Synthesis and fluorescence properties of 7-hydroxy-3-(2-pyridyl)coumarin derivatives

Article abstract of DOI:10.1007/s11164-011-0339-2

Three coumarin derivatives, 7-hydroxy-3-(2-pyridyl)coumarin (HPC), 7-(4-(5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl)benzyloxy)-3-(2-pyridyl) coumarin (Ox-PC), and 7-(4-(9H-carbazol-9-yl)butoxy)-3-(2-pyridyl)coumarin (Cz-PC), were synthesized and characte

Full text of DOI:10.1007/s11164-011-0339-2

Res Chem Intermed (2012) 38:215–222
DOI 10.1007/s11164-011-0339-2
Synthesis and fluorescence properties of 7-hydroxy-3-
(2-pyridyl)coumarin derivatives
•
Tianzhi Yu Shuxia Yang Yuling Zhao
•
•
•
•
•
Peng Zhang Zhe Lv Duowang Fan
Zhongrong Geng
Received: 11 May 2011 / Accepted: 7 June 2011 / Published online: 22 June 2011
Ó Springer Science+Business Media B.V. 2011
Abstract Three coumarin derivatives, 7-hydroxy-3-(2-pyridyl)coumarin (HPC),
7-(4-(5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl)benzyloxy)-3-(2-pyridyl)couma-
rin (Ox-PC), and 7-(4-(9H-carbazol-9-yl)butoxy)-3-(2-pyridyl)coumarin (Cz-PC),
1
were synthesized and characterized by elemental analysis, H NMR, FT-IR, and
UV–vis absorption spectra. The fluorescence behaviors of the compounds in
methanol solutions and solid states were investigated. HPC exhibits weak green
emission, whereas Cz-PC and Ox-PC show strong blue emissions in dilute
solutions.
Keywords Synthesis ꢀ 7-Hydroxy-3-(2-pyridyl)coumarin ꢀ Coumarin derivative ꢀ
Fluorescence
Introduction
Coumarins are a very large and important family of organic compounds, which have
been extensively investigated due to their applications in biological, chemical, and
physical fields [1–10]. Recently, some coumarin dyes with high quantum yield were
used as the leading fluorescent emitters in the field of organic light-emitting diodes
(OLEDs), which were investigated as blue, green and red dopants [11–13].
Coumarin dyes readily suffer from self-quenching in high concentration owing to
the strong intermolecular interactions and aggregations, and therefore they were
T. Yu (&) ꢀ S. Yang ꢀ P. Zhang ꢀ D. Fan ꢀ Z. Geng
Key Laboratory of Opto-Electronic Technology and Intelligent Control
(Lanzhou Jiaotong University), Ministry of Education, Lanzhou 730070, China
e-mail: ytz823@hotmail.com
S. Yang ꢀ Y. Zhao ꢀ Z. Lv
School of Chemical and Biological Engineering, Lanzhou Jiaotong University,
Lanzhou 730070, China
123

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T. Yu et al.
always doped in the host materials at appropriate concentrations to relieve the
aggregations between the dyes in fabricating OLEDs with reasonable luminescent
efficiency.
It is well known that the photophysical and spectroscopic properties of the
coumarin derivatives can be readily modified by the introduction of substituents in
parent coumarin, and thus matching the requirements of color purity, photostability
and high quantum yields. The coumarin derivatives substituted at the 7-position
with electron-releasing moieties and at the 3-position with electron-withdrawing
groups are a class of D-A structural molecules and exhibit strong emissions [6, 14,
15]. The search for new multifunctional coumarin derivatives which can efficiently
act as fluorescence dyes for various potential applications has been an active area of
modern research [16–19]. Some substituted coumarin derivatives with good
photoluminescence and electroluminescence properties have been designed and
synthesized in our laboratory [6, 7, 14, 20–22].
In order to get a better understanding of how the structural features of coumarins
may control the intrinsic luminescence property, we have rationally designed three
coumarin derivatives (Fig. 1), in which two compounds have been combined: an
electron-transporting 1,3,4-oxadiazole and a hole-transporting carbazole moiety.
The key purpose is to design and synthesize readily processible and thermally stable
emission and charge-transporting materials. We are reporting the syntheses,
characterization and spectroscopic studies of the compounds in this study.
Experimental
Materials and methods
Pyridine-2-acetonitrile, carbazole, 4-tert-butylbenzoyl chloride and 2,4-dihydroxybe-
nazldehyde were purchased from Alfa Aesar. P-toluic acid and N-bromosuccinimide
(NBS) were obtained from Shanghai Zhongqin Chemical Reagent and Tianjin
Guangfu Fine Chemical Research Institute, respectively. Tetrabutylammonium
N
N
N
O
O
O
HO
O
O
Cz-PC
HPC
N
N
N
O
O
O
O
Ox-PC
Fig. 1 Chemical structures of coumarin derivatives discussed in this study
123

Synthesis and fluorescence properties of 7-hydroxy-3-(2-pyridyl)coumarin
217
bromide (TBAB) was bought from Tianjin BASF Chemical Reagent. All other
chemicals were analytical grade reagents.
IR spectra (400–4,000 cm-1) were measured on a Shimadzu IRPrestige-21 FT-
1
IR spectrophotometer. H NMR spectra were obtained on Unity Varian-500 MHz.
C, H, and N analyses were obtained using an Elemental Vario-EL automatic
elemental analysis instrument. UV–vis absorption and photoluminescent spectra
were recorded on a Shimadzu UV-2550 spectrometer and Perkin Elmer LS-55
spectrometer, respectively. Melting points were measured by using an X-4
microscopic melting point apparatus made by Beijing Taike Instrument, and the
thermometer was uncorrected.
Synthesis and characterization of HPC, Ox-PC, and Cz-PC
The synthetic routes of HPC, Ox-PC, and Cz-PC are described in Scheme 1.
The intermediate, 2-[4-(bromomethyl)phenyl]-5-(4-tertbutylphenyl)-1,3,4-oxadiazole,
was obtained as previously described [23]. 7-Hydroxy-3-(2-pyridyl)coumarin
(HPC) was synthesized according to the method reported by Khilya et al. [24].
7-Hydroxy-3-(2-pyridyl)coumarin
Pyridine-2-acetonitrile (11.8 g, 0.1 mol) and 2,4-dihydroxybenazldehyde (13.8 g,
0.1 mol) were dissolved in 65 mL of anhydrous alcohol, and then piperidine (6 mL)
was added stepwise at room temperature. The mixture was stirred for 12 h at
room temperature, then treated with H2SO4 (50 mL, 3%) and refluxed for 8 h to
hydrolyze the iminocoumarin. When the reaction was finished, the acidic solution
was neutralized with aqueous ammonia until the pH was 7. The precipitate was
filtered off and washed throughly with water, and dried under reduced pressure.
The crude was recrystallized from methanol to yield 15.1 g (63%) of product. m.p.
258–260 °C (255–256 °C [24]). IR (KBr pellet cm-1): 3,286 (mAr–OH), 1,704
1
(m–C=O), 1,608, 1,581, 1,487, 1,462, 1,221. H NMR (DMSO-d6, d, ppm): 10.78 (s,
1H, –OH), 8.82 (s, 1H, H-4), 8.67 (d, 1H, J = 5 Hz, H-60), 8.26 (d, 1H, J = 8 Hz,
N
O
CHO
OH
N
N
OH
H+
H
+
CN
HO
O
N
HO
O
NH
HPC
N
O
N
N
O
O
O
O
Br
N
N
N
O
Ox-PC
K2CO3
Butanone
+
N
O
HO
O
Br
N
HPC
N
O
O
Cz-PC
Scheme 1 Synthetic routines to the compounds HPC, Ox-PC, and Cz-PC
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T. Yu et al.
H-30), 7.89 (t, 1H, J = 9.2 Hz, H-40), 7.77 (d, 1H, J = 10 Hz, H-5), 7.39 (t, 1H,
J = 6 Hz, H-50), 6.86 (d, 1H, J = 2 Hz, H-6), 6.79 (s, 1H, H-8). Anal. Calc. for
C14H9O3N (%): C, 70.29; H, 3.79; N, 5.86. Found: C, 70.20; H, 3.81; N, 5.84.
9-(4-Bromobutyl)-9H-carbazole
A solution of carbazole (4 g, 24 mmol) and 1,4-dibromobutane (15.56 g, 72 mmol)
in toluene (30 mL) was added to a mixture of TBAB (0.32 g, 1 mmol), and aqueous
50% sodium hydroxide (25 mL). The mixture was stirred at room temperature for
24 h. After the reaction was completed, the mixture was poured into 50 mL of water
and extracted with dichloromethane (3 9 100 mL). The organic phase was washed
with water and dried over anhydrous MgSO4. After removal of solvent, the crude
was purified by chromatography on silica gel using petroleum ether as the eluent to
give 9-(4-bromobutyl)-9H-carbazole (5.05 g, 69.8%). m.p. 98–100 °C. IR (KBr
pellet cm-1): 3,051 (Aryl–CH), 2,960, 2,928, 2,865 (Alkyl–CH), 1,627, 1,595,
1,483, 1,226, 753, 648. 1H NMR: (CDCl3, d, ppm): 8.12 (d, J = 7.6 Hz, 2H, Aryl–
H), 7.45 (m, 2H, Aryl–H), 7.41 (d, J = 8 Hz, 2H, Aryl–H), 7.22 (m, 2H, Aryl–H),
4.34 (t, J = 6.8 Hz, 2H, N–CH2), 3.36 (t, J = 6.4 Hz, 2H, Br–CH2), 2.05 (m, 2H,
CH2), 1.92 (m, 2H, CH2).
7-(4-(9H-Carbazol-9-yl)butoxy)-3-(2-pyridyl)coumarin
In a nitrogen-flushed flask, 7-hydroxy-3-(2-pyridyl)coumarin (0.4 g, 1.67 mmol),
9-(4-bromobutyl)-carbazole (0.51 g, 1.69 mmol) and K2CO3 (0.691 g, 5 mmol)
were dissolved in 30 mL of butanone and stirred at 70 °C for 72 h. The mixture was
poured into 50 mL of water and extracted with dichloromethane (3 9 100 mL). The
organic phase was washed with water and dried over anhydrous MgSO4. After
removal of solvent, the crude was purified by chromatography on silica gel using
ethyl acetate/petroleum ether (1:20, v/v) as the eluent to give Cz-PC (0.3 g, 40%).
m.p. 176–177 °C. IR (KBr pellet cm-1): 3,055, 2,949, 1,731, 1,610, 1,470, 1,273,
1,201, 1,131. 1H NMR: (CDCl3, d, ppm): 8.72 (s, 1H, H-4), 8.66 (d, 1H,
J = 4.5 Hz, H-60), 8.40 (d, 1H, J = 8.5 Hz, H-30), 8.11 (d, 2H, J = 9.0 Hz, Ar–H),
7.78 (t, 1H, J = 9.5 Hz, H-5), 7.51–7.42 (m, 5H, Ar–H), 7.28–7.22 (m, 3H, Ar–H),
6.83–6.79 (m, 2H, Ar–H), 4.43 (t, 2H, J = 8.0 Hz, –O–CH2–), 4.01 (t, 2H,
J = 8.0 Hz, –N–CH2–), 2.15–2.11 (m, 2H, –CH2–), 1.92–1.88 (m, 2H, –CH2–).
Anal. Calc. for C30H24O3N2 (%): C, 78.24; H, 5.25; N, 6.08. Found: C, 78.31; H,
5.21; N, 6.12.
7-(4-(5-(4-Tert-butylphenyl)-1,3,4-oxadiazol-2-yl)benzyloxy)-3-(2-pyridyl)coumarin
The preparation of Ox-PC was similar to that described for Cz-PC, which was
obtained from 2-[4-(bromomethyl)phenyl]-5-(4-tertbutylphenyl)-1,3,4-oxadiazole
and 7-hydroxy-3-(2-pyridyl)coumarin. The crude was purified by chromatography
on silica gel using ethyl acetate/petroleum ether (1:6, v/v). Yield: 51.5%. m.p.
226–227 °C. IR (KBr pellet cm-1): 3,063, 2,964, 1,724, 1,611, 1,505, 1,465, 1,379,
1
1,265, 1,195, 1,125, 1,012. H NMR: (CDCl3, d, ppm): 8.74 (s, 1H, H-4), 8.67 (d,
123

Synthesis and fluorescence properties of 7-hydroxy-3-(2-pyridyl)coumarin
219
1H, J = 5.5 Hz, H-60), 8.40 (d, 1H, J = 9.0 Hz, H-30), 8.19 (d, 2H, J = 9.0 Hz,
Ar–H), 8.07 (d, 2H, J = 9.5 Hz, Ar–H), 7.77 (t, 1H, J = 5.0 Hz, H-5), 7.63 (d, 2H,
J = 9.5 Hz, Ar–H), 7.59–7.55 (m, 4H, Ar–H), 6.98–6.95 (m, 2H, Ar–H), 5.25 (s,
2H, –O–CH2–), 1.38 (s, 9H, –CH3). Anal. Calc. for C33H27O4N3 (%): C, 74.84; H,
5.14; N, 7.93. Found: C, 74.78; H, 5.10; N, 7.97.
Results and discussion
UV–vis absorption of HPC, Ox-PC and Cz-PC
UV–vis absorption spectra of HPC, Ox-PC and Cz-PC in diluted methanol
solutions are given in Fig. 2. It is shown that the absorption spectrum of HPC
exhibits absorptions at 251 and 366 nm, respectively. The absorption spectrum of
Ox-PC shows absorptions at 294 and 350 nm, whereas the absorption spectrum of
Cz-PC displays absorptions at 248, 262, 294, and 346 nm, respectively. Compared
with HPC, the absorption bands of Ox-PC and Cz-PC located longer wavelengths
which were obviously blue-shifted. HPC possesses a hydroxyl group at the
7-position, and there are solvent effects and intermolecular interactions between
HPC molecules and between HPC molecules and solvent molecules via hydrogen
bondings, so the maximal absorption band of HPC is larger than that of Ox-PC and
Cz-PC. On the other hand, the electron-donating ability of hydroxy group is larger
than that of the alkoxy and benzyloxy groups, which enhances the electron density
of coumarin ring. Similar phenomena were reported by Khilya et al. [24] and
Azuma et al. [25].
3
HPC
Ox-PC
Cz-PC
2
1
0
250
300
350
400
450
500
Wavelength (nm)
Fig. 2 Absorption spectra of HPC, Ox-PC and Cz-PC in methanol solutions (C = 1.0 9 10-5 mol/L)
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T. Yu et al.
Fluorescence of HPC, Ox-PC and Cz-PC
Figure 3 shows the fluorescence spectra of HPC, Ox-PC and Cz-PC in dilute
methanol solutions (C = 1.0 9 10-4 mol/L). The fluorescence properties of HPC,
Ox-PC and Cz-PC varied greatly. HPC exhibits green emission with the maximum
peak at ca. 530 nm, whereas Ox-PC and Cz-PC both exhibit bright blue emissions
with the maximum peaks at ca. 422 nm. The emission peaks of Ox-PC and Cz-PC
were blue-shifted by 108 nm with respect to that of HPC. It is worth noting that at
the same concentration the relative emission intensities of HPC, Ox-PC and Cz-PC
were different from each other, the order of emission intensity being Cz-PC [
Ox-PC [ HPC. HPC holding a hydroxy group at 7-position exhibits weak
fluorescence, whereas Ox-PC or Cz-PC, which have a direct connection between
the coumarin ring and diaryl-1,3,4-oxadiazole moiety or carbazole moiety via
methylene or alkyl linker, shows strong fluorescence.
It was well known that electron-withdrawing groups at the 3-position and
electron-donating groups at the 7-position have been shown to enhance the
fluorescence intensity of coumarins [6, 15, 26]. Although the electron-donating
ability of hydroxy group is larger than that of the alkoxy and benzyloxy groups,
there are intermolecular interactions between HPC molecules and between HPC
molecules and solvent molecules via hydrogen bondings, which causes the
fluorescence of HPC to be quenched. Compared with Ox-PC and Cz-PC, HPC
is easily self-quenched due to the intermolecular interaction and aggregations at the
same concentration.
From Fig. 3, we can also see that the fluorescence intensity of Cz-PC is stronger
than that of Ox-PC. From the structures of the compounds, it is found that Ox-PC
was substituted by a diaryl-1,3,4-oxadiazole-contained benzyloxy group, whereas
Cz-PC was substituted by a carbazole-contained long chain alkoxy group. The
benzyloxy group of Ox-PC was a less powerful electron donor than the alkoxy
422 nm
HPC
800
Cz-PC
Ox-PC
600
422 nm
400
200
0
530 nm
300
350
400
450
500
550
600
650
Wavelength (nm)
Fig. 3 Fluorescence spectra of HPC, Ox-PC, and Cz-PC in methanol solutions (C = 1.0 9 10-4 mol/
L, kex, HPC = 366 nm, kex, Ox-PC = 350 nm, and kex, Cz-PC = 346 nm)
123

Synthesis and fluorescence properties of 7-hydroxy-3-(2-pyridyl)coumarin
221
1.0
HPC
Cz-PC
Ox-PC
444 nm
466 nm
0.5
537 nm
0.0
400
450
500
550
600
650
Wavelength (nm)
Fig. 4 Fluorescence spectra of HPC, Ox-PC, and Cz-PC in solid states (kex, HPC = 366 nm, kex, Ox-PC
350 nm and kex, Cz-PC = 346 nm)
=
group of Cz-PC due to the strong electron affinity of 1,3,4-oxadiazole moiety. This
would explain why Cz-PC shows stronger fluorescence with respect to Ox-PC.
Figure 4 shows the fluorescence spectra of HPC, Ox-PC and Cz-PC in solid
states. Compared with their fluorescence spectra in dilute methanol solutions, the
emission peaks of the solid states of HPC, Ox-PC and Cz-PC are observed at 537,
466, and 444 nm, respectively. The emission peaks of the solid states of HPC,
Ox-PC and Cz-PC were red-shifted in various degrees. It is also found that sharp
self-quenching and much broader emission bands of the solid states brought about
the compact pilling which occurred, suggesting that the compounds in solid states
have more intermolecular interaction than that in solution situations, and thus may
lead to changes of the emission bands of the compounds.
Conclusions
We used a known compound, 7-hydroxy-3-(2-pyridyl)coumarin, as parent com-
pound to design and synthesize two coumarin derivatives which have a direct
connection between the coumarin ring and diaryl-1,3,4-oxadiazole moiety or
carbazole moiety via benzyloxy or alkoxy linker. Their structures were confirmed
by means of element analysis, 1H NMR, FT-IR, and UV–vis absorption spectra. The
fluorescence behaviors of the compounds in methanol solutions and solid states
were investigated and discussed. 7-Hydroxy-3-(2-pyridyl)coumarin exhibits weak
green emission, whereas two new compounds show strong blue emissions in dilute
solutions. The compounds could be used as emitting dopants in OLED applications.
Acknowledgments This study was supported by the Natural Science Foundation of Gansu Province
(096RJZA086) and the National Natural Science Foundation of China (Grant 60776006), and also
supported by ‘Qing Lan’ Talent Engineering Funds (QL-05-23A) by Lanzhou Jiaotong University.
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