Physicochemical Investigation of 2,4,5-Trimethoxybenzylidene Propanedinitrile (TMPN) Dye as Fluorescence off-on Probe for Critical Micelle Concentration (CMC) of SDS and CTAB

Article abstract of DOI:10.1007/s10895-015-1662-6

2,4,5-trimethoxybenzylidene propanedinitrile (TMPN) was synthesized by Knoevenagel condensation. Structure of the TMPN was conformed by the elemental analysis and EI-MS, FT-IR, ¹H-NMR, ¹³C-NMR spectroscopy. Absorbance and emission sp

Full text of DOI:10.1007/s10895-015-1662-6

J Fluoresc
DOI 10.1007/s10895-015-1662-6
ORIGINAL ARTICLE
Physicochemical Investigation of 2,4,5-Trimethoxybenzylidene
Propanedinitrile (TMPN) Dye as Fluorescence off-on Probe
for Critical Micelle Concentration (CMC) of SDS and CTAB
Salman A. Khan1 & Abdullah M. Asiri1,2
Received: 1 June 2015 /Accepted: 14 September 2015
#
Springer Science+Business Media New York 2015
Abstract 2,4,5-trimethoxybenzylidene propanedinitrile
(TMPN) was synthesized by Knoevenagel condensation.
Structure of the TMPN was conformed by the elemental anal-
responsible for the color because molecules absorbs the visi-
ble light at certain wavelength [1].Various food colorings
dyes, pigments, cotton fibric dyes, pH-indicators, lycopene,
stains and β-carotene all contain chromophores [2, 3].
Donor and acceptor group such as N-(CH3)2, OCH3, OH
and CN, NO2 can easily give the lone pair of electron to the
acceptor [4]. Intramolecular charge-transfer (ICT) of mole-
cules are formed between a donor (D) group and an acceptor
(A) group in the ground states or exited states [5]. ICT mole-
cules have unique absorption bands in the region of ultravio-
let–visible spectra [6]. Due to this behavior donor –acceptor
containing chromophores have attracted current interests in
terms of materials science such as near-infrared dyes, nonlin-
ear optics, photonic imaging, electrochemical sensing, lang-
muir films, photoinitiated polymerization, dye synthesis solar
sell [7–9]. Photophysical and Physicochemical studies such
as, oscillator, florescent quantum yield strength,
solvatochromic, piezochromic, and photostability are one of
most important studies to determine the physicochemical be-
havior of the molecules [10]. Numerous reactions were report-
ed for the synthesis of donor- acceptor chromophores [11, 12].
However, Knoevenagel condensation is one of the most im-
perative reactions for the development of donor acceptor chro-
mophores by the reaction of carbonyl compounds with active
methylene carbon in the presence of some Lewis acids or
Lewis base followed by a nucleophilic addition and dehydra-
tion reaction [13]. Different synthetic methods were reported
for Knoevenagel reaction, such as normal/refluxing in the sol-
vent [14], ultrasonication, microwave radiation [15], solid-
phase reaction [16] and photosensitization [17]. Due to
numarus application of donor acceotet chrom,pres and con-
tinues work on the phtophyca studies in this paer we are
ropting the synthesis of donor acceptor chromophore 2,4,5-
trimethoxybenzylidene propanedinitrile (TMPN) and its
photophysical investigation.
1
ysis and EI-MS, FT-IR, H-NMR, 13C-NMR spectroscopy.
Absorbance and emission spectrum of the TMPN was studied
in different solvent provide that TMPN is good absorbent and
emission red shift in absorbance and emission spectra as po-
larity of the solvents increase. Photophysical properties in-
cluding, oscillator strength, extinction coefficient, transition
dipole moment, stokes shift and fluorescence quantum yield
were investigated in order to investigate the physicochemical
behaviors of TMPN. Dye undergoes solubilization in different
micelles and may be used as a probe to determine the critical
micelle concentration (CMC) of SDS and CTAB.
.
.
Keywords TMPN Dipole moment Fluorescence quantum
.
.
.
yield CMC SDS CTAB
Introduction
Long chain pi bond conjugated system is known as chromo-
phore. Organic compound containing with chromophores are
* Salman A. Khan
sahmad_phd@yahoo.co.in
1
Chemistry Department, Faculty of Science, King Abdulaziz
University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
2
Center of Excellence for Advanced Materials Research, King
Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia

J Fluoresc
Experimental
using the same excitation wavelength, the unknown quantum
yield is calculated using the following Eq. (1).
Chemicals and Reagents
2
Is  Ar  ns
Φf
¼
Φr
ð1Þ
Ir  As  n2
r
The 2,4,5-trimethoxybenzylidehyde and malononitrile were
acquired from Acros Organic. All solvents and reagents
(A.R.) were acquired commercially and utilized with no addi-
tional purification, excluding dimethylformamide (DMF),
ethanol and methanol.
where ϕ is the quantum yield, I is the integrated emission
intensity, A is the absorbance at excitation wavelength, and
n is the refractive index of the solvent. The subscript r refers to
the reference fluorophore of known quantum yield [18]. The
photochemical quantum yield (ϕc) was calculated using the
method that was described in details previously [19].
Apparatus
Thomas Hoover capillary melting apparatus was used to re-
cord the melting points of the synthesized compounds without
any correction. Nicolet Magna 520 FT-IR spectrometer was
utilized to record the FT-IR spectra. Brucker DPX 600 MHz
spectrometer with tetramethyl silane as internal standard at
Result and Discussion
Chemistry
1
room temperature was used to perform the H-NMR and
13C-NMR experiments in CDCl3. Shimadzu UV-160A spec-
trophotometer was utilized to gain the UV-Vis electronic ab-
sorption data and by using a 10 mm quartz cell absorption
spectra were collected. By using Shimadzu RF 5300
spectrofluorphotometer having a quartz cell of rectangular
shape with dimensions 0.2 cm × 1 cm for minimizing the
reabsorption. Emission spectra were observed at right angle.
Before proceeding data analyses, all fluorescence spectra were
blank subtracted.
The synthesis of 2,4,5-trimethoxybenzylidene
propanedinitrile (TMPN) is straight forward and the TMPN
was isolated in good yield. [18] The acquired TMPN is stable
in both solid as well as in the solution form. The structure of
synthesized compound shown in experimental section was
recognized by comparing spectral data R, H-NMR & 13C-
NMR. It is clear from the IR band positions that the formation
of the TMPN takes place. TMPN demonstrated intense bands
at 1570 cm-1 due to v (C=C) stretch, which proves the forma-
tion of TMPN by konovelgal; condensation. Additional con-
firmation for the formation of TMPN was acquired from the
1H-NMR spectra, which offer diagnostic tools for the posi-
tional illumination of the protons. Assignments of the signals
are based on the chemical shifts and intensity patterns. The
aromatic protons of TMPN is shown as two singlet (s) at 7.78
and 7.19 ppm . A Singlet due to =C-H proton in the TMPN at
8.14 ppm was observed at respectively.
I
1
Synthesis of 2,4,5-Trimethoxybenzylidene
Propanedinitrile (TMPN)
A mixture of the 2,4,5-trimethoxybenzylidehyde (0.025 mol,
0.5 g) and malononitrile (0.025 mol ) in absolute ethanol
(25 mL) of few drop of pyridine was refluxed at 80 °C for
3 h with continuous stirring. The reactions were monitored
through TLC using solvent system ethyl acetate: benzene
(2:8), when the reaction was found to be complete, then reac-
tion mixture was cooled in an ice bath and the product thus
formed was filtered washed with water and recrystallized by
distilled ethanol and chloroform. Mp. 166 oC; Yeild: 75 %; IR
(KBr) vmax cm-1: 3030 (C-H), 2930 (C-H), 2220 (C-CN),
1570 (C=C); 1H-NMR (CDCl3) δ: 8.14 (s, CH), 7.78 (s, 1H,
CHAromatic), 7.19 (s, 1H, CHAromatic), 3.93 (s, 3H, OCH3),
3.85 (s, 3H, OCH3), 3.02 (s, 3H, OCH3); 13C-NMR (CDCl3)
δ: 156.85, 156.52, 152.53, 143.57, 115.36, 114.49, 112.23 C-
Aromatic) , 95.44, 77.04, 76.83, 75.66, 56.49 (OCH3), 56.37
(OCH3), 56.39 (OCH3); EI-MS m/z (rel. int.%): 246 (76) [M+
1]+. ;Anal. calc. for C13H12N2O3: C, 63.93, H, 4.95, N, 11.47,
Found: C, 63,90, H, 4.92, N, 11.43.
13C NMR (CDCl3) spectra of TMPN was recorded in
CDCl3 and spectral signals are in good agreement with the
probable structures details of 13C-NMR spectra of TMPN and
data are given in the experimental section. Finally character-
istic peaks were observed in the mass spectra of TMPN by the
molecular ion peak. The mass spectrum of TMPN shows a
molecular ion peak (M+.) m/z 246.
Spectral behavior of 2,4,5-Trimethoxybenzylidene
Propanedinitrile (TMPN)
UV-vis electronic spectra of TMPN in ten organic solvent with
different polarites were studied. UV-vis absorption spectra of
the studied TMPN was measured in various non- polar, polar
aportic and polar protic solvents such as ethanol, methnol,
dimethylsulfoxide, dimethylformamide, chloroform, dichloro-
methane, carbon tetrachloride, acetonitrile, dioxan, tetrahydro-
furan. Fig. 1 shows absorption spectra of a1 × 10-5 mol dm-3
The fluorescence quantum yield (ϕf) was measured using
the optically diluted solution to avoid reabsorption effect (ab-
sorbance at excitation wavelength ≤ 0.1), relative method with
solution of quinine sulfate in 0.5 mol dm-3 H2SO4 (ϕ = 0.55),

J Fluoresc
0.33
0.30
0.27
0.24
0.21
0.18
0.15
0.12
0.09
0.06
0.03
0.00
nitrogen is enhanced in the ICT excited state. On the other
hand, type (b) hydrogen bonding is weakened on photoexci-
tation, because the charge densities at the OCH3 decrease in
the excited state.
The energy of absorption (Ea) and emission (Ef) spectra of
the TMPN in different solvents correlated with the empirical
Dimroth polarity parameter ET (30) [21] of the solvent
(Fig. 3). A linear correlation between the energy of absorption
and emission versius polarity of solvents was obtained (Eqs. 1
and 2), implying potential application of these parameters to
probe the microenvironment of TMPN.
EtOH
MeOH
DMSO
DMF
CHCl3
CH3CN
THF
Dioxan
n-Hexane
350
375
400
425
450
475
500
Ea ¼ 75:17–0:1032 Â ETð30Þ
Ef ¼ 68:28–0:257 Â ETð30Þ
ð2Þ
ð3Þ
Wavelength (nm)
Fig. 1 Electronic absorption spectra of 1 × 10-5 mol dm-3 of TMPN in
different solvents
solution of TMPN in these solvents as a sample. As it can be
seen from Fig. 1, in the all solvent tested the main band of
TMPN, located in the spectral range 395–421 nm. Shorter
wavelength band in UV region observed for studies TMPN
in different solvent system is assigned to π to π* transition of
the benzenoid system toward the OCH3 which is characterized
by high electron donation and nitrile electron group is
accepting character present in its structure. The emission spec-
tra, however, are broad and red shifted (Fig. 2) as the solvent
polarity increases. The red-shift from 446–503 nm in n-
Hexane to in DMSO indicates that photoinduced intramolecu-
lar charge transfer (ICT) occurs in the singlet excited state [19]
and therefore the polarity of TMPN increases on excitation.
The red shift of the fluorescence peak in alcoholic solvents
are assigned to solute – solvent hydrogen bonding interaction
in the singlet excited state which causes an extra red shift in the
observed spectra [20] (Table 1).
Determination of Oscillator Strength and Transition
Dipole Moment
The solvatochromic performance of TMPN allows to establish
the difference in the dipole moment between the excited sin-
glet and the ground state (Δμ = μe – μg). This variation can be
obtained using the simplified Lippert- Mataga equation as
follows [21, 22]:
ꢀ
ꢁ
2
2 μe − μg
À
Δν st
¼
Δf þ Const:
ð4Þ
ð5Þ
hca3
D −1
2D þ 1
À
n2−1
2n2 þ 1
Δf ¼
−
where Δν st is known as Stokes–shift which decreasing
with decreasing the solvent polarity indicating to week
stabilization of the excited state in non polar solvents
[21]. Δf is the orientation polarizability of the solvent,
μe and μg are the dipole moments in the excited and
ground state, respectively which measures both electron
mobility and dipole moment of the solvent molecule. c
is the speed of light in vacuum, a is the Onsager cavity
radius and h is Planck's constant, n and ε are the re-
fractive index and dielectric constant of the solvent for
Eq. 4 respectively. The Onsager cavity radius was cho-
sen to be 4.2 Å because this value is comparable to the
radius of a typical aromatic fluorophore [23].
A simplified description of hydrogen bonding of TMPN is
shown in Scheme 1 Type (a) hydrogen bonding is strength-
ened in the excited state, since the charge density at the nitrile
225
EtOH
MeOH
200
175
150
125
100
75
DMSO
DMF
CHCl3
THF
Dioxan
CH3CN
n-Hexane
À
Δν ss is the Stokes shifts of the TMPN in different solvents
were deliberate, as shown in Table 1, using the following the
equation [24]:
50
25
À
À
À
Δν ss ¼ ν ab–ν em
ð6Þ
À
0
425 450 475 500 525 550 575 600 625
À
À
where Δν ss is the difference between λmax of the ν ab and ν em
indicate the wavenumbers of absorption and emission maxima
(cm−1) respectively.
Wavelength (nm)
Fig. 2 Emission spectra of 1 × 10-5 mol dm-3 of TMPN in different
solvents

J Fluoresc
Table 1 Spectral data of TMPN in different solvents
Solvent
Δf
ET (dye)
λab(nm)
λem(nm)
ε
f
μ 12
Debye
Φf
ETN
Δν
M -1cm-1
(cm-1)
EtOH
MeOH
DMSO
DMF
0.305
0.308
0.266
0.263
0.217
0.274
0.148
0.263
0.0014
0.94
1.19
1.14
1.16
1.18
1.19
1.20
1.22
1.28
68.72
69.39
67.91
68.56
69.22
69.39
69.73
70.24
72.38
416
412
421
417
413
412
410
407
395
494
490
503
486
484
482
479
476
446
23000
25600
28060
31300
32100
29800
30400
27000
24700
0.33
0.39
0.43
0.42
0.45
0.42
0.42
0.42
0.42
5.39
5.83
6.19
6.09
6.27
6.05
6.04
6.01
4.84
3672
3863
3872
3404
3552
3525
3514
3562
2895
0.32
0.34
0.47
0.43
0.49
0.57
0.56
0.56
0.36
CHCl3
Acetonitrile
Dioxan
THF
n-Hexane
The change in dipole moments (Δμ) between the ex-
Oscillator strength values of TMPN in various solvents were
calculated from the Eq. 8 and reported in Table 1, [26].
ET (30) and ENT is the empirical Dimroth polarity parameter
of TMPN was also premeditated according to the following
equation [27].
cited singlet and ground state were calculated from the
À
slop of plot of Stokes shifts (Δν ss ) and orientation po-
larizability of the solvent (Δf) as 5.58 debye for TMPN
respectively, positive value indicating that the excited sate
is more polor than the ground state.
The change in transition dipole moments (Δμ12) be-
tween the excited singlet and ground state of TMPN in
various solvents were calculated as in Table 1, using the
Eq. 7 [25].
ET ðsolventÞ−30:7
ETN
¼
ð9Þ
32:4
28591
ET ðsolventÞ ¼
ð10Þ
λmax
where λmax corresponds to the peak wavelength (nm) in
the red region of the intramolecular charge transfer ab-
sorption of the bitain dye. TMPN has bathochromic
when solvent polarity increase from n-hexane to
DMSO indicates that the polarity of TMPN and photo-
induced intramolecular charge transfer (ICT) occurs in
f
2
μ12
¼
ð7Þ
4:72 Â 10−7 Â Emax
where Emax is the maximum energy of absorption in cm-1and f
is the oscillator strength.
The oscillator strength ( f ), can be calculated using the
following equation:
Z
f ¼ 4:32 Â 10−9 εðν Þ dν
ð8Þ
À
À
72
where ν represents the numerical value of wavenumber
(cm−1) and ε is the extinction coefficient (Lmol−1cm−1).
68
HOR
b
Ea
Ef
CH3
64
60
56
O
a
HOR
N
HOR
b
H3C
N
HOR
O
a
O
b
H3C
30
35
40
45
50
55
HOR
ET (30) kcal mol-1
Fig. 3 Plot of energy of absorption (Ea) and emission (Ef) versus ET (30)
of different solvents
Scheme 1 (TMPN)

J Fluoresc
0.60
0.55
0.50
0.45
0.40
0.35
0.30
the singlet excited state, therefore increasing the
excitation(Fig. 4).
Fluorescence Quantum Yield
The fluorescence quantum yield (ϕf) of TMPN depends
strongly on the solvent properties (Table 1). The fluorescence
quantum yield can be correlated with ET(30) of the solvent,
where ET(30) is the solvent polarity parameter introduced by
Reichardt [28] Fig. 5. The fluorescence quantum yield of
TMPN increases with increasing solvent polarity from 0.36
in a non-polar solvent n-Hexane to 0.57 in a moderately polar
solvent acetonitrile; with a further increase in solvent polarity
the fluorescence quantum yield seems to decrease, i.e., 0.32 in
a strongly polar solvent, C2H5OH. The fluorescence quantum
yield of TMPN varied between 0.36 in n-Hexane, 0.57 in
acetonitrile and 0.32 in C2H5OH solvent. This indicates the
occurrence of negative solvatokinetic effect and positive
solvatokinetic effect [29] during the course of increasing sol-
vent polarity. One main reason for the negative solvatokinetic
effect (increase ϕf with a suitable enhancement of ICT) could
be due to the biradicaloid charge transfer involving the un-
bridged double bonds and the other cause could be related to
the proximity effect for compounds with n-л* and л-л* elec-
tron configuration. In other words, in non-polar solvents, these
effects will result in effective nonradiative decay of the excited
states. In strong polar solvents, the fluorescence quantum
yield decreases, due to large degree of intramolecular charge
transfer, which causes an increase in the rate of radiationless
relaxation of an excited state, giving rise to positive
solvatokinetic effect (reduction in ϕf by strong ICT).
Moreover, the much lower fluorescence quantum yields in
proton solvents can be attributed to the hydrogen bond inter-
action between the molecule and surrounding solvent, which
30
35
40
ET(30)Kcal mol-1
Fig. 5 Plot of ϕf versus ET (30) of different solvents
45
50
55
results in an additional nonradiative decay as observed in oth-
er dipolar molecules.
Effect of Surfactant on Emission Spectrum of TMPN
A positively charged and cetyltrimethyl ammonium bromide
(CTAB) and negatively charged sodium dodecyl sulphate
(SDS) surfactants were selected for evaluating the emission
behavior of the TMPN dye. The two specified surfactants
were chosen because ionic charges possessed by TMPN dye
can be influenced by the positively charged CTAB and nega-
tively charged SDS. Thus, the charge attraction accounts for
the TMPN emission behavior. Fluorescence emission spectra
of TMPN in the absence and presence of CTAB and SDS were
measured. Fluorescence intensities of TMPN increase when
increasing the concentration of CTAB from 2 × 10−4 up to
1.6 × 10−3 M as shown Fig. 6. Such enhancement in the
fluorescence intensity of 1 × 10−5 M TMPN at fixed concen-
trations with an increase in the CTAB concentration may like-
ly be ascribed to the association mechanism of TMPN with
CTAB. The fluorescence intensity of TMPN is quenched with
an increase of the SDS concentration (2 × 10−3 up to
1.6 × 10−2 M Moreover, more significant reductions were
noticed in fluorescence intensities of TMPN with SDS. The
quenching of TMPN upon increasing SDS concentration can
likely be ascribed to the association of TMPN with SDS.
Fig. 7 represents the influence of SDS on the relative emission
intensity of 1 × 10−5 M TMPN. It can be observed that there
was a subsequent decrease in the relative emission intensity of
TMPN with an increase in the SDS concentration, strongly
providing that there was an interaction between TMPN and
SDS. It seems that the dye molecules located in the hydrocar-
bon core of CTAB aggregates, while in SDS, the dye located
at micelle – water interface, with quenching role of water. As
shown in Figs. 6 and 7, the emission intensity of TMPN
4000
3800
3600
3400
3200
3000
2800
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35
Fig. 4 Plot of ΔF versus Stokes shift (Δυ)

J Fluoresc
60
50
40
30
20
10
0
60
55
50
45
40
35
30
25
20
15
CMC
450 475 500 525 550 575 600 625
Wavelength (nm)
0.0000
0.0004
0.0008
[CTAB] mol dm-3
0.0012
0.0016
Fig. 6 Emission spectrum of 1 × 10-5 mol dm-3 of TMPN at different
Fig. 8 Plot of If versus the concentration of CTAB
concentrations of CTAB, the concentrations of CTAB at increasing
emission intensity are 0.0, 2 × 10-4, 4 × 10-4 6 × 10-4, 8 × 10-4,
,
10 × 10-4, 12 × 10-4, 16 × 10-4 and 18 × 10-4 mol dm-3
aqueous bulk solution to the palisade layer of micelle. The
decrease in polarity of the microenvironment around dye mol-
ecule results in the reduction of non-radiative rate from ICT
state to low-laying singlet or triplet state due to the enlarge-
ment the energy gap between them, which leads to an increase
in emission intensity.
increases with increasing the concentration of surfactant
CTAB, and emission intensity of TMPN decreases with in-
creasing the concentration of surfactant SDS an abrupt change
in fluorescence intensity is observed at surfactant concentra-
tion of 8.21 × 10-4 and 6.10 × 10-3 mol dm-3 which are very
close to the critical micelle concentration of CTAB and SDS
[30], thus TMPN can be employed as a probe to determine the
CMC of a surfactants Figs. 8 and 9. It was well known that
aromatic molecules were generally solubilized in the palisade
layer of micelle [31, 32]. Thus the enhancement of emission
intensity is attributed to the passage of dye molecule from the
Conclusion
TMPN dye displays red shift in fluorescence spectrum as sol-
vent polarity increases, further red shift was observed in polar
protic solvents due to intermolecular hydrogen bonding be-
tween solute and solvent, the hydrogen bond with solvent
enhances the radiationless processes of excited molecule.
TMPN undergoes solubilization in anionic (SDS) and cationic
(CTAB) micelle with abrupt change in fluorescence intensity
at CMC of micelle.
30
25
20
15
10
5
26
24
22
20
18
16
14
CMC
0
12
10
450
475
500
525
550
575
600
625
650
Wavelength (nm)
Fig. 7 Emission spectrum of 1 × 10-5 mol dm-3 of TMPN at different
concentrations of SDS, the concentrations of SDS at decreeing emission
intensity are 0.0, 2 × 10-3, 4 × 10-3, 6 × 10-3, 8 × 10-3, 10 × 10-3, 12 × 10-3
and 16 × 10-3 mol dm-3
0.000
0.004
[SDS] mol dm-3
Fig. 9 Plot of If versus the concentration of SDS
0.008
0.012
0.016

J Fluoresc
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