Highly efficient orange phosphorescent organic light-emitting diodes based on an iridium(III) complex with diethyldithiocarbamate (S^S) as the ancillary ligand

Article abstract of DOI:10.1039/c6ra12529h

A novel iridium(iii) complex Ir(dpp)₂(dta) (dppH = 4,6-diphenyl pyrimidine; dta = diethyldithiocarbamate) was synthesized and characterized. The complex displayed strong emissions at 575 nm with high photoluminescence quantum yields of 86% in the doped poly(methyl methacrylate) (PMMA) film at 1% doping concentration. The orange-yellow polymer light-emitting devices (PLEDs) based on Ir(dpp)₂(dta) as a triplet emitter with different concentrations (x = 1%, 3%, 5%, 7% and 10%) doped in a polymeric host 70% poly(N-vinylcarbazole) (PVK) + 30% 2,2′-(1,3-phenylene)bis[5-(4-tert-butylphenyl)-1,3,4-oxadiazole] (OXD-7) were fabricated with high luminance and efficiency. The device achieved an ideal turn-on voltage (<4 V) and superior luminance (81 918 cd m^-2) as well as electroluminescence efficiency (30.12 cd A^-1) at 3% doping concentration.

Full text of DOI:10.1039/c6ra12529h

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Highly efficient orange phosphorescent organic
light-emitting diodes based on an iridium(III)
complex with diethyldithiocarbamate (S^S) as the
ancillary ligand†
Cite this: RSC Adv., 2016, 6, 64003
Qunbo Mei, Chen Chen,a Ruqiang Tian,a Min Yang,a Bihai Tong,b Qingfang Hua,a
a
*
a
a
a
*
Yujie Shi, Quli Fan and Shanghui Ye
A novel iridium(III) complex Ir(dpp)2(dta) (dppH ¼ 4,6-diphenyl pyrimidine; dta ¼ diethyldithiocarbamate)
was synthesized and characterized. The complex displayed strong emissions at 575 nm with high
photoluminescence quantum yields of 86% in the doped poly(methyl methacrylate) (PMMA) film at 1%
doping concentration. The orange-yellow polymer light-emitting devices (PLEDs) based on Ir(dpp)2(dta)
as a triplet emitter with different concentrations (x ¼ 1%, 3%, 5%, 7% and 10%) doped in a polymeric host
70% poly(N-vinylcarbazole) (PVK) + 30% 2,20-(1,3-phenylene)bis[5-(4-tert-butylphenyl)-1,3,4-oxadiazole]
(OXD-7) were fabricated with high luminance and efficiency. The device achieved an ideal turn-on
voltage (<4 V) and superior luminance (81 918 cd mꢀ2) as well as electroluminescence efficiency (30.12
cd Aꢀ1) at 3% doping concentration.
Received 13th May 2016
Accepted 24th June 2016
DOI: 10.1039/c6ra12529h
www.rsc.org/advances
stands for an ancillary bidentate ligand.3 In general, the cyclo-
metalating ligands (C^N) are usually aromatic heterocyclic rings
containing C, N, S, O atoms, etc.4 Aromatic heterocyclic
Introduction
Organic light-emitting diodes (OLEDs) have become a hot
research eld and have attracted great attention due to their
potential applications in organic at-panel displays since Tang
reported efficient electroluminescence by using tris(8-
hydroxyquinolinato) aluminium (AlQ3) as an emitter in the
rst efficient small molecule OLEDs in 1987.1 The organic
electrophosphorescent materials play a critical role because
they can fully utilize both singlet and triplet excitons through
the strong spin-orbital coupling caused by heavy metal ions in
complexes. Theoretically, the internal quantum efficiency of the
phosphorescent OLEDs (PhOLEDs) can reach up to 100%.2
Among these emitting materials for OLEDs, iridium(III)
complexes are the most promising candidates. Iridium(III)
complexes are usually constituted by three bidentate ligands,
either homoleptic (C^N)3Ir or heteroleptic (C^N)2Ir(L^X), where
C^N represents a cyclometalating bidentate ligand and L^X
compounds containing N atoms such as pyrimidine derivates
and pyridine derivates are expected to be electron-decient
units which have been used to construct novel cyclometalated
organic ligand and electron-transporting materials.5 In 2003,
Tsuboyama et al. designed and synthesized a series of neutral
facial homoleptic cyclometalated iridium(III) complexes based
on pyridine derivates cyclometalating ligands (C^N) for use as
red-emissive materials in OLEDs.6 Cyclometalated organic
ligands containing pyrimidine unit has high electron affinities
and may possibly afford iridium(III) complexes with high
quantum efficiency.7 These pyrimidine-based iridium(III)
complexes have better properties than pyridine-based iridiu-
m(III) complexes, so iridium(III) pyrimidine complexes have
attracted much attention for their high device performances in
the past decade.5,8–11 Some new iridium(III) complexes using 2-
substituted pyrimidine derivatives as the cyclometalated
ligands were synthesized.5,8,9 Chang et al. designed 5-
substituted pyrimidine chelates and the corresponding iridium
complexes. With this new phosphor design, the best phospho-
aKey Laboratory for Organic Electronics and Information Displays & Institute of
Advanced Materials(IAM), Jiangsu National Synergetic Innovation Center for
Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, 9
Wenyuan Road, Nanjing 210023, China. E-mail: iamqbmei@njupt.edu.cn;
iamshye@jupt.edu.cn
rescent OLED exhibited a peak external quantum efficiency of
17.9%, a luminance efficiency (LE) of 38.0 cd Aꢀ1
. Jiang et al.
10
bCollege of Metallurgy and Resources, Anhui University of Technology, Ma'anshan,
used 4,6-disubstituted pyrimidine derivatives as the cyclo-
metalated ligands and synthesized pyrimidine-based iridiu-
m(III) complexes with high quantum efficiency.11 Among these
pyrimidine-based iridium(III) complexes, ancillary ligands (L^X)
are usually containing C, N, O atoms and the most common
Anhui 243002, P. R. China
† Electronic supplementary information (ESI) available: Experimental details,
structure characterization data of the ligands and iridium complexes
synthesized in this work, cyclic voltammogram and computational details.
CCDC 1402238. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c6ra12529h
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auxiliary ligands are acetylacetone (acac, L^X]O^O), 2-pyr-
idinecarboxylic acid (pic, L^X]N^O) and 2,20-bipyridine (bipy,
L^X]N^N). Sulfur atoms are rarely introduced into the auxil-
iary ligands for the iridium complexes used in OLEDs. In
recently, the selection of ancillary chelates has been extended to
the less accessible, tailor-made heterocycles such as dithiocar-
bamate, dithiophosphate, and benzamidinate. Dithiolate
compounds are very stable that can reduce the work function of
the emitter and reduce the threshold electric eld, therefore
reducing the turn on voltage.12 Iridium(III) complexes (TPQ)2-
Ir(Et2dtc) and (TPQ)2Ir(Et2dtp) used dithiolate (S^S) as the
ancillary ligand were synthesized, but two complexes showed
low photoluminescence quantum efficiencies of only 8.4% and
3.2%, respectively.12,13 In this paper, a highly efficient phos-
phorescent iridium(III) complex Ir(dpp)2(dta) with dieth-
yldithiocarbamate (S^S) as the ancillary ligand was successfully
designed and synthesized with emission ranged at 575 nm and
high photoluminescence quantum yield of 86%. We used
Ir(dpp)2(dta) as a triplet emitter doped in a polymeric host (70%
PVK: 30% OXD-7) in solution-processed method and prepared
yellow-orange OLED. When the doping density was 3%, the
device achieved the maximum luminance of 81 918 cd mꢀ2, the
highest EL efficiency of 30.12 cd Aꢀ1 and the external quantum
efficiency of 9.28%.
Fig. 1 General structure for the OLED devices and molecular struc-
tures of the relevant compounds.
The X-ray crystallography of Ir(dpp)2(dta)
Single crystal of Ir(dpp)2(dta) suitable for X-ray diffraction
analysis has been grown by slow diffusion of hexane into
a dichloromethane solution of the complex. ORTEP drawing of
the structure is shown in Fig. 2, and selected crystallographic
Results and discussion
Synthesis and characterization
˚
data are provided in Table S1† and the bond lengths [A] and
angles [deg] for Ir(dpp)2(dta) are listed in Table S2.† The
complex has the expected octahedral coordination geometry
around the iridium center by two cyclometalated ligands 4,6-
diphenyl pyrimidine and one ancillary ligand diethyl dithio-
carbamic acid sodium salt (dta) with the cis-C,C and trans-N,N
chelating disposition of the original chloro-bridged dimer as
reported in related examples.14 The Ir–C bond lengths are
shorter than the Ir–N bond distances as shown in Table S2.†
This result implies that there is a stronger trans inuence of the
phenyl group over that of the 4,6-diphenyl pyrimidine. The two
coordinated S atoms of ancillary ligand reside in the equatorial
plane trans to the metalated C (dpp) atoms.
As shown in Scheme 1, the cyclometalated ligand 4,6-diphenyl
pyrimidine was conveniently prepared by 4,6-dichloro-
pyrimidine and phenyl-boronic acid in the toluene solvent
according to the Suzuki procedures. The reaction was at 120 ꢁC
for 24 h and the yield of products was satisfactory. The synthesis
of the iridium(III) complex Ir(dpp)2(dta) included two steps.
First, the cyclometalated Ir(III) m-chloride bridged dimer was
synthesized by reacting IrCl3$3H2O with the cyclometalated
ꢁ
ligand at 125 C for 24 h under nitrogen. Then, the chloride-
bridged
dimer
was
reacted
with
sodium
dieth-
yldithiocarbamatodithioate in dichloromethane (CH2Cl2) solu-
tion to afford the target products in 60% yields. The structures
of these products were conrmed by NMR and mass.
UV-vis absorption and photoluminescence properties
The absorption and the photoluminescence spectra of the
complex Ir(dpp)2(dta) in CH2Cl2 solution at room temperature
were shown in Fig. 3. The strong absorption bands at 303 nm
and 402 nm were assigned to the spin-allowed 1p–p* transition.
In addition, the weak and broad absorption band at 510 nm can
be attributed to the typical spin-allowed and spin-forbidden
metal-to-ligand charge transfer bands (MLCT). It can be found
that the MLCT absorption bands of Ir(dpp)2(dta) were mostly
covered by the emission band of host material (PVK: OXD-7
¨
blend lmax ¼ 320 nm). It indicated that the Forster energy can
transfer from the singlet-excited states of the host to the MLCT
states of the iridium(III) complex Ir(dpp)2(dta) effectively. PL
spectrum of Ir(dpp)2(dta) in CH2Cl2 solution showed a strong
Scheme 1 Synthesis of Ir(dpp)2(dta).
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Table 1 Energy levels of Ir(dpp)2(dta)
Molecule
LUMOcal
HOMOcal
Eg[cal]
Transition
Ir(dpp)2(dta)
ꢀ1.8776
ꢀ5.0886
3.211
LC/MLCT/LLCT
Table 2 Electronic ground states of Ir(dpp)2(dta)
Molecule
LUMO
HOMO
Ir(dpp)2(dta)
Fig. 2 Perspective view of Ir(dpp)2(dta).
from the UV-vis absorption band, calculated according to the
formula: 1240/labs. Finally, the lowest unoccupied molecular
orbital (LUMO) energy level was calculated according to the
formula: HOMO + Eg, and to be ꢀ3 eV. It is clear that the HOMO
and LUMO levels of the complex are exactly between the HOMO
and LUMO levels of PVK (HOMO ¼ ꢀ5.8 eV and LUMO ¼ ꢀ2.2
eV). So the iridium(III) complex Ir(dpp)2(dta) can trap both elec-
trons and holes, and resulted in high device efficiency.
Theoretical calculations
To compare the experimentally obtained HOMO/LUMO levels
with the theoretical calculation, we have used the B3LYP density
functional theory (DFT) to calculate HOMO/LUMO energy levels
and the electronic ground states for (dfppy)2Ir(LN^O).16
Fig.
3 UV-vis absorption and photoluminescence spectra of
Ir(dpp)2(dta) at the solution of CH2Cl2.
As shown in Table 1 and 2, it is obvious that the LUMO of
Ir(dpp)2(dta) is locates in diphenyl pyrimidine. However the
HOMO of Ir(dpp)2(dta) distributes in Ir and cyclometalated
yellow-orange phosphorescent emission at 575 nm. The phos-
phorescence emission of the Ir(dpp)2(dta) complex was found to
mainly originate from the 3MLCT (metal-to-ligand charge
À
Á
ligand. The energy transfer consists dpðIrÞ/p*S^S MLCT,
3
transfer) excited states and/or the ligand-centered (LC) p–p*
À
Á
À
Á
3
pC^N/p*S^S LLCT and LC pC^N/pC*^N , especially between
their mixed valence.
excited states.15 Moreover, the absolute quantum efficiencies
(F) of Ir(dpp)2(dta) in solid power and doped PMMA were
measured. All data of the photophysical properties were listed
in Table S3.† The Fpower and Flm (in 1% PMMA lm) were 14%
and 86%, respectively. As showed in Fig. S7,† the phosphores-
cent life time of Ir(dpp)2(dta) in lm was 0.904 ms.
Thermal properties
The thermal properties of the iridium complex was examined by
thermal gravimetric analysis (TGA) under an N2 atmosphere at
Electrochemical properties
The electrochemical properties of the iridium(III) complex
Ir(dpp)2(dta) was investigated by cyclic voltammetry in CH2Cl2
solution at room temperature using ferrocene as the internal
standard. The conventional three-electrode cell with a Pt work
electrode of 2 mm diameter, a platinum-wire counter electrode,
and a Ag/AgCl reference electrode was employed. The scan rate is
0.1 V sꢀ1. At the end of the experiment, the ferrocene/ferricenium
(Fc/Fc+) couple was used as the internal standard as shown in
Fig. S8.† The HOMO energy levels (eV) of the complex is calculated
ox
according to the formula: ꢀ[4.8 + (E ꢀ E1/2(Fc/Fc+))] eV. The half-
1/2
wave oxidation potential (Eo1/x2) is 0.9 V vs. Ag/AgCl as shown in
Fig. S9.† The highest occupied molecular orbital (HOMO) energy
level was calculated to be ꢀ5.28 eV. The energy gap (Eg) derived Fig. 4 TG curves of iridium complexes.
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Fig. 5 EL spectra of different devices at 8 V.
Fig. 7 Current density–voltage–luminance curves of the devices.
a heating rate of 10 ꢁC minꢀ1 from room temperature to 700 ꢁC.
The results in Fig. 4 suggested that Ir(dpp)2(dta) exhibited good
thermal stability properties with high decomposition tempera-
tures (Td, corresponding to 5% weight loss) of 215 ꢁC, which was
benecial to the long-term stability of the OLED devices fabri-
cated from these materials.
host and the mass ratio of OXD-7 was 30% in the PVK:OXD-7
blend.
The HOMO/LUMO levels of the materials used in PLEDs
were described in Fig. 1. The PVK:OXD-7 blend material with
high HOMO levels as electron-donating and hole-transport
media, while TPBI with low LUMO levels as electron-accepting
and electron-transport media. Hole was injected from the
anode to HOMO levels of host material and electron was
injected in LUMO levels. The HOMO and LUMO level of
Ir(dpp)2(dta) was embedded between the HOMO of PVK (ꢀ5.80
eV) and its LUMO (ꢀ2.20 eV). Thus, efficient energy transfer to
Ir(dpp)2(dta) occured in the emitting layer (EML). The PLED
performance in luminance should be determined by the
intrinsic quantum efficiency of Ir(dpp)2(dta) in radiative decay.
Because the HOMO level of Ir(dpp)2(dta) was 0.52 eV higher
than that of PVK, it behaved as effective trapping sites for holes;
meanwhile, the LUMO level of Ir(dpp)2(dta) was 0.8 eV lower
than that of PVK, electrons injected from the TPBI layer into the
emitting layer were mostly transported through the PVK layer
and being composited in the iridium phosphor. Thus, varying
the concentration of Ir(dpp)2(dta) in the EML should signi-
cantly alter the degree of charge injection in PVK-based devices.
Electrophosphorescent properties
To investigate the EL properties of the iridium(III) complex
Ir(dpp)2(dta), the PLEDs using Ir(dpp)2(dta) as dopant with the
device structure of ITO/PEDOT:PSS (40 nm)/(70% PVK + 30%
OXD-7): x% Ir(dpp)2(dta) (70 nm)/TPBI (30 nm)/Ca (15 nm)/Ag
(100 nm) were fabricated. Fig. 1 shows the general structure
and molecular structure of the compounds used in the devices.
Indium tin oxide (ITO) on a substrate of electroluminescent
device is the anode and Ca/Ag as the cathode. PEDOT/PSS as
hole injection layer can smooth the ITO surface, reduce the
probability of electrical shorts, decrease the turn-on voltage and
prolong the operation lifetime of the device. PVK:OXD-7 blend
can be used as host materials, and TPBI as the hole-blocking
material can improve electron injection and electron trans-
port. In the emitting layer, the doping concentrations of
Ir(dpp)2(dta) were 1%, 3%, 5%, 7% and 10%, respectively. The
doping concentrations of Ir(dpp)2(dta) were according to the
Fig. 6 EL spectra of the device (3%) at different voltage.
64006 | RSC Adv., 2016, 6, 64003–64008
Fig. 8 EL efficiency–luminance of the devices.
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Table 3 Performances of the electrophosphorescent devices
The maxima of quantum efficiency
Turn-on voltage
(V)
Lmax
EQE
(%)
LE
Roll-off (%)
Voltage
(V)
CD
L
CIE
(x, y)
Device
(cd mꢀ2
)
(cd Aꢀ1
)
at 20 000 cd mꢀ2
(mA cmꢀ2
)
(cd mꢀ2
)
1(1%)
2(3%)
3(5%)
4(7%)
5(10%)
4
49 804.6
81 918.0
59 649.8
55 064.3
46 857.1
6.16
9.28
6.02
6.64
4.61
20.35
30.12
19.89
21.01
13.95
13.12
7.8
4.43
4.81
6.81
7.68
8.0
7.09
6.79
6.8
51.77
28.18
49.9
44.34
98.40
10 535.4
8488.9
9923.4
9318.0
13 726.3
(0.4577, 0.5088)
(0.4745, 0.5109)
(0.4792, 0.5074)
(0.4801, 0.5086)
(0.4787, 0.5035)
4.1
4.1
4
3.9
Therefore, the current density of the PVK-based devices efficiency decrease by 7.8% of its maximum upon 20 000 cd mꢀ2
exhibited a strong dependence on the doping concentration. (Table 3). The extra-high efficiency is an indication of balanced
The normalized electroluminescence (EL) spectra of electron and hole recombination in the host matrix, and
Ir(dpp)2(dta) doped PVK:OXD-7 devices in the doping concen- complete energy and charge transfer from the host material to
trations from 1% to 10% at 8 V are shown in Fig. 5, while others iridium(III) complex upon electrical excitation. The devices
such as 6 V, 10 V and 12 V are in the ESI in Fig. S11.† We noticed showed no obvious differences in turn-on voltage with
that there are weak emission peaks at about 420 nm from the increasing doping concentration from 1% to 10%. In order to
host PVK:OXD-7 in 1% doping concentration. Then the peak investigate the performances of these electrophosphorescent
decreased with an increase in doping concentration and nally devices with different doping concentrations, the parameters of
disappeared with increasing doping concentration to 3%. It can these devices were summarized in Table 3. The best device
be speculated that the energy is complete transfer from host to performance was obtained in the device 2 at 3% doping
Ir(dpp)2(dta) in higher doping concentration. But for the PL concentration.
spectra for different doping concentrations (Fig. S10†), even at
the doping concentration of 7%, complete quenching of the
PVK:OXD-7 emission is not observed. This implies that energy
Conclusions
transfer from the PVK:OXD-7 blend to the iridium complex is
inefficient under photo-excitation. The phenomenon with
complete quenching of the host EL emission in higher doping
In summary, we designed and synthesized a new iridium(III)
complex by introducing the 4,6-diphenylpyrimidine on the
frame of Ir(dpp)2(dta) to develop highly efficient yellow phos-
concentration (5–10%) indicating that excitons in the dopants
formed both by direct charge trapping in the dopant molecules
and energy transfer from the host to Ir(dpp)2(dta) is efficient
under electrical excitation. So the doping concentrations have
signicant inuence on the mixing single and triplet emission
of MLCT. EL spectra with peaks at about 575 nm are observed in
all of the devices. Meanwhile, with doping concentration
increasing, the EL spectra are steadily and the CIE plot were
located in the Fig. S13.† The Commission International del'E-
clairage (CIE) color coordinate is (0.4745, 0.5109) which is
almost closed to the standard yellow demanded by the National
Television System Committee (NTSC). Fig. 6 present selector
luminescence spectra of device (3%) at different voltage, while
others such as 1%, 5%, 7%, 10% are in the Fig. S12 in ESI.† With
the change of voltage from 6 V to 12 V, the spectrum is almost
unchanged and there are no residual emissions from the host
even at high voltage. It can be indicated that the device had
a good light-stability. Fig. 7 shows current density and lumi-
nance vs. voltage curves of Ir(dpp)2(dta) doped OLEDs from 1%
to 10% doping concentrations. The device at 3% doping
concentration achieved the maximum luminance, reaching
phors. In addition, the Ir(dpp)2(dta)-based device 2 still
possessed high EL efficiency of 30.12 cd Aꢀ1, maximum lumi-
nance of 81 918.0 cd mꢀ2 and EQE of 9.28% at extremely high
luminance of 8488.9 cd mꢀ2. To the best of our knowledge, this
efficiency was a higher ever reported for vacuum-deposited
yellow PhOLEDs.
Experimental
The synthesis of Ir(dpp)2(dta)17
A mixture of 4,6-dichloro-pyrimidine (0.3 g, 2 mmol), phenyl-
boronic acid (1.08 g, 4 mmol), Pd(pph3)4, TBAB, toluene (10
mL) and 2 M K2CO3 (10 mL) were stirred at 120 ꢁC for 24 h under
nitrogen. Then the reaction mixture was cooled to room
temperature, and followed poured into 200 mL water extracted
by ethyl acetate. The organic layer was washed with water and
dried over anhydrous Mg2SO4. Aer ltration, the solvent was
removed under reduced pressure. The residue was puried by
column chromatography (petroleum ether/ethyl acetate (6 : 1)
as eluent) to afford 4,6-diphenyl pyrimidine, yield ¼ 80%, mp ¼
1
ꢁ
80 C. H NMR (400 MHz, CDCl3) d 9.33 (s, 1H), 8.14 (dd, J ¼
13.2, 9.4 Hz, 5H), 7.59–7.50 (m, 6H). 13C NMR (CDCl3, 100 MHz)
d: 164.73, 159.23, 137.05, 130.97, 129.06, 127.22, 112.86. GC-MS
(m/z): 232.
81 918 cd mꢀ2
.
The maximum external quantum efficiency and the
maximum luminous efficiency of the device at 3% doping
concentration are 9.28% (Fig. S14†) and 30.12 cd Aꢀ1 (Fig. 8),
respectively. The EL device with a traditional EL structure shows
no obvious efficiency roll-off even at high luminance and the EL
A mixture of IrCl3$3H2O (0.70 g, 2 mmol) and 4,6-diphe-
nylpyrimidine (1.16 g, 5 mmol) were added in a mixture of 2-
methoxyethanol 12 mL and distilled water 4 mL. The mixture
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ꢁ
was stirred at 125 C for 24 h under nitrogen. Aer cooled to
room temperature, the precipitate was collected by ltration
and washed with water and ethanol, gave to the dimeric iri-
dium(III) complex [(dpp)2IrCl]2. Aer drying, the crude product
was directly used for next step without further purication.
Cyclometalated [(dpp)2IrCl]2 (1.38 g, 1 mmol), K2CO3 (1.38 g, 10
mmol) and 5 equivalents of diethyl dithiocarbamic acid sodium
salt in dichloromethane (10 mL) was stirred at room tempera-
ture under a nitrogen atmosphere for 3 h. Aer reaction, the
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aluminum oxide using dichloromethane/petroleum ether as the
eluent to afford the desired iridium(III) complex Ir(dpp)2(dta)
(4.8 mmol, 2.8 g). Yield: 60%. 1H NMR (400 MHz, CDCl3) d:
10.24 (d, J ¼ 0.9 Hz, 2H), 8.27 (dd, J ¼ 8.0, 1.3 Hz, 4H), 8.19 (d, J
¼ 0.8 Hz, 2H), 7.81 (dd, J ¼ 7.8, 0.9 Hz, 2H), 7.64–7.53 (m, 6H),
6.88 (dd, J ¼ 7.6, 1.0 Hz, 2H), 6.82 (dd, J ¼ 7.5, 1.2 Hz, 2H), 6.62–
6.54 (m, 2H), 3.76–3.70 (m, 2H), 3.62–3.46 (m, 2H), 1.25 (d, J ¼
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Acknowledgements
The authors acknowledge nancial support from the National
Basic Research Program of China (973 Program, 2012CB933301,
2012CB723402), the Ministry of Education of China (IRT1148),
the National Natural Science Foundation of China (BZ2010043,
20974046, 20774043, 51173081, 50428303, 61106017,
61136003), the Priority Academic Program Development of
Jiangsu Higher Education Institutions (PAPD, YX03001),
Jiangsu National Synergistic Innovation Center for Advanced
Materials (SICAM), the Nature Science Foundation of Jiangsu
Province (BK20131375) and the Research Fund for Nanjing
University of Posts and Telecommunications (NY215153).
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64008 | RSC Adv., 2016, 6, 64003–64008
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