Arylene-vinylene terpyridine conjugates: Highly sensitive, reusable and simple fluorescent probes for the detection of nitroaromatics

Article abstract of DOI:10.1039/c7tc04178k

A series of highly emissive arylene-vinylene conjugated 4′-(4-{2-[aryl]-ethenyl}phenyl)-2,2′:6′,2′′-terpyridines (aryl = 4-methylphenyl, 4-fluorophenyl, 1-naphthyl, and 9-anthralyl in P1-P4 respectively) have been explored for the detection of nitroaromat

Full text of DOI:10.1039/c7tc04178k

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ISSN 2050-7526
PAPER
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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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Arylene-vinylene Terpyridine Conjugates: Highly Sensitive,
Reusable and Simple Fluorescent Probes for the Detection of
Nitroaromatics
Received 00th January 20xx,
Accepted 00th January 20xx
Amit Sil,a Dipanjan Giri,a and Sanjib K. Patra*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
A series of highly emissive arylene-vinylene conjugated 4'-(4-{2-[aryl]-ethenyl}phenyl)-2,2':6',2''-terpyridines (aryl = 4-
methylphenyl, 4-fluorophenyl, 1-naphthyl, and 9-anthralyl in P1-P4 respectively) have been explored for the detection of
nitroaromatic compounds (NACs). The synthesized push-pull fluorescent probes show remarkable sensitivity towards NACs
in solution, vapor and contact mode as ‘turn-off’ sensor which can be visualized by naked eye. The photophysical studies
reveal that the electron transfer occurs from the electron rich arylene-vinylene conjugated terpyridine to the electron
deficient NACs through supramolecular complexation which is further illustrated from the time-resolved fluorescence and
1H NMR titration experiment. The arylene-vinylene conjugated terpyridines offer excellent sensitivity towards picric acid
(PA) exhibiting nanomolar detection in contact mode, and ppm level detection in solution state with association constants
in the range of 2.54-8.08×104 M-1. The HOMO-LUMO energy levels have been calculated from electrochemical and TDDFT
studies to understand the efficacy and the mechanism of electron transfer from probe to NACs leading to fluorescence
quenching. The reversibility, recyclability and contact mode detection of PA in nanomolar level demonstrates its practical
utility
as
solid
state
kit
for
onsite
detection
of
NAC
explosives.
nuclear quadruple resonance,6 ion mobility spectrometers,7
surface enhanced Raman spectroscopy,8 chromatography
using an ultraviolet absorption detector,9 mass spectrometry,10
thermal neutron analysis,11 and electrochemical assay,12 are
being used for NACs detection in solution and in vapor phase.
Among them, fluorescence signaling is advantageous because
of its high sensitivity, quick response, cost efficiency,
portability, and operational simplicity. A considerable number
of fluorescent probes including polymers,3c,5a,13 gels,14 small
molecule sensors,15 nanoparticles,16 nano fibers17 and MOFs18
have been developed for the detection of NACs. Although the
rigid π-conjugated polymers show good sensitivity towards
NACs through optical signal amplification effect, this class of
polymeric probes suffer from solubility, processability and
multistep synthetic routes in many cases which are not viable.
Moreover, the detection of NACs by MOFs and nanoparticles
are not often preferable for practical application as they suffer
from poor solubility, complex structure and solution instability.
Therefore development of novel, simple and efficient sensing
probes based on small molecules which can be accessed
through economic and easy synthetic strategy is now current
interest to the research communities. Moreover, its well
defined structure, monodispersity and convenient method of
purification allow a clear understanding of the structure
property relationship, and permits superior performance
towards detection of NACs. The high electron deficient
property of the NACs facilitates efficient photo-induced
electron transfer (PET) from the electron rich π-conjugated
Introduction
Nitro-rich compounds are essential energetic materials mostly
used as component for the preparation of landmines and
explosive materials.1 For example, trinitrotoluene and picric
acid are heavily used as nitroaromatic (NAC) explosives. On the
other hand, nitroaromatics can deteriorate the environment
through short-term or long-term exposure by contaminating
the soil and water at toxic levels thus causing hazardous
effects on human health such as headache, weakness, anemia,
respiratory disorders, carcinogenicity, skin irritation and liver
injury.2 Large scale use of NAC explosives by terrorist groups,
has prompted the scientific community to develop novel and
synthetically ease sensing materials for easy, rapid, sensitive,
selective and economic detection of explosives both in air and
in solution.3
Various type of analytical methods such as energy
dispersive X-ray diffraction,4 fluorescence signaling,3a-c,5
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fluorophores. There are a considerable number of small moiety are twofold. Firstly, it can efficiently act as donor unit
molecule chromophores based on pyrene, coumarins, inducing tunable ICT band emission. Secondly, being a Lewis
purpurin, fluorescein, triphenylamine, hetero oligophenylene, base, pyridyl ‘N’ can act as proton abstractor and as synthon
pentacenquinone, fluoranthene and iptycene which have been for H-bonding offering beneficial supramolecular interaction
employed for the detection of NACs.15
between probe and analyte. Moreover, the presence of highly
Arylene-vinylene conjugated terpyridines represent a new π-conjugated backbone with tpy unit favors the formation of
category of highly luminescent π-conjugated chromophores strong intermolecular and π-π interaction with electron
with chelating function and relatively long lived excited deficient NACs. Most importantly, the syntheses of the probes
states.19 Arylene-vinylene unit is advantageous to improve the are very simple and economic. A series of arylene-vinylene
π-conjugation enabling strong intermolecular and π-π conjugated terpyridines were synthesized by varying the
interaction with electron deficient NACs. In this work, different electron donating and withdrawing nature of the arylene
aryl groups have been introduced in 4'-(4-{2-[aryl]- groups to tune their photophysical and consequently the
ethenyl}phenyl)-2,2':6',2''-terpyridyl conjugates through the sensing properties. The synthetic protocol for preparing 4'-(4-
vinylene linkage to tune the electronic and optical properties. {2-[aryl]-ethenyl}phenyl)-2,2':6',2''-terpyridine
The extended π-conjugation in the arylene-vinylene outlined in Scheme 1 following a stepwise approach starting
conjugated terpyridine is beneficial to reduce the energy of from 4'-(4-methylphenyl)-2,2':6',2''-terpyridine ( ). Compound
LUMO and hence the band gap of the probes, enhancing was synthesized following Krӧhnke method involving the
(
P1-P4
)
is
1
1
detection sensitivity. In the present work, we report facile and condensation of 4-methylbenzaldehyde with two equivalent of
easy synthesis of highly fluorescent arylene-vinylene acetyl pyridine followed by the formation of the central
terpyridine conjugates by varying the arylene group and their pyridine ring in ethanol using ammonia as a base.20 NBS-
detection efficiency towards NACs in solution and solid state. mediated allylic bromination of
The unique structural modification on the arylene core and amount of 2,2′-azobisisobutyronitrile (AIBN) to furnish
extension of π-conjugation offer interesting and favorable which upon reacting with PPh3 in refluxing toluene yielded the
properties for detection of NACs, thereby improving the phosphonium ylide (
) as evidenced by 31P{1H} NMR showing a
1 was achieved by catalytic
2
,
3
optical signal response.
singlet at 23.7 ppm (in CDCl3). Finally, a series of highly
luminescent fluorophores appended with tpy was achieved by
Wittig condensation between the aryl aldehyde and
3 in
Results and discussion
presence of t-BuOK in solvent free mild condition by grinding
in a mortar as green and economic synthetic protocol reducing
the use of hazardous organic solvents.19g The luminescent
Synthesis and Characterization
arylene-vinylene conjugated terpyridines
(P1-P4) were
unambiguously characterized by multinuclear NMR and HRMS
(ESI, +ve) analyses. 19F{1H} NMR (CDCl3) spectrum of P2 shows
one characteristic singlet at -114.1 ppm. 1H NMR (CDCl3) of
compound P1 shows one singlet at 2.37 ppm for the methyl
protons. The naphthyl and anthracenyl protons in P3 and P4
Arylene-vinylene unit has been chosen as a sensing probe for
the detection of NACs for its enlarged π-conjugation to
enhance the fluorescence response and also for its emission in
the visible range. The reasons for selecting terpyridyl (tpy)
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resonate as multiplet in the region of 7.88-8.69 ppm and 7.89-
8.43 ppm respectively. In 13C{1H} NMR spectra, the methyl
carbon in probe P1 resonates at 21.6 ppm. The carbons of the
aromatic rings for all the probes appear in the region of 115.3–
156.6 ppm. More importantly, the two vinylene protons
resonate in the range of 6.97-7.38 ppm as doublets with J ≥
16.0 Hz in 1H NMR spectra, confirming the trans-vinylene
linkage (ESI†). The trans-vinylene linkage is necessary to
acquire the extended π-delocalization as also noticed from the
molecular structure of P4 and similar terpyridyl conjugate, 4'-
(4-(2-(4-(N,N-diphenylamino)phenyl)ethenyl)phenyl)-2,2':6',2''-
terpyridine
obtained
from
single
crystal
X-ray
crystallography.19d,g Despite several attempts, we were not
successful to grow crystals of the newly synthesized P2 and P3
probes, suitable for single crystal X-ray diffraction studies. The
formation of the probes was further confirmed by mass
spectrometric analyses (HRMS) exhibiting the molecular ion
peaks ([M+H]+) with the expected isotopic distribution pattern
(ESI†).
ESI†). The coexistence of multiple degree of aggregation in thin
film is supported by the broad nature of the absorption
spectra.21 The energy band gap of the probes was accounted
by inspecting the edge of the solid state absorption spectra,
using the equation Egopt(eV) = 1240/λcut off. The optical band
gap values of P1-P4 were found to be 3.0, 3.03, 2.95 and 2.66
eV respectively.
Photophysical properties
The emission spectra of P1-P4 were recorded in CHCl3 as
The very good solubility in common organic solvents allowed
us to study the photophysical properties of the π-electron rich
arylene-vinylene conjugated terpyridines. The photophysical
1×10-5 M concentration at ambient temperature (28 °C).
Interestingly, the probes showed a gradual bathochromic shift
in photoluminescence spectra with λem from 417 nm to 498
nm exhibiting blue to green fluorescence while going from P1
properties of the synthesized probes (P1-P4) were studied in
CHCl3 in 1×10-5 M concentration at ambient temperature. Each
of the probes showed a lower energy absorption band at λmax
of 334-390 nm in addition to a relatively higher energy
absorption in the region of 258-285 nm (λmax) as shown in Fig.
1. The high energy absorption is assigned as intra-ligand
charge transfer (ICT) transition while the low energy band is
ascribed as π to π* transition of the conjugated molecular
to P4. In case of P1-P3, the compounds showed blue
fluorescence with emission maxima at 417, 435 and 437 nm
respectively whereas P4 exhibited green fluorescence with
emission maxima at 498 nm as shown in Fig. 2. The solid state
emission maxima for all the arylene-vinylene conjugated
terpyridines (P1-P4) showed bathochromic shift from the
solution state emission maxima, and were found to be 435,
440, 458 and 504 nm respectively (Fig. S28 in ESI†).The
fluorescence quantum yields (Φ) of all the sensing probes were
measured in CHCl3 using quinine sulfate (Φ = 0.54) in 0.1 M
H2SO4 solution as a reference at ambient temperature (28
°C).22 The PL quantum yields of the fluorophores are in the
range of 0.23-0.36, showing a gradual enhancement with
increasing π-conjugation. The photophysical data of all the
arylene-vinylene conjugated terpyridines are summarized in
Table 1.
backbone. The low energy absorption maxima for P1, P3 and
P4 is gradually red shifted with increasing number of fused
aromatic rings indicating enhanced π-delocalization. The solid
state absorption spectra were recorded on thin film samples
made by spin-coating of the probe solution on quartz plates.
The solid state absorption of the arylene-vinylene conjugated
terpyridines (P1-P4) follow the similar trend but red shifted by
12–18 nm with relatively broad absorption spectra in
comparison to the solution state spectra, presumably due to
strong intermolecular π–π stacking interactions (Table S2,
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π−π stacking. All the arylene-vinylene conjugated terpyridines
displayed less aggregation behavior with concentration of 10-5
M as evidenced from DLS experiments and as well as
concentration-dependent fluorescence study (Fig. S30 in ESI†).
Hence, the sensing application was explored in 10-5
M
concentration to minimize the self-association behavior of the
probes.
Fig. 3 Concentration dependent (a) fluorescence and (b) DLS
spectra of P4 in CHCl3 solution at ambient temperature.
It is well known in literature that the π−π interaction in
planar polyaromatic compounds induces intermolecular
aggregation in solution.15e,23 The concentration dependent
emission spectra of the sensing probes was carried out in
CHCl3 to determine the optimum concentration undergoing
the minimum self-aggregation. We were unable to record the
concentration-dependent UV-Vis spectroscopic analysis as the
absorbance of the compounds exceeded the instrument limit
at higher concentration. For example, the intensity of the
emission band decreases with a red-shift in emission maxima
from 498 to 512 nm (λem) for P4 with increasing concentration
from 10−5 to 10−3 M. This clearly demonstrates that the self-
assembled aggregates are formed in higher concentration
(~10-3 M) due to π-π interaction inducing fluorescence
quenching. However, this band was blue shifted to 498 nm
when the solution was diluted to 10−5 M. This behavior of the
fluorescence spectra attributed the dissociation of the
aggregates to form unimers in lower concentrated solution.
The fluorescence intensity of the emission maxima decreased
when the compound solution was diluted to 10-6 M. The other
arylene-vinylene conjugated terpyridines also showed similar
concentration-dependent fluorescence spectral change in the
corresponding concentrated and dilute solutions (Fig. S29 in
ESI†). To further confirm the formation of aggregates, the
particle size of the probes was measured by dynamic light
scattering (DLS) experiment in solution. DLS studies of all the
probes were carried out by varying the concentration ranging
from 10−3 to 10−5 M, and different particle sizes were found as
expected (Fig. 3). The average aggregate size (DH) of all the
probes in relatively higher concentrated (10−3 M) solution was
larger (DH = 164-342 nm) with broad distribution illustrating
the presence of various size of aggregates in the medium.
When diluted, the distribution was relatively narrower and the
aggregate size also decreased ascribing the presence of
smaller particle size (DH = 58–141 nm) of the sensing probe in
solution.13a,15i,15u This variation in size occurs as the extent of
molecular self-assembly increases in the relatively higher
concentration of the probe molecules due to the favorable
The highly emissive and π-electron rich nature of the
arylene-vinylene conjugated terpyridines allowed us to explore
their potential application as a fluorescence chemosensor.
NACs are highly electron deficient molecules and can undergo
strong interaction with electron rich π-conjugated fluorescent
probes mainly through π-π interaction resulting change in
emission properties. Keeping this in mind, all the arylene-
vinylene conjugated terpyridines were employed for the
detection of different NACs in CHCl3. To explore the potential
application, all the probes were treated with different
nitroaromatic compounds such as nitrobenzene (NB),
nitrotoluene (NT), 4-hydroxy nitrobenzene (HNB), 4-
nitrobenzoic acid (NBA), 2,6-dinitrotoluene (DNT), and picric
acid (PA). Due to the electron deficient nature of the NACs, the
fluorescence intensity of the probes readily quenched upon
addition of NACs as a result of facile electron transfer between
the fluorophore and the NACs. Interestingly, after the addition
of various nitroaromatics (with concentration of ~1x10-5 M to
~1×10-3 M) to the solution of P1 P4 probes (~1×10-5 M),
–
different percentage of emission quenching was observed.
Increase in the number of nitro groups enhances the electron
deficiency in nitroaromatic molecules, and thus the extent of
fluorescence quenching is increased. Therefore, on addition of
picric acid the magnitude of quenching was higher than the
other nitroaromatics used in this study, indicating the efficient
detection sensitivity of the probes towards explosive picric
acid. To prove the selectivity only towards NACs, fluorescence
spectra of the sensing probes were also measured in the
presence of other aromatic compounds such as chlorinated
(such as 1,2-dichlorobenzene and 1,4-dichlorobenzene) and
alkyl aromatics (such as toluene and mesitylene), showing no
significant change in fluorescence quenching (Fig. 4).
Since the anthracene functionalized arylene-vinylene
conjugated terpyridine (P4) exhibits highest quantum yield
among the four probes in its solution state, the sensing
efficacy of P4 is discussed in detail here. To gain an insight into
the sensing mechanism in more depth and to ensure the
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phenomenon, fluorescence spectroscopic titrations were excited state proton transfer from PA to the pyridyl nitrogen of
performed with a continuous variation of concentrations of the probes was ruled out as no new band formation was
NACs. Upon incremental addition of NACs the emission observed in fluorescence spectra upon treating with excess
intensity of P4 steadily quenched and no residual emission equivalent of PA to the probe. To confirm the possibility of
band was observed. The addition of 25 equivalents of PA to excited state proton transfer to arylene-vinylene conjugated
the solution of P4 resulted almost complete quenching of terpyridines, 2 µL of strong non-aromatic acid (trifluoroacetic
fluorescence emission. The similar quenching behaviors of the acid) was added to P4 solution under the same set of condition
other probes (P1
(Fig. S31, ESI†). The relative emission quenching of the probes recorded. Interestingly, the emission band at 498 nm was
P1 P4 towards different NACs is depicted in Fig. 4, disappeared with concomitant appearance of a new band
-P3) in the presence of PA are provided in ESI as used for PA and the corresponding fluorescence data was
(
-
)
demonstrating maximum emission quenching for PA.
centered at 588 nm with low emission intensity ascribing the
slow proton transfer. The strong acidic nature of the
trifluoroacetic acid induces the proton transfer which is not
observed for aromatic picric acid. Under the same set of
conditions, identical results were observed for the other
arylene-vinylene conjugated terpyridyl probes upon addition
of PA and TFA (Fig. S32 in ESI†).
To gain insight of quenching rate, the emission response of
the sensing probes (P1-P4) was investigated using Stern-
Volmer relationship.15l,m The Stern–Volmer (SV) equation is
expressed as Io/I = 1 + KSV [Q], where Io and I are the
fluorescence intensities in the absence and presence of the
analyte (NACs), [Q] is the analyte concentration and KSV is the
Stern−Volmer rate constant. The SV plots reveal high binding
constants of probe-NAC adducts with the highest value of KSV
in the order of 104 M-1. The Stern-Volmer constants of the
arylene-vinylene conjugated terpyridines towards different
NACs are summarized in Table 2 (Fig. S34 in ESI†). The linear
quenching response from Stern-Volmer plot upon incremental
addition of NACs demonstrates the static quenching through
the excited state interaction as presented in Fig. 7. These
results suggest that the quenching phenomena of the
fluorescent aggregates can be explained by the electron
transfer and/or energy transfer process between the π-
electron rich sensing probe and the electron deficient NACs.15h-
Fig. 4 (a) Quenching efficiency of all the probes (P1-P4)
towards different NACs in CHCl3. Probe (1), Toluene (TOL),
Dichlorobenzene (DCB), Nitrobenzene (NB), Nitrotoluene (NT),
p-Hydroxy nitrobenzene (HNB), Nitrobenzoic acid (NBA), 2.6-
Dinitrobenzene (DNT), Picric acid (PA); (b) Fluorescence
titration of P4 upon incremental addition of PA; (inset)
Fluorescence quenching behavior of P4 at 498 nm.
The excellent ‘turn-off’ sensing behavior of P4 towards PA
encouraged us to investigate the visual change of P4 probe in
solution before and after addition of PA. Upon addition of PA,
the colour of P4 solution turned immediately from light green
to intense yellow probably due to the formation of charge
transfer complex with PA. More interestingly, the bright green
fluorescent solution of P4 turned to nonemissive when the
l,24
Moreover, the spectral overlap of the absorption and
emission bands of the sensing probes allows the energy
transfer from the excited state of the arylene-vinylene
solution was illuminated under 365 nm light (Fig. 5). Like P4
,
the other blue emissive probes (P1-P3) also turned to
conjugated terpyridines (P1–P4) to the ground state of PA,
nonfluorescent immediately after treating with PA,
demonstrating easy and efficient detection of PA in naked eye
(Fig. S42 in ESI†)
further increasing the fluorescence quenching efficiency (Fig.
S33 in ESI†).15f The lowest detectable concentration of PA was
estimated using the linear response of fluorescence intensity
of the probe with the concentration of PA to assess the
sensitivity of the probes. The limit of detection (LOD)15k of the
probes towards PA was found to be in the range of 1.31-
The possibility of forming protonated species originated by
Fig. 5 Visual appearance of P4 before and after addition of
PA under (a) ambient light and (b) UV illumination at 365
nm.
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2.94×10-7 M (31-70 ppm) signifying its applicability even in the gap (vide supra) following the equation, ELUMO (eV) = EHOMO
+
sub-micromolar range (Fig. S35 in ESI†).
Eg
.
opt 27 Thus, the ELUMO energy levels were calculated as -3.02, -
P4 respectively As expected,
The time resolved fluorescence spectra of P1-P4 in 3.04, -3.05 and -3.25 eV for P1
–
.
chloroform were also recorded to understand the nature of introduction of more numbers of fused aromatic rings in the
the excited states as depicted in Fig. 6. The fluorescence molecular backbone induces enhancement in π-conjugation
lifetime of all the probes were found to be in the range of resulting decrease in the HOMO-LUMO energy gap (Table 3).
0.94-2.88 ns as measured by the time correlated single photon The LUMO energy levels of electron deficient nitroaromatics
counting (TCSPC) method (Table 1). The fluorescence lifetime are higher than the HOMOs of the probe molecules but lower
values of P1, P3 and P4 probes increase with the extended π- than the LUMO levels of the probes. The facile photo-induced
conjugation. However, the fluorescence lifetime of P2 is electron transfer (PET) occurs from the LUMO of the excited
relatively less than that of the other probes presumably due to probes to LUMO of the NACs followed by reverse electron
the higher band-gap (Egopt = 3.03 eV). The fluorescence lifetime transfer to the HOMO of the sensing probes in a non-radiative
of the probes were also evaluated in the presence and absence process resulting in fluorescence quenching as depicted in Fig.
of PA to understand the fluorescence quenching mechanism. 8.5f,15f,28
. Consequently, higher degree of fluorescence
For all the probes, the fluorescence life time values were found quenching was observed for the utmost electron deficient
to be nearly invariant upon addition of incremental nitroaromatics such as PA due to its relatively lower LUMO
equivalents of PA suggesting of static quenching occurring energy levels. Furthermore, the presence of large π-
through electron transfer from the excited state of the probe conjugated backbone facilitates the formation of non-emissive
to electron deficient PA (Fig. S36 in ESI†).15l,m,25
charge transfer complex through π-π stacking interaction
To inspect the ‘turn-off’ sensing mechanism, HOMO and between the planar arylene-vinylene moiety and the
LUMO energy levels have been calculated from cyclic nitroaromatic ring favoring electron transfer. In order to gain
voltammetry experiments conducted in CH3CN solution using an insight of the structural properties and calculated
n-Bu4NPF6 (0.1 M) as supporting electrolytes, Pt disc working HOMO/LUMO energy levels of the probes, frontier orbitals,
electrode, Pt wire counter electrode and Ag/AgCl reference geometry optimization for all the arylene-vinyline conjugated
electrode (Fig. S46 in ESI†). The EHOMO energy levels of P1
-
P4 terpyridines was carried out using the Gaussian 03 program
with B3LYP/6-31g* basis sets (Fig. S48 in ESI†).29 The time-
dependent density functional theoretical (TDDFT) studies were
also performed to predict the excited state HOMO/LUMO
energy levels in chloroform at the same theory level. The
calculated electronic transition are summarized in Table S8
(ESI†). For P1-P4, the HOMO orbitals are delocalized on the
arylene-vinylene moiety whereas LUMO orbitals are largely
delocalized in the arylene-vinylene substituted phenyl
conjugates and the central pyridine ring. Detection limits are
based on the efficiency of the electron transfer process, which
can be improved by increasing analyte−fluorophore binding
interactions and matching the frontier molecular orbital
energies of the fluorophore with the LUMO of the NACs. From
the energy level diagram, we could observe that the optical
band gaps are in the range 2.66−3.03 eV showing slight
variation by changing arylene substituents on para-position of
the phenyl group in 4'-phenyl-2,2':6',2''-terpyridine core.
Fig. 6 Time resolved fluorescence spectra of the probes
(P1-P4) in CHCl3.
To further investigate the mode of supramolecular
interaction between the π-conjugated arylene-vinylene
conjugated terpyridines and NACs, 1H NMR spectroscopic
titration studies were carried out with incremental addition of
picric acid (PA) as presented in Fig. 9. The Ha,a' and Hb,b' pyridyl
protons resonated at 8.76 and 7.81 ppm in free P3
experienced downfield shift by 0.02 and 0.01 ppm respectively
in P3-PA supramolecular complex. This downfield behavior is
presumably due to the withdrawl of electron density from P3
by PA indicating the possibility of weak H-bonding interaction.
More importantly, the signal centered at 7.97 ppm for the
naphthyl protons (Hj,k,l) up fielded to 7.95 ppm upon addition
of PA suggesting π-π intermolecular interactions between the
naphthyl moiety and the aromatic ring of PA.15e,k For P4, the
similar observation was witnessed. However, signals
Fig. 7 (a) The Stern-Volmer plot for the P4 towards different
nitroaromatics; 1: NB, 2: NT, 3: HNB, 4: NBA, 5: DNT, 6: PA. (b)
Plot for evaluation of LOD for P4 towards PA.
were estimated as -6.02, -6.07, -6.0 and -5.91 eV respectively,
based on their onset oxidation potential by considering EHOMO
(eV) = -(Eoxonset + 4.71).26 As there was no prominent reduction
process for the probe molecules, the ELUMO energy levels were
estimated from the EHOMO energy levels and its optical band
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Fig. 9 Change in chemical shift of the protons in P3 probe
during 1H NMR titration in CDCl3 on incremental addition of PA
(* = residual CHCl3).
washed and dried films were recorded, and the whole process
was repeated multiple times. The initial fluorescence intensity
was significantly retained after repeated use of the sensing kit,
indicating high reproducibility and photostability of the probe.
The quenching efficiency of 42% was observed even after five
cycles of exposure to picric acid (Fig. 10). Moreover, identical
results were observed for the other arylene-vinylene
conjugated terpyridines and the quenching behavior has been
depicted in Fig. S39 (ESI†).
Nitroaromatics are also recognized as environmental
contaminants polluting soil and drinking water due to short-
term or long-term exposure in environment. Considering the
fact, the sensing ability of the thin film was measured towards
picric acid solution in submicromolar concentration. The thin
film was immersed separately into the different concentration
of aqueous picric acid solutions, and the percentage of
fluorescence quenching was monitored by measuring the solid
state emission. The study revealed that the fluorescence
emission intensity was quenched by 75% for P4 when the
concentration of the picric acid was 100 µM (Fig. 11), which
was clearly visible in naked eye.
corresponding to the anthracenyl protons were relatively
complex due to the overlapping of the aromatic proton
resonances. Interestingly, for P1 and P2 probes, the 4′-
phynelene ring was involved in π-π stacking interaction with
PA ring as manifested by the up field chemical shift of 4′-
phynelene protons (ESI†).
The excellent ‘turn-off’ sensing behavior of the probes
towards NACs motivated us to explore the practical application
as on-site detection of picric acid. In order to demonstrate the
potential utility of the sensing probes towards PA, solid state
emission study was investigated by making thin film of all the
arylene-vinylene conjugated terpyridine probes. The thin films
were fabricated by spin coating probe solution (2 mg probe in
0.2 mL chloroform) on a quartz plate with a rate of 1000 rpm.
The film was kept in a glass jar containing pinch of PA crystals
at ambient temperature to measure the fluorescence response
of the thin film towards PA vapour. The initial fluorescence
intensity of all the probes was found to decrease gradually as a
function of increasing exposure time. It was observed that the
initial emission of P4 was reduced to 37% after 60 s of
exposure time, and ultimately reached to its equilibrium at 540
s by reducing the emission intensity to 81% as shown in Fig.
10. To check the reusability of the films, it was exposed to the
vapor of picric acid for 420 s, and the emission spectrum was
recorded. After that, the film was washed several times with
milli-Q water and dried in hot air. The emission spectra of the
The scanning electron microscopic image analyses of all the
probe samples (prepared by drop casting of 5 x 10-4
M
chloroform solution on aluminum plate) were conducted in
presence and absence of picric acid to examine the change in
surface morphology. All the sensing probes show porous
surface morphology which is highly necessary to be a good
nitroaromatic sensor by enhancing the probe-analyte
interaction. P1 showed two-dimensional rod-shaped
morphology whereas leaf-like porous bed surface morphology
was observed for P2. Interestingly, for P3 and P4, hierarchical
structures constructed by the three dimensional flower-like
nano-objects with porous bed were detected. The porous
morphology of the probes was altered to irregular shape upon
treatment with PA as studied by FESEM analysis (Fig. 12).
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Fig. 10 (a) Quenching of fluorescence intensity of the thin film
of P4 upon exposure to PA vapor by varying exposure time; (b)
Repeatability test of P4 probe film demonstrating reusability.
Fig. 11 Emission spectra of thin film (P4) after exposing
aqueous PA solution by varying concentration.
Fig. 12 FESEM images of the sensing probes before (left) and
after (right) treating with PA a) P1, b) P2, c) P3, d) P4 (Samples
were prepared by drop casting of 5x10-4 M chloroform solution
on aluminum plate); Scale bar: 5 μm.
Recently contact mode approach for detecting trace
nitroaromatic explosives has received great attention for real
time field application. Considering the porous surface nature
of the probes and the efficient solution mode detection, we
carried out a surface sensing approach to test the efficiency of
our probes for the detection of trace nitroaromatics residue.
For contact mode sensing, the filter paper (Whatman 42) test
strips were dipped into the concentrated chloroform solution
of the probes and dried under reduced pressure. To test the
possibility, different concentration (10-3-10-9 M) of picric acid
solution in chloroform was prepared and 5 µL of each solution
was spotted on each fresh test paper strips. The visual change
of the fluorescence color was monitored under the
illumination of 365 nm light exhibiting dark black spots for 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 eye is even up to 10-9 M level as depicted in Fig. 13,
chemosensor for the detection of nitroaromatics both in
solution and solid state. The detail photophysical,
electrochemical and sensing studies including life time
measurement suggest that the fluorophores behave as ‘turn
off’ sensor through static quenching induced by PET from the
photo-excited probe to the electron deficient nitroaromatics,
as a result of favorable HOMO-LUMO energy levels, by
supramolecular complex formation as evidenced by 1H NMR
titration. The sensing ability of the probes P1-P4 towards NACs
as filter paper strips was successfully examined to
demonstrate the practical utility as on-site detection kit for
establishing the probes (P1-P4) as excellent and efficient
chemosensors for instant visual detection of trace picric acid in
solution and as well as in solid state (Fig. S41 in ESI†).
Conclusions
In conclusion, we have successfully demonstrated that the
highly emissive arylene-vinylene conjugated terpyridines with
tunable fluorescence properties by varying the aromatic
substituents on arylene-vinylene unit can act as efficient
Fig. 13. Visual detection of PA of different concentration by the
paper test strips coated with P4 probe (under 365 nm light)
demonstrating remarkable sensitivity towards trace PA upto
nanomolar level.
8 | J. Name., 2012, 00, 1-3
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nanomolar level visual detection of NACs. This work clearly chromatography (ethyl acetate: hexanes, 4:1) on silica gel (60-
reveals a novel platform for further development of simple, 120 mesh) to achieve analytically pure white solid product.
1
easily processable, selective, highly sensitive, economic and Yield: 0.74 g (56%); H NMR (CDCl3, 400 MHz): δ, 6.98 (d, J = 8
reusable chemosensor for explosive and environmentally Hz, 2H), 7.07-7.11 (m, 2H), 7.35-7.38 (m, 4H), 7.50 (d, J = 8 Hz,
pollutant NACs. For further improvement of sensitivity and 2H), 7.63 (d, J = 8 Hz, 2H), 7.89-7.94 (m, 4H, py), 8.68 (d, J = 8
efficacy, efforts to the structural modifications of the arylene- Hz, 2H, py), 8.74 - 8.77 (m, 4H, py) ; 13C{1H} NMR (CDCl3, 100
vinylene conjugates by fine tuning of electronic properties is MHz): δ 115.3, 115.6, 115.8, 118.6, 121.5, 124.0, 126.3, 126.9,
underway in our laboratory. Highly blue-green light emitting 127.7, 128.0, 129.1, 129.5, 129.5, 137.0, 137.3, 149.3, 156.1,
nature of this unique class of functionizable arylene-vinylene 156.5. 19F{1H} (CDCl3, 376 MHz): 114.1 ppm. HRMS (ESI+):
terpyridinyl conjugates in solution and in solid state with good C29H21N3F1, Calculated value 430.1720 ([M+H]+); experimental
photostability can also be explored for developing as active 430.1719 ([M+H]+); FTIR (KBr, cm-1): 2923 (ꢀŬC-H stretching), 1586
component in organic light emitting devices.
(
ꢀŬC=C stretching), 1514 (ꢀŬC=N stretching); λmax(ε): 339 nm (4.3 × 104 M-
1cm-1), 283 nm (3.2 × 104 M-1cm-1); λem: 435 nm (λex: 339 nm).
Experimental Section
4'-(4-{2-[2-Naphthyl]-ethenyl} phenyl)- 2,2':6',2''-terpyridine
(P3)
Materials and methods have been described in ESI†.
P3 was prepared analogously to P1 using 1-naphthaldehyde
(0.60 mL, 4.65 mmol), 4-(2,2':6',2''-terpyridyl-4')-benzyl
triphenylphosphonium bromide (1.0 g, 3.1 mmol) and
potassium tert-butoxide (2.08 g, 18.6 mmol). The pure product
was isolated as pale yellow solid after purification by column
chromatography (ethyl acetate: hexanes, 4:1) on silica gel (60-
Synthesis and characterization
Compounds 1-3 have been synthesized following the literature
procedure (ESI†).19c
Synthesis
of
4'-(4-{2-[p-Tolyl]-ethenyl}phenyl)-
2,2':6',2''-
terpyridine (P1)
1
120 mesh). Yield: 0.70 g (49%); H NMR (CDCl3, 400 MHz): δ
7.21 -7.26 (m, 2H), 7.36-7.39 (m, 2H), 7.50-7.58 (m, 2H), 7.74-
7.76 (m, 2H, py), 7.80-7.84 (m, 2H, py), 7.88-7.92 (m, 3H), 7.98-
8.02 (m, 3H), 8.28 (d, J = 8Hz,1H), 8.70 (d, J = 8 Hz, 2H, py),
8.75-8.76 (m, 2H, py), 8.8 (s, 2H, py); 13C{1H} NMR (CDCl3, 100
MHz): δ 118.9, 121.7, 124.0, 125.9, 126.5, 126.9, 127.1, 127.6,
127.9, 128.5, 128.9, 129.2, 129.9, 131.3, 131.7, 134.0, 135.1,
137.2, 137.9, 138.7, 149.3, 150.0, 156.2, 156.5; HRMS (ESI+):
C33H24N3, Calculated value 462.1970 ([M+H]+); experimental
462.1967 ([M+H]+); FTIR (KBr, cm-1): 2926 (ꢀŬC-H stretching), 1577
4-(2,2':6',2''-Terpyridyl-4')-benzyl
triphenylphosphonium
bromide (1.0 g, 3.1 mmol) and potassium tert-butoxide (2.08 g,
18.6 mmol) were combined using a mortar and pestle, and the
yellow medium was aggregated until a light orange powder
formed. To it 4-methylbenzaldehyde (0.56 mL, 4.65 mmol) was
added and the combined mixture was grinded vigorously for
about 30 min. After the mixture became sticky, 5ml of
dichloromethane was added and the mixture was continuously
grinded for another 10 min. After completion of the reaction
(monitored by TLC), the mixture was dispersed in 100 mL of
dichloromethane and worked up with brine solution followed
by water. The organic part was collected and dried over
anhydrous MgSO4, filtered and concentrated. The solid residue
was stirred in distilled methanol for overnight at room
temperature. The precipitated solid was isolated by vacuum
filtration, washed with water (3×10 mL), methanol (5×10 mL)
and diethyl ether (3×10 mL). The solid residue was further
purified by column chromatography (ethyl acetate: hexanes,
4:1) on silica gel (60 - 120 mesh) to achieve analytically pure
(
ꢀŬC=C stretching), 1566 (ꢀŬC=N stretching); λmax(ε): 344 nm (2.7× 104 M-
1cm-1), 284 nm (3.3 × 104 M-1cm-1); λem: 433 nm (λex: 344 nm).
4'-(4-{2-[9-Anthryl]-ethenyl}phenyl)- 2,2':6',2''-terpyridine
(P4)
P4 was prepared using a similar procedure as that for P1 using
9-anthraldehyde (0.9 g, 4.65 mmol), 4-(2,2':6',2''-Terpyridyl-
4')-benzyl triphenylphosphonium bromide (1.0 g, 3.1 mmol)
and potassium tert-butoxide (2.08 g, 18.6 mmol). The pure
product was isolated as bright yellow solid after purification by
column chromatography (ethyl acetate: hexanes, 4:1) on silica
gel (60-120 mesh). Yield: 0.32 g, (51%);; 1H NMR (CDCl3, 400
MHz): δ 7.04 (d, J = 16.8 Hz, 1H), 7.37-7.40 (m, 2H), 7.47-7.51 (
m, 4H), 7.82 ( d, J = 8.4 Hz, 2H), 7.89-7.93 (m, 4H), 8.02-8.06
(m, 4H), 8.39-8.43 (m, 3H), 8.68-8.77 (m, 4H, py), 8.83 (s, 2H,
py); 13C{1H} NMR (CDCl3, 100 MHz): δ 118.9, 121.6, 124.0,
124.3, 125.5,126.4, 126.9, 127.4, 127.7, 127.9, 128.4, 128.9,
129.3, 129.9, 131.8, 132.7, 137.1, 138.1, 149.4, 149.9, 156.2,
156.5; LCMS (ESI+): C37H28N3, Calculated value 512.2 ([M+H]+);
experimental 512.4 ([M+H]+); FTIR (KBr, cm-1): 2924 (ꢀŬC-H
stretching), 1584 (ꢀŬC=C stretching), 1569 (ꢀŬC=N stretching); λmax(ε): 391 nm
lemon yellow solid product. Yield: 0.71
g (54%). The
characterization data were identical to those previously
reported by us following a different synthetic approach
(ESI†).19h
4'-(4-{2-[4-Fluorophenyl]-ethenyl}phenyl)-2,2':6',2''
terpyridine (P2)
P2 was prepared using a similar procedure as that for P1 using
4-(2,2':6',2''-terpyridyl-4')-benzyl
triphenylphosphonium
bromide (1.0 g, 3.1 mmol) and potassium tert-butoxide (2.08 g,
18.6 mmol) and 4-fluorobenzaldehye(0.58 mg, 4.65 mmol).
The solid residue was further purified by column
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(1.5 × 104 M-1cm-1), 286 nm (3.1 × 104 M-1cm-1); λem: 498 nm
(λex: 391 nm).
9
(a) R. Hodyss and J. L. Beauchamp, Anal. Chem., 2005, 77
,
3607; (b) R. V. Taudte, A. Beavis, L. W. Wilde, C. Roux, P.
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10 (a) A. Popov, H. Chen, O. N. Kharybin, E. N. Nikolaev and R.
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through Fast Track scheme (SB/FT/CS-010/2012). SKP thanks
IIT Kharagpur for funding the purchase of the electrochemical
workstation through Competitive Research Infrastructure Seed 13 (a) S. Shanmugaraju, C. Dabadie, K. Byrne, A. J. Savyasachi,
D. Umadevi, W. Schmitt, J. A. Kitchenc and T.
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acknowledge IIT KGP for doctoral fellowship.
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Journal of Materials Chemistry C
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DOI: 10.1039/C7TC04178K
Arylene-vinylene Terpyridine Conjugates: Highly Sensitive, Reusable and
Simple Fluorescent Probes for the Detection of Nitroaromatics
Amit Sil,a Dipanjan Giri,a and Sanjib K. Patra*a
TOC
A
series of highly emissive arylene-vinylene terpyridine
conjugates have been developed for the detection of
nitroaromatic (NAC) explosives as efficient and reusable
fluorescent probes.