A sensitive TICT Probe exhibiting ratiometric fluorescence repose to detect hydrazine in solution and gas phase

Article abstract of DOI:10.1016/j.saa.2020.118153

A twisted intramolecular charge transfer (TICT) based probe, dicyanovinyl-9-phenylanthracene (DPA) has been designed and synthesized for the detection of hydrazine (N₂H₄) with good limit of detection (LOD, 7.85 nM (0.25 ppb)). Upon interaction with hydrazine the terminal electron withdrawing dicyanovinyl function is changed to electron donating amino/hydrazone function. Consequently, the significant change in the photophysical property of the probe is attributed to a change in orientation of charge propagation. The probe with hydrazine shows ratiometric fluorescence “turn-on” response as well as naked-eye sensitive color change in the medium. The surface morphology studies (SEM and TEM) suggested about amorphousness and crystalline nature of the probe DPA and derivative DPA-HDz, respectively. The conducting behavior of the probe decreases upon interaction with hydrazine because of decrease in amorphousness of the matrix and increase in relatively more rigid crystalline structure. Additionally, the probe been utilized to detect hydrazine vapor in solution and on test paper strip with good naked-eye sensitive responses.

Full text of DOI:10.1016/j.saa.2020.118153

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A sensitive TICT Probe exhibiting ratiometric fluorescence
repose to detect hydrazine in solution and gas phase
Ramesh C. Gupta, Sushil K. Dwivedi, Rashid Ali, Syed S. Razi,
Rudramani Tiwari, S. Krishnamoorthi, Arvind Misra
PII:
S1386-1425(20)30131-1
https://doi.org/10.1016/j.saa.2020.118153
SAA 118153
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date: 5 December 2019
Revised date:
10 February 2020
12 February 2020
Accepted
date:
Please cite this article as: R.C. Gupta, S.K. Dwivedi, R. Ali, et al., A sensitive TICT
Probe exhibiting ratiometric fluorescence repose to detect hydrazine in solution and gas
phase, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy(2020),
https://doi.org/10.1016/j.saa.2020.118153
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Graphical Abstract
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A sensitive TICT Probe exhibiting ratiometric fluorescence repose to detect Hydrazine in
solution and gas phase
Ramesh C. Gupta^, Sushil K. Dwivedi^, Rashid Ali, Syed S. Razi, Rudramani Tiwari,
S. Krishnamoorthi and Arvind Misra*
Department of Chemistry, Institute of Science, Banaras Hindu University,
Varanasi-221005, UP INDIA
Corresponding author: amisra@bhu.ac.in; arvindmisra2003@yahoo.com
Tel: +91-542-6702503; Fax: +91-0542-2368127, 2368175.
^ Authors have made equal contribution
Abstract
A twisted intramolecular charge transfer (TICT) based probe, dicyanovinyl-9-phenylanthracene
(DPA) has been designed and synthesized for the detection of hydrazine (N2H4) with good limit
of detection (LOD, 7.85 nM (0.25 ppb)). Upon interaction with hydrazine the terminal electron
withdrawing dicyanovinyl function is changed to electron donating amino/hydrazone function.
Consequently, the significant change in the photophysical property of the probe is attributed to a
change in orientation of charge propagation. The probe with hydrazine shows ratiometric
fluorescence “turn-on” response as well as naked-eye sensitive color change in the medium. The
surface morphology studies (SEM and TEM) suggested about amorphousness and crystalline
nature of the probe DPA and derivative DPA-HDz, respectively. The conducting behavior of the
probe decreases upon interaction with hydrazine because of decrease in amorphousness of the
matrix and increase in relatively more rigid crystalline structure. Additionally, the probe been
utilized to detect hydrazine vapor in solution and on test paper strip with good naked-eye
sensitive responses.
Keywords: TICT, fluorescence, ratiometric, conductivity, N2H2
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Introduction
Among the different kind of amines hydrazine (N2H4) has wide applications in industries such
as, corrosion inhibitor for boilers, pharmaceutical intermediates, catalysts, emulsifiers and
antioxidants [1-5]. Chemically, hydrazine (N2H4) is highly reactive base and has good reducing
property. Because of combustible nature it has uses in missile, rocket propulsion systems and as
reactant in fuel cells [6-8]. Hydrazine has also been drawn in terrorist incidents [8]. Also,
hydrazine is easily miscible in water and may reach to human body either orally or through the
skin. Some literature reports about the generation of hydrazine inside the body by the
metabolism of some drugs which increases the chance of intoxication [2, 9]. The high
concentration of hydrazine and/or its derivatives may cause irritation in eyes, nose and throat,
temporary blindness, dizziness, nausea, pulmonary edema, coma, blood abnormalities, defect in
DNA and even damage of some vital organs like, liver, lungs, kidneys, and human central
nervous system [10, 11]. The U.S. Environmental Protection Agency (EPA) identified hydrazine
as a potential hepatotoxic, mutagenic and carcinogenic agent for human body with threshold
limit value (TLV) of 10 ppb [12-14]. Therefore, there is increase attention among the scientific
community to develop some convenient methods for the detection of hydrazine. Analytical
methods like, spectrophotometry, fluorimetry, potentiometry, chromatography, voltammetry,
amperometry, and others [15-24] have been developed for the quantification of trace amount of
hydrazine but most of them require complicated pre-treatment process and involve use of
sophisticated instrumentation. Therefore, the development of selective and sensitive method that
can sense even a low concentration of hydrazine through naked-eye sensitive chromo and
fluorogenic response is worth to investigate. Also, the noninvasive fluorescence based methods
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are economical, sensitive, selective, and very much helpful in real-time monitoring of important
studies related to diagnostics, biological assays and environmental sciences [25-31].
During past few years, various classes of fluorescent probes have been developed for on line
detection of low concentration of specific metal ions, anions and biomolecules without any pre-
treatment of the sample [32-39]. The chemodosimeters are special type of chemosensor that
encourages sensing event intelligently through an irreversible chemical transformation [40-42].
In the same context, fluorescent chemodosimeter particularly, exhibiting ratiometric sensing
response are more precise in quantification of guest concentration at two wavelengths and
reduces the chance of environmental effect by self built in corrections of the two signals [40].
However, for amines like, hydrazine the number of such chemodosimeters are limited [43-51].
For instance, Kumar et al. [52] developed an intramolecular charge transfer (ICT) based
fluorescent probe in which electron withdrawing dicyanovinyl function transformed to
hydrazone and oxime functions in the presence of hydrazine and hypochlorite, respectively. Xie
et al. [53] developed dicyanovinyl-(diphenylamino)-anthracene based fluorescence turn-on TICT
probe for hydrazine. Kim et al. [54] explored naphthalene core containing electron donor
(D)−bridge−acceptor (A) type dipolar fluorophore to detect hydrazine by hydrazone formation.
Chang et al. [55] and Peng et al. [56] utilized coumarin chromophore to selectively detect
hydrazine by deprotection of levulinate group and hydrazone formation, respectively.
Thus, keeping in mind the importance aspects related to typical photophysical mechanism and of
hydrazine the present contribution describes synthesis of a sensor motif based on TICT
mechanism to detect hydrazine through a reaction based chemodosimeter approach and
ratiometric fluorescence response. We previously reported some molecular probes based on
tricyano-ethyl-phenyl-phenanthro-imidazole (TCPPI) [57], 2,2-dicyanovinly anthracene [58] and
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tetrasubstituted imidazole core containing derivatives [59] for cyanide anion through Michael
type nucleophilic addition and cyclization reactions. The developed probes have shown
sensitivity for fluorescence “turn –on” and “turn – off” responses due to modulation in ICT and
ESIPT process, respectively. Further, we thought that introduction of dicyanovinylphenyl as a
terminal substituent at anthracene (AN) chromophore will certainly enhance unidirectional
charge transfer (CT) feature from AN unit to electron withdrawing cyano function and generate
emission toward high wavelength region [60, 61]. Secondly, the dicyanovinyl (DCV) function of
the probe will work as a suitable receptor site to react with hydrazine via nucleophilic addition
reaction. And subsequent displacement of malononitrile unit from the probe will generate
electron donating, hydrazone and/or amino function at the terminal position [62, 63]. Thus, due
to change of polarity at the terminal position, by the generation of new electron rich N atom
containing functions, will restrict the original charge flow from AN to vinyl function. In other
word the orientation of CT will revert toward the AN chromophore unit. Consequently, the
significant change in photophysical properties of probe would favor ratiometric response
wherein the TICT emission will disappear and ICT emission dominate in blue region.
As expected, the molecular probe upon interaction with different class of amines showed high
selectivity for hydrazine in THF and displayed enhanced emission ratiometrically. Also, the
naked eye sensitive bright fluorescent red color of the probe changed to a blue color. Moreover,
the electrical properties of DPA revealed about high dc conductivity due to enhancement in
amorphousness of the matrix. The SEM and TEM images depict about the amorphous nature for
DPA while the morphology of the new derivative, DPA-HDz changes to crystalline flower like
structure wherein the probe gain more insulating character. The probe DPA has also been
explored to detect N2H4 vapor in solution and on test paper strips.
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Experimental Section: The experimental section including general methodology, estimation of
quantum yield, limit of detection, conductivity measurements, preparation of samples for
titration SEM, TEM studies, and synthesis of probe have been given in electronic supportive
information file (ESI).
Synthesis of probe DPA: Compound 2 (282 mg, 1 mmol) and malononitrile (66 mg, 1 mmol)
were taken in dichloromethane (10 mL) and few drops of triethylamine was added. The reaction
mixture was stirred at room temperature for 2-3hr. After completion of reaction the solvent was
evaporated in vacuum to afford orange-red color compound which was further purified by
column chromatography using EtOAc: hexane (1:9, v/v) as eluent. Yield 66%; 1H NMR
(500MHz, CDCl3), δ (ppm): 8.56 (s, 1H), 8.15-8.13 (d, 2H, J = 8.5 Hz), 8.08-8.07 (d, 2H, J = 8.5
Hz), 7.93 (s, 1H), 7.65-7.64 (d, 2H, J = 7.5 Hz), 7.56-7.54 (d, 2H, J = 8.5 Hz), 7.49-7.41 (t, 2H,
13
J1 = 8.5, J2 = 9.0 Hz), 7.40-7.39 (t, 2H, J1 = 5.0, J2 = 3.5 Hz). CNMR (125 MHz); δ (ppm):
159.5, 146.4, 134.2, 132.8, 131.3, 130.8, 129.7, 128.8, 127.9, 126.2, 125.9, 125.4, 123.9, 122.9.
IR (KBr) vmax (cm-1): 2226 and 2193 (-C≡N), 1588 (-C=C). HRMS m/z: [DPA + H]+ = 331.1235
(Calculated = 331.1230).
Synthesis of DPA-HDz: To a solution of DPA (82 mg, 0.25 mmol) in tetrahydrofuran (THF)
(5.0 mL) N2H4 (8.0 µL, 0.25 mmol) was added and stirred the reaction mixture at room
temperature for 1 hr. The reaction was monitored on TLC. After complete consumption of
starting reactants THF was evaporated under reduced pressure. The crude product was purified
by column chromatography using EtOAc:hexane (1:9, v/v) as eluent to afford desired compound.
1
Yield 73%; H NMR (500MHz, CDCl3), δ (ppm): 8.49 (s, 1H), 8.05-8.04 (d, 2H, J = 8.5 Hz),
7.86 (s, 1H), 7.68-7.67 (d, 2H, J = 8.5 Hz), 7.61-7.59 (d, 2H, J = 7.5 Hz), 7.49-7.45 (M, 3H),
7.36-7.30 (M, 3H), 5.65 (s, 2H). 13CNMR (125 MHz); δ (ppm): 142.9, 131.6, 131.4, 130.1,
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128.4, 127.8, 127.3, 126.2, 126.0, 125.4, 125.1, 123.6, 123.4. HRMS m/z: [DPA + H]+ =
297.1391 (Calculated = 297.1384).
Results and Discussion:
The synthesis of molecular probe, dicyanovinyl-phenyl-9-anthracene, (DPA) was carried out
conveniently as shown in scheme 1. Anthracene was brominated with N-bromosuccinimide to
obtain 9-bromoanthracene, 1. Suzuki coupling reaction was performed between compound 1 and
4-formylbenzeneboronic acid in the presence of catalytic amount of Pd(OAc)2/TBAB to obtain
compound 2 in good yield. The compound 2 was then reacted with malononitrile via
Knoevenagel condensation reaction to afford probe DPA. The compounds were well
characterized by 1H, 13C NMR, FT-IR and HRMS spectral data analysis (Figure S1-S8).
Scheme 1: Synthesis of probe DPA
Photophysical properties of probe DPA:
The photophysical properties of probe DPA (10.0μM) in solvent of different polarity is
illustrated in Table–1. In almost all studied solvent systems the absorption spectra consistently
showed broad high energy absorption maxima around at ~320 nm and the typical anthracene
absorption bands appeared between 345 nm to 385 nm. The broad absorption band appeared at ~
410 - 415 nm is attributed to n–π* charge transfer (CT) process (Figure S9a). Further the little
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variation observed in the absorption data is attributed to modulation in charge transfer process
due to generation of different conformers in solvents of different polarity.
Table 1: Photophysical properties of probe DPA
S.N.
Molar absorptivity
Stoke's Shift Quantum yield
(nm)
λ385
415, 510 510 33, 100 100 0.169 0.188
λem (nm)
λmax
(nm)
ε = 1 x 105 M-1cm-1
Φ
Solvent
λ385
λ415
λ385
λ415
λ415
Φ385
Φ415
1.
Hexane 382, 410 0.132
0.073
2.
3.
4.
5.
6.
7.
Benzene 384, 421 0.145
Dioxane 384, 410 0.155
CHCl3 384, 426 0.137
0.071
0.076
0.063
0.071
0.062
0.059
415, 553 553 31, 132 132 0.127
415, 560 560 31, 150 150 0.119
415, 590 590 31, 164 164 0.096
415, 600 600 30, 185 190 0.081
0.168
0.145
0.115
0.081
0.005
0.007
THF
385, 415 0.152
384, 415 0.152
ACN
415, ---
415, ---
---
---
31, ---
---- 0.024
MeOH 384, 415 0.151
31, --- ---- 0.025
The probe DPA upon excitation at 385 nm displayed broad high and low energy dual emission
bands at 415-425 nm and in the range of 510 - 600 nm. Upon changing the polarity of medium
we observed changes in relative fluorescence intensity, Stokes shift and quantum yield of the
probe DPA (Table 1). Also, under Uv light probe DPA exhibits different colors in solvents of
different polarity (Figure 1c). It is interesting to mention that upon changing the polarity of the
medium from nonpolar (hexane) to partial polar one (THF) the relative fluorescence intensity of
low energy band decreases (Figure 1b) along with significant shift toward high wave length (red)
region. Moreover, in relatively high polarity solvents such as, in acetonitrile (ACN) methanol
(MeOH) and water (fw = 5%, 10%) the low energy band disappeared completely. This is possibly
due to fast intramolecular electron transfer from the donor to the acceptor part of probe in the
polar environment in which, D–A units twist around the carbon–carbon bond to generate a
relaxed perpendicular state [53, 64-65]. Thus, the observed low energy emission band toward
high wavelength region (510-600 nm) is attributed to TICT state while the relaxed locally
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excited (LE) state generates high energy emission band at ~ 415-425 nm (Figure 1a). Also, in
semi polar solvents, including THF the dual emission appears due to equilibrium between the
relaxed perpendicular and coplanar conformers. Further, in polar medium like, MeOH, ACN and
water the equilibrium between LE and ET character disintegrate in which the LE character of
first CT state dominated and only high energy band appeared in the medium [60, 64-65].
Additionally, upon increasing the water fraction (fw) above 10% precipitation occurred.
Figure 1: (a) Jabłonski diagram shows TICT dynamics (b) Emission spectra of probe DPA (10
M; at ex = 385 nm) in different solvents (c) Image: Change in color of probe DPA in solvents
(under UV light at365 nm).
Response of probe toward anions and amines:
The probe (10 μM) upon interaction with different class of anions (15 equiv.) such as, F-, Cl-, Br-,
2-
2-
-
I-, SCN-, AcO-, CN-, CO3 , SO4 , NO3 , S2- showed good selectively for CN- anion in THF.
Upon interaction with CN- the absorption bands, centered at 310 and 415 nm disappeared and a
new absorption band appeared at 470 nm. Similarly, the emission spectra of probe DPA
exhibited decrease in relative fluorescence intensity, wherein the low energy emission band,
centered at 600 nm shift toward blue region to appear at 535 nm. However, the intensity of high
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energy band (at 415 nm) remained unchanged (Figure S10). Moreover, upon interaction with
cyanide the yellow-green color of the probe solution change to a pink-red color under visible
light while under Uv light (at 365 nm) fluorescent red color for probe DPA changed to a blue
color (Figure S10, images). The other tested anions revealed insignificant change in optical
properties of the probe. Thus, the preliminary anion interaction studies suggested about the
nucleophilic addition reaction of cyanide at dicyanovinyl function of the probe. The new adduct
formed in medium reduces extent of conjugation and inhibit the ICT process [57, 58].
Further, the probe DPA (10 µM) upon interaction with amines and amides (10.0 equiv.) such as,
methylamine, benzylamine, hydrazine (HDz, N2H4), phenylhydrazine, 2,4-dinitrophenyl
hydrazine, hydroxylamine, ethylenediamine, triethylamine, aminoethanol, piperazine, urea, and
thiourea, displayed high selectivity for hydrazine in THF. The electronic transition bands
centered at 315 nm (ε = 0.318 x 105 M-1 cm-1) decreased (ε = 0.131 x 105 M-1 cm-1) with
hypsochromic shift of about ~ 7 nm (308 nm). The ICT band centered, at 415 nm diminished
completely with a narrow rise in molar absorptivity of typical AN bands (345, 365, 385 nm)
(Figure 2a). Similarly, upon interaction with N2H4 we observed peculiar changes in the emission
spectra (Figure 2b). The relative fluorescence intensity of low energy TICT emission band of the
probe DPA (λex = 385 nm; ΦDPA = 0.081) centered, at λem. = 600 nm diminished and a new
emission of relatively high fluorescence intensity appeared at 435 nm (ΦDPA-HDz = 0.095).
Moreover, the naked-eye sensitive light yellow-green color of probe solution become colorless
while under UV light (365 nm) the fluorescent bright red color changed to a blue color (Figure 2,
images). The other tested amines/amides showed insignificant change in the optical behavior of
the probe, except hydroxylamine. Thus, the observed significant change in the photophysical
property of the probe in the presence of hydrazine and hydroxylamine suggested about the
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formation of hydrazone and oxime derivatives, respectively wherein, the TICT state disappeared
(off) and ICT process dominates in blue region [52]. Further an attempt was made to understand
the sensitivity of probe for hydrazine in aqueous medium. As mentioned (vide supra) upon
increasing the water fraction (fw) 5% and 10% to the solution of probe in THF the low energy
TICT band (at ~600 nm) disappeared completely. However, in aqueous THF medium (5% and
10%) probe interacts with hydrazine and in comparison to pure THF solution exhibited relatively
low fluorescence intensity ICT band, at ~ 435 nm (Figure S11). We could rationalize these
observations by the fact that in aqueous medium the nucleophilicity of hydrazine decrease due to
feasible interaction with water molecules.
1000
800
600
400
200
0
(b)
0.33
0.22
0.11
0.00
(a)
DPA + N2H4
DPA+other amine
DPA+N2H4
DPA + other amine
DPA + NH2OH
450
525
600
675
750
300
400
500
Wavelength (nm)
Wavelength (nm)
Figure 2: (a) Absorption and (b) emission spectra of probe DPA (10μM) upon interaction with
tested amines (15.0 equiv.) in THF. Inset: Change in color of probe upon interaction with
hydrazine.
The titration experiments were performed to understand the binding stoichiometry between
probe DPA and N2H4. Upon a sequential addition of N2H4 (0-1.2 equiv.) to a solution of probe
the absorption bands, centered at 315 and at 415 nm decreased gradually and an isosbestic point
appeared at 400 nm (Figure 3a). Similarly, the emission intensity (at 600 nm) of the probe
decreased gradually while the intensity of the new emission band, at 435 nm enhanced
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ratiometrically (Figure 3b). The formation of isoemissive point at 530 nm suggested about the
existence of more than two species in the medium. The observed significant change in optical
behavior of probe clearly suggested about the inversion in charge propagation (TICT–Off–On)
due to a change in nature of functional group at terminal end, wherein, the relatively more
electron-withdrawing dicyanovinyl function is substituted with a strong electron donating
hydrazone and/or amine function. The probe could able to detect hydrazine in solution with limit
of detection (LOD) 7.85 nM (0.25 ppb) (Figure S12a,b) which, was comparatively very much
low to the level recommended by EPA (TLV; 10 ppb) and also comparable with other reported
methods (Table S1). Time-resolved photoluminescence (TPRL) data for probe DPA and
DPA+N2H4 were acquired (λex = 385 nm) at room temperature in THF. The time-resolved
emission profile for probe DPA showed biexponential curve (Figure S12c) with average lifetime
of 4.5 ns. However, we could not able to get life time for probe after interaction with hydrazine.
1000
800
600
400
200
0
0.33
0.22
0.11
0.00
(b)
(a)
N2H4 (0-1.2 equiv)
N2H4 (0-1.2 equiv.)
450 525 600 675 750
Wavelength (nm)
300
400
500
Wavelength (nm)
Figure 3: (a) Absorption and (b) emission (at ex = 385 nm) titration spectra of DPA (10 M)
upon addition of N2H4 (0 - 1.2 equiv.) in THF.
The optimum response time for a chemical reaction between probe and hydrazine in the medium
has been realized by obtaining time-kinetics plot (Figure 4a,b). The change in relative
fluorescence intensity ratio (I435/I600) was monitored with respect to time (0-20 min).
Importantly, the observed ratiometric changes in the relative fluorescence intensities, at I435/I600
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reached to the saturation point within 10 min. It clearly indicated that the dicyanovinyl function
of the probe reacted with hydrazine quickly to form new hydrazone derivative in the medium.
Moreover, optical behavior of probe does not show any significant change in the pH range 2 to
12 (Figure S13). Furthermore, the competitive interference studies ensures about the high
selectivity of probe for hydrazine as the overall change in the emission ratio (at I435/I600) for
probable derivative, DPA.HDz was significantly high in the presence of other tested amino and
amide function containing organic compounds (Figure S14).
0 min.
1000
800
600
400
200
0
35
28
21
14
7
(a)
(b)
1 min.
2 min.
3 min.
4 min.
5 min.
6 min.
7 min.
8 min.
9 min.
0
0
3
6
9
12
15
450 525 600 675 750
Wavelength (nm)
Time (minute)
Figure 4: (a) Time-dependent fluorescence spectra and (b) Plot of I435/I600 versus time for DPA-
N2H4 (10 M) solution after addition of 1.2 equiv. N2H4 in THF.
Mechanism of Interaction with N2H4
1
To have a deep insight about the mode of interaction between probe and hydrazine HNMR
spectra were analyzed. In CDCl3 the characteristic proton resonances appeared at δ 8.56 (s, Hi), δ
8.15-8.13 (d, Hd), δ 8.08-8.07 (d, Hb), δ 7.93 (s, Ha, –HC=N– function), δ 7.65-7.64 (d, Hc), δ
7.56-7.54 (d, AN), δ 7.49-7.41 (t, AN), δ 7.40-7.39 (t, AN) ppm respectively (Figure 5a). Upon
interaction with hydrazine the Ha, Hb and aromatic ring protons shift toward upfield region while
a new resonance attributed to formation of –N–NH2 function appeared at δ 5.655 ppm. The broad
signal appeared at δ 2.776 ppm is due to unreacted excess hydrazine. In contrast, the resonance
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appeared at δ 3.600 ppm indicated about the formation of malononitrile entity in the medium
(Figure 5b). To confirm, we prepared the hydrazone derivative, DPA-HDz and analyzed the
1
1H/13C NMR, and HRMS data of the isolated product (Figure S15-S17). The HNMR spectrum
of DPA-HDz revealed typical signal corresponding to hydrazone (–C =N–NH2) function while
signals corresponding to free malononitrile and hydrazine were absent (Figure 5c). Moreover, the
molecular ion peak (HRMS; m/z) for DPA+H+ and DPA-HDz+H+ appeared at 331.1235
(Calculated = 331.1230) (Figure S8) and 297.1391 (Calculated = 297.1386) (Figure S17)
respectively. Thus, on the basis of above observations we proposed a mechanism (Scheme 2) for
the formation a new hydrazone derivative by the addition-elimination reaction in which, the
dicyanovinyl function is eliminated as malononitrile unit.
1
Figure 5: (a) H NMR (500 MHz) spectrum of DPA, (b) after addition of N2H4 (2.0 equiv.) to
the solution of DPA and (c) after isolation of DPA-HDz in CDCl3.
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Scheme 2. A plausible mechanism for the formation of hydrazone derivative
Quantum chemical calculations: In order to understand the involvement of orbitals of different
charge densities in the possible TICT process density functional theory (DFT) calculation
method was employed for probe, DPA and derivative DPA-HDz in a Gaussian 09 suits of
program employing basis set B3LYP/6-31+G** (Solvent = THF) (Figure 6). The dihedral angles
corresponding to anthracene (AN) and the phenyl ring and dipole moment of the optimized
structure of probe DPA were found to be 70.10 and 8.8980 Debye, respectively. For derivative,
DPA-HDz the dihedral angle between the phenyl and AN rings increased to 74.30 while the
dipole moment drastically reduced to 2.4485 Debye. Thus, the deviation obtained could be one
of reasons for TICT in which the charge distribution between the donor and acceptor units gets
distorted. Moreover, in the ground state the frontier molecular orbital of probe suggested that the
electron densities of HOMO (5.5568eV) are localized over AN unit while of LUMO (2.9064eV)
remains on phenyl and malononitrile (acceptor) units, respectively. The HOMO and LUMO
energy difference (ΔE) was found to be 2.6504eV. In contrast, for derivative DPA-HDz the
electron densities in occupied HOMO (5.0558eV) and LUMO (1.5445eV) (ΔE = 3.5113eV)
were mostly localized on AN unit only. It clearly suggested that due to generation of electron
donating hydrazone function (–C=N–NH2) at the terminal position there is substantial increase in
charge density of AN unit and a clear charge distribution state subdued. Consequently, the TICT
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band in the absorption and emission spectra disappeared and a new broad high intensity emission
was observed in blue region due to enhanced intramolecular charge transfer (ICT) process.
Figure 6: (a) DFT optimized structures and (b) HOMO-LUMO energy distribution of DPA and
DPA-HDz.
Conductivity measurement of DPA and DPA-HDz
To understand the conducting and insulating behavior of the probe and its derivative (DPA-HDz)
films of thickness around 1 x 10-2 cm were prepared on a copper plate. Further to compare the
conducting behavior another film, as a standard, was prepared in which probe was doped with
molecular iodine (I2). For I2 doped probe dc conductivity (σdc = l / Rb . A) was estimated by
obtaining Nyquist plot (Z′ vs. Z″) at different temperature (Figure 7a). Form the intercept of
semi-circular arc of the plot the value for bulk resistance (Rb) was obtained, for which, dc
conductivity was found to be 10-8 S/cm at 30oC. Similarly, for probe DPA the dc conductivity
was found to be 10-9 S/cm. However, for derivative, DPA-HDz the dc conductivity was very low
due to out of range bulk resistance. Moreover, the activation energy (Ea) for I2 doped DPA film
was estimated from the slope of Figure 7b and was found to be ~ 1.6eV. Thus, it is worth to
mention that temperature dependent dc conductivity of I2 doped DPA is similar to Arrhenius type
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thermally activated process and change in dc conductivity with the rise in temperature remained
linear (Figure 7b).
at 40oC
at 50oC
at 60oC
at 70oC
at 80oC
at 90oC
at 100oC
at 110oC
2.0M
1.5M
1.0M
-5
-6
(a)
(b)
-7
100000
80000
60000
40000
20000
0
Z''
-8
500.0k
0.0
-9
-10
0
20000
40000
60000
80000
100000
Z'
0.0
500.0k 1.0M
Z'
1.5M
2.0M
2.6
2.8
3.0
3.2
3.4
-1)
1000/T (K
Figure 7: (a) Nyquist plot (Z’ v/s Z”) at deferent temperatures and (b) Temperature dependent
dc conductivity plot for I2 doped DPA.
Further, the ac conducting behavior of I2 doped DPA film was analyzed at different
temperatures. The conductivity curve (Figure 8) shows that around temperature, ≥800C I2 doped
DPA film follows dual behavior with the formation of frequency independent plateau region
(100 Hz) and a high frequency dispersion region (5 MHz) while in the intermediate region
conductivity remains independent to the frequency. The absence of low frequency dispersion
region suggested about less availability of number of free ions. Notably, in high frequency
region, conductivity increases with the increase in frequency. The dc conductivity (σ0) values
was obtained by the extrapolation of plateau corresponding to pre-exponential factor (log ; on
y-axis) [66-68]. Further, it has been reported that for ion conducting matrix the value of power
exponent (n) should lie between 0 to 1 for the diffusion limited hoping and perfect long range
pathways [65]. We applied Jonscher’s power law (σ() = σdc + An) to calculate the values for
ζdc, A and n from the fitting of ac conductivity curve (Table 2).
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10-4
10-5
10-6
10-7
10-8
10-9
at 40oC
at 50oC
at 60oC
at 70oC
at 80oC
at 90oC
at 100oC
at 110oC
104
105
f (Hz)
106
Figure 8: AC conductivity plot for I2 doped DPA at deferent temperature.
For I2 doped DPA film the upward shift in the middle region of AC conductivity plot (Figure 8)
suggested about the thermally activated nature of ion dynamics. It means this frequency region is
coupled with free and correlated hopping of charge carriers and the dc conductivity (σ0) increase
+
with the rise of thermal activation at different temperature (Table 2). Thus, the DPA matrix (I3 /
I-) is susceptible to ionic conductivity and thermally activated ionic dynamics. Moreover,
relatively high dc conductivity of I2 doped probe suggested about the amorphousness of the
matrix. In contrast, the low conductivity of DPA-HDz derivative clearly shows that upon
substitution of dicyanovinyl function with hydrazine the derivative gain more insulating
character. This is due to fact that upon interaction with hydrazine probe DPA loses amorphous
character and gain more crystalline nature due to formation of derivative, DPA-HDz.
Consequently, increase in rigidity and relatively more distortion in planarity reduce the extent of
conductivity of crystalline DPA-HDz than the probe DPA.
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Table 2: Change in dc conductivity and other parameters with respect to temperature.
Temp (0C)
R2
n
A
dc conductivity
2.75 x 10-10
1.45 x 10-9
6.00 x 10-9
2.11 x 10-8
5.60 x 10-8
1.47 x 10-7
6.49 x 10-7
2.31 x 10-6
0.95631
0.97747
0.99179
0.99643
0.99798
0.99905
0.99953
0.99913
1.29031
1.27333
1.21692
1.13186
1.08658
1.2498
7.90 x 10-16
1.57 x 10-15
6.67 x 10-15
4.00 x 10-14
1.11 x 10-13
2.65 x 10-14
2.79 x 10-14
3.14 x 10-14
40
50
60
70
80
90
1.28923
1.33857
100
110
Surface morphology and Thermal stability: The surface morphology studies were performed
to understand the variation in nature of matrix by the change in functionality of probe at the
terminal position. The SEM images of probe DPA acquire micro cube like amorphous structure
while derivative, DPA-HDz attains crystalline flower like edifice of inconsistent size (Figure
9a,b). The observed variation in morphology of the probe is probably due to arrangement of
molecules in different inter-planer angles with distortion in planarity. The probe DPA have
nitrile (–CN) function at the terminal position whereas derivative, DPA-HDz contains terminal
hydrazone (=N–NH2) fragment. The hydrogen atom present in hydrazone function interacts with
other molecules to form inter molecular hydrogen bonds. Thus, the close packing and systematic
aggregation of DPA-HDz would increase the extent of crystalline rigid structure. To rationalize
this hypothesis dark field TEM images were acquired for crystalline phase analysis of DPA and
DPA-HDz. The observed lattice point clearly shows diffraction for DPA-HDz while such
diffraction was not observed in the image of DPA. Moreover, the thermo-gravimetric analysis
(TGA) for probe, DPA and DPA-HDz under nitrogen atmosphere suggested about the ~ 10%
weight loss around thermal decomposition temperatures (Td) at 380°C and 283°C respectively
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(Figure 9). It means the introduction of electron donating hydrazone function at the terminal
position reduced the thermal stability of the probe. Thus, it is worthy to mention that the
conducting behavior of DPA due to amorphousness of matrix decreased upon formation of
crystalline DPA-HDz derivative.
100
80
DPA
60
DPA-HDz
40
20
100 200 300 400 500 600
0
Temperature ( C)
Figure 9: SEM images of (a) DPA (b) DPA-HDz. Dark field TEM images of DPA (c) and DPA-
HDz (d) Thermo-gravimetric analysis of DPA and DPA-HDz measured at a heating rate of 10
°C/min under nitrogen atmosphere.
Sensitivity of probe DPA for hydrazine vapor in solution and on test paper strips
In order to ensure potential fluorogenic applicability of DPA to sense hydrazine vapor
selectively in solution N2H4, N2, NH3, and CO2 gases (1.0 mL) were passed to the solution of
probe for 15 min and monitored the absorption and emission spectra. Notably, with vapor of
hydrazine the changes observed in optical behavior of the probe were almost similar to the
changes observed in solution (Figure 10). However, other tested gases could not able to induce
any significant change in the optical behavior of the probe. Further, to calculate the limit of
detection a calibration curve was obtained between change in relative fluorescence intensity
ratio, I435/I600 and different concentration (0-10 µM) for hydrazine vapor. From the slope of
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fluorescence plot calibration sensitivity (m) was found to be 0.3279. In vapor phase the estimated
limit of detection (LOD = 3/m;  = 1.12) 10.24 µM was lower than the LOD estimated in pure
THF solution (Figure S18).
0.35
0.28
0.21
0.14
0.07
0.00
1000
750
500
250
0
(b)
N H Vapour
(a)
2
4
DPA+tested gases
N H Vapour
2
4
DPA+tested gases
450
600
750
300
400
500
Wavelength (nm)
Wavelength (nm)
Figure 10: Change in absorption (a) and emission (b) spectra of probe DPA in presence of N2H4,
N2, NH3 and CO2 gases.
Further to detect hydrazine gas on test paper strip (solid surface) probe modified small cellulose
paper strips were prepared (WhatmanTM 1.5 × 2.0 cm2 in THF) by dip stick method. The air dried
test strips were separately kept in different air tight vials and the calculated volume (~60 ppm; 1
mL vol.) of tested gases were passed with the help of graduated syringes. The treated strips were
left for 15 min and then visualized under UV light (365 nm). The strips exposed with hydrazine
gas showed naked eye sensitive color change in which the light red color of the paper strip
changed in to a blue color (Figure 11). Similarly, the probe modified cellulose paper discs upon
exposure to tested gases displayed similar changes in presence of N2H4 gas selectively. We also
given exposure of organic vapors such as, carbon tetrachloride, benzene, toluene and pertrolium
ether on to the test paper strips for about half an hour. The insignificant change in the color of the
strips demonstarted applicability and selectivity of the probe to detect N2H4 gas in solid state.
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Figure 11: Change in color of test paper strips of DPA (10 µM) in the presence of selected gases
(60 ppm; 15 min). Change in color of paper discs of DPA at two different concentrations (10 and
5 µM) in the presence of selected gases (60 ppm; 15 min).
Conclusion:
In conclusion, a rationally designed molecular probe displayed ratiometric fluorescence response
to detect hydrazine selectively through the chemodosimeter approach. The probe upon
interaction with hydrazine substituted dicyanovinyl function to form hydrazone derivative.
Consequently, the orientation of charge propagation reverted and TICT state disappeared while
the ICT process dominated. Thus, the ratiometric change in the emission (Turn – On) behavior of
the probe was observed along with naked-eye sensitive color changes in the medium. The probe
has shown good limit of detection and quick response time to detect hydrazine. Additionally the
amorphous nature of matrix provided high conductivity to the probe with temperature dependent
ion dynamics. Moreover, the generation of hydrazone function at the terminal position increased
the crystalline nature that reduces the conducting property of the probe. The probe has also been
explored to detect hydrazine vapor in solution and on the test paper strips.
Acknowledgement
The authors are thankful to the Council of Scientific and Industrial Research (CSIR
02(0349)/19/EMR-II) for financial assistance. Authors RCG and SKD acknowledge
University Grant Commission UGC, and CSIR New Delhi for fellowships.
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Captions
Scheme 1: Synthesis of probe DPA
Scheme 2. A plausible mechanism for the formation of hydrazone derivative
Table 1: Photophysical properties of probe DPA
Table 2: Change in dc conductivity and other parameters with respect to temperature.
Figure 1: (a) Jabłonski diagram shows TICT dynamics (b) Emission spectra of probe DPA (10
M; at ex = 385 nm) in different solvents (c) Image: Change in color of probe DPA in solvents
(under UV light at365 nm).
Figure 2: (a) Absorption and (b) emission spectra of probe DPA (10μM) upon interaction with
tested amines (15.0 equiv.) in THF. Inset: Change in color of probe upon interaction with
hydrazine.
Figure 3: (a) Absorption and (b) emission (at ex = 385 nm) titration spectra of DPA (10 M)
upon addition of N2H4 (0 - 1.2 equiv.) in THF.
Figure 4: (a) Time-dependent fluorescence spectra and (b) Plot of I435/I600 versus time for DPA-
N2H4 (10 M) solution after addition of 1.2 equiv. N2H4 in THF.
1
Figure 5: (a) H NMR (500 MHz) spectrum of DPA, (b) after addition of N2H4 (2.0 equiv.) to
the solution of DPA and (c) after isolation of DPA-HDz in CDCl3.
Figure 6: (a) DFT optimized structures and (b) HOMO-LUMO energy distribution of DPA and
DPA-HDz.
Figure 7: (a) Nyquist plot (Z’ v/s Z”) at deferent temperatures and (b) Temperature dependent
dc conductivity plot for I2 doped DPA.
Figure 8: AC conductivity plot for I2 doped DPA at deferent temperature.
Figure 9: SEM images of (a) DPA (b) DPA-HDz. Dark field TEM images of DPA (c) and DPA-
HDz (d) Thermo-gravimetric analysis of DPA and DPA-HDz measured at a heating rate of 10
°C/min under nitrogen atmosphere.
Figure 10: Change in absorption (a) and emission (b) spectra of probe DPA in presence of N2H4,
N2, NH3 and CO2 gases.
Figure 11: Change in color of test paper strips of DPA (10 µM) in the presence of selected gases
(60 ppm; 15 min). Change in color of paper discs of DPA at two different concentrations (10 and
5 µM) in the presence of selected gases (60 ppm; 15 min).
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Declaration of Interest Statement
The manuscript has not been previously published, is not currently submitted for review to any other
journal, and will not be submitted elsewhere before a decision is made by this journal.
The authors declare no competing financial interest.
The authors declare no conflict of interest.
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CREDIT_AUTHOR_STATEMENT
A sensitive TICT Probe exhibiting ratiometric fluorescence repose to detect Hydrazine in
solution and gas phase
Ramesh C. Gupta^, Sushil K. Dwivedi^, Rashid Ali, Syed S. Razi, Rudramani Tiwari,
S. Krishnamoorthi and Arvind Misra*
Department of Chemistry, Institute of Science, Banaras Hindu University,
Varanasi-221005, UP INDIA
Corresponding author: amisra@bhu.ac.in; arvindmisra2003@yahoo.com
Tel: +91-542-6702503; Fax: +91-0542-2368127, 2368175.
^ Authors have made equal contribution
29