L. Cisse et al. / Spectrochimica Acta Part A 79 (2011) 428–436
429
Table 1
5
8
4
Chemical yields and melting points (m.p.) of the synthesised 7,4-disubstituted-
coumarins under study.
1
0
6
7
3
◦
Compound
7-X
4-R
Yield (%)
m.p. ( C)
8
9
10
Cl
Br
OH
OH
Cl
CF3
CF3
CH3
CF3
CH3
75
70
75
70
80
112
2
122–124
190–192
180
9
O
O 11
1
1
1
2
1
148
Scheme 1. General molecular structure of coumarin.
The other 7,4-disubstituted coumarins, including compounds
–12, were synthesised by using the Pechmann reaction [3,4], as
environmental factors, including the solvent, pH and temperature
8
[
14,24,25,28–30]. In particular, knowledge of the solvent effects on
described elsewhere [38]. In the typical procedure, it was added
drop wise 3-susbtituted phenol (0.015 mol) and 2-acetyl ethyl
acetate or 2-trifluoroacetyl ethyl acetate (0.013 mol) to 17 mL of
the electronic absorption and fluorescence spectra presents a great
interest. Indeed, a solvent change is accompanied by a significant
modification of the polarity, dielectric constant, and/or polarizabil-
ity of the surrounding medium.
The quantitative effect of solvent on the UV–vis absorption and
fluorescence spectra can be used to evaluate the magnitude as
well as the direction of electric dipole moment of coumarins and
other dye molecules in their first electronically excited singlet-state
◦
concentrated sulfuric acid, the mixture being kept at 10 C in an ice
bath. This mixture was stirred at room temperature for at least 18 h,
and, then, poured in 70 mL of crushed ice. The obtained precipitate
was filtered under vacuum, washed subsequently with cold water,
and dissolved in a 20-mL 5% sodium hydroxide solution. This solu-
tion was filtered, and it was added a 2 M sulfuric acid solution to
acidify it. The crude product was recuperated by filtration with a
water ejector, washed with cold water, dried, and crystallized from
[
14,27–30]. It is worthwhile to note that the dipole moment of
an electronically excited state is considered as an important prop-
erty that provides information about the electronic and geometrical
structure of molecules in their short-lived excited state. Moreover,
the excited state dipole moments of fluorescent dye molecules have
practical applications since they determine the tunability range of
the emission energy as a function of the medium polarity [30]. In
these conditions, it is very useful to gather data on the excited-
state dipole moments for optimizing the performances of coumarin
laser dyes, which should also provide important information on
the polarity of the singlet excited-states [26], as well as on the
electronic and/or geometrical structure of the short-lived excited
species [19,20,30].
9
5% ethanol. In all cases, the obtained coumarins were character-
1
13
ized by IR, H NMR and C NMR spectroscopy [38]. The chemical
yields and melting points (m.p.) are presented in Table 1.
Cyclohexane (CyHx), 1,4-dioxan (Diox), diethyl ether (Et O),
2
ethyl acetate (EtOAc), ethanol (EtOH), acetonitrile (CH CN), N,N-
3
dimethylformamide (DMF), and dimethylsulfoxide (DMSO) of
spectroscopic grade were utilized as solvents.
2
.2. Apparatus
Electronic absorption spectra of coumarins were obtained
In the present work, we chose to investigate the solvent
effects on the electronic absorption and fluorescence spectra
of coumarins substituted by various electron-donating and/or
electron-withdrawing groups. All the selected coumarins have
important applications as laser dyes, and the knowledge of the sol-
vent effects on their photophysical properties and of their excited
singlet-state dipole moments is essential in order to improve the
performances of laser dyes. Therefore, we have studied the pho-
tophysical properties of four series of coumarins, namely those
carrying a methyl group in 4 position, those bearing a CF3 group
in 4 position, those carrying a COOH group in 3 position, and 7-
NEt2 substituted coumarin. More precisely, the twelve substituted
coumarins under study were: 7-NH -4-CH -coumarin (1), 7-NH -
in several solvents at room temperature with a Lambda 2
Perkin-Elmer UV–visible absorption spectrophotometer. Corrected
excitation and emission fluorescence spectra were recorded using
a Perkin-Elmer Luminescence Spectrometer LS-50 in various sol-
vents at room temperature. For all spectral measurements, the
solutions were kept in 1 cm × 1 cm quartz cells.
The statistical analysis of the solvatochromic data was per-
formed using a Microcal Origin 6.0 Professional program.
2.3. Ground-state dipole moments
Theoretical ground state dipole moments were calculated using
the AM1 method by means of the MOPAC program with a Chem 3D
Ultra 8.0 software.
2
3
2
4
-CF -coumarin (2), 7-NMe -4-CF -coumarin (3), 7-NEt -4-CH -
3 2 3 2 3
coumarin (4), 7-NEt -coumarin (5), 7-NEt -3-COOH-coumarin
2
2
(
6), 3-COOH-coumarin (7), 7-Cl-4-CF -coumarin (8), 7-Br-4-CF -
3
3
coumarin (9), 7-OH-4-CH -coumarin (10), 7-OH-4-CF –coumarin
2.4. Experimental excited singlet-state dipole moments
3
3
(
11) and 7-Cl-4-CH -coumarin (12). Also, we have determined
3
their dipole moments in the first singlet excited-state by using the
Bakhshiev [31], Kawski–Chamma–Viallet [32–34], Lippert [35,36],
and Reichardt [37] equations, based on the solvatochromic shift
method. The values of the singlet excited-state dipole moments
have been compared with the ground-state dipole moments calcu-
lated by means of the AM1 semi-empirical, theoretical approach.
Experimental excited singlet-state dipole moments were deter-
mined by the solvatochromic method, using the equations
proposed by Bakhshiev (Eq. (1)) [31], Kawski–Chamma–Viallet (Eq.
(2)) [32–34], Lippert–Mataga (Eq. (3)) [35,36] and Reichardt (Eq.
(11)) [37].
ꢀ
A
¯
− ꢀ¯ F = m F + constant
(1)
1 1
2
. Experimental
ꢀ¯ A + ꢀ¯ F
=
m F + constant
(2)
(3)
2
2
2
2.1. Reagents and chemical synthesis
ꢀ
A
¯
− ꢀ¯ F = m F + constant
3 3
Substituted coumarins 1–7 of laser grade (purity = 98–99%) were
purchased from Sigma–Aldrich Chemical Company and were used
without further purification.
where ꢀ¯ A and ꢀ¯ F are the absorption and fluorescence maxima
−
1
wavenumber (in cm ), respectively; F , F2 and F3 are the solvent
1
functions of Eqs. (1), (2) and (3), respectively; m , m2 and m3 are
1