Synthesis and optical properties of coumarins and iminocoumarins: Estimation of ground- and excited-state dipole moments from a solvatochromic shift and theoretical methods

Article abstract of DOI:10.1016/j.molliq.2014.02.029

The optical properties of coumarin and iminocoumarin were investigated in different solvents by UV/vis absorption and fluorescence spectroscopy at room temperature (298 K). These compounds were characterized by UV-vis, FT-IR, mass spectroscopy and NMR. A bathochromic shift is observed in absorption and fluorescence spectra of our molecules with increasing solvent polarity. Solvatochromic correlations were used to obtain the ground-state and excited-state dipole moments. The excited-state dipole moments for all the molecules were found to be larger than the corresponding ground-state dipole moments.

Full text of DOI:10.1016/j.molliq.2014.02.029

Journal of Molecular Liquids 195 (2014) 240–247
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Synthesis and optical properties of coumarins and iminocoumarins:
Estimation of ground- and excited-state dipole moments from a
solvatochromic shift and theoretical methods
Amal Rabahi a,b, Malika Makhloufi-Chebli a,c, Safouane M. Hamdi d, Artur M.S. Silva e, Djaffar Kheffache f,
Baya Boutemeur-Kheddis a, Maamar Hamdi a,
⁎
a
Laboratoire de Chimie Organique Appliquée (Groupe Hétérocycles) Faculté de Chimie Université des Sciences et de la Technologie BP32, El-Alia 16111 Bab-Ezzouar, Algiers, Algeria
Centre de Recherche Scientifique et Technique en Analyse Physico-Chimique (C.R.A.P.C) BP 248, Algiers 16004, Algeria
Université Mouloud Mammeri, Faculté des Sciences, Département de Chimie, Tizi Ouzou 15000, Algeria
Laboratoire de Biochimie-Purpan, Institut Fédératif de Biologie, CHU Toulouse 3330, Avenue de Grande Bretagne-F-31039, Toulouse Cedex 9, France
Department of Chemistry & QOPNA, University of Aveiro, Aveiro 3810-193, Portugal
Laboratoire de Chimie Théorique Computationnelle et Photonique, Faculté de Chimie Université des Sciences et de la Technologie BP32, El-Alia 16111 Bab-Ezzouar, Algiers, Algeria
b
c
d
e
f
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 January 2014
Received in revised form 11 February 2014
Accepted 14 February 2014
Available online 11 March 2014
The optical properties of coumarin and iminocoumarin were investigated in different solvents by UV/vis absorp-
tion and fluorescence spectroscopy at room temperature (298 K). These compounds were characterized by UV–
vis, FT-IR, mass spectroscopy and NMR. A bathochromic shift is observed in absorption and fluorescence spectra
of our molecules with increasing solvent polarity. Solvatochromic correlations were used to obtain the ground-
state and excited-state dipole moments. The excited-state dipole moments for all the molecules were found to be
larger than the corresponding ground-state dipole moments.
Keywords:
Iminocoumarin
Coumarin
© 2014 Elsevier B.V. All rights reserved.
Microwave
Photophysical properties
1. Introduction
microwave irradiation and reaction give excellent yield, have short
reaction times and were carried out in environmentally friendly
The coumarin (2-H-chromen-2-one) moiety is often found in
natural products [1]. In view of the ubiquity of this fragment in a
variety of biologically active compounds, the synthesis of various
2H-chromen-2-one analogs is important in gaging their potential as
a source of chemotherapeutics [2]. They have been shown to inhibit
the replication of HIV and thus exhibit a therapeutic potential [3]. A
large number of structurally novel coumarin derivatives have been re-
ported to show substantial cytotoxic and anti-HIV activities in vitro
and in vivo [4,5]. Coumarins have been extensively studied due to
their practical applications [6–10], which include uses as biological
and chemical sensors, fluorescent probes and laser dyes [11–14].
In the present work, we designed and synthesized a 2-imino-2H-
chromene-3-carbonitrile 3a, 7-hydroxy-2-imino-2H-chromene-3-
carbonitrile 3b, 2-oxo-2H-chromen-3- carbonitrile 4a and 7-hydroxy-
2-oxo-2H-chromen-3- carbonitrile 4b (Scheme 1).
conditions: which means less solvent, less added acid, and less energy
consumed for heating. All compounds were characterized with NMR, IR
and mass spectroscopy.
The photophysics and spectral properties of these compounds
(Scheme 1) were investigated in different solvents.
2. Experimental
2.1. Apparatus
Melting points were determined by an electrothermal Stuart sci-
entific SPM3 apparatus fitted with a microscope and are uncorrected.
IR spectra (KBr) were recorded on a FTIR — Perkin Elmer (Spectrum
One). 1H and 13C NMR spectra were recorded in deuteriochloroform
solutions and with DMSO-d6 with TMS as internal standard on a
BRUKER Avance 300, carbon: 75.47 MHz, proton: 300.13 MHz, and
the chemical shifts are expressed in δ (ppm). 13C assignments were
made using dept 135 and the mass spectra were recorded on a
MicroTOF-Q de Bruker Daltonics.
All these compounds are described in the literature [15]. Using
conventional heating, we have realized that these products under
⁎
Corresponding author at: Organic Chemistry at the University of Sciences and
All spectrophotometric measurements were conducted at ambient
temperature. UV/vis absorption spectra were recorded on a Varian
Technology USTHB, Algiers, Algeria. Tel./fax: +213 21 50 85 81.
E-mail address: prhamdi@gmail.com (M. Hamdi).
http://dx.doi.org/10.1016/j.molliq.2014.02.029
0167-7322/© 2014 Elsevier B.V. All rights reserved.

A. Rabahi et al. / Journal of Molecular Liquids 195 (2014) 240–247
241
H
O
CN
CN
O
i
ii
CNCH2CN
2
R
O
NH
R
O
R
3(a-b)
4(a-b)
1
R=H: 3a,4a,
R= OH: 3b,4b
(i)NaHCO3,RT;(ii)MW irradiation,300W,9min,HCl 2N
Scheme 1. Synthesis of coumarins and iminocoumarins
CARY 50 conc. The estimated experimental error was 2 nm on the band
maximum and 5% on the molar extinction coefficient. Fluorescence
work was performed on a Varian CARY Eclipse spectrofluorometer. All
excitation and emission spectra were corrected. The fluorescence quan-
tum yields were determined using fluorescein disodium salt (Φ = 0.9)
as standard.
2.2. Materials
For synthesis, salicylaldehyde, malonitrile, and 2,4-dihydroxy
benzaldehyde were purchased from Aldrich. The solvents used in
the present study, namely acetonitrile, dimethylsulfoxide, chloroform,
ethyl acetate, cyclohexane and tetrahydrofurane were of HPLC and
spectroscopic grade (purchased from Fluka and Labosi). The required
solutions were prepared at a fixed solute concentration (3.2 ∙ 10−5 M).
The fluorescence quantum yields (Φ) were determined using the
classical formula:
ꢀ
ꢁ ꢀ
ꢁ
3. Results and discussion
2
2
ΦX
¼
Φs:As:Fx:nx = Ax: Fs:ns
:
3.1. UV/vis absorption spectra
Where “A” is absorbance at the excitation wavelength, “F” is the
The UV/vis absorption spectra of all the compounds 3a–b and 4a–b
(concentration 3.2 × 10−5 M) were recorded immediately after dissolu-
tion of the products and filtration through paper. The magnitudes of
Stokes shift vary between 3369 and 5248 cm−1. The values of the Stokes
shift are also indicative of the charge transfer transition.
area under the fluorescence curve and “n” is the refractive index of
the solvents used. Subscripts “s” and “x” refer to the standard and to
the sample of unknown quantum yield, respectively. The excitation
source was a long life Xenon flash lamp.
a
b
1,6
1,0
DMSO
1,4
Cyclohexane
0,9
Acetonitril
1,2
0,8
DMSO
0,7
1,0
0,6
Chloroform
Acetonitril
0,8
0,5
Cyclohexane
Chloroforme
0,6
0,4
0,3
0,2
0,1
0,4
0,2
0,0
0,0
300
350
400
260 280 300 320 340 360 380 400 420
Wavelength nm
Wavelength nm
d
c
2,2
0,45
0,40
0,35
0,30
0,25
0,20
0,15
0,10
0,05
0,00
DMSO
DMSO
2,0
1,8
Acetonitril
1,6
1,4
1,2
Acetonitril
1,0
Cyclohexane
Chloroforme
0,8
0,6
0,4
0,2
0,0
260 280 300 320 340 360 380 400 420 440
260 280 300 320 340 360 380 400 420 440 460
Wavelength nm
Wavelength nm
Fig. 1. (a–d): Absorption spectrum of all compounds (a) 3a, (b) 4a, (c) 3b and (d) 4b in different solvents.

242
A. Rabahi et al. / Journal of Molecular Liquids 195 (2014) 240–247
300
280
260
240
220
200
180
160
140
120
100
80
250
DMSO
DMSO
THF
200
Cyclohexane
150
Ethylacetate
Acetate d'ethyl
100
THF
Cyclohexane
50
0
60
40
20
0
400
500
600
350
400
450
500
550
600
Wavelength nm
Wavelength nm
Compound 3a
Compound 4a
400
350
300
250
200
150
100
50
DMSO
350
300
250
200
150
100
50
DMSO
THF
THF
Ethyl Acetate
Acetonitril
Chloroforme
Cyclohexane
Cyclohexane
0
0
350
400
450
500
550
600
350
400
450
500
550
600
Wavelength nm
Wavelangth nm
Compound 3b
Compound 4b
Fig. 2. Emission spectrum of all compounds 3a, 4a, 3b and 4b in different solvents.
The absorption spectrum of all the compounds in tetrahydrofurane,
chloroform, acetonitrile, cyclohexane, ethyl acetate and dimethyl-
The absorption spectrum exhibited a band at around 290–300 nm,
attributed to the n–π transition and associated with the imine and car-
⁎
sulfoxide displays two distinct bands (Fig. 1(a)–(d)).
bonyl groups. A large band at long wavelengths lying around 347, 355,
Table 1
Spectroscopic and photophysical characteristics of the derivatives of compounds 3(a–b) and 4(a–b) in different solvents.
Compounds
3a
(Solvent)
λ
abs (nm)
ε (M−1 cm−1
)
λ
ex (nm)
λ
em (nm)
ðva‐vf Þ (cm−1
)
Фf
DMSO
Acetonitrile
THF
Ethyl acetate
Chloroform
DMSO
Acetonitrile
THF
Ethyl acetate
Chloroform
Cyclohexane
DMSO
Acetonitrile
THF
Ethyl acetate
Chloroform
DMSO
Acetonitrile
THF
347
348
350
351
352
334
336
338
339
340
342
355
356
360
362
364
351
352
355
356
359
361
22,405
22,231
10,496
5581
7974
17,307
9246
8387
7455
7440
6433
13,549
11,275
10,829
13,087
18,036
58,565
50,206
19,771
32,765
14,407
30,435
406
407
409
410
411
405
407
409
410
411
412
409
408
412
414
415
401
402
405
406
408
411
4187.90
4165.61
4121.55
4099.78
4078.19
5248.76
5191.88
5135.92
5108.28
5080.86
4967.92
3719.13
3580.08
3505.93
3469.72
3376.14
3552.37
3533.46
3477.57
3459.34
3345.35
3369.93
1.24 ∙ 10−3
1.19 ∙ 10−3
6.81 ∙ 10−2
8.42 ∙ 10−2
9.46 ∙ 10−2
2.20 ∙ 10−3
1.82 ∙ 10−3
2.06 ∙ 10−2
9.61 ∙ 10−2
0.58 ∙ 10−2
2.07 ∙ 10−2
2.20 ∙ 10−3
8.60 ∙ 10−3
2.62 ∙ 10−2
7.20 ∙ 10−2
3.42 ∙ 10−2
1.23 ∙ 10−3
0.59 ∙ 10−3
1.02 ∙ 10−3
5.97 ∙ 10−2
5.09 ∙ 10−2
1.05 ∙ 10−2
273
265
4a
3b
4b
314
311
Ethyl acetate
Chloroform
Cyclohexane
λabs: Absorbance wavelength; ε: corresponding molar extinction coefficient; λex: maximum excitation wavelength; λem: maximum emission wavelength and shoulders; va‐vf : Stokes
shift; and Φf: fluorescence quantum yield with excitation at the maximum absorption wavelength.

A. Rabahi et al. / Journal of Molecular Liquids 195 (2014) 240–247
243
2-imino-2H-chromene-3-carbonitrile3a
3.2. Fluorescence spectra
N
N
The fluorescence of compounds 3a–b and 4a–b in all solvents used
H
above (3.2 × 10−5 M) was observed at room temperature.
O
N
H
O
N
molecule-3a1
molecule-3a2
7-hydroxy-2-imino-2H-chromene-3-carbonitrile 3b
3.2.1. Steady-state emission spectra
N
N
For all compounds, the shape and position of the emission spectra
were independent of the excitation wavelength, which confirms that
only one species emits in each solution.
The emission spectra underwent a strong blue shift when the
polarity of the solvent increased.
The peak moved from 406 in DMSO to 407 nm in acetonitrile to
409 nm in THF and to 411 nm in chloroform for compound 3a (Fig. 2).
The emission spectra for compound 3b (Fig. 2) showed wavelength
bands at 409 nm in DMSO, at 410 nm in chloroform and at 415 nm in
CHCl3.
However large variations were found in the shape of the spectra
according to the medium. For all the compounds, the intensity of
the peak increased with the polarity of the solvent.
H
O
O
O
N
H
O
N
H
molecule-3b2
H
molecule-3b1
N
N
H
H
H
O
O
O
H
N
O
N
molecule-3b3
molecule-b4
2-oxo-2H-chromene-3-carbonitrile4a
The fluorescence quantum yields were measured (Table 1). It
ranges from 10−2 to 10−3 being divided by ten when passing from
cyclohexane to DMSO. Our results are comparable to those described
in the literature [14,16] where the authors have shown that the
N
O
O
quantum yield of the chromene dye varies between 10−2 and 10−3
.
2-Imino-2H-chromene-3-carbonitrile 3a, 7-hydroxy-2-imino-2H-
chromene-3-carbonitrile 3b, 2-oxo-2H-chromen-3-carbonitrile 4a
and 7-hydroxy-2-oxo-2H-chromen-3-carbonitrile 4b are character-
ized by good fluorescence efficiency in aprotic apolar media.
molecule-4a1
7-hydroxy-2-oxo-2H-chromene-3-carbonitrile4b
N
N
O
O
O
H
O
O
3.3. Theoretical calculations of ground-state dipole moments
O
molecule-4b1
H
molecule-4b2
3.3.1. Computational methods
Fig. 3. Schematic representation of the different 2H-chromene derivatives including their
The B3LYP [17,18] density functional method as implemented in the
Gaussian 03 package [19] was used throughout the whole study. All
structures were fully optimized by using the 6–31 + G (d,p) basis set.
The harmonic vibrations were also calculated in order to characterize
the structures of rotameric forms of different chromene derivatives
and thermodynamic properties. Solute–solvent interaction was evaluat-
ed by the use of the conductor-like polarizable continuum model
(CPCM) [20], in which the cavity was built up using the united atom
topological model (UATM). The CPCM calculations were performed as
single points on the gas phase geometries. The different solvents were
modeled using their dielectric constant.
rotamers. Adopted nomenclatures are presented.
334 and 351 nm for 3a, 3b, 4a and 4b, in different solvents associated
⁎
with the π–π , the latter being associated with the cyano group.
This band can be attributed to the charge transfer that occurs be-
tween the electron donor group (the hydroxyl group) and the electro-
withdrawing pole, constituted by the carbonyl, imino and the cyano
groups that reinforce the electron-attracting strength on this side of
these molecules.
Table 2
Relative energies ΔE(ZPVE) (kcal/mol) in the gas phase, relative energies ΔE0 in solvent phases, and dipole moment (Debye) calculated at the B3LYP / 6–31 + G(d,p) level of theory.
Compounds
Gas phase (ε = 1.000)
ΔE(ZPVE)
2-Imino-2H-chromene-3-carbonitrile 3a
DMSO (ε = 46.826)
Acetonitrile (ε = 35.688)
Chloroform (ε = 4.7113)
μ
ΔE0
μ
ΔE0
μ
ΔE0
μ
Molecule-3a1
Molecule-3a2
01.23
00.00
06.35
07.48
01.91
00.00
09.05
10.61
01.83
00.00
09.01
10.59
0.003
00.00
08.37
09.83
7-Hydroxy-2-imino-2H-chromene-3-carbonitrile 3b
Molecule-3b1
Molecule-3b2
Molecule-3b3
Molecule-3b4
00.06
00.00
00.80
01.15
08.72
07.63
06.05
07.94
00.00
00.07
01.81
01.75
12.71
11.00
08.73
11.59
00.00
00.07
01.73
01.72
12.69
10.98
08.70
11.56
00.00
0.0002
01.45
01.54
11.70
10.15
08.05
10.67
2-Oxo-2H-chromene-3-carbonitrile 4a
Molecule-4a1
–
08.34
–
11.85
–
11.81
–
10.98
7-Hydroxy-2-oxo-2H-chromene-3-carbonitrile 4b
Molecule-4b1
Molecule-4b2
00.36
00.00
09.89
08.20
00.00
00.03
14.36
11.81
00.00
00.01
14.32
11.78
00.14
00.00
13.24
10.90

244
A. Rabahi et al. / Journal of Molecular Liquids 195 (2014) 240–247
Table 4
3.3.2. Results and discussion
Summary of solvent properties and calculated values of solvent polarity parameters f(ε,n)
and Φ(ε,n).
3.3.2.1. Relative stability. Fig. 3 depicts the schematic representation of
the different 2H-chromene derivatives including their rotamers when
they exist. Relative energies for different rotamers computed at the
B3LYP / 6–31 + G(d,p) level are summarized in Table 2. An inspection
of the relative energies of rotamers reported in Table 2 shows a relative-
ly small difference between rotamers and reveals that the variation of
the spatial orientation of the \OH and/or _NH groups in different
2H-chromenes has an insignificant effect on the relative stabilities. The
very low relative rotamer energies suggest that many rotamers are
present in the samples at room temperature.
Probably one of the most crucial factors determining the rotamer
distribution is the environment. The study of interaction between
solute and solvent is normally a complicated process. A CPCM allows
one to take into account long-range interactions between solute and
solvent. The continuum model is computationally efficient for the
prediction of stability trends in the liquid phase. The energetic pa-
rameters of the rotamer forms in different solvents calculated at
the B3LYP / 6–31 + G(d,p) level of theory are also listed in Table 2.
Except for 2-imino-2H-chromene-3-carbonitrile, the introduction
of solvent in 7-hydroxy-2-imino-2H-chromene-3-carbonitrile and
7-hydroxy-2-oxo-2H-chromene-3-carbonitrile modifies the hierar-
chy between rotamers. It is worth mentioning that the relative ener-
gies of rotamers remain very small. The molecular radius a0 (A°) of
the spherical cavity obtained for optimized geometries for different
chromenes are calculated at the B3LYP/6-31+G(d,) (Table 3).
No.
Solvent
μ
ε
n
f(ε,n)
0.841
0.863
0.5492
0.489
Φ(ε,n)
1
2
4
5
6
7
Dimethylsulfoxide
Acetonitrile
Tetrahydrofurane
Ethyl acetate
Chloroform
3.9
46.7
37.5
1.478
1.344
1.407
1.372
1.446
1.426
1.488
1.331
1.102
0.996
0.972
0.575
3.45
1.75
1.88
1.15
0.00
7.58
6.02
4.81
2.02
0.371
−0.003
Cyclohexane
moments of different molecules. The equations [Eqs. (1) and (2)]
that afford the best results in the change of dipole moments of an ex-
cited molecule were suggested by A. Kawski and P. Bojarski [21–23]:
The difference va−vf :
νa‐νf ¼ S1 fðε; nÞ þ const:
ð1Þ
ð2Þ
ð3Þ
And the sum νa þ νf :
νa þ νf ¼ ‐S2Φðε; nÞ þ const:
Where
Φðε; nÞ ¼ f ðε; nÞ þ 2gðnÞ:
And
3
2
n4‐1
n2 þ 2
gðnÞ ¼
:
ð4Þ
ꢂ
ꢃ
2
3.3.2.2. Dipole moment. Dipole moment reflects the degree of charge
distribution in a molecule. Habitually it is used to clarify a variety
of physical and chemical properties. The dipole moments of different
chromene derivatives have been computed at the B3LYP / 6–31 + G(d,p)
level of theory in different environments and are shown in Table 2. As can
be seen, there is an important variation in dipole moment, while going
from gaseous to solvent phase environments. Dipole moments of the
studied compound have higher values in the solvent phase. This is due
to the interactions between the molecules of solvents and the dipolar
molecule of compounds. The value of the dipole moments increases
with dielectric constant in the following order:
In these Eqs., va and vf are the absorption and fluorescence maxima
(cm−1), respectively, and n and ε are the refractive index and the di-
electric constant of solvents, respectively.
Table 5
Spectroscopic and photophysical characteristics of 3a–b and 4a–b in different solvents.
Solvent
νa (cm−1
)
vf (cm−1
)
va‐vf (cm−1
)
va þ vf (cm−1
)
DMSO
3a
4a
3b
4b
28,818.44
29,940.12
28,169.01
28,490.03
24,630.54
24,691.36
24,449.88
24,813.89
4187.90
5248.76
3719.13
3552.37
53,448.98
54,631.48
52,618.89
53,427.68
DMSONacetonitrileNchloroformNgas phase:
In the case of chromenes having different rotamers it can be seen
that, under the liquid phase, there is a direct correlation between the
dipole moment and the order of stability. Polar rotamers have a larg-
er dipole moment in a polar solvent, and consequently the stability of
the molecular system is increased.
Acetonitrile
3a
4a
3b
4b
28,735.63
29,673.59
28,089.88
28,490.03
24,449.88
24,509.80
24,570.02
24,875.62
4165.61
5191.88
3580.08
3533.46
53,305.66
54,331.93
52,599.69
53,284.71
THF
3a
4a
3b
4b
3.4. Determination of dipole moments in the excited-state in several
solvents using the solvatochromic method
28,653.29
29,585.80
27,624.31
28,248.59
24,390.24
24,449.88
24,570.02
24,570.02
4121.55
5135.92
3505.93
3477.57
53,021.31
54,035.68
52,049.62
52,860.37
The dipole moment of a molecule in the excited-state is determined
by the effect of electric field (internal or external) on its spectral
band position. The solvent dependence of the absorption and fluo-
rescence band maxima is used to estimate the excitation-state dipole
Ethyl acetate
3a
4a
3b
4b
28,571.43
29,498.52
27,548.21
28,169.01
24,271.84
24,330.90
24,509.80
24,570.02
4099.78
5108.28
3469.72
3459.34
52,880.27
53,888.78
51,778.89
52,720.43
Chloroform
Table 3
3a
4a
3b
4b
28,409.09
29,411.76
27,548.21
28,169.01
24,213.07
24,330.90
24,271.84
24,449.88
4078.19
5080.86
3376.14
3345.35
52,739.99
53,742.66
51,568.91
52,364.96
Molecular radius a0 (Å) of the spherical cavity obtained for optimized geometries for
different chromenes calculated at the B3LYP / 6–31 + G(d,p) level of theory. We
considered only the most stable rotamers obtained in the liquid phase.
Compounds
a0 (Å) molecular radius
Cyclohexane
2-Imino-2H-chromene-3-carbonitrile 3a
4.53
4.45
4.60
4.46
3a
4a
3b
4b
28,248.59
29,239.76
27,397.26
28,089.89
–
–
–
7-Hydroxy-2-imino-2H-chromene-3-carbonitrile 3b
2-Oxo-2H-chromene-3-carbonitrile 4a
24,271.84
–
4967.92
–
53,511.61
–
7-Hydroxy-2-oxo-2H-chromene-3-carbonitrile 4b
24,154.59
3369.93
52,031.73

A. Rabahi et al. / Journal of Molecular Liquids 195 (2014) 240–247
245
The expressions for the solvent parameters f(ε,n) and Φ(ε,n) are
given by A. Kawski and colleagues [21–23]:
molecular volume of dye molecules according to Suppan's equa-
tion [24,25]: a0 = (3M/4πδN)1/3, where δ is the density of the solute
molecule, M is the molecular weight of solute and N is Avogadro's
number.
The ground- and excited-state dipole moments are estimated by
means of Eqs. (9)–(11). Based on Eqs. (7) and (8) and assuming that
the symmetry of the investigated solute molecules remains unchanged
upon electronic transition and the ground- and excited-state dipole mo-
ments are parallel, one obtains:
"
#
2n2 þ 1 ε‐1
n2‐1
n2 þ 2
fðε; nÞ ¼
‐
ð5Þ
n2 þ 2
ε þ 2
ꢀ
ꢁ
0
1
!
3 n4−1
n2 þ 2
2n2 þ 1 ε−1
n2−1
@
A
ꢂ
ꢃ
Φðε; nÞ ¼
−
þ
:
ð6Þ
ꢂ
ꢃ
2
n2 þ 2
n2 þ 2
ε þ 2
!
1=2
jS2‐S1j hca30
μg
¼
¼
ð9Þ
2
2S1
The calculated values of solvent polarity parameters f(ε,n) and
Φ(ε,n) are given in Table 4.
Table 4 summarizes solvent properties and calculated values of sol-
vent polarity parameters f(ε,n) and Φ(ε,n).
From Eqs. (1) and (2) the expression for slopes S1 and S2 are given
by:
!
1=2
jS2 þ S1j hca30
μe
ð10Þ
ð11Þ
2
2S1
ꢀ
ꢁ
2
2 μe‐μg
μe jS2 þ S1j
¼
Slope S2NSlope S1:
S1
¼
¼
ð7Þ
ð8Þ
μg
jS2‐S1j
hca30
The slopes S1 and S2 were graphically determined by plotting Stokes
ꢀ
ꢁ
shifts ðνa‐νf Þ and ðνa þ νf Þ versus the bulk solvent polarity functions
2 μ2e‐μ2g
f(ε,n) and Φ(ε,n) respectively, for different solvents.
S2
:
hca30
Solvent properties and polarity values f(ε,n) and Ф(ε,n) for various
solvents used in this study are presented in Table 1. The absorption
and emission maxima, Stokes shift ðνa‐νf Þ and arithmetic mean of
Stokes shift ðνa þ νf Þ for all the molecules in different solvents are
given in Tables 5. Typical absorption and emission spectra of the com-
pounds are given in Table 1.
Where μg and μe are ground- and excited-state dipole moments of
the solute molecule. The symbols h and c are Planck's constant and
the light velocity in vacuum, respectively. a0 is the Onsager cavity ra-
dius of the solute molecule and the values were calculated from the
a 4200
b
5260
5240
5220
5200
5180
5160
5140
5120
5100
5080
5060
5040
5020
5000
4185
4170
4155
4140
4125
4110
4095
4080
4065
4050
4035
4020
4005
0,3
0,4
0,5
0,6
0,7
0,8
0,9
0,3
0,4
0,5
0,6
0,7
0,8
0,9
f (ε,n)
f (ε,n)
c
d
3920
3850
3780
3710
3640
3570
3500
3430
3360
3290
3220
3150
3560
3540
3520
3500
3480
3460
3440
3420
3400
3380
3360
3340
3320
3300
3280
0,3
0,4
0,5
0,6
0,7
0,8
0,9
0,0
0,2
0,4
0,6
0,8
1,0
f (ε,n)
f (ε,n)
Fig. 4. (a–d): The variation of Stokes shift with f(ε,n) by using Eq. (1) in different solvents: (a) 3a, (b) 4a, (c) 3b and (d) 4b.

246
A. Rabahi et al. / Journal of Molecular Liquids 195 (2014) 240–247
a
b
55000
53690
53620
53550
53480
53410
53340
53270
53200
53130
53060
52990
52920
52850
52780
52710
52640
52570
52500
54800
54600
54400
54200
54000
53800
53600
53400
53200
53000
0,9
1,0
1,1
1,2
1,3
1,4
1,5
1,0
1,2
1,4
1,6
φ (ε,n)
φ (ε,n)
c
d
54000
53800
53600
53400
53200
53000
52800
52600
52400
52200
52000
51800
51600
53600
53400
53200
53000
52800
52600
52400
52200
52000
51800
51600
51400
51200
51000
0,9
1,0
1,1
1,2
1,3
1,4
1,5
1,0
1,2
1,4
1,6
φ (ε,n)
φ (ε,n)
Fig. 5. (a–f): The variation of arithmetic sums of Stokes shift with Φ(ε,n) by using Eq. (2) in different solvents: (a) 3a, (b) 4a, (c) 3b and (d) 4b.
Figs. 4(a)–(f) and 5(a)–(d) show the graph of ðνa þ νf Þ versus f(ε,n)
The discrepancies between the experimental and theoretical ex-
cited dipole moments result from Kawski et al. [21–23] and all equa-
tions used for the treatment of the solvatochromic effect (Table 7)
and at the B3LYP / 6–31 + G(d,p) level of theory in different envi-
ronments (Table 2) may explain that the dipole moment of organic
molecules depends on the transfer process within the molecule.
These fluorescent probes show that they are more polar in the excit-
ed state than the ground state.
and ðνa þ νf Þ versus Φ(ε,n) and the slopes S1 and S2 are obtained from
these graphs, respectively.
The correlation coefficients, slopes and intercepts of the fitted
lines are given in Table 6. Good correlation coefficient was obtained
for all cases. The excited-state dipole moments μe of four compounds
have been estimated by computing the values of μg obtained by
Eqs. (9)–(10) and the values are presented in Table 7. The higher
values of μe and μg and the variation in the dipole moment for all
compounds may be understood in terms of their possible resonance
structures as shown in Fig. 6. The carbonitrile substituent in 3a and
4a compounds does not produce a considerable change in the π elec-
tron mobility. Upon excitation the carbonyl group becomes a strong
electron donor. Hydroxyl atom contributes better to the resonance of
the mobility of π electrons. This explains the higher value of dipole
moment in both excited states during the charge transfer process;
the OH group may become more polar relative to the S0 state.
4. Conclusion
In the present study, we described the solvent effect on the absorp-
tion and emission spectra of 2-imino-2H-chromene-3-carbonitrile
3a, 7-hydroxy-2-imino-2H-chromene-3-carbonitrile 3b, 2-oxo-2H-
chromen-3-carbonitrile 4a and 7-hydroxy-2-oxo-2H-chromen-3-
Table 7
Dipole moments in the ground and excited states and correlation coefficient of 3a–b and
4a–b using a0 calculated with ChemOffice (i) and a0 calculated with B3LYP (ii).
Table 6
Statistical treatment of the correlations of solvent spectral shifts of 3a–b and 4a–b.
d
Molecules Radius
(A°)
μ
g (D)a
μ
e (D)b
Δμ (D)c
μe/μg
Compounds
Slope
Intercept cm−1
Correlation
coefficient
Number
of data
(i)
(ii)
(i)
(ii)
(i)
(ii)
(i)
(ii)
(i)
(ii)
Eq. (1) correlation
3a
4a
3b
4b
3.79 4.53 12.04 10.82 16.51 14.84 4.47 4.02
1.37 1.37
3a
4a
3b
4b
202.56701
289.74356
530.52448
230.66489
4004.47968
4972.73407
3199.87424
3336.76768
0.97
0.94
0.90
0.88
5
6
5
6
3.70 4.60
3.75 4.45
8.41 11.66 13.52 18.75 5.11 7.089 1.61 1.61
8.83 11.42 15.06 19.46 6.22 8.04
1.70 1.70
1.33 1.33
3.67 4.46 11.95 16.00 15.89 21.29 3.94 5.28
Debye = 3.33564 × 10−30 C∙m = 10−18 esu C∙m. i, a0 calculated with ChemOffice. ii, a0
Eq. (2) correlation
calculated with B3LYP.
3a
4a
3b
4b
1293.3488
1242.71372
2036.43365
1628.97352
51,555.93578
52,684.87309
49,724.68844
51,026.69919
0.92
0.97
0.96
0.97
5
6
5
6
a
Experimental ground-state dipole moment calculated from Eq. (9).
Experimental excited-state dipole moments calculated from Eq. (10).
Change of dipole moment calculated from Eq. (7).
b
c
d
Ratio of μe and μg is calculated from Eq. (11).

A. Rabahi et al. / Journal of Molecular Liquids 195 (2014) 240–247
247
N
C
X
O
O
1
N
H
C
X
1
2
O
H
O
N
C
2
3b, X= NH
4b, X=O
O
O
X
H
Fig. 6. Schematic representation of the resonance structures for the 7-hydroxy-2H-chromene derivatives.
carbonitrile 4b and estimated their dipole moments. A bathochromic
shift is observed upon increasing the polarity of the solvent for all the
molecules indicating a π–π* transition. We found that all molecules
possess higher dipole moment values in the excited-state than in the
ground-state and the dipole moment of the 2-imino-2H-chromene-3-
carbonitrile 3a was higher than the other derivatives. This could be
attributed to the intense electron resonance effect of substituents
carbonile and imine in the excited singlet state. The interactions be-
tween systems π and chromophores CN lead to a strong delocalization
of the excited state that makes molecules much more polar, thus
resulting in greater values of dipole moments. The same is true for the
mesomeric donor effect of the hydroxyl function which shows the
same effect.
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Acknowledgment
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[26] Malika Makhloufi-Chebli, M. Safouane, Hamdi Amal Rabahi, M.S. Artur, Silva
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We thank Cristina Barros and Hilário Tavares (Department of Chem-
istry, University of Aveiro, 3810-193, Aveiro, Portugal) for performing
the mass and NMR spectra.