Synthesis of light-emmting materials bis-[2′-2″-(9H-fluoren-2-yl)-vinyl-8-hydroxyquinoline] zinc(II) and bis-[2′-4″-(4,5-diphenyl-1H-imidazol-2-yl)styryl-8-hydroxyquinoline] zinc(II)

Article abstract of DOI:10.1016/j.tet.2009.06.025

Two novel light-emitting materials bis-[2′-2″-(9H-fluoren-2-yl)-vinyl-8-hydroxyquinoline] zinc(II) (3) and bis-[2′-4″-(4,5-diphenyl-1H-imidazol-2-yl)styryl-8-hydroxyquinoline] zinc(II) (4) containting 8-hydroxyquinoline and fluorene or imidazole moieties

Full text of DOI:10.1016/j.tet.2009.06.025

Tetrahedron 65 (2009) 6325–6329
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis of light-emmting materials bis-[20-200-(9H-fluoren-2-yl)-vinyl-8-
hydroxyquinoline] zinc(II) and bis-[20-400-(4,5-diphenyl-1H-imidazol-
2-yl)styryl-8-hydroxyquinoline] zinc(II)
,
b
,
d
a
Ting-Ting Wang a, Gong-Chang Zeng c, He-Ping Zeng a , Peng-Yi Liu , Rui-Xiang Wang ,
*
Zhong-Jun Zhang a, Yan-Ling Xiong a
a Institute of Functional Molecule, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, People’s Republic of China
b School of Chemistry & Environment, South China Normal University, Guangzhou 510631, People’s Republic of China
c Department of Chemistry, McMaster University, 1280 Main Street West, Hamilton, ON L8S 4M1, Canada
d Department of Physics, Jinan University, Guangzhou 510632, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Two novel light-emitting materials bis-[20-200-(9H-fluoren-2-yl)-vinyl-8-hydroxyquinoline] zinc(II) (3)
and bis-[20-400-(4,5-diphenyl-1H-imidazol-2-yl)styryl-8-hydroxyquinoline] zinc(II) (4) containting 8-hy-
droxyquinoline and fluorene or imidazole moieties have been synthesized. The optical properties of
these complexes were influenced by the styryl substituents, and exhibited orange-emission. They have
higher fluorescence quantum yields than Alq3, and good stabilities with thermal decomposition tem-
peratures 395 ꢀC and 435 ꢀC. The single-layer OLED fabricated by 3 emitted lemon-yellow, and exhibited
good device performance with a maximum luminance of 489 cd mꢁ2, and luminance efficiency of up to
0.41 cd Aꢁ1. The single-layer OLED fabricated by 4 emitted yellow-green, and exhibited good device
Article history:
Received 28 February 2009
Received in revised form 5 June 2009
Accepted 5 June 2009
Available online 12 June 2009
Keywords:
Single-layer organic light-emitting devices
Fluorene
Imidazole
performance with a maximum luminance of 323 cd mꢁ2, and luminance efficiency of up to 0.54 cd Aꢁ1
.
Ó 2009 Elsevier Ltd. All rights reserved.
8-Hydroxyquinoline derivatives
1. Introduction
results, we designed and synthesized the following two new 8-
hydroxyquinoline derivatives containing imidazole or fluorene unit
and 8-hydroxyquinoline with pi-conjugated bonds. It is expected
that a combination of fluorine or imidazole and 8-hydroxyquinoline
could not only remain the thermal and morphological stability but
also improve the quantum efficiency. On the other hand, compli-
cated configurations of an organic light-emitting diode, multi-
layers,8,9 doped10,11 or phosphorescent12,13 OLEDs showed their
industrial limits in terms of costly and time consuming fabrication
process and durability due to interfacial or phase separation prob-
lems.14–16 Thus, there are many attempts to fabricate a promising
single-layer device.17–22
We report herein the first synthesis of light-emmting molecules
bis-[20-200-(9H-fluoren-2-yl)-vinyl-8-hydroxyquinoline] zinc(II) (3)
and bis-[20-400-(4,5-diphenyl-1H-imidazol-2-yl)styryl-8-hydroxy-
quinoline] zinc(II) (4) which containing the fluorene or imidazole
moiety and 8-hydroxyquinoline derivative. The synthetic strategies
for fluorene- and imidazole- based light-emmting molecules 3 and
4 are outlined in Scheme 1. We also investigated their thermal
stabilities, fluorescence properties and use in single-layer OLEDs,
where the results indicated that the organic materials have better
thermal stability with luminescent properties for promising single-
layer OLEDs.
To have an efficient and practical OLED device, the requirements
for the materials include low ionization potentials, highly thermal
stability, ease of forming amorphous glass and high quantumyield of
photoluminescence.1,2 Undoubtedly, approaches for improving the
thermal stability of organic molecules without reducing fluores-
cence are needed. Imidazole derivatives show unique chemical and
physical properties because they contain imidazole heterocycle
which has better thermal stability, and benzene rings can increase
the degree of conjugation of the organic molecule. Fluorene de-
rivatives also show high quantum yield of photoluminescence and
high thermal stability,3 as well as their high hole mobility which
have been extensively used as hole-transporting materials in
OLEDs.4 In addition, research suggests that the zinc analog of light-
emitting materials AlQ3,5,6 zinc bis(8-hydroxyquinoline) (ZnQ2) has
a better injection efficiency, a lower operating voltage, and higher
quantum yields than the commonly used AlQ3.7 Inspired by these
* Corresponding author. Tel./fax: þ86 20 85212511.
E-mail address: zenghp@scnu.edu.cn (H.-P. Zeng).
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.06.025

6326
T.-T. Wang et al. / Tetrahedron 65 (2009) 6325–6329
O
O
+
OHC
CHO
N
CHO
CHO
N
H
Ac2O
N
CH3
OH
N
N
N
H
N
HO
HO
2
1
ZnAc2
N
N
H
N
Zn
N
O
O
O
Zn
H
N
O
N
N
N
4
3
Scheme 1.
2. Results and discussion
The thermal properties of the zinc(II) complexes 3 and 4 were
determined by differential scanning calorimetry (DSC) and thermal
gravimetric analysis(TGA) measurement from 20 to 1000 ꢀC under
nitrogen atmosphere with a heating rate of 10 ꢀC/min. The thermal
decomposition temperature of compound 3 is 395 ꢀC, while com-
pound 4 is 435 ꢀC due to the good stability of imidazole ring. And
the glass transition temperatures of compound 3 and 4 are 187 and
144 ꢀC, respectively. The results indicate that both of them possess
good thermal stability.
2.1. Synthesis and characterization
Scheme 1 shows the synthetic route to chemically modify
8-hydoxyquinoline, allowing to fine-tune its optical and thermal
properties. The two complexes were derived from substituted
styryl-8-hydroxyquinol bearing fluorene and imidazole group
which were from the reaction of 2-methyl-8-hydroxyquinoline and
corresponding aldehydes in acetic anhydride. The substitution of an
electron-donating substituent at the 4- or 2-position of 8-hydroxy
quinoline will provoke a blue shift of the absorption spectrum of
the corresponding complexes. Imidazole derivatives show unique
chemical and physical properties because they contain imidazole
heterocycle which has better thermal stability, and benzene rings
can increase the degree of conjugation of the organic molecule.
From simple MOPAC molecular modeling, two benzene rings twist
out of the imidazole plane, which may help prevent intermolecular
electrostatic interactions between the molecules.23 Although thus
structure may decrease the degree of the conjugation of the
molecule, the imidazole moieties are electron rich and electron
donating due to the lone-pair electrons in the sp3 nitrogen atoms.
2.2. Photophysical properties
The absorption maximum (labsmax) of the two complexes in
DMF were 351 nm and 377 nm, respectively, and showed a blue-
shift compared to ligands (360 nm and 384 nm). The emission
spectra of the ligands 1 and 2 in different solvents showed large
solvatochromism, which indicates that these molecules permit
strong intramolecular charge transfer from the donor to the ac-
ceptor. The emission maximum lem of compound 2 increases as the
solvent polarity increases, and the emission color ranges from blue
to green. As shown in Figure 1, the solid-state PL maximum of 3 and
4 located at 598 and 607, respectively, which is red-shifted

T.-T. Wang et al. / Tetrahedron 65 (2009) 6325–6329
6327
607nm
557nm
(b)
1.2
(a)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
575nm
515nm
527nm
598nm
PL of 4 in DMF
solid PL of 4
EL of 4
466nm
solid PL of 3
1.0
0.8
0.6
0.4
0.2
0.0
EL of 3
PL of 3 in DMF
Solid PL of Zn(mHQ)2
400
500
600
700
Wavelength/nm
800
900
400
500
600
Wavelength/nm
700
800
Figure 1. PL emission spectra and EL spectra of complex 3(a) and 4(b).
compared to the solution emission (466, 527 nm) and the solid-
state emission of Zn(mHQ)2 (515 nm).
centered at 557 nm, which showed a red shift compared with device
III fabricated by Zn(mHQ)2, as shown in Figure 2a. Note, however, that
the electroluminescence (EL) spectrum of devices fabricated using 4
showed a shift toward shorter wavelengths with a maximum at
557 nm compared to a maximum of 607 nm in solid-state photo-
luminescence spectra, and a shift toward longer wavelengths com-
pared to a maximum of 527 nm in solution photoluminescence
spectra. It may be excimer formation between adjacent lumophors in
solid and intermolecular processes that are greatly reduced in dilute
solution, leading to a blue-shift of the emission spectra.
The fluorescence quantum yields (FPL) of the complexes 3 and 4
were measured in DMF solutions at concentrations of 1ꢂ10ꢁ6 mol/L
and then calculated by Eq. 1, where 4s and 4std stand for the
quantum yields of unknown and standard samples (4std¼0.71, for
RhB in ethanol),24 As (<0.05) and Astd are the solution absorbance at
the excitation wavelength (lex), Is and Istd are the integrated emis-
sion intensities, and hs and hstd are the refractive indices of the
solvents for unknown and standard samples respectively. The
emission quantum yields of the complexes 3 and 4 measured in
DMF are 0.168 and 0.273, respectively, which are higher than that of
Alq3 in DMF (0.116).25
150
500
Device I
2
Device II
4s
¼
4stdðAstd=AsÞðIs=IstdÞðhs
=
hstd
Þ
(1)
400
Device III
100
2.3. Electroluminescence
300
In order to investigate the EL properties of bis-[20-200-(9H-fluoren-
2-yl)-vinyl-8-hydroxyquinoline] zinc(II) (3) and bis-[20-400-(4,5-
diphenyl-1H-imidazol-2-yl) styryl-8-hydroxyquinoline] zinc(II) (4),
two single-layerdevices (as showninFig. 2) of tin oxide(ITO)/3 or 4/Al
were fabricated with a contrastive device fabricated by bis(2-methyl-
8-hydroxyquinoline)zinc(II) [i.e., Zn(mHQ)], marked with device I, II
and III.
Zinc complex compounds 3 and 4 having high emission quantum
yields, were subjected to EL studies on an indium tin oxide (ITO) an-
ode. Figure 3 shows the electroluminescence properties of the single-
layer OLEDs with the structures of ITO/3, 4 or Zn(mHQ)2 (50 nm)/
Al(100 nm), which were fabricated by the conventional vacuum de-
position and subsequently investigated for their electroluminescent
(EL) properties. The single-layer device I fabricated by 3 exhibited
lemon-yellow emissionwith the peak centered at 575 nm, and device
II fabricated by 4 exhibited yellow-green emission with the peak
200
50
100
0
0
0
5
10
Voltage/V
15
20
Figure 3. Current density–voltage–luminance curves (J–V–L) characteristics for the
three single-layer OLEDs. Red lines stand for the current density-voltage curves and
blue lines stand for luminance-voltage curves.
Figure 3 shows the current density and luminance versus applied
voltage characteristics for the three single-layer devices. The single-
layer device I exhibited good performance with a maximum
N
O
Zn
N
O
Al (100nm)
50nm
H3C
N
3
O
Zn
O
or
N
CH3
N
ITO
N
N
Zn
Glass
H
O
O
H
N
4
N
N
Figure 2. Structure of the single-layer OLED.

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T.-T. Wang et al. / Tetrahedron 65 (2009) 6325–6329
luminance of 489 cd mꢁ2 and a luminance efficiency of up to
0.41 cd Aꢁ1, whereas device II exhibited a maximum luminance of
323 cd mꢁ2 and a luminance efficiency of up to 0.54 cd Aꢁ1, which
are higher than the device III (114 cd mꢁ2, 0.1 cd Aꢁ1). From the re-
sults, we can see that fluorene-based complex 3 and imidazole-
based complex 4 are good light-emitting materials to fabricate sin-
gle-layer OLED. The curves indicate that the luminance under the
same potential increased in the order III<II<I, suggesting that using
these light-emitting materials as the active organic layer can en-
hance the performance in single-layer device. The external quantum
efficiency, hext, which is defined as the ratio of number of emitted
electrons in the forward viewing direction to the number of injected
electrons, is useful in understanding the fundamental physical
mechanisms responsible for light emission within an OLED. In this
paper, the external quantum efficiencies of the three devices are
0.56, 0.62 and 0.2 at maximum luminance, respectively. However, in
the present study, the conditions have not been optimized, such as
electrodes and the thickness of the active organic layer, so the cur-
rent densities in the devices are low, which may be due to the in-
sufficiency of the carriers.
4.2. Synthesis
4.2.1. Synthesis of 20-200-(9H-fluoren-2-yl)-vinyl-8-hydroxyquinoline
(1) and 20-400-(4,5-diphenyl-1H-imidazol-2-yl)styryl-8-
hydroxyquinoline (2)
A mixture of 2-methyl-8-hydroxyquinoline (0.954 g, 6 mmol),
2-fluorene carboxaldehyde (1.164 g, 6 mmol) or 4-(4,5-diphenyl-
1H-imidazol-2-yl)benzaldehyde (1.95 g, 6 mmol)and acetic anhy-
dride (25 mL) was stirred and heated at 125 ꢀC–130 ꢀC for 40 h
under nitrogen atmosphere. After cooling, the mixture was poured
into ice water (100 mL) and stirred 4–5 h, and a brown black solid
was obtained by filtration. The solid was washed with water twice,
and then purified by silica gel (100–200 mesh) column chroma-
tography using ethyl acetate/petroleum ether as an eluent.
20-200-(9H-Fluoren-2-yl)-vinyl-8-hydroxyquinoline (1): yellow
solid. Yield 0.715 g, 75%; mp 160–162 ꢀC; Rf¼0.45 (ethyl acetate/
petroleum ether, V:V¼1:5); IR (KBr)
n: 3407.69, 3050.32, 2927.29,
1633.02, 1555.06, 966.25; MS (ESI) m/z (%): 337.40 [(MþH)þ]; 1H
NMR (CDCl3):
d
8.13 (d, J¼8.80 Hz, 1H), 7.67 (d, J¼8.80 Hz, 2H), 7.58
(d, J¼7.2 Hz, 1H), 7.178 (d, J¼6.4 Hz, 1H), 7.28–7.36 (m, 2H), 7.38–
7.48 (m, 3H), 7.79–7.85 (m, 4H), 3.97 (s, 2H); 13C NMR (CDCl3):
d
153.72, 151.98, 143.82, 143.67, 142.54, 141.20, 137.96, 136.34,
3. Conclusion
134.95, 134.71, 129.71, 128.37, 127.37, 127.29, 127.05, 126.58, 125.29,
125.08, 123.55, 120.32, 120.09, 117.65, 110.13, 36.83. Calculated for
C24H17NO: C, 85.94%; N, 4.18%; H, 5.11%. Found: C, 85.90%; N, 4.20%;
H, 5.15%.
Two newly synthesized light-emmting materials bis-[20-200-
(9H-fluoren-2-yl)-vinyl-8-hydroxyquinoline] zinc(II) (3) and bis-
[20-400-(4,5-diphenyl-1H-imidazol-2-yl)styryl-8-hydroxyquinoline]
zinc(II) (4) containing 8-hydroxyquinoline and fluorene or imid-
azole have been used in the single-layer OLEDs for the first time.
The two materials have good stabilities and their thermal de-
composition temperatures are 395 ꢀC and 435 ꢀC. They also ex-
hibit better fluorescence quantum yields (0.168 and 0.273) than
Alq3 (0.116). The optical properties of these complexes are influ-
enced by the styryl substituents, and emit orange in solid state,
which is red-shifted compared with the solution emission. The
two complexes exhibit intriguing ambipolar carrier transport
properties and can be used to fabricate single-layered EL devices
possessing good performance, compared with that of Zn(mHQ)2.
Because of good luminescence properties and high thermal sta-
bility, the two compounds 3 and 4 show great potential for use as
an active organic layer in single-layer OLEDs.
20-400-(4,5-Diphenyl-1H-imidazol-2-yl)styryl-8-hydroxyquino-
line (2): yellow solid. Yield 0.95 g, 34%, mp 228–230 ꢀC, Rf¼0.22
(ethyl acetate/petroleum ether, V:V¼3:8). MS (ESI) m/z:
467[(MþH)þ]; UV (in DMF) lmax: 294, 384 nm; IR (KBr, cmꢁ1
) n:
3342.28, 3049.45,1623.94,1506.72,1243.32, 968.51, 836.42, 766.45,
697.62; 1H NMR (DMSO-d6)
d
(ppm): 12.85 (s, 1H), 8.32 (d, J¼8.4 Hz,
1H), 8.19 (m, 3H), 7.84 (d, J¼13.5 Hz, 1H), 7.82 (d, J¼13.5 Hz, 1H),
7.59–7.33 (m, 14H), 7.11 (d, J¼7.2 Hz, 1H); 13C NMR (101 MHz,
DMSO)
d 153.81, 153.38, 145.65, 138.63, 136.95, 136.71, 134.26,
130.66, 128.85, 128.61, 128.13, 128.00, 127.51, 126.02, 121.49, 118.01,
111.64. Calculated for C32H23N3O: C, 82.56%; N, 9.03%; H, 4.98%.
Found: C, 82.95%; N, 8.98%; H, 5.07%.
4.2.2. Synthesis of the metal complexes 3 and 4
Zn(CH3COO)2 (0.092 g, 0.5 mmol) was dissolved in tetrahydro-
furan (15 mL), and then added drop-wise to the solution of com-
pound 1 (0.336 g, 1 mmol) and 2 (0.466 g, 1 mol) in tetrahydrofuran
(50 mL). After stirred for 24 h under nitrogen atmosphere, yellow
precipitates were obtained, and then filtered off, washed with
tetrahydrofuran and methanol 3–5 times respectively, and dried in
vacuum. Due to the solubility of the zinc(II) complexes 3 and 4 is
not good, 1H NMR spectra didn’t obtain.
4. Experimental section
4.1. Chemicals and instruments
4-(4,5-Diphenyl-1H-imidazol-2-yl)benzaldehyde was synthe-
sized by Weidenhagen reaction according to the literature.26 2-
Methyl-8-hydroxyquinoline and 2-Fluorenecarboxaldehyde were
purchased from Tokyo Kasei Kogyo Co. All chemicals were used
without further purification. Solvents were distilled prior to use. All
reactions were carried out under nitrogen atmosphere. The exper-
imental course was monitored by thin layer chromatography (TLC).
Melting points were determined using an XT-4 microscope melting
point inspection instrument and the thermometer was uncorrected.
Infrared (IR) spectra were recorded on a BRUKE Tensor27 Fourier
transform infrared (FTIR) spectrometer and measured as KBr pellets.
1H nuclear magnetic resonance (1H NMR) spectra and 13C nuclear
magnetic resonance (13C NMR) were recorded on a Bruker DRX-
400 MHz spectrometer with TMS as internal standard and DMSO-
d6/CDCl3 as solvent. ESI-MS spectra were performed with DECA XP
MAX LCQ. UV visible spectra were measured with a Shimadzu UV-
2550 spectrophotometer. Fluorescence spectra and efficiency were
acquired at 28 ꢀC on an Edinburgh Co. FL920 spectrofluorimeter.
Thermal analyses were performed with a TGA/DSC Q600 SDT TA
Instuments apparatus at an elevation temperature of 10 ꢀC/min.
Bis-[(E)-20-200-(9H-fluoren-2-yl)-vinyl-8-hydroxyquinoline] zinc
(II) (3). Yield 0.33 g, 91%, mp >300 ꢀC, IR (KBr)
n: 3050.40, 2920.55,
2854.32, 1622.48, 1555.06, 1102.20, 520.50, 468.06; FABMS: m/z:
732.18[(MꢁH)þ]. Calculated for C48H32N2O2Zn: C, 78.53%; N, 3.82%;
H, 4.39%. Found: C, 78.23%; N, 3.80%; H, 4.33%.
Bis-[20-400-(4,5-diphenyl-1H-imidazol-2-yl)
styryl-8-hydroxy
quinoline] zinc(II) (4). Yield 0.32 g, 64%, mp >300 ꢀC, IR (KBr,
cmꢁ1): 3048.08, 2920.45, 2854.33, 1628.11, 1598.22, 1503.66,
1433.51, 1102.08, 960.99, 834.25, 696.30; FABMS: m/z: 993. Calcu-
lated for C64H44N6O2Zn: C, 77.30%; N, 8.45%; H, 4.46%. Found: C,
77.41%; N, 8.39%; H, 4.38%.
4.3. Devices fabrication and charaterization
In order to investigate the EL properties of bis-[(E)-20-200-(9H-
fluoren-2-yl)-vinyl-8-hydroxyquinoline] zinc(II) (3) and bis-[20-400-
(4,5-diphenyl-1H-imidazol-2-yl) styryl-8-hydroxyquinoline] zinc(II)
(4), two single-layer devices (as shown in Fig.1) of tin oxide(ITO)/3 or

T.-T. Wang et al. / Tetrahedron 65 (2009) 6325–6329
6329
4/Al were fabricated with a contrastive device fabricated by bis(2-
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Supplementary data
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.tet.2009.06.025.