Spectroscopic study of 3-aryl-7-methoxy-coumarin, iminocoumarin and bis-iminocoumarin derivatives in solution

Article abstract of DOI:10.1016/j.dyepig.2013.06.003

3-Phenyl-5,7-dimethoxycoumarin and 3-(2-thienyl)-7-methoxycoumarin were strongly fluorescent in organic solution, with quantum yields reaching 0.84. In contrast, the iminocoumarin analogs of these dyes were poorly emissive. This shows that the behaviour o

Full text of DOI:10.1016/j.dyepig.2013.06.003

Dyes and Pigments 99 (2013) 594e598
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Spectroscopic study of 3-aryl-7-methoxy-coumarin, iminocoumarin
and bis-iminocoumarin derivatives in solution
Kacem Khemakhem a,b, , Houcine Ammar , Souhir Abid , Rachid El Gharbi ,
Suzanne Fery-Forgues
a
c
a
a
a
b,c,d,e,
*
Laboratory of Applied Chemistry HCGP, Faculty of Science, University of Sfax, 3038 Sfax, Tunisia
Université de Toulouse, ITAV-USR 3035, F31106 Toulouse, France
CNRS, ITAV-USR 3035, F31106 Toulouse, France
Université de Toulouse, IMRCP-UMR 5623, F31062 Toulouse, France
CNRS, IMRCP-UMR 5623, F31062 Toulouse, France
b
c
d
e
a r t i c l e i n f o
a b s t r a c t
Article history:
3-Phenyl-5,7-dimethoxycoumarin and 3-(2-thienyl)-7-methoxycoumarin were strongly fluorescent in
organic solution, with quantum yields reaching 0.84. In contrast, the iminocoumarin analogs of these
dyes were poorly emissive. This shows that the behaviour of coumarins cannot be systematically
transposed to their iminocoumarin analogs. The spectroscopic properties of iminocoumarin dimers were
also examined. These dimers present little spectroscopic interest in comparison with the parent com-
pounds. Above all, this study indicates the untapped potential of fluorescent 3-aryl-7-
methoxycoumarins, which have scarcely been studied. In particular, 3-(2-thienyl)-7-methoxycoumarin
exhibited good emission efficiency in a wide range of solvents, and could be very useful as a fluores-
cent label including in protic media.
Received 21 May 2013
Accepted 2 June 2013
Available online 1 July 2013
Keywords:
Coumarin
Dimer
Iminocoumarin
7-Methoxycoumarin
bis-Iminocoumarin
Ó 2013 Elsevier Ltd. All rights reserved.
Fluorescence
1. Introduction
probes for microenvironments [9]. It seems that these character-
istics are not retained for 7-alkoxycoumarins that possess extended
In the large family of coumarin dyes, those that bear an electron-
donor group in the 7-position are most commonly used for fluo-
rescence applications. This substitution pattern is known to favour
electron delocalization and the electron-donating strength of the
substituent borne in the 7-position drastically influences the opti-
cal properties [1]. For instance, the 7-diethylamino or 7-julolidino-
coumarin derivatives, which bear a strong electron donor group,
generally show outstanding fluorescence properties in various
aprotic media and have found wide-spread applications as fluo-
rescent probes [2,3]. In contrast, when coumarins are substituted in
the 7-position by a weak electron donor group, such as an alkoxy
group, another behaviour appears. The first members of the series,
for example 7-methoxycoumarin, are virtually non-emissive in
organic solvents, but are markedly fluorescent in aqueous solution
conjugated system, since 3-aryl-7-alkoxycoumarins are fluorescent
in organic solvents [10,11]. Despite their interest, these dyes have
not been much studied until now, although they find applications
as optical brighteners for polyester fabrics [12] and as the light-
emitting material in electroluminescent devices [13,14].
Comparison of the behaviour of 7-alkylamino- and 7-alkoxy-
coumarins with that encountered in the neighbouring family of
iminocoumarins also reveals some interesting features. Some of our
previous work, as well as that of other teams, has shown that good
fluorescence efficiency is common to both 7-diethylaminocoumarins
and their iminocoumarin analogs [15e27]. This is particularly
attractive since the imino group allows the molecule to be easily
functionalized for variousapplications such as i.e. fluorescent polarity
probes [15e18], ion sensors [26,28,29], phosphatase activity probes
[24,25] dye-sensitized solar cells [23], fluorescent nanofibers and
crystals [19]. Regarding 7-alkoxy-iminocoumarins, only the simplest
derivatives have been studied, showing no more optical interest in
organic solvents than the corresponding coumarins [15,27]. No in-
formation is available about 3-aryl-7-alkoxy-iminocoumarins, and
the question is to know whether these compounds have kept the
fluorescence properties of their coumarin analogs.
[4e7], hence their use as derivatization agents of biological sub-
stances for high-performance liquid chromatography [8] and
*
Corresponding author. Institut des Technologies Avancées en sciences du Vivant
(
ITAV), CNRS USR 3505, Centre Pierre Potier, Oncopole, BP 50624, 31106 Toulouse
cedex 1, France. Tel.: þ33 582 991 035; fax: þ33 582 991 001.
E-mail address: suzanne.fery-forgues@itav-recherche.fr (S. Fery-Forgues).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.06.003

K. Khemakhem et al. / Dyes and Pigments 99 (2013) 594e598
595
In this context, the present study aims to examine the spectro-
n
¼ 1614 (C]C), 1713 (C]O). Anal. calcd (%) for C17
H
14
O
4
: C, 72.33;
scopic behaviour of 3-aryl-7-alkoxy-coumarins and iminocoumarins,
through the investigation of six compounds (Fig. 1). To do so, two 7-
methoxy-coumarin derivatives, bearing either a phenyl group (1a) or
a thienyl group (2a) in the 3-position, were studied and compared
with their iminocoumarin analogues (1b and 2b, respectively). Sec-
ondly, our attention was turned towards two dimers (1c and 2c) in
which the iminocoumarin moieties are borne by a diphenylmethane
linker. It is interesting to see if these iminocoumarin dimers can lead
to the same applications as coumarin dimers [30,31], which proved
to be useful in the field of laser dyes, photosensitizers and organic
light-emitting diodes.
H, 5.00. Found: C, 72.21; H, 4.80; HRMS: m/z calcd for C17
283.0970 [M þ H ]; found: 283.0969.
H
15
O
4
:
þ
2.1.2. 5,7-Dimethoxy-3-phenyl-2H-1-benzopyran-2-imine (1b)
ꢀ
1
Yield: 62%. M.p. 150 C. H NMR (300 MHz, DMSO-d6):
(ppm) ¼ 3.83 (s, 3H, CH ), 3.87 (s, 3H, CH ), 6.16 (s, 1H, H6), 6.40
(s, 1H, H8), 7.28e7.60 (m, 5H, H2 H3 H4 H5 H6 ), 7.63 (s, 1H, H4),
d
3
3
0
0
0
0
0
13
8.24 (s, 1H, NH). C NMR (75 MHz, DMSO-d6):
56.11, 89.79, 93.83, 104, 125.23, 127.56, 128.03, 128.91, 134.59, 155,
158, 159.24, 162.37. IR (cm ):
(NH). Anal. calcd (%) for C17
d
(ppm) ¼ 55.75,
ꢁ1
n
¼ 1611 (C]C), 1648 (C]N), 3276
3
H15NO : C, 72.58; H, 5.37; N, 4.98.
Found: C, 71.96; H, 5.89; N, 5.05. HRMS: m/z calcd for C17
282.1130 [M þ H ]; found: 282.1119.
H
16NO
3
:
þ
2
. Experimental
2
.1. Materials
2.1.3. 4,4’-Methylenebis[N-[5,7-dimethoxy-3-phenyl-2H-1-
benzopyran-2-ylidene]-benzenamine (1c)
ꢀ
1
Toluene and chloroform (SDS), ethylacetate and dime-
Yield: 45%. M.p. 248 C.
(ppm) ¼ 3.81 (s, 6H, CH ), 3.89 (s, 6H, CH
Ar), 6.29 (s, 2H, H6), 6.44 (s, 2H, H8), 7.07 (d, J ¼ 8.5 Hz, 4H, Ar), 7.23
H
NMR (500 MHz, DMSO):
thylsulphoxide (Fisher Scientific), ethanol and methanol (VWR)
were analytical grade and used as-received. Iminocoumarin dyes
1
d
3
3
), 3.94 (s, 2H, AreCH e
2
0
0
0
b and 2b, and bis-iminocoumarin derivatives 1c and 2c were
(d, J ¼ 8.5 Hz, 4H, Ar), 7.34e7.45 (m, 6H, H2 H4 H6 ), 7.71 (d, 4H,
0
0
ꢁ1
synthesized as previously described [32]. Coumarins 1a and 2a
were prepared by hydrolysis of 1b and 2b, respectively, in a mixture
of acetic acid and ethanol (30:70 v/v). After heating during 1 h, the
reaction mixture was left to cool down, yielding a crystalline pre-
cipitate that was recovered by filtration and recrystallized in
ethanol, yielding 1a and 2a. The IR, H and C NMR spectra were in
line with the literature data [33e35]. All compounds gave a single
spot by TLC using ethylacetate as eluentand were further checked
for purity by HRMS and elemental analysis.
H3 H5 ), 8.55 (s, 2H, H4). IR (cm ):
n
¼ 1591 (C]C), 1644 (C]N).
Anal. calcd (%) for C47 : C, 77.67; H, 5.27; N, 3.85. Found: C,
38 2 6
H N O
39 2 6
77.34; H, 5.06; N, 3.74; HRMS: m/z calcd for C47H N O : 727.2808.
[M þ Hþ ]; found: 727.2828.
1
13
2.1.4. 7-Methoxy-3-(2-thienyl)-2H-1-benzopyran-2-one (2a)
ꢀ
1
Yield: 66%. M.p. 188 C. H NMR (500 MHz, DMSO-d6):
d
(ppm) ¼ 3.89 (s, 3H, CH
3
), 7.02 (dd, J ¼ 9 Hz, J ¼ 2.5 Hz, 1H, H6),
0
7.08 (d, J ¼ 2.5 Hz, 1H, H8), 7.18 (dd, J ¼ 5 Hz, J ¼ 3.5 Hz, 1H, H4 ),
0
7
.65 (dd, J ¼ 5 Hz, J ¼ 1 Hz, 1H, H5 ), 7.71 (d, J ¼ 9 Hz, 1H, H5), 7.80
0
13
2.1.1. 5,7-Dimethoxy-3-phenyl-2H-1-benzopyran-2-one (1a)
(dd, J ¼ 3.5 Hz, J ¼ 1 Hz, 1H, H3 ), 8.54 (s, 1H, H4). C NMR
ꢀ
1
Yield: 70%. M.p. 179 C.
(ppm) ¼ 3.90 (s, 3H, CH ), 3.94 (s, 3H, CH
H6), 6.49 (d, J ¼ 2.5 Hz, 1H, H8), 7.38e7.47 (m, 3H, H2 H4 H6 ), 7.74
H
NMR (500 MHz, CDCl
3
):
(125 MHz, DMSO-d6):
d
(ppm) ¼ 56.52, 100.84, 113.28, 113.60,
d
3
3
), 6.34 (d, J ¼ 2.5 Hz, 1H,
117.62, 126.22, 127.78, 128.39, 130.03, 136.23, 137.25, 154.54, 159.71,
162.84. IR (cm ):
C
calcd for C14
calcd for C14
0
0
0
ꢁ1
n
¼ 1601 (C]C), 1705 (C]O). Anal. calcd (%) for
0
0
13
(
m, 2H, H3 H5 ), 8.15 (s, 1H, H4). C NMR (125 MHz, CDCl
3
):
14
H
10
O
3
S: C, 65.10; H, 3.90. Found: C, 64.54; H, 3.61; HRMS: m/z
þ
d
(ppm) ¼ 55.87, 56.01, 94.46, 94.94, 104.84, 122.76, 128.19, 128.38,
H
H
10
O
O
3
SNa: 281.0248 [M þ Na ]; found: 281.0248. m/z
ꢁ1
þ
1
28.43, 128.44, 135.46, 156.19, 157.14, 161.16, 163.52. IR (cm ):
11
3
S: 259.0429 [M þ H ]; found: 259.0431.
1
1
a (X = O)
b (X = N)
2a (X = O)
2b (X = N)
1
c
2
c
Fig. 1. Chemical structure of the coumarin, iminocoumarin and bis-iminocoumarin derivatives.

596
K. Khemakhem et al. / Dyes and Pigments 99 (2013) 594e598
2
.1.5. 7-Methoxy-3-(2-thienyl)-2H-1-benzopyran-2-imine (2b)
Yield: 68%. M.p. 192 C. H NMR (500 MHz, DMSO-d6):
The spectroscopic characteristics of the six compounds were
ꢀ
1
studied in chloroform and are collated in Table 1. For all com-
pounds, the absorption spectra showed only one intense band
above 300 nm, without any fine vibronic structure. The dimers
absorbed at slightly longer wavelengths than the corresponding
iminocoumarins, as could be expected from the extension of the
delocalized electron system. In contrast, the fluorescence excitation
spectra of all compounds were very similar. For the coumarin de-
rivatives, they were strictly superimposable with the absorption
spectra. This indicates that all the species visible on the absorption
spectra are responsible for fluorescence. For the iminocoumarin
monomers and, even more so, for the dimers a discrepancy was
noticed. It can be seen for example in Fig. 2 that the fluorescence
excitation spectrum of 2c was narrower than the absorption
spectrum and shifted to a shorter wavelength (Fig. 2). An expla-
nation would be that the absorption spectrum arises from a con-
tinuum of conformations, some of them being non-emissive.
The emission spectra of the six compounds were single, unre-
solved bands of similar shape. In both series of molecules, the
coumarin, iminocoumarin and bis-iminocoumarin derivatives had
almost the same emission maximum (Fig. 3). This suggests that the
emissive species have a similar conformation. The different elec-
tron withdrawing character of the oxygen and nitrogen atoms is not
discernible in this solvent. Interestingly, the thienyl derivatives 2a-c
emitted at longer wavelength than the phenyl derivatives 1a-c. This
is in line with the literature data. Actually, it has been reported that
direct attachment of thiophene subunits instead of phenyl groups
onto conjugated systems of Bodipy and porphyrin derivatives re-
sults in a bathochromic shift of the absorption and emission
wavelengths [37,38]. Such a shift cannot be explained on the basis
d
(ppm) ¼ 3.82 (s, 3H, CH
3
), 6.74 (d, J ¼ 2 Hz, 1H, H8), 6.82 (dd,
0
J ¼ 9 Hz, J ¼ 2 Hz, 1H, H6), 7.13 (dd, J ¼ 5 Hz, J ¼ 3.5 Hz,1H, H4 ), 7.49
0
(
d, J ¼ 9 Hz,1H, H5), 7.57 (d, J ¼ 5 Hz,1H, H5 ), 7.82 (d, J ¼ 3.5 Hz,1H,
0
13
H3 ), 7.95 (s, 1H, H4), 8.64 (s, 1H, NH). C NMR (125 MHz, DMSO):
(ppm) ¼ 56.16, 100.31, 111.21, 113.37, 120.15, 126.14, 127.07, 129.37,
d
ꢁ1
1
30.24, 135.19, 137.42, 154.01, 161.78, 163.19. IR (cm ):
n
¼ 1608
S: C,
5.35; H, 4.31; N, 5.44. Found: C, 65.16; H, 4.10; N, 5.27; HRMS: m/z
(
C]C), 1651 (C]N), 3282 (NH). Anal. calcd (%) for C14H11NO
2
6
þ
þ
calcd for C14
H11NO
2
S þ H : 258.0589 [M þ H ]; found: 258.0593.
2
.1.6. 4,4’-Methylenebis[N-[7-methoxy-3-(2-thienyl)-2H-1-
benzopyran-2-ylidene]-benzenamine (2c)
ꢀ
1
Yield: 43%. M.p. 165 C.
H
NMR (500 MHz, DMSO):
eAr), 6.78 (d,
d
(ppm) ¼ 3.82 (s,6H,2OCH
3
), 3.98 (s, 2H, AreCH
2
J ¼ 2.5 Hz, 2H, H8), 6.87 (dd, J ¼ 8.5 Hz, J ¼ 2.5 Hz, 2H, H6), 7.15 (dd,
0
J ¼ 5 Hz, J ¼ 4 Hz, 2H, H4 ), 7.28 (d, J ¼ 8.5 Hz, 4H, Ar), 7.31 (d,
J ¼ 8.5 Hz, 4H, Ar), 7.54 (d, J ¼ 8.5 Hz, 2H, H5), 7.62 (d, J ¼ 5 Hz, 2H,
0
0
13
H5 ), 7.84 (d, J ¼ 4 Hz, 2H, H3 ), 8.14 (s, 2H, H4). C NMR (125 MHz,
DMSO):
d
(ppm) ¼ 40.14, 56.44,100.29,112.60,113.51,121.01,123.70,
1
25.91, 126.92, 129.31, 129.65, 130.35, 136.83, 137.64, 143.36, 146.69
ꢁ
1
(
C2), 153.38 (C9), 162.04 (C7). IR (cm ):
n
¼ 1599 (C]C), 1652 (C]
N). Anal. calcd (%) for C41 : C, 72.54; H, 4.45; N, 4.13.
Found: C, 72.23; H, 4.67; N, 3.49; HRMS: m/z calcd for
C
30 2 4 2
H N O S
þ
41 31
H N
2
O
4
S
2
: 679.1725 [M þ H ]; found: 679.1738.
2
.2. Apparatus
ꢀ
Spectroscopic measurements were conducted at 20 C in a
temperature-controlled cell. The step between two measurements
was 2 nm. UVeVis absorption spectra were recorded on Hewlette
Packard 8452A and Helios Omega ThermoScientific spectropho-
tometers. Corrected steadystate fluorescence spectrawere recorded
with a Photon Technology International (PTI) Quanta Master 1
spectrofluorometer and a Perkin Elmer LS55 spectrofluorometer.
of inductive effect alone. More likely is the strong pep orbital
overlap of the thienyl ring and chromophore conjugated system.
The quantum yields showed large differences from one com-
pound to the other. They were quite high for the coumarin de-
rivatives (
F
¼ 0.58 and 0.84 for 1a and 2a, respectively), but they
were drastically reduced by one order of magnitude when passing
to iminocoumarins. For bis-iminocoumarins, the quantum yield of
1c was lower than that of the corresponding iminocoumarin, while
that of 2c was markedly higher (Fig. 3). The lifetimes were around
3 ns for every compound.
The fluorescence quantum yields (
F) of solutions were determined
2
2
using the classical formula:
F
x
¼ (A
s
ꢂ F
x
ꢂ n
x
ꢂ
F
s
)/(A
x
ꢂ F
s
s
ꢂ n )
where A is the absorbance at the excitation wavelength, F the area
under the fluorescence curve and n the refraction index [36]. Sub-
scripts s and x refer to the standard and to the sample of unknown
From the values of the fluorescence quantum yield and lifetime,
quantum yield, respectively. Coumarin 6 in ethanol (
F
¼ 0.78) was
the radiative k
calculated as k
that a high value for k
r
and non-radiative knr deactivation constants were
and knr ¼ (1 ꢁ )/ . It is generally admitted
indicates that the energy levels of the
taken as the standard [36]. The fluorescence quantum yields were
measured by exciting the samples at their absorption maximum.
The absorbance of the solutions was equal or below 0.05 at the
excitation wavelength. The error on the quantum yield values was
estimated to be about 5%. Fluorescence decay was measured with
the stroboscopic technique using a Strobe Master fluorescence
lifetime spectrophotometer from PTI. The excitation source was a
flash lamp filled with a mixture of nitrogen and helium (30/70).
Excitation was performed at 337 nm, and the fluorescence signal
was collected at the maximum emission wavelength of each com-
pound. Data were collected over 200 channels with a time-base of
r
¼
F
/s
F s
r
molecule favour high fluorescence efficiency, whereas a high value
for knr indicates that non-radiative deactivations, such as rotations
or vibrations, take place in the molecule and offer deactivation
channels that compete with the fluorescence process. In the pre-
sent case, as far as the monomers are concerned, the knr value was
higher for the phenyl derivatives than for the thienyl derivatives.
The most obvious possibility of non-radiative deactivation in the
monomeric compounds is the rotation of the ring in the 3-position.
0
.1 ns per channel. Analysis of fluorescence decay was performed
Table 1
using the multiexponential method software from PTI.
Maximum absorption (
), fluorescence quantum yields (
vation constants of the six compounds in chloroform.
l
abs) and emission (
l
em) wavelengths, fluorescence lifetime
(s
F
), radiative (k
r
) and non-radiative (knr) deacti-
3
. Results and discussion
labs (nm)
l
em (nm)a
Fa
s
(ns)
k
r
(s
ꢁ1
)
k
ꢁ1
nr (s )
The iminocoumarins 1b and 2b were prepared by reacting 2-
8
7
6
8
7
8
8
8
8
7
8
8
1
1
1
2a
2b
a
b
c
348
342
350
372
366
374
433
433
435
452
452
452
0.58
0.054
0.009
0.84
0.15
0.29
3.1 ꢃ 0.1
2.9 ꢃ 0.1
2.8 ꢃ 0.2
3.5 ꢃ 0.2
3.3 ꢃ 0.2
2.9 ꢃ 0.2
1.87 ꢂ 10
1.86 ꢂ 10
3.21 ꢂ 10
2.40 ꢂ 10
4.54 ꢂ 10
1.00 ꢂ 10
1.35 ꢂ 10
3.26 ꢂ 10
3.54 ꢂ 10
4.57 ꢂ 10
2.58 ꢂ 10
2.45 ꢂ 10
hydroxybenzaldehydes with arylacetonitriles, and the bis-imino-
coumarins were obtained by coupling iminocoumarins 1b and 2b
with p-phenylenediamine. Ion-exchange resins were used as cat-
alysts in both steps of this synthesis [32]. Coumarins 1a and 1b
were obtained by hydrolysis of 1b and 2b, respectively, in acidic
conditions.
2c
a
Excitation at
l
abs
.

K. Khemakhem et al. / Dyes and Pigments 99 (2013) 594e598
597
5
4
3
2
1
0
0
0
0
0
0
100
80
60
40
20
0
For the dimers, the introduction of the phenyl groups on the
imino nitrogen atom leads to new possibilities of rotations. How-
ever, no obvious variation in the knr value was observed with
respect to the corresponding iminocoumarins. This feature,
together with the similarity of the fluorescence spectra, suggest
that the emissive conformation would be close to that of the
monomers, which would imply that the phenyl group borne by the
imino nitrogen atom hardly participates to the conjugated system
in the excited state. Here again, differences appear with the k
r
value, indicating that the dimer of the thienyl series is intrinsically
more emissive than that of the phenyl series. Circumspection must
also be kept in mind when considering the dimers, because these
compounds are quite unstable in solution and rapidly generate
some amounts of the corresponding coumarin, the presence of
which can strongly affect the spectroscopic properties.
It appeared here that the compounds that bear a thienyl group
in the 3-position are more fluorescent in chloroform than the
corresponding phenyl analogs. It seemed instructive to study the
evolution of their fluorescence properties in several solvents. The
results are gathered in Table 2.
Concerning the position of the fluorescence spectra, it can be
seen that for the three compounds the emission maximum was
shifted to long wavelengths with increasing the solvent polarity.
This positive solvatochromic effect indicates an increase of the
intramolecular charge transfer upon excitation, which is perfectly in
line with the presence of an electron donor group in the 7-position.
An analogycan be made with 3-phenyl-7-methoxycoumarin, whose
ratio of the excited state dipole moment to the ground state dipole
moment was reported to be z 3.4 D [10]. Interestingly, the Stokes
2
00
300
400
500
600
Wavelength (nm)
Fig. 2. Absorption spectrum (dotted line), and normalized excitation (
and emission (
l
em ¼ 453 nm)
l
ex ¼ 374 nm) spectra (plain line) of bis-iminocoumarin derivative 2c in
ꢁ5
chloroform. Dye concentration for absorption: 1.5 ꢂ 10 M (molar absorption coef-
ꢁ
1
ꢁ1
cm ). Dye concentration for fluorescence:
ficient
1
3
¼
49,600
ꢃ
2000 mol
.0 ꢂ 10ꢁ M.
6
1
00
8
6
4
2
0
2
a
0
0
0
2c
2
b
shift (
n
abs
-1
ꢁ
nem) was particularly large in the present case, reaching
0
5340 cm for 2a in methanol. It seemed to be even larger for 2b, but
4
00
450
500
Wavelength (nm)
550
600
the values may be overestimated because the maximum absorption
wavelength was lower than the maximum excitation wavelength.
This Stokes shift was larger than that usually found for 7-
diethylaminocoumarin derivatives [2,16].
Fig. 3. Fluorescence emission spectra of 3-thienyl-7-methoxycoumarin (2a), imino-
coumarin (2b) and bis-iminocoumarin (2c) in chloroform, excitation at 370, 362 and
74 nm, respectively. The area under the spectra is proportional to the fluorescence
3
The fluorescence quantum yields were measured in selected
solvents. The quantum yield of 2a was high whatever the solvent. In
this respect, this 7-methoxycoumarin derivative differs from the 7-
diethylaminocoumarins, whose quantum yield generally collapses
when passing in protic media, due to hydrogen bonding in-
teractions [2,3]. Regarding the iminocoumarin derivative 2b, the
fluorescence quantum yield was low in every solvent investigated.
The behaviour of both 3-thienyl-7-methoxycoumarin and imino-
coumarin is thus very different from that of simple 7-
alkoxycoumarins that are not substituted in the 3-position. For
the latter compounds, the fluorescence efficiency was considerably
enhanced when passing from apolar to polar and protic solvents,
due to a change in the nature of the lowest excited singlet state
quantum yield.
In the presence of the methoxy group in the 7-position, the reso-
nance structure is planar for the phenyl derivatives [10]. It can be
assumed that the thienyl derivatives are also planar, because the
five membered meso thienyl ring is smaller than the six membered
phenyl ring, and generates less steric hindrance. Both phenyl and
thienyl groups can freely rotate. It can be thought that the reso-
nance structure is better stabilized in the thienyl derivatives
because of extensive conjugation. In each series, the knr value was
moderately increased when passing from the coumarin to the
iminocoumarin derivatives, suggesting that rotations of the ring
occurs more easily in the latter case. Regarding now the radiative
1
1
(non-emissive (n-p*) vs emissive (pep*)) upon formation of H-
deactivation constant k
7 ꢁ1
constant value was in the 10 s range for coumarins and 10 s
r
, a spectacular variation was noted. This
bonds [4,5]. Nothing comparable was found here. On the contrary,
the fluorescence bathochromic shift observed with increasing the
8
ꢁ1
for iminocoumarins, indicating that the latter compounds are
intrinsically less emissive than the former.
solvent polarity clearly indicates that the transition is of p / p*
type for both 2a and 2b whatever the solvent.
Table 2
Maximum absorption (
pounds 2a, 2b and 2c in various organic solvents. Excitation at
l
abs), excitation (
l
ex) and emission (
l
em) wavelengths, Stokes shift (
n
abs
ꢁ
nem) and fluorescence quantum yields (F) of the thienyl-substituted com-
l
abs for emission spectra, emission near
2b
abs (nm)
lem for excitation spectra.
Solvent
2a
2c
l
abs (nm)
lex (nm)
lem (nm)
n
abs
ꢁ
n
em
F
l
lex (nm)
l
em (nm)
n
abs
ꢁ
n
em
F
labs (nm)
lex (nm)
lem (nm)
n
abs
ꢁ
n
em
F
Toluene
Ethylacetate
Dimethylsulphoxide 374
Ethanol
372
368
372
368
374
370
369
445
445
460
458
458
4410
4702
4999
5266
5340
0.74 368
0.68 362
0.84 366
0.70 360
0.71 364
372
368
374
370
366
442
444
456
454
456
4698
5102
5393
5751
5543
0.058 379
0.048 378
0.073 386
0.054 377
0.066 371
370
368
376
368
369
442
444
450
448
449
3761
3932
3684
4204
4682
0.206
0.194
0.426
0.495
0.413
369
368
Methanol

5
98
K. Khemakhem et al. / Dyes and Pigments 99 (2013) 594e598
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1
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The authors acknowledge the French Ministry of Foreign and
European Affairs and the Ministry of Higher Education, Scientific
research and Technology in Tunisia for their financial support (PHC-
Utique program, managed by CMCU, action intégrée n 11G 1204).
SFF also thanks financial support from ERA-NET Euronanomed
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