Comparative Photophysical Study of Differently Substituted Cinnamaldehyde-Based Chalcones: From Intramolecular Charge Transfer to Fluorogenic Solvent Selectivity

Article abstract of DOI:10.1021/acs.jpca.9b03437

We synthesized three cinnamaldehyde-based chalcone derivatives, (2E,4E)-1-(2-hydroxyphenyl)-5-phenylpenta-2,4-dien-1-one (HPD), (2E,4E)-5-(4-(dimethylamino)phenyl)-1-phenylpenta-2,4-dien-1-one (DPPD), and (2E,4E)-5-(4-(dimethylamino)phenyl)-1-(2-hydroxyph

Full text of DOI:10.1021/acs.jpca.9b03437

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A: Kinetics, Dynamics, Photochemistry, and Excited States
Comparative Photophysical Study of Differently Substituted
Cinnamaldehyde based Chalcones: from Intramolecular
Charge Transfer to Fluorogenic Solvent Selectivity
Arghyadeep Bhattacharyya, Subhash Chandra Makhal, and Nikhil Guchhait
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b03437 • Publication Date (Web): 09 Jul 2019
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Comparative Photophysical Study of Differently Substituted Cinnamaldehyde based
Chalcones: from Intramolecular Charge Transfer to Fluorogenic Solvent Selectivity
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Arghyadeep Bhattacharyya, Subhash Chandra Makhal and Nikhil Guchhait*
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Department of Chemistry, University of Calcutta, 92, A.P.C. Road, Kolkata-700009, India
Abstract: We have synthesized three cinnamaldehyde based chalcone derivatives, (2E,4E)-1-(2-
hydroxyphenyl)-5-phenylpenta-2,4-dien-1-one (HPD), (2E,4E)-5-(4-(dimethylamino)phenyl)-1-
phenylpenta-2,4-dien-1-one
(DPPD),
(2E,4E)-5-(4-(dimethylamino)phenyl)-1-(2-
hydroxyphenyl)penta-2,4-dien-1-one (DPHPD). The molecule HPD is totally non-fluorescent.
Exclusion of phenolic –OH moiety from HPD along with introduction of dimethylamino moiety
in DPPD results in excellent excited state charge transfer property in the solution phase.
Interestingly, introduction of phenolic –OH and dimethylamino moiety in DPHPD results in
solvent selectivity in the excited state. DPHDP behaves as a strong fluorophore only in carbonyl
or thiocarbonyl constituting solvents like dimethylsulfoxide (DMSO), dimethylformamide
(DMF) and dimethylacetamide(DMAC) and shows prominent red colour under UV-light.
However, no emission was observed in similar carbonylated solvents like acetone or formamide,
or in viscous medium like glycerol. The difference in solvent response was probed by various
spectroscopic techniques and analyzed using crystal structure of three chalcones along with
theoretical calculations. The practical utility of DPHPD has been explored by detecting
percentage of water in DMSO solvent.
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1. Introduction
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Molecules capable of exhibiting excited state intramolecular charge transfer (ESICT)
phenomenon has garnered undisputed attention from theoretical and experimental chemists alike
since the observation of dual emission bands in 4-dimethylaminobenzonitrile (DMABN)1,2.
Many groups have been engaged in purposefully designing new and novel ESICTmolecules3-
6and studying their photophysical properties along with analytical application. Our group has
been engaged with designing many ESICT probes5, 6-8, and the main feature of them being the
substituent change at the acceptor side of the molecules. In all such self designed molecules, the
compounds largely resembled 4-dimethylaminocinnalmaldehyde (DMACD) in structure. The
principal reason behind this was the interesting charge transfer phenomenon observed in
DMACD9. Inspired by this, we designed a chalcone derivative (Scheme 1) DPPD from
DMACD and it turned out to be an excellent ESICT probe, showing a sharp increment in
Stokes’ Shift value (~3120 cm-1 in Heptane to ~7738 cm-1 in MeOH) in solution phase. Our
literature survey revealed that Zhu et al and Tang et al.10, 11reported a similar chalcone derived
from 4-dimethylaminobenzaldehyde and 2-hydroxy acetophenone (HCAP) which showed
Aggregation Induced Emission (AIE) coupled ESIPT emission. In those compounds, the probe
was poorly emissive in all solvents except water, where there was enhanced emission. Weak but
important interactions in the aggregated state cause restriction of intramolecular motion (RIR)
which was attributed to be the major contributing factor for enhanced emission apart from the
poor solvation of the probe by water molecules in aqueous solution. Furthermore, the
hydrophobic nature of the chalcone bond in the previously mentioned compounds could be
another contributing factor behind the poor solvation of HCAP by water. Inspired by HCAP, we
then synthesized the probe DPHDP since this probe is one with ethylene unit longer than HCAP
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and can thus be a potential probe to portray ESIPT-AIE-ESICT coupling as DPPD shows ESICT
phenomenon. However, DPHDP was poorly emissive in all common solvents as well as in
water, which eliminated any possibility of the said probe to be an AIE-gen. Interestingly,
DPHPD was strongly emissive only in dimethylsulfoxide, dimethylformamide and
dimethylacetamide indicated by the sharp increment in the quantum yield values. The emission
maxima were in total correspondence to the same exhibited by DPPD in the aforesaid triad of
solvents. The solvent selective emissive behaviour of DPHPD was further probed by steady state
and time resolved emission fluorescence measurements. Thus, we could induce solvent selective
emission in a chalcone derivative by simple inserting an extra functional group in a charge
transfer probe. To further validate this, control compound HPD was synthesized which proved to
be essentially non-emissive in all solvents. The reason behind this solvent selectivity was probed
by obtaining crystal structures of the three chalcone derivatives and subsequent theoretical
calculations. Finally, we devised an analytical application of this solvent selective behavior of
DPHDP by finding out the water content in a dry DMSO sample by simple fluorescence
quenching method.
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Me2N
O
OH
O
DPPD (ESICT active)
HPD (non-emissive)
Me2N
Me2N
O
OH
O
OH
HCAP (shows coupling of AIE and ESIPT)
DPHPD (emissive only in DMSO, DMF and dimethylacetamide)
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Scheme 1.Molecular structures of our synthesized probes and HCAP reported by Tang et. al.
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2. Experimental Details
2.1. Chemicals
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All UV-grade solvents and dimethylaminocinnamaldehyde, cinnamaldehyde, acetophenone and
2-hydroxy acetophenone were purchased from Merckand used as received.
2.2. Apparatus
NMR spectra were recorded on aBruker Advanced Supercon 300 MHz NMR spectrophotometer
using TMS as internal standard. Steady State UV-Vis absorption profiles were recorded on a
Hitachi U-3501 spectrophotometer. Steady state emission and excited state decay profiles were
recorded on Perkin Elmer LS-55 and Horiba JobinYvonFluorocube-01-NL, respectively.Mass
spectrometrywas carried out inWatersXevo G2-S Q TOF mass spectrometer. During steady state
absorption and emission measurements, the concentration of probe was maintained at micro
molar range to avoid aggregation, re absorption or self-quenching respectively. All data were
recorded at room temperature. The excited state lifetime measurements were carried out by
exciting the sample using a picoseconds laser diode of 375nm and 450nm. The signals were
obtained by setting the emission polarizer at magic angle (54.7º) to prevent anisotropy and the
decays were deconvoluted using DAS6 software. The goodness of fitting was judged using χ2
criteria. The average lifetime was calculated using standard procedures12. The quantum yield
values were calculated using Coumarin 153 as standard13.The single crystals ofHPD, DPPD and
DPHPD was mounted on a Bruker-AXS SMART APEX II diffractometerequipped with a
graphite monochromator and Mo Kα(λ = 0.71073 A) radiation. The crystal was placed 60 mm
from theCCD and 360 frames were measured with a counting time of 10s.The structure was
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solved using the Patterson method using SHELXS97. Subsequent difference Fourier synthesis
and least-square refinementrevealed the positions of the remaining non-hydrogen atoms.Non-
hydrogen atoms were refined with independent anisotropic displacementparameters. Hydrogen
atoms were placed in idealized positionsand their displacement parameters were fixed to be 1.2
timeslarger than those of the attached non-hydrogen atoms. Successful convergencewas
indicated by the maximum shift/error of 0.001 for thelast cycle of the least squares refinement.
Absorption corrections werecarried out using SADABS program. All calculations were
carriedout using SHELXS 97, PLATON 99, ORTEP-32 and WinGX systemVer-1.64.14 Mass
spectrum was recorded on Waters XevoG2-S Q TOF mass spectrometer. For cell imaging
studies, the cell survivability assay was done by MTT assay. Micro plate reader from Biotek,
USA and fluorescence microscope (Leica) were used for MTT assay and cell imaging
respectively.
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2.3. Theoretical calculations
All theoretical calculations were performed using Gaussian 09package15. For calculations in
vaccum 6-311++G (d, p)16 was used. For calculations in solvent, the Polarizable continuum
model was employed (PCM).
3. Results and discussions
The syntheses of the probes have been carried out according to Scheme 2. The probes were
1
13
characterized using H NMR, C NMR, ESI-MS (ESI, Figures S1-S9) and X-ray single crystal
structure analysis. The characterization data has been provided in the supplementary information.
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The single crystals for all the probes were obtained by slow evaporation of saturated solutions of
the chalcones in chloroform over a period of few days.
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R2
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R2
O
Pyrolidine
EtOH
O
R1
R1
CHO
R1=OH,R2=NMe2; DPHPD
R1=H,R2=NMe2; DPPD
R1=OH,R2=H; HPD
Scheme 2.Synthetic outline of the chalcone derivatives
3.1. Crystal structures of the chalcones. To have an idea of the absolute structure of the
chalcones, the single crystals of all of them were obtained. Figure 1shows the ORTEP diagrams
of the chalcones. The crystallographic details of the crystal structures are provided in Tables S1-
S3 in the supplementary information section. Both DPHPD and HPD are involved in formation
of a 6-membered intramolecular hydrogen bonded rings in the solid state (Figure 2). However,
HPD adopts a non-linear configuration in the solid state as evidenced by the interplanar angles
between the cinnamoyl and the acetophenone fragment (29.54º). The same value reduces to
15.02º in DPPHPD. However, in DPPD where there is no possibility of hydrogen bonding
interaction due to absence of -OH group, the interplanar angle value attains a value of 9.62º.
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Figure 1. ORTEP diagrams of a) HPD, b) DPPD and c) DPHPD. (The probabilities of thermal
ellipsoids are each set at 40%)
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Figure 2. H-bonding interactions and interplanar angles in a) HPD, b) DPHPD and c)DPPD.
3.2. Absorption spectra. The steady state absorption spectra of HPD in various solvents
consisted of a single absorption band which varies from 348 nm to 368 nm based on the solvent
polarity (Table 1, Fig. 3). The absorption spectra of DPPD also consisted of single absorption
band. However, introduction of dimethylamino moiety in DPPD resulted in bathochromic shift
of the absorption band compared to HPD (Figure 3). DPPD also showed solvent dependant
shifting of its absorption maxima (Table 1). The absorption profile of DPHPD was qualitatively
similar to the other two chalcones, the bathochromic shift in its absorption profile being largest
owing to presence of two electron donating groups in the form of –OH and –NMe2 moieties
(Table 1, Figure 3). In accordance with previous reports of cinnamaldehyde derived probes8, 9 the
single absorption bands of the three chalcones were assigned to be π-π*. Addition of acid
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(Trifluoroacetic acid) to the acetonitrile solutions of DPPD as well as DPHPD caused
diminishing of the red shifted absorption bands along with generation of a new band at 325 nm
for DPPD and at 341 nm for DPHDP (Figure 4). In presence of acid, the dimethylamino moiety
gets protonated, thereby decreasing the chromophore stability upon loss of conjugation. Hence
the blue shifting occurs. The protonated form of DPHPD resembled HPD. Hence, the absorption
maxima of DPHD-acid ensemble resembled that of HPD.HPD showed no change in its
absorption profile upon adding acid. All the three chalcone derivatives were silent to base
addition as far as absorption spectra are concerned.
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Figure 3. Steady state absorption spectra of HPD,DPPDand DPHPD in various solvents.
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Figure 4. Effect of acid on the absorption spectra of DPPD and DPHPD in acetonitrile.
3.3. Steady State emission spectra
HPD was almost non-emissive in a range of solvents. However, the chalcone DPPD was found
to be a promising fluorophore. The emission maxima of DPPD underwent an excellent red shift
from ~480 nm in n-heptane to ~615 nm in isopropanol (Figure 5). The appreciable quantum
yield (Table 1) of DPPD in all solvents enabled us to observe the colour of DPPD under UV-
light (Figure 5). The larger extent of red shifting of emission maxima of DPPD compared to the
same in ground state indicates an increment of the excited state dipole moment of DPPD. The
excited state dipole moment was calculated using the Lippert-Mataga plot (Figure 6)17 as given
below:
The terms bear their usual significance. The Onsager cavity radius was calculated using the
volume calculation of DPPD using Gaussian package, the value of which was 5.45 Å. The
ground state dipole moment of DPPD was also found to be 7.52 Debye. Upon calculation, the
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excited state dipole moment was found to be 26.49 Debye. The polarity dependence of the
excited state of DPPD was further established by the emission maxima vs. Reichardt Scale (ET
(30)) scale18 using non protic solvents, which showed a linear variation (Figure 6).Furthermore,
addition of acid to DPPD in acetonitrile caused a steady quenching of the emission. Hence, the
protonation of the donor dimethylamino affects the quantum yield of DPPD, a characteristic of
ESICT probes.17Thus; the steady state emission data reveals DPPD to be an excited state
intramolecular charge transfer (ESICT) probe. The emission profiles (ex=450 nm) of DPHPD in
various solvents (Figure 7) reveal that DPHPD is poorly emissive in almost all solvents
excluding DMSO, DMF and dimethylacetamide. Upon increasing the temperature of solution
(Figure 7) of DPHPD in DMSO, the emission intensity experiences a decrement. The colour of
solutions of DPHPD in various solvents observed under UV-light corroborates to the
observation as well as from the quantum yield value of DPHPD in various solvents (Table 1).
Interestingly, if the quantum yield is observed for DPHPD in various solvents, the values are
greater in DMSO, DMF, dimethylacetamide compared to others. It was the first hint that
DPHPD has some affinity towards carbonylated (S=O, C=O) solvents. Furthermore, the
emission maxima of DPHPD in these three solvents were almost identical to those for DPPD.
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Figure 5. (a) Normalized emission spectra of DPPD in various solvents (ex=420 nm; excitation
slit=2 nm, emission slit=2 nm). (b) Colour of DPPD as observed under UV-light in various
solvents: (from left) n-heptane, chloroform, 1,4-Dioxane, acetonitrile, DMSO, methanol,
isopropanol.
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Figure 6. (a)Lippert Mataga plot for DPPD (b) Plot of emission maxima vs. ET (30) plot. (c)
effect of addition of acid on the emission profile of DPPD in acetonitrile medium.
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Figure 7. (a) Emission profiles of DPHPD in various solvents (ex=450 nm; excitation slit=2
nm, emission slit=2 nm). (b) Effect of temperature on emission profile of DPHPD in DMSO. (c)
Colour of DPHPD observed under UV-light in various solvents: (from left) n-heptane,
chloroform, 1, 4-Dioxane, acetonitrile, dimethylsulfoxide, methanol, isopropanol. (d) Colour of
DPHPD in carbonylated solvents under UV-light: (from left) DMSO, DMF, Formamide,
acetamide, dimethylacetamide, acetone.
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Table 1. Steady state parameters for HPD, DPPD and DPHPD
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maxUV(nm)
347
maxFl(nm)
Compound
HPD
Solvents
Φ
n-heptane
Non-
Not
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cyclohexane
dichloromethane
chloroform
348
fluorescent in applicable
all solvents
360
362
acetonitrile
356
1,4-dioxane
357
dimethylsulfoxide
364
dimethylformamide 364
methanol
369
369
407
409
420
439
450
433
458
450
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DPPD
n-heptane
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492
574
553
590
527
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608
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0.008
0.010
0.011
0.015
0.012
0.016
0.075
0.072
0.082
cyclohexane
dichloromethane
chloroform
acetonitrile
1,4-dioxane
acetone
dimethylsulfoxide
dimethylformamide 442
dimethylacetamide 427
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methanol
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430
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459
458
451
446
435
468
622
615
-
0.004
isopropanol
n-heptane
0.013
DPHPD
-
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cyclohexane
dichloromethane
chloroform
acetonitrile
1,4-dioxane
acetone
-
-
-
-
-
-
620
-
0.002
-
-
-
dimethylsulfoxide
608
609
600
-
0.092
0.074
0.083
-
dimethylformamide 467
dimethylacetamide 465
methanol
isopropanol
water
449
471
467
462
466
600
-
0.005
-
glycerol
PEG400
642
624
0.002
0.004
3.4. Excited state lifetime measurements
Since the charge transfer probes show marked variation of their excited state lifetime with
solvent polarity, fluorescence time resolved experiments were performed for DPPD in a range of
solvents whereas for DPHPD the experiments were performed only in carbonylated solvents,
where sufficient fluorescence intensity was observed (Figure 8). In all solvents, a biexponential
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decay pattern is observed. Among the two components, the faster component experienced an
increment in its contribution to the total excited state population as the solvent polarity is
increased (Table 2). Thus, this component is due to the charge transferred, polarity sensitive
form of DPPD. The lifetime components as well as the average lifetime values experience an
increment in solvents like DMSO, DMF, acetamide and isopropanol. This could be an outcome
of the viscosity of the said solvents as it is known that average lifetime increases in viscous
medium.19The abrupt drop in the lifetime components as well as the average lifetime in methanol
(Table 2) could be an outcome of the strong H-bonding ability and low viscosity of the
solvent.20The slower component in various solvents could be due to some excited state
complexation between DPPD and solvents.. The average lifetime of DPPD when plotted against
the Reichard parameter (ET (30)) value of the solvents (Fig. 8), produced a satisfactory linear
correlation (R2=0.874). For DPHPD, the same trend of excited state lifetime values was
displayed in DMSO, DMF and acetamide (Table 2).
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Figure 8.TCSPC decay profiles of (a) DPPD (ex=375 nm)and (b) DPHPD in various solvents.
(Ex=450 nm). (c) Plot of average lifetime of DPPD vs. Reichard Parameter.
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Table 2. Excited state decay parameters of DPPD (ext=375nm ) and DPHPD (ext=450nm).
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Species
DPPD
Solvent
τ1 (ns)/B1(%) τ2(ns)/ B2(%) τav(ns) χ2
chloroform
dichloromethane
1,4-dioxane
acetonitrile
acetone
0.21/71
0.46/93
0.11/97
0.44/55
0.81/89
1.29/73
0.48/29
0.94/7
0.29
0.48
0.12
0.67
0.88
1.53
1.19
1.15
0.13
0.99
0.28
0.39
0.67
1.23
1.21
1.24
1.09
1.23
1.07
1.02
1.16
1.25
1.03
1.00
1.05
1.03
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0.50/3
0.83/45
3.61/11
2.17/27
1.95/14
1.55/44
0.12/98
1.89/8
dimethylsulfoxide
dimethylformamide 1.07/86
dimethylacetamide 0.84/56
methanol
0.61/02
0.91/92
isopropanol
DPHPD
dimethylformamide 0.17/85
dimethylacetamide 0.23/82
0.93/15
1.13/18
1.60/30
dimethylsulfoxide
0.28/70
Standard Deviation for fitting analysis is ±5%
3.5. Discussion: From the above experimental observations, we concluded that HPD was a non-
emissive compound, DPPD was an efficient ICT probe and DPHPD was strongly fluorescent in
only a few solvents, the common factor in all the solvents being the presence of a carbonyl
moiety in them. The main task at our hand was to find the reason behind such behaviour of the
said compounds. The crystal structures of the compounds were crucial to explain the behaviour
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of the compounds. HPD already having a bent structure (Figure 2a) would further experience
bond rotation around the single bond in it in the solution phase, thereby activating non radiative
decay channels21which results in the totally non-emissive behaviour of HPD. Focusing on
DPPD, the angle between the cinnamoyl moiety and the acetophenone moiety was much less
(Figure 2a) compared to HPD. Thus, the more planar structure of DPPD facilitates charge
transfer from the dimethylamino moiety to the acetophenone moiety. The decreased quantum
yield of DPPD in polar protic solvents could be an outcome of Hydrogen bonding interactions,
thereby activating non radiative decay channels. When DPHPD is considered, the aforesaid
angle attains a value of 15.02º. Hence, the angular structure could explain the poor emissive
behaviour of DPHPD in several solvents as observed. However, the viscosity of solvents like
dimethylsulfoxide, dimethylacetamide and dimethylformamide probably aids in DPHPD
attaining a linear structure, thereby facilitating charge transfer from dimethylamino moiety to the
carbonyl unit. It is this lack of viscosity which leads to DPHPD being practically non-emissive
in similar carbonylated solvents like acetone and formamide. But consideration of viscosity
alone could not explain the poorly emissive character in highly viscous solvents like
polyethylene glycol 400 and glycerol. Thus, the dielectric constants were also considered. An
examination of the dielectric constant values of polyethylene glycol 400 (14.1)22could explain
the poor emissive nature of DPHPD in the said solvent. However, in a similar viscous solvent
glycerol has a dielectric constant value of ~4723 which is much greater than the aforementioned
carbonylated solvents where DPHPD shows emission strong enough to be visible through naked
eye under UV light, consideration of dielectric constant values alone is also insufficient to
explain experimental observations. This also holds for polar protic solvents whose dielectric
constants are considerably greater than the aprotic solvents. Thus, the effect of hydrogen bonding
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must be considered to rationalize the experimental observations. In polar protic solvents
including the highly viscous glycerol as well as in carbonylated solvent formamide, there is
strong H-bonding interaction with the dimethylamino moiety of DPHPD which reduces the
availability of lone pair on dimethylamino moiety on the said compound to be delocalized to the
carbonyl acceptor. Shifting our focus on the carbonylated solvents in which DPHPD is strongly
emissive, certain features appear to be common: i) strong dielectric constant values facilitate
charge transfer in DPHPD. ii) Viscosity of the said solvents ‘flattens’ the bent structure of
DPHPD, aiding charge transfer in the same molecule. The effects of the aforementioned factors
were best realized upon recording the emission profile of DPHPD in DMSO at varying
temperatures (Figure 7). The emission maximum does not change at all. But the emission
intensity decreases with increasing temperature. This signifies that increment in temperature
decreases the viscosity of DMSO; hence non-radiative decay channels would be activated.
However, as the dielectric constant is not changed that much, the maximum does not alter. The
above factors when considered could rationalize the increased emissive property of DPHPD in
dimethylsulfoxide, dimethylformamide and dimethylacetamide.
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3.6. Theoretical Calculations
The principal conclusion that was derived from preceding discussions was that charge transfer
was facile in solvents like dimethylsulfoxide, dimethylformamide and so on. To confirm our
conclusions, theoretical calculations were performed for DPHPD and DPHD. From the
optimized structures (ESI, Figures S10, S11 and S12) of the two chalcones, the HOMO LUMO
diagrams of the compounds as well (ESI, Figures S10, S11 and S12) as the Mulliken Charge
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distributions were considered (Table S4). The electron densities on the HOMO as well as LUMO
diagrams of DPPD showed (ESI, Figures S10) a swing in the electron density from the dimethyl
amino moiety to the ketophenyl group, hinting charge transfer. On the other hand, the HOMO-
LUMO diagram of DPHPD showed a swing in electron density only in case of DMSO (Figures
S13) solvent. This swing in electronic charge in DPHPD in DMSO was comparable to that of
DPPD. This could indirectly explain why DPHPD behaves similarly to DPPD in DMSO. A
perusal of the values of the charge densities on nitrogen atom and carbonyl oxygen atom of
DPHPD revealed that in DMSO the charge gradient between the said two atoms was much
greater compared to the same in vaccum. Similarly, the same in DPPD was already greater
compared to DPHPD, indicating facile transfer of charge density from nitrogen to carbonyl
oxygen in the same. Thus, we could corroborate our conclusions from theoretical calculation
results.
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3.7. Practical application of DPHPD
Among the solvents in which DPHPD shows high emissive behaviour, dimethylsulfoxide holds
special importance owing to its widespread use as NMR solvent as well as in various reactions.
Since DPHPD shows very low emission in water, we decided to find out the water content in
DMSO using simple fluorimetric method. A dry DMSO solution of DPHPD (17 µM) was
titrated with water (Figure 9). Addition of water resulted in gradual diminishing of the intense
emission band of DPHPD, with red shift to ~640 nm. The red shifting occurs due to greater
dielectric constant of the DMSO-water mixture, whereas the diminished emission intensity is
observed due to hydrogen bonding interaction of DPHPD with water present in the system.
Interestingly, the emission maxima shift with increase in water content of the system. This is
indeed an interesting feature of the water detection method as just by noting the emission
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maxima of DPHPD in a DMSO sample, which nullifies the concentration of DPHPD to be fixed
for water determination of a DMSO sample. The emission maxima when plotted against the
percentage of water provided reasonable linearity. Hence, DPHPD can act as a potential tool to
detect water in a DMSO sample. The practical utility was established by detecting the water
content in DMSO by recording the emission spectrum of three simulated DMSO samples and
finding out the water concentration from the calibration curve in Figure 9. The results have been
provided in Table 3. The simplicity by which DPHPD can detect the water content in DMSO
overcomes the complexities of earlier reported processes involving Karl-Fischer Titration24 as
well as that involving freeze-thaw pump cycles.25 Moreover; the Stern- Volmer plot was
constructed to find out the quenching constant (ESI, Figure S13). The high value of quenching
constant26 (in the order of 103 mol-1) suggested the efficient quenching of DPHPD in DMSO in
presence of water. Furthermore, the quenching mechanism was investigated in details by steady
state as well as time resolved methods. The Stern Volmer plot showed a curvature (ESI, Figure
S13). We provided a global fit as well as individual fits, three of each having an intercept value
close to 1.0. Such non-linear pattern hinted presence of static as well as dynamic modes of
quenching26. This was further verified by recording absorption profiles of DPHPD in DMSO
containing varying water percentage (ESI, Figure S13). It was observed that the absorption
profile underwent mild red shift to ~479 nm. This indicated the formation of a ground state
complex between DPHP in DMSO and water. This was further verified by recording decay
profiles of DPHPD in DMSO water mixtures. The average lifetime of DPHPD was unaffected
till 6% water content. This is the signature of static quenching26. However, steady decrement was
observed at post 6% water content, indicating excited state collision leading to dynamic
quenching26. This conclusively proved the presence of dynamic as well as static state quenching
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modes. The limit of detection of water in DMSO by DPHPD was calculated to be 2.8% (ESI,
Figure S14) using IUPAC recommended guidelines27 It is worth mentioning at this juncture that
although the plot of emission intensity vs. water content showed excellent linearity, the sole
purpose of plotting the emission wavelength vs. water content was to nullify instrument specific
intensity problem. The emission wavelength would effectively be constant when recoded by any
instrument, whereas the intensity may vary.
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Figure 9.(a)Emission titration of DPHPD in DMSO by water, (b) Plot of emission maxima vs.
% of water in DMSO.
Table 3. Determination of water content in spiked DMSO solutions
Sample Name Added Water content Recovered water content Relative error (in %)
W1
W2
W3
25.00%
20.00%
10.00%
23.97%
19.45%
9.55%
4.29
2.83
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3.8. Conclusions: Three chalcones HPD, DPPD and DPHPD bearing cinnamaldehyde and
acetophenone were synthesized. The substitution effect changes the nature of the chalcones from
charge transfer to solvent selective emission. The structural consequences of substitution effect
have been depicted by single crystal structure analysis of the chalcones. The high dielectric
constant along with viscosity facilitates DPHPD to behave as a strong emitter in carbonylated
solvents like DMSO, DMF and acetamide. The practical utility of DPHPD has been
demonstrated by determination of percentage water content in simulated DMSO samples.
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Author Information
*Corresponding author. Tel.: +913323508386; fax: +913323519755.
E-mail address: nguchhait@yahoo.com (N. Guchhait).
The authors declare no conflict of interest.
Acknowledgements
AB and SM would like to thank CSIR and UGC for granting fellowships. NG thanks DST, India
(Project No. EMR/2016/004788) and CSIR, India (Project No. 01(2920)18/EMR-II) for financial
support.
Associated Content
Supporting Information. Synthetic procedure, NMR, ESI-MS, crystallographic information of
HPD, DPPD and DPHPD, optimized structures of DPPD and DPHPD, Mulliken charge
distributions on DPPD and DPHPD, solvent abbreviation index.
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