The synthesis, structure and photoluminescence of coumarin-based chromophores

Article abstract of DOI:10.1016/j.dyepig.2010.03.003

Coumarin-based chromophores with azo and pyrazoline moieties were synthesized and their structures and properties elucidated using spectroscopy. The coumarins were luminescent, having solid state emission at wavelengths ranging from 400 to 750?nm, dependi

Full text of DOI:10.1016/j.dyepig.2010.03.003

Dyes and Pigments 87 (2010) 109e118
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The synthesis, structure and photoluminescence
of coumarin-based chromophores
,
b
a
b
a
a
b
Yi-Feng Sun a , Shu-Hong Xu , Ren-Tao Wu , Zhu-Yuan Wang , Ze-Bao Zheng , Ji-Kun Li , Yi-Ping Cui
*
a Department of Chemistry, Taishan University, Taian 271021, China
b Advanced Photonics Center, School of Electronic Science and Engineering, Southeast University, Nanjing 210096, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Coumarin-based chromophores with azo and pyrazoline moieties were synthesized and their structures
and properties elucidated using spectroscopy. The coumarins were luminescent, having solid state
emission at wavelengths ranging from 400 to 750 nm, depending on structure. The relationships
between the solid state photoluminescence and both chemical and crystal structures are discussed.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 2 December 2009
Received in revised form
27 February 2010
Accepted 1 March 2010
Available online 15 March 2010
Keywords:
Coumarin
Azo
Pyrazoline
Crystal structure
Photoluminescence
Quantum chemical calculations
1. Introduction
activities, including antimicrobial, anticonvulsant, anti-inflamma-
tory, analgesic, anticancer, antitubercular and herbicidal [17,18].
Coumarin derivatives have long been recognized to possess
multiple biological activities [1e5], especially antioxidant and anti-
inflammatory activities and the coumarin unit can be found in
many natural and synthetic drug molecules. Moreover, as an
important class of organic heterocyclic dyes, coumarin derivatives
exhibit unique photochemical and photophysical properties, which
render them useful in a variety of applications such as 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 [6e12].
More importantly, coumarin dyes have also been used as blue,
green and red dopants in organic light-emitting diodes (OLEDs).
For example, 10-(2-benzothiazolyl)-1,1,7,7 -tetrainethyl-2,3,6,7-
tetrahydro-1H,5H,11H-[1]benzo pyrano[6,7,8-ij]quinolizin-11-one
(C-545T) (Fig. 1), which belongs to the highly fluorescent class of
coumarin laser dyes, is one of the best green fluorescent dopants,
and has widely been used as a doped green emitter in OLEDs
[13e16].
Furthermore, pyrazolines show strong fluorescence and have
high hole-transport efficiency and excellent blue emitting
a
properties. Therefore, pyrazoline derivatives have widely been
used as whitening or brightening reagents for synthetic fibers, as
fluorescence probes in chemosensors, as fluorescent chemo-
sensors for recognition of transition metal ions, as hole-transport
materials in electrophotography and electroluminescence fields
[19e22]. For instance, 1,5-diphenyl-3-(1-naphthyl)-2-pyrazoline
(DPNPZ) (Fig. 1) has a strong blue fluorescence emission, and can
be used as an emissive dopant in an electroluminescent (EL)
device [23].
On the other hand, azo compounds, while being a well-docu-
mented class of commercial dyes, are attracting ever increasing
attention in the areas of optical data storage, non-linear optical
materials, photoswitches, photoprobes, ink-jet printing, biochem-
ical assays, and photocontrol of biological processes [24e26]. The
main reason is that these azo chromophores can undergo light-
driven, reversible transecis isomerisation of the azo bond with
concomitant change of the structure, dipole moment and optical
properties [27].
Similarly, pyrazoline derivatives generate strong interest
stemming from their broad spectrum of pharmacological
In view of the considerable importance of these compounds
work, the focus of the current research workers turned towards the
synthesis of coumarin derivatives containing an azo or pyrazoline
* Corresponding author. Tel.: þ86 538 6715546; fax: þ86 538 6715536.
E-mail address: sunyf50@yahoo.com.cn (Y.-F. Sun).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.03.003

110
Y.-F. Sun et al. / Dyes and Pigments 87 (2010) 109e118
a Horiba Jobin-Yvon LabRam HR800 Raman microspectrometer
under a 325 nm HeeCd laser excitation. Single crystal was char-
acterized by Bruker Smart 1000 CCD X-ray single crystal diffrac-
tometer. All the chemicals are commercially available and they
were used without further purification.
N
O
S
N
N
N
O
2.2. Synthesis of the coumarinechalcone hybrids (2)
C-545T
DPNPZ
Fig. 1. The structures of C-545T and DPNPZ.
Three coumarinechalcone hybrids were synthesized from 3-
acetyl coumarins and cinnamaldehyde or 2,20-bithiophene-5-
carbaldehyde in the presence of piperidine in ethanol under
microwave irradiation following the method reported previ-
ously [28]. 3-((5-Phenyl)pent-2,4-dienoyl)-2H-1-naphtho[2,1-b]
pyran-2-one (2b) and 3-(3-(2-(thiophen-2-yl)thiophen-5-yl)
prop-2-enoyl)-2H-1-benzopyran-2-one (2c) have been reported
in previous papers [28,31].
moiety. It was envisaged that compounds containing these moie-
ties in the same molecule may show enhanced biological and
optical properties. In previous papers we have reported the
synthesis, crystal structures and preliminary spectroscopic prop-
erties of various coumarin derivatives [28e30]. However, as the
solid state photoluminescence properties of the dyes have not been
investigated, in keeping with our interest in the synthesis, crys-
tallography and optical evaluation of coumarin-based chromo-
phores, this paper concerns the synthesis and spectroscopic
characterization of novel, coumarin-based chromophores based
either on an azo or pyrazoline core. In addition, the photo-
luminescence properties of the compounds are discussed in the
solid state and the structures of 2c and 4a were studied both
experimentally and theoretically. The synthetic pathway and the
structures of target molecules are shown in Figs. 2e4.
2.2.1. 3-((5-Phenyl)pent-2,4-dienoyl)-2H-1-
benzopyran-2-one (2a)
Recrystallization from ethanoleacetone gave the title compound
2a as yellow crystals, yield 62%; mp 182e184 ꢁC; 1H NMR (400 MHz,
CDCl3/TMS) d: 7.02e7.10 (m, 2H), 7.31e7.40 (m, 5H), 7.47e7.52
(m, 3H), 7.63e7.70 (m, 3H), 8.56 (s, 1H); IR (KBr) n: 1732, 1651, 1612,
1574, 1451, 1348, 1232, 1183, 1143, 997, 751, 689, 577 cmꢀ1. Anal.
calcd. for C20H14O3: C 79.46, H 4.67; found C 79.25, H 4.74.
2.3. Synthesis of the coumarinepyrazoline derivatives (3)
2. Experimental
Two coumarinepyrazoline derivatives were synthesized from
coumarinechalcone hybrids (2) and 2-hydrazino-1,3-benzothia-
zole in ethylene glycol under microwave irradiation according to
the method reported previously [29].
2.1. General
1H NMR spectra were recorded with a Bruker AVANCE-400 or
Varian INOVA-600 NMR spectrometer and chemical shifts
expressed as
d (ppm) values with TMS as internal standard. The IR
2.3.1. 1-(Benzothiazol-2-yl)-3-(-2-oxo-2H-1-benzopyran-
3-yl)-5-styryl-2-pyrazoline (3a)
spectra were measured on a Nicolet/Nexus-870 FT-IR spectrometer
with KBr pellets in the range 4000e400cmꢀ1. Element analysis was
taken with a PerkineElmer 240 analyzer. Mass spectra (MS) were
measured on an LCQ Advantage MAX or VG ZAB-HS mass spec-
trometer. The melting points were determined with a WRS-1A
melting point apparatus and are uncorrected. The Raman spectra
were recorded using a Horiba Jobin-Yvon LabRam HR800 Raman
microspectrometer, with an excitation laser at 514 nm, and
a 600 groove mmꢀ1 diffraction grating. Signals were recorded in
the range from 200e2500 cmꢀ1. The UVevis absorption spectra
were recorded using a Helios Alpha UVeVis scanning spectropho-
tometer. The photoluminescence spectra were measured using
Recrystallization from chloroform gave the title compound 3a as
yellow solid, yield 63%; mp > 240 ꢁC; 1H NMR (400 MHz, CDCl3/
TMS)
d
: 3.62 (dd, J ¼ 5.6, 18.6 Hz, 1H), 3.98 (dd, J ¼ 11.7, 18.6 Hz, 1H),
5.46e5.53 (m, 1H), 6.41 (dd, J ¼ 6.6,15.9 Hz,1H), 6.76 (d, J ¼ 15.9 Hz,
1H), 7.16 (t, J ¼ 7.6 Hz, 1H), 7.24e7.42 (m, 8H), 7.57e7.71 (m, 4H),
8.51 (s,1H). IR (KBr) n: 1728, 1601, 1567,1551, 1533, 1455, 1443, 1360,
1277, 1243, 1139, 1106, 959, 881, 747, 720, 697, 638 cmꢀ1. MS m/z:
450.6 (M þ 1). Anal. calcd. for C27H19N3O2S: C 72.14, H 4.26, N 9.35;
found: C 72.38, H 4.61, N 9.13.
2.3.2. 1-(Benzothiazol-2-yl)-3-(-2-oxo-2H-1-naphtho
[2,1-b]pyran -3-yl)-5-styryl-2-pyrazoline (3b)
Ar
Recrystallization from chloroform gave the title compound 3b as
O
red solid, yield 52%; mp 216 ꢁC (decompose); 1H NMR (600 MHz,
N
i
ii
N
RCOCH
3
R
Ar
CDCl3/TMS)
d
: 3.65 (dd, J ¼ 5.4, 18.6 Hz, 1H), 4.02 (dd, J ¼ 11.7,
S
N
R
18.6 Hz, 1H), 5.50e5.53 (m, 1H), 6.41 (dd, J ¼ 6.6, 15.6 Hz, 1H), 6.76
1a, 1b
2a-2c
3a, 3b
(d, J ¼ 15.6 Hz, 1H), 7.07e8.42 (m,15H), 9.25 (s, 1H). IR (KBr)
n: 1726,
1598,1568,1527,1440,1281,1214,1143, 994, 810, 743, 687 cmꢀ1. MS
m/z: 500.2 (M þ 1). Anal. calcd. for C31H21N3O2S: C 74.53, H 4.24, N
8.41; found: C 74.33, H 4.37, N 8.26.
R =
Ar =
a
,
O
O
2.4. Synthesis of the coumarineazo derivatives (4)
b
c
R =
R =
Ar =
Ar =
,
O
O
Azocoumarins were prepared according tothe method described
earlier [30] which was modified. Aromatic amines (10 mmol) were
dissolved in 6 M HCl (8 mL) and diazotised using sodium nitrite
(12 mmol in 5 mL of water) solution at 0 ꢁC with stirring. On
completion of diazotization (30e40 min), the resulting diazonium
salt solution was added dropwise to a cold, alkaline solution of
salicylaldehyde (or 3-methoxysalicylaldehyde) (10 mmol) in 2% aq
,
O
O
S
S
Fig. 2. Synthesis of coumarinepyrazoline hybrids. Reagents and reaction conditions:
(i) ArCHO, Piperidine, EtOH, reflux; (ii) 2-Hydrazino-1,3-benzothiazole, Ethylene glycol,
MW.

Y.-F. Sun et al. / Dyes and Pigments 87 (2010) 109e118
111
Ar
N
N
CHO
OH
Ar
N
N
COOC H
2
5
_
iii
iv
v
+
Ar NH
Ar N
2
Cl
2
O
O
R
R
4a-4c
a
c
F
C
O N
Ar =
R = H , Ar =
R = OCH
,
b
R = H , Ar =
3
;
;
COOH
Fig. 3. Synthesis of the coumarine-azo dyes. Reagents and reaction conditions: (iii) NaNO2, HCl, 0e5 ꢁC; (iv) Salicylaldehyde or 3-Methoxysalicylaldehyde, pH ¼ 8e9, 0e5 ꢁC;
(v) CH2(COOC2H5)2, EtOH, Piperidine, Glacial acetic acid, reflux.
NaOH solution (50 mL) with stirring. During coupling, the reaction
temperature was maintained at 0e5 ꢁC and the pH at 8e9 by the
addition of 0.5% aq NaOH solution. On completion of coupling the
precipitate was filtered and washed thoroughly with water and
dried. The product was further purified by recrystallization from
acetone or ethanol.
To a mixture of arylazosalicylaldehydes (5 mmol) and diethyl
malonate (5 mmol) in ethanol (20 mL), glacial acetic acid (0.3 mL)
and piperidine (0.3 mL) were added under rapid stirring. The
reaction mixture was heated under reflux for 5 h and, after cooling,
the solid was filtered and recrystallized from a suitable solvent to
afford the pure product.
2.4.2. Ethyl 6-(2-carboxylphenyl)azo-2H-1-benzopyran-
2-one-3-carboxylate (4b)
Recrystallization from acetone gave the title compound 4b as
yellow crystals, yield 52%; mp 214e215 ꢁC; 1H NMR (600 MHz,
DMSO-d6/TMS)
d
: 1.33 (t, J ¼ 7.2 Hz, 3H), 4.32 (q, J ¼ 7.2 Hz, 2H), 7.57
(d, J ¼ 7.8 Hz, 1H), 7.62 (t, J ¼ 7.2 Hz, 1H), 7.66 (d, J ¼ 8.4 Hz, 1H), 7.71
(t, J ¼ 7.5 Hz,1H), 7.83 (d, J ¼ 7.2 Hz,1H), 8.14 (dd, J ¼ 8.7, 2.7 Hz,1H),
8.49 (d, J ¼ 2.4 Hz, 1H), 8.95 (s, 1H), 13.19 (s, 1H). IR (KBr)
n: 1742,
1690, 1619, 1562, 1363, 1296, 1250, 1148, 1025, 974, 789, 682,
579 cmꢀ1. FAB-MS m/z: 367 (M þ 1). Anal. calcd. for C19H14N2O6: C
62.30, H 3.85, N 7.65; found: C 62.45, H 3.96, N 7.41.
2.4.3. Ethyl 8-methoxy-6-(4-nitrophenyl)azo-2H-
2.4.1. Ethyl 6-(4-trifluoromethylphenyl)azo-2H-1-
1-benzopyran-2-one-3-carboxylate (4c)
benzopyran-2-one-3-carboxylate (4a)
Recrystallization from ethanoleDMF gave the title compound
Recrystallization from ethanol gave the title compound 4a as red
4c as yellow solid, yield 59%; mp 265e266 ꢁC; 1H NMR (600 MHz,
crystals, yield 68%; mp 206e208 ꢁC; 1H NMR (600 MHz, CDCl3/
CDCl3/TMS)
d
: 1.44 (t, J ¼ 7.2 Hz, 3H), 4.09 (s, 1H), 4.45 (q,
TMS)
d
: 1.44 (t, J ¼ 6.6 Hz, 3H), 4.45 (q, J ¼ 7.2 Hz, 2H), 7.51 (d,
J ¼ 7.2 Hz, 2H), 7.78 (d, J ¼ 1.8 Hz, 1H), 7.93 (d, J ¼ 1.8 Hz, 1H),
8.07 (d, J ¼ 8.4 Hz, 2H), 8.42 (d, J ¼ 9.0 Hz, 1H), 8.64 (s, 1H). IR
J ¼ 9.0 Hz,1H), 7.81 (d, J ¼ 7.8 Hz, 2H), 8.03 (d, J ¼ 7.8 Hz, 2H), 8.24 (s,
1H), 8.26 (d, J ¼ 8.4 Hz, 1H), 8.65 (s, 1H). IR (KBr)
n
: 1746, 1706, 1614,
(KBr) n: 1764, 1698, 1612, 1574, 1522, 1471, 1383, 1342, 1246, 1142,
1557, 1475, 1322, 1245, 1066, 846, 794, 605 cmꢀ1. FAB-MS m/z: 391
(M þ 1). Anal. calcd. for C19H13F3N2O4: C 58.47, H 3.36, N 7.18;
found: C 58.33, H 3.49, N 7.02.
1107, 1026, 964, 878, 810, 692 cmꢀ1. MS m/z: 398.4 (M þ 1). Anal.
calcd. for C19H15N3O7: C 57.43, H 3.81, N 10.58; found: C 57.29,
H 3.92, N 10.43.
Fig. 4. The structures of the coumarin-based dyes.

112
Y.-F. Sun et al. / Dyes and Pigments 87 (2010) 109e118
2.5. X-ray crystallography
acetic acid and piperidine (Fig. 3). The azosalicylaldehydes were
prepared by diazotisation of aromatic amines and then a coupling
reaction with either salicylaldehyde or 3-methoxysalicylaldehyde.
The structure and purity of the resulting new compounds were
confirmed by spectral data and elemental analysis. In the IR spectra
of these compounds, the strong bands at around 1726e1764 cmꢀ1
confirm the presence of a coumarin skeleton. Comparison of the IR
spectrum of 2a (1732 and 1651 cmꢀ1) with that of compound 3a
Suitable single crystals of 2c and 4a for X-ray structural analysis
were obtained by slow evaporation of the ethanol or DMF solutions.
The diffraction data for both structures were collected with a Bruker
Smart Apex 1000 CCD X-ray single crystal diffractometer using
a graphite monochromated Mo K
a
radiation (
l
¼ 0.071073 nm) at
273(2) K. The structures were solved by direct methods with
SHELXS-97 program and refinements on F2 were performed with
SHELXL-97 program by full-matrix least-squares techniques
with anisotropic thermal parameters for the non-hydrogen atoms.
All H atoms were initially located in a difference Fourier map. The
methyl H atoms were then constrained to an ideal geometry, with
CeH ¼ 0.096 nm and Uiso(H) ¼ 1.5Ueq(C). All other H atoms were
placed in geometrically idealized positions and constrained to ride
on their parent atoms with CeH distances 0.093e0.097 nm and
Uiso(H) ¼ 1.2Ueq(C). A summary of the crystallographic data and
structure refinement details is given in Table 1.
16000
14000
12000
10000
4a
8000
6000
4000
2000
0
3. Results and discussion
3.1. Synthesis
Compounds 2 and 3 were prepared using the method reported
by us previously [28,29], as depicted in Fig. 2. ClaiseneSchmidt
condensation of 1 with cinnamaldehyde or 2,20-bithiophene-5-
carbaldehyde yielded the corresponding chalcones 2. When the
chalcones were treated with 2-hydrazino-1,3-benzothiazole in
ethylene glycol under microwave irradiation, the pyrazoline
derivatives 3 were obtained in 52e63% yield.
-2000
250
500
750
1000
1250
1500
1750
2000
2250
2500
-1
Raman shift (cm )
20000
17500
15000
12500
10000
7500
5000
2500
0
The azocoumarins (4ae4c) were synthesized by condensation of
azosalicylaldehydes and diethyl malonate in the presence of glacial
4b
Table 1
Crystal data and structure refinement.
Compound
2c
4a
Empirical formula
Formula weight
Temperature (K)
Crystal system,
Space group
Unit cell dimensions
a (nm)
C20H12O3S2
364.42
273(2)
Monoclinic
P21/c
C19H13F3N2O4
390.31
273(2)
Triclinic
P1
0.52115 (10)
0.95258 (18)
3.2937 (7)
90
91.797 (3)
90
1.6343 (6), 4
1.481
0.342
0.69007 (19)
0.7746 (2)
1.6975 (5)
88.797 (5)
81.253 (4)
73.407 (4)
0.8592 (4), 2
1.509
0.128
400
-2500
b (nm)
250
500
750
1000
1250
1500
1750
2000
2250
2500
c (nm)
-1
Raman shift (cm )
a
b
g
(ꢁ)
(ꢁ)
(ꢁ)
Volume (nm3), Z
Dcalc (Mg/m3)
20000
17500
15000
12500
10000
7500
5000
2500
0
Absorption coefficient (mmꢀ1
F (0 0 0)
)
752
Crystal size (mm)
q
0.21 ꢂ 0.16 ꢂ 0.12
2.23 to 25.05
ꢀ6 ꢃ h ꢃ 6
ꢀ8 ꢃ k ꢃ 11
ꢀ37 ꢃ l ꢃ 39
8385/2879
[Rint ¼ 0.0284]
0.9601 and 0.9316
2879/0/227
1.040
0.21 ꢂ 0.16 ꢂ 0.12
2.43 to 25.05
ꢀ8 ꢃ h ꢃ 7
ꢀ9 ꢃ k ꢃ 8
ꢀ20 ꢃ l ꢃ 17
4534/3016
[Rint ¼ 0.0190]
0.9848 and 0.9736
3016/0/254
1.057
range for data collection (ꢁ)
Limiting indices
4c
Reflections collected/unique
Max. and min. transmission
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2sigma (I)]
R1 ¼ 0.0403;
wR2 ¼ 0.0955
R1 ¼ 0.0616;
wR2 ¼ 0.1089
0.0037 (7)
197 and ꢀ328
R1 ¼ 0.0601;
wR2 ¼ 0.1756
R1 ¼ 0.0694;
wR2 ¼ 0.1881
0.047 (7)
R indices (all data)
-2500
Extinction coefficient
250
500
750
1000
1250
1500
1750
2000
2250
2500
Largest diff. peak
692 and ꢀ413
-1
Raman shift (cm )
and hole (e nmꢀ3
)
CCDC
750 904
750 905
Fig. 5. Raman spectra of the coumarin-azo dyes (4ae4c).

Y.-F. Sun et al. / Dyes and Pigments 87 (2010) 109e118
113
in the IR spectrum appears at 1343 cmꢀ1 in the Raman spectrum as
a strong band.
The structures of the prepared compounds were also confirmed
by 1H NMR spectroscopy. The 1H NMR spectra of these coumarin
dyes show
a
clearly distinguishable intense singlet at
d
¼ 8.51e9.25 ppm for H-4 of the coumarin nucleus. Such a signal is
characteristic for coumarin system protons [33].
In the 1H NMR spectra of compounds 3 signals of the CH2eCH
fragment of the pyrazoline ring were present. These protons appear
as two doublets of doublets and a multiplet, respectively, at the
d
¼ 3.58e3.68, 3.94e4.04 and 5.46e5.53 ppm regions, integrating
for one proton each. Moreover, the 1H NMR spectra also exhibited
the presence of two trans-olefinic protons at
(J ¼ 15.6e15.9 Hz).
d 6.41 and 6.76
Fig. 6. (a) The molecular structure of 2c, showing the atom-labelling scheme.
Displacement ellipsoids are drawn at the 50% probability level. H atoms are shown as
small spheres of arbitrary radius. (b) The side elevation of 2c. H atoms are omitted for
clarity.
Additionally, the azocoumarins 4b and 4c gave a characteristic
doublet at
nucleus, while the corresponding C-5 proton in 4a appeared as
a singlet at 8.24. For compound 4b a characteristic carboxyl
proton singlet at
13.19 was apparent in the 1H NMR spectrum.
d 7.93e8.49, which was assigned to H-5 of the coumarin
d
d
clearly indicated the absence of a band around 1651 cmꢀ1 due to
C]O of chalcone, which clearly confirmed that
a cyclo-
3.2. Crystal structure
condensation with 2-hydrazino-1,3-benzothiazole had taken place.
In the case of 4c, the IR spectra show two strong bands located
around 1522 and 1342 cmꢀ1, which can be ascribed to the presence
of the eNO2 group.
Although compound 2c has been studied previously by us [28],
the crystal structure of 2c has not been reported. So, in order to
study the structureeproperty relationship, its crystal structure was
elucidated. The crystal structure of 2c is shown in Fig. 6. For 2c the
coumarin ring and 2,20-bithiophene ring system are essentially
planar with maximum deviations of 0.00282 and 0.00402 nm for
atoms C1 and S2, respectively. The dihedral angle between the 2,20-
bithiophene ring plane and the ring plane of coumarin is 5.0ꢁ,
which indicates that the whole molecule is nearly planar (Fig. 6).
This is in marked contrast to the orientations in 3-(3-(4-N,N-
diphenyl aminophenyl)prop-2-enoyl) -2H-1-naphtho[2,1-b]pyran-
2-one (DPNP) (Fig. 4), which were previously reported by us [28], in
which three phenyl rings all are rotated significantly out of the
plane of the rest of the molecule.
In addition, molecule 2c adopts a trans configuration about the
central olefinic bond, as in DPNP. Moreover, the linkage between
the 3-actylcoumarin system and 2,20-bithiophene ring system
with bond lengths of C10eC11: 0.1465(3) nm, C11eC12: 0.1321
(3) nm and C12eC13: 0.1440(3) nm, suggests that all non-
hydrogen atoms between donor (2,20-bithiophene ring system)
The Raman spectra of the solid samples of these compounds (2,
3 and 4) have been measured. However, unfortunately, among
these molecules, the Raman spectra of compounds 2 and 3 cannot
be obtained due to their strong fluorescence interference. The
Raman spectra of 4 are shown in Fig. 5. Generally, in the Raman
spectra, v(N]N) is normally a quite intense single peak, compared
with the extremely weak corresponding band in the IR [32]. Thus,
in the Raman spectrum, the corresponding band (v(N]N)) is more
easily recognized by its high intensity, which is due to the large
polarizability derivative associated with the N]N stretch. Obvi-
ously, all three azocoumarin derivatives have a strong band (v(N]
N)) in the region 1439e1475 cmꢀl as shown in Fig. 5. The strong
Raman activity of v(N]N) in this series of compounds suggests that
Raman spectroscopy is a preferred means of identifying v(N]N) in
azo systems, especially in view of the difficulty in distinguishing v
(N]N) from aromatic absorptions in the IR. Additionally for
compound 4c, the vibration of the eNO2 group seen at 1342 cmꢀ1
Fig. 7. The chain structure formed via hydrogen bonds in 2c. The dashed lines indicate hydrogen bonds.

114
Y.-F. Sun et al. / Dyes and Pigments 87 (2010) 109e118
Fig. 8. A packing diagram for 2c, viewed down the a axis.
and acceptor (3-carbonylcoumarin system) are highly conjugated,
leading to a
-bridge for the charge transfer from the 2,20-
0.1250(3) nm is indicative of double-bond character. Similar
geometry has been observed in related azo dye ethyl 6-(4-
nitrophenyl)azo-2H-1-benzopyran-2-one-3-carboxylate (NPABC)
(Fig. 4), which were reported previously by us [30]. But
compared with 4a, the two ring systems in NPABC is almost
coplanar and form a dihedral angle of 3.7ꢁ. Thus, the significant
deviation from coplanarity in 4a might be attributed to the
presence of the fairly bulky p-trifluoromethyl moiety.
p
bithiophene ring system to 3-carbonylcoumarin system. These
characteristics are in good agreement with those of DPNP.
Meanwhile, the molecules are linked together by the weak
intermolecular CeH.O hydrogen bonds (C20eH20.O1: CeH ¼
0.093 nm, H20.O1 ¼ 0.2432 nm, C20.O1 ¼ 0.3356(3) nm,
C20eH20.O1 ¼172ꢁ) into a one dimensional zigzag chain running
along the b-axis (Fig. 7), which are cross-linked into a two-dimen-
sional framework by the weak intermolecular CeH.O hydrogen
bonds (C3eH3.O3i: CeH ¼ 0.093 nm, H3.O3 ¼ 0.2556 nm,
C3.O3 ¼ 0.3405(3) nm, C3eH3.O3 ¼ 152ꢁ; symmetry code:
(i) 2 ꢀ x, 2 ꢀ y, ꢀz;). Another interesting feature of the intermo-
Additionally, the weak intermolecular CeH.O hydrogen bonds
(C17eH17.O2ii: CeH
¼
0.093 nm, H17.O2
¼
0.2369 nm,
lecular interaction may be
pep stacking interactions between
molecules in the crystal lattice. As a result, the molecules are further
self-assembled to form an extended network structure (Fig. 8).
In 4a although the coumarin moiety is essentially planar, the
coumarin and phenyl rings, which adopt
a distorted trans
configuration about the N]N double-bond, are not coplanar with
a dihedral angle of 43.1ꢁ. As a result, the whole compound is not
a planar molecule (Fig. 9). Moreover, the N1]N2 bond length of
Fig. 9. (a) The molecular structure of 4a, showing the atom-labelling scheme.
Displacement ellipsoids are drawn at the 50% probability level. H atoms are shown as
small spheres of arbitrary radius. (b) The side elevation of 4a. H atoms are omitted for
clarity.
Fig. 10. The one dimensional structure formed via hydrogen bonds in 4a. The dashed
lines indicate hydrogen bonds.

Y.-F. Sun et al. / Dyes and Pigments 87 (2010) 109e118
115
Fig. 11. A packing diagram for 4a, viewed down the a axis.
C17.O2 ¼ 0.3256(3) nm, C17eH17.O2 ¼ 159ꢁ; C18eH18.O3iii:
CeH ¼ 0.093 nm, H18.O3 ¼ 0.2513 nm, C18.O3 ¼ 0.3382(3) nm,
C18eH18.O3 ¼ 156ꢁ; symmetry code: (ii) 2 ꢀ x, 2 ꢀ y, 1 ꢀ z; (iii)
1 ꢀ x, 2 ꢀ y, 1 ꢀ z) are also observed in the structure. The molecules
are linked together by such intermolecular hydrogen bonds into
a one dimensional structure extending along the a axis (Fig. 10),
appears around 305 nm with a shoulder towards longer wave-
length, and exhibits a blue-shift of 2e9 nm compared with the
corresponding peaks that of 4b and 4c. The results above imply that
less
p-electrons and shorter p-conjugated structure may be
involved in 4a than those in 4b and 4c.
which are further held together via
pep stacking interactions to
3.4. Photoluminescence spectra
form a two-dimensional supramolecular architecture. Finally, the
molecules are assembled to form a layer structure (Fig. 11).
The formation of ordered structures from the dyes 2c and 4a is
expected to have a potentially large impact on their optoelectronic
properties in the solid state.
The photoluminescence (PL) of coumarin dyes were examined
with a HeeCd laser of 325 nm at room temperature. To probe the
relationship between structure and spectral properties, the pho-
toluminescence of 2c is also discussed for comparative purposes
(Fig. 13). The coumarin dyes 3 and 4 exhibit different emission
colors when in the solid state, as shown in Fig. 14. The solid PL
spectrum for 2c exhibits (Fig. 13) three primary color emission
bands, a strong narrow blue emission at 404 nm and a strong green
emission at 524 nm, as well as a broad red emission at 697 nm
(Fig. 13(b)). These PL spectral characteristics are similar to those
observed in the corresponding chloroform solution of 2c (Fig. 13
(a)), in which the three corresponding emission bands were
located at 420, 534 and 737 nm, respectively. Noticeably, in the
solid state case we did not observe the splitting of the blue band,
which was noted for the chloroform solution. A blue-shift of about
10e40 nm in the blue, yellow and red emission maximum is
observed from solution to solid state.
3.3. UVeVis absorption spectra
The structures of the target molecules are shown in Fig. 4. As
seen from Fig. 4, for two coumarinepyrazolines, 3a and 3b, struc-
tural modification occurs only in one terminal unit, where
a coumarin moiety was replaced by a benzocoumarin system. On
the other hand, three molecules 4ae4c contain both a coumarin
core and an azo group. These molecules are related, but their
chemical structures are differentiated by introduction of various
substituents.
UVevis absorption spectra of these molecules in diluted chlo-
roform solutions (2 ꢂ 10ꢀ5 M) are given in Fig. 12. Compound 2a
shows a sharp absorption peak at 375 nm with a shoulder at
315 nm. The two coumarinepyrazolines (3a and 3b) have similar
absorption spectra, and three sharp absorption peaks located at
264, 294 and 421 nm, are observed. The first absorption band
0.7
4a
2a
0.6
(
lmax ¼ 421 nm) of pyrazoline 3a exhibits a red-shift of 46 nm
4c
compared to that of 2a (lmax ¼ 375 nm), which further confirmed
the formation of pyrazoline ring system. However, compared with
3a, a shoulder peak around 470 nm on the longer wavelength side
of the main peak is clearly evident in 3b. It is well known that
pyrazoline compounds are typical intra-molecular charge transfer
compounds [23,27]. Thus, this difference might be attributed to
the larger conjugation for benzocoumarin relative to coumarin
system. Additionally it is noted that lmax (421 nm) of the first
absorption band of pyrazoline 3a is very close to that of 1-(ben-
zothiazol-2-yl)-3-(6,8-di-tert-butyl -2-oxo-2H-1-benzopyran-3-
yl)-5-(4-N,N-dimethylaminophenyl)-2-pyrazoline (BDOBP) (Fig. 4)
0.5
3b
0.4
3a
0.3
4b
0.2
0.1
0.0
(
lmax ¼ 423 nm), reported by us previously [29], owing to their very
-0.1
similar structure.
250
300
350
400
450
500
550
Compound 4b exhibits absorption spectral features that are
similar to those of 4c with two major absorption bands at around
314 and 362 nm. But, in the case 4a, only one sharp absorption peak
Wavelength (nm)
Fig. 12. Absorption spectra of the coumarin-based dyes in chloroform solution.

116
Y.-F. Sun et al. / Dyes and Pigments 87 (2010) 109e118
of the emission peaks possibly result from the strong interactions
130
120
110
100
90
a
between molecules of 2c. The presence of the intermolecular
interactions have been demonstrated by X-ray diffraction crystal
structure analysis in this work. As mentioned above, the crystal
structure analysis results reveal that 2c is nearly planar, and the
main driving forces in the formation of the extended network
80
structure are intermolecular hydrogen bonding and
pep stacking
70
interactions. Consequently, crystal 2c with interchromophore
interactions displays the blue-shifts of the PL peaks, especially the
red band, which was enhanced, broadened and blue-shifted up to
40 nm compared to that in chloroform solution.
In contrast, only an emission peak could be observed in the
photoluminescence spectra of molecules 3 (Fig.14), which indicates
that the emission occurs from the lowest excited state with the
largest oscillator strength. Compound 3a shows a yellow emission
band centered at 562 nm while 3b displays a specific red emission
band centered at 635 nm. The emission peak of 3b is significantly
shifted to longer wavelength with respect to that of 3a, and both
60
50
40
30
20
10
0
350
400
450
500
550
600
650
700
750
800
Wavelength (nm)
exhibit red-shifted emission as compared to BDOBP (lmax
529 nm) [29].
¼
b1100
1000
The three azocoumarins 4ae4c have very similar photo-
luminescence spectra, and present a broad emission band with the
shoulder peak centered in the 590e650 nm wavelength region, the
maximum emission peaks undergo a red-shift from 4a (580 nm) to
4b (640 nm), and to 4c (650 nm). Obviously, the maximum emis-
sion peak of 4b is red-shifted by 60 nm, when compared to 4a, but
blue-shifted by about 10 nm with respect to that of 4c. It should be
noted that compound 4a has a very broad emission band spanning
the entire visible spectrum from 400 to 750 nm, which can be
attributed to intermolecular interactions within the crystal of 4a.
900
800
700
600
500
400
300
200
100
0
3.5. Quantum chemical calculations
-100
To gain a deeper insight into the electronic and luminescent
properties of 2c, we performed quantum chemical calculations.
The geometry structure of 2c was optimized at the ab initio Har-
treeeFock (HF) level with the standard 3-21G basis set (HF/3-21G),
all structure optimizations and energy calculations were performed
with the GAUSSIAN 98 program. The optimal structure of 2c is
depicted in Fig. 15.
It was found that the calculated results are very close to the
experimental values obtained from the X-ray single crystal
diffraction. As described above, in crystal structures, molecule of 2c
is nearly planar with a dihedral angle of 5.0ꢁ between the coumarin
ring and 2,2-bithiophene ring system. The HF calculations predict
the planar arrangement of the two ring systems for 2c, reflecting
the strong conjugation across the 3-carbonylcoumarin acceptor
group. The reason for the slight discrepancy is probably that
theoretical results belong to the isolated molecule and the experi-
mental values are attributed to the molecule in solid state. The
geometry of the solid state structure is subject to intermolecular
350
400
450
500
550
600
650
700
750
800
Wavelength (nm)
Fig. 13. PL spectra of 2c in chloroform solution (a) and in the solid state (b).
Such results may be due to the fact that the PL spectrum of
a luminescent molecule in dilute solution reflect mainly its single
molecular characteristic, while the PL spectrum of the same lumi-
nescent molecule in an organic crystal results from the interaction
of large numbers of molecules. So, in a crystal of 2c, the blue-shifts
13000
3b
12000
11000
10000
9000
3a
8000
7000
6000
5000
4b
4000
3000
4c
2000
4a
1000
0
350
400
450
500
550
600
650
700
750
800
Wavelength (nm)
Fig. 14. Solid state PL spectra of the coumarin-based dyes.
Fig. 15. Optimized structure of 2c by HF/3-21G method.

Y.-F. Sun et al. / Dyes and Pigments 87 (2010) 109e118
117
4. Conclusions
In summary, we have described the synthesis and detailed
characterization of some organic chromophores based on the
coumarin moiety. The PL spectra of these molecules are discussed
in the solid state. The resulting coumarins cover the emission
region from 370 to 750 nm, depending on the effective conjugation
length of chromophores. Significantly, the solid state PL spectrum
for 2c exhibits three primary color emission bands, having maxima
at 404, 524, and 697 nm, respectively, roughly corresponding to
blue, greenish blue, and red light. To the contrary, 4a displays
a quite different PL spectrum, which exhibits a very broad emission
band spanning the entire visible spectrum from 400 to 750 nm.
X-ray structural analysis show 2c to be monoclinic, space group
P21/c, while 4a is triclinic, space group P1. At the same time, the
molecular structure of 2c is optimized using HF/3-21G and the
HOMO and LUMO levels are deduced.
The coumarins are extremely variable in structure, due to the
various types of substituents in their basic structure, which can
influence their optical properties. Thus, the totality of the results
suggests that this system might be effective for the development of
long wavelength emissive and white-light-emitting chromophores,
and the coumarins are promising candidates for applications in
molecular electronics and biological imaging. Our work is currently
focused on the further development of these coumarin systems.
Fig. 16. Molecular orbital surface and energies of the main frontier molecular orbitals
for 2c.
Supplementary material
forces, such as van der Waals interactions, crystal packing forces
and hydrogen bond forces [34]. Although there is a slight difference
between the calculated and experimental values, the general
qualitative conclusion drawn from our calculations is consistent
with the experimental one.
In an effort to find some clues about the experimental results,
The highest occupied molecular orbital (HOMO) and lowest unoc-
cupied molecular orbital (LUMO) energy levels diagrams of dye 2c
are also calculated to check the smooth transfer of electron density
when the dye is excited by light. HOMO and LUMO levels of 2c are
shown in Fig. 16.
The crystallographic data (excluding structure factors) of 2c and
4a have been deposited with the Cambridge Crystallographic
Center as supplementary publication no. 750904 and 750 905. Copy
of this information may be obtained free of charge via www: http://
www.ccdc.cam.ac.uk or from The Director, CCDC, 12 Union Road,
Cambridge CB221EZ, UK (fax: þ44 1223/336 033; email: deposite@
ccdc.cam.ac.uk). Structural factors are available on request from the
authors.
Acknowledgements
Generally, the energy values of LUMO and HOMO and their
energy gap reflect the chemical activity of the molecule. HOMO as
an electron donor represents the ability to donate an electron,
while LUMO as an electron acceptor represents the ability to obtain
an electron. The smaller the LUMO and HOMO energy gaps, the
easier it is for the HOMO electrons to be excited [35].
This project was supported by the National Natural Science
Foundation of China (No. 60877024 and 60708024) and the Foun-
dation of Taishan University (No. Y04-2-02).
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These findings are important for the better understanding of its
UVeVis absorption and PL properties.
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