Fluorescent coumarin derivatives with large stokes shift, dual emission and solid state luminescent properties: An experimental and theoretical study

Article abstract of DOI:10.1016/j.dyepig.2011.09.023

Coumarin derivatives containing 8-benzothiazole (C-2) and its difluoroboron bound derivative (C-3) were prepared. Both derivatives show dual emission at 322 nm and 513 nm and large Stokes shift (188 nm), compared to the unsubstituted coumarin (C-1), which shows emission at 356 nm with small Stokes shift of 46 nm. C-2 and C-3 show fluorescence in solid state, in contrast the C-1 is non-fluorescent in the solid state. The excited state intramolecular proton transfer (ESIPT) process of C-2 was fully rationalized by DFT/TDDFT calculations with optimization of the ground state (S₀) and excited state (S ₁) geometries. TDDFT calculations propose that the large Stokes shift of C-2 and C-3 are due to the re-distribution of the frontier molecular orbitals at excited states. Study of the potential energy curve of C-2 indicated that the dual emission of the C-2 is due to the simultaneous S₁ and S₃ emission, not the rotamer of the enol form.

Full text of DOI:10.1016/j.dyepig.2011.09.023

Dyes and Pigments 92 (2012) 1361e1369
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Fluorescent coumarin derivatives with large stokes shift, dual emission and solid
state luminescent properties: An experimental and theoretical study
a
,
b
,
*, Yinghui Chen a b, Wenting Wu , Huimin Guo , Jianzhang Zhao ,*, Xuerong Yu b
,
b
b
b
*
Lijuan Xie
a
Institute of Molecular Medicine, Huaqiao University, 269 Chenghua North Road, Quanzhou 362021, PR China
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, E-208 West Campus, 2 Ling-Gong Road, Dalian,
b
Liaoning Province 116024, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 August 2011
Received in revised form
Coumarin derivatives containing 8-benzothiazole (C-2) and its difluoroboron bound derivative (C-3)
were prepared. Both derivatives show dual emission at 322 nm and 513 nm and large Stokes shift
(
188 nm), compared to the unsubstituted coumarin (C-1), which shows emission at 356 nm with small
27 September 2011
Stokes shift of 46 nm. C-2 and C-3 show fluorescence in solid state, in contrast the C-1 is non-fluorescent
in the solid state. The excited state intramolecular proton transfer (ESIPT) process of C-2 was fully
rationalized by DFT/TDDFT calculations with optimization of the ground state (S ) and excited state (S )
0 1
Accepted 28 September 2011
Available online 6 October 2011
geometries. TDDFT calculations propose that the large Stokes shift of C-2 and C-3 are due to the re-
distribution of the frontier molecular orbitals at excited states. Study of the potential energy curve of
C-2 indicated that the dual emission of the C-2 is due to the simultaneous S and S emission, not the
1 3
rotamer of the enol form.
Keywords:
Coumarin
ESIPT
Fluorescence
Density functional theory
Photochemistry
Photophysics
Ó 2011 Elsevier Ltd. All rights reserved.
1
. Introduction
Stokes shift [12,13]. We found that the emission wavelength was
extended from 356 nm for the parent compound (C-1) to the 513 nm
for the new compounds (C-2 and C-3). The Stokes shift of the
derivatives is up to 188 nm, compared to the small Stokes shift of
46 nm for the parent compound C-1. Furthermore, the fluorescence
quantum yields of the derivatives are higher than the parent
compound (C-1). Significant dual emission (at 363 nm and 513 nm)
was observed for C-2. Based on density functional theory (DFT)
calculations for ground state and excited state, the ESIPT process was
fully rationalized. We exclude the previously proposed possibility of
the rotamer to be responsible for the dual emission [14]. Instead, we
Coumarin derivatives have been widely used in fluorescent
molecular probes, electroluminescence, light-harvesting molecular
assemblies or photovoltaics [1e7]. However, the unsubstituted
coumarin (7-hydroxyl-4-methyl-coumarin) fluorophore suffers
from disadvantages of small Stokes shift, which is detrimental to
the applications of coumarin, such as for fluorescent molecular
probes, etc [1,4]. Therefore, it is of great interest to prepare
coumarin derivatives that show large Stokes shifts. Concerning this
aspect, one approach is to extend the p-conjugation framework of
the fluorophore [5,8e10]. However, with red-shifted emission
wavelength, the fluorescence quantum yield usually decreases, due
to the energy gap law [11]. Thus a strategy has to be employed to
extend the emission to the red-end of the spectra, but at the same
time, to maintain high fluorescence quantum yields.
propose that the dual emission is due to the radiative decay of the S
and S excited states of C-2. This assumption is supported by the dual
1
3
emission of C-3, for which the enol structure does not exist. Our
complementary experimental and theoretical investigation of the
coumarin fluorophores will be useful for design of new coumarin
based fluorophores that show red-shifted emission, large Stoke shift
and solid state luminescence property.
In order to address the aforementioned challenges, herein we
prepared benzothiazole-containing coumarin derivatives (C-2, C-3
3
was prepared by complexation with BF . Scheme 1) to employ the
excited state intramolecular proton transfer (ESIPT) to produce large
2. Experimental section
2.1. General information
*
Corresponding author. Tel./fax: þ86 411 8498 6236.
*
* Corresponding author. Institute of Molecular Medicine, Huaqiao University, 269
NMR spectra were measured on a 400 MHz Varian Unity Inova
Chenghua North Road, Quanzhou 362021, PR China.
E-mail addresses: xielijuan@hqu.edu.cn (L. Xie), zhaojzh@dlut.edu.cn (J. Zhao).
spectrophotometer. Mass spectra were recorded with a Q-TOF
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.09.023

1362
L. Xie et al. / Dyes and Pigments 92 (2012) 1361e1369
OH
a
O
O
b
+
O
HO
O
O
HO
O
CHO
O
OH
C-1
HO
O
O
O
F B
F
O
O
c
d
N
S
N
S
C-2
C-3
ꢀ
ꢀ
Scheme 1. Synthesis of the compounds C-1, C-2 and C-3. (a) THF, 96% H
2
SO
4
, reflux for 3 h. (b) Hexamine, glacial acetic acid, 95 C, 5.5 h; 20% HCl, 70 C, 1 h. (c) ethyl cyanoacetate,
ꢁ
benzoic acid, n-butanol, reflux for 5 h. (d) BF
3
OEt
2
, TEA, CH
2
Cl
2
.
Micro MS spectrometer. UVeVis spectra were carried on an Agilent
HP8453 UVevisible spectrophotometer. Fluorescence spectra were
recorded on a JASCO FP-6500 and Shimadzu 5301PC spectrofluo-
rometer. Luminescence quantum yields were measured with
2.2.2. 8-Formyl-7-hydroxy-4-methyl-coumarin
7-Hydroxy-4-methyl-coumarin (5.3 g, 0.03 mol) and hexamine
(9.8 g, 0.07 mol) in glacial acetic acid (50 mL) were refluxed for
5.5 h, and then 20% HCl (75 mL) was added and the solution was
heated for 1 h. After cooling, the reaction mixture was extracted
with ether, and the combined organic layers were evaporated
under reduced pressure. The residue was poured into ice water, and
pale yellow solid of 8-formyl-7-hydroxy-4-methyl-coumarin was
obtained. The further purification was carried out by recrystalli-
zation with hot ethanol to obtain a light yellow powder (612.1 mg,
quinine sulfate as the standard (
Luminescence lifetimes were measured on an OB920 fluorescence
F
F
¼ 54.6% in 0.05 M H
2 4
SO ).
lifetime spectrometer (Edinburgh Instruments, U.K.).
2
2
.2. Synthesis
ꢀ
ꢀ
ꢀ
ꢀ
1
Yield 10%). M.p. 180 Ce181.0 C (lit. 120.0 Ce122.0 C) [15].
NMR (400 MHz, CDCl ), 12.23 (s, 1H); 10.63 (s, 1H); 7.52 (d, 1H,
J ¼ 9 Hz); 6.92 (d, 1H, J ¼ 9 Hz); 6.21 (s, 1H); 2.43 (s, 3H).
H
.2.1. C-1 [15]
3
d
Concentrated sulphuric acid (2 mL) was added into the solution
of resorcinol (11.0 g, 0.1 mol) in tetrahydrofuran (THF, 40 mL), then
acetoacetic ester (13.0 g) was added dropwise. The reaction
mixture was refluxed for 3 h and monitored by TLC until the
resorcinol was completely consumed. After cooling, the solution
was poured into ice water (100 mL), yellow precipitate was
collected by filtration and washed with cold water. Pale yellow solid
was obtained by recrystallization with 95% ethanol (11.3 g, Yield: 64
2.2.3. C-2
A mixture of 8-formyl-7-hydroxy-4-methyl-coumarin (204.0 mg,
mmol), ethyl cyanoacetate (0.11 mL, 1 mmol) and o-amino-
1
thiophenol (0.11 mL, 1 mmol) in n-butanol (10 mL) containing ben-
zoic acid (0.3 mmol) was refluxed for 5 h. After cooling, the solid was
filtered and washed with water. Further purification was employed
ꢀ ꢀ
1
%
). M.p. 185.0 Ce186.0 C. H NMR (400 MHz, CD OD), d 7.61
3
(
2
d, 1H, J ¼ 8.9 Hz), 6.83 (d, 1H, J ¼ 10 Hz), 6.70 (s, 1H), 6.10 (s, 1H),
þ
.42 (s, 3H). TOF MS EI calcd 176.0473, found 176.0469.
5
4
3
2
1
00
00
00
00
00
0
C-3
C-2
0.9
0.6
0.3
0.0
C-3
C-2
C-2
C-3
C-1
C-1
300
400
500
600
300
350
400
Wavelength / nm
Wavelength / nm
ꢃ5
ꢃ5
Fig. 1. UVeVis absorption of C-1, C-2 and C-3 (1.0 ꢂ 10 mol/L) in toluene at room
Fig. 2. Excitation spectra and emission spectra of C-1, C-2 and C-3 (1.0 ꢂ 10 mol/L) in
ꢀ
ꢀ
temperature 20 C.
toluene at room temperature.
l
ex/C-1 ¼300 nm,
l
ex/C-2 ¼ 310 nm,
l
ex/C-3 ¼ 310 nm 20 C.

L. Xie et al. / Dyes and Pigments 92 (2012) 1361e1369
1363
Table 1
3. Results and discussion
3.1. UVevis absorption and fluorescence emission spectra
Photophysical properties of C-1, C-2 and C-3.
ꢃ1
cm )a
ꢃ1
a
s
F
(ns)a
Compounds
l
abs
l
em
ε (M
F
F
/nm)a
(/nm)
a
(
C-1
C-2
325
322
356
15,000
46,000
2.56%
4.07%
11.84
8.73 (360 nm),
The UVevis absorptions of the compounds were presented in
Fig. 1. It was observed that the absorption of C-2 and C-3 were
enhanced compared to that of C-1. Furthermore, the absorption of
C-2 and C-3 are relatively red-shifted compared to the absorption of
C-1 [9]. The molar extinction coefficients of C-1 at 325 nm is
363, 514
340, 514
2
.44 (530 nm)
9.97 (360 nm),
.59 (530 nm)
C-3
322
34,000
7.69%
2
a
Measurements were performed in toluene solvent. Fluorescence quantum
ꢃ1
ꢃ1
1
5,000 M cm . C-2 and C-3 show
3
value of 45,000 and
yields were measured with quinine sulfate (
reference.
F
F
¼ 54.6% in 0.05 M H
2 4
SO ) as the
ꢃ1
ꢃ1
34,000 M cm at the same wavelength.
The fluorescence excitation and emission spectra of the
compounds were studied (Fig. 2). The parent compound C-1 shows
excitation band at 310 nm and emission band at 356 nm. Thus the
Stokes shift is 46 nm. For C-2, however, the excitation maximum is
at 325 nm and the emission band is significantly red-shifted to
with column chromatography (silica gel, dichloromethane), afford-
ing C-2 as a pale yellow solid (170.0 mg, Yield 55%) [10]. M.p.
ꢀ ꢀ
1
2
40.0 Ce240.8 C. H NMR (400 MHz, CDCl ), d 8.07 (d, 1H,
3
513 nm. Thus the Stokes shift of C-2 is 188 nm. C-3 shows excitation
J ¼ 8.1 Hz); 8.02 (d, 1H, J ¼ 7.9 Hz); 7.64 (d, 1H, J ¼ 8.8 Hz); 7.59
and emission bands similar to that of C-2. To the best of knowledge,
the Stokes shifts of C-2 and C-3 are larger than most of the
coumarin fluorophores [1,4,9]. Since the spectra were measured
under the same conditions thus we can anticipate high fluores-
cence quantum yields for C-2 and C-3 compared to C-1.
Interestingly, dual emission bands at 363 nm and 514 nm were
observed for C-2. Similarly minor emission band at 340 nm was
observed for C-3, along the major emission band at 514 nm. To the
best of knowledge, very few coumarin fluorophores show the dual
emission properties. The multi emission of organic fluorophores is
in particular interesting for applications such as white light elec-
troluminescence or luminescent bioimaging [9,17e19].
(
m, 1H, J ¼ 8.3 Hz, 7.0 Hz); 7.50 (m, 1H, J ¼ 7.3 Hz, 8.7 Hz); 7.11 (d, 1H,
þ
J ¼ 8.9 Hz); 6.22 (s, 1H); 2.47 (s, 3H). TOF MS EI calcd 309.0461,
found 309.0460.
2
.2.4. C-3
-benzothiazoly-7-hydroxy-4-methyl-coumarin (61.8 mg, 0.2
mmol) was resolved in CH Cl (10 mL), and then one drop of trie-
thylamine and BF .OEt (0.1 mL) were added successively. The
reaction solution was stirred for 30 min. Then crude product was
purified by silica gel column chromatography (eluted with CH Cl
to give C-3 as a pale yellow solid (17.9 mg, Yield 25%). M.p.
8
2
2
3
2
2
2
)
ꢀ ꢀ
1
2
33.7 Ce234.1 C. H NMR (400 MHz, CDCl ), d 8.07 (d, 1H,
3
The photophysical properties of the compounds were summa-
rized in Table 1. Interestingly, C-2 shows unexpected high fluores-
J ¼ 9.1 Hz); 8.02 (d, 1H, J ¼ 7.9 Hz); 7.64 (d, 1H, J ¼ 8.7 Hz); 7.59
(
(
3
m, 1H, J ¼ 6.7 Hz, 8.2 Hz); 7.50 (m, 1H, J ¼ 7.9 Hz, 7.6 Hz); 7.11
þ
cence quantum yields (
that show excited state intramolecular proton transfer (ESIPT),
which usually give quantum yield of less than 1% [12].
F
F
¼ 4.07 %) than most of the fluorophores
d, 1H, J ¼ 8.9 Hz); 6.22 (s, 1H); 2.48 (s, 3H). TOF MS EI calcd
09.0461, found 309.0460.
The solvent polarity dependence of the compounds’ emission is
studied (Fig. 3). For C-1, the emission is greatly red-shifted in highly
polar solvents such as methanol and water. For C-2, the emission
intensity is decreased by increasing the solvent polarity. We
noticed that the dual emission of C-2 is persistent in most solvents.
Interestingly, we found that the intensity varied with the solvent
polarity, but the emission wavelength did not change. This property
is different from the normal behavior of organic fluorophores that
gives red-shifted emission in polar solvents [20]. We tentatively
assign the decreased emission intensity of C-2 in polar solvents to
a non-radiative decay channel which is enhanced by high polarity.
Considering the potential ESIPT, the dual emission can be
2
.3. Computational methodology
All the calculations were based on density functional theory
(
DFT) with B3LYP functional and 6-31G(d) basis set. Toluene was
used as solvent in all the calculations (PCM model). The UVevis
absorption of the compounds (vertical excitation) was calculated
with the TDDFT methods based on the optimized ground state
geometry (S
wavelength was calculated based on the optimized excited states
geometries (S , S or S state). All these calculations were per-
0
state). For the fluorescence emission, the emission
1
2
3
formed with Gaussian 09W [16].
a
b
c
2
50
00
PhCH3
DCM
THF
in PhMe
400
2
in CH Cl2
2
in CH Cl
2
2
00
00
0
300
2
EtOAc
in PhCH3
in H O
CH CN
150
00
2
3
in EtOAc
in THF
in MeOH
in EtOAc
in THF
2
00
00
0
1
in H O
2
in MeOH
1
in H O
1
2
in MeCN
in MeOH
5
0
MeCN
0
400
500
600
400
500
600
3
50 400 450 500 550
Wavelength / nm
Wavelength / nm
Wavelength / nm
ꢃ5
Fig. 3. Emission spectra of (a) C-1 and (b) C-2 (1.0 ꢂ 10 mol/L) in different kinds of solvent at room temperature.
l
ex/C-1 ¼300 nm. ex/C-2 ¼ 310 nm. (c) Emission spectra of C-3
l
ꢃ5
ex/C-3 ¼ 310 nm, 20 ꢀC.
(
1.0 ꢂ 10 mol/L) in different kinds of solvent at room temperature.
l

1364
L. Xie et al. / Dyes and Pigments 92 (2012) 1361e1369
Table 2
Electronic excitation energies (eV) and corresponding oscillator strengths (f), main
configurations and CI coefficients of the low-lying electronically excited states of C-2
calculated by TDDFT//B3LYP/6-31G(d), based on the DFT//B3LYP/6-31G(d) optimized
ground state and excited state geometries.
Electronic TDDFT//B3LYP/6-31G(d)
a
transition
Energy
fb
Compositionc
CId
Excitation
S
S
0
0
/ S
/ S
1
2
3.65 eV/ 0.2620 HOMO / LUMO
0.5982
0.1717
0.3191
0.5910
0.2943
0.2230
3
40 nm
HOMO ꢃ 1 / LUMO
HOMO / LUMO þ 1
3.80 eV/ 0.2571 HOMO ꢃ 1 / LUMO
326 nm
HOMO / LUMO
HOMO / LUMO þ 1
Emission
S
S
S
0
0
0
/ S e
1
2.51 eV/ 0.2138 HOMO / LUMO
95 nm
3.42 eV/ 0.0000 HOMO ꢃ 2 / LUMO
63 nm
3.44 eV/ 0.3541 HOMO / LUMO þ 1
61 nm
0.7024
4
/ S
/ S
2
0.6938
3
HOMO ꢃ 2 / LUMO þ 1 0.1041
0.6713
f
3
3
a
Only selected excited states were considered. The numbers in parentheses are
the excitation energy in wavelength.
Fig. 4. Emission spectra of solid C-1, C-2 and C-3 at room temperature.
ꢀ
l
ex/C-1 ¼300 nm,
l
ex/C-2 ¼ 310 nm,
l
ex/C-3 ¼ 310 nm 20 C.
b
Oscillator strength.
Only the main configurations are presented.
c
d
Coefficient of the wavefunction for each excitations. The CI coefficients are in
tentatively assigned to the enol rotamer and the cis keto form
absolute values.
e
(
produced by the ESIPT) [14].
Based on the optimized S
Based on the optimized S
1
3
state geometry.
state geometry.
f
We found that C-2 and C-3 are highly fluorescent in solid state
Fig. 4). Intense greenish or blue emission were observed upon
(
illumination of the solid with the handhold UV lamp (365 nm). By
comparison C-1 is non-fluorescent in the solid state. Materials
fluorescent in solid state could be used for electroluminescence
devices [21].
3.2. DFTcalculations: rationalize the excitation and emission spectra
Recently theoretical calculations are becoming attractive for
study of the fluorescence or molecular probes [22,23]. Previously
Fig. 5. Optimized ground state (S
0
state, top row) and the singlet excited state (S
1
state, bottom row) geometry of the compounds C-1, C-2 and C-3. Solvent toluene was considered
in the calculation (PCM model). Calculated at B3LYP/6-31g(d) level with Gaussian 09W.

Fig. 6. Frontier MOs involved in the excitation and emission of C-2 and rationalization of the dual emission of C-2 and the ESIPT process. The vertical excitation (UVevis absorption) is
calculated based on the optimized ground state geometry (S ) and the emission is calculated based on the optimized excited state geometry. Calculated at DFT//B3LYP/6-31G(d) level with
Gaussian 09W. ESIPT stands for excited-state-intramolecular-proton-transfer; CT stands for conformation transformation; GSIPT stands for ground state intramolecular proton transfer.
0

1366
L. Xie et al. / Dyes and Pigments 92 (2012) 1361e1369
with DFT calculations we studied the fluorescence sensing mech-
anism of thiol probes [24e27], the emission of phosphorescent
transition metal complex, etc [28]. The UVevis absorption and the
fluorescence emission (include the dual emission) of the
compounds described herein were investigated by theoretical
calculations based on the density functional theory (DFT) method.
Table 3
Electronic excitation energies (eV) and corresponding oscillator strengths (f), main
configurations and CI coefficients of the low-lying electronically excited states of
enol rotamer C-2 calculated by TDDFT//B3LYP/6-31G(d), based on the DFT//B3LYP/6-
31G(d) optimized ground state geometries.
Electronic
transition
TDDFT//B3LYP/6-31G(d)
Energya
fb
Compositionc
CId
Firstly geometry of the compounds at ground state (S
singlet excited state (S ) were optimized (Fig. 5). Generally the
compounds take coplanar geometry at the S state. For C-2, we
0
) and the
Excitation
S
0
/ S
1
3.97 eV/
0.1648
HOMO / LUMO
0.5958
0.1805
0.2961
0.3238
0.6118
1
3
12 nm
HOMO ꢃ 1 / LUMO
HOMO / LUMO þ 1
HOMO / LUMO
HOMO / LUMO þ 1
0
found that the optimized ground state (S
in line with the reported result of the 2-(2 -hydroxynaphthyl)
0
) is the enol form. This is
S
S
0
0
/ S
/ S
2
1
4.18 eV/
297 nm
0.2589
0.1267
0
benzazoles compounds [29]. The lowest-lying excited state (S
the compounds were also optimized. Interestingly, we found that
that C-2 changed to the keto form at the S excited state, as
1
) of
Emission
3.35 eV/
HOMO / LUMO
0.6559
0.2065
0.1311
0.6800
0.1365
3
70 nm
HOMO / LUMO þ 1
HOMO ꢃ 1 / LUMO
HOMO ꢃ 1 / LUMO
HOMO / LUMO
1
a result of the ESIPT (please note the initial structure for the
optimization is enol form). This theoretical prediction is in full
agreement with the previous experimental results of the similar
compounds [29,30].
In order to study the UVevis absorption and the fluorescence
emission of the compounds, the time-dependent DFT (TDDFT)
calculations were carried out (Table 2). The calculated UV-vis
absorption is in good agreement with the experimental results
S
0
/ S
2
3.71 eV/
34 nm
0.6068
3
a
Only selected excited states were considered. The numbers in parentheses are
the excitation energy in wavelength.
b
Oscillator strength.
Only the main configurations are presented.
Coefficient of the wavefunction for each excitations. The CI coefficients are in
c
d
absolute values.
(
Table 1). For example, the calculated absorption maxima is at
40 nm, which is very close to the experimentally observed
absorption at 325 nm (Fig. 1). The frontier molecular orbitals (MOs)
are distributed on the whole -conjugation framework (Fig. 6).
In order to study the emission of C-2, the S / S excitation
energy was calculated with the TDDFT method based on the opti-
mized S state geometry. The calculated S eS energy gap, i.e. the
emission wavelength, is 495 nm (Table 2), which is in good agree-
ment with the experimental results (513 nm, Fig. 2). The frontier
MOs involved in the emission of C-2 were presented in Fig. 6.
3
more stabilized by 0.33 eV than the LUMO at the S0 geometry.
However, the HOMO at the S state is destabilized by 0.73 eV (based
1
p
on optimized S state geometry). The much smaller HOMO-LUMO
1
0
1
energy gap at the S1 geometry than that at the S0 geometry can
be used to rationalize the large Stokes shift of C-2. We attribute the
different HOMOeLUMO energy gap to the ESIPT process, which is
clearly predicted by the DFT/TDDFT calculations.
1
0
1
For C-1, however, no such large geometry changes were
observed for S and S state, and a small Stokes shift was observed
1
0
Compared to the MOs of the ground state (S
state is more localized. For example, the HOMO is localized in the
coumarin core and the LUMO is more localized on the benzothia-
0
state), the MOs at S
1
for C-1. This theoretical approach to predict the emission property
of organic fluorophores can be used for design of new fluorophore
with large Stokes shift.
zole moiety, which are different from the case of MOs at the S
state. Furthermore, we found that the LUMO of C-2 at the S state is
0
We found a higher excited state S with emission wavelength of
361 nm, this is consistent with the emission band of C-2 at 363 nm.
3
1
Fig. 7. Geometry of the C-2 enol rotamer at the ground state (S
Calculated at B3LYP/6-31g(d) level with Gaussian 09W. The dihedral angle: S
0 1
, left) and the singlet excited state (S , right). Solvent toluene was considered in the calculation (CPCM model).
ꢀ ꢀ
1
state, dihedral angle: 60 ; S state, dihedral angle: 33.9 .
0

L. Xie et al. / Dyes and Pigments 92 (2012) 1361e1369
1367
Considering the large energy gap between the S
3
and S
1
state, we
3.3. Is rotamer responsible for the emission at shorter wavelength
propose that emissive transition of S / S is possible [31]. This
3
0
of the dual emissive C-2?
postulation is supported by the emission of C-3, for which there is
no ESIPT (e.g. no enol rotamer exist), but the compound still show
dual emission. This postulation is against the previous claim that
the minor emission at shorter wavelength is due to the enol
rotamer at the ground state [29,30].
In order to study the property of the anticipated enol rotamer of
C-2, the proposed enol rotamer structure at the S and S state were
0 1
optimized (Fig. 7). For the ground state geometry, the coumarin
core and the benzothiazole of the molecule take a twisted
Fig. 8. The frontier MOs involved in the vertical excitation and the emission of C-2 enol rotamer. Calculated with TDDFT based on the optimized ground state (S
0
) and the lowest-
lying singlet excited state (S
transformation.
1
) geometry. Solvent toluene was considered in the calculation. Calculated at B3LYP/6-31g(d) level with Gaussian 09W. CT stands for conformation

1368
L. Xie et al. / Dyes and Pigments 92 (2012) 1361e1369
ꢀ
conformation, with dihedral angle of 60 . At the S
1
state, however,
angle between the coumarin core and the benzenthioazole moiety
indicated a very small barrier for the transformation of the rotamer
to the enol form (0.8 kJ mol ), thus the previously proposed
the two components takes a more coplanary geometry, the dihedral
ꢀ
ꢃ1
angle is 33.9 .
The UVevis absorption and the emission of enol rotamer were
calculated based on the optimized S state and S state geometry,
0 1
rotamer form is impossible to co-exist with the enol form of C-2.
Therefore we propose the dual emission of the C-2 is due to the
respectively, with the TDDFT method (Table 3). The calculated
absorption band is located at 312 nm, which is much shorter than
the normal enol form (Table 2). The HOMO and LUMO involved in
the vertical excitation were presented in Fig. 8. HOMO is localized
in the coumarin core and the LUMO is localized in the coumarin and
1 3
simultaneous S and S emission, which are fully rationalized by
TDDFT calculations (with optimization of the excited states). Our
findings will be useful for design of new fluorescent dyes with large
Stokes shift and to study the photophysical properties of ESIPT
0
fluorescent dyes, such as those with 2-(2 -hydroxyphenyl)-benzo-
the benzothiazole moeity and basically S
0
/ S
1
is a charge transfer
thiazole structures.
excitation. The emission of the enol form is calculated as 370 nm,
which is in good agreement with the experimentally observed
emission band of C-2 at 363 nm.
Acknowledgments
In order to study the possibility of the co-existence of the
rotamer (Fig. 7) and the enol form (Fig. 5), we studied the potential
energy curve of the C-2 with variation of the dihedral angle of the
CeC bond which connects the coumarin core and the benzothiazole
moiety (Fig. 9). Although the rotamer as a local minimum was
obtained with DFT geometry optimization (Fig. 7), the potential
We thank the National Natural Science Foundation of China
(NSFC) (20972024 and 21073028), Fundamental Research Funds for
the Central Universities (DUT10ZD212 and DUT11LK19), The Royal
Society (UK) and NSFC (China) for the ChinaeUK Cost-Share
Program (21011130154), the State Key Laboratory of Fine Chem-
icals (KF0802) and the Ministry of Education of China (SRFDP-
200801410004 and NCET-08-0077) for financial support.
ꢃ1
energy curve gives a very small energy barrier of 0.8 kJ mol for
the transformation of rotamer to the intramolecular hydrogen
bonded enol form (Fig. 5). Thus it is impossible for the rotamer
Appendix. Supplementary data
(Fig. 8) to exist at room temperature. Therefore, we propose that
the dual fluorescence of C-2 is due to the S / S and S / S
1
0
3
0
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.dyepig.2011.09.023.
emission of the enol form of C-2, not the emission from the enol
form and the rotamer. The dual emission of C-2 is fully rationalized
by the TDDFT calculations (Table 2).
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