Synthesis and 1.1 μm near-infrared electrophosphorescence properties of a phenoxy-substituents copper phthalocyanine

Article abstract of DOI:10.1134/S0036023609030139

A novel copper phthalocyanine bearing phenoxy-substituents was prepared and characterized by MS and Elemental analysis. Its UV/Vis absorption and photoluminescence (PL) spectra were investigated. Organic light-emitting devices (OLEDs) were demonstrated by

Full text of DOI:10.1134/S0036023609030139

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2009, Vol. 54, No. 3, pp. 407–412. © Pleiades Publishing, Inc., 2009.
COORDINATION
COMPOUNDS
Synthesis and 1.1 mm Near-Infrared Electrophosphorescence
Properties of a Phenoxy-Substituents Copper Phthalocyanine1
Wei Hea, Chuan-Hui Chenb, Shu-Kun Yub, Zhao-Qi Fanb, Xi-Guang Duc, and Guo-Tong Dua
a Department of Physics, Dalian University of Technology, Dalian 116023, P. R. China
b State Key Laboratory of Integrated Optoelectronics, Jilin University, Changchun 130021, P. R. China
c Department of Chemistry, Northeast Normal University, Changchun 130000, P. R. China
E-mail: heweicherr79@yahoo.com.cn
Received February 04, 2008
Abstract—A novel copper phthalocyanine bearing phenoxy-substituents was prepared and characterized by
MS and Elemental analysis. Its UV/Vis absorption and photoluminescence (PL) spectra were investigated.
Organic light-emitting devices (OLEDs) were demonstrated by employing this copper phthalocyanine doped
into 4, 4'-N,N'-dicarbazole-biphenyl (CBP). Room-temperature electrophosphorescence was observed at about
1.1 µm due to transitions from the first excited triplet state to the singlet ground state (T1–S0) of this CuPc. The
intensity of NIR emission at lower doping concentrations (about 10 wt %) was extremely high compared with
devices doped with fluorescent dyes. The results indicated that direct charge trapping appears to be the domi-
nant mechanism.
DOI: 10.1134/S0036023609030139
1
Since the first report of multi-layer organic light-emitting
diodes (OLEDs) by Tang and Van Slyke [1] OLEDs have
been developed remarkably because their applications in
full-color flat-panel displays [2–4]. Most of the research
focused on devices that emitted visible light. Recently NIR
OLEDs have received attention due to their potential appli-
cation in laser technology, optical sensors and optical com-
munication. However, most of materials for use in NIR
OLEDs were organic complexes with trivalent rare earth
ions such as Er3+, Nd3+, Tm3+ and Yb3+ [5–11]. Only few
organic materials containing no rare earth ions showed EL
characteristic in near-infrared region [12].
EXPERIMENTAL
Instruments
Mass spectrum (MS) was obtained on a LDI-
1700-TOF mass spectrometer (Linear Scientific Inc.,
USA). H nuclear magnetic resonance (NMR) spec-
1
tra were recorded on a Bruker AV 500 spectrometer.
Elemental analysis was performed on a Flash
EA1112 Elemental Analyzer (ThermoQuest, Italy).
UV/Vis spectra were carried out on a UV-3100 UV-
Vis-NIR Recording Spectrophotometer (SHI-
MADZU, Japan). Current (I) versus voltage (V)
measurements were obtained on a Keithley 2400 cur-
rent-voltage source. The NIR photoluminescence
(PL) spectra were determined on a PL9000 Photolu-
minescence System (BIO-RAD Micromeasurements
Phthalocyanine, which was first developed as a pigment,
has found widespread applications in materials science. Ltd, UK). The NIR EL signals were focused into a
monochromator and detected with a liquid-nitrogen-
cooled Ge detector, using standard lock-in tech-
niques.
Many potential applications are expected for these molecu-
lar materials which have a high thermal and chemical stabil-
ity, for instance, solar cell functional materials [13], gas sen-
sors [14], nonlinear optical limiting devices [15], photody-
namic therapy agents [16]. Recently, Pcs have been used as
light-emitting material.
Synthesis of 4-(2-isopropyl-5-
methylphenoxy)phthalonitrile
A mixture of 2-isopropyl-5-methylphenol (1.5 g,
10 mmol), 4-nitrophthalonitrile (1.73 g, 10 mmol)
and LiOH · H2O (0.42 g, 10 mmol) in anhydrous
DMSO (30 mL) was stirred at room temperature for
72 h. The reaction mixture was poured into NaCl
solution (10%, 400 mL) and the precipitate was iso-
In this paper, a novel copper-tetra (2-isopropyl-5-meth-
ylphenoxy)phthalocyanine (TIMP PcCu) was synthesized
and characterized by MS, elemental analysis and UV/Vis
absorption. In order to study its electroluminescent (EL)
properties, we prepared single-layer devices and multiple- lated by filtration. Further purification was achieved
by column chromatography on silica gel with petro-
layer devices.
leum/ether (9 : 1) as the mobile phase to afford col-
1
1
The article is published in the original.
orless crystals. Yield: 83%. H NMR (500 MHz,
407

408
HE et al.
CDCl3): d = 7.708 (d, 1 H, J = 8.5 Hz, ArH), 7.295 (d, 1 H, single-layer and the type ITO/NPB/CBP: CuPc/Alq3/Al
J = 8.5 Hz, ArH), 7.219 (s, 1 H, ArH), 7.189 (d, 1 H, J =
8 Hz, ArH), 7.109 (d, 1 H, J = 8 Hz, ArH), 6.745 (s, 1 H,
ArH), 2.958 (m, 1 H, CH), 2.331 (s, 3 H, ArCH3), 1.154 (d,
6 H, J = 6.5 Hz, 2CH(CH3)2).
were fabricated by vacuum evaporating with a base pressure
of 3 × 10–4 Pa. A 35-nm-thick film of N, N'-di-1-naphthyl-
N, N'-diphenylbenzidine (NPB) served as the hole-transport
layer (HTL). A 30-nm-thick TIMP PcCu doped CBP layer
was deposited as light-emitting-layer (EML) by simulta-
neous evaporation from two separate sources. A 15-nm-
Synthesis of Copper-Tetra (2-isopropyl-5-
methylphenoxy)phthalocyanine
thick
2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline
(BCP) was deposited as hole-blocking layer (HBL). After
that, a 20-nm-thick tris-(8-hydroxyquinoline) aluminum
(Alq3) was used to electrons-transport-layer (ETL). Finally
a shadow mask with 2 × 2 mm openings was used to define
the 120-nm-thick Al cathode.
A mixture of 4-(2-isopropyl-5-methylphenoxy)phtha-
lonitrile (1.104 g, 4 mmol), Cu(OAc)2 (0.20 g, 1 mmol), 1-
pentanol (6 mL) and a few drops of 1,8-diazabicy-
clo[5.4.0]undec-7-ene (DBU) was heated to 135°C for 12 h.
After cooling to room temperature the reaction mixture was
poured into 100 mL of methanol, precipitation was col-
lected and adequately extracted by anhydrous methanol in a
Soxlet extractor. The obtained mixture was purified by col-
umn chromatography on silica gel. The target product was
eluted twice by CHCl3/methanol (40 : 1) as a blue band.
Yield: 607 mg, (52%). UV-Vis (chloroform, λ max/nm)
340, 616, 684; MS (LDI-TOF): m/z 1168.6 (M + H+); anal.,
calcd. for C72H64CuN8O4 (1168.4): C, 73.87; H, 5.51; N,
9.57. Found: C, 73.54; H, 5.70; N, 9.23.
RESULTS AND DISCUSSION
Synthesis
The synthetic route of TIMP PcCu is outlined in
Scheme 1.
Copper-tetra(2-isopropyl-5-methylphe-
noxy)phthalocyanine was synthesized according to the pre-
vious published methods [17] with the corresponding phth-
alonitrile derivatives, namely 4-(2-isopropyl-5-methylphe-
noxy)phthalonitrile. Tetrasubstituted phthalocyanines are
usually obtained as a mixture of four constitutional isomers
(D2h : Cs : C2v : C4h). According to reports of Prof.
M. Hanack [18, 19] its isomers should be in the expected
statistical proportion (D2h : Cs : C2v : C4h = 1 : 4 : 2 : 1). The
Device Preparation and Characterization
The structure of devices used in this study is traditional.
The indium-tin-oxide (ITO) with a resistance of 10 Ω/cm
on glass was cleaned sequentially by acetone, ethanol, conclusions of this paper do not depend on having a pure
deionized water and supersonic treatment. The OLEDs of isomer.
CN
CN
CN
OH
DMSO
LiOHi · H2O
+
O
CN
O2N
O
O
N
[–pentanol], DBU
Cu(OAc)
N
N
N
N
2
Cu
N
N
N
O
O
Scheme 1. The synthetic route of copper-tetra (2-isopropyl-5-methylphenoxy)phthalocyanine.
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SYNTHESIS AND 1.1 µm NEAR-INFRARED ELECTROPHOSPHORESCENCE PROPERTIES
409
8000
CBP PL
6000
ABS
TIMPPcCu
4000
2000
0
300
400
500
600
700
800
Wavelength (nm)
Fig. 1. The absorption spectrum of TIMPPcCu film and PL spectrum of CBP at room temperature (excited using 325 nm line from
a He–Cd laser).
1000
800
600
10%
400
15%
5%
600
200
0
400
200
0
100%
600
1000 1400 1800
Wavelength (nm)
800
1000
1200
1400
1600
Wavelength (nm)
Fig. 2. The NIR EL spectra of devices based on TIMPPcCu with different concentrations in CBP (measured under the same condi-
+
tions). Inset: The PL spectra of TIMPPcCu tablet excited using the 488 nm line from an Ar laser at 10 K temperature.
Optical Properties
singlet excited state of the host to the singlet excited
state of the guest.
The UV-Vis absorption spectra of TIMPPcCu film
and PL spectrum of CBP presented in Fig. 1. The
absorption spectrum exhibit the characteristic B band
(corresponding to 340 nm) and Q band (the region of
600–700 nm) of phthalocyanines, which assigned to be
π-π* transition of conjugated systems. Figure 1 also
shows the PL spectrum of CBP. There was an overlap
between the PL spectrum of CBP and the B band of the
TIMPPcCu which indicated that the CBP (host) doped
with TIMPPcCu (guest) system may meet the require-
The inset of Fig. 2 shows the PL spectrum of
TIMPPcCu tablet excited using the 488 nm line from an
Ar+ laser at 10 K temperature. The tablet emitted a
sharp peak centered at around 1180 nm. There exists a
broad band at 1300–1400 nm which can not be
assigned. According the repot before [21], the lumines-
cence of CuPc without any substituents is due to transi-
tions from the first excited triplet state to the ground
state (T1–S0). Phenoxy-substituents does not change the
large spin-orbit coupling, the center metal (Cu) atom is
ment for efficient Förster energy transfer [20] from the not changed yet. So we can conclude the luminescence
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 54 No. 3 2009

410
HE et al.
5000
4000
3000
2000
1000
0
0%
5%
10%
20%
200
300
400
500
600
Wavelength (nm)
Fig. 3. The UV-vis EL spectrum of the doped OLEDs with different TIMPPcCu concentrations.
of TIMPPcCu tablet can be attributed to the phospho- tions. There exists the blue emitting of NPB centering
rescence of the first excited triplet state to the ground
state (T1–S0). PL spectrum at room temperature shows
almost no emitting, which can also prove that the lumi-
nescence of TIMPPcCu tablet is phosphorescence.
at about 440 nm in all the devices. In the doped devices
there almost exists no emitting of CBP centering at
about 380 nm. The results can be understood as fol-
lows. In the case of the low doping concentrations hole
injection from the NPB highest occupied molecular
orbital (HOMO) into the CBP HOMO was energeti-
cally unfavorable because of the large energy differ-
ence between the HOMO levels of NPB and CBP
(0.8 eV) (Fig. 4b). The recombination of the holes
accumulated at the interface of NPB, CBP and elec-
trons injected from CBP layer leads to blue NPB emis-
sion. The HOMO and lowest unoccupied molecular
orbital (LUMO) of TIMPPcCu are –4.97 eV and –3.3 eV
Electroluminescent Properties
The single-layer devices of TIMPPcCu between
ITO and Al electrodes do not show any measurable EL,
the reason for which we attribute to unbalanced charge
injection and transport at the electrode/organic inter-
face. Figure 2 shows the NIR EL spectra of the devices
made by different concentrations of TIMPPcCu doped
in CBP at 10 mA at room temperature. As can be seen, (measured by CV), falling within the band gap of the
the emission at about 1.1 µm coincides with the PL
spectra of TIMPPcCu. The difference of wavelength
between EL and PL is about 80 nm. This difference can
be attributed to the form (tablet or film) and even the
crystal type (α, β, single crystal or polycrystal [21]).
When the concentration of TIMPPcCu is low
(<10 wt %), the intensity of NIR emission increases
with the increasing doping concentration. When the
concentration of TIMPPcCu is high (>10 wt %), the
intensity of NIR emission decreases with the increasing
doping concentration. The reason may be that the effect
of triplet-triplet annihilation becomes remarkable at
higher concentration. The optimum concentration is
10 wt %, which is extremely high compares with
devices doped with other fluorescence dyes.
CBP. So the TIMPPcCu functioned as both a hole trap
and an electron trap. Thus, we concluded that exciton
formation occurred on TIMPPcCu by sequential elec-
tron and hole capture. It is more efficient with increas-
ing of TIMPPcCu concentrations, which leading to the
decrease of NPB EL intensity. The weak emission of
CBP from undoped device indicated that only a small
number of exciton (singlet or triplet) formation
occurred on CBP molecules. That is to say only a small
number of exciton (singlet or triplet) can be involved in
energy transfer of the host (CBP) to guest (TIMPPcCu).
The results suggested that Förster and Dexter energy
transfers play a minor role in these devices, and direct
charge trapping appears to be the dominant mechanism.
The I–V characteristics of OLEDs (Fig. 4a) also
present an evident of the direct charge trapping process.
In the case of lower doping concentrations (<10 wt %)
there appear to be higher driving voltages. When the
doping concentrations were higher than 10 wt % direct
hole injection from NPB into TIMPPcCu and direct
There are two possible ways for electrically exciting
TIMPPcCu in the doped OLED: either by direct carrier
trapping and exciton formation on the TIMPPcCu, or
by exciton formation in the host and energy transfer to
TIMPPcCu guest by Förster [20] or Dexter [22] pro-
cess. Figure 3 shows the UV-vis EL spectrum of the
doped OLEDs with different TIMPPcCu concentra- electron injection from BCP into TIMPPcCu may lead
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SYNTHESIS AND 1.1 µm NEAR-INFRARED ELECTROPHOSPHORESCENCE PROPERTIES
411
3.0
(a)
2.5
0%
5%
2.0
1.5
1.0
0.5
0
10%
20%
100%
0
4
8
12
16
Voltage (V)
LOMO Level
TIMPPcCu
(b)
2.3 eV
NPB
2.9 eV
3.3 eV
3.1 eV
4.1 eV
Al
4.7 eV
ITO
CBP
Alq
3
BCP
5.2 eV
4.97 eV
5.8 eV
6.0 eV
HOMO Level
6.4 eV
Fig. 4. (a) The current-voltage characteristics of OLEDs based on TIMPPcCu. (b) The energy level diagram of the devices contain-
ing TIMPPcCu.
2. C. H. Chen, J. Shi, and C. W. Tang, Coord. Chem. Rev.
to the lower driving voltage. When the doping concen-
tration was 100%, the driving voltage is lowest.
171, 161 (1998).
3. U. Mitschke and P. Bauerle, J. Mater. Chem. 10, 1471
In summary we have prepared a copper-tetra(2-iso-
propyl-5-methylphenoxy)phthalocyanine which was
characterized by MS and Elemental analysis. We have
fabricated the NIR OLEDs emitting at about 1.1µm by
employing the TIMPPcCu. The dominant mechanism
in NIR EL should be direct charge trapping.
(2000).
4. L. S. Hung and C. H. Chen, Mater. Sci. Eng. 39, 143
(2002).
5. W. P. Gillin and R. J. Curry, Appl. Phys. Lett. 74, 798
(1999).
6. R. G. Sun,Y. Z. Wang, Q. B. Zheng, et al., J. Appl. Phys.
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ACKNOWLEDGMENTS
7. Harrison B.S., Foley T.J., Bouguettaya M. et al., Appl.
Phys. Lett. 79 (2001) 3770.
This work was supported by the National Science
Foundation of China under grant no. 20472014.
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