Four novel ketocoumarin-based chromophores: Synthesis, photoluminescent behaviors and theoretical studies

Article abstract of DOI:10.1016/j.molstruc.2012.01.007

Four novel coumarin derivatives were synthesized and characterized via MS, ¹H NMR, FT-IR, and UV-vis absorption spectra. The fluorescence behaviors of the compounds in DFM solutions were observed. The compounds exhibited strong blue and green e

Full text of DOI:10.1016/j.molstruc.2012.01.007

Journal of Molecular Structure 1012 (2012) 177–181
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Four novel ketocoumarin-based chromophores: Synthesis, photoluminescent
behaviors and theoretical studies
⇑
Jing Li, Xianggao Li , Shirong Wang
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
a r t i c l e i n f o
a b s t r a c t
Four novel coumarin derivatives were synthesized and characterized via MS, 1H NMR, FT-IR, and UV–vis
absorption spectra. The fluorescence behaviors of the compounds in DFM solutions were observed. The
compounds exhibited strong blue and green emissions under ultraviolet light excitation. Calculations
performed using a combined time-dependent density functional theory (TD-DFT) and conductor-like
Article history:
Received 22 October 2011
Received in revised form 8 January 2012
Accepted 8 January 2012
Available online 13 January 2012
polarizable continuum model (C-PCM) reproduced the
p ?
pꢀ type absorption bands of the four com-
pounds. Resonance frequency calculations were also performed to study the IR spectra of the compounds.
The calculated results are in good accordance with the experimental values.
Ó 2012 Elsevier B.V. All rights reserved.
Keywords:
Coumarin
Synthesis
UV–vis
Fluorescence
DFT studies
1. Introduction
rin derivatives, 3a–3d, containing a series of substituents on the
coumarin ring. These derivatives exhibit strong blue and green
Coumarins, representing an important class of organic heterocy-
cles, have received considerable attention in the past few years. This
compound group can be found in many natural or synthetic drug
molecules and possess versatile biological and medical activities
[1]. This compound group is used for the treatment of brucellosis,
rheumatic disease and burns [2], and it also possesses anti-HIV
[3], antiinflammatory [4], anticoagulant [5] and antimicrobial activ-
ities [6]. Moreover, coumarin derivatives exhibit unique photo-
chemical and photophysical properties, including sufficient
fluorescence in a visible light range, large Stokes shift, high quantum
yield of photoluminescence and reasonable solubility [7–10], all of
which make them useful in a variety of applications. These applica-
tions include optical brighteners, laser dyes, non-linear optical
chromophores, solar energy collectors, fluorescent labels and
probes in biology and medicine, as well as two-photon absorption
(TPA) materials [11–13]. More importantly, coumarin dyes have
also been used as blue, green, and red dopants in organic light-emit-
ting diodes (OLEDs). For example, ketocoumarins, which belong to
the highly fluorescent class of coumarin laser dyes, comprise a ser-
ies of excellent fluorescent dopants and have been widely used as
doped emitters in OLEDs. Many ketocoumarin-based electrolumi-
nescent (EL) materials have been developed at present [14–17].
In the current work, we report the synthesis, photophysical
properties, and theoretical investigation of four novel ketocouma-
emissions under ultraviolet light excitation.
2. Experimental
2.1. Materials and methods
IR spectra (400–4000 cmꢁ1) were measured on a Nicolet 380
spectrophotometer. 1H NMR spectra were obtained using a Varian
Inova 500 spectrometer (at 500 MHz). Mass spectra were recorded
on a micrOTOF-Q II mass spectrometer. Melting points were taken
on a RY-1 micro melting apparatus; the thermometer was uncor-
rected. UV–vis absorption and emission spectra were recorded
using a Thermo Evolution 300 spectrometer and a Cary Eclipse
spectrometer, respectively. All the chemicals were commercially
available and were used without further purification. All the sol-
vents were dried using standard methods before use. The starting
compounds 1a–1d were synthesized according to Refs. [18,19].
2.2. Synthesis and characterization of compounds 2a–2d and 3a–3d
The synthetic routes to compounds 2a–2d and 3a–3d are shown
in Scheme 1.
The general process for the synthesis of compounds 2a–2d is as
follows: in a reaction flask, 1 mol of compounds 1a–1d and 2 mol
benzene-1,4-dicarboxaldehyde were dissolved in 200 mL glacial
acetic acid. Three drops of piperidine were added as catalyst. The
mixture was heated and refluxed for approximately 5 h, and then
⇑
Corresponding author. Tel.: +86 22 27404208; fax: +86 22 27892279.
E-mail address: lixianggao@hotmail.com (X. Li).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2012.01.007

178
J. Li et al. / Journal of Molecular Structure 1012 (2012) 177–181
O
O
O
OHC
CHO
NCCH2COOH
R
R
AcOH, Piperidine
EtOH, Piperidine
O
O
O
CHO
1a-1d
2a-2d
CN
COOH
a: R=H;
b: R=4-OH
O
c: R=8-OCH3; d: R=7-N(CH3)2
R
O
O
3a-3d
Scheme 1. Synthetic routes to compounds 3a–3d.
cooled to room temperature. The precipitate was collected via fil-
tration and recrystallized twice in ethanol/acetonitrile.
J = 7.5 Hz, 1H), 7.353 (t, 1H), 3.933 (s, 3H). HRMS (ESI+): m/z: calcd.
for C23H15NO6: 424.0792 [M + Na+]; found: 424.0711.
2a: m.p. 204–205 °C. IR (KBr pellet cmꢁ1): 3040, 1720, 1690,
3d: Yield: 70.3%. m.p. 204–205 °C. IR (KBr pellet cmꢁ1): 1712,
1670, 1560, 984 cmꢁ1
.
1H NMR (CDCl3, d, ppm): 10.049 (s, 1H),
1592, 1502, 1348, 1178, 1064 cmꢁ1 1H NMR(DMSO, d, ppm):
.
8.631 (s, 1H), 8.078 (d, J = 16.0 Hz, 1H), 7.927 (d, J = 8.0 Hz, 2H),
7.871 (d, J = 15.5 Hz, 1H), 7.826 (d, J = 8.0 Hz, 2H), 7.686 (q,
J = 13.3 Hz, 2H), 7.394 (m, 2H).
8.595 (s, 1H), 8.273 (s, 1H), 8.056 (q, J = 1.5 Hz, J = 9.5 Hz, 3H),
7.877 (d, J = 8.5 Hz, 2H), 7.689 (d, J = 4.5 Hz, 1H), 7.664 (d,
J = 2.0 Hz, 1H), 6.802 (q, J = 2.0 Hz, J = 2.0 Hz, 1H), 6.597 (d,
J = 2.0 Hz, 1H), 3.493 (q, 4H), 1.141 (t, 6H). HRMS (ESI+): m/z: calcd.
for C26H22N2O5: 465.1421 [M + Na+]; found: 465.1472.
2b: m.p. 209–210 °C. IR (KBr pellet cmꢁ1): 3040, 1730, 1700,
1630, 1540, 982 cmꢁ1 1H NMR (CDCl3, d, ppm): 10.060 (s, 1H),
.
8.544 (d, J = 15.5 Hz, 1H), 8.108 (q, J = 7.8 Hz, 1H), 8.037 (d,
J = 16.0 Hz, 1H), 7.947 (d, J = 8.0 Hz, 2H), 7.866 (d, J = 8.0 Hz, 2H),
7.720 (m, 1H), 7.350 (m, 2H).
2.3. Quantum chemical calculations
2c: m.p. 203–204 °C. IR (KBr pellet cmꢁ1): 3050, 1720, 1690,
The full geometry optimization, resonance frequencies, lowest
energy electronic transitions, and absorption spectra of compounds
3a–3d were calculated with the density functional theory (DFT) and
the time-dependent DFT (TD-DFT) from Becke’s three-parameter
hybrid functional (B3LYP) [20] method using two different basis
sets 6-311+Gꢀꢀ and 6-31Gꢀ. The solvent polarity effects were in-
cluded in the conductor-like polarizable continuum model (C-
PCM) [21]. The solvent used in our calculations was N,N0-dimethyl-
formide (DMF). All calculations were performed using the Gaussian
03 program.
1660, 1610, 962 cmꢁ1 1H NMR (CDCl3, d, ppm): 10.048 (s, 1H),
.
8.600 (s, 1H), 8.078 (d, J = 16.0 Hz, 1H), 7.923 (d, J = 8.0 Hz, 2H),
7.865 (d, J = 16.0 Hz, 1H), 7.822 (d, J = 8.0 Hz, 2H), 7.253 (m, 3H),
4.008 (s, 3H).
2d: m.p. 205–206 °C. IR (KBr pellet cmꢁ1): 3090, 1750, 1700,
1621, 1600, 1060 cmꢁ1 1H NMR (CDCl3, d, ppm): 10.035 (s, 1H),
.
8.570 (s, 1H), 8.272 (d, J = 16.0 Hz, 1H), 7.905 (d, J = 8.0 Hz, 2H),
7.814(t, 3H), 7.442 (d, J = 9.0 Hz, 1H), 6.648 (q, J = 2.5 Hz,
J = 2.5 Hz, 1H), 6.502 (d, J = 2.0 Hz, 1H), 3.479 (q, 4H), 1.261 (t, 6H).
The general process for the synthesis of compounds 3a–3d is as
follows: a mixture of compounds 2a–2d (1 mol), cyanoacetic acid
(1 mol), and piperidine (2 mL) in acetonitrile (200 mL) was placed
in a three-necked flask under a nitrogen atmosphere and heated to
reflux for approximately 1 h. After cooling, the precipitate was col-
lected and recrystallized from acetonitrile to obtain the pure com-
pounds 3a–3d.
3. Results and discussion
3.1. UV–vis absorption and fluorescence of compounds 3a–3d
The UV–vis absorption spectra of compounds 3a–3d in dilute
DMF solutions are shown in Fig. 1. These molecules consist of a
3a: Yield: 52.4%. m.p. 204–205 °C. IR (KBr pellet cmꢁ1): 3442,
2366, 1729, 1606, 1177 cmꢁ1
.
1H NMR(DMSO, d, ppm): 8.827 (s,
typical D–
and cyanoacrylic acid groups act as donor (D),
ter ( ), and acceptor (A) moieties, respectively. Structural modifi-
p–A structure, wherein 3-carbonylcoumarinyl, styryl,
1H), 8.696 (s, 1H), 8.148 (s, 1H), 8.023 (d, J = 8.5 Hz, 2H), 7.942
(d, J = 7.5 Hz, 1H), 7.904 (d, J = 8.0 Hz, 2H), 7.766 (s, 2H), 7.743 (d,
J = 8.5 Hz, 1H), 7.493 (d, J = 8.0 Hz, 1H), 7.430 (t, 1H). HRMS
(ESI+): m/z: calcd. for C22H13NO5: 394.0686 [M + Na+]; found:
394.0611.
p-conjugated cen-
p
cation only occurs in the coumarin moiety, where the 4-, 7-, and
8-positions are replaced with hydroxyl, diethylamino, and meth-
oxyl groups, respectively. Such modifications are expected to
change the electron-donating capability of the 3-carbonylcou-
marinyl group, as well as cause red-shifts in the absorption and
emission spectra.
The degree of red-shift in compound 3d is greater than that in
compound 3c because the electron-repelling group at position 7
leads to electron displacement in the 3-carbonylcoumarinyl group
(Fig. 2). This displacement enhances the electron-donating capabil-
ity of the 3-carbonylcoumarinyl group. Although electron displace-
ment occurs (Fig. 3) in the hydroxyl group at position 4 of
3b: Yield: 49.1%. m.p. 279–280 °C. IR (KBr pellet cmꢁ1): 3424,
2363, 1709, 1613, 1433, 1276 cmꢁ1 1H NMR(DMSO, d, ppm):
.
8.342 (t, 2H), 8.110 (d, J = 8.0 Hz, 2H), 8.023 (t, 2H), 7.936 (d,
J = 7.5 Hz, 2H), 7.826 (t, 1H), 7.430 (t, 2H). HRMS (ESI+): m/z: calcd.
for C22H13NO6: 410.0635 [M + Na+]; found: 410.0614.
3c: Yield: 62.8%. m.p. 269–270 °C. IR (KBr pellet cmꢁ1): 3427,
1727, 1606, 1470, 1274, 1179 cmꢁ1 1H NMR(DMSO, d, ppm):
.
8.664 (s, 1H), 8.332 (s, 1H), 8.087 (d, J = 8.0 Hz, 2H), 7.936 (d,
J = 8.5 Hz, 2H), 7.775 (s, 2H), 7.473 (d, J = 7.5 Hz, 1H), 7.429 (d,

J. Li et al. / Journal of Molecular Structure 1012 (2012) 177–181
179
3.2. Quantum chemical calculations
1.0
0.8
0.6
0.4
0.2
0.0
3a
3b
3c
3d
Quantum chemical calculations were performed to further
investigate the electronic and luminescent properties of com-
pounds 3a–3d. The B3LYP method using two different basis sets,
namely, 6-311+Gꢀꢀ and 6-31Gꢀ, were used to obtain reliable spectral
data to investigate the absorption spectra of the four compounds.
The experimental and theoretical kmax values of the four compounds
are shown in Fig. 7. The error between the calculated and experi-
mental results ranges from 17 nm to 22 nm for 6-311+Gꢀꢀ, and
28 nm to 32 nm for 6-31Gꢀ. These values reveal that the
6-311+Gꢀꢀ basis set is more suitable than the 6-31Gꢀ basis set for
studying the absorption spectra of the four compounds. The results
obtained with the introduction of the solvent reaction field and the
individual combination of C-PCM and the 6-311+Gꢀꢀ and 6-31Gꢀ ba-
sis sets are much closer to the experimental values. 6-311+Gꢀꢀ/TD-
DFT(C-PCM) and 6-311+Gꢀꢀ(C-PCM)/TD-DFT(C-PCM) were deter-
mined using the optimized gas phase geometries and solvated
geometries of the solvent, respectively. Similarly, 6-31Gꢀ/TD-
DFT(C-PCM) and 6-31Gꢀ(C-PCM)/TD-DFT(C-PCM) were determined
using optimized gas phase geometries and solvated geometries in
the solvent, respectively. The kmax optimized by 6-311+Gꢀꢀ(C-
PCM)/TD-DFT(C-PCM) reduces the absolute deviation to 3 nm for
compound 3a, 1 nm for compound 3b, 2 nm for compound 3c, and
6 nm for compound 3d. These results indicate that the accuracy
remarkably increases when the solvent effects are taken into ac-
count because of the intermolecular hydrogen bonding interaction
in the corresponding electronic excited state. The electronic transi-
tion energy from the ground state to the excited state generally de-
creases, which leads to a red-shift in the electronic spectrum
because of strengthening of the excited hydrogen bond. In contrast,
when the hydrogen bond is weakened in the excited state, the de-
crease in energy level of the excited state is less than that of the
ground state. Thus, the electronic transition energy from the ground
state to the excited state increases, which indicates an electronic
spectral blue-shift because of weakening of the excited hydrogen
bond [24].
300
400
500
600
Wavelength (nm)
Fig. 1. UV–vis absorption spectra of compounds 3a–3d in DMF at room temper-
ature. (C = 5.00 ꢂ 10ꢁ5 mol/L).
O
O
O
O
N
N
O
N
N
O
O
or
O
O
O
O
O
Fig. 2. Electron displacement in compound 3d.
compound 3b, a strong intramolecular hydrogen bond between the
hydroxyl group and the ketonic oxygen weakens this interaction,
which results in the maximum absorption peak in compound 3b
similar to that in compound 3c.
The main orbital compositions of the computed lower-lying sin-
glet excited states and the transition feature of compounds 3a–3d
obtained at the 6-311+Gꢀꢀ(C-PCM)/TD-DFT(C-PCM) level are listed
in Table 1. The lowest energy absorption in the four compounds
The major absorption band of the four compounds corresponds
to the intramolecular charge transfer from the 3-carbonylcoumari-
nyl group to the cyanoacrylic acid group in the whole electronic
system. However, another minor absorption band is observed in
the absorption spectrum of compound 3d, which is attributed to
can be attributed to the electronic
p ?
pꢀ transition from HOMO
to LUMO. In addition, the calculated frontier orbitals of compounds
3a–3d are nearly the same. For example, the HOMO and LUMO dia-
grams (Fig. 8) of compound 3d show that it is likely to exhibit an
efficient electron transfer from the 3-carbonylcoumarinyl group
of the HOMO to the chalcone group of the LUMO if electronic tran-
sitions occur. The HOMO for the compound is localized at the 3-
carbonylcoumarinyl region, whereas the LUMO is located at the
chalcone region. Therefore, when electrons transfer from HOMO
to LUMO, the electron density significantly decreases in the elec-
tron-donating 3-carbonylcoumarinyl moiety, accompanied by an
increase in the electron density of the electron-accepting chalcone
moiety. This result indicates that the electrons transfer from the
3-carbonylcoumarinyl group to the cyanoacrylic acid group
through the styryl group.
The resonance frequency calculation for compound 3d was con-
ducted at the B3LYP/6-311+Gꢀꢀ level to perform a preliminary
study of its IR spectra. The observed and calculated data of the IR
spectra of compound 3d are given in Table 2. The calculated IR
spectral data are slightly higher than the experimental values. A
possible reason is that the result obtained from the calculation is
that of the harmonic oscillator frequency, whereas the experimen-
tal value contains the frequency of an anharmonic oscillator. The
data were modified using 0.9613 [25] as the frequency scaling
factor. The consistent relation graph between the calculated and
the
p ?
pꢀ transition in the 3-carbonylcoumarinyl moiety. The
main absorption band of compound 3d is red-shifted when the sol-
vent polarity is increased. This result further shows the strong
intramolecular charge transfer in the structure of compound 3d,
which is in good agreement with the findings of a previous study
(Fig. 4) [22].
Fig. 5 shows the photoluminescence spectra of compounds 3a–
3d. Compounds 3a–3c exhibit bright blue emissions with emission
peaks at around 411, 439, and 432 nm, respectively. Compound 3d
exhibits a bright green emission with a peak at 580 nm. The degree
of bathochromic shift in compound 3d is greater than those of
compounds 3b and 3c because the effect of electron displacement
in the 3-carbonylcoumarinyl group causes a stronger intramolecu-
lar charge transfer. The emission band of compound 3d red1-shifts
and it gradually broadens with increasing solvent polarity. This re-
sult demonstrates that the compound also has an effective intra-
molecular charge transfer characteristic [23] (Fig. 6).
1
For interpretation of color in Figs. 1, 5 and 7–9, the reader is referred to the web
version of this article.

180
J. Li et al. / Journal of Molecular Structure 1012 (2012) 177–181
OH
O
OH
O
O
O
OH
O
O
O
OH
O
O
O
or
O
O
Fig. 3. Electron displacement in compound 3b.
1.2
1.0
0.8
0.6
0.4
0.2
0.0
480
440
400
360
a b c d e
Exp.
6-311+G**(C-PCM)/TD-DFT(C-PCM)
6-311+G**/TD-DFT(C-PCM)
6-311+G**/TD-DFT
6-31G*(C-PCM)/TD-DFT(C-PCM)
6-31G*/TD-DFT(C-PCM)
6-31G*/TD-DFT
320
300
350
400
450
500
550
3a
3b
3c
3d
Wavelength (nm)
Fig. 7. Plot of experimental and theoretical kmax of compounds 3a–3d in DMF.
Fig. 4. Effect of the polarity of the solvent upon the absorption spectra of compound
3d. Solvents: Petroleum ether (a); cyclopentane (b); toluene (c); chloroform (d);
N,N0-dimethylformide (e).
Table 1
Main orbital compositions and excitation energies of compounds 3a–3d.
1.0
Compounds State Transition
feature
6-311+Gꢀꢀ/C-PCMa
fc
3a
Transition
characterb
D
(eV)
E
3b
3c
3d
0.8
0.6
0.4
0.2
0.0
3a
3b
3c
3d
1A
1A
1A
1A
p
p
p
p
?
?
?
?
pꢀ
pꢀ
pꢀ
pꢀ
H ? L (85.3%)
H ? L (82.7%)
H ? L (84.1%)
H ? L (87.3%)
4.07
3.83
3.81
3.11
0.883
0.812
0.853
0.861
a
Solvent is DMF solution.
H and L stand for HOMO and LUMO, respectively, and the proportion of the
main transition is given in the parentheses.
b
c
Oscillator strength coefficients.
400
500
600
700
800
Wavelength (nm)
Fig. 5. Fluorescence emission spectra of compound 3a (kex = 341 nm), compound 3b
(kex = 361 nm), compound 3c (kex = 361 nm), and compound 3d (kex = 465 nm) in
DMF at room temperature (C = 5.00 ꢂ 10ꢁ5 mol/L).
1.2
a b c d e
1.0
0.8
0.6
0.4
0.2
0.0
HOMO
500
600
700
800
Wavelength (nm)
LUMO
Fig. 6. Effect of the polarity of the solvent upon the fluorescence emission spectra of
compound 3d. Solvents: petroleum ether (a); cyclopentane (b); toluene (c);
chloroform (d); N,N0-dimethylformide (e).
Fig. 8. Sketch of the main molecular orbitals for compound 3d obtained at the
6-311+Gꢀꢀ(C-PCM)/TD-DFT(C-PCM) level.

J. Li et al. / Journal of Molecular Structure 1012 (2012) 177–181
181
Table 2
These derivatives exhibited strong blue and green emissions in di-
lute solutions. The 6-311+Gꢀꢀ(C-PCM)/TD-DFT(C-PCM) method
provided a reliable description of the molecular absorption charac-
Comparison of the calculated and experimental infrared data of compound 3d.
No. Exp.
Calcd.
Vibratory feature
ters and reproduced the
p ?
pꢀ type absorption bands of the com-
Freq.a Int. (IR)b Non scaled Scaledc Int. (IR)
pounds. Resonance frequency calculation was also performed to
study the IR spectra of the compounds, and the calculated results
were consistent with the experimental values. Furthermore, the
four compounds are potential candidates for use in opto- or opto-
electronic blue and green-emitting devices. Their other character-
istics will be further investigated in future studies.
1
2
3
4
5
6
7
8
9
1064
1178
1275
1348
1502
1592
1712
2361
3432
w
m
w
m
s
1113
1209
1347
1377
1581
1642
1824
2436
3584
1070
1163
1295
1324
1520
1579
1754
2342
3446
167.13
271.98
140.91
205.42
456.79
474.29
612.72
119.38
150.98
vC@C
vC@C
vCAN
vC@C
vC@C (CH@CH)
vC@O (CO)
vC@O (COO)
vC„N
v
m
w
w
Acknowledgment
vOAH (COOH)
a
b
c
Frequencies in cmꢁ1
m: middle, s: strong, v: very strong, w: weak.
Scaling factor using 0.9613.
.
This work was financially supported by National Natural Sci-
ence Foundation of China (21176180).
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4000
3500
3000
Non-scaled
Scaled
Y
= 1.0433X - 2.1998
non-scaled
2500
2000
1500
1000
Y
= 1.0030X - 1.9585
scaled
1000 1500 2000 2500 3000 3500
Experimental wavenumber (cm-1)
Fig. 9. Consistency of the wavenumbers of the calculated and experimental IR
spectra main peaks of compound 3d.
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experimental data of compound 3d is shown in Fig. 9. It could be
seen that the linear slope and intercept were 1.0433, 2.1998 for
the non-scaled data and 1.0030, 1.9585 for the scaled, respectively.
These results indicate that the description error clearly decreased
after the introduction of the frequency-scaling factor, and the cal-
culated data are in good agreement with the experimental value.
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4. Conclusions
Four novel ketocoumarin derivatives (3a–3d) were successfully
synthesized and their photophysical properties were investigated.
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