porous film like surface morphology which is essential to be a
good nitroaromatic sensor (ESI†). The porous morphology of
the probes changed to irregular shape morphology upon
treatment with picric acid in FESEM analysis as shown in Figure
S45-S47. The Energy Dispersive X-ray (EDX) line analysis showed
the presence of bromine while treating with the derivative of
Fig. 11 a) Quenching of fluorescence intensity of thin film
samples of P2 upon exposure to PA vapor by varying
exposure time. b) Superiority of P2 over P1 and P3 as
indicated by relative quenching efficiency (in %) as a
function of the exposure time to picric acid vapor.
The excellent ‘turn-off’ sensing behaviour of the probes (P1
-
P3) towards NACs motivated us to explore their practical
application of the polymer probes for on-site detection of picric
acid. The potential utility of the sensing probes toward PA was
investigated in solid state by making a thin film of all the
synthesized A-alt-B type copolymers. Thin films of the probes
were casted from chlorobenzene solution (5 mg in 1 ml
chlorobenzene) onto quartz plates through spin coating at a
rate of 1200 rpm. The sensory response of polymer thin films
towards nitroaromatics was carried out by the decrease in
fluorescence intensity of polymer thin films upon increasing of
the exposure time with the vapor of picric acid in atmospheric
pressure (Fig. 11a). It was observed that after exposing the
polymer film for about 200 s, the fluorescent intensity was
diminished by 91% for P2, while for P1 and P3 the quenching
was relatively lower to 53% and 48% respectively (Fig. 11b),
demonstrating superiority of P2 probe over the others. To
Fig. 12 Cycles of emission quenching and recovery by
exposing the probe film to PA vapor for 200 s
demonstrating reusability.
8 | J. Name., 2012, 00, 1-3
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PA (2,4,6-trinitrophenyl 3-bromobenzoate), further confirming conjugated polymers efficiently recognize electron deficient
View Article Online
DOI: 10.1039/D0TC03310C
the formation of polymer:NAC adduct.
nitroaromatics both in solution and as solid state through
fluorescence quenching with high sensitivity and efficiency. The
In recent days, contact mode approach has received great facile HOMO/LUMO energy levels and photophysical studies
attention for real time application in practical field to find the suggest static fluorescence quenching through PET mechanism
trace amount of nitroaromatics explosive residue. Therefore, to by supramolecular complex formation between the polymer
1
demonstrate its practical applicability, the filter paper probe and NAC molecules, as manifested by H NMR titration.
(Whatman 42) test strips were dipped into a concentrated All the polymers show high sensitivity towards picric acid
chloroform solution of the polymer probes and dried under exhibiting ppb level detection with binding constant in the order
reduced pressure. Picric acid crystals were placed on the of 104 M-1. The sensing ability of the polymer probes towards
polymer coated test strips for 10-15 s. To visualize the contact trace NACs as filter paper strips has also been demonstrated for
mode responses of nitroaromatics, the polymer coated strips its practical utility as on-site detection kit by naked eye. Further
were illuminated under 365 nm light with the appearance of development of the π-conjugated polymeric probes for
black spots on the contact area with picric acid. The visual detection of NAC explosives with enhanced sensitivity is
change in the fluorescence intensity of the polymer coated film underway.
strip provides an easy way to detect nitroaromatics. To
demonstrate the trace detection of NAC analytes in solution
state, different concentrations (10-3–10-9 M) of picric acid in
chloroform were prepared and 2 µL of each solution was
spotted on each freshly prepared test paper strip (Fig. 13). The
visual colour change in fluorescence responses with different
concentration of PA were recorded upon exposure to 365 nm
light exhibiting dark black spot of PA. The black spots were
found to be prominent for the concentrated analyte solution
and faded upon dilution. Remarkably the minimum detection
limit of the picric acid by the naked eyes is even up to 10-9 M
level for polymer P2, showing an excellent chemosensor for the
instant visual detection for practical application in detecting
trace amount of picric acid in solution as well as in vapor phase.
Similar experiment was also carried out for the polymer P1 and
P3 (Figure S37-S38).
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Authors thank to ER & IPR, DRDO, New Delhi (ERIP/ER/DG-
ACE/991115202/M/01/1734) for financial support. SKP thanks
IIT Kharagpur (IIT KGP) for funding the purchase of
multidetector
GPC
through
Competitive
Research
Infrastructure Seed Grant (IIT/SRIC/CHY/NPA/2014-15/81).
Central Research Facility, IIT KGP is gratefully acknowledged for
the analytical facility. DG acknowledges IIT KGP for doctoral
fellowship.
Experimental Section
Syntheses
Pd(PPh3)4 was synthesized following the procedure described in the
literature.87 Details syntheses and characterization of Copper(II)-
hydroxyapatite(Cu-HAP),88 compounds 1, 2,89 and the monomers
M1–M3 have been described in ESI†. Compounds 5, 6,90 7, 8,91 9 and
1092 were synthesized from the commercially available carbazole,
fluorene and 3-bromo thiophene respectively following the
previously reported methods.
Compound 3: Cu-HAP (1.2 g, 25 wt%) was added to a stirring mixture
of 2-Methyl-4-(trimethylsilyl)-3-butyn-2-ol (4.5 g, 29.2 mmol), benzyl
bromide (5.0 g, 29.2 mmol), TBAF (catalytic amount) and sodium
azide (2.5 g, 37.9 mmol) in THF/H2O (1:1) mixture at room
temperature. The reaction mixture was stirred for 14 h and after that
the progress of the reaction was monitored by TLC
(hexane/ethylacetate eluent 4:1). After completion of the reaction,
the desired product was separated by extraction with DCM from the
reaction mixture. The product was obtained after concentration of
the DCM solution as highly pure crystalline compounds with 76%
Fig. 13 Visual appearance of P2 coated filter paper before
and after the drop casting of different concentrations of PA
by contact mode method under illumination at 365 nm.
Conclusions
In conclusion, we have synthesized a series of highly
emissive carbazole, fluorene and thiophene containing A-alt-B
type π-conjugated copolymers appended with 1,2,3-triazolyl
moiety via Suzuki polymerization. The synthesized polymers
have been unambiguously characterized by various
spectroscopic tools and their detail photophysical properties
have been investigated. The triazolyl functionalized π-
1
yield (4.5 g). H NMR (400 MHz, CDCl3): δ (ppm) 7.40-7.37 (m, 3H,
phenyl), 7.34 (s, 1H, triazolyl), 7.29-7.27 (m, 2H, phenyl), 5.50 (s, 2H,
-CH2-Ph), 1.61 (s, 6H, -CH3). 13C{1H} NMR (100 MHz, CDCl3): δ (ppm)
134.1, 131.4, 129.1, 128.8, 128.1 (aromatic carbons), 120.7 (triazol –
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CH-), 73.4(-C(CH3)2), 54.2 (-CH2-Ph), 27.1 (-CH3). MALDI-TOF (m/z): 30.0, 29.2, 27.3, 23.9, 22.6(-CH2, heptyl), 14.1(-CH3, heptyl). UV-vis
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4
C12H15N3O, calculated value 218.539 (M+H)+, found 218.534 (M+H)+. (THF): λmax (ε, M-1cm-1): 325 nm (2.11 × 10 ). PL(CHCl3): λem(λex): 458
DOI: 10.1039/D0TC03310C
Compound 4: To solution of 2,5-Dibromo-3- (350). GPC: Mw = 21.9 kDa, Mn = 16.4 kDa, PDI = 1.34, Td = 340 °C.
a
(bromomethyl)thiophene (2) (500 mg, 1.3 mmol), in anhydrous DMF Polymer P2: 1H NMR (600 MHz, CDCl3): δ 7.82-7.73 (m, 2H, fluorene),
(10 ml), tBuOK (240 mg, 2.0 mmol) and 2-(1-Benzyl-1H-1,2,3-triazol- 7.59-7.53 (m, 4H, fluorene), 7.42 (bs, 1H, triazolyl), 7.35-7.27 (m, 6H,
4-yl)propan-2-ol (357 mg, 1.6 mmol) were added at 0 °C. The reaction phenyl + thienyl), 5.52-5.51 (bs, 2H, -CH2-O-), 4.44-4.42 (bs, 2H, -
mixture was stirred at 90 °C for 12 h. After consumption of thiophene CH2Ph), 2.08-2.03 (m, 4H, octyl), 1.76-1.74 (m, 6H, octyl), 1.12-1.10
substrate (the reaction was monitored by TLC, n- (m, 25H, octyl), 0.82-0.79 (m, 6H, octyl). 13C{1H} NMR (125 MHz,
hexane:ethylacetate was used as the eluent) the reaction mixture CDCl3): δ 153.3, 151.8, 151.4, 143.2, 141.4, 140.3, 139.8, 135.6,
was poured into cold water (200 ml), extracted with ethyl acetate (3 134.7, 133.1, 131.9, 128.7, 128.6, 127.9, 127.2, 126.0, 125.6, 124.7,
× 50 ml) and dried over anhydrous MgSO4. The solvent was removed 123.5, 120.1, 120.0 (aromatic rings), 119.6 (-CH, triazole), 59.3(-CH2-
under diminished pressure and the residue was purified by column O), 55.4, 54.1, 53.4, 40.6, 31.8, 30.0, 29.2, 27.3, 23.6, 22.6(-CH2,
chromatography eluting with hexanes to afford a white solid with 78 octyl), 14.1(-CH3, octyl). UV-vis (THF): λmax (ε, M-1cm-1): 391 nm (2.15
1
% yield (470 mg). H NMR (400 MHz, CDCl3): δ (ppm) 7.39-7.37 (m, × 104), PL(CHCl3): λem(λex): 464, 489 (400). GPC: Mw = 26.2 kDa, Mn =
4H, phenyl, triazolyl), 7.30-7.28 (m, 2H, phenyl), 6.88 (s, 1H, thienyl), 18.4 kDa, PDI = 1.42, Td = 342 °C.
5.53 (s, 2H, -CH2-O-), 4.15 (s, 2H, -CH2-), 1.46 (s, 6H, methyl protons). Polymer P3: 1H NMR (600 MHz, CDCl3): δ 7.37-7.29 (m, 5H, phenyl),
13C{1H} NMR (100 MHz, CDCl3): δ (ppm) 152.7, 139.7, 134.6, 131.0, 7.21 (bs, 1H, triazolyl), 7.04-6.98 (m, 2H, thienyl), 5.53-5.47 (m, 2H, -
129.1, 128.8, 128.1,111.0, 109.2 (aromatic carbons), 120.7 (triazolyl CH2-O-), 4.24-4.19 (m, 2H, -CH2-Ph), 2.60-2.46 (m, 2H, hexyl), 1.73-
-CH), 73.4, 59.6 (-CH2-O-), 54.2(-CH2-Ph), 27.1(-CH3). HRMS (m/z): 1.61 (m, 4H, hexyl), 1.31-1.25 (m, 12H, hexyl), 0.89-0.84 (m, 3H,
C17H17Br2N3OS, calculated value 471.9537 (M+H)+, found 471.9539 hexyl). 13C{1H} NMR (125 MHz, CDCl3): δ 153.0, 143.4, 139.6, 134.8,
(M+H)+.
129.1, 127.8, 126.8, 125.4, 123.6 (aromatic rings), 119.8 (-CH,
General synthesis of polymers: Compound 4 (0.18 mmol) and triazole), 61.0(-CH2-O), 59.1, 54.1, 31.6, 30.7, 29.7, 29.0, 27.1, 23.4,
M1/M2/M3 (0.18 mmol) were dissolved in 10 mL of distilled THF in 22.4(-CH2, hexyl), 14.1(-CH3, hexyl). UV-vis (THF): λmax (ε, M-1cm-1):
a 100 mL Schlenk flask under argon atmosphere. The reaction flask 406 nm (2.03 × 104), PL(CHCl3): λem(λex): 551 (400). GPC: Mw = 25.1
was degassed three times via freeze-pump-thaw technique. In an kDa, Mn = 20.1 kDa, PDI = 1.25, Td = 360 °C.
another 100 mL Schlenk flask, K2CO3 (0.74 g, 5.35 mmol) was
dissolved in 4 mL of mili-Q water and degassed by purging Ar through
a needle for about 30 min. After that, the K2CO3 solution was
transferred to the reaction mixture through a cannula. Next,
Pd(PPh3)4 (4 mg, 0.0036 mmol, 2 mol%) was added to it and the
Caution: The permissible exposure limit of picric acid was strictly
followed for sensing studies.
reaction mixture was warmed to 65 °C for 72h while stirring. The
colour of the reaction mixture turned to dark green during the course
of the reaction. After cooling to room temperature, the solvent was
concentrated to minimum volume of ca. 2 mL, and polymer was
Notes and references
1
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precipitated to
a stirring methanol solution. After complete
precipitation the methanol was removed and the brown colour
polymer was washed another two times with methanol, followed by
drying under vacuum. Next, the polymer was purified through
Soxhlet extraction using distilled hexanes, methanol and at last
collected in dry and distilled chloroform. The chloroform part was
evaporated to get the dark greenish brown colored polymer. The
polymers were finally purified by fractional precipitation in cold
distilled hexanes from a concentrated dichloromethane solution to
achieve well-defined and pure P1, P2 and P3 with yield of 58%, 62%
and 55% respectively.
Polymer P1: 1H NMR (600 MHz, CDCl3): δ 8.40-8.32 (m, 2H,
carbazole), 7.76-7.62 (m, 2H, carbazole), 7.45 (bs, 1H, triazolyl), 7.42-
7.37 (m, 2H, carbazole), 7.19-7.05 (m, 6H, phenyl, thienyl), 5.28 (bs,
2H, -CH2-O-), 4.40-4.25 (m, 4H, -N-CH2-, -CH2Ph), 1.95-1.88 (m, 2H,
heptyl), 1.76-1.69 (m, 6H, heptyl), 1.28-1.25 (m, 10H, heptyl), 0.89-
0.86 (heptyl). 13C{1H} NMR (125 MHz, CDCl3): δ 143.2, 141.4, 140.3,
139.8, 135.6, 134.7, 133.1, 131.9, 128.8, 128.1, 127.6, 125.6, 124.7,
123.5, 120.1, 120.0 (aromatic rings), 119.6 (-CH, triazole), 73.3 (-
NCH2, heptyl), 59.3 (-CH2-O), 55.4(-CH3), 54.1(-CH3), 53.4, 40.6, 31.8,
4
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