Metal-free phthalocyanine single crystal: Solvothermal synthesis and near-infrared electroluminescence

Article abstract of DOI:10.1016/j.cclet.2016.01.009

A metal-free purple H₂Pc single crystal was synthesized by a facile solvothermal method, and its solubility and near-infrared (NIR) optical properties were also investigated due to its potential applications as a light-emitting layer for OLEDs. The H₂Pc single crystal is insoluble in 1-chlorine naphthalene and other organic solvents. It gives a wide absorption in the range from 620 nm to 679 nm and a wide emission in near 922 nm. As an active light-emitting layer, H₂Pc was employed to fabricate electroluminescent (EL) devices with a structure of ITO/NPB (30 nm)/Alq₃:H₂Pc (30 nm)/BCP (20 nm)/Alq₃ (20 nm)/Al. The emission center is at 936 nm when the H₂Pc doping concentration is 20 wt%. The doping concentration strongly governs the emission intensity. When doping concentration decreases from 10 wt% to 1 wt%, the emission intensity remarkably fades, and simultaneously the emission center undergoes a blue shift.

Full text of DOI:10.1016/j.cclet.2016.01.009

Chinese Chemical Letters 27 (2016) 764–768
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Chinese Chemical Letters
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Original article
Metal-free phthalocyanine single crystal: Solvothermal synthesis
and near-infrared electroluminescence
a
a
a
a
Qing-Long Bai a, , Chun-Hua Zhang , Juan-Juan Song , Jing-Hai Liu , Yan-Chun Feng ,
*
Li-Mei Duan a, Chuan-Hui Cheng b
a Inner Mongolia Key Lab of Chemistry of Natural Products and Synthesis of Functional Molecules, College of Chemistry and Chemical Engineering,
Inner Mongolia University for Nationalities, Tongliao 028000, China
b School of Physics and Optoelectronics Technology Dalian University of Technology Dalian 116024, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A metal-free purple H2Pc single crystal was synthesized by a facile solvothermal method, and its
solubility and near-infrared (NIR) optical properties were also investigated due to its potential
applications as a light-emitting layer for OLEDs. The H2Pc single crystal is insoluble in 1-chlorine
naphthalene and other organic solvents. It gives a wide absorption in the range from 620 nm to 679 nm
and a wide emission in near 922 nm. As an active light-emitting layer, H2Pc was employed to fabricate
electroluminescent (EL) devices with a structure of ITO/NPB (30 nm)/Alq3:H2Pc (30 nm)/BCP (20 nm)/
Alq3 (20 nm)/Al. The emission center is at 936 nm when the H2Pc doping concentration is 20 wt%. The
doping concentration strongly governs the emission intensity. When doping concentration decreases
from 10 wt% to 1 wt%, the emission intensity remarkably fades, and simultaneously the emission center
undergoes a blue shift.
Received 21 July 2015
Received in revised form 1 October 2015
Accepted 24 December 2015
Available online 19 January 2016
Keywords:
Solvothermal synthesis
H2Pc single crystal
Optical properties
Near-infrared
ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Electroluminescence
1. Introduction
phosphorescent peak at 1150 nm. Copper phthalocyanine single
crystal has only a sharp phosphorescent peak around 1120 nm.
Phthalocyanine and phthalocyanine complexes have a special
two-dimensional conjugated electronic structure and strong
The photoluminescence properties in near-infrared region of
phthalocyanine indicated that it could be used in organic NIR
electroluminescent (EL) devices as the light-emitting layer
material. Assour et al. have examined the fluorescence of
phthalocyanine (H2Pc) in 1-hydrogen chloride naphthalene solu-
tion [16], and observed three peaks located at 699 nm, 735 nm, and
778 nm. Fujii et al. reported single-layer H2Pc light-emitting
devices with emission of near-infrared light at 920 nm [17]. Despite
these advances, phthalocyanine (H2Pc) single crystals are difficult
to obtain, and their corresponding fundamental solubility and
optical properties have until now not been rigorously investigated.
Here, we reported a facile solvothermal synthesis to prepare
single-crystalline H2Pc. It gives poor solubility in various organic
solvents, such as 1-chlorine naphthalene. UV–vis spectroscopy and
PL spectroscopy of solid-state H2Pc single crystals show wide
absorption in the range from 620 nm to 679 nm and a wide
emission near 922 nm. The strong aggregations of H2Pc single
crystals weaken fluorescence intensity due to concentration
quenching. Thus, H2Pc was doped with Alq3 as a light-emitting
layer to fabricate multilayer EL devices by a vacuum deposition
method with a structure of ITO/NPB/Alq3:H2Pc/BCP/Alq3/Al. The
emission center is at 936 nm when the H2Pc SC doping
p
p–p
electronic interactions caused by their conjugate ring structures,
giving this variety of compounds their characteristic optical,
electrical, and magnetic properties. Thus, they have potential
applications in nonlinear optical materials [1], optical limiting
complex materials [2,3], molecular electronic components [4,5],
electrochromic materials [6,7], liquid crystal display materials [8,9]
and optical dynamic anticancer treatment drugs [10–12]. Phthalo-
cyanine complex single crystals are accessible and have been
extensivelyusedinOLEDsastheholeinjectionlayermaterial[13,14]
or as a hole buffer layer located between the ITO anode and the
hole transport layer to improve device half-life. Yoshino et al.
studied the near-infrared (NIR) emission spectroscopy of several
phthalocyanine complex single crystals and their corresponding
evaporation deposited films [15]. Zinc phthalocyanine single crystal
has fluorescence peaks at 760 nm and 1000 nm, and a sharp
*
Corresponding author.
E-mail address: baiql1968@sina.com (Q.-L. Bai).
http://dx.doi.org/10.1016/j.cclet.2016.01.009
1001-8417/ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.

Q.-L. Bai et al. / Chinese Chemical Letters 27 (2016) 764–768
765
The samples were stimulated by a He–Cd laser UV light with
emission wavelength at 325 nm. Emitted light was focused by two
lenses, passed through the slit of a grating monochromator, then
detected by a photomultiplier tube to be recorded by the data
acquisition system. The signal was enlarged by using standard phase-
locked amplifier technology. When the near infrared electrolumi-
nescence spectrum of the device was tested, a JT-1 transistor
characteristic tracer generated a 100 Hz saw tooth wave to launch
100 Hz pulse light. The light was aimed at the optical grating
monochromator slits, and the signal was detected by liquid nitrogen
cooling germanium detectors after passing through a monochroma-
tor spectrometer, and the data was recorded by a characteristic X–Y
recorder after a phase-locked amplifier. When the sample was tested
in the near-infrared range, 532 nm Nd YAG green laser was used as
the excitation light source, and chopped into 25 Hz pulses of light.
The light emitted by the samples was focused by two lenses, passed
through the slit of a grating monochromator, and detected by a liquid
nitrogen cooled germanium detector. The data was recorded by
characteristic X–Y recorder after a phase-locked amplifier finally. I–V
characteristic of the device was tested on Keithley 2400 current–
voltage source.
Scheme 1. Synthesis of H2Pc single crystal.
concentration is 20 wt%. The H2Pc doping concentrations influence
emission intensity and blue shift the emission center.
2. Experimental
2.1. Materials and equipments
Solvents were purified according to standard procedures. All
chemicals were obtained commercially and used without further
purification.
Metal aluminum electrode was evaporated in
a vacuum
Absorption spectra were taken on a UV-3600 230 VCE recording
spectrophotometer (Shimadzu, Japan). Versus voltage (V) mea-
surements were obtained using a Keithley 2400 current–voltage
source. NIR PL spectra were measured on a PL 9000.
Photoluminescence System (Bio-Rad Micromeasurements Ltd.,
UK); the NIR EL signals were focused into a monochromator and
detected with a liquid-nitrogen-cooled Ge detector, using standard
lock-in techniques. All the measurements were performed in air at
room temperature.
deposition machine of DM-300B type, which was produced in
Beijing scientific instrument factory. We usually evaporate alumi-
num film when the vacuum reaches 2 ꢀ 10ꢁ3 Pa, and its thickness
was also monitored by quartz vibrator film thickness gauge.
3. Results and discussion
3.1. Synthesis and structure determination
Quinoline is a high boiling point solvent which can be used as
both the solvent for solvothermal synthesis and the culture
solution for single crystal growth. Here, the H2Pc single crystal was
synthesized with phthalonitrile as the original material.
2.2. Synthesis of H2Pc single crystal
The mixture of phthalonitrile (0.051 g, 0.4 mmol), ammonium
molybdate (0.022 g, 0.1 mmol), and urea (0.024 g, 0.4 mmol) in
quinoline (20 mL) was kept at 180 8C in the autoclave for 8 h, and
then cooled back to room temperature (Scheme 1). After removing
the solvent, the target product of H2Pc single crystal (0.0246 g,
48.24% yield) was obtained as a purple crystal.
The single crystal with size of 0.08 mm ꢀ 0.02 mm ꢀ 0.01 mm
was chosen for test by four-circle X-ray single crystal diffractometer.
H2Pc single crystal belongs to monoclinic system with formula of
N8C32H16, space group of P2(1)/n, and crystal cell parameters as
´
´
´
˚
˚
˚
follows: a = 14.768(3) A, b = 4.7209(9) A, c = 17.331(3) A,
a = 908,
´
3
˚
b
= 104.240(3)8,
g
= 908, V = 1171.2(4) A , R1 = 0.0468, wR2 = 0.1209.
2.3. H2Pc electroluminescent devices
The structure of H2Pc is shown in Fig. 1. The central 16-member
ring consists of eight N atoms and eight C atoms, and the C–N
Organic film was prepared by a multiple source organic
molecule vapor deposition system, and the instrument was
produced by the Shenyang institute of Sida vacuum technology.
The gas was pumped with a vacuum pump at a pressure of
4 ꢀ 10ꢁ4 Pa. For the evaporation of organic materials, the vacuum
needs to be maintained at less than 1 ꢀ 10ꢁ3 Pa. The evaporation
rate and film thickness were monitored in real-time through a
quartz vibrator film thickness gauge.
˚
˚
bonds range from 1.320(2) A to 1.370(2) A, which are similar to
those observed in other H2Pc structures. In H2Pc ring, the distances
˚
between two relative N1 atoms is 3.928(3) A and the distances
˚
between two N3 atoms is 3.926(3) A. The packing diagram of H2Pc
is shown in Fig. 2, the H2Pc rings are stacked in a herringbone
fashion, similar to that of the unsubstituted phthalocyanine. It is
found that the intermolecular
p–p stacking is present between
˚
two H2Pc molecules with central-to-central distances of 4.721(2) A
Fig. 1. Structure of H2Pc single crystal (a) and crystal-packing diagram of H2Pc (b).

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Q.-L. Bai et al. / Chinese Chemical Letters 27 (2016) 764–768
1.6
1.2
0.8
0.4
0.0
2500
H2Pc single crystal in solid pellet
H2Pc power in 1-chloronaphthalene solution
2000
1500
1000
500
0
600
800
1000
1200
1400
1600
400
500
600
700
800
900
Wavelength (nm)
Wavelength (nm)
Fig. 3. Photoluminescence (PL) spectra of H2Pc single crystal.
Fig. 2. UV–vis absorption spectra of H2Pc single crystal (blue curve) and H2Pc
power1-chlorine naphthalene as reference (red curve).
along the crystallographic b axis. As one of important types of
the corresponding monomers, it is more suitable for applications in
near infrared organic light-emitting diodes (OLED) [19].
supramolecular force,
p–p stacking shows a specific structural
requirement for substrate recognition or the arrangement of
complicated architectures.
3.4. Electroluminescent performance
3.2. The solubility of H2Pc single crystal
ITO glass was cleaned in acetone and ethanol, then subse-
quently sonicated in acetone, anhydrous ethanol, and deionized
(DI) water for 5 min each. After drying under nitrogen, it was
immediately placed in the multi-source organic molecule vacuum
deposition system. The system was pumped at a pressure below
1 ꢀ 10ꢁ3 Pa, and the material was heated to 50 8C to remove
moisture. After 15 min, temperatures were adjusted to evaporate
and deposit the organic layers in order. Growth thickness and rate
were monitored in real-time by a quartz crystal oscillator film
thickness gauge. The doping process was implemented by growing
two materials at the same time, according to the designed
concentration. Growth rate of each material in the evaporation
process can be regulated by adjusting the source temperature until
they meet the required ratio. A 2 mm ꢀ 2 mm open mask plate was
covered on the organic layers after evaporation to control the light-
emitting area of the device. Various properties were tested at room
temperature immediately after finishing the device fabrication.
Fig. 4 shows the structure of the device, where Alq3 was
employed as the host material, H2Pc as a light-emitting layer
(30 nm), the ITO as the anode, and aluminum (Al, 120 nm) as the
cathode. N,N0-di-1-naphthyl-N,N0-diphenylbenzidine (NPB) and
2,9-dimethyl-4,7-diphenyl-1,10-phenanthrolien (BCP) work as
hole transport layer (30 nm) and a hole blocking layer (20 nm),
respectively. Alq3 functions as electron transfer and injection layer
(20 nm).
H2Pc single crystal is insoluble in common organic solvents
such as CHCl3, CH2Cl2, DMF, DMSO, THF, toluene, and pyridine,
because it has no substituent introduced and no center metal ion
coordinated. Aggregation occurs due to the
interactions between phthalocyanine molecules, which results in
even poor solubility in large electron conjugated system and
p-conjugate system
p
strong polar 1-chlorine naphthalene solvents.
3.3. The optical properties of the H2Pc single crystal
As shown in Fig. 2 wide absorption peak from 620 nm to
679 nm was observed for H2Pc single crystal in the solid-state
ultraviolet–visible (UV–vis) spectra, corresponding to the two Q
zone (664 nm and 699 nm) of H2Pc powder in 1-chlorine
naphthalene (reference). The blue shift and broadening of the
peaks in the single-crystalline H2Pc compared to the dilute
solution are the result of interactions that arise from aggregated
molecules compared to dissolved monomers [18]. Another strong
absorption peak is located at 335 nm, corresponding to the B zone
in the UV–vis spectra of the H2Pc solution.
PL spectra were obtained from single-crystal H2Pc ground with
KBr and pressed into pellets, and the spectra showed a broad
emission band at 922 nm (Fig. 3). This 223 nm red shift and peak
broadening compared to the dilute solution in 1-chlorine
naphthalene is again due to the different state of the molecules.
The fluorescence in dilute 1-chlorine naphthalene solution is from
H2Pc monomers, but in the single crystal, it is from the molecules
packing state. H2Pc molecules are closely stacked, and the effective
distances between molecules are very small. Strong intermolecular
forces between molecules influence molecular behavior and
restrain molecules. Thus, the emission was derived from the
H2Pc condensed state, not produced by any individual molecules.
The most common aggregation is composed of two molecules.
When two identical molecules interact and emit a photon, the two
molecules form an excimer, which can generate a new, stronger
and wider emission peak at a longer wavelength range. Single
crystal PL spectra may be from the H2Pc excimers. As the
luminescence wavelength for H2Pc single crystal was longer than
Al
Alq3
BCP
Alq3:H2Pc
NPB
ITO
Glass
Fig. 4. Structures of device with H2Pc as light-emitting active material.

Q.-L. Bai et al. / Chinese Chemical Letters 27 (2016) 764–768
767
4200
3600
3000
2400
1800
1200
600
20 wt%
4000
3000
2000
1000
0
20 wt%
10 wt%
5 wt%
10 wt%
5 wt%
1 wt%
1 wt%
0
0
2
4
6
8
10
12
600
700
800
900
1000 1100 1200 1300 1400
Voltage (V)
Wavelength (nm)
Fig. 7. Current density–voltage characteristics of device.
Fig. 5. The near infrared (NIR) electroluminescence (EL) spectra of devices with
different H2Pc concentrations.
holes on H2Pc directly form excitons, which gives luminescence
after excited state transition to the ground state. Fo¨rster energy
transfer is mainly from the dipole interaction of donor and
receptor, which depends on the distance between the donor and
acceptor molecules. The occurrence probability is proportional to
the degree of overlap between donor emission spectra and
receptor absorption spectra. Due to the small overlap between
absorption spectrum of H2Pc and Alq3 light spectrum (Fig. 1),
Fo¨rster energy transfer gives little contribution into the system.
In the energy level diagrams in Fig. 6, the HOMO and LUMO of
H2Pc are 5.46 eV and 3.35 eV, respectively, [21] which is located
within the Alq3 forbidden band. This result indicates that the H2Pc
in the device can work as both the hole trap and electron trap,
which determines the direct space charge limited mechanism.
Holes were injected from the ITO, passed through NPB layer, and
captured by H2Pc, while electrons were injected from Al cathode,
penetrated into BCP, and captured by H2Pc. The trapped holes and
electrons on H2Pc generate H2Pc excited state. Fig. 7 shows the
current density–voltage (I–V) characteristics for the device with
different H2Pc doping concentrations. The results illustrate a low
driving voltage of around 4 V. The driving voltage for the device
decreases as H2Pc doping concentrations increases from 1 wt% to
20 wt%. This originated from the close effective distances and
enhanced interactions of densely distributed H2Pc molecules for
high doping concentration, which also supports the direct charge
space charge limited mechanism in device luminescence.
Fig. 5 shows electroluminescent (EL) spectra of the device in
near infrared at 8 mA working current with different H2Pc doping
concentrations. When the doping concentration is 20 wt%, a wide
and strong emission band with the center of emission wavelength
of near 936 nm was observed, which is similar to the PL peak.
Therefore, the emission band can be derived from H2Pc molecular
aggregation. EL intensity is also affected by the doping concentra-
tion. The EL intensity decreased as the doping concentrations of
H2Pc decreased from 10 wt% to 1 wt%. As for the EL spectra, a blue
shift of the emission wavelength was observed at the same time.
The blue shift may be induced by the various aggregation degrees
of H2Pc molecular at different doping concentrations [20].
When the device is powered, the excitons formed in the light-
emitting layer will convey the energy to H2Pc molecular
aggregates, and radiative transitions in the aggregates gives
luminescence. When doping H2Pc into Alq3, Alq3 works as the
diluent. When the H2Pc concentration is high, the distribution of
H2Pc molecules is dense, and the molecule interactions are strong.
With decreasing H2Pc concentration, the distribution is looser, and
the interaction between molecules is weaker as the distance
between the molecules becomes bigger. Therefore, H2Pc doping
concentrations give rise to the blue shift of the emission
wavelength and the weakening of EL intensity.
H2Pc electroluminescence (EL) mechanism in OLED devices
might be a space charge limited mechanism. H2Pc works as both
the hole trapping center where the holes are injected from ITO
electrode and transmitted into each functional layer, and
the electron trapping center where the electrons pass through
Al electrodes into each functional layer. The captured electrons and
4. Conclusion
H2Pc single crystal was successfully synthesized by one-step
solvothermal method. Compared with absorption spectroscopy
and optical luminescence of H2Pc solution, H2Pc single crystal
exhibits a broadening absorption peak with a large blue shift, and a
broadening PL peak with a significant red shift at around 922 nm.
With H2Pc doped Alq3 as a light-emitting layer, the EL device gives
an emission center at 936 with doping concentration of 20 wt%.
The device can be driven at low voltage of 4 V. The EL emission
intensity and blue shift are affected by H2Pc doping concentration
in Alq3. The facile synthesis of H2Pc crystal and the near-infrared EL
properties of H2Pc based light-emitting layer will enhance the
applications of H2Pc crystal in OLEDs.
Vacuum level
2.3eV
LUMO
electron
-2.9eV
-3.1eV
-3.1eV
-3.35
-4.1eV
Al
hole
-4.7eV
ITO
5.2eV
-5.46
H2Pc
-5.8eV
NPB -5.8eV
Alq3
HOMO
-6.4eV Alq3
BCP
Acknowledgments
H2Pc:Alq3
mixed layer
This work was financially supported by the Ph.D. Initiative
Science Foundation of Inner Mongolia University for the Nationalities
Fig. 6. Energy level diagram of the device.

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Q.-L. Bai et al. / Chinese Chemical Letters 27 (2016) 764–768
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