Synthesis, structural characterization, and electrophosphorescent properties of rhenium(I) complexes containing carrier-transporting groups

Article abstract of DOI:10.1021/ic061645o

Two novel diimine rhenium(I) carbonyl complexes with the formula [Re(CO)₃(L)Br], where L = 1-(4-5′-phenyl-1,3,4- oxadiazolylbenzyl)-2-pyridinylbenzoimidazole (1) and 1-(4-carbazolylbutyl)-2- pyridinylbenzoimidazole (2), have been successfully synthesized and characterized by elemental analysis, ¹H NMR, and IR spectra. Their electrochemical, photophysical, and electroluminescent behaviors, along with the X-ray crystal structure analysis of 2, are also described. White electrophosphorescent devices were fabricated using 1 and 2 as emitters. The devices based on carbazole-containing (hole-transporting group) 2 with the structure ITO/m-MTDATA (30 nm)/NPB (20 nm)/2:CBP (8%, 30 nm)/Bphen (20 nm)/Alq₃ (20 nm)/LiF (0.8 nm)/Al (200 nm) exhibit Commission Internationale de L'Eclairage coordinates of x = 0.34, y = 0.33 with a maximum brightness of 2300 cd/m² at 580 mA/cm². When a brightness of 1500 cd/m² appears at 230 mA/cm², the devices based on 10 wt % 2 still possess 56% of the maximum efficiency which appeared at 2.7 mA/cm². These performances are among the best reported for devices using Re(I) complexes as emitters. By comparison of the electroluminescent properties of the devices based on 1 and 2, we conclude that the introduction of the carbazole group into the ligand improves the performance of 1-doped devices.

Full text of DOI:10.1021/ic061645o

Inorg. Chem. 2007, 46, 6155−6163
Synthesis, Structural Characterization, and Electrophosphorescent
Properties of Rhenium(I) Complexes Containing Carrier-Transporting
Groups
Zhenjun Si,†,‡ Jiang Li,§ Bin Li,*,† Feifei Zhao,§ Shiyong Liu,*,§ and Wenlian Li†
Key Laboratory of Excited State Processes, Changchun Institute of Optics, Fine Mechanics and
Physics, and Graduate School of the Chinese Academy of Sciences, Chinese Academy of Sciences,
Changchun 130033, P. R. China, and State Key Laboratory of Integrated Optoelectronics,
Jilin UniVersity, Changchun 130023, P. R. China
Received August 30, 2006
Two novel diimine rhenium(I) carbonyl complexes with the formula [Re(CO)3(L)Br], where L
) 1-(4-5′-phenyl-
1,3,4-oxadiazolylbenzyl)-2-pyridinylbenzoimidazole (1) and 1-(4-carbazolylbutyl)-2-pyridinylbenzoimidazole (2), have
been successfully synthesized and characterized by elemental analysis, 1H NMR, and IR spectra. Their
electrochemical, photophysical, and electroluminescent behaviors, along with the X-ray crystal structure analysis of
2, are also described. White electrophosphorescent devices were fabricated using 1 and 2 as emitters. The devices
based on carbazole-containing (hole-transporting group) 2 with the structure ITO/m-MTDATA (30 nm)/NPB (20
nm)/2:CBP (8%, 30 nm)/Bphen (20 nm)/Alq3 (20 nm)/LiF (0.8 nm)/Al (200 nm) exhibit Commission Internationale
de L’Eclairage coordinates of x
) 0.34, y )
0.33 with a maximum brightness of 2300 cd/m2 at 580 mA/cm2.
When a brightness of 1500 cd/m2 appears at 230 mA/cm2, the devices based on 10 wt % 2 still possess 56% of
the maximum efficiency which appeared at 2.7 mA/cm2. These performances are among the best reported for
devices using Re(I) complexes as emitters. By comparison of the electroluminescent properties of the devices
based on 1 and 2, we conclude that the introduction of the carbazole group into the ligand improves the performance
of 1-doped devices.
Introduction
emission quantum efficiencies (theoretically up to 100%)
compared to that of fluorescent materials because of the
Interest in next generation displays and lighting technolo-
gies has stimulated research on organic light-emitting devices
(OLEDs).1 Especially, white-emitting OLEDs (WOLEDs)
based on phosphorescent materials have seen significant
improvement.2 As a result, d6 and d8 transition metal
phosphorescent complexes3-5 have been studied intensively
throughout the world. This class of compounds afford high-
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* To whom correspondence should be addressed. E-mail: lib020@
ciomp.ac.cn (B.Li.); syliu@mail.jlu.edu.cn (S.Liu.). Fax: +86 0431
86176935 (B.Li.).
† Changchun Institute of Optics, Fine Mechanics and Physics.
‡ Graduate School of the Chinese Academy of Sciences.
§ Jilin University.
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10.1021/ic061645o CCC: $37.00
Published on Web 06/21/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 15, 2007 6155

Si et al.
strong spin-orbit coupling in the luminescence process. With
the aim of further exploring novel phosphorescent materials,
several groups paid attention to Re(I) complexes,6 of which
the d6 electronic configuration is identical to that of the
corresponding Os(II) and Ir(III) systems. However, most of
the previously reported phosphorescent Re(I) complexes used
in the OLEDs have the disadvantage of the saturation of
emission sites resulting from triplet-triplet annihilation
which leads to low efficiency at high current density.7
Therefore, it is pressing to explore new Re(I) complexes to
overcome the shortcoming mentioned above. Two methods
are available to develop novel Re(I) complexes: (i) the
application of novel diimine ligands with different ligand
field strengths6d,7b,8 to adjust the molecular orbital energy
levels in the Re(I) complexes which are related to the
photochemical, photophysical, and electrochemical events
and (ii) introduction of functional groups with electro-
donating6c,7c,9 or -accepting6a,10 properties into diimine ligands,
which is helpful to improve the carrier-transporting ability
of Re(I) complexes in OLEDs and avoid the triplet-triplet
annihilation because of the steric hindrance effect. To date,
the first tactic has been successfully employed to obtain
electroluminescent (EL) Re(I) complexes without functional
groups. But Re(I) complexes containing carrier-transporting
groups are rarely used in OLEDs. In this article, we
successfully applied the second tactic to design and synthe-
size two novel Re(I) complexes which contain the electron-
transporting moiety 2,5-diphenyl-1,3,4-oxadiazole and the
hole-transporting moiety carbazole. The synthesized com-
plexes, [1-(4-5′-phenyl-1,3,4-oxadiazolylbenzyl)-2-pyridin-
ylbenzoimidazole]Re(CO)3Br (1) and [1-(4-carbazolylbu-
tyl)-2-pyridinyl-benzimidazole] Re(CO)3Br (2), are capable
of being vacuum deposited to construct EL devices. The
optical, electrochemical, thermal, and electroluminescent
properties of complexes 1 and 2 are also reported, ac-
companied by the X-ray crystal structure analysis of 2.
Experimental Section
Materials. All starting chemicals and charge-transporting materi-
als used in the process of the OLEDs devices fabrication, 4,4′,4′′-
tris[3-methylphenylphenylamino]triphenylamine (m-MTDATA), 4,7-
diphenyl-1,10-phenanthroline (Bphen), 4,4′-bis[N-(1-naphthyl)-N-
phenylamino]biphenyl (NPB), 4,4′-dicarbazolyl-1,1′-biphenyl (CBP),
tris(8-hydroxy-quinoline)aluminum (Alq3), and LiF, were com-
mercially available and were used without further purification unless
otherwise noted. The organic precursor of 2-4′-methylbenzyl-5-
benzyloxadiazole (a) was synthesized according to a published
method;11 the ligands of 1-(4-carbazolylbutyl)-2-pyridinylbenzoimi-
dazole (L1),12 1-(4-5′-Phenyl-1,3,4-oxadiazolylbenzyl)-2-pyridi-
nylbenzoimidazole (L2),13 and the corresponding Re(I) complexes14
were prepared according to the modified procedures. All reactions
and manipulations were carried out under N2 with the use of
standard inert atmosphere and Schlenk techniques. Solvents used
for synthesis were dried by standard procedures and stored under
N2. Solvents used in luminescent and electrochemical studies were
of spectroscopic and anhydrous grades, respectively.
Synthesis of 2-(4-Bromomethylphenyl)-5-phenyl-1,3,4-oxa-
diazole (c). Fifteen grams of (0.097 mol) 4-methylbenzoyl chloride
was added dropwise to a solution of 13.20 g (0.097 mol) of
benzohydrazide, 9.80 g (0.097 mol) of triethylamine, and 150 mL
of chloroform at room temperature (RT). The resulting mixture was
stirred for 1 h and then filtered. The collected solid was washed
with water and methanol to give 23.43 g (yield 95%) of the product
N′-benzoyl-4-methylbenzohydrazide. A mixture of 20.00 g of N′-
benzoyl-4-methylbenzohydrazide and 250 mL of POCl3 in a 500
mL flask was refluxed under nitrogen for 5 h. The excessive POCl3
was then distilled out, and the residue was poured into water. The
crude solid product was collected by filtration and purified by
recrystallization from chloroform/methanol to give 15.80 g of 2-4′-
tolyl-5-phenyl-1,3,4-oxadiazole (a) as needlelike crystals (85%).
A mixture of 14.16 g (0.06 mol) of compound a, 11.68 g (0.066
mol) of N-bromosuccinimide (NBS), 0.13 g of benzoyl peroxide,
and 200 mL of CCl4 in a 500 mL flask was refluxed for 5 h. The
mixture was filtered while it was still hot. The solid was washed
with hot chloroform. The residual solid was recrystallized from
tetrahydrofuran (THF)/methanol to give 8.00 g of product as white
1
needlelike crystals (40%). H NMR (CDCl3, 500 MHz): δ 4.570
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Thompson, M. E. Chem. Mater. 1999, 11, 3709. (b) Chan, S. C.; Chan,
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2006, 45, 137. (e) Zhuang, W.; Zhang, Y.; Hou, Q.; Wang, L.; Cao,
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Li, Y.; Liu, Y.; Guo, J.; Wu, F.; Tian, W.; Li, B.; Wang, Y. Synth.
Met. 2001, 118, 175. (c) Fu, C.; Li, M,; Su, Z.; Hong, Z.; Li, W.; Li,
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(2H, s), 7.334 (2H, d, J ) 8), 7.534-7.552 (3H, m), 8.026 (2H, d,
J ) 8), 8.133 (2H, d, J ) 8), Anal. Calcd for C15H11BrN2O: C,
57.16; H, 3.52; N. 8.89. Found: C, 57.04; H, 3.70; N, 8.96. IR
(KBr): ν 631, 1610, 3018 cm-1
.
Synthesis of 9-4′-Bromobutylcarbazole (d). NaH (0.74 g, 0.020
mol) and 9H-carbazole (b) (3.34 g, 0.020 mol) were added to 100
mL of anhydrous N,N-dimethylformamide (DMF) in a 250 mL
flask; the solution became clear after 1 h, and 3.0 mL (0.022 mol)
of 1,4-dibromobutane was added dropwise. The reaction mixture
was stirred for another 6 h and then poured into 300 mL of cool
water; the organic components were extracted with CH2Cl2 (3 ×
50 mL). The organic phase was washed with water and dried over
anhydrous sodium sulfate. After the solvent was removed by rotary
evaporation, the residue was purified by silica gel column chro-
matography with petroleum ether and acetic acid ethyl ester (v/v
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C.; Cheung, K. K. Eur. J. Inorg. Chem. 2003, 25, 4035. (b) Wang,
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3353. (c) Li, B.; Li, M.: Hong, Z.; Li, W.; Yu, T.; Wei, H. Appl.
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99, 257. (b) David, G.; Walsh, P. J.; Gordon, K. C. Chem. Phys. Lett.
2004, 383, 292.
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Shen, J. App. Phys. Lett. 2004, 84, 148.
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Appl. Phys. Lett. 2003, 83, 365. (b) Chan, W. K.; Ng, P. K.; Gong,
X.; Hou, S. Appl. Phys. Lett. 1999, 75, 3920.
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Kiggen, N. M. M.; Langeveld, B. M. W. J. Am. Chem. Soc. 2004,
126, 6035.
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Organometallics 2001, 20, 4911.
6156 Inorganic Chemistry, Vol. 46, No. 15, 2007

Re(I) Complexes with Carrier-Transporting Groups
1
) 10: 1) to yield 3.00 g white needle crystals. H NMR (CDCl3,
unknown sample according to the equation
500 MHz): δ 1.91 (m, 2H), 2.05 (m, 2H), 3.37 (t, 2H), 4.34 (t,
2H), 7.23 (m, 2H,), 7.38 (d, 2H, J ) 8.0 Hz), 7.46 (t, 2H), 8.09 (d,
2H, J ) 8.0 Hz). Anal. Calcd for C16H16BrN: C, 63.59; H, 5.34;
N, 4.63. Found: C, 63.45; H, 5.28; N, 4.74. IR (KBr): ν 3048,
2
Φunk ) Φstd(Iunk/Aunk)(Astd/Istd)(ηunk/ηstd
)
where Φunk is the luminescence quantum yield of the sample, Φstd
is the luminescence quantum yield of quinine sulfate and is taken
as 0.546,15 Iunk and Istd are the integrated fluorescence intensities
of the unknown sample and quinine sulfate, respectively, and Aunk
and Astd are the absorbances of the unknown sample and quinine
sulfate at excitation wavelengths of 420 nm for 1 and 364 nm for
quinine sulfate and 2. The ηunk and ηstd terms represent the refractive
indices of the corresponding solvents (pure solvents were assumed).
The excited-state lifetimes were detected by a system equipped with
a TDS 3052 digital phosphor oscilloscope pulsed Nd:YAG laser
with a THG 355 nm output and a computer-controlled digitizing
oscilloscope according to the reported method.16 The Origin 7.0
program by OriginLab Corporation was used for the curve-fitting
analysis.
Crystallography. Yellowish single crystals of complexes 2
suitable for X-ray diffraction studies were grown from slow
evaporation of a CH2Cl2 solution. The crystals were measured on
a Bruker Smart Apex CCD single-crystal diffractometer using λ-
(Mo KR) radiation, 0.7107 Å at 293 K. An empirical absorption
was based on the symmetry-equivalent reflections and applied to
the data using the SADABS program. The structure was solved
using the SHELXL-97 program.17 The crystallographic refinement
parameters of 2 are summarized in Table 1, while the selected bond
distances and angles are given in Table 2.
2935, 2858, 1587, 1450, 1370 cm-1
.
Synthesis of L1. NaH (0.37 g, 0.01 mol) and 2-pyridinylben-
zoimidazole (1.95 g, 0.01 mol) were added to 100 mL of stirring
anhydrous DMF in a 250 mL flask; 3.45 g (0.011 mol) of 2-(4-
bromomethylphenyl)-5-phenyl-1,3,4-oxadiazole was added when
the solution was clear. The mixture was stirred for 24 h, and then
it was poured into 300 mL of cool water. The resulting white
powder was collected by filtration and purified by silica gel column
chromatography with petroleum ether and acetic acid ethyl ester
1
(v/v ) 10:1) to yield 2.10 g of yellow powder (60%). H NMR
(CDCl3, 500 MHz): δ 6.299 (2H, s), 7.364 (6H, m), 7.553 (3H,
m), 7.875 (1H, t, J ) 8), 7.909 (1H, d, J ) 8), 8.045 (2H, d, J )
8), 8.115 (2H, d, J ) 6.5), 8.469 (1H, d, J ) 7.5), 8.629 (1H, d, J
) 3.5) Anal. Calcd for C27H19N5O: C, 75.51; H, 4.46; N, 16.31.
Found: C, 75.60; H, 4.52; N, 16.20. IR (KBr): ν 1446, 1549, 1770
cm-1
.
Synthesis of L2. The procedure was similar to that of compound
5. Yield: 55%.1H NMR (CDCl3, 500 MHz): δ 2.016 (4H, m, J )
6.5), 4.340 (2H, t, J ) 6.5), 4.846 (2H, t, J ) 6.5), 7.240-7.343
(8H, m), 7.448-7.477 (2H, m), 7.828-7.888 (2H, m), 8.114 (2H,
d, J ) 7.5), 8.408 (1H, d, J ) 8), 8.560 (1H, d, J ) 4). Anal.
Calcd for C28H24N4: C, 80.74; H, 5.81; N, 13.45. Found: C, 80.89;
H, 6.00; N, 13.11. IR (KBr): ν 3046, 2924, 1584, 1446, 1390 cm-1
.
Synthesis of 1. L1 (0.09 g, 0.210 mmol) and Re(CO)5Br (0.08
g, 0.200 mmol) were refluxed in 15 mL of toluene for 6 h. After
the mixture was cooled to RT, 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 (v/v ) 10:1). Yield: 0.124 g (80%). 1H NMR
(CDCl3, 500 MHz): δ 5.994 (2H, s), 7.319-7.333 (3H, d, J ) 7),
7.513-7.652 (7H, m), 7.838 (2H, d, J ) 7.5), 7.939 (1H, m,), 8.098
(2H, d, J ) 7.5), 8.180 (2H, d, J ) 33.5), 8.196 (1H, d, J ) 13),
9.238 (1H, d, J ) 5.5). Anal. Calcd for C30H19BrN5O4Re: C, 46.22;
H, 2.46; N, 8.98. Found: C, 46.22; H, 2.44; N, 9.10. IR (KBr): ν
Thermal Analysis. Thermogravimetric analysis (TGA) was
performed on ∼2 mg of 1 and 2 using a Perkin-Elmer thermal
analyzer. The samples were dried under vacuum at 56.5 °C before
being heated from 40 to 605 °C at a heating rate of 10.0 °C/min.
A 10 mL/min flow of dry nitrogen was used to purge the sample
at all times.
Cyclic Voltammetry. Cyclic voltammetry measurements were
conducted on a voltammetric analyzer (CH Instruments, Model
620B) with a polished Pt plate as the working electrode, Pt mesh
as the counter electrode, and a commercially available saturated
calomel electrode (SEC) as the reference electrode at a scan rate
of 0.1 V/s. The voltammograms were recorded using CH3CN
sample solutions with ∼10-3 M 1 and 2 and 0.1 M ammonium
hexafluorophosphate (AHFP) as the supporting electrolyte. Prior
to each electrochemical measurement, the solution was purged with
nitrogen for ∼10-15 min to remove the dissolved O2 gas.
Fabrication of EL Devices. Prepatterned ITO substrates with a
resistivity of <30 Ω and an effective individual device area of 4
mm2 were cleaned by sonication in detergent solution, water, and
ethanol, sequentially. After they were blown dry with nitrogen, the
ITO substrates were treated with oxygen plasma for 1 min before
being loaded into the vacuum chamber. All layers in the diodes
are deposited by a resistive heating method. The pressure of the
chamber is below 3 × 10-4 Pa. LiF (0.8 nm) was deposited as the
electron-injection layer, which was capped with 200 nm of Al as
the cathode. The EL spectra and Commission Internationale de
L’Eclairage (CIE) coordinates of the devices were measured by a
2018, 1898 cm-1
.
Synthesis of 2. The procedure is similar to that of 1. The yellow
solid was purified by silica gel column chromatography with
1
dichloromethane and THF. Yield: 80%. H NMR (CDCl3, 500
MHz): δ 1.979 (2H, m), 2.148 (2H, m), 4.335 (2H, t, J ) 7.5),
4.417 (2H, t, J ) 7.5), 7.133 (2H, d, J ) 8.5), 7.277 (2H, d, J )
8), 7.376 (2H, d, J ) 8), 7.463 (4H, t, J ) 8), 7.525 (1H, t, J ) 7),
7.636 (1H, d, J ) 8), 7.727 (1H, d, J ) 8), 8.051 (1H, d, J ) 7.5),
8.119 (2H, d, J ) 7.5). Anal. Calcd for C31H24BrN4O3Re: C, 48.57;
H, 3.16; N, 7.31. Found: C, 48.30; H, 3.31; N, 7.20. IR (KBr): ν
2019, 1897 cm-1
.
Spectroscopy. The IR spectra were acquired using a Magna560
FT-IR spectrophotometer. Element analyses were performed using
1
a Vario Element Analyzer. H NMR spectra were obtained using
a Bruker AVANCE 500 MHz spectrometer with tetramethylsilane
as the internal standard. A suitable amount of sample for emission
studies was dissolved in the appropriate solvent to make the
concentration 10-5 mol/L, and the absorbance of the solution was
measured. This concentration provided enough material for data
acquisition but excluded self-quenching processes. The corrected
PL spectra of the degassed solution were collected by a Shimadzu
RF-5301PC spectrophotometer. The luminescence quantum yields
were measured by comparison of the fluorescence intensities
(integrated areas) of a standard sample (quinine sulfate) and the
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Instruments Inc.: Madison, WI, 1998. (b) SMART and SAINT; Siemens
Analytical X-ray Instruments Inc.: Madison, WI, 1995. (c) Sheldrick,
G. M. SADABS; University of Go¨ttingen: Go¨ttingen, Germany, 1996.
Inorganic Chemistry, Vol. 46, No. 15, 2007 6157

Si et al.
Table 1. Crystal Data for 2‚CH2Cl2
selected bond distances and angles for the complexes are
listed in Table 2. The coordination geometry at the Re atom
is a distorted octahedron with the three carbonyl ligands
arranged in a facial fashion. The distances of C(1), C(2),
and C(3) to Re(1) are 1.863(13), 1.994(11), and 1.904(11)
Å, respectively, and the average Re-N bond distance is
2.175 (8) Å. The C-C bond distance for the bridgehead
carbon atoms of L2 (C(8)-C(9)) is 1.464 (12) Å, which is
in agreement with that for the 2-pyridinylbenzoimidazole
(Pybm) congener.7b The bond angles in Table 2 clearly
indicate that the CO ligands are linearly coordinated. The
bond angles between adjacent CO carbon atoms are 88.3-
(5)-93.3(5)°, which is close to 90°, but the bond angle
between the coordinated nitrogen atoms of ligand L2 is 72.7-
(3)°, which is much less than 90°. This is not only a result
of the steric requirement of the bidentate coordination of L2
but also the result ofa twist of the imidazole ring relative to
the pyridine plane. This twist is clearly verified by the fact
that the plane of the pyridine ring intersects that of the
imidazole ring at an angle of 5.0° in normal. C(1) is
constrained to be isotropic because of its disorder in the
crystals formed at RT. All other bond distances and angles
are comparable to those found for the related Re(I) com-
plexes.19
formula
fw
T (K)
wavelength (Å)
cryst syst
space group
a (Å)
C32H26Cl2N4O3ReBr
851.58
293(2)
0.71073
monoclinic
P21/n
14.7407(8)
9.1414(5)
23.4181(13)
90
98.2320(10)
90
3123.1(3)
4
1.811
5.381
1656
yellow
0.311× 0.287 × 0.193
1.54-28.24
18 724
7347
95.10%
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
F
calcd (Mg/m3)
µ (mm-1
)
F(000) (e)
cryst color
cryst size (mm)
range of transm factors (deg)
reflns collected
unique
completeness to θ ) 28.24
data/restraints/params
GOF on F2
R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)
7347/0/415
1.065
0.0636, 0.1483
0.1166, 0.1713
Table 2. Selected Bond Bengths (Å) and Angles (deg) for 2‚CH2Cl2
C(1)-Re(1)
C(2)-Re(1)
C(3)-Re(1)
C(9)-N(1)
1.863(13)
1.994(11)
1.904(11)
1.322(10)
N(1)-Re(1)
N(2)-Re(1)
Br(1)-Re(1)
C(8)-N(2)
2.149(7)
2.195(8)
2.5956(12)
1.338(11)
Thermal Analysis. TGA was performed on complexes 1
and 2 in a N2 atmosphere to investigate their stable
characteristics, and the TGA traces of 1 and 2 are presented
in Figure 2. Complex 1 began to lose weight when it was
heated to 328 °C, which should be attributed to the loss of
the bromine anion and carbonyl groups. L1 began to
dramatically disassociate or sublimate at 487 °C and totally
decomposed at 600 °C. Complex 2 shows thermal charac-
teristics similar to those of 1 in a N2 atmosphere: it began
to decompose gradually at 365 °C probably because of the
loss of the carbonyl groups and bromine anion. At temper-
atures higher than 457 °C, the weight loss is probably caused
by the unassisted thermolysis of L2 or simply its sublimation.
This process finished at 550 °C. Therefore, both 1 and 2 are
stable enough to be sublimated to construct OLED devices
at ∼300 °C.
O(1)-C(1)-Re(1)
O(2)-C(2)-Re(1)
O(3)-C(3)-Re(1)
C(9)-N(1)-Re(1)
C(10)-N(1)-Re(1)
C(8)-N(2)-Re(1)
C(4)-N(2)-Re(1)
C(1)-Re(1)-C(3)
C(3)-Re(1)-C(2)
C(1)-Re(1)-C(2)
C(3)-Re(1)-N(2)
177.1(11)
C(1)-Re(1)-N(2)
C(2)-Re(1)-N(2)
C(3)-Re(1)-N(1)
C(1)-Re(1)-N(1)
C(2)-Re(1)-N(1)
N(1)-Re(1)-N(2)
C(3)-Re(1)-Br(1)
C(1)-Re(1)-Br(1)
C(2)-Re(1)-Br(1)
N(1)-Re(1)-Br(1)
N(2)-Re(1)-Br(1)
170.3(4)
93.6(4)
168.7(4)
99.7(4)
96.9(3)
72.7(3)
92.2(3)
88.2(4)
178.4(3)
82.38(18)
84.85(18)
174.7(13)
178.0(10)
117.8(6)
133.9(6)
119.0(6)
122.5(7)
90.0(5)
88.3(4)
93.3(5)
97.0(4)
PR650 spectrometer. All measurements were carried out at RT
under ambient conditions.
Results and Discussion
Synthesis and Characterization. Ligands L1 and L2 and
the corresponding rhenium(I) complexes 1 and 2 were
prepared according to literature methods, and their structures
are shown in Scheme 1. The purity and composition of the
Electrochemistry. Cyclic voltammograms of complexes
1 and 2 are shown in Figure 3 and exhibit irreversible metal-
centered oxidation and ligand-based reduction in CH3CN
solution, which are consistent with the redox behavior of
Pybm-based Re(I) diimine complexes reported in the
literature.7b Complex 1 shows irreversible anodic waves at
V1/2 ) +1.26 V with an onset oxidation potential of +1.19
V vs SCE and irreversible cathodic waves at E1/2 ) -1.50
V with an onset reduction potential of -1.20 V vs SCE.
The anodic waves are associated with a ReI-based oxidation
process (ReI/ReII), and the cathodic waves are associated with
a ligand-based reduction process ([ReICl(CO)3(L)]/ [ReICl-
(CO)3(L•)]-).20
1
complexes were confirmed by H NMR, IR, and elemental
1
analyses. The H NMR spectra of complexes 1 and 2 have
some differences with those of corresponding free diimine
ligands L1 and L2. Compared to the free ligands, these
signals were shifted toward low field because of the
formation of new coordination bonds between the synthesized
ligands and the Re metal centers.18
Structure of 2‚CH2Cl2. Using the single-crystal X-ray
diffraction method, we obtained solid evidence to support
the structure of 2‚CH2Cl2. An ORTEP diagram of 2 is shown
in Figure 1, and the crystal data are presented in Table 1;
(19) Rajendran, T.; Manimaran, B.; Lee, F. Y.; Lee, G. H.; Peng, S. M.;
Wang, C. C.; Lu, K. L. Inorg. Chem. 2000, 39, 2016.
(18) Zhang, M.; Lu, P.; Wang, X.; He, L.; Xia, H.; Zhang, W.; Yang, B.;
Liu, L.; Yang, L.; Yang, M.; Ma, Y.; Feng, J.; Wang, D.; Tamai, N.
J. Phys. Chem. B 2004, 108, 13185.
(20) (a) Berger, S.; Klein, A.; Kaim, W.; Fiedler, J. Inorg. Chem. 1998,
37, 5664. (b) Moya, S. A.; Guerrero, J.; Pastene, R.; Schmidt, R.
Sariego, R. Inorg. Chem. 1994, 33, 2341.
6158 Inorganic Chemistry, Vol. 46, No. 15, 2007

Re(I) Complexes with Carrier-Transporting Groups
Scheme 1. Synthetic Route to Complexes 1 and 2a
a (i) NBS, BPO, CCL4, 120 °C; (ii) NaH, DMF, RT; (iii) Re(CO)5Br, toluene, 80 °C.
Figure 1. ORTEP drawing of a crystal of 2 with displacement ellipsoids
at the 30% probability level.
Figure 2. TGA traces for complexes 1 and 2.
- Eonset(Red) (Eonset(Red) ) -1.30 V for 2). These calcula-
tions give HOMO and LUMO energy levels of -5.93 and
-5.81 eV and -3.54 and -3.44 eV for 1 and 2, respectively.
The gaps between the LUMO and the HOMO energy levels
of 1 and 2 were thus determined to be 2.39 and 2.37 eV,
respectively, which are in good agreement with those (2.38
eV) obtained from UV-vis spectroscopy. The electron
affinity of -2.55 eV for 2,5-diphenyl-1,3,4-oxadiazole and
the ionization potential of -5.78 eV for carbazole indicate
they possess electron- and hole-transporting abilities, re-
spectively. As a result, the introduction of them into the
parent complex (Pybm)Re(CO)3Br (ELUMO ) -3.47 eV,
Complex 2 shows a similar redox behavior. The energy
levels of the highest-occupied molecular orbital (HOMO)
and the lowest-unoccupied molecular orbital (LUMO) are
calculated from the onset oxidation (Eonset(Ox)) and reduction
(Eonset(Red)) potentials with the formula EHOMO ) -4.74 -
E
onset(Ox) (-4.74 V for SCE with respect to the zero vacuum
level21 and Eonset(Ox) ) +1.07 V for 2) and ELUMO ) -4.74
(21) (a) Bard, A. J.; Faulkner, L. R. Electrochemical Methods Fundamental
and Applications; Wiley: New York, 1980. (b) Wu, D. G.; Huang,
C. H.; Gan, L. B.; Zheng, J.; Huang, Y. Y.; Zhang, W. Langmuir
1999, 15, 7276. (c) Ashwini, K. A.; Samson, A. J. Chem. Mater. 1996,
8, 579.
Inorganic Chemistry, Vol. 46, No. 15, 2007 6159

Si et al.
(CO)3Br] in dichloromethane. The broad PL spectra in CH2-
3
Cl2 solution only show the MLCT transition bands with
maximum values of 601 nm for 1 and 577 nm for 2, which
implies that the effects of the 2,5-diphenyl-1,3,4-oxadiazole
and carbazole groups on the energy level of the π* orbital
of the parent complex (Pybm)Re(CO)3Br are different. It was
reported that the effects should be as follows: the energy of
the π* orbital is lowered by the presence of an electron-
withdrawing moiety (2,5-diphenyl-1,3,4-oxadiazole) and
heightened by the introduction of an electron-donating moiety
(carbazole).6a These effects are also verified by the difference
of the excitation spectra of 1 and 2 in CH2Cl2 solution: the
broad and low-energy MLCT band in the absorption spectra
partially resolves into two bands in the scanning excitation
spectrum, namely, the 1MLCT and 3MLCT states at 379 and
420 nm and 370 and 415 nm for 1 and 2, respectively. As
can be seen from Figure 4b, the PL spectra of (Pybm)Re-
(CO)3Br and complex 2 are almost the same, indicating the
effect of carbazole group on the energy of π* orbital is
negligible in complex 2, which should be attributed to the
longer distance (5.38 Å) between the carbazole and (Pybm)-
Re(CO)3Br moieties.
Figure 3. Cyclic voltammograms of complexes 1 and 2 measured in CH3-
CN (vs SCE) at a scan rate of 0.1V/s. A polished Pt plate and a Pt mesh
were used as the working electrode and the counter electrode, respectively.
AHFP was taken as supporting electrolyte.
Table 3. Absorption and Emission Spectral Data of 1 and 2 in CH2Cl2
and the Solid State at 293 K
excitation
emission
abs [λ, nm
λ
λmax
φ
complex medium (ꢀ, ×10-3 mol-1cm-1)]
(nm)
(nm) (degass)
1
CH2Cl2 284 (41.6), 340 (22.8), 379, 420 601 2.6 × 10-3
420 (4.7)
Excitation at either the π f π* absorption region (∼280
nm) or the MLCT absorption region (∼370 nm) leads to the
same MLCT emission (∼578 nm) at RT for 2, which
indicates there is potential surface crossing from the higher
π f π* state to the lower MLCT state, and the major
contribution of the observed emission is from the MLCT
state.23 However, the same potential surface crossing does
not exist for 1 because the excitation spectrum of 1 in CH2-
Cl2, measured by monitoring the emission λmax at RT, did
not show the π f π* excitation band probably because of
the lower emission energy of the ligand L1 shown in Figure
S1. This character of the photoluminescence is further
revealed by the measurement of the excited-state lifetimes.
As indicated in Figure 5, the PL spectrum of complex 2 is
composed of two exponential decays with lifetimes of τ1 )
0.91 µs (A1 ) 0.25) and τ2 ) 0.22 µs (A2 ) 7.25).24 In
general, if there is potential surface crossing from the higher
ligand π f π* state to the lower MLCT state, one can
expect, on the basis of the energy gap law, a shorter decay
lifetime from the π f π* state than the lower MLCT state.25
However, the potential surface crossing for 2 from the higher
π f π* state to the MLCT state is not efficient. Therefore,
the assignment of the major, short-lived decay component
to emission from the 3MLCT state and the minor, long-lived
component to the emission from the π f π* state should be
reasonable. Complex 1 also exhibits a biexponential decay
pattern with lifetimes of τ1 ) 0.41 µs (A1 ) 1.00) and τ2 )
0.12 µs (A2 ) 1139.70), confirming there is hardly any
potential surface crossing for 1 from π f π* state to the
solid
420, 468 569
2
CH2Cl2 260 (40.9), 292 (30.9), 276, 370, 577 3.1 × 10-3
328 (27.2), 340 (25.4), 415
366 (6.4)
solid
252, 282, 536
436
EHOMO ) -5.84 eV) makes the difference in the redox
potentials of the Re(I)-Re(II) couples between 1 and 2,
suggesting that the property of the functional group affects
the stability of the metal orbital energy levels.
Photophysical Properties. The absorption and emission
spectral data for 1 and 2 along with lifetimes and emission
quantum yields in deoxygenated dichloromethane at 293 K
are summarized in Table 3. Figure 4a shows UV-vis
absorption spectra and excitation spectra of complexes 1 and
2 in dichloromethane. For both 1 and 2, the dominant
absorption bands in the 260-360 nm region were assigned
to an admixture of intraligand (π f π* (L)) transitions of
the carrier-transporting moieties and the Pybm moiety. These
assignments were based on the absorption spectra of the free
ligands shown in Figure S1 (in the Supporting Information)
and similar Re(I) complexes.22 These bands are also ac-
companied by lower-energy features extending into the
visible region from 360 to 520 nm which are tentatively
assigned to the metal-to-ligand charge-transfer dπ(Re) f
π*(N-N) (MLCT) transitions. It is obvious that all of the
MLCT absorption bands of complexes 1 and 2 are similar
to those observed in the related Re(I) complexes and show
little difference from each other which may be caused by
the weak MLCT transition absorbance.
(23) Sacksteder, L.; Zipp, A. P.; Brown, E. A.; Streich. J.; Demas, J. N.;
DeGraff, B. A. Inorg. Chem. 1990, 29, 4335.
(24) Ranjan, S.; Lin, S.-Y.; Hwang, K. C.; Chi, Y.; Ching, W.-L.; Liu,
C.-S.; Tao, Y.-T.; Chien, C.-H.; Peng, S.-M.; Lee, G.-H. Inorg. Chem.
2003, 42, 1248.
(25) (a) Shaw, J. R.; Schmehl, R. H. J. Am. Chem. Soc. 1991, 113, 389.
(b) Wallace, L.; Rillema, D. P. Inorg. Chem. 1993, 32, 3836. (c)
Striplin, D. R.; Crossby, G. A. Coord. Chem. ReV. 2001, 211, 163.
Figure 4b exhibits the emission spectra of complexes 1,
2, and (2-pyridinylbenzoimidazole)Re(CO)3Br [(Pybm)Re-
(22) (a) Lo, K. K.-W.; Lau, J. S.-Y.; Fong, V. W.-Y.; Zhu, N. Organo-
metallics 2004, 23, 1098. (b) Guerrero, J.; Piro, O. E.; Wolcan, E.;
Feliz, M. R.; Ferraudi, G.; Moya, S. A. Organometallics 2001, 20,
2842.
6160 Inorganic Chemistry, Vol. 46, No. 15, 2007

Re(I) Complexes with Carrier-Transporting Groups
Figure 4. (a) UV-vis absorption spectra and excitation spectra of complexes 1 and 2 in CH2Cl2 solution. (b) Emission spectra of complexes 1, 2, and
(Pybm)Re(CO)3Br (0) in CH2Cl2 solution.
Table 4. Key Parameters of the OLEDs Based on Complexes 1 and 2
and (Pybm)Re(CO)3Br
a
b
c
Bmax
λmax
ηmax
complexes
(cd/m2)
(nm)
(cd/A)
1
2
1774
2300
1240
∼590
∼570
∼564
0.53
1.60
0.75
(Pybm)Re(CO)3Br
a The maximum brightness. b The maximum EL emission wavelength.
c The maximum current efficiency.
Figure 5. Photoluminescence lifetime decay measurement of the solid
thin film of complex 2.
3
emission bands attributed to the MLCT emission from the
Re(I) complexes show red shifts from 572 to 600 nm for 1
and from 568 to 588 nm for 2, accompanying the decrease
of the emission from CBP with the increase of the doping
concentration from 6 to 20 wt %. The difference of the EL
emission from Re(I) complexes can also be attributed to the
effect of the carrier-transporting groups as described above.
The red shift should be explained by the interaction of the
dopant molecules at higher doping concentration.26 When
the doping concentration of 1 and 2 is higher than 10 wt %,
the emission from CBP almost disappears, and the EL spectra
only demonstrate the broad emission bands centered at 600
nm for 1-based devices and 588 nm for 2-based ones. The
CIE coordinates for the devices with 10 wt % 1 and 2 are x
) 0.519 ((0.002), y ) 0.46 ((0.002) from J ) 4.29 to
381.68 mA/cm2 and x ) 0.504 (no shift), y ) 0.474 ((0.003)
from J ) 0.73 to 476.86 mA/cm2, respectively. The devices
with 8 wt % 1 and 2 were found to be good WOLEDs which
demonstrate CIE coordinates of (0.35, 0.32) at 11 V and
(0.34, 0.33) at 15 V, respectively, both of which are close
to those of pure white light (0.33, 0.33). These CIE
coordinates also show minor shifts at different current
densities. For 1-based devices, the CIE coordinates show ∆x
) ( 0.02, ∆y ) ( 0.04 from J ) 7.14 to 180.10 mA/cm2,
and for 2-doped devices, the CIE coordinates show the same
shifts from J ) 80.00 to 372.14 mA/cm2.
MLCT state. Finally, the luminescence quantum yields in
deaerated dichloromethane were measured to be 2.6 × 10-3
and 3.1 × 10-3 for complexes 1 and 2, respectively.
EL Properties. The OLEDs with complexes 1 and 2 as
dopants in the CBP emissive layer were fabricated with the
structure ITO/m-MTDATA (30 nm)/NPB (20 nm)/dopant:
CBP(× wt %, 30 nm)/Bphen (20 nm)/Alq3 (20 nm)/LiF (0.8
nm)/Al (200 nm).9a m-MTDATA is used as the hole-injection
layer; NPB is the hole-transporting and electron-blocking
layer, and Bphen and Alq3 are employed as the exciton-
blocking layer and the electron-transporting layer, respec-
tively. Carrier recombination occurring in the CBP layer is
necessary to generate CBP excitons. The Bath layer with a
LUMO level of -2.4 eV and the NPB layer with a HOMO
level of -5.2 eV pave the ways for electrons and holes to
transport from Alq3 (LUMO level ) -3.1 eV) and m-
MTDATA (HOMO ) -5.1 eV) to the CBP layer (HOMO
) -5.5 eV, and LUMO ) -2.0 eV), respectively. Because
the PL spectrum of CBP centered at 380 nm shows large
spectral overlap with the MLCT absorption bands of
complexes 1 and 2, there should exist efficient Fo¨ster-type
energy transfer in the constructed light-emitting devices,
which can emit stable white light at a suitable doped
concentration. To study the effect of the functional groups
on the performance of the electrophosphoresent devices based
on Re(I) complexes, the EL properties of 1- and 2-based
devices are reported in this article. The key parameters of
the OLEDs based on (Pybm)Re(CO)3Br, 1, and 2 are listed
in the Table 4.
Figure 7 presents the luminance-voltage (L-V) charac-
teristics of the devices based on 1 (a) and 2 (b) with doping
concentrations of 6, 8, 10, 20 wt %. As can be observed,
the maximum brightness achieved from the devices based
on 8 wt % of complexes 1 and 2 reached 1774 cd/m2 at 17
V and 2300 cd/m2 at 16 V, respectively. The current-voltage
Figure 6 shows the EL spectra for the devices based on 1
and 2 at an applied voltage of 13 V. The EL emission band
centered at 410 nm originates from CBP. The lower-energy
(26) Polleux, J.; Pinna, N.; Antonietti, M.; Niederberger, M. AdV. Mater.
2004, 16, 432.
Inorganic Chemistry, Vol. 46, No. 15, 2007 6161

Si et al.
Figure 6. EL spectra of the OLEDs based on 1 (a) and 2 (b) at 13 V.
Figure 7. L-V and I-V (inset) characteristics of the OLEDs based on 1 (a) and 2 (b).
The saturation is responsible for the fast decrease of the EL
efficiency at high current density, exceeding 300 mA/cm2.
The devices with a doping concentration of 10 wt % exhibit
the maximum efficiency; the efficiency of 2-based devices
at 230 mA/cm2 stays as high as 56% of the maximum value
which appears at 2.7 mA/cm2, and that of 1-based devices
at 230 mA/cm2 is 51% of the maximum value at 4.2 mA/
cm2. These data suggest that the devices based on 1 and 2
not only exhibit excellent white emission but also are stable
in a large current range.
As can be observed from the Table 4 and the Figure 8,
the order of the maximum efficiency of the OLEDs is as
follows: complex 2 > (Pybm)Re(CO)3Br > complex 1. The
explanation might be that when (Pybm)Re(CO)3Br is linked
with the hole-transporting group of carbazole, the charge
carrier injection balance in the devices are improved,
resulting in the effective carrier recombination. In contrast,
the introduction of the electron-transporting group of 2,5-
diphenyl-1,3,4-oxadiazole into (Pybm)Re(CO)3Br will lower
the device’s efficiency. It also implies that (Pybm)Re(CO)3Br
may possess effective electron-transporting and weak hole-
transporting ability. Therefore, unlike other metal com-
plexes,13,27 modifying some Re(I) complexes with a suitable
hole-transporting moiety would lead to a better carrier-
injection and -transportation balance and, consequently, to
the improvement of the device performances.
Figure 8. Current efficiency of the OLEDs based on 1 and 2 as a function
of current density.
(I-V) characteristics of these devices are also presented in
the inset of Figure 7. When the doping concentration is not
higher than 10 wt %, the current density at a given voltage
increases with the doping concentration, indicating the good
carrier-transporting ability of both 1 and 2 in the CBP
emissive layer. The I-V curves for the devices based on 20
wt % Re(I) complexes lie below the other I-V curves, which
may be caused by phase separation at higher doping
concentration.
Figure 8 presents the characteristic of the EL efficiency-
current density of the devices based on 1 and 2. The
efficiency of these devices decreases slowly at first with an
increase of the current densities, suggesting that the saturation
of the phosphorescence sites is not severe because of the
steric hindrance effect of the functional groups in 1 and 2.
(27) Wang, J. F.; Wang, R. Y.; Yang, J.; Zheng, Z. P.; Carducci, M. D.;
Cayou, T.; Peyghambarian, N.; Jabbour, G. E. J. Am. Chem. Soc. 2001,
123, 6179.
6162 Inorganic Chemistry, Vol. 46, No. 15, 2007

Re(I) Complexes with Carrier-Transporting Groups
Summary
is believed that the modification of some diimine rhenium-
(I) carbonyl complexes with suitable hole-transporting
moieties would lead to a better carrier-injection and -trans-
portation balance and, consequently, to the improvement of
the OLEDs’ performances, and we are in the progress of
exploring these possibilities.
Rhenium(I) complexes 1 and 2 were successfully synthe-
sized according to the molecular engineering, and they are
stable enough to be sublimated during EL-device fabrication.
The lifetime measurements confirmed that both 1 and 2
display biexponential phosphorescent emission with triplet
excited-state lifetimes of 0.41 and 0.12 µs for 1 and 0.91
and 0.22 µs for 2. When the doping concentration is 8 wt
%, the devices demonstrate CIE coordinates of x ) 0.35, y
) 0.32 with a maximum brightness of 1774 cd/m2 at 17 V
for the electron-transporting group 2,5-diphenyl-1,3,4-oxa-
diazole-containing 1 and CIE coordinates of x ) 0.34, y )
0.33 with a maximum brightness of 2300 cd/m2 at 16 V for
hole-transporting group carbazole-containing 2. The perfor-
mance of the devices based on 2 is notably superior to that
of the complex 1-based devices for a given doping level; it
Acknowledgment. The authors gratefully thank the One
Hundred Talents Project from Chinese Academy of Sciences
and the NSFC (Grant 20571071) for financial support.
Supporting Information Available: Figure showing the ab-
sorption spectra and PL spectra of ligands L1 and L2 and
crystallographic data in CIF format. This material is available free
of charge via the Internet at http://pubs.acs.org.
IC061645O
Inorganic Chemistry, Vol. 46, No. 15, 2007 6163