Azine based AIEgens with multi-stimuli response towards picric acid

Article abstract of DOI:10.1039/d0nj01324b

Multifunctional aggregation induced emission luminogens (AIEgens) are novel materials which have significant applications in various fields including biomedical, optoelectronics and sensors. Enormous efforts have been directed towards the strategic development and characterisation of AIEgens, in order to better understand the AIE mechanism. Herein, we studied the photophysical properties of symmetrical azine (D-π-D) based AIEgens in detail by varying the solvent polarity and viscosity. The AIE behaviour of the azine derivatives relies on the substituents on the nitrogen atom of the amine moiety. Further, these azine derivatives sense the picric acid (PA) with high selectivity and sensitivity in both monomer and aggregated forms. The hydrogen bonding interaction of PA with any one side of the amine nitrogen leads to the formation of a new intramolecular charge transfer state which results in the formation of new absorption and emission spectra. Interestingly in the presence of PA, azine monomers exhibit fluorescence enhancement in pure THF, while aggregated azine molecules show fluorescence quenching in THF-water mixtures.

Full text of DOI:10.1039/d0nj01324b

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Azine based AIEgens with multi-stimuli response towards picric acid
a
a
a
Munusamy Sathiyaraj , Kumaravelu Pavithra and Viruthachalam Thiagarajan *
aSchool of Chemistry, Bharathidasan University, Tiruchirappalli-620 024, India
E-mail: v.thiagarajan@bdu.ac.in; vthiags@gmail.com
Phone: +91-4366-2407053
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Abstract
Multifunctional aggregation induced emission luminogens (AIEgens) are novel materials which
have significant applications in various fields including biomedical, optoelectronics and sensors.
Enormous efforts have been directed towards the strategic development and characterisation of
AIEgens, in order to better understand the AIE mechanism. Herein, we studied the photophysical
properties of symmetrical azine (D-π-D) based AIEgens in detail by varying the solvent polarity
and viscosity. The AIE behaviour of the azine derivatives rely on the substituents on the nitrogen
atom of the amine moiety. Further, these azine derivatives sense the picric acid (PA) with high
selectivity and sensitivity in both monomer and aggregated forms. The hydrogen bonding
interaction of PA with any one side of the amine nitrogen leads to the formation of new
intramolecular charge transfer state which results in the formation of new absorption and emission
spectra. Interestingly in the presence of PA, azine monomers exhibit fluorescence enhancement in
pure THF, while aggregated azine molecules show fluorescence quenching in THF-water mixture.
1
. Introduction
In recent years, great attention has been given to the synthesis of simple organic luminescent
molecules, having a well-defined morphology with tuneable fluorescent behaviour in the aggregate
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state. These luminescent materials have potential applications in OLED , sensors , photoemitters
etc. The emission efficiency of most of the organic molecules decreases in the solid state due to
aggregation caused quenching. The fluorescence of the fluorophore is usually quenched through
non-radiative pathways such as π-π stacking in the aggregate state which limits many practical
applications.4-9
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Aggregation induced emission(AIE) the converse phenomenon of aggregation caused
quenching was first noted by Tang et al. in 2001 from a solution of 1-methyl-1,2,3,4,5-
pentaphenylsilole.10 This behaviour is shown by a number of molecules like tetraphenylethene
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(
TPE) , 4-butoxybiphenyl (BBP) , maleimide , benzoxazole dyes
and others. These types of
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molecules which are non-emissive in their solution state become emissive in their aggregated state
due to the restriction in the intramolecular rotations/vibrations. Analogous to this there is the
development of another phenomenon known as aggregation induced emission enhancement
(AIEE), in which those molecules that are weakly emissive in the solution state become highly
luminescent in the aggregate state. It can be achieved by the connection of two or more aromatic
or conjugated moieties by rotatable C-C, C-N, and N-N single bonds.20-22 The restriction in the
intramolecular rotation has been found to be the main root for this effect. Typical examples of this
kind of AIEE molecules include conjugated cyanostilbene derivatives23,24, boron compounds25-29
,
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salicylaldehyde azines , hexa-peri-hexabenzocoronene and polyacetylenes.
Conformational restrictions of the small molecules to produce a change in the fluorescence
by altering the photophysical phenomenon have been gaining more attention especially in the
sensing areas. AIE has paved the way for the conformational restricted sensing mechanism among
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various other mechanisms like photoinduced electron transfer(PET), intramolecular charge
transfer(ICT),34 fluorescence resonance energy transfer(FRET)35 and excimer/exciplex
formation.36 Several AIE based azine molecules were reported in the literature and their
mechanisms were studied.37-45
Picric acid is a powerful explosive and a water soluble pollutant. It has extensive
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applications in pharmaceuticals , chemical laboratories, fireworks, glass and leather industries.
It is strong irritant to skin/ eye, and causes damage to kidney and organs involved in the respiratory
system.48-51 On account of concern for human health, as well as for environmental protection and
national security52-54, there has been an enormous demand for a suitable, sensitive and cost-
effective sensors for the detection of picric acid.
In this aspect, we synthesized different azine based derivatives with an excellent AIE
behaviour. It is evident that the substituent in the amine donor moiety determines the photophysical
and AIE properties of azine derivatives. Further, the azine derivatives selectively detect picric acid
in both aqueous as well as organic media. The fluorescence output of these derivatives in the
presence of picric acid depends on the solvent medium (fluorescence on/off).
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. Experimental section
.1 Materials
All the chemicals were directly used for the synthesis without further purification. Benzaldehyde,
and 4-(dimethylamino) benzaldehyde, were purchased from Avra synthesis, India. 4-
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dipropylaminobenzaldehyde55
was
prepared
by
previous
report,
and
4-
diphenylanimobenzaldehyde was purchased from Alfa Aesar India. Hydrazine hydrate was
purchased from Spectrochem, India. All the solvents were of HPLC grade and purchased from
Merck, India. 2-Nitrophenol, 3-nitrophenol, 4-nitrophenol, 2,4-dinitrobenzoic acid, nitrobenzene
and picric acid were purchased from Sigma Aldrich India. Hydrochloric acid and sodium
hydroxide were purchased from Merck India. Quinine sulphate was purchased from Sigma Aldrich
India.
2
.2 Methods
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H and C-NMR spectra were recorded on a Bruker Seoul 400 at 400 MHz instrument. Mass
spectra were obtained with Thermo scientific exactive plus, HESI mode. Absorption spectra were
recorded using Varian Cary Bio 100 UV-Visible spectrophotometer. The corresponding
concentration of picric acid was used as a reference for absorption measurements in the presence
of picric acid. Fluorescence studies were carried out using JASCO FP-6500 fluorescence
spectrophotometer. Actual particle size distribution was determined using a NanoPlus-3
Micromeritics instrument. SEM images were obtained using LEO Scanning Electron Microscope
435 VP (Leo Electron Microscopy. Ltd., Cambridge, UK). PA sensing studies were carried out
with both monomer as well as aggregated forms of azine molecules. In pure THF, azine molecules
are in the monomer form whereas in THF-water mixture azine molecules are in the aggregated
form. With reference to the PA binding studies in pure THF, PA was added to the non-aggregated
forms of azine molecules, while in the case of THF-water mixture, PA was added to the aggregated
forms of azine molecules.
All the five azine derivatives were prepared by the previously reported procedure and presented in
scheme 1.56,57 One equivalent of hydrazine monohydrate in ethanol was added dropwise to the two
equivalents of the corresponding aldehyde dissolved in 5mL of ethanol along with stirring. The
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above reaction mixture was stirred for 6 h at 80 C and then the formed product was filtered, washed
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several times with ethanol and dried. The final dried product was recrystallized using
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dichloromethane-ethanol mixture. The H, C-NMR and mass spectral data of all azine derivatives
are presented in the supporting information.
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Scheme 1. Synthesis of azine derivatives BA, DMBA, DEBA, DPBA, and DAPA.
3. Results and Discussion
3
.1 Solvent effect
The photophysical properties of different azine derivatives in various aprotic and protic solvents
have been carried out and the representative spectra in THF are shown in Figure 1. Table 1 and
S1 present the absorption and emission data of azine derivatives in various solvents. Absorption
spectra of azine derivatives are dependent on the electron donating substituents in the aryl group,
and show a strong absorption band around 390 nm and is due to the intramolecular charge transfer
from the electron rich N,N-dialkyl/diaryl donor to the electron deficient imine acceptor moiety. In
addition to that a shoulder peak observed around 290-325 nm and is due to the intramolecular
transition within the substituted aryl group.
Fluorescence properties of azine derivatives have been studied in various solvents by
exciting at its longer wavelength absorption maximum and are noticed to be depend on the electron
donating properties of the N, N-dialkyl/diaryl group. The emission maximum observed around 470
nm shifts towards lower energy with increase in solvent polarity and this shift is due to the
stabilization of the charge transfer between N, N-dialkyl/diaryl donor to the imine acceptor moiety.
As expected, the molecule without the donor group (BA) does not show any emission. However,
the fluorescence quantum yield of all the azine derivatives in THF are very low (<0.006). It can be
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mainly due to the azine derivatives experiencing minimal interaction between each other in
solution, the photoisomerization namely the planarity of the molecule is reduced due to rotation
via the N-N flexible bond. Upon excitation, the excitation energy is used for rotation which makes
the molecule return back to the ground state via a non-radiative pathway, which exhibits weak
fluorescence in the solution state.
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Figure 1. (a) and (b) Absorption and emission spectrum of azine derivatives in THF solvent.
Table 1. Photophysical properties of compounds in THF
a
Compounds
λ
abs (nm)
λ
em (nm)
Stoke’s
shift (cm )
Φ
-
f
–
1
BA
300
384
390
392
402
-
-
DMBA
DEBA
DPBA
DABA
467
471
473
482
4628
4410
4369
4129
0.002 ± 0.001
0.005 ± 0.001
0.003 ± 0.001
0.003 ± 0.001
a
a
Parameter indicate fluorescence quantum yields (Φ ) in THF
f
Quantum yields relative to quinine sulphate in 0.1 N H
2
SO
4
(Φ = 0.546)
f
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.2 Aggregation Induced Emission
In order to investigate, the fluorescence properties of azine derivatives upon aggregation
THF/water mixture was chosen. All the compounds show excellent solubility in organic solvents,
and due to their hydrophobic nature, they have poor solubility in the aqueous medium. The
absorption and emission spectra of DMBA in various THF/ water fractions are presented in Figure
2 (All other azine derivatives are presented in Figure S1 and S2). On increasing the water fraction
upto 50%, there is no significant change in the absorption and emission spectra of azine
derivatives. The slight increase in absorbance and fluorescence intensity until 40% of water
fraction may be due to the polarity of the medium. After 40% of water fraction, the absorbance
decreases along with a red shift in absorption maximum. After 80% of water fraction, the
aggregates precipitate which in turn results in a red edged absorption band due to scattering of
light by particles/precipitate.
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Figure 2 (a) Absorption and (b) emission spectrum of DMBA (5 µM) in different THF -water
fraction, λexc = 392 nm. (c) Plot of fluorescence intensity vs water fraction, (d) DMBA in 0% and
80% water fraction under 365 nm UV illumination.
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Upon excitation at 392 nm, the emission intensity increases along with a red shift in
emission maximum (467 to 500 nm) with increase in water fraction. Around 80% of water
fraction, a 1923-fold fluorescence enhancement was observed for DMBA. DEBA, DPBA and
DABA show 1022, 467 and 77-fold fluorescence enhancement respectively(Table S2). After 85%
of water fraction, the emission intensity decreases due to the precipitation of aggregated molecules
into the solution. This observation may be explained as in low water fraction, azine molecules
experience minimal interaction between each other which results in free rotation around N-N
single bond and N, N-dialkyl/diaryl groups. Upon excitation, this free rotation causes the molecule
to return back to the ground state via a non-radiative process and it exhibits weak fluorescence. At
higher water fraction, the azine molecules aggregate together which in turn brings steric effects.
The closeness of the neighbouring azine molecules block the intramolecular rotations as well as
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the relaxation via non-radiative processes, and they exhibit strong fluorescence. Within the azine
derivatives, DMBA shows maximum fluorescence enhancement, due to lesser steric effect which
favours stable aggregates. The amount of fluorescence enhancement decreases along with increase
in the size of the substituent at –NR
the aggregates. In general, the rotation of –NR
which in turn results in weak fluorescence. This kind of rotation will be restricted in highly viscous
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and N-N bonds lead to a non-radiative pathway
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solvents, and due to this, the non-radiative pathway suppresses with increase in viscosity. To
analyse the influence of viscosity on the azine derivatives, the fluorescence studies were carried
out in methanol containing varying percentages of glycerol. The emission spectra given in Figure
S3 clearly shows the increase in the emission intensity along with a red shift as the percentage of
glycerol increases. The increase in viscosity restricts the intramolecular rotations and this induces
the radiative process which leads to the strong emission around 500 nm similar to one in the
aggregated state. With viscosity studies, we can confirm that the restriction of intramolecular
rotation leads to increase in fluorescence enhancement via a decrease in non-radiative deactivation
pathways. The formation of the aggregates in the aqueous mixtures at high water fractions was
also confirmed by DLS and SEM measurements. DLS result shows that no particles were observed
in pure THF solution. In contrast, on increasing the water fraction to 70% clear aggregates with an
average diameter of ~1000 nm was formed. The SEM measurements of corresponding aggregate
showed clear morphology of the aggregated states as shown in Figure 3(b).
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Figure 3. (a) Dynamic light scattering measurement of DMBA with particle size distribution in
THF-water mixture (70%). SEM image of DMBA (b) Aggregates recovered from 90% water
fraction.
3
.3 Detection of picric acid in THF
The absorption and emission spectra of DMBA in the presence and absence of picric acid in THF
are presented in figure 4. In the absence of picric acid, DMBA shows an absorption maximum at
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84 nm. On increasing the picric acid concentration, the absorbance at 384 nm decreases along
with the formation of a new absorption peak at 480 nm. The formation of a clear isosbestic point
at 415 nm shows the single equilibrium between DMBA and picric acid.
On exciting at the isosbestic point, the emission spectrum of DMBA in THF shows an
emission peak at 467 nm. The emission spectra show the formation of a new emission band centred
at 576 nm on increasing the picric acid concentration. Increasing the picric acid concentration leads
to a hydrogen bonding interaction between the picric acid and DMBA molecule which in turn leads
to a new stabilized charge transfer absorption and emission band. A similar trend was observed
with other azine derivatives (DEBA and DPBA) except DABA, and their corresponding spectra
are presented in Figure S4 and S5. Due to the presence of bulky electron stabilizing N, N-diaryl
group in DABA, the hydrogen bonding interaction between DABA and picric acid may be very
weak. Within the azine derivatives (DMBA, DEBA and DPBA), the strong electron donating
N,N-dipropyl containing DPBA shows higher fluorescence enhancement in the presence of picric
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acid. Figure 4a inset shows the colour change from colourless to an intense orange generated upon
the addition of picric acid to DMBA in THF. In contrast, no such absorption and emission changes
were observed in the presence of 2-nitrophenol, 3-nitrophenol,4-nitrophenol, 2,4-dinitrobenzoic
acid and nitrobenzene to azine derivatives in THF (Figure S6). This shows the highly selective
nature of azine derivatives towards picric acid sensing. The detection limit of PA by azine
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molecules are calculated based on fluorescence titration data from the earlier report. The limit of
detection of PA using DMBA in THF is found to be 26 µM. The photophysical data of all azine
derivatives in the presence and absence of picric acid in THF are presented in Table S3.
Figure 4. (a) and (b) Absorption and emission spectrum of DMBA (5 μM) in the presence and
absence of picric acid in THF, λexc= 415 nm.
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.4 Mechanism of picric acid sensing in THF
It can be expected that the hydrogen bonding interaction of analyte with the donor or acceptor
moiety of azine derivatives will change the photophysical properties by affecting the
intramolecular charge transfer pathway.61,62 Addition of picric acid to azine derivatives in THF,
brings in hydrogen bonding interaction with one of the N,N-dialkyl nitrogen atom, which converts
that N,N-dialkyl moiety to an electron accepting one instead of an electron donor. The binding
mode of picric acid with azine derivative is presented in Scheme 2.
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Scheme 2. Proposed mechanism for the spectral response of DMBA in the presence of azine
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This results in long range charge transfer from one of the N,N-dialkyl group to the other side
of protonated N,N-dialkyl group instead to the imine moiety which in turn leads to red shift of the
absorption and emission spectrum along with an increase in emission intensity. The above
photophysical changes are dependent on the substituents in the N,N-di substituted moiety which
reflects in the different fold fluorescence enhancement observed within the azine derivatives.
The above results are further confirmed by time dependent density functional theory (TD DFT)
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calculations as well as H NMR titrations of DMBA in the presence and absence of picric acid.
TD DFT calculations were carried out using Gaussian B3LYP/6-31G* level on the DMBA, mono
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and bi protonated DMBA species (protonation at single side and both sides). The TD-DFT
studies indicate that the electronic transition is charge transfer in nature originating from the N,N-
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dialkyl/diaryl amine to imine moiety (Figure 5 and Figure S7). The present calculations confirm
that the formation of the new absorption band observed in the presence of picric acid is originated
from the monoprotonated species and has excitation energy of 2.49 eV which is comparable to that
of the experimental energy of 2.57 eV in THF. In addition to that, it confirms that bi protonated
species was not formed in the presence of picric acid (bi protonated species lead to blue shift (299
nm) in the absorption spectrum with an excitation energy of 4.14 eV). In addition to that, TD DFT
calculations are carried out in THF using PCM solvation method in the presence and absence of
chloride counterion and the results are presented in Table S4. All theoretical result support the
proposed any one side of the amine nitrogen protonation mechanism.
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Figure 5. Molecular orbital plots for the DMBA, mono and bi protonated DMBA forms.
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In order to further confirm the binding mode, we studied the H NMR change of DMBA before
and after addition of picric acid (Figure S8). Azine compounds show symmetry structure and it
contains four different proton environment indicated as a, b, c and d. Addition of picric acid to
DMBA results in downfield shifts of the N,N-dimethyl protons in CDCl , representing the
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formation of hydrogen bonding interaction of picric acid with the nitrogen of N,N-dimethyl
moiety. In addition to that upfield shift and peak broadening of the aromatic protons (b, c) and
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imine protons (d) were observed due to decrease in ring current. Control studies with picric acid
in THF were carried out with BA molecule (without the donor moiety) in order to confirm the
mode of hydrogen bonding interaction between PA and azine derivatives. There is no significant
change in the absorption spectrum of BA in the presence of PA in THF which confirms that the
hydrogen bonding interaction occurs only at the donor amine nitrogen not at the imine nitrogen
(Figure S9).
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.5 Detection of picric acid in aqueous environment
The absorption spectra of azine derivatives in THF/water (3:7) mixture show an absorption
maximum around 400 nm. In the presence of picric acid, the absorbance at 400 nm decreases along
with the formation of a new absorption band around 490 nm (Figure 6). It confirms that the ground
state complex formation between the azine molecules with PA via hydrogen bonding in THF/water
mixture as similar to the behaviour observed in pure THF. By exciting at the isosbestic point, in
the absence of picric acid, azine derivatives show a strong emission around 490 nm in THF/water
(3:7) mixture due to AIE phenomenon. In contrast to pure THF, an increase in the PA concentration
leads to decrease in emission intensity at 490 nm emission band. At higher PA concentrations, two
emission bands are visible (490 nm and 540 nm) in THF/water (3:7) mixture (Figures 6 and S10).
The decrease in emission intensity at 490 nm with increase in PA concentration is due to the
disturbance of aggregates by the hydrogen bonding interaction between nitrogen of N,N-dialkyl
moiety and PA and also by the inner filter effect due to self-absorption of protonated species.64,65
The new longer wavelength emission band observed at 540 nm at higher concentration of PA is
due to the formation of new ICT state as similar to one observed in pure THF. The formation of
new absorption and emission band in the presence of picric acid confirms that the fluorescence
quenching is not only due to the inner filter effect. The photophysical data and spectra of all other
azine derivatives in the presence and absence of PA in THF/water (3:7) mixture are presented in
Table S5 and Figures S11 and S12. As similar to the trend observed in THF, there is no significant
change in absorption and emission spectra were observed in the presence of 2-nitrophenol, 3-
nitrophenol,4-nitrophenol, 2,4-dinitrobenzoic acid and nitrobenzene to azine derivatives in
THF/water (3:7) mixture (Figure S13). This again confirms the selective binding of azine
derivative towards PA. The limit of detection of PA using DMBA in THF/water (3:7) mixture is
found to be 38 µM. There is no significant change in the absorption spectrum of BA in the presence
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of PA in THF-water mixture that confirms that the hydrogen bonding interaction occurs only at
the donor amine nitrogen not at the imine nitrogen (Figure S14).
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Figure 6. (a) Absorption and (b) emission spectrum of DMBA (5 μM) in the presence and absence
of picric acid in THF/water (3:7) mixture, λexc = 443 nm.
In order to confirm the new absorption band observed at 490 nm is due to the protonated
species, absorption and emission spectral studies were carried out for DMBA in the pH range 9 to
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in THF/water (3:7) mixture and presented in Figure 7. The pH of the solution was adjusted by
adding HCl or NaOH to THF/water (3:7) mixture and recorded the pH immediately using pH
meter. To the final pH solution, DMBA was added and recorded the absorption and emission
spectrum. Figures 7a and 7b show the absorption and emission spectrum of DMBA in the pH range
of 9 to 2. There is no significant change in the absorption spectrum were observed until the pH
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range of 9 to 5. On further decrease in pH (from 5 to 2) lead to decrease in absorbance at 400 nm
band along with the formation of a new absorption band at 490 nm. By exciting at the isosbestic
point, the emission intensity decreases with decrease in pH from 5 to 2 as similar to the one
observed in the presence of PA Figure 7. It clearly confirms the formation of new longer
wavelength absorption band in the presence of PA is due to the hydrogen bonding interaction
between the nitrogen atom of N, N-dialkyl moiety and PA in THF/water (3:7) mixture. The
decrease in emission intensity may be due to both the inner filter effect (self-adsorption of
protonated species) as well as disturbance of aggregates in THF/water (3:7) mixture.
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Figure 7. (a) Absorption and (b) emission spectrum of DMBA (5 μM) in different pH. λexc = 433 nm in
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There is no such new longer wavelength absorption was observed with the control
compound BA, azine molecule without the donor moiety until pH 2 (Figure S15). With further
decrease in pH from 2 to 1.5, the 303 nm absorption of BA decreases along with the formation of
a new absorption band at 250 nm. This new blue shifted absorption band may be due to the
protonation of imine nitrogen. All azine derivatives except BA and DABA shows a longer
wavelength absorption band around 490 nm which clearly indicates the protonation takes place at
the nitrogen of N,N-dialkyl group (Figure S16). In contrast to other azine derivatives, DABA
shows decrease in absorbance at 410 nm along with slight increase in absorbance at longer
wavelength absorption on decreasing the pH from 9 to 2. This result confirms that there is a weak
protonation at the amine nitrogen that might be due to the unavailability of electron density at the
amine center as well as the steric hinderance induced by aryl groups. It supports the weak
interaction between the amine nitrogen and PA in DABA molecule.
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. Conclusions
We report azine based D-π-D systems which exhibit a good optical property as well as aggregation
induced emission behaviour. Among the different azine derivatives, DMBA showed higher
fluorescence enhancement (1923 fold) due to the restriction of intramolecular rotation in the
aggregated state. These azine derivatives act as fluorescent as well as colorimetric sensors for PA
in a highly selective and sensitive manner. In the presence of picric acid, the hydrogen bonding
interaction of PA with the donor amine moiety results in the formation of new intramolecular
charge transfer state which in turn leads to new absorption as well as emission band. The
mechanism of optical signalling was confirmed by pH based experiments followed by theoretical
studies. Interestingly, in the presence of picric acid, azine derivatives show fluorescence
enhancement in pure THF and fluorescence quenching in THF-water mixture due to the formation
of new ICT and formation of new ICT state along with inner filter effect respectively. The limit of
detection of PA using DMBA is found to be in the micromolar region.
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Conflicts of interest
There are no conflicts of interest to declare.
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Acknowledgement
This research was supported by Science and Engineering Research Board (SERB), New Delhi,
India [Grant no. YSS/2014/00026] and DST Nanomission [Grant no. DST/NM/NB/2018/10(G)].
VT thanks UGC, New Delhi, for a start-up grant and UGC FRP faculty award [F. 4-5(24-
FRP)/2013(BSR)]. MS acknowledges UGC-RFSMS for fellowship. We thank DST, New Delhi
for providing 400 MHz NMR, TCSPC and HRMS facilities under FIST program. We acknowledge
Dr. K. Thirumoorthy, VIT for valuable suggestions and theorical calculations.
Corresponding Author information:
Viruthachalam Thiagarajan, School of Chemistry, Bharathidasan University, Tiruchirappalli-620
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24, India
E-mail: v.thiagarajan@bdu.ac.in; vthiags@gmail.com
Orcid ID: https://orcid.org/0000-0003-4197-2255
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Azine based AIEgens with multi-stimuli response towards picric acid
Munusamy Sathiyaraj, Kumaravelu Pavithra and Viruthachalam Thiagarajan*
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Selective detection of picric acid using AIEgens via fluorescence enhancement and
quenching in the monomer and aggregated from respectively