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

Journal of Molecular Structure 1011 (2012) 19–24

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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

Article history: Received 30 August 2011 Received in revised form 20 November 2011 Accepted 21 November 2011 Available online 14 December 2011 Keywords: Coumarin Synthesis Fluorescence TD-DFT studies

a b s t r a c t 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), 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 excitedstate 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 density 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 calculated results are in very good agreement with the experimental data. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Coumarin and its derivatives have attracted much interest in several fields and represent an important class of organic heterocycles that can be found in many natural or synthetic drugs. These compounds possess versatile biological activities, such as antioxidant [1] and anti-inflammatory activities [2]. They also have outstanding optical properties [3–6], such as sufficient fluorescence in the visible light range, large Stokes shift, high photoluminescence quantum yield, and superior photostability. In addition, the absorption and fluorescence characteristics of the coumarin derivatives can be readily modified by changing the nature of the substituents, 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, fluorescent 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]. Recently, azo compounds have also gained strong interest because of their unique photochemical and photophysical properties. Azo chromophores can undergo light-driven, reversible trans–cis

isomerization of the azo bond, with concomitant change in the structure, dipole moment, and optical properties [15–17]. 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-formylphenyl)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 cm1) 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 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 solvents were dried using standard methods before use. 2.2. Synthesis and characterization of compounds 1a–1c

⇑ Corresponding author. Tel.: +86 22 27404208; fax: +86 22 27892279. E-mail address: [email protected] (X. Li). 0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2011.11.034

A 20% aq. NaNO2 solution (20 mL) was slowly added to a solution of aniline (p-methoxyaniline, 0.05 mol) in 37% aq. HCl

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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 refluxed for 5 h. After cooling, the solid was filtered and recrystallized from ethanol to obtain the pure compounds 1a–1c. 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). 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). 2.3. Synthesis and characterization of compounds 2a–2c In a reaction flask, compounds 1a–1c (0.05 mol) and an appropriate 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 filtration and recrystallized twice in ethanol/acetonitrile to produce the pure compounds 2a–2c. 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 cm1): 1731 (ester C@O); HRMS (ESI+): m/z: calcd for C25H16N2O4: 431.1002 [M+Na+]; found: 431.1049. 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 cm1): 1726 (ester C@O); HRMS (ESI+): m/z: calcd for C26H18N2O5: 461.1008 [M+Na+]; found: 461.1026. 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 cm1): 1729 (ester

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) calculations, based on the CIS-optimized structures of the first excited state, were used to predict the emission wavelength. The absorption 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. 3. Results and discussion 3.1. Synthesis Compounds 1a–1c and 2a–2c were prepared using the method in Refs. [18,19] (Scheme 1). Diazotization of aniline (p-methoxyaniline) and a coupling reaction with salicylaldehyde (3-methoxysalicylaldehyde) yielded the corresponding azosalicylaldehydes. Knoevenagel condensation of azosalicylaldehydes with ethylacetoacetate yielded compounds 1a–1c. Finally, Claisen–Schmidt condensation of compounds 1a–1c with 1,4-phthalaldehyde yielded the corresponding 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 lactone C@O stretching at around 1731, 1726, and 1729 cm1, respectively, confirming the presence of the coumarin skeleton. The 1H NMR spectra of compounds 2a–2c show clear, distinguishable, intense 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 coumarin 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 corresponding 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. 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. 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° min1 in a nitrogen atmosphere. The thermograph 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, compound 2b in particular shows good thermal stability up to 291 °C.

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O

i

2

ii

2

Ar N N

CHO

iii

Ar N N O

OH

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.

120

Weight (%)

100

2b 2c 2a

80 60 40 20 0 200

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

2a 2b 2c

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0 250

300

350

400

450

500

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

Intensity (a.u.)

Absorbance (a.u.)

1.0

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 because 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 substituent 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, respectively. 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 electronrepelling methoxyl group in their molecular structures.

0.0 550

Wavelength (nm) Fig. 2. Normalized UV–vis absorption and emission spectra of compounds 2a–2c in diluted dichloromethane solutions at room temperature. (C = 5  105 mol/L, kex,2a = 336 nm, kex,2b = 346 nm, and kex,2c = 350 nm).

Us ¼ UR 

 2 AR F s ns  F R 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 refers to the sample, and R denotes the reference. The standard fluorophore for the quantum yield measurements is quinine sulfate in 0.2 mol dm3 H2SO4 (UR = 0.51) [22]. In the current study, the US 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 US,2a, US,2b and US,2c are increased because of the presence of the methoxyl group.

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Table 1 Comparison of calculated and experimental absorption wavelengths (kmax, in nm).

a b c d e

Compounds

Route 1a

Route 2b

Route 3c

Route 4d

Exp.e

2a 2b 2c

310 321 326

329 338 342

320 329 335

333 348 354

336 346 350

PBE1PBE/TD-DFT. PBE1PBE/TD-DFT(PCM), solvent is dichloromethane. B3LYP/TD-DFT. B3LYP/TD-DFT(PCM), solvent is dichloromethane. Solvent is dichloromethane.

Table 2 Comparison of calculated and experimental emission wavelengths (kmax, in nm). Cal.a

Compounds

2a 2b 3c a b

Exp.b

Without solvent

Dichloromethane

415 420 434

429 435 451

431 437 449

CIS/TD-DFT. Solvent is dichloromethane.

Table 3 Main orbital compositions and vertical excitation energies of compounds 2a–2c. Compound

2a 2b 2c

Transition feature

B3LYP/6-31G(d) Transition charactera

EHOMO

ELUMO

f

p ? p⁄ p ? p⁄ p ? p⁄

H ? L (82.5%) H ? L (87.1%) H ? L (83.5%)

6.3827 6.2809 6.2344

2.2224 2.3835 2.3973

0.867 0.872 0.846

a H and L stand for HOMO and LUMO, respectively, and the proportion of the main transition is given in the parentheses.

3.5. Quantum chemical calculations 3.5.1. UV–vis absorption and fluorescence spectra The UV–vis absorption spectra of the three compounds were calculated using TD-DFT at the B3LYP/6-31G(d) and PBE1PBE/631G(d) levels. Solvent effects were considered in the PCM model. The experimental and calculated kmax (nm) values for the lower-lying singlet state of compounds 2a to 2c are listed in Table 1. Comparison 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. The emission energies calculated with the TD-DFT/6-31G(d) level of theory using the CIS-optimized geometries and the experimental kmax values are listed in Table 2. The absolute errors between 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 to the p ? p⁄ excitation of the solely highest occupied (HOMO) to lowest unoccupied (LUMO) molecular orbital with the largest transitional proportion. 3.5.2. Assignment of the calculated transition The main orbital compositions of the computed lower-lying singlet excited states and the transition feature of compounds 2a to 2c obtained at the B3LYP/6-31G level are listed in Table 3. The calculated 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 region (Fig. 3). However, HOMO  1 and LUMO + 1 are localized mainly on the benzene ring of the azo and chalcone groups, respectively. Thus, the transition from HOMO to LUMO is easier than that

Fig. 3. HOMO and LUMO electron distributions of compound 2a.

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Fig. 4. Optimized structures of compound 2a in the ground state (left) and the first excited state (right).

Table 4 Comparison of the calculated and experimental infrared data of compound 2a. No.

Exp. Freq.

1 2 3 4 5 6 7 8 9 10 11 12 13 a b c

Calcd. a

578 680 766 826 987 1192 1553 1607 1661 1699 1731 2852 3046

Int. (IR) m m m m s s s v s s v m w

b

Vibratory feature

Non scaled

Scaled

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

c

Frequencies in cm1. m: Middle, s: strong, v: very strong, w: weak. Scaling factor using 0.9613.

Int. (IR) 60.74 27.45 25.87 22.59 257.20 214.55 84.71 97.75 200.02 311.97 704.39 161.79 11.25

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

3500

Caculated wavenumber (cm-1)

from HOMO  1 to LUMO, HOMO to LUMO + 1, or HOMO  1 to LUMO + 1. This phenomenon also elucidates why the lowest energy 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. 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 because 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 because 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 calculated as 6.3827 and 2.2224 eV, respectively, whereas the HOMO–LUMO energy gap is 4.1603 eV. For compound 2b, the results 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 difference in the HOMO and LUMO energies is reduced and the transition from HOMO to LUMO becomes easier for compounds 2b and

3000 2500

Ynon-scaled = 1.0403X + 3.2984 Non-scaled Scaled

2000 1500

Yscaled = 1.0002X + 2.6718

1000 500 500

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.

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).

3.5.3. Calculation and experimental comparison of IR spectra The full geometry optimization and resonance frequency calculation 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 contained the harmonic oscillator frequency, whereas the experimental 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.

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3.5.4. Ionization potential 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 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 increase 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. 4. Conclusions Three new ketocoumarin derivatives (compounds 2a to 2c) containing the azo group were successfully synthesized, and their photophysical properties were investigated. All derivatives showed excellent thermal stability, high fluorescence quantum yield, and bright blue emissions. Calculations that included solvent polarity effects enabled the absorption and emission spectra to fit well with experimental measurements. The B3LYP/PCM method provided a reliable description of the molecular absorption characters and reproduced the p ? p⁄ type absorption bands of the three compounds. Resonance frequency calculations were undertaken to study the IR spectra of compound 2a, and the calculated results were in good agreement with the experimental values. Compound 2b had a better electron-transfer property than the other compounds. The synthesized compounds are potential candidates for use in optoor optoelectronic blue-emitting devices. However, the other characteristics of these compounds need further investigation.

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