Synthesis, photoluminescence properties of novel cationic Ir(III) complexes with phenanthroimidazole derivative as the ancillary ligand

Article abstract of DOI:10.1016/j.poly.2017.09.014

Two novel cationic Ir(III) complexes, [(nbt)₂Ir(L)](PF₆) and [(CF₃-bt)₂Ir(L)](PF₆), where nbt = 2-(1-naphthyl)benzothiazole; CF₃-bt = 2-phenyl-6-(trifluoromethyl)-benzothiazole and L = 1-(4

Full text of DOI:10.1016/j.poly.2017.09.014

Accepted Manuscript
Synthesis, photoluminescence properties of novel cationic Ir(III) complexes
with phenanthroimidazole derivative as the ancillary ligand
Fuzhi Yang, Tianzhi Yu, Yuling Zhao, Hui Zhang, Yuying Niu
PII:
S0277-5387(17)30591-0
https://doi.org/10.1016/j.poly.2017.09.014
POLY 12824
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
20 July 2017
13 September 2017
Please cite this article as: F. Yang, T. Yu, Y. Zhao, H. Zhang, Y. Niu, Synthesis, photoluminescence properties of
novel cationic Ir(III) complexes with phenanthroimidazole derivative as the ancillary ligand, Polyhedron (2017),
doi: https://doi.org/10.1016/j.poly.2017.09.014
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Synthesis, photoluminescence properties of novel cationic Ir(III) complexes
with phenanthroimidazole derivative as the ancillary ligand
Fuzhi Yang1, Tianzhi Yu1,*, Yuling Zhao2, Hui Zhang1, Yuying Niu1
1Key Laboratory of Opto-Electronic Technology and Intelligent Control (Ministry of Education),
Lanzhou Jiaotong University, Lanzhou 730070, China
2School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070,
China
Abstract: Two novel cationic Ir(III) complexes, [(nbt)2Ir(L)](PF6) and [(CF3-bt)2Ir(L)](PF6),
where nbt = 2-(1-naphthyl)benzothiazole; CF3-bt = 2-phenyl-6-(trifluoromethyl)-benzothiazole and
L = 1-(4-(tert-butyl)phenyl)-2-(5-(9-(p-tolyl)-9H-carbazol-3-yl)pyridin-2-yl)-1H-phenanthro
[9,10-d]imidazole, were successfully synthesized and characterized. The structures of L and
[(nbt)2Ir(L)](PF6) were confirmed by single crystal X-ray diffraction. Their photophysical,
electrochemical properties and thermal stabilities were investigated systematically. The results
showed that the Ir(III) complexes had higher thermal stabilities, and exhibited strong red emission
and orange emission, respectively.
Keywords: Cationic Ir(III) complex; Phenanthroimidazole derivative; Crystal structure;
Photoluminescence
*Corresponding Author. Tel. +86-931-4956935; Fax: +86-931-4938756; e-mail: yutzh@mail.lzjtu.cn (T.Z. Yu)
1

1. Introduction
In the last decades, the neutral Ir(III) complexes have been widely used as phosphorescent
emitters in organic light-emitting diodes (OLEDs) due to their high phosphorescent efficiencies and
tunable emission color [1-9]. However, the OLEDs based on Ir(III) complexes are highly sensitive
to the doping concentration and devices suffer from reduced emission efficiency while increasing
the concentration of phosphorescent emitters, thus Ir(III) complexes as electroluminescent (EL)
materials are always doped in the host materials at appropriate concentration to fabricate OLEDs. In
addition, the use of neutral Ir(III) complexes in OLEDs requires a complicated multilayered
structure for charge injection, transport, and light emission [10-12].
Recently, the use of cationic iridium(III) complexes has attracted tremendous attentions due to
their ability to be used in bio-imaging [13], light-emitting electrochemical cells (LECs) [14-16] as
well as OLEDs [17-20]. Compared to the iridium(III) complexes, the cationic iridium complexes
have many advantages such as easy preparation, facile tuning of the emission and the energy levels
via the ancillary ligands, and good solubility in polar solvents. Moreover, the presence of mobile
counter ions in the cationic Ir(III) complexes can form accumulation of negative charge near one
electrode under applied voltage, which causes high electric fields at the electrodes, thus minimizing
carrier injection barrier for electron and hole and enhancing electronic charge injection ultimately
allowing for the use of a single-layer architecture operating at low voltages [21,22]. However, the
large size of the cations renders the complexes to be essentially stationary in the device, meaning
that upon initial application of a bias, the cation density at the cathode is initially much lower than
the anion concentration at the anode, leading to an imbalance of charge injection into the device. By
2

doping in small amounts of alkali metal cation hexafluorophosphate salts (e.g., LiPF6), it could lead
to more balanced charge injection that ultimately improved not just response times, but also
luminance values and EQEs [23]. In the current research state of cationic Ir(III) complexes, the
OLEDs based on the cationic Ir(III) complexes have current efficiencies in the 12-31 cd/A range,
much inferior to the neutral Ir(III) complexes based OLEDs [17,24-28]. For example, the Cao group
reported highly efficient polymer light-emitting diodes based on yellow-green light-emitting
cationic
Ir(III)
complexes
with
1,10-phenanthroline
derivatives
containing
oxadiazole-triphenylamine, oxadiazole or triphenylamine unit as neutral auxiliary ligands, in which
the devices fabricated from the cationic Ir(III) complex containing bipolar unit of
oxadiazole-triphenylamine exhibited higher EL performance with the maximum luminance
efficiency of 30.6 cd/A and the maximum external quantum efficiency of 1.02% due to its balance
of hole and electron fluxes [17]. Wong et al. [24] reported OLEDs based on sublimable cationic
iridium complexes with rigid pyridinylfluorene derivatives as dopants, the devices exhibited yellow
light emission with a maximum luminance efficiency of 20 cd/A and a quantum efficiency of 7%.
Benzimidazole derivatives have good luminescent and electron-transporting properties, and
strong coordination ability to transition metal ions. In especial, 2-(2'-pyridyl)benzimidazole and its
derivatives have aroused considerable interest due to their structural similarity to the widely studied
2,2'-bipyridine, 1,10-phenanthroline, 2,2'-biimidazole, etc., in containing the –N=C–C=N– group.
Thus they have been often used as the powerful bidentate chelating agents to form stable
luminescent complexes with various transition metal ions [29-31]. In previous studies we have
reported the synthesis, crystal structures and luminescent properties of Cu(I) and Ln(III) complexes
3

with 1-substituted pyridyl benzimidazole ligands containing carbazole or oxadiazole unit [32-34].
As part of our continuous research interests in the metal complexes based on the functionalized
benzimidazole ligands, in this paper a phenanthroimidazole derivative was synthesized as the
neutral auxiliary ligand to prepare two new cationic Ir(III) complexes. The photophysical,
electrochemical properties and thermal stabilities of the cationic Ir(III) complexes were reported.
The synthetic routes of the neutral auxiliary ligand L and the cationic Ir(III) complexes were
shown in Scheme 1.
2. Experimental
2.1 Materials and methods
Carbazole and p-Iodotoluene were purchased from Shanghai Zhongqin Chemical Reagent Co.
Ltd (China). 5-Bromopicolinaldehyde, phenanthrene-9,10-dione, 4-(tert-butyl)aniline, tetrabutyl
ammonium bromide (TBAB), bis(pinacolato)diboron, tetrakis(triphenylphosphine)palladium
(Pd(PPh3)4) and [1,1'-bis(diphenylphosphino)ferrocene]dichloropalladium(II) (PdCl2(dppf)) were
obtained from Energy Chemical (China). N-bromobutanimide (NBS), piperidine and
1,10-phenanthroline were bought from Tianjing Chemical Reagent Co. Ltd (China). All the other
chemicals were analytical grade reagent. All of the organic solvents used in this study were dried
over appropriate drying agents and freshly distilled prior in the related reaction.
1H NMR spectra were recorded on Varian Mercury Plus 400 MHz and Agilent Technologies
DDZ 600 MHz. Mass spectra were recorded using a Thermo Scientific Orbitrap Elite mass
spectrometer. Thermogravimetric analysis (TGA) was performed on a PerkinElmer Pyris system.
4

UV-vis absorption and photoluminescent spectra were recorded on a Shimadzu UV-2550
spectrometer and on a Perkin-Elmer LS-55 spectrometer, respectively. Cyclic voltammetry (CHI
Instruments 760 B) was performed with a 0.10 mol/L solution of tetrabutylammonium
hexafluorophosphate (Bu4NPF6) in dichloromethane, with the analyte present in a concentration of
10-3 mol/L and employing a scan rate of 50 mV/s at room temperature. A glassy carbon electrode
was used as the working electrode, while a Pt wire and an Ag/Ag+ electrode were used as the
counter electrode and reference electrode, respectively.
The cyclometalated Ir(III) μ-chlorobridged dimer of (nbt)2Ir(μ-Cl)2Ir(nbt)2 was prepared as
previously described [35]. The cyclometalated Ir(III) μ-chlorobridged dimer of
(CF3-bt)2Ir(μ-Cl)2Ir(CF3-bt)2 was synthesized according to the procedure described in the
literature [36].
2.2 Synthesis and characterization of the ancillary ligand (L) and the cationic Ir(III)
complexes
9-(p-Tolyl)-9H-carbazole: Carbazole (5.0 g, 29.90 mmol), 4-iodotoluene (7.82 g, 35.89 mmol)
and potassium hydroxide (2.52 g, 44.91 mmol) were dissolved in p-xylene (100 mL), stirring for 1 h
under the protection of nitrogen. Then 1,10-phenanthroline (0.27 g, 1.50 mmol) and CuI (0.285 g,
1.495 mmol) were added, after reacting 30 minutes, slowly heated to reflux for 48 h. The reaction
mixture was cooled to room temperature and filtrated. The extracted solution was extracted with
dichloromethane (3× 100 mL). The combined organic layer was dried by anhydrous MgSO4 and
concentrated under reduced pressure. The residue was purified by column chromatography on silica
gel using petroleum ether as eluent to give the white product (9-p-tolyl-9H-carbazole) (6.93 g, 90%).
5

1HNMR (400 MHz, CDCl3, δ, ppm): 8.14 (d, J = 8.2 Hz, 2H, Ar-H), 7.43 (d, J = 8.0 Hz, 2H, Ar-H),
7.39 (d, J = 7.8 Hz, 4H, Ar-H), 7.27 (t, J = 7.8 Hz, 4H, Ar-H), 2.48 (s, 3H, -CH3).
3-Bromo-9-(p-tolyl)-9H-carbazole: 3.00 g (11.66 mmol) of 9-p-tolyl-9H-carbazole was
dissolved in absolute dichloromethane (20 mL). Under the ice bath, 2.08 g (11.69 mmol) of NBS
dissolved in 40 mL of absolute dichloromethane was dropwise added in the reaction mixture, then
the reaction mixture was stirred for overnight at room temperature. After the reaction was finished,
the mixture was poured into 100 mL of water and extracted with dichloromethane (3× 100 mL), and
then the combined organic layer was dried by anhydrous MgSO4 and concentrated under reduced
pressure. The residue was purified by column chromatography on silica gel using
dichloromethane/petroleum ether (1:80, v/v) as eluent to obtain oyster white powder (3.72 g,
94.9%). m.p.: 136-138 oC. 1H NMR (400 MHz, CDCl3, δ, ppm): 8.24 (s, 1H, Ar-H), 8.08 (d, J = 8.0
Hz, 1H, Ar-H), 7.48-7.34 (m, 6H, Ar-H), 7.30-7.22 (m, 3H, Ar-H), 2.49 (s, 3H, -CH3).
3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9-(p-tolyl)-9H-carbazole
(I):
3-Bromo-9-(p-tolyl)-9H-carbazole (1.90 g, 5.65 mmol), bis(pinacolato)diboron (2.30 g, 9.06 mmol),
PdCl2(dppf) (0.20 g, 0.27 mmol) and potassium acetate (1.78 g, 18.14 mmol) were mixed in 60 mL
of 1,4-dioxane solution. The mixture was stirred vigorously for 48 h under nitrogen atmosphere at
85 oC. After cooling, the reaction mixture was poured into cold water (300 mL), and extracted with
dichloromethane (100 mL × 3). The combined organic phase was dried over anhydrous Na2SO4.
After filtering, the filtrate was evaporated to dryness under reduced pressure. The crude was
purified by chromatography on silica gel using petroleum ether/dichloromethane (3:1, v/v) as the
1
eluent to give yellow powder (1.09 g, 50.3 %). H NMR (400 MHz, CDCl3, δ, ppm): 8.64 (s, 1H,
6

Ar-H), 8.17 (d, J = 8 Hz, 1H, Ar-H), 7.85 (d, J = 9.6 Hz, 1H, Ar-H), 7.43-7.24 (m, 8H, Ar-H), 2.48
(s, 3H, Ar-CH3), 1.40 (s, 12H, -CH3).
5-(9-(p-tolyl)-9H-carbazol-3-yl)picolinaldehyde (II): The compound I (1.00 g, 2.61 mmol),
5-bromopicolinaldehyde (0.53 g, 2.85 mmol ), Pd(pph3)4 (0.15 g, 0.13 mmol), Na2CO3 (0.83 g, 7.83
mmol), TBAB (0.04 g, 0.124 mmol) and distilled water (4 mL) were mixed in 60 mL of toluene,
o
and the mixture was stirred vigorously at 110 C for 24 h. After cooling, a small quantity of water
was added, and the mixture was extracted with dichloromethane (50 mL × 3). The organic phase
was dried over anhydrous Na2SO4. After filtering, the filtrate was evaporated to dryness under
reduced pressure. The crude was purified by chromatography on silica gel using
dichloromethane/ethyl acetate (10:1, v/v) as the eluent to give pale yellow powder (0.76 g, 80.38 %).
1H NMR (400 MHz, CDCl3, δ, ppm): 10.15 (s, 1H, -CHO), 9.14 (s, 1H, Ar-H), 8.43 (s, 1H, Ar-H),
8.19 (dd, J = 7.2 Hz, J = 8.8 Hz, 2H, Ar-H), 8.06 (d, J = 8.0 Hz, 1H, Ar-H), 7.68 (d, J = 8.0 Hz, 1H,
Ar-H), 7.50-7.40 (m, 7H, Ar-H), 7.34 (t, J = 6.8 Hz, 1H, Ar-H), 2.51 (s, 3H, Ar-CH3).
1-(4-(tert-butyl)phenyl)-2-(5-(9-(p-tolyl)-9H-carbazol-3-yl)pyridin-2-yl)-1H-phenanthro[9,
10-d]imidazole (L): The compound II (1.21 g, 3.34 mmol), 9,10-phenanthrenequinone (0.70 g,
3.36 mmol), 4-(tert-butyl)aniline (0.8 mL, 5.02 mmol) and ammonium acetate (3.15 g, 40.87 mmol)
were mixed in 120 mL of acetic acid and stirred vigorously for 12 h at 120 ℃. After cooling, the
reaction mixture was poured into cold water (200 mL), and extracted with dichloromethane (150
mL × 3). The combined organic phase was dried over anhydrous Na2SO4. After filtering, the filtrate
was evaporated to dryness under reduced pressure. The crude was purified by chromatography on
silica gel using petroleum ether/ethyl acetate (2:1, v/v) as the eluent to obtain white solid product
7

o
1
(1.23 g, 54 %). m.p.: 259-262 C. H NMR (400 MHz, CDCl3, δ, ppm): 8.95 (d, J = 8.0 Hz, 1H,
Ar-H), 8.80 (s, 1H, Ar-H), 8.76 (d, J = 8.4 Hz, 1H, Ar-H), 8.70 (d, J = 8.0 Hz, 1H, Ar-H), 8.33 (s,
1H, Ar-H), 8.17 (d, J = 7.6 Hz, 1H, Ar-H), 8.06 (d, J = 8.4 Hz, 1H, Ar-H), 7.99 (d, J = 8.0 Hz, 1H,
Ar-H), 7.75 (t, J = 7.6 Hz, 1H), 7.69-7.59 (m, 4H, Ar-H), 7.53-7.49 (m, 3H, Ar-H), 7.45-7.38 (m,
8H, Ar-H), 7.32-7.24 (m, 3H, Ar-H), 7.19 (d, J = 8.0 Hz, 1H, Ar-H), 2.49 (s, 3H, Ar-CH3), 1.46 (s,
13
9H, -CH3). C NMR (100 MHz, CDCl3, δ, ppm): 152.54, 149.38, 147.64, 147.47, 141.56, 140.98,
137.67, 137.51, 136.77, 136.43, 134.69, 134.25, 130.55, 129.56, 129.11, 128.89, 128.45, 128.39,
127.35, 127.21, 126.89, 126.46, 126.35, 126.23, 125.62, 125.02, 124.31, 124.00, 123.26, 123.08,
122.90, 121.29, 120.37, 120.13, 118.78, 110.41, 110.01, 34.97, 31.42, 21.25. HR EI-MS: Calcd. for
C49H38N4, 682.31; Found: 683.32 [M+1]+.
[(nbt)2Ir(L)](PF6): The chloro-bridged dimer complex (nbt)2Ir(μ-Cl)2Ir(nbt)2 (0.30 g, 0.20
mmol) and L (0.27 g, 0.40 mmol) were refluxed in 60 mL of dichloromethane/methanol (1:1, v/v)
under nitrogen atmosphere at 75 ℃ for 4 h with keeping in dark place. After cooling, an excess of
KPF6 (0.07 g, 0.38 mmol) was added and the mixture was stirred for 2 h at room temperature. After
completion of the reaction, the solids in the solution were removed by filtration. The residue was
purified through chromatography on silica gel using ethyl acetate/petroleum ether (3: 1, v/v) as the
eluent to obtain the target product as a yellow solid (0.25 g, 40.48%). 1H NMR (600 MHz, CDCl3, δ,
ppm): 8.92 (d, J = 8.8 Hz, 1H, Ar-H), 8.60 (d, J = 8.4 Hz, 1H, Ar-H), 8.48 (s, 1H, Ar-H), 8.32 (d, J
= 8.4 Hz, 1H, Ar-H), 8.27 (d, J = 8.8 Hz, 1H, Ar-H), 8.18 (d, J = 8.6 Hz, 1H, Ar-H), 8.07 (d, J = 8.4
Hz, 1H, Ar-H), 7.93-7.89 (m, 3H, Ar-H), 7.85-7.82 (m, 3H, Ar-H), 7.75 (t, J = 7.2 Hz, 2H, Ar-H),
7.68 (s, 1H, Ar-H), 7.64 (d, J = 8.4 Hz, 1H, Ar-H), 7.60-7.55 (m, 2H, Ar-H), 7.52 (t, J = 7.2 Hz, 1H,
8

Ar-H), 7.47 (t, J = 7.8 Hz, 1H, Ar-H), 7.43-7.35 (m, 9H, Ar-H), 7.33 (d, J = 7.8 Hz, 1H, Ar-H),
7.31-7.27 (m, 4H, Ar-H), 7.25-7.20 (m, 3H, Ar-H), 7.15 (d, J = 8.8 Hz, 1H, Ar-H), 6.92 (t, J = 7.8
Hz, 1H, Ar-H), 6.83 (dd, J = 7.8, 8.4 Hz, 2H, Ar-H), 6.69 (d, J = 8.4 Hz, 1H, Ar-H), 6.47 (d, J = 8.4
Hz, 1H, Ar-H), 6.17 (t, J = 7.8 Hz, 1H, Ar-H), 6.08 (d, J = 8.4 Hz, 1H, Ar-H), 2.47 (s, 3H, Ar-CH3),
13
1.52 (s, 9H, -CH3). C NMR (150 MHz, CDCl3, δ, ppm): 178.76, 177.26, 160.52, 156.31, 152.06,
148.51, 147.32, 147.07, 145.23, 141.69, 141.54, 139.68, 137.90, 136.34, 135.44, 135.04, 134.11,
133.59, 132.85, 132.07, 131.93, 131.69, 131.36, 131.28, 131.15, 130.98, 130.55, 130.04, 129.77,
128.97, 128.91, 128.78, 128.72, 128.49, 127.90, 127.73, 127.08, 126.65, 126.60, 126.49, 126.37,
125.95, 125.53, 125.05, 124.98, 124.69, 124.48, 123.97, 123.36, 122.87, 122.62, 122.45, 122.42,
121.65, 121.26, 120.94, 120.41, 120.20, 119.51, 117.75, 117.29, 110.93, 110.03, 35.43, 31.23,
21.26. HR EI-MS: Calcd. for [C83H58IrN6S2]+, 1395.38; Found, 1395.3788.
[(CF3-bt)2Ir(L)](PF6): The preparation of the complex [(CF3-bt)2Ir(L)](PF6) was similar to
that described for the complex [(nbt)2Ir(L)](PF6), which was obtained from the reaction between
the chloro-bridged dimer (CF3-bt)2Ir(μ-Cl)2Ir(CF3-bt)2 (0.24 g, 0.15 mmol) and L (0.20 g, 0.30
mmol). The crude was purified by chromatography on silica gel using ethyl acetate/petroleum ether
(1:1, v/v) as the eluent to give pale yellow powdery [(CF3-bt)2Ir(L)](PF6) in 39.4 % yield (0.19 g).
1H NMR (600 MHz, CDCl3, δ, ppm): 8.66 (d, J = 8.4 Hz, 1H, Ar-H), 8.56 (d, J = 8.4 Hz, 1H, Ar-H),
8.31 (dd, J = 8.4, 7.8 Hz, 3H, Ar-H), 8.18 (s, 1H, Ar-H), 8.09 (d, J = 7.2 Hz, 1H, Ar-H), 8.06-7.95
(m, 5H, Ar-H), 7.87 (d, J = 8.4 Hz, 1H, Ar-H), 7.8 (dd, J = 7.8, 7.8 Hz, 3H, Ar-H), 7.44-7.39 (m,
6H, Ar-H), 7.35-7.29 (m, 7H, Ar-H), 7.25-7.22 (m, 4H, Ar-H), 7.09 (t, J = 8.8 Hz, 1H, Ar-H),
7.03-6.99 (m, 2H, Ar-H), 6.90 (d, J = 8.4 Hz, 1H, Ar-H), 6.74 (t, J = 7.8 Hz, 1H, Ar-H), 6.62 (d, J =
9

7.8 Hz, 1H, Ar-H), 6.46 (dd, J = 7.8, 8.4 Hz, 2H, Ar-H), 6.15 (d, J = 7.8 Hz, 1H, Ar-H), 2.48 (s, 3H,
Ar-CH3), 1.51 (s, 9H, -CH3). 13C NMR (150 MHz, CDCl3, δ, ppm): 184.64, 182.65, 156.39, 152.14,
151.79, 151.56, 150.62, 148.17, 146.60, 145.29, 141.78, 140.36, 138.00, 135.11, 133.79, 133.31,
133.17, 132.91, 132.68, 131.49, 131.34, 130.61, 130.42, 130.16, 129.32, 129.08, 128.45, 128.12,
127.35, 126.75, 126.10, 125.48, 125.07, 124.74, 124.44, 124.16, 124.06, 123.86, 123.41, 123.09,
122.83, 122.69, 122.38, 122.03, 121.34, 120.63, 120.41, 120.09, 118.62, 117.94, 110.95, 110.21,
35.23, 31.21, 21.02. HR EI-MS: Calcd. for [C77H52F6IrN6S2]+, 1431.32; Found, 1431.3251.
2.3 Crystallography
Suitable single crystal of the ancillary ligand L was obtained from its methanol solution,
whereas the single crystal of the cationic Ir(III) complex [(nbt)2Ir(L)](PF6) was obtained by the
vapor diffusion of diethyl ether into its dichloromethane solution. The diffraction data were
collected with a Bruker Smart Apex CCD diffractometer equipped with graphite-monochromatic
Mo Kα radiation (λ = 1.54178 Å or λ = 0.71073 Å) at 293(2) or 298(2) K. The structures were
solved by direct methods and refined by full-matrix least-squares method on F2 using SHELXL. All
hydrogen atoms were added theoretically.
3. Results and discussion
3.1 Synthesis and characterization
The synthesis of the cationic Ir(III) complexes from Scheme 1 began first with preparing the
ancillary ligand L. The ligand L was successfully obtained through five step reactions. First, the
N-arylation of carbazole precursor was synthesized by the reaction of carbazole with 4-iodotoluene
in high yield, and then further processed by using 1.10 equivalent of NBS to produce the
10

brominated intermediate. The brominated intermediate was converted into its arylboronic ester I.
The intermediate II was Suzuki coupling reaction of the arylboronic ester I with
5-bromopicolinaldehyde. Finally, in the presence of ammonium acetate, the target L was prepared
via reacting the intermediate II with phenanthrene-9,10-dione and 4-(tert-butyl)aniline in refluxing
acetic acid. The ligand L was purified by column chromatography to give the pure product, and
characterized using 1HNMR, MS and X-ray crystallography.
The cationic Ir(III) complexes [(nbt)2Ir(L)](PF6) and [(CF3-bt)2Ir(L)](PF6) were synthesized
by reacting the corresponding Ir(III) chloro-bridged dimers with the ancillary ligand L, and
–
followed by a counter ion-exchange reaction from Cl– to PF6 . The two Ir(III) complexes were
structurally characterized by 1H NMR and the high resolution electro spray ionization mass spectra
(HR ESI-MS), and the structure of [(nbt)2Ir(L)](PF6) was confirmed by X-ray crystallography.
The crystal structure and packing diagram of L is given in Fig. 1. The corresponding
crystallographic data are summarized in Table 1. The crystal of L belongs to the triclinic space
group P-1, a = 9.0069(5) Å, b = 11.0409(6) Å, c = 19.3194(12) Å, α = 74.556(2)°, β = 87.910(3)°, γ
= 82.95(2)°, U = 1837.81(18) Å3, Z = 2, Dc = 1.234 g/cm3, μ = 0.557 mm-1. As shown in Fig. 1(a),
the pyridine ring is not coplanar with the adjacent carbazole ring, and the dihedral angle is 29.48o.
While the pyridine ring is nearly coplanar with the adjacent phenanthroimidazole skeleton, the
dihedral angle is 3.49o. The dihedral angle between carbazole ring and phenanthroimidazole
skeleton is 30.0o. In N-arylation of carbazole moiety, the dihedral angle between the benzene ring
and the carbazole ring is 50.77o. In N-arylation of phenanthroimidazole moiety, the benzene ring is
nearly perpendicular to the phenanthroimidazole skeleton, and the dihedral angle is 89.18o. From its
11

packing diagram (Fig. 1(b)), it is found that there is strong intermolecular π–π interaction between
two anti-paralleled molecules in a unit crystal cell, the interplanar distance is about 3.637 Å.
However the intermolecular interactions are disconnected between different unit cells in the crystal
lattices and thus no intermolecular π–π stacking exist in L crystal.
The crystal structure of [(nbt)2Ir(L)](PF6) is shown in Fig. 2. Its corresponding
crystallographic data is given in Table 1, and the selected bond lengths and angles are listed in
Table 2. The crystal of [(nbt)2Ir(L)](PF6) also belongs to the triclinic space group P-1, a =
15.6730(13) Å, b = 16.5860(14) Å, c = 16.9220(14) Å, α = 94.2760(10)°, β = 111.502(3)°, γ =
103.676(2)°, U = 3913.0(6) Å3, Z = 2, Dc = 1.308 g/cm3, μ = 1.840 mm-1. From Fig. 2, it was found
that the coordination center of iridium(III) is surrounded by two N atoms and two C atoms from the
cyclometalated ligands (nbt, i.e. C^N ligand) and two N atoms from the ancillary ligand L (N^N
ligand), and the iridium(III) center adopts a distorted octahedral coordination geometry. Two C
atoms and two N atoms from the cyclometalated ligands exhibit cis-C,C and trans-N,N chelate
mode, and two N atoms from the ligand L locate cis-N,N chelate disposition. Moreover, it can be
seen from Table 2 that the distances of Ir-C (C^N ligand) bonds (2.021(7) and 1.968(8) Å) are
slightly shorter than the Ir-N (C^N ligand) bonds (2.066(5) and 2.068(5) Å). The Ir-N (N^N ligand)
distances (2.239(6) and 2.175(5) Å) are longer than the distances of Ir-N (C^N ligand).
3.2 Photophysical, electrochemical and thermal properties of the cationic Ir(III) complexes
The UV-vis absorption spectra of the free ligands and the complexes, and the
photoluminescence spectra of the ligand L and the complexes [(nbt)2Ir(L)](PF6) and
[(CF3-bt)2Ir(L)](PF6) in degassed dichloromethane solution are shown in Fig. 3. As shown in Fig.
12

3, the absorption bands of the free ligands mainly located below 400 nm. The ligand L exhibits four
intense absorption bands at 260, 295, 348 and 368 nm, while the ligands nbt and CF3-bt only show
one intense absorption band at 320 and 290 nm, respectively. These absorption bands of the free
ligands could be attributed to the π–π* transitions. In addition, with increasing the π-conjugation of
the ligand (L>nbt>CF3-bt), the longwave absorption bands of nbt and L were red-shifted by about
30 and 78 nm, respectively, compared to the ligand CF3-bt. For the two cationic Ir(III) complexes,
the intense absorption bands before 300 nm in their absorption spectra are assigned to the
1
spin-allowed (π–π*) ligand-centered transition of the cyclometalated primary ligand and the
ancillary ligand in the complexes. The following relatively weak absorption band at 352 nm for
[(nbt)2Ir(L)](PF6) could be attributed to both spin-allowed metal-to-ligand charge-transfer (1MLCT)
and ligand-to-ligand charge-transfer (1LLCT) transitions, while that for [(CF3-bt)2Ir(L)](PF6) is
blue-shifted to 327 nm due to an electron withdrawing group (-CF3) attached in its primary ligand.
Moreover, the weaker absorption bands that found at lower energy region ranging from 400 to 500
3
nm in the absorption spectra of the Ir(III) complexes are attributed to the spin-forbidden MLCT
3
and LLCT [37-39]. Table 3 summarized the photophysical data of the two cationic Ir(III)
complexes.
From Fig. 3, under ultraviolet light excitation, the ligand L displays bright blue emission with
peak at 430 nm in dilute solution. Under the excitation of 427 nm, the complex [(nbt)2Ir(L)](PF6)
exhibits red emission with a maximum peak at 601 nm and a shoulder peak at 651 nm in degassed
dichloromethane solution at room temperature, while the complex [(CF3-bt)2Ir(L)](PF6) shows
orange emission with a maximum peak at 579 nm. The CIE coordinates of [(nbt)2Ir(L)](PF6) and
13

[(CF3-bt)2Ir(L)](PF6) are (0.627, 0.370) and (0.495, 0.494), respectively. Compared with the
similar Ir(III) complex (CF3-bt)2Ir(acac) (λem = 554 nm) [36], the maximum emission peak of
[(CF3-bt)2Ir(L)](PF6) was red-shifted by about 25 nm. It could be that varying the ancillary ligand
3
from acac to pyridylbenzimidazole in the octahedral Ir(III) system led to a lowering of the MLCT
energy gap, giving a change from yellow to orange emission. In addition, the photoluminescence
quantum yields of [(nbt)2Ir(L)](PF6) and [(CF3-bt)2Ir(L)](PF6) at room temperature were
measured to be 5.51% and 1.72% in dichloromethane solution (ca. 10-6 mol/L) through an absolute
method using the Edinburgh Instruments spectrometer (FLS920) integrating sphere excited at 427
nm with a Xe lamp.
The phosphorescence lifetime (τ) is the crucial factor that determines the rate of triplet-triplet
annihilation in OLEDs, and the longer τ values usually cause greater triplet-triplet annihilation [40].
The photoluminescence decay lifetimes of [(nbt)2Ir(L)](PF6) and [(CF3-bt)2Ir(L)](PF6) in
dichloromethane solution were measured to be 1.10 μs and 1.40 μs with a time-correlated single
photon counting spectrometer using an Edinburgh Instruments spectrometer (FLS920) with a
microsecond flash lamp as the excitation source (repetition rate 90 Hz) at room temperature.
In order to investigate the frontier orbitals of the cationic Ir(III) complexes, cyclic voltammetry
(CV) was carried out to investigate their electrochemical behavior. Their HOMO (highest occupied
molecular orbital) energy levels were calculated from the onset of the oxidation potentials of their
cyclic voltammetry measurements, while their LUMO (lowest unoccupied molecular orbital)
energy levels were deduced from their HOMO energy levels and optical band gaps determined from
their extrapolated UV-Vis absorption edges [41]. The cyclic voltammograms of [(nbt)2Ir(L)](PF6)
14

and [(CF3-bt)2Ir(L)](PF6) were measured in degassed dichloromethane solution with Bu4NPF6 (0.1
mol/L) as the electrolyte (Fig. 4). A platinum working electrode and a saturated Ag/AgCl reference
electrode were used. Ferrocene was used for potential calibration. As shown in Fig. 4, the first
oxidation potentials of [(nbt)2Ir(L)](PF6) and [(CF3-bt)2Ir(L)](PF6) were at ca.1.43 and 1.45 V,
respectively. Under the same conditions, the oxidation and reduction potentials of ferrocene were
observed at 0.43 and 0.31 V, respectively, then the E1/2 (Fc/Fc+) is 0.37 V. Thus, the HOMO energy
levels of [(nbt)2Ir(L)](PF6) and [(CF3-bt)2Ir(L)](PF6) were calculated to be -5.86 and -5.88 eV,
regarding the energy level of ferrocene/ferrocenium as -4.80 eV. The optical band edges of
[(nbt)2Ir(L)](PF6) and [(CF3-bt)2Ir(L)](PF6) were estimated to be at ca. 502 and 482 nm, which
corresponds to 2.47 and 2.57 eV. Therefore, the LUMO energy levels of [(nbt)2Ir(L)](PF6) and
[(CF3-bt)2Ir(L)](PF6) were calculated to be -3.39 and -3.31 eV, respectively.
The thermal properties of the cationic Ir(III) complexes were investigated using
thermogravimetric analysis (TGA) under a nitrogen atmosphere. The results of their TGA
measurements are shown in Fig. 5. From their TGA curves in Fig. 5, [(nbt)2Ir(L)](PF6) and
o
[(CF3-bt)2Ir(L)](PF6) exhibited good thermal stability up to 320 C (˂ 3% weight loss), with
increasing temperature, [(nbt)2Ir(L)](PF6) and [(CF3-bt)2Ir(L)](PF6) began to decompose at 360
oC accompanied by a sharp weight loss in their TGA curves. The result indicates that the Ir(III)
complexes are potential emitting material for the fabrication of stable OLEDs.
4. Conclusion
In summary, two novel cationic iridium(III) complexes with phenanthroimidazole derivative as
15

the ancillary ligand were synthesized and characterized. The cationic iridium(III) complexes exhibit
high thermal stability, and display strong red and orange emissions with photoluminescence
quantum efficiency of 5.51% and 1.72%, respective, suggesting that they are potential emitting
materials for the fabrication of stable OLEDs.
Appendix A. Supplementary data
CCDC 1556705 and 1563488 contain the supplementary crystallographic data for the ancillary
ligand L and the complex [(nbt)2Ir(L)](PF6) in this paper. These data can be obtained free of
charge via http:www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
Electronic supplementary information (ESI) available: the 1H, 13C NMR and mass spectra for
the synthesized L and Ir(III) complexes.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant
51563014).
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22

Legends
Scheme 1. Synthetic routes of the ligand (L) and the Ir(III) complexes.
Fig. 1. Crystal structure (a) and packing diagram (b) of L.
Fig. 2. Crystal structure of [(nbt)2Ir(L)](PF6).
Fig. 3. Normalized UV-vis absorption spectra of the free ligands and the complexes, and
photoluminescence spectra of the ligand L and the complexes [(nbt)2Ir(L)](PF6) and
[(CF3-bt)2Ir(L)](PF6) in degassed CH2Cl2 solution at room temperature. (C = 1.0 × 10-5 mol/L, λex
= 427 nm for [(nbt)2Ir(L)](PF6) and [(CF3-bt)2Ir(L)](PF6))
Fig. 4. Cyclic voltammograms of ferrocene, [(nbt)2Ir(L)](PF6) and [(CF3-bt)2Ir(L)](PF6) (Scan
rate: 10 mV/s; solvent: dichloromethane).
Fig. 5. Thermogravimetric analyses (TGA) of [(nbt)2Ir(L)](PF6) and [(CF3-bt)2Ir(L)](PF6) in
nitrogen atmosphere (heating rate: 10 oC/min).
Table 1. Crystallographic data for L
Table 2. Selected bond lengths and bond angles for [(nbt)2Ir(L)](PF6)
Table 3. Photophysical, thermal and electrochemical properties of the cationic Ir(III) complexes
23

(nbt)2Ir(μ-Cl)2Ir(nbt)2
(CF3-bt)2Ir(μ-Cl)2Ir(CF3-bt)2
Scheme 1
24

a
b
Fig. 1
25

Fig. 2
26

L (UV-vis)
nbt (UV-vis)
CF3-bt (UV-vis)
L (PL)
[(nbt)2Ir(L)](PF6) (PL)
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
[(CF3-bt)2Ir(L)](PF6) (PL)
[(nbt)2Ir(L)](PF6) (UV-vis)
[(CF3-bt)2Ir(L)](PF6) (UV-vis)
300
400
500
600
700
Wavelength (nm)
Fig. 3
27

Ferrocene
[(nbt) Ir(L)](PF )
2
6
10
5
[(CF -bt) Ir(L)](PF )
3
2
6
0
-5
0.0
0.5
1.0
1.5
2.0
+
Potential (V, vs. Ag/Ag )
Fig. 4
28

100
80
[(nbt) Ir(L)](PF )
2
6
60
[(CF -bt) Ir(L)](PF )
3
2
6
0
200
400
600
o
Temperature ( C)
Fig. 5
29