New rhenium(i) complexes with substituted diimine ligands for highly efficient phosphorescent devices fabricated by a solution process

Article abstract of DOI:10.1039/c2jm14222h

A series of new carbonyl rhenium(i) complexes chelated by a substituted 1,10-phenanthroline ligand with the general formula Re(CO)₃LCl, where L = 2-(2′-methoxyphenyl)-1,10-phenanthroline (L₁), 2-(4′-methoxyphenyl)-1,10-phenanthroline

Full text of DOI:10.1039/c2jm14222h

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PAPER
New rhenium(I) complexes with substituted diimine ligands for highly efficient
phosphorescent devices fabricated by a solution process†
b
a
a
a
a
Xiaoming Liu,ab Hong Xia, Wei Gao, Qiaolin Wu, Xiao Fan, Ying Mu and Chunsheng Mab
*
*
Received 28th August 2011, Accepted 25th November 2011
DOI: 10.1039/c2jm14222h
A series of new carbonyl rhenium(I) complexes chelated by a substituted 1,10-phenanthroline ligand
with the general formula Re(CO)3LCl, where L ¼ 2-(20-methoxyphenyl)-1,10-phenanthroline (L1),
2-(40-methoxyphenyl)-1,10-phenanthroline (L2) and 2-(40-diphenylaminophenyl)-1,10-phenanthroline
(L3), have been systemically synthesized. The molecular structure of complex 1 was determined by
single crystal X-ray diffraction studies, showing that the complex adopts a distorted octahedral
geometry. The electrochemical, photophysical, and thermal properties, as well as the
electroluminescent behaviors of three rhenium(I) complexes, were investigated. The solution
processable complex 1, 2 or 3 was used as a yellow emitting dopant to fabricate electrophosphorescent
devices with a polymer host. The device based on complex 3 exhibits a maximum current efficiency of
12.2 cd Aꢀ1 and a peak brightness in excess of 7300 cd mꢀ2, respectively. Even at a high luminance of
5000 cd cmꢀ2 with current density of 81 mA cmꢀ2, the current efficiency of this device remains as high as
6.4 cd Aꢀ1. These results represent the best values reported for electophosphorescent devices based on
solution processable rhenium(I) complexes.
different ligands have been reported,6–9 and parts of them have
been applied as emitters in OLEDs.6,7 For example, Li et al.
Introduction
In recent years, heavy metal complexes have been extensively
reported efficient devices based on (2,9-dimethyl-1,10-phena-
throline)Re(CO)3Cl doped into 4,40-N,N0-dicarbazole-biphenyl
(CBP) with a high current efficiency of 7.15 cd Aꢀ1 6a
. Liu et al.
studied as electrophosphorescent materials to fabricate highly
efficient organic light-emitting devices (OLEDs) since the orig-
inal works of Thompson, Forrest and Ma.1,2 The OLEDs based
€
achieved yellow electoluminescence devices with a maximum
current efficiency of 17.6 cd Aꢀ1 and a brightness of 6500 cd mꢀ2
by doping (3-ethyl-2-(40-diphenylaminophenyl)imidano[4,5-f]
1,10-phenanthroline)Re(CO)3Cl into CBP.7a Recently, the
groups of Li and Chu7b reported a more efficient OLED based
on (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline)Re(CO)3Br
with a maximum EL efficiency and luminance of 21.8 cd Aꢀ1 and
8315 cd mꢀ2, respectively. Utilizing the vacuum-deposition
method, the rhenium(I) complexes based OLEDs show high
efficiency and good luminance. However, it is well known that
the sublimation process has critical drawbacks including
considerable loss of the expensive materials during evaporation,
complex manufacturing process and high manufacturing costs.
Therefore, solution processed OLEDs are attracting much
attention as potential candidates for large area flat panel
displays, owing to their simple processing route and low
manufacturing cost. Up to now, to the best our knowledge,
rhenium(I) complexes based devices made by a solution process
were less reported and showed relatively low efficiency and
luminescence, which may be attributed to neutral rhenium(I)
complexes having poor solubility.10 In this context, the rational
design of new rhenium complexes with good solubility and the
exploitation of their electroluminescence properties of solution
on phosphorescent materials show remarkable enhancement in
their electroluminescent (EL) performance due to the use of both
singlet and triplet excitons caused by strong spin-orbital coupling
in the existence of the heavy metal atoms, and exhibit higher
internal quantum efficiency of up to 100% in principle. Iridiu-
m(III),3 platinum(II),4 and ruthenium(II)5 complexes have been
mainly exploited in OLEDs. Recent research results indicate that
rhenium(I) complexes6,7 also serve as a class of electro-
phosphorescent materials for OLEDs with features of high room
temperature phosphorescence quantum yield, relatively short
excited state lifetime and excellent thermal, chemical, and
photochemical stability.8 So far, some rhenium(I) complexes with
aState Key Laboratory for Supramolecular Structure and Materials,
School of Chemistry, Jilin University, 2699 Qianjin Avenue, Changchun,
130012, P.R.China. E-mail: ymu@jlu.edu.cn; Fax: +86 431 85168376;
Tel: +86 431 85168376
bState Key Laboratory on Integrated Optoelectronics, College of Electronic
Science and Technology, Jilin University, 2699 Qianjin Avenue,
Changchun, 130012, P.R.China. E-mail: hxia@jlu.edu.cn; Fax: +86 431
85168242 8121; Tel: +86 431 85168242 8121
† Electronic supplementary information (ESI) available. CCDC
reference number 841691. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c2jm14222h
This journal is ª The Royal Society of Chemistry 2012
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processed OLEDs is also an important project. Hence, we have
synthesized three new soluble rhenium(I) complexes and found
that highly efficient phosphorescent devices can be fabricated
with these complexes by the spin casting solution process. In this
paper, we report the synthesis, characterization, electrochemical,
photophysical, and thermal properties, as well as EL
behaviors, of three new rhenium(I) complexes Re(CO)3LCl [L ¼
2-(20-methoxyphenyl)-1,10-phenanthroline (1), 2-(40-methoxy-
phenyl)-1,10-phenanthroline (2) and 2-(40-diphenylamino-
phenyl)-1,10-phenanthroline (3)].
Results and discussion
Synthesis and characterization of compounds
Fig. 1 ORTEP drawing of a crystal of 1 with displacement ellipsolids at
Ligands L1–L3 were prepared according to a known procedure
(Scheme 1).11 Of them, L3 is a new compound while ligands L1,
and L2 are known compounds. The new ligand L3 was synthe-
sized by the reaction of dry 1,10-phenanthroline with 4-diphe-
nylaminophenyllithium in moderate yields after oxidative
rearomatization. The 4-diphenylaminophenyllithium was
prepared by treatment of 4-diphenylamino-bromobenzene with
n-BuLi. These ligands were characterized by 1H NMR spec-
troscopy along with elemental analysis. Reaction of Re(CO)5Cl
with one equivalent of ligands L1–L3 in toluene at refluxing
temperature afforded the corresponding complexes 1–3 in high
yields (>80%) as pale yellow solids (Scheme 1). Three complexes
were all characterized by 1H NMR spectroscopy, Fourier
transform infrared spectroscopy (FT-IR) and elemental analyses,
the 30% probability level.
structure of 1 is shown in Fig. 1. Selected bond lengths and angles
are given in Table 1. The X-ray analysis reveals that complex 1
adopts a distorted octahedron geometry with the metal center
coordinated by one chloride atom, three carbonyl ligands and
two nitrogen atoms of the ligand L1. The bond distances of C(1)–
Re, C(2)–Re, and C(3)–Re are 1.898(8), 1.892(7), and 1.921(7),
respectively. These values are consistent with the bond lengths
found in related octahedral rhenium(I) carbonyl complexes.13
ꢀ
The average Re–N bond distances are 2.189 A, which are longer
than the ones previously reported for related octahedral rhe-
nium(I) complexes.7c,14 The C–Re–C bond angles are in the range
87.2–90.6ꢁ, being close to the ideal 90ꢁ bond angle for octahedral
complexes, while the N–Re–N bond angle is 75.51ꢁ. In complex
1, the dihedral angle between the phenyl ring and the phenan-
throline plane is 72.2ꢁ. In addition, there are abundant weak
interactions, such as p–p stacking interactions, hydrogen
bonding and CH/p interactions, in the packing structure of 1.
The hydrogen bonding between a carbonyl oxygen atom in one
molecule and a hydrogen atom of the phenanthroline group in an
1
and satisfactory analytic results were obtained. The H NMR
spectra of complexes 1–3 show that the resonances of the protons
on the phenanthroline aromatic rings shift toward low field
compared to the corresponding signals in their free ligands,
indicating the formation of the coordination bonds between the
ligand and the rhenium(I) center (Fig. S1, ESI†).12 The FT-IR
spectra of these complexes show three bands in the 1869–2021
cmꢀ1 region, which can be attributed to the stretching vibrations
of the three carbonyl groups at different positions around the
rhenium(I) metal. Complexes 1–3 are moderately soluble in
dimethylsulphoxide, dichloromethane (CH2Cl2), N,N-dime-
thylformamide and tetrahydrofuran, while insoluble in saturated
hydrocarbon solvents.
ꢀ
adjacent molecule with an H/O distance of 2.56 A, and between
its chlorine atom and a hydrogen atom of the phenanthroline
group in another adjacent molecule with a H/Cl distance of
ꢀ
2.79 A, can be observed. The hydrogen bonding interactions link
the molecules of 1 into one-dimensional chains. Meanwhile, the
one-dimensional chains are bound together into two-dimen-
sional sheets by weak CH/p and hydrogen bonds between
Crystal structure
ꢀ
molecules in adjacent chains with distances of 2.81 A for the
ꢀ
CH/p interaction and 2.90 A for the hydrogen bond, respec-
tively (Fig. S2, ESI†).
The molecular structure of complex 1 was determined by X-ray
crystallographic analysis. Samples of the complex suitable for
X-ray crystal structure determination were grown from CH2Cl2
at room temperature. The ORTEP drawing of the molecular
ꢁ
ꢀ
Table 1 Selected bond lengths (A) and bond angles ( ) for complex 1
Re(1)–C(1)
Re(1)–C(2)
Re(1)–C(3)
Re(1)–N(2)
Re(1)–N(2)
O(1)–C(1)
O(2)–C(2)
O(3)–C(3)
1.898(8)
1.892(7)
1.921(7)
2.166(5)
2.213(5)
1.148(9)
1.154(9)
1.156(9)
C(2)–Re(1)–C(1)
C(2)–Re(1)–C(3)
C(3)–Re(1)–C(1)
C(2)–Re(1)–N(1)
C(1)–Re(1)–N(2)
N(2)–Re(1)–N(1)
C(1)–Re(1)–Cl(1)
C(3)–Re(1)–Cl(1)
88.9(3)
90.6(3)
87.2(3)
95.2(3)
94.2(3)
75.51(19)
92.6(3)
91.6(2)
Scheme 1 Synthesis of ligands and their rhenium(I) complexes.
3486 | J. Mater. Chem., 2012, 22, 3485–3492
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Photophysical properties
metal (dp) to ligand (p*) charge transfer (MLCT).17 The MLCT
absorption bands of three complexes are similar to those previ-
ously reported for rhenium(I) complexes with similar ligands.18
Excitations at the p–p* and MLCT absorption frequencies do
not lead to the same MLCT emission at room temperature for 1–
3, indicating that potential surface crossing from the higher p–p*
state to the lower MLCT state is not efficient.19 The emission
spectra of complexes 1–3 show a similar profile in CH2Cl2 with
exited wavelength at peak of corresponding MLCT absorption
(Fig. 3a). The emission peaks at about 570 nm for these
The absorption and emission spectral data along with lifetimes
and emission quantum yields for the free ligands L1–L3 in both
solution and the solid state are summarized in Table S1, ESI.†
All free ligands display an intense absorption band with the
absorption maxima about 283 nm, which can be attributed to the
p–p* electronic transition of the phenanthroline moiety in these
ligands. An additional absorption band observed at 382 nm for
L3 may result from a charge transfer from the triphenylamine
donor to phenanthroline acceptor. Upon irradiation with an
excited light, the free ligands L1–L3 emit a blue light in CH2Cl2
solution with emission peaks at 460–470 nm, respectively (Fig. 2.
and Table S1, ESI†). The fluorescence decay lifetime of L1–L3
were found to be in the range from 3.16 to 3.24 ns. In comparison
with the ligands L1 and L2,11 the emission frequency of L3 show
strong solvent-dependence. For example, the emission lmax of L3
appears at 435 nm in toluene, 470 nm in CH2Cl2, and 495 nm in
acetonitrile (CH3CN), respectively. The emission spectra of L3 in
various solvents and their photos under irradiation of light at 365
nm are shown in Fig. S4 and S5, ESI.† It should also be noted
that no significant solvatochromism is observed in the UV-visible
absorption spectra of L3. The solvent-dependent emission
demonstrates the presence of a highly polarized exited state15 in
which the lowest unoccupied molecular orbital (LUMO) level
may be dominated by the phenanthroline unit, while the highest
occupied molecular orbital (HOMO) can be dominated by the
triphenylamine group.16 L3 was found to be a very bright emitter
in solution, and its quantum yield was determined to be 0.49, 0.40
and 27.7 in toluene, CH2Cl2 and CH3CN respectively, which are
higher than the data of other ligands (Table S1, ESI†). L3 in the
solid state emits weak fluorescence and its emission peak is red
shifted about 23 nm in comparison with emission in CH2Cl2
solution (Fig. 2). The red shift phenomenon can be attributed to
the p-stacking of the aromatic rings in these molecules in the
solid state.
3
complexes should predominantly be as a result of the MLCT
exited state. In the solid state, an emission band at about 550 nm
can be observed at room temperature for complexes 1–3
(Fig. 3b). Their maximum emission peaks display pronounced
hypsochromic shifts in comparison to the ones observed in
CH2CI2. This phenomenon is typical for phosphorescent emis-
sion from the 3MLCT excited state, which has been observed in
other rhenium(I) complexes in the literature.20,21 The excited state
lifetime of 1–3 at 570 nm in CH2Cl2 is listed in Table 3. The
values of the lifetime in the range 13–80 ns were determined by
exponential decay curve-filling analysis (Fig. S6, ESI†). The
short excited state lifetime may provide the opportunity to use
them for highly efficient OLEDs.
Electrochemistry
The electrochemical properties of complexes 1–3 were investi-
gated by cyclic voltammetry (CV) measurements and the results
are given in Table 2. Three complexes show a reduction reaction
with reduction potentials in the range ꢀ1.43 V to ꢀ1.57 V in
CH3CN solution, which can be assigned to the redox couple of
the phenanthroline ligand. For complexes 1 and 2, an oxidation
reaction with a potential of about +1.00 V was observed, which
process takes place on a centred metal rhenium(I) associated with
the Re(I)/Re(II) redox couple. In contrast to 1 and 2, two
oxidation reactions were observed for 3. The first oxidation
reaction at the low potential of around +0.65 V could be ascribed
to the oxidation of the triphenylamine unit in its ligand, while the
second oxidation reaction with a potential of about +1.07 V
should be for the Re(I)/Re(II) redox couple. As an example, the
CV curves of 2 and 3 are shown in Fig. 4. The HOMO energy
levels of complex 3 estimated from its first oxidation potential is
ꢀ5.36 eV. This data reveals that the introduction of the triphe-
nylamine unit to the ligand lowers the barrier height of these
complexes for hole injection, thereby facilitating the transfer of
holes.
The photophysical properties of 1–3 are listed in Table 2. The
absorption spectra of three complexes in dilute CH2Cl2 are
shown in Fig. 3a. These spectra display a strong absorption
around 280 nm and a broad absorption between 330 and 490 nm.
The absorption at high energy is primarily due to the p–p*
transition of the phenanthroline moiety in their ligands, and the
lower energy absorption band is typically from the spin-allowed
Thermal analysis
Thermogravimetric analysis (TGA) was performed on 1–3 to
investigate their thermal stability. The thermal decomposition
temperatures of these complexes are given in Table 2. All
complexes 1–3 show high thermal decomposition temperatures
ꢁ
(307–343 C), which demonstrates their good thermal stability.
The TGA traces of three complexes exhibit two thermal
decomposition processes. As examples, the first weight loss step
ꢁ
starts from 343 C, and the second weight loss step starts from
ꢁ
436 C for complex 3 (Fig. S7, ESI†). The first thermal decom-
Fig. 2 Absorption and emission spectra of L3 in solution and solid state.
This journal is ª The Royal Society of Chemistry 2012
position process might be attributed to the loss of the carbonyl
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Table 2 Photophysical, electrochemical, thermal and infrared data for complexes 1–3
a
b
d
ox
onset
red
g
Complex labs (nm) 3 ꢂ 103 Mꢀ1cmꢀ1 lex (nm) lemc (nm) lem (nm) Lifetime (ns) E
e (v) E
e (v) Ered f (v) Td (ꢁC) FT-IR (cmꢀ1) CO
onset
1
2
3
275(11.3), 394(2.8)
276(27.4), 343(5.1), 390(2.1) 315, 346 571
282(17.3), 392(4.7) 382, 431 570
344, 418 570
550
549
551
42.82
66.74
79.49
0.98
1.00
ꢀ1.58
ꢀ1.58
ꢀ1.57
ꢀ1.54
ꢀ1.43
310
307
343
1886, 1913, 2020
1869, 1909, 2021
1882, 1913, 2017
0.65, 1.03 ꢀ1.55
a
b
c
Maximum absorption wavelength, measured in CH2Cl2 solution. Maximum excitation wavelength, measured in CH2Cl2. Maximum emission
d
e
wavelength, measured in CH2Cl2. Maximum emission wavelength, measured in the solid state. Estimated by CV using a platinum disk as the
working electrode, platinum wire auxiliary electrode, Ag/Ag+ reference electrode with ferrocene as the internal standard, Bu4NBF4 as supporting
f
ox
electrolyte in CH3CN. These data were calculated using the E
onset
and the optical bandgap. g Thermal decomposition temperature.
Fig. 4 CV curves of complex 2 (a) and 3 (b) measured in CH3CN at 0.1
v/s scan rate.
Electroluminescence
Polymer-based light-emitting diodes with the new rhenium(I)
complex 1, 2 or 3 as a dopant were fabricated by the spin casting
and their EL performances were investigated. To our knowledge,
there are very few reports on EL devices using solution
processable rhenium(I) complexes as the emission layer and their
EL performances are not expected.10 The crystal structure of 1
reveals these complexes have a strong aggregation tendency in
the solid state. If these rhenium(I) complexes are dispersed as
a guest in a polymer host, it can suppress the intermolecular
interaction of the dopant and increase the performances of the
EL device. The polymer blend PVK–PBD (40 wt%) was selected
as the host, for which the emission band overlaps with the
absorption bands of these rhenium(I) complexes. PVK is a good
hole-transporting material while PBD is an electron-transporting
material. Employment of the PVK–PBD mixture host would
enhance balance of carriers in the light-emitting layer and
thus enhance the emitting efficiency of devices.22 Indium-tin-
oxide (ITO) glass and aluminum were used as the anode
and cathode, respectively. In addition, poly(3,4-ethyl-
enedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and
LiF were used as materials for facilitating the hole and electron
injection from the two electrodes, and 2,20,200-(1,3,5-benzenetriyl)
tris[1-phenyl-benzimidazole] (TPBI) was chosen as an
Fig. 3 Absorption and emission spectra of complexes 1–3 in CH2Cl2 (a)
and in the solid state (b).
Table 3 Summary of the performance of complexes 1–3 based OLEDs
a
h1
b
c
d
Complex
Concentration
h2
Bmax
h3
1
2
3
0.5
1.0
0.5
1.0
0.25
0.5
1.0
2.0
7.10
10.74
8.85
6.79
8.1
10.8
12.2
9.8
5.42
7.08
6.79
5.41
7.0
9.5
11.4
8.1
3140
3074
2742
4219
5233
6490
7308
6237
3.19
4.82
4.19
2.91
2.83
4.30
2.74
2.48
a
b
Maximum efficiency (cd Aꢀ1). Efficiency at 1000 cd mꢀ2 (cd Aꢀ1).
c
d
Maximum brightness (cd mꢀ2). Maximum power efficiency (lm/W).
groups and partial decomposition of their ligands, while the
second weight loss process is likely to be caused by the loss of
their ligands. The good thermal stability of new rhenium(I)
complexes is important for application in OLEDs.
3488 | J. Mater. Chem., 2012, 22, 3485–3492
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electron-transporting material. The structure of the devices are as
follows: ITO/PEDOT:PSS (40 nm)/PVK–PBD (40 wt%): x wt%
rhenium(I) complex (80 nm)/TPBI (40 nm)/LiF (0.7 nm)/Al
(100 nm). The rhenium(I) complex was doped into the host PVK–
PBD in the range 0.25–2.0 wt% as the light-emitting layer. The
thickness of the doped light-emitting layer is about 80 nm. The
TPBI layer and the electrode layers were deposited by a resistive
heating method. Atomic force microscopy (AFM) was employed
to explore the surface image of light-emitting layer containing
rhenium(I) complexes guest and PVK–PBD host. The AFM
images reveal similar smooth and uniform topographies, free of
pinholes, particle aggregation, or phase separation, with root-
mean-square surface roughnesses of about 0.60 nm for these
complexes (Fig. S8, ESI†). This suggested that new rhenium(I)
complexes possess good miscibility to the PVK–PBD blend.
The selected performance parameters of the OLEDs based on
1, 2 and 3 are summarized in Table 3. The device performance
shows a dependence on the doping concentration, when 1, 2 and
3 were blended with PVK–PBD in a mass ratio of 1.0%, 0.5% and
1.0% respectively, the devices demonstrate the best perfor-
mances. Fig. 5a presents the current density–brightness–voltage
characteristics of the optimized devices. The turn-on voltage,
that is the voltage needed to reach 1 cd mꢀ2, is 7 V, 7 V and 8 V
for optimized devices of 1, 2 and 3, respectively. The maximum
luminance of optimized device is 3074, 2742 and 7308 cd mꢀ2 at
a corresponding driving voltage. These values of maximum
brightness observed for the EL devices based on new rhenium(I)
complexes are higher than the 960 cd mꢀ2 previously reported for
a device based on the [Re2(m-Cl)2(CO)6(m-diazine)] complex
incorporated into PVK polymer with a device structure of ITO/
PEDOT:PSS/PVK:Re2(m-Cl)2(CO)6(m-diazine)/TPBI/Ba/Al,10b
and higher than 730 cd mꢀ2 for a rhenium(I) complex with
derivative 2,20-bipyridine doped in host material of poly-
carbonate (PC) and the device configuration of ITO/
PVK:Re:PC/Al.10c Fig. 5b shows the current efficiency–current
density characteristics of the optimized devices and all devices
exhibit a slowly decrease in the efficiency with an increase in the
current density. These devices based on new rhenium(I) display
high efficiency, which is 10.74 cd Aꢀ1 for 1 and 8.85 cd Aꢀ1 for 2,
respectively. In particular, a maximum efficiency as high as 12.2
cd Aꢀ1 was obtained from the device doped with 1.0 wt% of 3.
The value of maximum current efficiency observed for the EL
devices studied in this paper was enhanced about 6-fold
compared to the best previously reported value (2.1 cd Aꢀ1).10d
For these devices, the current efficiencies at the benchmark
luminance of 100 cd mꢀ2 are close to their maximum efficiencies.
When the luminance reaches to 1000 cd mꢀ2, the 1.0 wt % of 3
doped device still exhibits a maximum current efficiency of 11.4
cd Aꢀ1. Even at a much higher luminance of 5000 cd mꢀ2 with
current density of 81 mA cmꢀ2, the current efficiency of this
device remains as high as 6.4 cd Aꢀ1, which is higher than half of
the peak value. The slow decrease in current efficiency with
increasing the current density has been attributed to the fact that
the saturation of the phosphorescence sites under these condi-
tions is not severe6c due to the short triplet lifetime of the light-
emitting material.23 Nevertheless, the performances of these
devices are greatly improved in comparison with the previously
reported results from OLEDs fabricated by solution processable
rhenium(I) complexes.9 It seems that the introduction of the
methoxyphenyl or the diphenylaminophenyl group on the 1,10-
phenanthroline skeleton can decrease molecular interaction of
rhenium complexes, and restrain the effect of triplet–triplet
annihilation. In particular, the triphenylamine moiety in 3 can
also improve the hole injection ability of this complex. In addi-
tion, efficient exciton formation on the rhenium(I) molecules by
direct electron and hole capturing is also responsible for the
excellent performances. Generally, this can be estimated from the
analysis of the PL and EL spectra of PVK films with different
concentrations of 3 and energy levels of emitting and trans-
porting materials.
€
To test the efficiency of Forster energy transfer, PVK–PBD
films containing different concentrations of 3 were prepared and
their PL spectra were determined with an excitation radiation of
325 nm. The normalized PL spectra of these films are shown in
Fig. 6a. There are two emission peaks in these PL spectra: the one
centred at ꢃ425 nm is from the PVK–PBD host; the other peak
with lower energy is a result from 3. The emission at 425 nm
decreases significantly with the increasing in doping concentra-
tion of 3, and disappears at doping concentrations of 3 higher
than 2.0 wt%. The decrease in the host emission and the simul-
€
taneous increase in the emission of 3 are consistent with Forster
energy transfer from the PVK–PBD host to 3.24 Fig. 6b shows the
corresponding EL spectra obtained with the devices made from
the same solutions. The emission of the host disappears at doping
concentrations of 3 higher than 0.5 wt%. The host EL emission is
completely converted into the emission of 3 at lower doping
concentration than that observed for the PL emission, suggesting
that the molecules of 3 act as recombination centers. The EL
Fig.
5 Current density–brightness–voltage curves (a) and current
efficiency–current density curves (b) of 1 (1.0 wt%), 2 (0.5 wt%) and 3
(1.0 wt%) based devices.
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a high luminance of 5000 cd mꢀ2, the high current efficiency of 6.4
cd Aꢀ1 can be maintained. To the best our knowledge, these
performances are the best reported for devices by spin casting
technology employing solution rhenium(I) complexes as the
emission layer. We suggest that short triplet lifetime, enhanced
charge injection, and efficient charge capturing are responsible
for the excellent performances of devices fabricated with solution
processable complexes as the luminescent layer.
Experimental section
General information
All reactions were performed using standard Schlenk techniques
in an atmosphere of high-purity nitrogen or glove-box tech-
niques. Toluene and diethyl ether were dried by refluxing over
sodium and benzophenone and distilled under nitrogen prior to
use. Solvents used in luminescent and electrochemical studies
were anhydrous and of spectroscopic grade. CH3CN, Bu4NBF4,
PVK, PBD, TPBI, PEDOT:PSS, 1,10-phenanthroline and nBuLi
were purchased from Aldrich and used as received. The FT-IR
spectra were acquired using a Magna560 spectrophotometer.
NMR spectra were measured using a Varian Mercury-300 NMR
spectrometer. The elemental analysis was performed on a Perkin-
Elmer 2400 analyzer. UV-vis absorption spectra were recorded
on a UV-3100 spectrophotometer. Fluorescence measurements
were carried out on an RF-5301PC. Room-temperature lumi-
nescence quantum yields were measured at a single excitation
wavelength (360 nm) referenced to quinine sulfate in sulfuric acid
aqueous solution (F ¼ 0.546). The quantum yields were calcu-
lated using known procedures.25 Time-resolved fluorescence
measurements were performed by the time-correlated single
photon counting (TCSPC) system under right-angle sample
geometry. A 379 nm picosecond diode laser (Edinburgh Instru-
ments EPL375, repetition rate 20MHz) were used to excite the
sample. The emission was detected by a photomultiplier tube
(Hamamatsu H5783p) and a TCSPC board (Becker&Hickel
SPC-130). The instrument response function (IRF) is aboutꢁ220
ps. All measurements were done at room temperature (22 C).
TGA was performed on ꢃ2 mg of 1–3 using a Perking-Elmer
thermal analyzer in air. The samples were dried under vacuum at
60 ꢁC before being heated to 600 ꢁC at a heating rate of 10.0 ꢁC
minꢀ1. Electrochemical measurements were performed with
a BAS 100W bioanalytical system, using a platinum disk (4 ¼ 3
mm) as the working electrode, a platinum wire as the auxiliary
electrode, and a porous glass wick Ag/Ag+ as the reference
electrode with ferrocenium–ferrocene (Fc+/Fc) as the internal
standard. CV studies of 1–3, PBD, TPBI and PVK were carried
out at a scan rate of 100 mV sꢀ1 in CH3CN containing, 0.1 M
Fig. 6 PL spectra (a) and EL spectra (b) of 3 with different concentra-
tions in PVK–PBD (40 wt%) films.
emission spectra dominated by 3 have a central wavelength at
560 nm and the maximum emission peaks do not vary with the
change in doping concentration (Fig. 6b).
The HOMO and LUMO energies of PVK, PBD, TPBI and 3
were estimated by CV measurements. By comparing the HOMO
energies of 3 (ꢀ5.36 eV) and PVK (ꢀ5.55 eV), it is evident that
the guest molecule of 3 would be a hole trap with a depth of ca.
0.19 eV (Fig. S9, ESI†). On the other hand, the LUMO energy of
3 (ꢀ3.16 eV) is lower than the ones of TPBI (ꢀ2.70 eV) and PBD
(ꢀ2.54 eV), which indicate that 3 would also be a good electron
trap. So the holes and electrons in the emissive layer can be
trapped directly by molecules of 3. Fig. S10, ESI,† shows the
current–voltage (I–V) characteristics of devices with different
doping concentrations of 3. The turn-on voltage increases with
the increase in the doping concentration of 3, which is clear
evidence for charge trapping in the doped system.24a
Conclusions
In summary, three new rhenium(I) complexes with substituted
diimine ligands were synthesized, characterized and their pho-
tophysical and electroluminescent properties have been studied.
Crystal structure analysis reveals rhenium(I) complexes adopt
distorted octahedron geometry and have a strong aggregation
tendency in the solid state. These rhenium(I) complexes exhibit
bright yellow luminescence with relatively short triplet lifetime.
The high-efficiency polymer-based electrophosphorescent light-
emitting devices have been fabricated using rhenium(I)
complexes as dopant and PVK–PBD as host. In particular, 3
doped devices exhibited a maximum current efficiency up to 12.2
cd Aꢀ1, and a peak brightness as high as 7308 cd mꢀ2. Even at
Bu4NBF4 as the supporting electrolyte. From the onset, the
ox
anodic peak potential for the oxidation process, E
, and the
red
onset
onset cathodic peak potential for the reduction process, E
,
onset
and the HOMO and LUMO energy levels of compound were
estimated according to the following formula: EHOMO ¼ ꢀ4.71 ꢀ
ox
onset
red
onset
E
and ELUMO ¼ ꢀ4.71 ꢀ E
.
Device fabrication
ITO-coated glass with a sheet resistance of <50ꢁU,ꢀ1 was used
as substrate. The substrate was pre-patterned by
3490 | J. Mater. Chem., 2012, 22, 3485–3492
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photolithography to form the pattern of devices in size of 4 mm2.
Pre-treatment of ITO includes a routine chemical cleaning using
detergent and alcohol in sequence, followed by oxygen plasma
cleaning. Spin-coating with solutions containing rhenium(I)
complexes (0.5–2.0 wt%) in polymer host (PVK–PBD, 40 wt%)
(10 mg mlꢀ1) in chloroform on ITO substrate gives the active
layer with a thickness of 80 nm. The TPBI, LiF and Al was
deposited by thermo-evaporation. The luminance of devices was
recorded on a PR650 spectrometer. Current–voltage and light
intensity measurements were carried out at room temperature
and ambient conditions.
(2.18 g, 12.12 mmol) degassed Et2O (20 mL), and a wine-red
solution was obtained immediately. The resultant mixture was
refluxed for 12 h and cooled in an ice bath, then deionized water
(30 mL) was added to hydrolyze the products. The yellow
organic phase was separated and stirred with MnO2 for 48 h,
then filtered and dried with MgSO4. The solid was obtained upon
concentration of the solution, and chromatography on silica gel
with dichloromethane as the eluent afforded a light yellow
powder. Yield: 55%. Anal. Calcd for C30H21N3 (423.51): C,
1
85.08; H, 5.00; N, 9.92. Found: C, 85.22; H, 4.83; N, 9.95.
H
NMR (300 MHz, CDCl3, 293 K): d 7.06 (t, J ¼ 4.5 Hz, 2H), 7.17
(b, J ¼ 4.8 Hz, 4H), 7.22 (b, J ¼ 5.1 Hz, 2H), 7.29 (t, J ¼ 2.1 Hz,
4H), 7.67 (s, 1H), 7.77 (b, J ¼ 5.4 Hz, 1H), 7.83 (b, J ¼ 5.4 Hz,
1H), 8.07 (b, J ¼ 5.1 Hz, 1H), 8.28 (b, J ¼ 5.1 Hz, 4H), 9.27 (s,
X-ray structure determinations of 1
Single crystals of 1 suitable for X-ray structural analysis were
obtained from CH2Cl2. Diffraction data were collected at 293 K
on a Rigaku R-AXIS RAPID IP diffractometer equipped with
13
1H) ppm. C NMR (75 MHz, CDCl3, 293 K): d 120.1, 122.9,
123.2, 124.9, 125.4, 125.9, 126.8, 127.3, 128.8, 129.1, 129.4, 133.3,
136.6, 136.8, 145.7, 147.6, 149.3, 149.8, 157.5 ppm.
ꢀ
graphite-monochromated Mo-Ka radiation (l ¼ 0.71073 A) for
two complexes. Details of the crystal data, data collection and
structure refinements are summarized in Table 4. The structure
was solved by direct methods26 and refined by full-matrix least-
squares on F2. All non-hydrogen atoms were refined anisotrop-
ically and the hydrogen atoms were included in idealized posi-
tion. All calculations were performed using the SHELXTL27
crystallographic software package.
Synthesis of 1. L1 (80.1 mg, 0.28 mmol) and Re(CO)5Cl (100
mg, 0.27 mmol) were refluxed in 15 mL of toluene for 10 h. After
the mixture was cooled to room temperature, the solvent was
removed in a water bath under reduced pressure. The resulting
yellow solid was purified by silica gel column chromatography
with acetic acid ethyl ester and dichloromethane. Yield: 0.13 g
1
(85%). H NMR (300 MHz, DMSO-d6, 293 K): d 3.76 (s, 3H,
OCH3), 7.19 (t, J ¼ 3 Hz, 1H), 7.27 (d, J ¼ 6 Hz, 1H), 7.37 (d, J ¼
3 Hz, 1H), 7.60 (t, J ¼ 6 Hz, 1H), 8.05 (d, J ¼ 3 Hz, 1H), 8.11 (t,
J ¼ 3 Hz, 1H), 8.96–9.00 (m, 2H), 9.45 (d, 1H) ppm. Anal. Calcd
for C22H14ClN2O4Re (592.02): C, 44.63; H, 2.38; N, 4.73; Found:
Synthesis
Synthesis of L3. A solution of nBuLi (7.57 ml, 12.12 mmol) in
hexane was added to a solution of 4-bromotriphenylamine
(3.93 g, 12.12 mmol) in Et2O (15 mL) under a nitrogen atmo-
sphere at ꢀ78 ꢁC. The mixture was allowed to warm to room
temperature and stirred overnight. The resulting mixture was
added dropwise to an ice-cooled solution of 1,10-phenanthroline
C, 44.58; H, 2.46; N, 4.65. IR (KBr): n 1886, 1913, 2020 cmꢀ1
.
Synthesis of 2. The procedure is similar to that of 1. 1 H NMR
(300 MHz, DMSO-d6, 293 K): d 3.88 (s, 3H, OCH3), 7.19 (d, J ¼
6 Hz, 2H), 7.66–7.68 (m, 2H), 8.10 (d, J ¼ 3 Hz, 1H), 8.16 (d, J ¼
6 Hz, 1H), 8.36 (d, J ¼ 6 Hz, 2H), 8.97 (d, J ¼ 6 Hz, 1H), 9.00 (d,
Table 4 Crystal data and structural refinement details for complex 1
J
¼
6 Hz, 1H), 9.47 (s, 1H) ppm. Anal. Calcd for
1
C22H14ClN2O4Re (592.02): C, 44.63; H, 2.38; N, 4.73; Found: C,
44.57; H, 2.40; N, 4.77. IR (KBr): n 1869, 1909, 2021 cmꢀ1
.
Formula
Fw
Temp. (K)
C22H14ClN2O4Re
592.00
293(2)
0.71073
Triclinic
P-1
9.808(2)
11.209(2)
12.043(2)
79.92(3)
87.34(3)
65.46(3)
1185.2(4)
2
1.897
Synthesis of 3. The procedure is similar to that of 1. 1 H NMR
(300 MHz, DMSO-d6, 293 K): d 7.14–7.19 (m, 8H), 7.40 (t, J ¼ 6
Hz, 4H), 7.57–7.65 (m, 2H), 8.11 (dd, J ¼ 3 Hz, 1H), 8.19 (d, J ¼
6 Hz, 1H), 8.35 (dd, J ¼ 3 Hz, 2H), 8.97 (d, J ¼ 3 Hz, 1H), 8.99
(d, J ¼ 6 Hz, 1H), 9.47 (d, J ¼ 3 Hz, 1H) ppm. Anal. Calcd for
C33H21ClN3O3Re (729.20): C, 54.35; H, 2.90; N, 5.76; Found: C,
ꢀ
Wavelength (A)
Crystal system
Space group
ꢀ
a (A)
ꢀ
b (A)
ꢀ
c (A)
a (ꢁ)
b (ꢁ)
g (ꢁ)
V (A )
Z
54.43; H, 2.76; N, 5.71. IR (KBr): n 1882, 1913, 2017 cmꢀ1
.
3
ꢀ
Acknowledgements
dc (Mg mꢀ3)
F(000)
Crystal size (mm)
q range for data collection
limiting indices
568
This work was supported by the National Natural Science
Foundation of China (No. 21074043, 20772044 and 60978062).
0.25 ꢂ 0.23 ꢂ 0.21
3.07ꢁ to 24.71ꢁ
ꢀ12 # h # 11
ꢀ14 # k # 14
ꢀ15 # l # 15
4033/0/284
1.088
Notes and references
data/restraints/parameters
goodness-of-fit on F2
Final R indices (I > 2s(I))
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R1 ¼ 0.0344
b
wR2 ¼ 0.0927
P
P
P
P
a
b
R1 ¼ kFo| ꢀ |Fck/ |Fo|. wR2 ¼ [ [w (Fo ꢀ Fc )2]/ [w (Fo2)2]]1/2
.
2
2
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J. Mater. Chem., 2012, 22, 3485–3492 | 3491

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