Evaluating the merit of a diethylamino coumarin-derived thiosemicarbazone as an intramolecular charge transfer probe: Efficient Zn(II) mediated emission swing from green to yellow

Article abstract of DOI:10.1039/c9pp00108e

We report the synthesis and photophysical properties of a coumarin based probe (1E)-1-(1-(7-(diethylamino)-2-oxo-2H-chromen-3-yl) ethylidene) thiosemicarbazide (DIDOT). DIDOT shows a polarity dependent change in the emission maxima in the solution phase. This is explained by the increased dipole moment in the excited state by an intramolecular charge transfer (ICT) process. DIDOT can successfully detect Zn(ii) in aqueous methanol by a shift in the charge transfer emission maxima from approximately 506 to approximately 535 nm. This shift led to a change in the color of the emission from green to yellow under UV-light. The mechanism of Zn(ii) detection has been delineated using electrospray ionization-mass spectrometry (ESI-MS), Fourier-transform infrared spectroscopy (FTIR) and fluorescence time resolved studies coupled with theoretical calculations. The increment in the charge transfer in the Zn(ii) complex of DIDOT over the bare receptor as a consequence of conformational locking was determined to be the underlying cause of the cation detection phenomenon. The limit of detection and binding constant values of DIDOT towards Zn(ii) were approximately 3 × 10^-8 M and 2.35 × 10⁵ M^-1 respectively. Finally, the practical utility of DIDOT has been demonstrated by successful detection and quantification of Zn(ii) in spiked water samples.

Full text of DOI:10.1039/c9pp00108e

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Evaluating the merit of a diethylamino coumarin-derived thiosemicarbazone as an
intramolecular charge transfer probe: efficient Zn (II) mediated green to yellow emission
swing
Arghyadeep Bhattacharyya, Subhash Chandra Makhal, and Nikhil Guchhait*
Department of Chemistry, University of Calcutta, 92, A.P.C. Road, Kolkata-700009, India
*
Corresponding author. Tel.: +913323508386; fax: +913323519755.
E-mail address: nguchhait@yahoo.com (N. Guchhait).
1

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Abstract: We report the synthesis and photophysical properties of a coumarin based probe (1E)-
-(1-(7-(diethylamino)-2-oxo-2H-chromen-3-yl) ethylidene) thiosemicarbazide (DIDOT).
1
DIDOT shows polarity dependant change on the emission maxima in solution phase. This is
explained by the considering increased dipole moment in the excited state by intramolecular
charge transfer (ICT) process. DIDOT can successfully detect Zn (II) in aqueous methanol by
shifting of its charge transfer emission maxima from ~506 nm to ~535 nm. The shifting could be
conceived by change in color from green to yellow under UV-light. The mechanism of Zn (II)
detection has been delineated by ESI-MS, FTIR and fluorescence time resolved studies coupled
with theoretical calculations. Increment in charge transfer in the Zn (II) complex of DIDOT over
the bare receptor as a consequence of conformational locking was attributed to be the underlying
cause of the cation detection phenomenon. The limit of detection and binding constant values of
-8
5
-1
DIDOT towards Zn (II) are respectively ~3 x 10 M and 2.35×10 M respectively. Finally, the
practical utility of DIDOT has been demonstrated by successful detection and quantification of
Zn (II) in spiked water samples.
2

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Introduction
The selective and sensitive detection of Zn in its dipositive form is a relevant area of research.
The relevance arises due to the effect Zn (II) exerts on the human body as well as on
1
environment. Being the second most abundant element in the human body , Zn (II) is of utmost
2
3
4
5
importance as far as cell division , clotting of blood , repairmen of DNA and protein synthesis
processes are concerned. However, an excess accumulation of the same within the body
6
7
8
adversely affects hair growth , brain functioning and blood cholesterol level . Zinc is also
9
reported to be involved in causing neurodegenerative diseases like Alzheimer’s . Leaching of Zn
(II) in water makes it smelly and muddy, whereas Elevated levels of Zn in soil inhibit microbial
1
0
activity, thereby exerting phytotoxic effects . WHO has thus recommended limit for Zn (II) in
1
1
water to be 76 µM . Hence, selective and sensitive detection of Zn (II) dissolved in water along
with proper quantification by synthesized chemosensors thus remains a challenge. While
detecting Zn (II), the fluorimetric12 as well as colorimetric13 mode of detection are widely
practiced due to the cost effective and easy handling of instruments. The challenge arises mainly
due to competition with other similar sized cation Cd (II) 14-17, which offers similar spectral
response to Zn (II) as was reported previously by various groups. Although there are large
numbers of reports of selective chemosensors for Zn (II), the mode of response in such reports
deals with the variation of the emission intensity of the receptors, a phenomenon which can also
1
8
be affected by surrounding environment due to the sensitive nature of emission spectroscopy .
For this purpose, probes which show a shifting in their emission maxima would be more accurate
for the detection for Zn (II). Previously, we reported a coumarin based probe which underwent a
1
9
4
4 nm red shifting of its emission maxima in presence of Zn (II) . The current report is an
advanced modification to our previous reports, in which we synthesized thiosemicarbazone of 7-
3

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diethylamino 3-ethylcoumarin, named (1E)-1-(1-(7-(diethylamino)-2-oxo-2H-chromen-3-yl)
ethylidene) thiosemicarbazide (DIDOT) for the detection of Zn (II) ion. The choice of 7-
2
0
diethylamino coumarin was principally due to strong emissive nature of its derivatives .
Thiosemicarbazide was chosen due to the affinity of the said molecule and its derivatives
2
1
towards Zn (II) . DIDOT was previously utilized as an intermediate in the synthesis of some
2
2
antibacterial agents by Yuan et al . However, its spectroscopic characterization as well as
sensing aptitude had not been explored prior to the current report. Due to presence of hard donor
atoms in the form of N as well as O enabled DIDOT to be a Zn (II) specific probe13, 19. DIDOT
shows excellent solvatochromic shift in its emission profile depending on solvent polarity and
can detect Zn (II) by a remarkable shift in its emission maxima from ~506 nm to ~535 nm upto a
-9
lowest detection limit of ~3 x 10 M. Most importantly, this shifting in emission maxima was
evident when observed under UV-light, where the solution of DIDOT turns yellow from green
in presence of Zn (II). This increases the efficacy of DIDOT over other available Zn-selective
probes as the change in color under UV-light enables one to detect Zn (II) easily by observing
yellow coloration. Furthermore, by utilizing DIDOT as a Zn (II) specific probe, we have
1
9
improved our own previous report as the previous report afforded turn on detection whereas the
current one causes emission swing from green to yellow in presence of Zn (II). The binding
nature and mechanism of Zn (II) detection were well established by ESI-MS, Job’s plot analysis,
Benesi-Hildebrand (B-H) plot and time-resolved emission studies. Zn (II) detection by DIDOT
was applied by successful detection and quantification of the cation in spiked water sample.
Thus, green to yellow emission switching in presence of Zn (II) along with the low detection
limit towards the aforesaid metal ion makes DIDOT a lucrative receptor for detection of Zn (II).
4

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Furthermore, the interesting excited state solvatochromism of DIDOT makes it a potential
fluorescent probe for future applications.
Experimental
Chemicals
4
-diethylamino salicylaldehyde, ethyl acetoacetate, thiosemicarbazide, perchlorate salts of
cations, sodium salts of anions were purchased from Sigma Aldrich and used as received. All
spectroscopic studies were performed using spectroscopic grade solvents from Spectrochem.
Triple distilled water was used for preparation of the solutions of cations and anions.
Syntheses
Synthesis of DIDOT
The synthesis of DIDOT was achieved by two simple steps (Scheme 1) as discussed below.
Step I. Synthesis of 7-diethylamino 3-acetyl coumarin (1)
4
-diethylamino salicylaldehyde (1.93 g, 10 mmol) was dissolved in 20 mL ethanol, followed by
addition of equimolar amount of ethyl acetoacetate. 10 mmol of piperidine was added to the
reaction mixture while being stirred at room temperature. After stirring overnight, bright yellow
crystalline solid separated from the reaction mixture. It was filtered under suction, washed with
1
13
5
0% aqueous ethanol and dried in open air. The yield was 90%. H NMR and C NMR spectra
1
were recorded in CDCl for the purpose of characterization. H –NMR (ESI, Fig. S1) (300 MHz,
3
CDCl , 290 K, TMS): 8.35(s, 1H), 7.30(d, 1H), 6.53-6.57 (m, 1H), 6.39(d, 1H), 3.419-3.348(q,
3
1
3
4
H), 2.598(s, 3H) and 1.142-1.190(t, 6H). C NMR (ESI, Fig. S2) (75 MHz, CDCl , 290 K,
3
TMS): 195.6, 160.8, 158.7, 152.9, 147.7, 131.8, 116.2, 109.9, 108.2, 96.7, 45.0, 30.5 and 12.4.
5

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O
O
O
piperidine
ethanol
CHO
OH
OEt
Et N
2
O
Et N
2
O
3
-acetyl-7-(diethylamino)-2H-chromen-2-one(1)
O
O
S
H
N
NH2
ethanol
reflux
H N
N
2
N
NH2
H
S
Et N
2
O
Et N
O
O
2
(
1E)-1-(1-(7-(diethylamino)-2-oxo-2H-chromen-3-
yl)ethylidene)thiosemicarbazide (DIDOTT)
Scheme 1. Synthesis of (1E)-1-(1-(7-(diethylamino)-2-oxo-2H-chromen-3-yl) ethylidene)
thiosemicarbazide (DIDOT)
1
(2.0 mmol) was dissolved in ethanol by continuously stirring 1 at 80º C in a 50 mL round
bottom flask. Then solid thiosemicarbazide (0.2 g) was added to the hot ethanolic solution of 1 in
ethanol, followed by addition of 50 µL of acetic acid. The reaction mixture was refluxed with
stirring overnight, resulting in separation of a brick red powder. It was filtered under suction,
washed with 50% aqueous ethanol several times to remove traces of thiosemicarbazide, if any
and finally dried at 80º C for 6 hours. DIDOT was obtained in 65% yield as fine brick red
1
13
powder. It was characterized by FTIR (ESI, Fig. S3), HNMR (ESI Fig. S4), C NMR (ESI, Fig.
-1
S5) and ESI-MS (ESI, Fig. S6). FTIR (KBr, cm ): 3404, 3243, 2971, 1715, 1618, 1595, 1567,
1
1
7
1
1
508 and 1230. H -NMR (300 MHz, DMSO-d , 290 K, TMS): 10.25 (s, 1H), 8.29-8.32 (d, 2H),
6
.86 (s, 1H), 7.45-7.48 (s, 1H), 6.71-6.75 (m, 1H), 6.53 (s, 1H), 3.4-3.47 (q, 4H), 2.22 (s, 3H),
1
3
.09-1.14 (t, 6H). C -NMR (75 MHz, DMSO-d , 290 K, TMS): 179.3, 160.4, 156.9, 151.5,
6
47.7, 143.2, 130.7, 117.8, 109.9, 108.3, 96.5, 44.6, 16.5, 12.8. ESI-MS: molecular formula for
+
[
M-H] C H N O S, calculated m/z value, 333.1385, obtained 333.1367.
1
6
21
4
2
6

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Synthesis of DIDOT-Zn (II) complex
DIDOT (0.5 mmol) was suspended in 10 mL ethanol and 1.0 mmol of Zinc perchlorate
hexahydrate dissolved in 2 mL water was added drop wise, resulting in a clear dark red solution.
Overnight stirring at room temperature furnished a dark red powder. It was confirmed to be the
-1
Zn complex of DIDOT by FTIR (ESI, Fig. S7) and ESI-MS (ESI, Fig. S8). FTIR (KBr, cm ):
3
433, 2924, 1620, 1574, 1510, 1420, 1345, 1109, 1086, 805, 626. ESI-MS: calculated 413.0626,
obtained 413.2697.
Instruments
1
13
H-NMR and C-NMR were recorded on Bruker Advanced Supercon 300 MHz and chemical
shifts are expressed in ppm using TMS as internal standard. FTIR spectrum was recorded on
Perkin Elmer spectrum-100 and UV-Vis experiments were conducted in Hitachi U-3501
Spectrophotometer. Mass spectrometry was carried out in Waters Xevo G2-S Q TOF.
Fluorescence titration and excited state lifetime measurements using Time Correlated Single
Photon Counting (TCSPC) method were carried on in Perkin Elmer LS-55 and Horiba Jobin
Yvon Fluorocube-01-NL respectively.
Theoretical Calculations
DIDOT as well as its Zn (II) complex was optimized using Gaussian 09W using B3LYP hybrid
2
3
functional (6-311++g** basis set) . The structures were manually constructed with the aid of
Gauss view prior to optimization.
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Results and discussions
Photophysical properties of DIDOT
The photophysical properties of DIDOT were investigated in solution phase in various solvents.
All measurements were carried out using micro molar solutions of DIDOT to avoid reabsorption
effect in the absorption spectroscopy and self-quenching in the emission spectroscopic studies.
The absorption profiles of DIDOT in various solvents (Fig. 1) showed a single absorption band
ranging from 400-450 depending on the polarity of the solvents (Table 1). In non-polar medium
like heptane, a sharp absorption band was obtained at 410 nm. However, in water, a broad band
centered at around 450 nm was obtained. The red shift in the absorption maxima with solvent
polarity could be due to different extent of stabilization of the excited state compared to the
2
4
*
ground state . The single absorption band is assigned to be due to π-π transition by accounting
2
5
for the polarity dependence of the said band . The emission profiles in various solvents were
recorded by exciting DIDOT at ~410 nm (Fig. 1). In non-polar solvent heptane the emission
profile was broad in nature with maxima around 450 nm. The maximum was further shifted in
chloroform and attained a value of ~475 nm. This red shifting was observed as more polar
solvents were observed and in water, the emission band was centered on 515 nm. In butanol, the
emission maxima underwent blue shifting compared to methanol and water. This may be due to
the cumulative effect of H-bonding and viscosity of the solvent in question. Interestingly, a blue
sided shoulder emission band was observed over the red shifted emission maxima in solvents
like acetone, acetonitrile and DMSO. As polarity dependant shift in emission maxima indicated
the greater dipolar character of the excited state in DIDOT, a higher value of the excited state
dipole moment is expected for the probe in question. To verify this assumption, we constructed
2
6
Lippert plot as well as Lippert-Mataga plots to find out the ground and excited state dipole
8

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moments (Fig. 2) of DIDOT. In both cases reasonable linearity was obtained. This was assured
by plotting the Stokes’ Shift ( -  ) values of DIDOT in various solvents. The Onsager cavity
a
e
radius required for the calculation of the dipole moments was found out to be 5.81 Å from
theoretical volume calculation of DIDOT for the ground state optimized geometry (DFT
calculation with B3LYP hybrid functional and 6-311++g** basis set). The calculated values of
ground and excited states were 3.2 D and 6.8 D respectively. Furthermore, a plot of the emission
maxima values against the E (30) values of solvents furnished two slops according to the
T
proticity of the solvents, suggesting both dipolar as well as hydrogen bonding interactions are
2
7
prevalent with the solvent(s) in the excited state of DIDOT (Fig. 3a) . A reasonable linear
relation was obtained between the emission wave number  and the solvent hydrogen bonding
e
2
8
parameter (α) (Fig.3b) . The enhanced value of the excited state dipole moment along with the
previously discussed results suggested that the excited state photophysical properties of DIDOT
may be guided by relatively higher excited state dipole moment which may possible through
intramolecular charge transfer (ICT) from diethylamino moiety to the imine moiety of the
molecule(ICT) (Scheme 2). The fluorescence excitation spectra were recorded (ESI, Fig. S9)
and resembled the absorption profiles of DIDOT in various solvents. Interestingly, the excitation
spectra for the shoulder band and the maxima furnished a single band, suggesting both the bands
were furnished by excitation from the same electronic state29, 30. The quantum yields of DIDOT
in solvents were also calculated (Table 1). Interestingly, the quantum yield values increased
somewhat steadily till DMSO and experienced a decrement in protic solvents. This could be
rationalized owing to the stabilization of the excited state charge transfer in polar aprotic
solvents, whereas deexcitation triggered by Hydrogen bonding interactions in polar protic
3
1
solvents could account for the aforementioned observation .
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Table1. Spectral Information of DIDOT in various solvents (solvent abbreviation provided in
ESI)
Solvent
λabsmax(nm)
λemmax(nm)
Fluorescence
Quantum
Yield (Φf)
0.020
Hep
Cy
399
402
418
416
411
415
423
422
430
440
449
453
0.030
CHCl3
DCM
475
0.040
483
0.021
Acetone
ACN
475, 502
475, 509
475, 512
506
0.010
0.052
DMSO
MeOH
n-BuOH
Water
0.090
0.065
475, 492
515
0.035
0.011
1
0

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a)
b)
Wavelength (nm)
Figure 1: (a) Absorption profiles of DIDOT in various solvents. (b) Normalized Emission
profiles of DIDOT in various solvents (λ =410 nm).
ex
Figure 2: (a) Lippert plot. (b) Lippert-Mataga plot.
1
1

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Figure 3: (a) Plot of emission maxima against E (30) parameter. (b) Plot of emission wave
T
number values against Hydrogen bonding parameter of solvents.
Excited State Lifetime measurements
To probe the excited state dynamics of DIDOT, TCSPC experiments were conducted. The decay
profiles obtained were deconvoluted using DAS-6 software and the goodness of fit was based on
χ2 criteria. The relative population of excited state species and average lifetime values were
3
1
calculated using standard protocol . DIDOT was excited using a 375 nm LASER diode and the
decays were monitored at the corresponding emission maxima according to the solvent. DIDOT
showed a bi-exponential decay irrespective of the nature of the solvent (Fig. 4, Table 2). There
existed a component having lifetime in the order of picoseconds, whereas a slower component
having lifetime in the order of nanoseconds was recorded. In case of polar solvents the emission
decay was calculated to have faster lifetime with larger amplitude (Table 2). In non-polar
solvents like heptane and cyclohexane, the slow component shows larger amplitude. Since
thiosemicarbazone bearing probes are often used as chemosensors for metal ions, we
consequently checked the response of DIDOT towards metal ions.
1
2

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Scheme 2: Calculated optimized planar and non-planar forms of DIDOT (DFT level with
B3LYP hybrid functional and 6-311++g** basis set).
Figure 4: Decay profiles of DIDOT in various solvents (λ =375 nm).
ex
1
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Table2. Excited state decay parameters of DIDOT in various solvents
Solvent(λmon
)
τ1(s)/B1
τ2(s) /B2
τav(ps)
χ2
-
10
-9
Hep (449)
3.98×10 /35
2.21×10 /65
851
1.23
1.18
1.20
-
10
-9
Cy (453)
4.35×10 /40
2.25×10 /60
844
972
-
10
-9
CHCl (475)
3.34×10 /27
3.15×10 /73
3
-
10
-9
ACN (475)
ACN (510)
2.00×10 /80
2.73×10 /20
173
173
362
304
159
1.30
1.12
1.08
1.13
1.15
-
10
-9
1.63×10 /95
3.04×10 /5
-
10
-9
DMSO (475)
DMSO (512)
MeOH (506)
2.66×10 /70
2.49×10 /30
-
10
-9
2.77×10 /90
2.91×10 /10
-
10
-9
1.45×10 /91
1.86×10 /9
-
10
-9
n-BuOH (475)
n-BuOH (492)
Water (515)
4.93×10 /77
4.75×10 /23
625
498
435
1.00
0.94
-
10
-9
4.39×10 /87
4.21×10 /13
-
10
-9
4.19×10 /96
3.58×10 /4
Cation recognition by DIDOT
The response of DIDOT towards metals was initially checked by observing the naked eye color
change of a 50 µM methanolic solution of DIDOT after separately adding 10 equivalents of
metal perchlorate salt. The yellow color of DIDOT solution turned reddish in presence of Cu
(II), yellowish in presence of nickel (II) and cadmium (II) and there was a deepening in color in
presence of almost all other metal ions (Fig. 5). Thus, no selectivity was assured from naked eye
1
4

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color change. Then the changes in absorption profile of DIDOT were checked in presence of
metals.
Figure 5: Change in color of a 50 µM methanolic solution of DIDOT in presence of 10
equivalents of various metal ions observed under ambient light.
Addition of all metal ions (2 equivalents) to DIDOT (10 µM) perturbed the absorption profile of
the latter (ESI, Fig. S10). Thus, no selectivity could be ensured for any metals from the naked
eye color change as well as UV-Visible experiments. Finally, we checked whether there is any
selectivity as far as the fluorogenic response is concerned. For this purpose, color of solutions
containing DIDOT as well as mixture of DIDOT and metal salts were checked under UV-light
first. Interestingly, DIDOT itself shows a prominent green color under UV-light. However,
addition of only Zn (II) afforded a change in color from green to bright yellow. Addition of other
metals did not cause any such change in color as Zn (II) (Fig. 6). The emission profile of DIDOT
also experienced a red shift in accordance to naked eye color change under UV light in presence
of Zn (II) (Fig. 7).
1
5

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Figure 6: Change in color of a 50 µM methanolic solution of DIDOT in presence of 10
equivalents of various metal ions observed under UV-light.
The emission maxima of a 5 µM methanolic solution of DIDOT undergoes a red shift from ~506
nm to ~535 nm upon addition of 2 equivalents of Zn (II). Addition of other metal ions afforded
simple quenching or no change in the emission profile of the probe. An emission titration was
conducted by addition of Zn (II) to DIDOT where the new band at ~534 nm increased in
intensity upon gradual addition of the metal ion. Presence of an isoemissive point at ~480 nm
ensured the presence of equilibrium between DIDOT and its Zn (II) complex. Using standard
3
σ
3
2
protocol the limit of detection was calculated using the equation,LOD =
, where σ is the
Slope
standard deviation and slope is that5 of the calibration curve. The limit of detection towards Zn
-8
(
II) was calculated to be in the order of ~3 x 10 (M) (ESI, Fig. S11). The low limit of detection
well establishes the efficacy of DIDOT when compared to the limits of detection of previously
reported Zn (II) specific probes (ESI, Table S1).
1
6

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Figure 7: (a) Emission profiles of DIDOT and DIDOT+ various metal ions (λ =410 nm). (b)
ex
Emission titration of 5 µM solution of DIDOT with 0- 2 equivalents Zn (II); (inset) change in
color of solution of DIDOT after Zn (II) titration as observed under UV-light.
Binding nature of DIDOT with Zn (II)
3
3
The binding pattern of DIDOT with Zn (II) was first explored by Job’s plot analysis , where a
1
:1 binding pattern of the two species (Fig. 8). The binding constant value was calculated by
3
4
constructing B-H plot assuming 1:1 stoichiometry of binding using the emission titration data
Fig. 8). An excellent linear fit was obtained and the binding constant value was thus calculated
(
5
-1
to be 2.355×10 M . Such a high binding constant value accounts for the affinity of DIDOT
towards Zn (II). Further evidence of 1:1 complexation was obtained upon recording the ESI-MS
of DIDOT-Zn complex where a mass corresponding to 1:1 complex formation was observed
(Fig. S8). While analyzing the results of mass spectrometric analysis, the observed mass could be
explained only by assuming the deprotonation of the amine proton in DIDOT while undergoing
complexation with Zn (II). Presence of a water molecule as observed from the mass spectrum
completes the co- ordination sphere of the Zn (II) ion undergoing complexation. This was
1
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supported by FTIR analyses. If the FTIR spectra of DIDOT and its Zn (II) complexes are
compared, the sharp signal for –NH group in DIDOT is changed to a broad signal in the
2
-1
complex. Furthermore, the shift in the values of imine signal (1715 cm in free DIDOT to 1620
-1
cm in Zn (II) complex) indicates the involvement of the aforesaid moiety in the complexation
process. The involvement of coumarin nucleus is also expected as a change in the emission
maxima of DIDOT in presence of Zn (II). The only possible mode of binding of coumarin
moiety with Zn (II) is by means of forming a co-ordinate covalent bond between the ester
carbonyl moiety of DIDOT and Zn (II). From all the above observations, it was concluded that
DIDOT forms a 1:1 complex with Zn (II) forming 6 member and 5 member chelate rings
(Scheme 3). However, coordination through Sulfur centre could not be possible as i) the soft
Sulfur donor and hard Zn (II) ion would not form a very stable bond. ii) For a chelate ring
bearing an endocyclic double bond, a six member ring would be thermodynamically favoured
3
5
over a 5- member ring .
Figure 8: a) Job’s plot and b) B-H plot for DIDOT and Zn (II).
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Mechanism of Zn (II) detection
To probe the mechanistic pathway of Zn (II) detection by DIDOT, TCSPC experiments were
conducted. As can be seen from Table 2, bare DIDOT showed an average lifetime of 159 ps.
Upon addition of 1 equivalent of Zn (II), the average lifetime was calculated to be 1727ps (Fig.
9
, Table 3). Apart from increment in the lifetime, the faster lifetime component (τ~145ps) in
DIDOT underwent a considerable increment to attain a value of ~1590ps in presence of Zn (II)
whereas the component having higher lifetime value (τ~1860ps) showed lifetime in the same
order in presence of Zn (II) (τ~3700ps). Thus, from our earlier assignment of lifetime
components of DIDOT, the planar form of the same experiences enhancement in the lifetime
upon complexation with Zn (II). This could be justified based on the attainment of rigidity in the
flexible structure of DIDOT post complexation, as encountered often in case of Chelation
Induced Enhancement of Fluorescence (CHEF) mechanism36-37. To verify this, the emission
profile of DIDOT in polyethylene glycol-400 (PEG400), a viscous solvent, was recorded
(Fig.10a). However, the emission profile of DIDOT in PEG400 exhibited in only enhancement
of the fluorescence intensity, without any noticeable shift in the emission intensity. Thus, the
attainment in rigidity of DIDOT upon complexation only accounted for emission intensity
enhancement. But the shift in emission maxima was yet to be explained. Since DIDOT contains
a diethyl amino group as a donor and a keto imine as an acceptor, there is charge transfer from
the former to the latter, owing to the electron withdrawing character of the imine group.
However, electron donation from the thiosemicarbazone linkage adjacent to the imine moiety
partially quenches the ICT in DIDOT, reflected by the small increment in the excited state
dipole moment value of DIDOT (Scheme 3). However, the engagement of the imine as well as
the thiosemicarbazone moieties along with the coumarin nucleus in complexation process with
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Zn (II) is expected to facilitate the charge transfer from the diethylamino moiety to the imine
moiety (Scheme 3). This assumption was verified by recording the emission profiles of DIDOT-
Zn complex in acetonitrile and methanol (Fig. 10b). The results showed that the emission profile
in acetonitrile was ~525 nm, whereas the same in methanol was ~534 nm. This shift in emission
maxima was consistent keeping in mind the E (30) values of the two solvents (~46.0 for
T
acetonitrile and ~56 for methanol). Thus, the shift in emission maxima of DIDOT after
complexation with Zn (II) is a consequence of increased charge transfer from diethylamino
moiety to the imine moiety. This was also supported from considering the Mulliken charge
densities of DIDOT and its Zn (II) complex, explained in details in the subsequent theoretical
calculation section. The increment in lifetime of DIDOT-Zn (II) ensemble is due to the rigid
structure of the same, which results in freezing of non-radiative decay channels in the said
species. Thus, DIDOT detects Zn (II) by arresting of the conformation in presence of Zn (II)
followed by enhanced ICT from diethylamino moiety to the imine carbon. The fluorescence
intensity enhancement is due to CHEF operative in the rigid structure of the DIDOT-Zn
complex.
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Figure 9: Decay Profiles of DIDOT and its Zn (II) ensemble monitored at 530 nm (λ =375
ex
nm).
H
H
N
NH2
N
NH2
N
N
S
S
Et N
O
I
O
Et N
O
II
O
2
2
H
N
S
N
NH
Et N
O
III
O
2
Zn
Scheme 3: (I) Donation of charge from diethylamino moiety to the imine moiety, (II) Donation
of charge from thiosemicarbazone moiety to the imine, hindering charge transfer, (III) Facile
ICT post complexation of DIDOT with Zn (II).
Wavelength (nm)
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Figure 10: (a) Comparative emission profiles of DIDOT in MeOH and PEG400, (b)
Comparative emission profiles of DIDOT-Zn complex in ACN and MeOH.
Table 3. Excited state decay parameters of DIDOT-Zn in MeOH (λmon=530 nm)
Species
τ1(ps)/B1
182/91
τ2(ps)/B2
1940/9
τav(ps)
198
χ2
DIDOT
1.20
1.09
DIDOT-Zn
1590/87
3810/13
1727
Theoretical calculations
DIDOT (ESI, Fig.S12) and its Zn (II) complex were optimized and the important structural
parameters are provided in Table S2 of ESI. Two conformers were optimized for DIDOT and
designated as planar form (P and non-planar form (NP) respectively (Scheme 2). The
designation has been done considering the dihedral angle (ϴ ) values of the two conformers
d
0
about the imine-thiosemicarbazone linkage. In P form, the ϴ value is 25 whereas the same in
d
0
NP form is 38 . The free inter conversion between the two conformers as proposed earlier is
justified from calculating the energy gap (EHOMO-LUMO) between the Highest Occupied
Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) for each
receptor (ESI, Fig. S13). The energy gap is comparable in both conformers, allowing conversion
between the two. The charge density analyses on the two conformers (ESI, Table S3) show that
for P form and NP form, the Mulliken charges on the carbon adjacent to the imine moiety are -
0
.168 and -0.145 respectively (Table S3). However, in case of the Zn (II) complex, from all the
experimental results, P form was concluded to undergo complexation with the metal. In the
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complex, the charge on the same carbon attained a value of +0.075 (Table S2, Fig. S13), which
supports our proposal that greater charge transfer, occurs from diethylamino group to the imine
group in DIDOT post complexation with Zn (II). The dihedral angle value of the complex (220)
indicates that the complex bears structural resemblance to the P form. Hence, charge transfer is
expected to be enhanced in DIDOT post complexation with Zn (II). The stability attained by the
Zn (II) complex of DIDOT is supported from decreased HOMO-LUMO gap compared to either
conformers of DIDOT (ESI, Fig. S14). Hence, the theoretical calculations aid in understanding
the photophysical modulation of DIDOT upon complexation with Zn (II).
Practical application of DIDOT
In spite of the remarkable photophysical modulation of DIDOT in presence of Zn (II), the main
bottleneck in using it for practical purposes was the use of methanol as the sensing medium. To
overcome this limitation, an alternative approach was considered. Two 10 mL spiked water
samples containing Zn (II) were prepared with varying concentrations of Zn (II) in them. An
aliquot of 50 µL was added from both the samples to the methanolic solution of DIDOT. The
emission profiles were recorded, each point being recorded as triplicate. DIDOT was able to
detect the Zn (II) in both water samples (Fig. 11) and the results are provided in Table S4 of ESI.
In both the cases, the relative error was well below 10%, ensuring accuracy of measurement.
Hence, the fluorogenic detection and quantification of Zinc in water samples was achieved by
DIDOT.
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Figure 11: Emission response of DIDOT towards two spiked water samples labeled SWS1 and
SWS2.
Conclusion
In conclusion, the current work involves Zn (II) detection by interesting modulation of the
excited state photophysics of a coumarin based ICT compound DIDOT. The photophysical
characterization of DIDOT has been conducted thoroughly along with proper explanation of the
Zn (II) detection mechanism by theoretical and experimental methods. The limit of detection of
DIDOT towards Zn (II) is ~30 nM. DIDOT can successfully detect and quantify water in
drinking water sample.
Conflict of interest
There are no conflicts of interest.
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Acknowledgement
A.B and S.C.M gratefully acknowledge senior research fellowships from CSIR and UGC, New
Delhi, Government of India, respectively. NG likes to acknowledge DST, India (Project No.
EMR/2016/004788) for financial support.
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