Synthesis, photoluminescent behaviors, and theoretical studies of three novel ketocoumarin derivatives containing an azo moiety

Article abstract of DOI:10.1016/j.molstruc.2011.11.034

Three new coumarin derivatives, namely, 6-phenylazo-3-[3-(4-formylphenyl) prop-2-enoyl]-coumarin (2a), 6-phenylazo-8-methoxy-3-[3-(4-formylphenyl)prop-2- enoyl]-coumarin (2b), and 6-(4-methoxyphenyl)azo-3-[3-(4-formylphenyl)prop-2- enoyl]-coumarin (2c), w

Full text of DOI:10.1016/j.molstruc.2011.11.034

Journal of Molecular Structure 1011 (2012) 19–24
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, photoluminescent behaviors, and theoretical studies of three novel
ketocoumarin derivatives containing an azo moiety
⇑
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
Article history:
Received 30 August 2011
Received in revised form 20 November 2011
Accepted 21 November 2011
Available online 14 December 2011
Three new coumarin derivatives, namely, 6-phenylazo-3-[3-(4-formylphenyl)prop-2-enoyl]-coumarin
(2a), 6-phenylazo-8-methoxy-3-[3-(4-formylphenyl)prop-2-enoyl]-coumarin (2b), and 6-(4-methoxy-
phenyl)azo-3-[3-(4-formylphenyl)prop-2-enoyl]-coumarin (2c), were synthesized and characterized.
The three compounds exhibited high fluorescence quantum yields, strong blue emissions under UV light
excitation, and high thermal stabilities, as determined by thermogravimetric (TG) analysis. The excited-
state geometry was optimized at the CIS level of theory, whereas the B3LYP level of theory was applied
for the ground state. The absorption and emission spectra were calculated using the time-dependent den-
sity functional theory (TD-DFT) in combination with the polarized continuum model (PCM). Resonance
frequency calculations were undertaken to study the infrared spectra of the three compounds. The calcu-
lated results are in very good agreement with the experimental data.
Keywords:
Coumarin
Synthesis
Fluorescence
TD-DFT studies
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
isomerization of the azo bond, with concomitant change in the
structure, dipole moment, and optical properties [15–17].
Coumarin and its derivatives have attracted much interest in
several fields and represent an important class of organic heterocy-
cles that can be found in many natural or synthetic drugs. These
compounds possess versatile biological activities, such as antioxi-
dant [1] and anti-inflammatory activities [2]. They also have out-
standing optical properties [3–6], such as sufficient fluorescence
in the visible light range, large Stokes shift, high photolumines-
cence quantum yield, and superior photostability. In addition, the
absorption and fluorescence characteristics of the coumarin deriv-
atives can be readily modified by changing the nature of the sub-
stituents, the substituted position in the coumarin ring, and the
solvents used [7,8], giving them an increased flexibility to fit well
in various applications, including as optical brighteners, laser dyes,
non-linear optical chromophores, solar energy collectors, fluores-
cent labels and probes in biology and medicine, electroluminescent
(EL) materials, and two-photon absorption (TPA) materials [9–11].
For example, ketocoumarins, which contain a chalcone group in
the 3-position, are a family of excellent fluorescent dopants widely
used as doped emitters in organic light-emitting diodes (OLEDs). A
number of ketocoumarin-based EL materials have been developed
in recent years [12–14].
In the current study, three new ketocoumarins, namely,
6-phenylazo-3-[3-(4-formylphenyl)prop-2-enoyl]-coumarin (2a),
6-phenylazo-8-methoxy-3-[3-(4-formylphenyl)prop-2-enoyl]-
coumarin (2b), and 6-(4-methoxyphenyl)azo-3-[3-(4-formylphe-
nyl)prop-2-enoyl]-coumarin (2c), which all contain an azo group
in the 6-position, were synthesized and their UV–vis absorption
and PL properties were investigated in detail.
2. Experimental
2.1. Materials and methods
IR spectra (400–4000 cmꢀ1) 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. TG analysis was carried
out on a Q500 thermal analysis instrument. Melting points were
taken on a RY-1 micro melting apparatus; the thermometer was
uncorrected. UV–vis absorption and emission spectra were re-
corded using a Thermo Evolution 300 spectrometer and a Cary
Eclipse spectrometer, respectively. All the chemicals were commer-
cially available and were used without further purification. All the
solvents were dried using standard methods before use.
Recently, azo compounds have also gained strong interest be-
cause of their unique photochemical and photophysical properties.
Azo chromophores can undergo light-driven, reversible trans–cis
2.2. Synthesis and characterization of compounds 1a–1c
⇑
A 20% aq. NaNO2 solution (20 mL) was slowly added to a solu-
tion of aniline (p-methoxyaniline, 0.05 mol) in 37% aq. HCl
Corresponding author. Tel.: +86 22 27404208; fax: +86 22 27892279.
E-mail address: lixianggao@hotmail.com (X. Li).
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.11.034

20
J. Li et al. / Journal of Molecular Structure 1011 (2012) 19–24
(10 mL) at 0–5 °C and stirred for 1 h. The resulting bright yellow
diazonium salt solution was added dropwise to a cold, alkaline
solution of salicylaldehyde (3-methoxysalicylaldehyde, 0.05 mol)
in 2% aq. NaOH solution (100 mL). The crude brown solid was then
filtered and recrystallized from ethanol to obtain the pure yellow
product.
Piperdine (2 mL) was added to a mixture of azosalicylaldehydes
(0.05 mol) and ethylacetoacetate (6.3 mL, 0.05 mol) in ethanol
(200 mL) under rapid stirring, and the reaction mixture was re-
fluxed for 5 h. After cooling, the solid was filtered and recrystal-
lized from ethanol to obtain the pure compounds 1a–1c.
C@O); HRMS (ESI+): m/z: calcd for C26H18N2O5: 461.1008
[M+Na+]; found: 461.1056.
2.4. Quantum chemical calculations
The ground-state geometries and resonance frequencies of
compounds 2a to 2c were calculated using the B3LYP method with
the 6-31G(d) basis set. The structures for the first singlet excited
state were optimized using the CIS method with the 6-31G(d) basis
set. Time-dependent density functional theory (TD-DFT) calcula-
tions, based on the CIS-optimized structures of the first excited
state, were used to predict the emission wavelength. The absorp-
tion wavelengths were predicted based on the PBE1PBE- and
B3LYP-optimized ground-state geometries. Solvent effects were
considered using the polarized continuum model (PCM) model.
All calculations were performed using the Gaussian 03 program
package.
2.2.1. 3-Acetyl-6-phenylazo-coumarin (1a)
Yield: 52.3%; mp 213–214 °C; 1H NMR (DMSO-d6, d, ppm):
8.624 (s, 1H), 8.231–8.264 (t, 2H), 7.942–7.956 (d, 2H), 7.499–
7.557 (m, 4H), 2.760 (s, 3H).
2.2.2. 3-Acetyl-6-phenylazo-8-methoxy-coumarin (1b)
Yield: 56.1%; mp 205–206 °C; 1H NMR (DMSO-d6, d, ppm):
8.598 (s, 1H), 7.938–7.958 (q, 2H), 7.872–7.876 (d, 1H), 7.779–
7.783 (d, 1H), 7.525–7.574 (m, 3H), 4.091 (s, 3H), 2.759 (s, 3H).
3. Results and discussion
3.1. Synthesis
2.2.3. 3-Acetyl-6-(4-methoxyphenyl)azo-coumarin (1c)
Yield: 57.3%; mp 221–222 °C; 1H NMR (DMSO-d6, d, ppm):
8.805 (s, 1H), 8.452 (s, 1H), 8.159–8.175 (d, 1H), 7.905–7.921 (d,
2H), 7.616–7.633 (d, 1H), 7.144–7.161 (d, 2H), 3.870 (s, 3H),
2.595 (s, 3H).
Compounds 1a–1c and 2a–2c were prepared using the method
in Refs. [18,19] (Scheme 1). Diazotization of aniline (p-methoxyan-
iline) and a coupling reaction with salicylaldehyde (3-methoxysal-
icylaldehyde) yielded the corresponding azosalicylaldehydes.
Knoevenagel condensation of azosalicylaldehydes with ethylace-
toacetate yielded compounds 1a–1c. Finally, Claisen–Schmidt con-
densation of compounds 1a–1c with 1,4-phthalaldehyde yielded
the corresponding compounds 2a–2c.
2.3. Synthesis and characterization of compounds 2a–2c
In a reaction flask, compounds 1a–1c (0.05 mol) and an appro-
priate amount of 1,4-phthalaldehyde (0.1 mol) were dissolved in
glacial acetic acid (200 mL), and piperidine (2 mL) was added as
a catalyst. The mixture was refluxed for about 30 h, and then
cooled to room temperature. The precipitate was collected via fil-
tration and recrystallized twice in ethanol/acetonitrile to produce
the pure compounds 2a–2c.
The structure and purity of compounds 2a–2c were confirmed
via FT-IR, 1H NMR spectroscopy, and MS spectrometry. The FT-IR
spectra of compounds 2a–2c display characteristic bands for the lac-
tone C@O stretching at around 1731, 1726, and 1729 cmꢀ1, respec-
tively, confirming the presence of the coumarin skeleton. The 1H
NMR spectra of compounds 2a–2c show clear, distinguishable, in-
tense singlets at d = 10.035, 10.034, and 10.035 ppm, respectively,
corresponding to the formyl proton, and at d = 8.857, 8.831, and
8.837 ppm, respectively, which are attributed to H-4 of the couma-
rin nucleus. Additionally, compounds 2a and 2b give characteristic
doublets at d = 8.493–8.497 and d = 8.120–8.124, respectively,
which are assigned to H-5 of the coumarin nucleus, whereas the cor-
responding C-5 proton in compound 2c appears as a singlet at
d = 8.421 ppm. The peak shape and chemical shift of one of the three
kinds of protons found in the spectra are different because of the
presence of methoxyl groups in the structures of compounds 2b
and 2c, which show characteristic methoxyl proton singlets at
d = 4.045 and d = 3.864, respectively.
2.3.1. 6-Phenylazo-3-[3-(4-formylphenyl)prop-2-enoyl]-coumarin
(2a)
Yield: 20.3%; mp 239–240 °C; 1H NMR (DMSO-d6, d, ppm):
10.035 (s, 1H), 8.857 (s, 1H), 8.495 (d, J = 2.0 Hz, 1H), 8.225 (q,
J = 2.5, 2.5 Hz, 1H), 7.975 (s, 4H), 7.909 (d, J = 6.5 Hz, 2H), 7.838
(d, J = 16.0 Hz, 1H), 7.776 (d, J = 16.6 Hz, 1H), 7.682 (d, J = 9.0 Hz,
1H), 7.611 (d, J = 7.5 Hz, 3H); IR (KBr pellet cmꢀ1): 1731 (ester
C@O); HRMS (ESI+): m/z: calcd for C25H16N2O4: 431.1002
[M+Na+]; found: 431.1049.
Moreover, the 1H NMR spectra also exhibit the presence of two
trans-olefinic protons at d = 7.838 (J = 16.0 Hz) and d = 7.776
(J = 16.6 Hz) for compound 2a, d = 7.834 (J = 16.0 Hz) and d = 7.775
(J = 16.0 Hz) for compound 2b, and d = 7.835 (J = 16.0 Hz) and
d = 7.776 (J = 16.0 Hz) for compound 2c.
2.3.2. 6-Phenylazo-8-methoxy-3-[3-(4-formylphenyl)prop-2-enoyl]-
coumarin (2b)
Yield: 21.3%; mp 255–256 °C; 1H NMR (DMSO-d6, d, ppm):
10.034 (s, 1H), 8.831 (s, 1H), 8.122 (d, J = 2.0 Hz, 1H), 7.973 (s,
4H), 7.913 (q, J = 2.0, 1.0 Hz, 2H), 7.834 (d, J = 16.0 Hz, 1H), 7.809
(d, J = 2.0 Hz, 1H), 7.775 (d, J = 16.0 Hz, 1H), 7.609 (d, J = 7.5 Hz,
3H), 4.045 (s, 3H); IR (KBr pellet cmꢀ1): 1726 (ester C@O); HRMS
(ESI+): m/z: calcd for C26H18N2O5: 461.1008 [M+Na+]; found:
461.1026.
3.2. Thermal properties of compounds 2a–2c
To investigate the thermal stability of compounds 2a–2c, TG
experiments were conducted at the 30–800 °C temperature range
at a heating rate of 10° minꢀ1 in a nitrogen atmosphere. The ther-
mograph is shown in Fig. 1. The TG curves show a clear plateau
followed by a sharp decomposition curve, indicating that the
decomposition of compounds 2a–2c mainly proceeds within the
260–376 °C temperature range, with a corresponding weight loss
of 78%, 52%, and 64%, respectively. Of the three compounds, com-
pound 2b in particular shows good thermal stability up to 291 °C.
2.3.3. 6-(4-Methoxyphenyl)azo-3-[3-(4-formylphenyl)prop-2-enoyl]-
coumarin (2c)
Yield: 25.6%; mp 249–250 °C; 1H NMR (DMSO-d6, d, ppm):
10.035 (s, 1H), 8.837 (s, 1H), 8.421 (s, 1H), 8.178 (d, J = 9.0 Hz,
1H), 7.976 (s, 4H), 7.908 (d, J = 8.0 Hz, 2H), 7.835 (d, J = 16.0 Hz,
1H), 7.776 (d, J = 16.0 Hz, 1H), 7.655 (d, J = 8.5 Hz, 1H), 7.145 (d,
J = 8.5 Hz, 2H), 3.864 (s, 3H); IR (KBr pellet cmꢀ1): 1729 (ester

J. Li et al. / Journal of Molecular Structure 1011 (2012) 19–24
21
O
O
Ar N N
Ar N N
CHO
OH
iii
i
ii
2
2
O
R
R
O
Ar N N
3
iv
O
O
CHO
R
OCH3
Scheme 1. Synthetic routes to compounds 2a–2c. Reagents and reaction conditions: (i) NaNO2, HCl, 0–5 °C; (ii) salicylaldehyde or 3-methoxysalicylaldehyde, pH 8–9, 0–5 °C;
(iii) ethylacetoacetate, ethanol, piperidine, reflux; (iv) 1,4-phthalaldehyde, glacial acetic acid, reflux.
that of coumarin, which has a sharp absorption peak at 281 nm
with a shoulder at 320 nm [20]. The reason for the difference is
that the intramolecular charge transfer induced by the electron
push–pull system becomes stronger after the introduction of an
electron-withdrawing chalcone group in the 3-position and an
electron-donating azo group in the 6-position. The spectral shapes
of compounds 2b and 2c are similar to that of compound 2a be-
cause of their very similar structures. However, the absorption
peak at 346 nm of compound 2b and the absorption peak at
350 nm of compound 2c are red-shifted compared with that of
compound 2a as a result of the electron-releasing methoxyl substi-
tuent in their structures, which increase the conjugative effect of
the molecules. Note that the absorption peak of compound 2c is
red-shifted by 4 nm compared with that of compound 2b. Thus,
the conjugative effect of the methoxyl group in compound 2c is
generally larger than that in compound 2b.
Fig. 2 also shows the emission spectra of compounds 2a–2c in
diluted dichloromethane solutions. Compounds 2a–2c exhibit
bright blue emissions with peaks at 431, 437, and 449 nm, respec-
tively. The maximum emission wavelength of compounds 2b and
2c is bathochromically shifted by about 6 and 18 nm, respectively,
compared with that of compound 2a, because of the electron-
repelling methoxyl group in their molecular structures.
120
100
80
60
40
20
0
2b
2c
2a
200
250
300
350
400
450
Temperature (°C)
Fig. 1. TG curves of compounds 2a–2c.
3.3. UV–vis absorption and fluorescence of compounds 2a–2c
The UV–vis absorption spectra of compounds 2a–2c in diluted
dichloromethane solutions are given in Fig. 2. Compound 2a clearly
has only one intense absorption band with a maximum absorption
peak at 336 nm, which is bathochromically shifted compared to
3.4. Fluorescence quantum yield (US,2a, US,2b, and US,2c) of the
compounds
The fluorescence quantum yields of compounds 2a–2c in
dichloromethane solutions were estimated from the following
equation [21]:
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
2a
2b
2c
ꢀ
AR Fs ns
ꢁ
ꢁ
2
Us
¼
UR
ꢁ
FR AS nR
where UR is the quantum yield of the standard, A is the absorbance
at the excitation wavelength, F is the integrated intensity of the
emission spectra, n is the refractive index of the solution, index S re-
fers to the sample, and R denotes the reference. The standard fluo-
rophore for the quantum yield measurements is quinine sulfate in
0.2 mol dmꢀ3 H2SO4
(UR = 0.51) [22]. In the current study, the US
250
300
350
400
450
500
550
of compounds 2a–2c are 0.60, 0.65, and 0.72, respectively. Both
the azo and chalcone groups in the coumarin ring cause the high
fluorescence quantum yield in compounds 2a–2c. Compared with
Wavelength (nm)
Fig. 2. Normalized UV–vis absorption and emission spectra of compounds 2a–2c in
diluted dichloromethane solutions at room temperature. (C = 5 ꢂ 10ꢀ5 mol/L,
kex,2a = 336 nm, kex,2b = 346 nm, and kex,2c = 350 nm).
US,2a
,
US,2b and US,2c are increased because of the presence of the
methoxyl group.

22
J. Li et al. / Journal of Molecular Structure 1011 (2012) 19–24
Table 1
3.5. Quantum chemical calculations
Comparison of calculated and experimental absorption wavelengths (kmax, in nm).
Route 1a
Route 2b
Route 3c
Route 4d
Exp.e
3.5.1. UV–vis absorption and fluorescence spectra
Compounds
The UV–vis absorption spectra of the three compounds were
calculated using TD-DFT at the B3LYP/6-31G(d) and PBE1PBE/6-
31G(d) levels. Solvent effects were considered in the PCM model.
The experimental and calculated kmax (nm) values for the lower-ly-
ing singlet state of compounds 2a to 2c are listed in Table 1. Com-
parison of absolute deviations reveals that B3LYP is more suitable
than PBE1PBE in studying the absorption spectra of compounds 2a
to 2c. The remarkable increase in accuracy is consistent with the
experimental measurements when the solvent effects are
considered.
2a
2b
2c
310
321
326
329
338
342
320
329
335
333
348
354
336
346
350
a
b
c
PBE1PBE/TD-DFT.
PBE1PBE/TD-DFT(PCM), solvent is dichloromethane.
B3LYP/TD-DFT.
B3LYP/TD-DFT(PCM), solvent is dichloromethane.
Solvent is dichloromethane.
d
e
The emission energies calculated with the TD-DFT/6-31G(d)
level of theory using the CIS-optimized geometries and the exper-
imental kmax values are listed in Table 2. The absolute errors be-
tween the theoretical and experimental emission kmax values
decrease evidently when solvent effects are considered with the
TD-DFT/PCM method. The calculated outcomes are also in good
agreement with the experimental data. In addition, there is only
one main absorption, as well as one emission, wavelength with
the strongest oscillator strength. These wavelengths correspond
Table 2
Comparison of calculated and experimental emission wavelengths (kmax, in nm).
Compounds
Cal.a
Exp.b
Without solvent
Dichloromethane
2a
2b
3c
415
420
434
429
435
451
431
437
449
a
CIS/TD-DFT.
Solvent is dichloromethane.
b
to the
p ?
p⁄ excitation of the solely highest occupied (HOMO)
to lowest unoccupied (LUMO) molecular orbital with the largest
transitional proportion.
Table 3
Main orbital compositions and vertical excitation energies of compounds 2a–2c.
3.5.2. Assignment of the calculated transition
Compound Transition
feature
B3LYP/6-31G(d)
The main orbital compositions of the computed lower-lying sin-
glet excited states and the transition feature of compounds 2a to 2c
obtained at the B3LYP/6-31G level are listed in Table 3. The calcu-
lated frontier orbitals of compounds 2a to 2c are nearly the same.
The HOMO of compound 2a is localized on the unity of the azo and
coumarin regions, whereas the LUMO is located on the chalcone re-
gion (Fig. 3). However, HOMO ꢀ 1 and LUMO + 1 are localized
mainly on the benzene ring of the azo and chalcone groups, respec-
tively. Thus, the transition from HOMO to LUMO is easier than that
Transition
charactera
EHOMO
ELUMO
f
2a
2b
2c
p
p
p
?
?
?
p⁄
p⁄
p⁄
H ? L (82.5%)
H ? L (87.1%)
H ? L (83.5%)
ꢀ6.3827 ꢀ2.2224 0.867
ꢀ6.2809 ꢀ2.3835 0.872
ꢀ6.2344 ꢀ2.3973 0.846
a
H and L stand for HOMO and LUMO, respectively, and the proportion of the
main transition is given in the parentheses.
Fig. 3. HOMO and LUMO electron distributions of compound 2a.

J. Li et al. / Journal of Molecular Structure 1011 (2012) 19–24
23
Fig. 4. Optimized structures of compound 2a in the ground state (left) and the first excited state (right).
from HOMO ꢀ 1 to LUMO, HOMO to LUMO + 1, or HOMO ꢀ 1 to
3500
3000
2500
2000
1500
1000
500
LUMO + 1. This phenomenon also elucidates why the lowest en-
ergy absorption is a charge transfer transition from HOMO to
LUMO and mainly an electronic transition. Therefore, the electron
density decreases significantly in the electron-donating azo and
coumarin systems when electrons transfer from the HOMO to
the LUMO. This phenomenon is accompanied by an increase in
the electron density of the electron-accepting chalcone moiety.
This result indicates that the electrons transfer from the unity of
the azo and coumarin systems to the chalcone group.
Y
= 1.0403X + 3.2984
non-scaled
Non-scaled
Scaled
Y
= 1.0002X + 2.6718
scaled
The two structures of compound 2a in the ground state and in
the first excited state are illustrated in Fig. 4. The azo group is
coplanar with the parent coumarin plane in the ground state be-
cause of the average electron distributions in the unity region.
The coplanar structure of the HOMO region is destroyed when
electrons transfer from the HOMO to the LUMO in the first excited
state. The spatial structure of the chalcone group also changes be-
500
1000
1500
2000
2500
3000
3500
Experimental wavenumber (cm-1)
Fig. 5. Consistency of the wavenumbers of calculated and experimental IR spectra
main peaks of compound 2a.
cause of the
p ?
p⁄ electron repulsion between the two carbonyl
groups of the chalcone and coumarin moieties.
From Table 3, it can also be seen that based on the optimized
structure, the HOMO and LUMO energies for compound 2a are cal-
culated as ꢀ6.3827 and ꢀ2.2224 eV, respectively, whereas the
HOMO–LUMO energy gap is 4.1603 eV. For compound 2b, the re-
sults are ꢀ6.2809, ꢀ2.3835, and 3.8974 eV, respectively, and for
compound 2c, the results are ꢀ6.2344, ꢀ2.3973, and 3.8371 eV,
respectively. Compared with compound 2a, the methoxyl group
substituted in the structures of compounds 2b and 2c both raise
the HOMO energies and lower the LUMO energies. Hence, the dif-
ference in the HOMO and LUMO energies is reduced and the tran-
sition from HOMO to LUMO becomes easier for compounds 2b and
Table 5
Ionization potential (Ip) of compounds 2a–2c obtained by B3LYP/6-31G(d) method.
Compound
IPV (eV)
IPa (eV)
2a
2b
2c
7.75
6.85
7.49
7.52
6.71
7.22
2c than for compound 2a because their energy gaps are lower than
that of compound 2a. This also leads to the bathochromical shift of
the absorption peaks of compounds 2b and 2c (Section 3.3).
Table 4
Comparison of the calculated and experimental infrared data of compound 2a.
No. Exp.
Calcd.
Vibratory feature
3.5.3. Calculation and experimental comparison of IR spectra
The full geometry optimization and resonance frequency calcu-
lation of compound 2a were performed at the B3LYP/6-31G level.
The major vibrational modes (experimental and theoretical) and
their assignments are listed in Table 4. The calculated IR spectral
data are slightly higher than the experimental values and con-
tained the harmonic oscillator frequency, whereas the experimen-
tal values contained the anharmonic oscillator frequency. Fig. 5
establishes a comparison between the calculated and experimental
vibrational wavenumbers of compound 2a using 0.9613 [23] as the
frequency scaling factor. The linear slope and intercept are 1.0403
and 3.2984, respectively, for the non-scaled data and 1.0002 and
2.6718, respectively, for the scaled data. These results indicate that
the error is reduced by the introduction of the frequency scaling
factor and shows a good agreement between the experimental
and theoretical frequencies.
Freq.a Int. (IR)b Non scaled Scaledc Int. (IR)
1
2
578
680
m
m
m
m
s
s
s
v
s
s
v
m
w
596
703
792
863
966
1285
1609
1689
1743
1798
1853
2916
3181
573
676
761
829
928
1234
1546
1623
1675
1729
1781
2803
3058
60.74
27.45
25.87
d@CH
d@CH
d@CH
d@CH
vC-O-C
vC@C
vC@C
vC@C (CH@CH)
vC@O (CO)
vC@O (CHO)
vC@O (COO)
v@CH (CHO)
v@CH
3
766
4
826
22.59
5
987
257.20
214.55
84.71
6
7
8
9
10
11
12
13
1192
1553
1607
1661
1699
1731
2852
3046
97.75
200.02
311.97
704.39
161.79
11.25
Frequencies in cmꢀ1
m: Middle, s: strong, v: very strong, w: weak.
Scaling factor using 0.9613.
.
a
b
c

24
J. Li et al. / Journal of Molecular Structure 1011 (2012) 19–24
3.5.4. Ionization potential
Acknowledgements
The ionization potential indicates the energy required to lose
electrons. The calculated ionization potentials (IP) of compounds
2a to 2c using the B3LYP/6-31G(d) level are listed in Table 5. There
This work was financially supported by National Natural Sci-
ence Foundation of China (21176180).
are two parameters for ionization potentials, namely, vertical (IPV
)
and adiabatic (IPa). IPV denotes the energy difference between the
cation and molecule at the base of the optimized structure of a
neutral molecule. IPa denotes the energy difference between the
molecule and cation at the base of the optimized structures of
the neutral molecule and cation, respectively. The IPV and IPa in-
crease evidently from compound 2b to 2a, i.e., the ability to lose
electrons is reduced gradually (2b < 2c < 2a). This finding signifies
that compound 2b can act as a good electron-donating material.
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