Poly(benzodithieno-imidazole-alt-carbazole) based π-conjugated copolymers: Highly selective and sensitive turn-off fluorescent probes for Hg2+

Article abstract of DOI:10.1016/j.polymer.2018.10.069

In this work, highly emissive and soluble π-conjugated poly(benzodithieno-imidazole-alt-carbazole) π-conjugated copolymers (P1 and P2) have been synthesized through Pd(0)-catalyzed Suzuki coupling polymerization protocol from the corresponding monomers. For solubilizing in common organic solvents, heptyl (in P1) or 2-(2-(2-methoxyethoxy)ethoxy)ethyl (in P2) substituents have been introduced at benzodithieno-imidazole and 9-position of carbazole unit. The well-defined and soluble π-conjugated polymers have been characterized by multinuclear NMR spectra as well as by tetradetector GPC studies showing molecular weight (M_n) of 16.5 and 14.3 kDa with the polydispersity indices of 1.36 and 1.31 respectively. The emissive π-conjugated polymers show remarkable selectivity and sensitivity towards Hg²⁺ ion as turn-off fluorescent probes as a result of molecular wire effect, without interference by other competing metal ions. The detailed photophysical and sensing studies have been demonstrated to understand the insight of polymer-metal ion interaction which is responsible for selective fluorescence quenching of the π-conjugated polymers. DFT studies have been performed to gain the insight of the electronic properties of the sensor by determining the frontier molecular orbitals and the HOMO-LUMO energy levels for a model probe (M1). The polymeric probes (P1 and P2) are reversible in nature, and can also act as secondary sensors for S^2-. The limit of detection of the polymer probes towards Hg²⁺ is found to be in the range of 40–50 ppb with high association constant in the order of 10⁵ M^?1 suggesting its high applicability as chemosensor for Hg²⁺. The polymers have also been explored as thin film polymeric sensor towards Hg²⁺ to demonstrate its practical utility.

Full text of DOI:10.1016/j.polymer.2018.10.069

Accepted Manuscript
Poly(Benzodithieno-imidazole-alt-carbazole) based π-conjugated copolymers: Highly
2+
selective and sensitive turn-off fluorescent probes for Hg
Dipanjan Giri, Arindam Bankura, Sanjib K. Patra
PII:
S0032-3861(18)31003-6
DOI:
https://doi.org/10.1016/j.polymer.2018.10.069
JPOL 21018
Reference:
To appear in: Polymer
Received Date: 13 August 2018
Revised Date: 25 October 2018
Accepted Date: 30 October 2018
Please cite this article as: Giri D, Bankura A, Patra SK, Poly(Benzodithieno-imidazole-alt-carbazole)
2+
based π-conjugated copolymers: Highly selective and sensitive turn-off fluorescent probes for Hg
Polymer (2018), doi: https://doi.org/10.1016/j.polymer.2018.10.069.
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ACCEPTED MANUSCRIPT
Poly(Benzodithieno-imidazole-alt-carbazole) based π-conjugated copolymers: Highly
selective and sensitive turn-off fluorescent probes for Hg2+
Dipanjan Giria and Sanjib K. Patra*a
aDepartment of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur-721302, WB,
INDIA, E-mail: skpatra@chem.iitkgp.ac.in; Tel: +913222283338
Graphical abstract
Poly(benzodithieno-imidazole-alt-carbazole) based π-conjugated copolymer probes with
nitrogen and sulphur binding sites exhibit remarkable selectivity and sensitivity towards Hg2+ as
turn off fluorescent chemosensor.

ACCEPTED MANUSCRIPT
Poly(benzodithieno-imidazole-alt-carbazole) based π-conjugated copolymers: Highly
selective and sensitive turn-off fluorescent probes for Hg2+
Dipanjan Giri,a Arindam Bankurab and Sanjib K. Patra*a
aDepartment of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur-721302, WB,
INDIA, E-mail: skpatra@chem.iitkgp.ac.in; Tel: +913222283338
bDepartment of Chemistry, Central University of South Bihar, Gaya-824236, Bihar, INDIA
Abstract
In this work, highly emissive and soluble π-conjugated poly(benzodithieno-imidazole-alt-
carbazole) π-conjugated copolymers (P1 and P2) have been synthesized through Pd(0)-catalyzed
Suzuki coupling polymerization protocol from the corresponding monomers. For solubilizing in
common organic solvents, heptyl (in P1) or 2-(2-(2-methoxyethoxy)ethoxy)ethyl (in P2)
substituents have been introduced at benzodithieno-imidazole and 9-position of carbazole unit.
The well-defined and soluble π-conjugated polymers have been characterized by multinuclear
NMR spectra as well as by tetradetector GPC studies showing molecular weight (Mn) of 16.5 and
14.3 kDa with the polydispersity indices of 1.36 and 1.31 respectively. The emissive π-
conjugated polymers show remarkable selectivity and sensitivity towards Hg2+ ion as turn-off
fluorescent probes as a result of molecular wire effect, without interference by other competing
metal ions. The detailed photophysical and sensing studies have been demonstrated to
understand the insight of polymer-metal ion interaction which is responsible for selective
fluorescence quenching of the π-conjugated polymers. DFT studies have been performed to gain
the insight of the electronic properties of the sensor by determining the frontier molecular
orbitals and the HOMO-LUMO energy levels for a model probe (M1). The polymeric probes
(P1 and P2) are reversible in nature, and can also act as secondary sensors for S2-. The limit of
detection of the polymer probes towards Hg2+ is found to be in the range of 40-50 ppb with high
association constant in the order of 105 M-1 suggesting its high applicability as chemosensor for
Hg2+. The polymers have also been explored as thin film polymeric sensor towards Hg2+ to
demonstrate its practical utility.
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Introduction
In the recent years, the development of chemosensors for the recognition of biologically
and environmentally concerned toxic metal ions, especially Hg2+, has attracted considerable
research interest, due to its detrimental effects on the human health and natural ecosystem [1].
Mercury contaminations are originated from both natural and anthropogenic activities such as
volcanic eructation, mining waste, leather tanning and from several industrial products such as
battery, painting, electrical equipments [2]. Marine aquatic organisms convert inorganic mercury
into neurotoxin methyl-mercury, that bioaccumulates through the food chain in the tissues of fish
and marine mammals. The results lead to a severe human health problems such as brain damage,
kidney failure, Minamata disease, Hunter-Russell syndrome, damaging to the central nervous
system, various cognitive and motion disorders [3]. United States Environmental Protection
Agency (EPA) fixes the upper limit of Hg2+ in drinking water as 10 nM (2 ppb) [4]. In this
context, the environmental contamination resulting from mercury ion is alarming attention
throughout the world. Therefore, it is necessary to build up convenient and straightforward
sensors for rapid, sensitive, and versatile detection of mercury ion with no interference.
Various research groups have developed different analytical methods for the detection of
toxic analytes, such as atomic absorption spectroscopy [5], cold vapour atomic fluorescence
spectroscopy [6], gas chromatography [7], inductively coupled plasma mass spectroscopy [8],
neutron activation analysis [9], and electrochemical [10] with good sensitivities and selectivities.
However, each approach has some drawbacks due to bulky instrumentation, operational
inconvenience, analysis cost and test speed of detection limiting their applications. In the recent
years, design and construction of novel and simple fluorescent and colorimetric chemosensors
for selective detection of environmentally toxic heavy metal ions including Hg2+ have paid much
more attention due to their quick response to the analyte, operational simplicity, high sensitivity
and selectivity, cost-effectiveness and synthetic ease [11]. Therefore, the interest in developing
simple and highly sensitive optical fluorescent chemosensors with turn-off or turn-on fluorescent
strategy for specific recognition of a heavy toxic analyte has grown steadily for chemical,
biological, and medical applications.
For the detection of toxic Hg2+ a number of chemosensors have been developed by
various groups. Most of the fluorescent chemosensors for Hg2+ are based on small molecules,
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such as cationic porphyrin core [12], BODIPY based [13], Rhodamine B [14], coumarin [15],
pyrene core [16], cyclodextrin based probes [17], DNA-functionalized probes [18],
functionalized calixarene [19], fluorescein [20], aza-oxacrown and cryptand derivatives [21].
However, compared to the small molecular organic chemosensor, π-conjugated polymers based
optical chemosensors possesses several important advantages. The high sensitivity of the π-
conjugated polymers originates from the sensory signal amplification which attributed to energy
migration along the π-conjugated polymer backbone upon light excitations as a result of
molecular wire effect as demonstrated by the pioneering work by Swagar [22]. In the recent
years, a number of polymer based probes for the recognition of metal ions have been developed
by various research groups [23]. In the recent past, significant efforts have been focused on the
design and construction of conjugated polymer based sensory materials with appropriate
receptors unit to detect the toxic mercury metal ions [24]. However, there are few reports for
sensing toxic Hg2+ by π-conjugated polymer based sensor as efficient “turn-off” fluorescence
strategy with high selectivity and sensitivity [25].
Herein, we report design and synthetic protocol of highly soluble and emissive
poly(benzodithieno-imidazole-alt-carbazole) π-conjugated copolymers through Pd(0)-catalyzed
Suzuki coupling polymerization. The copolymer probes having nitrogen and sulphur donor
atoms act as appropriate chelating sites for selective sensing of Hg2+ ions. For solubilizing in
common organic solvents and in semi-aqueous media, heptyl or 2-(2-(2-
methoxyethoxy)ethoxy)ethyl side groups have been introduced respectively to benzodithieno-
imidazole and carbazole skeleton. The corresponding polymers have been investigated for
sensory applications towards Hg2+ as a thin-film polymeric sensor.
Results and discussion
Synthesis and characterization
A good and appropriate chelating or a binding site attached to π-conjugated polymer
backbone is essential to be a selective and efficient fluorescent chemosensor for toxic metal ions.
The favorable binding-site improves the binding constant and as well as enhancing the
fluorescence response towards the targeted toxic metal ions. For this reason, the benzodithieno-
imidazole based ligand containing nitrogen and sulphur donors are unique choice in designing
probe for Hg2+ [23 k,l]. Alkylation to rigid-rod π-conjugated polymer is essential to improve the
solubility and solution processability of the synthesized polymer. Accordingly, in this work we
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have designed the monomers in such a way that one building block (known as binding domain)
bears a suitable chelating-site for Hg2+ binding. It is well known that soft Hg2+ ions (soft acid)
can preferentially interact with sulphur or nitrogen (of N-heterocyclic aromatic ring) donor
atoms. The other block (signaling domain) of 9-Heptyl-9H-carbazole having π-conjugated
delocalized backbone with alkyl substituent enhance the emission and sensing signal through
molecular wire effect [22] and as well as solubility.
The diboronic ester functionalized carbazole monomers (4 and 5) with heptyl and
TEGME groups, for Suzuki polymerization were obtained by borylation of 3,6-dibromo-9- alkyl-
9H-carbazole on treatment with n-BuLi followed by addition of 2-isopropoxy-4,4,5,5-
tetramethyl-1,3,2- dioxaborolane (iPrO-Bpin) in dry THF at -78 °C as depicted in scheme 1 [26].
The characterization and purity of the monomers (2, 3, 4 and 5) were determined by multinuclear
NMR and mass spectrometry analysis (ESI). Analytically pure 8 and 9 substituted with heptyl
(8) and 2-(2-(2- methoxyethoxy)ethoxy)ethyl (TEGME) (9) were synthesized (scheme 1) with
50-60% yield following our previously reported work [27]. In 1H NMR, the four thienyl protons
in 8 and 9 resonate as multiplets in the range of 8.03-7.78 and 7.86- 7.77 ppm respectively. The
four benzene protons resonate as doublet at 7.65 and 7.57 ppm. The triplet signal centered at 4.60
ppm is attributed to the -CH2- protons, attached to nitrogen of the imidazole moiety. For the
compound 9, the TEGME protons resonate in the range of 4.73 to 3.31 ppm. The characteristic
signal at 113.3 ppm in 13C{1H} NMR, and the band centred at 2221 cm-1 in FTIR spectrum
confirm the presence of –CN group. Formation of compounds 8 and 9 was further confirmed by
HRMS study showing the molecular ion peak at 430.1414 and 478.1232 respectively. The
bromination of 8 and 9 using N-bromosuccinamide yielded the dibromo derivative, 10 and 11 as
depicted in scheme 1. Analytically pure 10 and 11 were obtained through chromatographic
purification (silica 60-120) using ethylaceate:hexane (20-30%) as an eluent with 85-90 % yield.
The thienyl protons resonate as singlet at 7.74 and 7.67 ppm for 10 and 7.73 and 7.68 ppm for 11
1
respectively in H NMR. The -CH2- group of the heptyl attached to nitrogen resonates as triplet
centered at 4.41 ppm, whereas the -CH2- group of TEGME group attached to nitrogen of 11
appears as triplet at 4.62 ppm. MALDI-TOF study shows the molecular ion peaks at 588.581 and
635.785 for 10 and 11 respectively.
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Scheme 1. Synthetic route for the monomers 4-5 and 10-11.
Scheme 2. Synthesis of polymers (P1 and P2), and the model compound (M1).
To access the corresponding polymer, poly(benzodithienoimidazole-alt-carbazole) (P1)
bearing heptyl groups, polycondensation of analytically pure monomers 4 and 10 was carried out
in degassed THF using aqueous potassium carbonate as a base and Pd(PPh3)4 as a catalyst
following Suzuki coupling polymerization protocol as demonstrated in scheme 2. The
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polymerization reaction was furnished under refluxing condition with vigorous stirring for 48 h
under inert atmosphere. The polymer P2, substituted with TEGME unit to the 9-position of
carbazole was also synthesized by following the similar strategy from the corresponding
monomer 5 and 11. Quenching of polymerization by adding few drops of methanol followed by
precipitation from its concentrated THF solution in ice-cold methanol yielded both the crude
polymers (P1 and P2). The polymers P1 and P2 were further purified through Soxhlet extractor
by using hexanes, followed by MeOH to remove the oligomers and catalyst residue and finally
with distilled CHCl3. The CHCl3 part was collected and evaporated to get the dark-brown
powder of polymer P1 and P2. The synthesized polymers bearing long heptyl or TEGME groups
as side chain are readily soluble in common organic solvents (such as toluene, THF, and
chlorinated solvents) and exhibit strong greenish blue to cyan fluorescence emission in solution.
Interestingly, both the synthesized poly(benzodithieno-imidazole-alt-carbazole copolymers
exhibit appreciable amount of fluorescence even in solid state (as thin film).
Fig. 1. Tetradetector GPC trace (refractive index response) of the synthesized polymers (P1 and
P2) using THF as eluent.
The synthesized luminescent π-conjugated polymers were characterized by multinuclear
1
NMR, FTIR and tetradetector Gel Permeation Chromatography (GPC). H NMR spectrum (in
CDCl3) of polymer P1 exhibits two multiplets in the region of 7.84-7.77 and 7.73-7.60 ppm,
attributed the six aromatic protons of the carbazole unit. The signal at 7.58-7.45 and 7.41- 7.29
ppm as multiplet are assigned to the aromatic protons of thienyl and phenyl group respectively.
The -CH2 protons of the heptyl groups attached to 9-position of carbazole and benzodithieno-
imidazole groups resonate at 4.44-4.15 ppm as multiplet. The remaining alkyl protons resonate in
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13
the region of 1.89 to 0.87 ppm. In C{1H}NMR spectrum, the aromatic carbon of P1 resonates
in the region of 134.7 to 118.2 ppm, whereas the –CN carbons resonate at 113.2 ppm. The heptyl
carbons appear in the region of 46.5 to 13.9 ppm. Similarly, polymer P2 shows multiplets at the
region of 8.14-8.08 and 7.84-7.81 ppm ascribing to the carbazole protons, whereas the aromatic
protons of thienyl and benzyl unit resonates as multiplets in the region of 7.80-7.745 and 7.69-
7.45 ppm. The TEGME protons resonate in the region of 4.61 to 3.30 ppm. In 13C{1H}NMR
spectrum, the signal in the range of 151.3 to 118.7 ppm attribute the aromatic carbons of the
polymer P2, whereas the carbons of the TEGME unit resonates in the region of 71.8 to 46.3
ppm. In FTIR spectra, the signal at 2228 cm-1 attributes the presence of -CN group in the
polymer (P1 and P2). The formation of the polymers was further confirmed by tetradetector Gel
Permeation Chromatography (in THF with polystyrene standards) as shown in Fig. 1. The
number-average molecular weights (Mn) of the well-defined π-conjugated polymers (P1 and P2)
are 16.5 and 14.3 kDa with the very good polydispersity indices of 1.36 and 1.31 respectively
(Table 1) [28]. The lower Mn can be attributed to the introduction of bulky monomers with long
chain alkyl groups during polymerization. The molecular weight of the synthesized π-conjugated
copolymers (P1 and P2) is high enough to demonstrate efficient sensing properties. The thermal
property of the polymer was also investigated by thermogravimetric analysis (TGA) in a nitrogen
atmosphere at heating rate of 10 ºC/min to 700 ºC, revealing only 5% weight loss at relatively
higher temperature of 356 and 305 °C, for P1 and P2 respectively (Fig. S51). A model
monomeric probe (M1) has also been synthesized and characterized to establish the coordination
and mechanistic studies of Hg2+ sensing (vide infra).
Table 1 Characterization data for P1 and P2
c
Mn(kDa)
16.5
Mw(kDa)a
PDIa
1.36
1.31
Td (oC)b
356 oC
305 oC
22.4
P1
P2
14.3
18.7
a obtained from GPC, bfrom TGA studies.
Photophysical study
Excellent solubility of the synthesized polymers allowed us to evaluate the photophysical
properties with UV-vis and fluorescence spectra. The photophysical properties of the synthesized
π-conjugated copolymers P1 and P2 were conducted in THF in 2 × 10−5 M concentration as
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depicted in Fig. 2. In solution (THF), polymer P1 exhibited absorption maxima at 305 and 361
nm while the polymer P2 showed similar absorption at λmax of 297 and 356 nm with ε values in
the range of 2.02 × 104 -2.12 × 104 M−1cm−1, which were attributed to the π-π* electronic
transition of the π-conjugated polymer backbone. The relatively high energy absorption at λmax of
241 and 237 nm was assigned as π-π* transition for the appended benzonitrile substituent. The π-
conjugated polymers (P1 and P2) exhibited strong cyan to blue emission centered at 499 nm (λex
= 361 nm) and 494 nm (λex= 356 nm) for P1 and P2 respectively. The photoluminiscent quantum
yields (Ф) of the polymers are in the range of 22 - 21% in THF as measured by relative method
with quinine sulphate (in 0.1 M sulphuric acid) as reference [29]. The absorption and emission
properties of the model compound (M1) are very much similar to those of P1 suggesting that the
lowest singlet excited state and the π-electron delocalization are almost confined to a single
repeating unit in the polymer backbone. The small bathochromic shift in λem by varying the
solvent polarity suggests that the ICT fluorescence is not dominant in these systems (Table S1
and Fig. S62. In thin film (made by spin coating on quartz substrate), the absorption maxima of
the polymer P1 and P2 were observed at 365 nm and 361 nm. More interestingly, both the
polymers are highly emissive in green to blue spectral region even as thin film showing λem at
527 nm and 535 nm respectively with a Stoke's shift of 7660 and 7846 cm-1 respectively. In the
solid state (as film) photophysical studies, a red shift by ca. 5 nm (in absorption spectra) and 30-
40 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.
Fig. 2. (a) Solution state UV-vis and PL spectra for the polymers P1 and P2 in THF with a
concentration of ~10-5 M (b) Solid state UV-vis and PL spectra for the films (spin coated on
quartz plate from CHCl3 solution of polymers) of the polymers P1 and P2 at 28 °C.
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Table 2 Photophysical data of the polymers (P1, P2) and the model compound (M1).
Absorbance (nm)
Emission (nm)
Solution
Stoke’s shift (cm-1)
Polymer
Solution
Solid
Solid
Solution Solid
λmax (ε x104 in
M−1cm−1)
λmax
λem
Φ (%)a λem
361 (2.09), 305
365,
499
494
494
22
21
18
527
535
-
7660
7846
7660
8272
9009
-
P1
(2.12), 241 (3.15)
309, 247
356 (2.02), 297
361,
P2
(2.09), 237 (3.13)
306, 257
356 (2.01), 303 (2.10),
236 (2.95)
363,
303, 256
M1
aMeasured by relative method with quinine sulphate (in 0.1 M sulphuric acid) as the reference (Φ =
0.54).
To estimate the HOMO and LUMO of the polymers, the cyclic voltammetric experiments
of the synthesized π- conjugated polymers were conducted (Fig. S72). For this purpose, films of
the polymers were prepared by drop casting the corresponding DCM solution on GC-disc
electrode. The measurement was carried out in CH3CN solution using n-Bu4NPF6 (0.1 M) as
supporting electrolytes, Pt wire counter electrode and Ag/AgCl reference electrode. The
electrochemical data are summarized in Table 3. HOMO energy levels of P1 and P2 are
calculated as -4.92 and -5.02 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) = [Eonset - Eox(ferrocene)] - 4.80 [30]. We have calculated the ELUMO levels from the
EHOMO levels and its optical band gaps (Egopt) using the relation, ELUMO = EHOMO + Egopt [31]. The
optical energy band gap (Egopt) of the polymer probes was calculated by examining the edge of
the solid state absorption spectra by using the equation Egopt(eV) = 1240/λcutoff. The optical
energy bandgap was found to be 2.54 and 2.55 eV for P1 and P2 respectively.
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Table 3 Electrochemical data and HOMO and LUMO energy levels of
P1 and P2.
Polymer
Eox, V
EHOMO,(eV)a
-4.92
ELUMO,(eV)b Egopt(eV)c
0.63
-2.38
-2.47
-2.38
2.54
2.55
2.58
P1
0.73
-5.02
P2
0.67
-4.96
M1
aFrom the onset of the oxidation potential, and using the equation,
b
EHOMO (eV) = [Eonset - Eox(ferrocene)] - 4.80. (ELUMO = EHOMO + Egopt).
cEgopt from solid state absorption spectra.
Selective recognition to Hg2+
The excellent emissive nature of the polymer P1 and P2 having dithieno-imidazole
binding sites encouraged us to explore their potential application as a fluorescence chemosensor
for the selective detection of toxic metal ions. To explore the potential application, the
absorbance and fluorescence responses of the highly emissive probes P1 and P2 were
investigated in 1:1 and 2:3 THF-H2O respectively in 5×10-5 M concentrations. The photophysical
responses of P1 and P2 are similar to that in 5 × 10-5 M THF solution.
Fig. 3 (a) Change in PL response of P1 towards various metal ions (1 eqv.). (b) Selectivity
profile; Blue solid bars: Free sensor or treated with metal ions; Red solid patterned bars: Treated
with competing metal ions followed by 1 eqv. of Hg2+; 1–15 represent 1) P1 2) Na+ 3) K+ 4)
Mg2+ 5) Ca2+ 6) Cd2+ 7) Co2+ 8) Cr3+ 9) Ni2+ 10) Pb2+ 11) Cu2+ 12) Hg2+ 13) Zn2+ 14) Fe3+ 15)
Ag+ 16) Ba2+ 17) Sr2+ 18) Al3+ 19) all. For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.).
The high stability and the electron rich π-conjugated network of the synthesized polymers
make a supreme candidate for sensing of toxic metal ion through the fluorescence quenching
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taking advantage of molecular wire effect of the π-conjugated polymers. To determine the
optimum concentration in which self-aggregation of both the π-conjugated polymeric probes
through π-π stacking is minimal, we have recorded concentration dependent emission spectra as
well as dynamic light scattering (DLS) studies. With increasing concentration from 10-5 to 10-3
M in THF the emission intensity of P1 and P2 decreased significantly (Fig. S52). The emission
intensity of the probes also decreased when the solution was diluted to 10-6 M. DLS studies of
the probes were carried out by varying the concentration, ranging from 10-3 to 10-5 M (Fig. S53).
The average aggregate size (DH) of 30-50 nm for both the probes confirmed the negligible self-
aggregation and the presence of mainly unimers of the polymeric probes in relatively dilute
solution of ca. 10-5 M in THF. Henceforth, the sensing application was explored in 10-5 M
concentration to avoid the self-association behavior of the probes. The metal ion recognition
study through the absorbance and fluorescence response was investigated qualitatively by
treating the polymer probes in THF:H2O with various biologically and environmentally relevant
metal ions including alkali metal ions (K+, Na+), alkaline-earth metal ions (Ca2+, Mg2+, Sr2+,
Ba2+), and heavy/transition metal ions (such as Cu2+, Pb2+, Cd2+, Ni2+, Zn2+, Ag+, Co2+, Fe3+, and
Cr3+). Upon addition of these metal ions to the polymer probe P1, absorption band remain
unperturbed or no significant change in the absorption intensity was noticed. Similar result was
obtained for TEGME functionalized probe P2. However, upon treatment with one equivalent of
Hg2+ per benzodithienoimidazole unit, a remarkable turn-off fluorescence response for both the
polymer probes was observed. The investigation clearly shows that the polymers P1 and P2
exhibit excellent fluorescent probes for detection of Hg2+. By contrast, other metal ions only
caused marginally fluorescence discrepancies in contrast to Hg2+, establishing an efficient sensor
for Hg2+ ion as illustrated in Fig. 3a. Both the polymers P1 and P2 exhibited partial quenching
(ca. 10%) upon addition of Cu2+, however change in fluorescence intensity was indistinguishable
by naked eye (ESI). The essential criterion for a selective sensing probe is the ability to sense the
particular metal ion in the vicinity of other competing metal ions. The synthesized π–conjugated
polymers (P1 and P2) exhibited fluorescence quenching by Hg2+, which was remained intact in
the presence of other competing metal ions and even in more complex mixtures. This
observation accomplishes the criterion for an efficient selective probe towards Hg2+ as portrayed
in Fig. 3b. Moreover, the fluorescence responses to different mercury (II) salts with various
-
-
2-
counter anions (such as NO3 , OAc-, S2-, ClO4 , SCN-, SO4 , Br-, I- and Cl-) were also
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investigated to ensure the anion-independent ‘turn-off’ response (Fig. S59). Thus, it can be
established that both the polymer sensors unveiled excellent selectivity towards different Hg2+
salts irrespective of counter ions and in the vicinity of the other competing metal ions.
Fig. 4. (a) Fluorescence titration of P1 upon incremental addition of Hg2+ metal ion with visual
change in fluorescence (b) Job’s plot of M1:Hg2+ complexes in 1:1 THF:H2O medium.
To obtain an insight into the sensing mechanism in depth and to find the binding
stoichiometry of the probe with Hg2+, spectroscopic titration was accompanied with a continuous
variation of concentrations of metal ion (Hg2+).Upon addition of increasing equivalents of Hg2+,
the corresponding emission spectrum showed a gradual decrease in intensity as illustrated in Fig.
4a, and at a higher concentration (2 x 10-4 M) of Hg2+ the fluorescence was completely
quenched, which could be presumed to the complete formation of benzodithieno-imidazole-
bounded Hg2+-metallopolymer. The selective quenching of the probe (P1 and P2) towards Hg2+
is easily visualized by naked eyes. Interestingly, after addition of one equivalent of Hg2+ per
benzodithieno-imidazole unit to the polymer solution leads to almost complete quenching
(98.6% for P1 and 98.1% for P2) of fluorescence and the emission intensity almost retained
intact after addition of excess equivalent of Hg2+ presumably due to the unavailability of the
vacant coordination sites in the polymer backbone. The fluorescence intensity at 527 and 535
nm(λem) of the probe towards Hg2+ remained unperturbed in the pH range of 5.4–8.5 and 5.6–8.7
for P1 and P2 respectively, establishing the sensing capability in the physiological condition
(Fig. S60-S61). The stoichiometric ratio of benzodithieno-imidazole:Hg (II) in P1-
Hg2+metallopolymer formed through coordination of Hg2+ to the benzodithieno-imidazole units
was estimated to be 0.88 from the saturation behavior of emission spectra, suggesting a 1:1
benzodithieno-imidazole:Hg2+-coordination behavior. To further prove the 1:1 coordination, the
model monomeric probe (M1) has been treated with Hg2+ in a similar set of condition as
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performed sensory studies for the polymers. Job’s plot indicated the 1:1 M1:Hg2+ stoichiometry
as depicted in Fig. 4b.
The coordination of Hg2+ to the chelating sites of the probe was investigated by IR
1
spectra and H NMR titration for the model compound (M1). Probably the soft and borderline
binding sites having sulphur and nitrogen donor atoms can interact preferentially to the soft
Hg2+. In the probe, three recognition sites are available for Hg2+ binding, i.e. “S,N-R”, “-C≡N”
and “N,S”. The probability of the binding in the “S, N-R” site was ruled out by considering the
steric hindrance of the heptyl substituent. In the IR spectral titration, the stretching frequency of
“-C≡N” group at 2221 cm-1 remains intact even after the addition of 1.5 equivalent of Hg2+ metal
ion. Hence, the Hg2+ recognition towards the -C≡N site was further excluded the possibility of
coordination. Therefore, the Hg2+ ion presumably coordinate with the “N,S” coordinating site.
The stretching vibration band for C=N (imidazole ring) appeared at 1605 and 1468 cm-1, which
were gradually shifted to lower energy region upon addition of Hg2+ ion. The C-N stretching
vibration band along with the C-S bond (of dithieno unit) also shifted to lower energy region
1
from 1235 cm-1 and 800 cm-1 respectively with incremental addition of Hg2+ (Fig. S70) [32]. H
NMR titration also supports this presumption. As illustrated in Fig. S71, after incremental
addition of Hg2+, the signal for thienyl (Hb) and carbazole protons (Hc and Hd) experienced slight
1
downfield shift by 0.03 ppm. Thus, the results of FTIR spectral and H NMR titration also
suggest coordination of Hg2+ to “N,S” chelating site of dithieno-imidazole unit with 1:1 binding
stoichiometry .
To further confirm the coordination mode between the “N,S”-chelating site of M1 and
Hg2+, the density function distribution analysis was carried out. The geometry of M1 and
[M1•Hg(NO3)2] were fully optimized by Becke’s three parameter hybrid functional using the
LYP correlation functional (B3LYP). The LanL2DZ basis set was employed for Hg element
while the 6-31G(d,p) basis set was assigned to nonmetal elements (C, H, O, N and S) [33]. To
reduce the computational time, the long alkyl groups are replaced by -CH3 group. The DFT
geometry optimization studies also support 1:1 coordination as presumed from fluorescence
titration studies (Fig. 5a). Furthermore, the formation of 1:1 M1:Hg2+ complex was
unambiguously confirmed by MALDI-TOF analysis showing molecular ion peak, [M1·Hg(II)-
NO3]+ at 1219.847. The experimental isotopic distribution pattern for the molecular ion peak is
in well agreement with the simulated one as presented in Fig. 5b.
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Fig. 5. (a) Proposed Hg2+-coordination with model sensing probe M1 (obtained from DFT
computed optimized structure). (b) Simulated and experimental isotopic distribution pattern for
the molecular ion peak, [M1·Hg(II)NO3]+ obtained from MALDI-TOF mass spectrometric
analysis.
Fig. 6. (a) Stern-Volmer plot for the quenching of P1 by incremental amount of Hg2+; (b) The
linear dynamic fluorescence response of P1 against Hg2+ to determine the limit of detection
(LOD).
The emission quenching process was also analyzed by the Stern-Volmer (SV) equation
I0/I = 1+KSV[Q] [34], where I0 and I are the fluorescence intensities in the absence and presence
of the quencher [Q] and KSV is the Stern-Volmer fluorescence quenching constant. In the
analysis, the relative fluorescence intensities (I0/I) were plotted versus the concentration of the
quencher ions (Hg2+) and the rate of quenching was determined. The decrease in the fluorescence
intensity with the linear SV-plot suggests for static fluorescence quenching of the probes in the
presence of Hg2+ metal ion with KSV value of 4.75 × 105 and 4.28 × 105 M-1 for the polymers P1
and P2 respectively, indicating thermodynamically stable binding between Hg2+ and the
chelating site attached to π-conjugated polymer. The fluorescence life time studies of both the
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polymer probes also supports the static quenching phenomenon as the life time is invariant with
different concentration of the metal ions (Fig. S56). It is noteworthy to mention that the limit of
detection (LOD) [35] of P1 and P2 for Hg2+ (1.35 × 10-7 M-1 and 2.17 × 10-7 M-1 respectively) is
sufficiently low for practical utility as fluorescent probe, as calculated from the concentration
dependent fluorescence titration study shown in Fig. 6.
The plausible sensing mechanisms could be due to the chelating quenched fluorescence
(CHQF) by Hg2+ as demonstrated elegantly by Schreckenbach′s and Huang’s groups [36]. The
HOMO orbital is delocalized over the carbazole and benzodithieno-imidazole moiety while the
LUMO is mainly distributed over the benzonitrile group. The important frontier orbitals of the
free M1 and Hg2+-coordinated complexes are presented in Fig. 7 (and Fig. S76). In the
coordination process, the M1 acts as an electron pair donor, while the Hg2+ ion serve as a an
electron pair acceptor and coordinates through the available vacant s-orbital. The vacant s-orbital
of the Hg2+ (-3.34 eV) metal ion is in between the HOMO (-4.92 eV) and LUMO (-1.72 eV) of
M1 as evidenced from DFT studies. The data in Table 3 indicate similar situation for the
polymeric probes P1 and P2. Hence, the chelating quenched fluorescence (CHQF) is induced
resulting radiationless decay of the excited state and thus the polymers act as efficient turn off
probe for toxic Hg2+ (Fig 8 and Fig. S77) [36].
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Fig. 7. Selected frontier molecular orbitals for M1 and [M1·Hg(NO3)2]. Estimated HOMO-
LUMO energy levels (eV) from DFT calculations, HOMO: -4.92, -5.66; LUMO: -1.72 and -3.34
respectively. To reduce the computational time, the heptyl group has been replaced by -CH3.
Fig. 8. Proposed sensing mechanism for the polymer probe, P1.
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Fig. 9. (a) Change in PL intensity after incremental addition of S2- to Hg2+-metallopolymer in
THF-H2O medium (reversibility study) (b) Cycles of emission quenching and recovery by
exposing the polymer probe (as film on quartz substrate) towards S2- solution demonstrating
reusability.
The reversibility of a sensing probe is another important aspect in view of the reusability
and cost-effectiveness of the sensing probes. Reversibility of the chemosensors was
demonstrated by the addition of S2- to the Hg2+-metallopolymer complex (λex = 361 nm, λem
=
499 nm). The S2- ion is well known to bind strongly with Hg2+. Incremental addition of S2- to a
solution of Hg2+-metallopolymer showed the opposite trend in the fluorescence intensity due to
the formation of stable HgS complex, and subsequently regain of free probe demonstrating
reversible nature of the P1 and P2 probes. (Fig. 9a). Furthermore, the Hg2+-metallopolymers of
P1 and P2 can act as a secondary sensor to detect the S2-.
After the successful sensing studies in solution phase, the recognition of Hg2+ was also
investigated for both the polymers P1 and P2 as thin film probe. Thin films of the probes for the
solid state measurements were casted from THF solution onto quartz plates. Solid state PL
studies of the polymeric films were conducted. Cycles of emission quenching and recovery by
exposing the probe towards successive treatment of Hg2+ and S2- demonstrated reusability. Most
importantly, the reversible process could be repeated by several times (more than 10 times) with
little loss of fluorescent efficiency (Fig. 9b).
Solid state PL studies of the Hg2+-coordinated polymeric films were also conducted by
immersing the polymer film into the various concentrations of Hg2+ aqueous solution (100µM–
1µM). Fig. 10a demonstrates a significant amount of quenching of the emission centered at 527
and 535 nm for the polymer probes with a gradually decrease of emission intensity as a function
of increasing concentration of Hg2+. To investigate the practical and real time application of
chemosensor, the excellent turn-off sensing behaviour of the polymer sensing probes to Hg2+ was
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employed. In order to demonstrate the potential utility of the sensing probes towards Hg2+, test-
paper experiment was conducted. The test strips were prepared by immersing filter papers into a
THF solution of polymer (0.1 M) and then drying in air. The test strips containing polymer probe
was immersed into the aqueous solution of Hg2+ by varying different concentration (10-4 – 10-7
M) and the obvious change from green emission to dark green could be detected by naked eye
(under 365 nm UV lamp) as shown in Fig. 10b. Similar response was observed for polymer P2 in
the similar set of experimental conditions (Fig. S69). This result demonstrates that P1 and P2 are
excellent chemosensors for the immediate visual detection of Hg2+ in solution and as test paper
strip.
Fig. 10. (a) Emission quenching of polymer P1 by micromolar aqueous solution of Hg2+ metal
ion demonstrating trace detection. (b) Change on P1 test paper strips after treating with different
samples of Hg2+ varying concentration (10-4–10-7 M-1), under 365 nm light.
Fig. 11. FESEM images and EDX line trace analysis of P1 (left) and after treatment with Hg2+
(right) revealing binding of Hg2+ to polymers.
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Fig. 12. Bright-field TEM micrographs of (a) monodisperse spherical self-assembled structures
of P1 formed in 45% (v/v) MeOH/THF (b) ribbon like self-assembly after addition of 150 µL of
Hg2+ metal ion solution (as 5 x 10-5 M in water). (c) STEM mapping showing the presence of
Hg2+ in the vicinity of S and N, further proving Hg(II) binding to N,S sites. (d) Comparison of
overlaying integrated EDX point spectra. Carbon coated copper grid (300 mesh) was used for the
TEM.
The scanning electron microscope image analysis of the polymer probe films (prepared
by drop casting of 5 x 10-4 M chloroform solution on aluminium substrate) were performed to
observe the change in surface morphology of the polymers before and after coordination of Hg2+.
Typical FESEM images of the morphology are shown in Fig. 11. Line EDX analysis of the film
was also carried out before and after Hg2+ ion coordination, which shows the presence of
elemental Hg, confirming the formation of the Hg2+-coordinated metallopolymer. The TEM
analysis (Fig. 12a,b) was also carried out to understand the self-assembly of the Hg2+-
metallopolymer complex. In order to study solution self-assembly, the polymer P1 (1.5 mg/ml)
was dissolved in dry THF, (a good solvent for the polymer) to give clear greenish-yellow
solutions at room temperature (28 ºC). After 2 hours, dry and filtered MeOH (passing through a
syringe filter, PTFE 0.45 µm) was then added dropwise until the solution consisted of ~45%
(v/v) MeOH in THF. This process was accompanied by a colour change from clear greenish-
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yellow to turbid yellow coloration. Transmission electron microscopy (TEM) analysis showed
the formation of sphere-like self-assembled structure with diameter in the range of 800-950 nm.
Interestingly, after Hg2+ coordination, the sphere self-assembled structure transferred to ribbon
like aggregates as monitored by TEM analysis. The STEM element mapping of an area of 284 by
284 µm with a pixel resolution of 256×256 (Fig. 12c,d) also clearly revealed the formation of
Hg2+-metallopolymer by showing the signal of N, S and Hg [37].
Conclusion
In conclusion, we have designed and synthesized well-defined soluble π-conjugated
copolymer (P1 and P2) consisting of carbazole and bezodithieno-imidazole in alternating
fashion, through Suzuki coupling polymerization. The synthesized polymers exhibit a simple,
facile, low-cost and efficient chemosensor towards Hg2+ through fluorescence quenching
responses via CHQF mechanism. Furthermore, no obvious interference was observed upon
addition of various metal ions, indicating excellent selective recognition of Hg2+ by the
polymeric probe in semi-aqueous environment. The limit of detection of the polymer probes
towards Hg2+ was found to be in ppb level with high association constant in the order of 105 M−1.
Naked eyes can detect the distinct and rapid change in fluorescence under 365 nm light. The
coordination of the Hg2+ to the “N,S”-chelating site of the dithieno-imidazole unit was
1
investigated though IR spectra and H NMR spectroscopic titration. To gain an insight into
coordination mode of Hg2+ to “N,S”-chelating site, DFT geometry optimization studies was also
performed. The favorable energy of LUMO of Hg2+, which is in between the HOMO and LUMO
of sensors, induces chelating quenched fluorescence (CHQF). Moreover, the polymers serve as a
reusable and recyclable probe on successive addition of Hg2+ and S2-. The paper strip test study
was also inspected, to demonstrate its practical utility as convenient and efficient Hg2+ test kit.
This work clearly reveals a novel platform to further develop easily processable π-conjugated
fluorescent polymer based simple, selective, highly sensitive, low cost and reversible probes by
judicious design of coordination site for sensing not only metal ions but also different
environmental and biological relevant analytes.
Materials and Instrumentation
All the air and moisture sensitive reactions and manipulations were carried out under an
atmosphere of pre-purified N2 or Ar by using standard Schlenk techniques. The glassware were
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oven-dried (at 180 ºC) and cooled under vacuum. Tetrahydrofuran and diethyl ether were dried
over Na/benzophenone. All chemicals were purchased from Sigma Aldrich unless otherwise
noted. Silica gel (60–120 and 100–200 mesh) used for column chromatography, was purchased
from Merck. Pd(PPh3)2Cl2 and Pd(PPh3)4 were synthesized following the literature method [38].
1H (600 MHz and 400 MHz), 13C{1H} (150 MHz and 100 MHz) NMR spectra were
obtained from Bruker Lambda spectrometer using CDCl3 unless otherwise mentioned. Spectra
were internally referenced to residual solvent peaks (δ = 7.26 ppm for proton and δ = 77.23 for
carbon (middle peak) in CDCl3). All coupling constants (J) are given in Hz. The HRMS was
recorded in ESI+ mode (70 eV) in Waters mass spectrometer (Model: Xevo-G2QTOF). The
absorption and fluorescence spectra were collected using a Shimadzu (Model UV-2450)
spectrophotometer and a Hitachi (Model F-7000) spectrofluorimeter, respectively. FTIR
spectroscopy was recorded in Spectrum-BX (Perkin Elmer). Solid state PL spectra were recorded
in Flurolog Horiba (Model FL-1016, Spectracq). MALDI-TOF study was performed by using
Bruker MALDI- TOFUltrafleXtreme instrument. TEM and EDX-STEM analysis of polymer
were carried out by JEOL JEM2100F (Japan) Transmittance Electron Microscope with JEOL
EDS detector. Scanning Electron Microscope (SEM) analysis was carried out with ZEISS
MERLIN (GEMINI2) FESEM with Oxford EDS detector. Thermogravimetric analysis (TGA)
was carried out using Perkin Elmer Pyris Diamond TG/DTA instrument. The thermal stabilities
of the samples under nitrogen were determined by measuring their weight losses while heating at
a rate of 5 °C/min. Molecular weights and polydispersity indices (PDI = Mw/Mn) of polymers
were obtained by Gel Permeation Chromatography (GPC) using a Viscotek VE 2001 Triple-
Detector Gel Permeation Chromatograph equipped with automatic sampler, pump, injector,
inline degasser, column oven (30 °C), styrene/divinylbenzene columns with pore sizes of 500 Å
and 100,000 Å, VE 3580 refractometer, four-capillary differential viscometer and 90° angle laser
and low angle laser (7°) light scattering detector (VE 3210 & VE270). HPLC grade THF was
used as the chromatography eluent, at a flow rate of 1.0 mL/min. Samples were dissolved in the
eluent (1 mg/ mL) and filtered with a Ministart SRP 15 filter (polytetrafluoroethylenemembrane
of 0.2µm pore size) before analysis. Calibration of all three detectors (refractive index, laser light
scattering and viscometry) was performed using polystyrene standards (Viscotek). This
equipment allows the absolute measurement of homopolymer molecular weights and PDIs.
Cyclic voltammetric studies were performed on a BASi Epsilon electrochemical workstation in
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acetonitrile with 0.1 M tetra-n-butylammonium-hexafluorophosphate (TBAPF6) as the
supporting electrolyte at room temperature. The working electrode was a BASi Pt disc electrode,
the reference electrode was Ag/AgCl and the auxiliary electrode was a Pt wire. The
ferrocene/ferrocenium couple occurs at E1/2 = +0.51(70) V versus Ag/AgCl under the same
experimental conditions. The dynamic light scattering (DLS) was carried out using Malvern Zeta
sizer Nano series equipped with a laser with wavelength of 633 nm and a detector oriented at
173º to the incident radiation.
Experimental
3,6-Dibromo-9-(2-(2-(2-methoxyethoxy)ethoxy)ethyl)-9H-carbazole (3): To a solution
of di-bromo-9H-carbazole (1) (1 g, 3.07 mmol), in anhydrous DMF (15 ml), NaH (160
mg, 4 mmol) and 1-bromo-2-(2-(2-methoxyethoxy)ethoxy)ethane (840 mg, 3.7 mmol)
were added. The reaction mixture was stirred at 90 ºC for 24 h. After consumption of
carbazole substrate (the reaction was monitored by TLC, n-hexane: ethyl acetate was used
as the eluent) the reaction mixture was poured into water (200 ml), extracted with ethyl
acetate (3 × 50 ml) and dried over anhydrous MgSO4. The solvent was removed under
diminished pressure and the residue was purified by column chromatography eluting with
n
-hexane: ethyl acetate (1:4) to afford colourless liquid with 70% yield (1.01 g). 1H NMR
(400 MHz, CDCl3): δ 8.12 (s, 2H, carbazolyl), 7.54 (d, 2H, = 8 Hz, carbazolyl), 7.34 (d,
2H, = 8 Hz, carbazolyl), 4.43 (t, 2H, = 2 Hz, -NCH2-), 3.83 (t, = 2 Hz, -NCH2CH2-
J
J
J
J
O-), 3.50-3.40 (m, 8H), 3.34 (s, 3H, -OCH3). 13C{1H} NMR (100 MHz, CDCl3): δ 139.4,
128.9, 123.3, 122.3, 112.0 (carbazolyl carbon attached to bromine), 110.7 (carbazolyl
carbon attached to bromine), 71.8 (alkyl carbon attached to N), 70.9, 70.5, 70.1, 69.3,
58.9, 43.4 (-OCH3). MALDI-TOF (m/z): C19H21Br2NO3, calculated value 471.986 (M)+,
found 471.521 (M)+.
9-heptyl-3,6-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole
(
4
):
-BuLi (1.6 M in hexanes, 9.3 mL, 14.8 mmol) was added dropwise to a
(2.03 g, 4.8 mmol) in THF (50 mL) kept at -78 °C. The obtained mixture
A
solution of
solution of
n
2
was vigorously stirred for 1 h at -78 °C before the addition of 2-isopropoxy-4,4,5,5-
tetramethyl-[1,3,2]-dioxaborolane (2.85 g, 15.2 mmol) in one portion. The resulting
solution was then allowed to reach room temperature and to react overnight. After
removing the solvent under vacuum, CH2Cl2 (50 mL) was added and the obtained
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solution was washed with water (3 × 50 mL) and dried over Na2SO4. The solvent was
removed under vacuum and the obtained crude product was purified by flash column
chromatography (silica gel, petroleumether 40-60 ºC /CH2Cl2 ¼ 1/1 v/v) to give
3 (1.7 g,
1
68 %) as a white solid. H NMR (400 MHz, CDCl3): δ 8.67 (s, 2H, carbazolyl), 7.90 (d,
2H,
J = 8 Hz, carbazolyl), 7.39 (d, 2H, J = 8 Hz, carbazolyl), 4.30 (t, 2H, J = 2 Hz, -
NCH2-), 1.88-1.83 (m, 2H, -NCH2CH2-), 1.41-1.37 (m, 32H), 0.89-0.85 (m, 3H). 13C{1H}
NMR (100 MHz, CDCl3): δ 142.7, 132.1, 128.0, 122.8, 108.1, 83.5, 43.0, 31.7, 29.1,
28.9, 27.2, 26.3, 25.4, 24.9, 22.6, 21.0, 14.2.
9-(2-(2-(2-methoxyethoxy)ethoxy)ethyl)-3,6-bis(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-9H-carbazole
(
5): A solution of
n-BuLi (1.6 M in hexanes, 2.5 mL,
4.04 mmol) was added dropwise to a solution of
4
(615 mg, 1.3mmol) in THF(10 mL)
kept at -78 ºC. The obtained reaction mixture was vigorously stirred for 1 h at -78 ºC
before the addition of 2-isopropoxy-4,4,5,5-tetramethyl-[1,3,2]-dioxaborolane (787 mg,
4.17 mmol) in one portion. The resulting solution was then allowed to reach room
temperature and to react 24 h. After removing the solvent under vacuum, CH2Cl2 (50 mL)
was added and the obtained solution was washed with water (3 × 50 mL) and dried over
Na2SO4. The solvent was removed under vacuum and the obtained crude product was
purified by flash column chromatography (silica gel 60-120 mesh, petroleum ether 40 -
60 ºC /CH2Cl21/1 v/v) to give
CDCl3): δ 8.64 (s, 2H, carbazolyl), 7.89 (d, 2H,
Hz, carbazolyl), 4.51 (t, 2H, = 2 Hz, -NCH2-), 3.87 (t, 2H,
5
(442 mg, 60%) as a white semisolid.1H NMR (400 MHz,
= 8 Hz, carbazolyl), 7.45 (d, 2H, = 8
= 2 Hz, -NCH2CH2O-),
J
J
J
J
3.50-3.33 (m, 8H), 3.33 (s, 3H, -OCH3), 1.39 (s, 24H, tetra-methyldioxaboralane proton).
13C{1H} NMR (100 MHz, CDCl3): δ 142.8, 140.6, 132.2, 127.8, 125.7, 122.8, 120.3,
119.5, 108.7, 83.5, 71.8, 70.9, 70.5, 70.1, 69.2, 58.8, 43.6 (-OCH3), 24.9 (tetra-
methyldioxaboralane carbon).
Synthesis of 7: A mixture of 4-cyano-benzaldehyde (315 mg, 2.4 mmol), benzo[1,2-
b:4,3-b']dithiophene-4,5-quinone (484 mg, 2.2 mmol), ammonium acetate (5.5 g, 72.6
mmol) and acetic acid (20 mL) was heated to 100 ºC for overnight. The green solution
was cooled to room temperature, and 15 mL water was added to stir for 15 min at room
temperature. The solution was filtered in a Buchner funnel. The product was washed
thoroughly in water and hexane, dried and taken to the next step.
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General synthesis of 8 and 9: To an oven dried Schlenk, previously prepared
compounds (7) (265 mg, 0.8 mmol) in DMF (30 ml) was taken, K2CO3 (300 mg, 2.1
mmol) was added and heated to 95 ºC for 3 h and then cooled to room temperature. To it
1-iodoheptane (203 mg, 0.9 mmol) or 1-bromo-2-(2-(2-methoxyethoxy)ethoxy)ethane
(204 mg, 0.9 mmol) were added slowly. Reaction mixture was heated to 95 ºC for
overnight. After cooling to room temperature the reaction mixture was poured in 20 mL
water. Organic phase was extracted by ethyl acetate via repeated washing in water and
dried over anhydrous MgSO4. The solvent was removed under rotary evaporation. Crude
product was purified by silica gel column chromatography (Ethyl acetate:Hexane =
10:90) to give solid product (8/9) (50-60% yield).
1
Compound 8: H NMR (400 MHz, CDCl3): δ 8.03-7.87 (m, 2H, thienyl), 7.86-7.77 (m,
2H, thienyl), 7.65 (d, 2H, J = 4 Hz, benzyl), 7.57 (d, 2H, J = 4 Hz, benzyl ), 4.60 (t, 2H, J
= 2 Hz, -NCH2-), 1.94-1.93 (m, 2H), 1.28-1.21 (m, 8H), 0.86-0.83 (m, 3H, -CH3).
13C{1H} NMR (100 MHz, CDCl3): δ 149.4, 133.1, 132.5, 132.1, 130.1, 127.9, 124.6,
123.9, 122.5, 120.8, 118.3, 113.3 (-CN), 46.7 (alkyl carbon attached to N), 31.6, 31.4,
28.6, 26.4, 22.5, 14.0 (-CH3). HRMS-ESI+ (m/z): C25H23N3S2 calculated value 430.1367
(M+H)+, found 430.1414 (M+H)+. FTIR (KBr, cm-1): 2958, 2855 (ꢀꢁC-H stretching), 2226
((ꢀꢁC≡N stretching), 1609, 1451 (ꢀꢁC=N imidazole stretching), 1262 (ꢀꢁC-N stretching).
1
Compound 9: H NMR (400 MHz, CDCl3): δ 8.16 (m, 2H, thienyl), 7.84 (m, 4H, -Ph),
7.55 (m, 2H, thienyl), 4.73 (t, 2H,
J
= 2 Hz, -NCH2-), 4.05 (t, 2H, J = 2 Hz, -NCH2CH2O-
13
), 3.51-3.38 (m, 8H), 3.31 (s, 3H, -OCH3). C{1H} NMR (100 MHz, CDCl3): δ 150.9,
136.0, 135.1, 132.5, 131.1, 129.6, 127.8, 124.8, 123.7,122.7, 118.7, 113.2 (-CN), 72.0,
71.0, 70.7, 70.6, 70.1, 59.1, 46.6 (-OCH3). HRMS-ESI+ (m/z): C25H23N3O3S2 478.1241
(M+H)+, found 478.1232 (M+H)+. FTIR (KBr, cm-1): 2910, 2811 (ꢀꢁC-H stretching), 2221
((ꢀꢁC≡N stretching), 1609, 1421 (ꢀꢁC=N imidazole stretching), 1248 (ꢀꢁC-N stretching).
General synthesis of 10 and 11: Solution of
8 (812 mg, 1.7 mmol)/9 (730 mg, 1.7 mmol)
in THF (10 mL) was taken in 100 mL oven dried Schlenk RB flask. N-bromosuccinamide
(676 mg, 3.8 mmol) was added portion wise in 10 min intervals with continuous stirring
at room temperature. Completion of the reaction was monitored by TLC. After
completion of the reaction the solvent was then removed by evaporation under reduced
24

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pressure and the crude product was purified by silica gel column chromatography (hexane
as eluent) to give a brown solid of 10
Compound 10: 1H NMR (400 MHz, CDCl3): δ 7.90 (d, 2H,
= 8 Hz, -Ph), 7.74 (s, 1H, thienyl), 7.67 (s, 1H, thienyl), 4.41 (t, 2H,
/11 (85-90% yield).
J
= 8 Hz, -Ph), 7.85 (d, 2H,
J
J
= 2 Hz, -NCH2-),
1.92-1.84 (m, 2H), 1.26-1.22 (m, 8H), 0.87-0.84 (m, 3H, -CH3). 13C{1H} NMR (100
MHz, DMSO-d6): δ 151.0, 134.5, 133.2, 131.6, 130.7, 129.6, 124.4, 127.1, 127.0, 118.8 (-
CN), 112.8 (carbazolyl carbon attached to bromine), 112.2 (carbazolyl carbon attached to
bromine), 31.8 (alkyl carbon attached to N), 30.7, 28.1, 25.9, 24.2, 22.3, 14.2 (-CH3).
MALDI-TOF (m/z): C25H21Br2N3S2, calculated value 588.392 (M)+, found 588.581 (M)+.
FTIR (KBr, cm-1): 2961,2851 (ꢀꢁC-H stretching), 2223 ((ꢀꢁC≡N stretching), 1605, 1465 (ꢀꢁC=N
imidazole stretching).
Compound 11: 1H NMR (400 MHz, CDCl3): δ 8.12 (d, 2H,
= 8 Hz, -Ph), 7.74 (s, 1H, thienyl), 7.67 (s, 1H, thienyl), 4.62 (t, 2H,
4.01 (t,
= 2 Hz, -NCH2CH2O-), 3.51-3.40 (m, 8H), 3.32 (s, 3H, -OCH3). 13C{1H} NMR
J
= 8 Hz, -Ph), 7.84 (d, 2H,
J
J
= 2 Hz, -NCH2-),
J
(100 MHz, DMSO-d6): δ 151.7, 148.6, 134.3, 133.8, 132.9, 131.5, 130.7, 127.2, 127.0,
126.8, 122.2 (-CN), 112.6 (carbazolyl carbon attached to bromine), 112.2 (carbazolyl
carbon attached to bromine), 71.8 (alkyl carbon attached to N), 70.4, 70.0, 69.9, 69.3,
58.3, 43.4 (-OCH3). MALDI-TOF (m/z): C25H21Br2N3O3S2, calculated value 635.390
(M)+, found 635.785 (M)+. FTIR (KBr, cm-1): 2925, 2871 (ꢀꢁC-H stretching), 2229 ((ꢀꢁC≡N
stretching), 1608, 1474 (ꢀꢁC=N imidazole stretching), 1094, 1058, 1011 (ꢀꢁC-O stretching),
800 (ꢀꢁC-S stretching).
Synthesis of M1: Compound 14 (200 mg, 0.52 mmol) and
9 (152 mg, 0.26 mmol) were
dissolved in 7 mL of distilled THF in a 100 mL Schlenk flask under argon atmosphere.
The reaction flask was degassed three times by freeze-pump-thaw technique. In an
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 niddle for about 45 min. After that, the
K2CO3 solution was transferred to the reaction mixture through a cannula. Next,
Pd(PPh3)4 (18 mg, 0.015 mmol, 3 mol %) was added to it and the reaction mixture was
heated to 65 °C for 36 h. The colour of the reaction turned to dark green during the course
of the reaction. After cooling to room temperature, the solvent was concentrated to
minimum volume and extracted with ethyl acetate (3 × 50 ml) and dried over anhydrous
25

ACCEPTED MANUSCRIPT
MgSO4. The solvent was removed under diminished pressure and the residue was purified
by column chromatography eluting with hexane:ethylacetater to afford brown solid with
58 % yield (280 mg). 1H NMR (600 MHz, CDCl3): δ 8.48-8.40 (m, 2H, carbazolyl), 8.20-
8.16 (m, 2H, carbazolyl), 7.90-7.86 (m, 10H, carbazolyl), 7.51-7.47 (m, 4H, -Ph), 7.44-
7.41 (m, 2H, thienyl), 4.37-4.28 (m, 6H, -NCH2-), 1.94-1.83 (m, 6H, -NCH2CH2-), 1.30-
1.26 (m, 24H, alkyl proton), 0.88-0.85 (m, 9H, -CH3).13C{1H} NMR (150 MHz, CDCl3):
δ 150.0, 149.5, 149.1, 140.9, 139.5, 133.3, 132.5, 132.3, 130.0, 125.6, 125.123.4, 120.4,
118.9, 112.5, 109.1, 109.0, 108.8, 46.5, 43.2, 31.7, 31.5, 29.1, 29.0, 27.3, 22.6, 22.5, 22.4,
14.1, 14.0 (-CH3). MALDI-TOF (m/z): C63H65N5S2, calculated value 956.471 (M)+, found
956.481 (M)+.UV-Vis (THF): λmax (ε, M-1cm-1): 357 nm (2.03 × 104).
Synthesis of model complex M1·Hg2+: A clear solution of M1 (100 mg, 0.10 mmol) in 5 mL
THF was taken into a 50 ml RB flask. 2 mL solution of Hg(NO3)2·3H2O in MeOH was added.
The resulting mixture was stirred for 8 h. The brown coloured product was precipitated, filtered
off and washed with diethyl ether three times. The brownish coloured [M1.Hg2+(NO3)2] was
dried in vacuum (yield: 70%). MALDI-TOF (m/z): C63H65HgN6O3S2, calculated value 1219.426
+
1
for [M1·Hg(NO3)] , found 1219.847. H NMR (400 MHz, DMSO-d6): δ 8.65-8.51 (m, 2H,
carbazolyl), 8.30-8.22 (m, 2H, carbazolyl), 8.06-7.98 (m, 10H, carbazolyl), 7.70-7.58 (m, 4H, -
Ph), 7.50-7.43 (m, 2H, thienyl), 4.52-4.40 (m, 6H, -NCH2-), 1.79-1.70 (m, 6H, -NCH2CH2-),
1.20-1.07 (m, 24H, alkyl proton), 0.80-0.75 (m, 9H, -CH3). FTIR (KBr, cm-1): 2956, 2832 (ꢀC-H
̅
stretching), 2229 ( ꢀ C
̅
stretching), 1608, 1470 ( ꢀ C=N
̅
imidazole stretching), 1219 ( ꢀ C-N
̅
≡
N
stretching), 773 (
ꢀ̅C-S stretching).
Synthesis of P1: Compound
3
(178 mg, 0.33 mmol) and 9 (195 mg, 0.33 mmol) were
dissolved in 7 mL of distilled THF in a 100 mL Schlenk flask under argon atmosphere.
The reaction flask was degassed three times by freeze-pump-thaw technique. In an
another100 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 niddle for about 30 min. After that, the
K2CO3 solution was transferred to the reaction mixture through a cannula. Next,
Pd(PPh3)4 (7 mg, 0.006 mmol, 2 mol%) was added to it and the reaction mixture was
heated to 65 ºC for 72 h. The colour of the reaction turned to dark green during the course
of the reaction. After cooling to room temperature, the solvent was concentrated to
minimum volume and polymer was precipitated to a stirring methanol solution. After the
26

ACCEPTED MANUSCRIPT
complete precipitation, the methanol was removed and the brown colour polymer was
washed another two times with methanol. After drying under vacuum, 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 brown
coloured polymer. The polymer was finally purified by fractional precipitation in cold
distilled hexanes from a concentrated dichloromethane solution to achieve well-defined
P1. Yield: 260 mg (71%). . 1H NMR (600 MHz, CDCl3): δ 7.84-7.77 (m, 2H, carbazolyl),
7.73-7.60 (m, 4H, carbazolyl), 7.58-7.45 (m, 2H, thienyl), 7.41-7.29 (m, 4H, -Ph), 4.44-
4.15 (m, 4H, -NCH2-), 1.89-1.73 (m, 4H, -NCH2CH2-), 1.30-1.26 (m, 16H, alkyl proton),
0.91-0.87 (m, 6H, -CH3). 13C{1H} NMR (100 MHz, CDCl3): δ 134.7, 134.3, 132.4, 131.3,
130.5, 129.9, 127.1, 125.6, 125.1, 124.1, 123.3, 121.7, 120.5, 119.2, 118., 118.0,
113.3,112.4, 109.0 46.5 (-NCH2-), 43.2 (-NCH2-), 31.7, 31.5, 1.3, 29.0, 28.5, 27.2, 26.,
22.6, 22.5, 22.4, 14.0, 13.9 (-CH3). FTIR (KBr, cm-1): 2950, 2854 (ꢀꢁC-H stretching), 2228
((ꢀꢁC≡N stretching), 1605, 1468 (ꢀꢁC=N imidazole stretching), 1235 (ꢀꢁC-N stretching), 800
(
ꢀꢁC-S stretching). UV-Vis (THF): λmax (ε, M-1cm-1): 361 nm (2.09 × 104). GPC: Mw = 22.4
kDa, Mn = 16.5 kDa, PDI = 1.36, Td = 356 ºC.
Synthesis of P2: Compound (160 mg, 0.28 mmol) and 11 (179 gm, 0.28 mmol) were
5
dissolved in 7 mL of distilled THF in a 100 mL Schlenk flask under argon atmosphere.
The reaction flask was degassed three times by freeze-pump-thaw technique. In an
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 niddle for about 30 min. After that, the
K2CO3 solution was transferred to the reaction mixture through a cannula. Next,
Pd(PPh3)4 (8 mg, 0.005 mmol, 2 mol %) was added to it and the reaction mixture was
heated to 65 ºC for 72 h. The colour of the reaction turned to dark green during the course
of the reaction. After cooling to room temperature, the solvent was concentrated to
minimum volume and polymer was precipitated to a stirring methanol solution. The
polymer P2 was further purified following the procedure described for P1. Yield: 280 mg
1
(65%). H NMR (600 MHz, CDCl3): δ 8.14-8.08 (m, 2H, carbazolyl), 7.84-7.81 (m, 4H,
carbazolyl), 7.80-7.745 (m, 2H, thienyl), 7.69-7.45 (m, 4H, -Ph), 4.61-4.56 (m, 4H, -
NCH2-), 3.98-3.96 (m, 4H, -CH2O-), 3.56-3.40 (m, 16H,), 3.33-3.30 (m, 6H, -OCH3).
13C{1H} NMR (100 MHz, CDCl3): δ 151.3, 151.0, 141.0, 134.9, 134.6, 132.3, 130.8,
27

ACCEPTED MANUSCRIPT
126.9, 125.6, 125.0, 123.9, 122.9, 121.8, 118.7, 113.4, 112.3, 109.0, 108.6, 71.8(-NCH2-),
70.5, 70.4, 69.8, 69.3, 58.9, 46.3 (-CH3). FTIR (KBr, cm-1): 2960, 2842 (ꢀꢁC-H stretching),
2229 (ꢀꢁC≡N stretching), 1608, 1475 (ꢀꢁC=N imidazole stretching), 1260 (ꢀꢁC-N stretching).
UV-Vis (THF): λmax (ε, M-1cm-1): 356 nm (2.02 × 104). GPC: Mw = 18.7 kDa, Mn = 14.3
kDa, PDI = 1.31, Td = 305 ºC.
Acknowledgements
Authors acknowledge the support from the DST, Govt. of India
(DST/TM/SERI/FR/193). SKP thanks IIT Kharagpur for funding the purchase of
tetradetector GPC through Competitive Research Infrastructure Seed Grant (SGDRI,
IIT/SRIC/CHY/NPA/2014-15/81). DG acknowledges IIT KGP for doctoral fellowship.
Conflicts of interest
There are no conflicts to declare.
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