Metallophosphors of iridium(III) containing borylated oligothiophenes with electroluminescence down to the near-infrared region

Article abstract of DOI:10.1016/j.jorganchem.2013.01.001

The synthesis, structural, electrochemical and photophysical studies of a series of cyclometalated iridium(III) complexes with functionalized borylated oligothiophene ligands are reported. The effect of oligothiophene chain length on the photophysical properties of these metal complexes was examined in detail and their excited state properties can be controlled by increasing the number of thienyl rings in the cyclometalating ligand, thus allowing the emission to be tuned from red to near-infrared. All of the iridium(III) complexes were utilized in the fabrication of phosphorescent organic light-emitting diodes and their device performances were investigated. These electrophosphors emit in the red to near-infrared regions. The present work enables the rational design of new borylated iridium phosphors by synthetically tailoring the structure of oligothiophene-based pyridine ring that can permit color tuning toward near-infrared phosphorescence.

Full text of DOI:10.1016/j.jorganchem.2013.01.001

Accepted Manuscript
Metallophosphors of iridium(III) containing borylated oligothiophenes with
electroluminescence down to the near-infrared region
Cheuk-Lam Ho, Bing Yao, Baohua Zhang, Ka-Leung Wong, Wai-Yeung Wong,
Zhiyuen Xie, Lixiang Wang, Zhenyang Lin
PII:
S0022-328X(13)00012-0
10.1016/j.jorganchem.2013.01.001
JOM 17846
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 25 November 2012
Revised Date: 2 January 2013
Accepted Date: 4 January 2013
Please cite this article as: C.-L. Ho, B. Yao, B. Zhang, K.-L. Wong, W.-Y. Wong, Z. Xie, L. Wang, Z.
Lin, Metallophosphors of iridium(III) containing borylated oligothiophenes with electroluminescence
down to the near-infrared region, Journal of Organometallic Chemistry (2013), doi: 10.1016/
j.jorganchem.2013.01.001.
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Table of Content Page
Metallophosphors of iridium(III) containing borylated oligothiophenes with
electroluminescence down to the near-infrared region
Cheuk-Lam Ho*, Bing Yao, Baohua Zhang, Ka-Leung Wong, Wai-Yeung Wong*,
Zhiyuen Xie**, Lixiang Wang, Zhenyang Lin
New borylated iridium(III) complexes containing oligothiophene-based cyclometalating
ligand were synthesized and characterized. The electron-withdrawing boryl moiety is
effective to cause a red-shift of the phosphorescence to the red or even near-infrared.
Red to near-infrared emitting organic light-emitting devices were fabricated.

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Highlights
> Metallophosphors of iridium with borylated oligothiophene ligands > Red to
near-infrared OLEDs were fabricated > Effect of thiophene ring on the emission.

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J. Organomet. Chem. (Special issue for ICOMC)
Metallophosphors
of
iridium(III)
containing
borylated
oligothiophenes with electroluminescence down to the near-infrared
region
b
b
a
a,
Cheuk-Lam Ho a,*, Bing Yao , Baohua Zhang , Ka-Leung Wong , Wai-Yeung Wong *,
Zhiyuen Xie b,**, Lixiang Wang b, Zhenyang Lin c
a
Institute of Molecular Functional Materials, Department of Chemistry and Institute of Advanced
Materials, Hong Kong Baptist University, Waterloo Road. Kowloon Tong, Hong Kong, P. R. China
b
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, P. R. China.
c
Department of Chemistry, The Hong Kong University of Science & Technology, Clear Water Bay,
Kowloon, Hong Kong, P. R. China
______________________________
*
Corresponding authors. Tel.: +852 3411 7074; fax: +852 3411 7348.
** Corresponding author. Tel.: 86-431-85262819
E-mail addresses: clamho@hkbu.edu.hk (C.-L. Ho); rwywong@hkbu.edu.hk (W.-Y. Wong);
xiezy_n@ciac.jl.cn (Z.-Y. Xie)
Dedicated to Professor F. G. A. Stone in the special issue of 25th ICOMC
1

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Abstract
The synthesis, structural, electrochemical and photophysical studies of a series of
cyclometalated iridium(III) complexes with functionalized borylated oligothiophene ligands are
reported. The effect of oligothiophene chain length on the photophysical properties of these
metal complexes was examined in detail and their excited state properties can be controlled by
increasing the number of thienyl rings in the cyclometalating ligand, thus allowing the emission
to be tuned from red to near-infrared. All of the iridium(III) complexes were utilized in the
fabrication of phosphorescent organic light-emitting diodes and their device performances were
investigated. These electrophosphors emit in the red to near-infrared regions. The present work
enables the rational design of new borylated iridium phosphors by synthetically tailoring the
structure of oligothiophene-based pyridine ring that can permit color tuning towards
near-infrared phosphorescence.
Keywords: Boron · Organic light-emitting diodes · Iridium · Phosphorescence · Near-Infrared
emission
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1.
Introduction
There is a growing interest in the near-infrared (NIR) emission from metal complexes
through photoluminescence [1-5] or electroluminescence [6-10]. Sources of near-infrared
emission are of interest for a variety of biological and biomedical applications, including
glucose monitoring [11] and immunoassay [12,13]. Furthermore, there are also potential uses
for NIR light sources in telecommunications [14] and defense applications [15]. The first class
of complexes showing notable NIR luminescence have to be credited to the lanthanide
complexes [6,16-19] Sharp emission signals for the lanthanide complexes are expected simply
because the 4f orbitals of the lanthanide elements are buried deep within the atom. However,
these complexes would not be seriously coupled with the ligand vibrational motions, so that
their emission properties are not easily tuned by changing the functionality of the attached
ligands. Moreover, due to their low optical cross section arising from the forbidden transition,
direct emissions from these ions are always weak and inefficient. Other classes of metal
complexes exhibiting NIR phosphorescence are Pd porphyrinate or polypyridyl complexes
involving the Re(I), Ru(II) and Os(II) metal ions with the d6 electronic configuration [20-23].
These complexes absorb appreciably in the low-energy region of the spectra but show very
weak luminescence [20-23]. Other attempts to achieve NIR electroluminescence (EL) (such as
low band-gap organic chromophores [24-28] and polymers [29,30]) also suffer from the same
drawbacks.
Herein, we have designed and synthesized new neutral Ir(III) complexes with very robust
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and stable dimesityboryl-oligothiophene groups on the cyclometalated ligands which can extend
the emission wavelength to the NIR region. As compared to the structurally analogous Re(I),
Ru(II) and Os(II) complexes, Ir(III) complexes usually display longer excited state lifetimes,
higher luminescence efficiencies and the strengthened metal−ligand interaction can increase the
stability and luminous efficiency of the corresponding Ir(III) complexes [31,32]. Therefore,
Ir(III) tris-chelate complexes with extended π-systems have generated much interest recently
because of their potential use in a variety of photonic applications, including photocatalysis [33],
photoelectrochemistry [34] and electroluminescence [35]. The nature of the cyclometalating
ligand was found to affect strongly the energy of the emissive state so that the emission color
could be tuned [35]. One of the advantages of using boron center in the cyclometalating ligand
is the availability of its empty pπ orbital which can act as excellent electron acceptors. When we
combine the boryl group with thiophene donor, a donor-acceptor (D-A) approach not only
enhances the charge transfer properties of the complexes, but also extends the emission of the
resulting Ir(III) complexes to the NIR region. Also, the bulky mesityl groups also promote the
formation of amorphous films with high glass transition temperatures, which can greatly
improve the performance and the long term stability of optoelectronic devices such as OLEDs
by removing grain boundaries [36-38]. Four Ir(III) borylated complexes were synthesized here
and their application in organic light emitting diodes (OLEDs) will also be investigated and
discussed.
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2.
Experimental
2.1. General information
All reactions were performed under nitrogen. Solvents were carefully dried and distilled
from appropriate drying agents prior to use. Commercially available reagents were used without
further purification unless otherwise stated. All reagents for the chemical syntheses were
purchased from Aldrich or Acros Organics. All reactions were monitored by thin-layer
chromatography (TLC) with Merck pre-coated glass plates. Flash column chromatography and
preparative TLC were carried out using silica gel from Merck (230−400 mesh). Fast atom
bombardment (FAB) mass spectra were recorded on a Finnigan MAT SSQ710 system. NMR
spectra were measured in CDCl3 on a Varian Inova 400 MHz FT-NMR spectrometer and
chemical shifts are quoted relative to tetramethylsilane for 1H and 13C nuclei.
2.2. Physical measurements
UV-vis spectra were obtained on a HP-8453 spectrophotometer. The photoluminescent
properties and lifetimes of the compounds were probed on the Photon Technology International
(PTI) Fluorescence Master Series QM1 system. The quantum yields were determined in CH2Cl2
solutions at 293 K against Rhodamine 6G (Φ = 0.95) as a reference [39]. A conventional
three-electrode configuration consisting of a glassy carbon working electrode and Pt-wires as
the counter and reference electrodes, was used. The solvent in all measurements was THF, and
the supporting electrolyte was 0.1 M [Bu4N]PF6. Ferrocene was added as a calibrant after each
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set of measurements, and all potentials reported were quoted with reference to the
ferrocene-ferrocenium (Fc/Fc+) couple at a scan rate of 100 mV/s. Thermal analyses were
performed with Perkin-Elmer TGA6 thermal analyzers.
2.3. Preparation of compounds
2.3.1. Synthesis of L1
A flask containing a mixture of L1-I (500 mg, 1.09 mmol) and 2-(tributylstannyl)pyridine
(422 mg, 1.31 mmol) and 50 mL toluene was charged with nitrogen, after which a catalytic
o
amount of Pd(PPh3)4 (30 mg) was added. The mixture was stirred for two days at 110 C. The
mixture was then allowed to cool to room temperature and water was added. The solution was
extracted with ethyl acetate and the organic layer was separated, dried by MgSO4, filtered and
concentrated. After purification by preparative TLC plate (hexane:CH2Cl2, 1:1 v/v), the title
compound was obtained as a pale yellow oil in 48% yield (214 mg).
L1: Spectral Data: 1H NMR (CDCl3): δ = 8.56–8.55 (d, J = 4.8 Hz, 2H, Ar), 7.71–7.66 (m,
2H, Ar), 7.44–7.43 (m, 1H, Ar), 7.17–7.14 (m, 1H, Ar), 6.81 (s, 4H, Ar), 2.25 (s, 6H, mesityl),
2.13 (m, 12H, mesityl). 13C NMR (CDCl3): δ = 156.11, 152.26, 149.73, 140.99, 140.78, 138.54,
136.70, 128.13, 126.64, 122.64, 119.67 (Ar), 23.10, 21.18 (mesityl). FAB-MS: m/z = 409 (M+).
2.3.2. Synthesis of L2-Br
To a solution of 2,2’-dibromobithiophene (1.0 g, 3.10 mmol) in ether (30 mL) was added
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a hexane solution of nBuLi (1.6 M, 2.13 mL, 3.41 mmol) dropwisely at –78 ºC. The mixture was
stirred at the same temperature for 3 h. A solution of dimesitylboron fluoride (1.11 g, 3.72 mmol)
in ether (20 mL) was added to the reaction mixture via syringe. The reaction mixture was
gradually warmed to room temperature and stirred overnight. After addition of water (20 mL),
the organic layer was separated and the aqueous layer was extracted with diethyl ether for three
times. The combined organic layer was dried over MgSO4, filtered, and evaporated under
reduced pressure. The mixture was purified by a silica gel column chromatography with hexane
to afford L2-Br in 91% yield (1.40 g) as a bright yellow solid.
L2-Br: Spectral Data: 1H NMR (CDCl3): δ = 7.34–7.33 (d, J = 4.0 Hz, 1H, Ar), 7.22–7.21
(m, 1H, Ar), 7.00–6.95 (m, 2H, Ar), 6.84 (s, 4H, Ar), 2.31 (s, 6H, mesityl), 2.14 (s, 12H,
mesityl). 13C NMR (CDCl3): δ = 148.57, 141.38, 140.79, 138.78, 138.63, 130.87, 128.18,
125.81, 124.89, 112.51 (Ar) 23.45, 21.21 (mesityl). FAB-MS: m/z = 492 (M+).
2.3.3. Synthesis of L2
A flask containing a mixture of L2-Br (1.36 g, 2.75 mmol) and 2-(tributylstannyl)pyridine
(1.07 g, 3.31 mmol) and 50 mL toluene was charged with nitrogen, after which a catalytic
o
amount of Pd(PPh3)4 (40 mg) was added. The mixture was stirred for two days at 110 C. The
mixture was then allowed to cool to room temperature and water was added. The solution was
extracted with ethyl acetate and the organic layer was separated, dried by MgSO4, filtered and
concentrated. After purification by column chromatography (hexane:CH2Cl2, 1:1 v/v), the title
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compound was obtained as a yellow oil in 59% yield (800 mg).
L2: Spectral Data: 1H NMR (CDCl3): δ = 8.56 (d, J = 4.0 Hz, 1H, Ar), 7.64–7.59 (m, 2H,
Ar), 7.44 (d, J = 4.0 Hz, 1H, Ar), 7.36 (m, 2H, Ar), 7.23 (m, 1H, Ar), 7.13–7.10 (m, 1H, Ar),
6.84 (s, 4H, Ar), 2.34 (s, 6H, mesityl), 2.16 (s, 12H, mesityl). 13C NMR (CDCl3): δ = 151.98,
149.69, 149.57, 144.71, 141.58, 140.76, 138.97, 138.50, 136.55, 128.98, 128.13, 126.06, 125.79,
125.20, 122.04, 118.51 (Ar) 23.45, 21.18 (mesityl). FAB-MS: m/z = 491 (M+).
2.3.4. Synthesis of iridium(III) chloro-bridged dimer
IrCl3.nH2O and 2.5 molar equivalents of cyclometalating ligand (L1 or L2) were added to a
mixture of 2-ethoxyethanol (6 mL) and water (2 mL) and the resulting mixture was heated under
a nitrogen atmosphere for 16 h. The reaction mixture was allowed to cool to room temperature
and water was added (20 mL). The mixture was filtered and the residue was washed with
ethanol, followed by hexane and dried under vacuum completely. The Ir(III) dimers were used
without further purification.
2.3.5. Synthesis of homoleptic iridium(III) complex Ir-1
To a solution of L1 (197 mg, 0.473 mmol) and its corresponding Ir-1 dimer (232 mg, 0.111
mmol) in 2-ethoxyethanol (8 mL) was added silver trifluoromethane sulfonate (60.5 mg, 0.235
mmol). The mixture was stirred at 110 oC for 3 days in the dark. After that, the reaction mixture
was gradually cooled to room temperature and was filtered. After addition of water, the organic
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layer was separated and the aqueous layer was extracted with chloroform for three times. The
combined organic layer was dried over Na2SO4, filtered, and concentrated under reduced
pressure. The mixture was purified by silica gel column chromatography (hexane:CH2Cl2, 1:4,
v/v) of Ir-1 in 41% yield as a red solid.
Ir-1: Spectral Data: 1H NMR (CDCl3): δ = 7.57–7.46 (m, 9H, Ar), 6.76–6.62 (m, 18H, Ar),
2.25 (s, 18H, mesityl), 1.81 (s, 36H, mesityl). 13C NMR (CDCl3): δ = 161.94, 159.52, 153.49,
149.47, 148.46, 148.08, 140.87, 137.75, 136.43, 127.76, 120.55, 118.92 (Ar), 23.07, 21.30
(mesityl). FAB-MS: m/z = 1417 (M+). Anal. Calc. for C81H81N3B3S3Ir: C, 68.64; H, 5.76; N,
2.96. Found: C, 68.72; H, 5.79; N, 3.10 %.
2.3.6. Synthesis of heteroleptic iridium(III) complex Ir-2
A mixture of crude iridium chloro-bridged dimer (170 mg, 0.0813 mmol) derived from
L1, acetylacetone (0.029 mL, 0.244 mmol) and Na2CO3 (86 mg, 0.813 mmol) in
o
2-ethoxyethanol (8 mL) was stirred at 100 C under nitrogen for 16 h. After the removal of all
volatile components, the compound was purified by silica column chromatography using
hexane:CH2Cl2 (1:5, v:v) as the eluent to afford the target compound as a red solid (28 %, 87
mg).
Ir-2: Spectral Data: 1H NMR (CDCl3): δ = 8.32–8.31 (d, J = 5.6 Hz, 2H, Ar), 7.50–7.49 (m,
4H, Ar), 6.88–6.70 (m, 10H, Ar), 6.36 (s, 2H, Ar), 5.28 (s, 1H, acac), 2.24 (s, 12H, Ar), 1.96 (s,
24H, Ar), 1.86 (s, 6H, CH3). 13C NMR (CDCl3): δ = 184.67 (CO), 164.06, 152.43, 150.94,
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148.70, 147.53, 145.49, 141.65, 140.84, 137.95, 137.40, 127.81, 120.41, 118.73 (Ar), 100.73
(acac) 28.66, 23.88 (mesityl), 21.25 (CH3). FAB-MS: m/z = 1108 (M+). Anal. Calc. for
C59H61N2B2O2S2Ir: C, 63.95; H, 5.55; N, 2.53. Found: C, 63.86; H, 5.69; N, 2.77%.
2.3.7. Synthesis of homoleptic iridium(III) complex Ir-3
Similar synthetic procedures as complex Ir-1 was adopted but ligand L2 (35 mg, 0.351
mmol) was used instead. The isolated complex was red solid (62 %, 35 mg).
Ir-3: Spectral Data: 1H NMR (CDCl3): δ = 7.46–7.44 (d, J = 7.6 Hz, 3H, Ar), 7.38–7.37 (m,
6H, Ar), 7.29–7.25 (m, 6H, Ar), 6.79 (s, 12H, Ar), 6.72–6.69 (m, 6H, Ar), 2.28 (s, 18H, mesityl),
13
2.13 (s, 36H, mesityl). C NMR (CDCl3): δ = 161.64, 160.09, 150.90, 148.51, 147.70, 141.57,
141.18, 140.78, 140.13, 138.26, 136.76, 136.62, 133.82, 128.06, 126.14, 119.39, 117.74 (Ar),
23.46, 21.19 (mesityl). FAB-MS: m/z = 1663 (M+). Anal. Calc. for C93H87N3B3S6Ir: C, 67.14; H,
5.27; N, 2.53. Found: C, 67.32; H, 5.32; N, 2.77%.
2.3.8. Synthesis of heteroleptic iridium(III) complex Ir-4
Similar synthetic procedures as complex Ir-2 was employed and the title complex was
isolated as a red solid (70%, 30 mg).
Ir-4: Spectral Data: 1H NMR (CDCl3): δ = 8.26–8.24 (d, J = 5.5 Hz, 2H, Ar), 7.63–7.59
(m, 2H, Ar), 7.44–7.43 (m, 2H, Ar), 7.23–7.15 (m, 4H, Ar), 6.94–6.91 (m, 2H, Ar), 6.79–6.77 (s,
8H, Ar), 6.33 (s, 2H, Ar), 5.21 (s, 1H, acac), 2.26 (s, 12H, mesity), 2.05 (s, 24H, mesity), 1.79 (s,
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6H, CH3). 13C NMR (CDCl3): δ = 184.56 (CO), 164.13, 150.68, 149.12, 148.59, 148.36, 141.34,
140.86, 139.94, 138.35, 137.73, 137.60, 130.51, 128.05, 125.82, 118.89, 117.61 (Ar), 100.64
(acac), 28.55, 23.36 (mesity), 21.20 (CH3). FAB-MS: m/z = 1272 (M+). Anal. Calc. for
C67H65N2B2O2S4Ir: C, 63.25; H, 5.15; N, 2.20. Found: C, 63.40; H, 5.34; N, 2.28%.
2.4. X-ray crystallography
X-Ray diffraction data for Ir-2 were collected at 173 K using graphite-monochromated
Mo−Kα radiation (λ = 0.71073 Å) on a Bruker Axs SMART 1000 CCD diffractometer. The
collected frames were processed with the software SAINT+ [40] and an absorption correction
(SADABS) [41] was applied to the collected reflections. The structure was solved by the Direct
or Patterson methods (SHELXTL) [42] in conjunction with standard difference Fourier
techniques and subsequently refined by full-matrix least-squares analyses on F2. Hydrogen
atoms were generated in their idealized positions and all non-hydrogen atoms were refined
anisotropically. Crystal data for Ir-2 2CH2Cl2: C61H65N2B2Cl4O2S2Ir Mw = 1277.89, monoclinic,
space group P21/n, a = 14.554(1), b = 24.898(2), c = 16.445(1) Å,
Å3, Z = 4, ρcalcd = 1.424 mg m−3, (MoKα) = 2.533 mm−1, F(000) = 2592. 28707 reflections
measured, of which 10383 were unique (Rint = 0.0434). Final R1 = 0.0416 and wR2 = 0.1022 for
7905 observed reflections with I > 2 (I).
β = 90.258(1)°, V = 5959.0(8)
µ
σ
2.5. OLED fabrication and measurements
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Commercial indium tin oxide (ITO) coated glass with sheet resistance of 20−30 Ω/ was
used as the starting substrates. Before device fabrication, the ITO glass substrates were cleaned
by ultrasonic baths in organic solvents followed by ozone treatment for 10 min. Each device
was assembled in the following sequence: ITO on glass substrate (anode), 50 nm of PEDOT, 80
nm of the emitting layer made of CBP host and phosphorescent dopant (x%), 15 nm of BCP, 30
nm Alq3, 1 nm of LiF and 100 nm of Al (cathode). The organic layers were evaporated and
laminated in the above sequence less than 10–6 Torr without breaking vacuum between each
vacuum deposition process. The layer thickness was monitored in situ using a quartz crystal
oscillator. The EL spectra and CIE coordinates were measured with a PR650 spectra colorimeter.
The L–V–J curves of the devices were recorded by a Keithley 2400/2000 source meter and the
luminance was measured using a PR650 SpectraScan spectrometer. All the experiments and
measurements were carried out at rt under ambient conditions.
2.6. Molecular orbital calculations
Density functional calculations at the B3LYP level were performed based on their
experimental geometry obtained from the X-ray data. The basis set used for C, N, B, O, and H
atoms was 6-31G while effective core potentials with a LanL2DZ basis set were employed for
the Ir atom [43,44]. All the calculations were carried out using the Gaussian 03 program [45].
All the MO plots were made with the use of Molden 3.5 [46].
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3. Results and discussion
3.1. Synthetic methodologies and chemical characterization
The chemical structures and synthetic procedures of new dimesityboryl-oligothiophene
cyclometalating ligands L1−L2 and their Ir(III) complexes Ir-1−Ir-4 are shown in Schemes 1
and 2. For use as optoelectronic materials and fabrication of their devices, the boron center must
be rendered stable to nucleophilic attack and hydrolysis. This can be readily achieved by
functionalization of the boron center with bulky aryl substituents, such as mesityl
(2,4,6-trimethylphenyl group). The bromo precursor L2-Br can be obtained from selective
monolithiation of 2,2’-dibromobithiophene followed by reaction with dimesityboron fluoride.
Synthesis of ligands L1−L2 was readily achieved by successive coupling of L1-I or L2-Br with
2-tributylstannylpyridine via Stille-type reaction conditions [47]. L1−L2 were easily converted
to the iridium-µ-chloro-bridged dimers by the reaction of iridium trichloride hydrate with
ligands [36]. It is worth noting that the homoleptic Ir(III) complexes are typically synthesized by
reacting Ir(acac)3 with cyclometalating ligand at 200oC in glycerol under N2 [48]. However, our
boron based ligands tend to decompose under such high temperature and hence result in very
low yields. Prompted by the recent work of Crutchley and coworkers [49] on a lower
temperature process for the preparation of new tris(cyclometalated) Ir(III) complexes, we have
succeeded in preparing Ir-1 and Ir-3 by reacting the Ir(III) dimer with an excess of the
corresponding ligands at 110oC in the dark. Here, the reaction is run with an excess of ligand,
which is easily recovered and reused. On the other hand, the diiridium(III) complexes were then
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converted to mononuclear iridium(III) complexes Ir-2 and Ir-4 by replacing the two bridging
chlorides with bidentate monoanionic acetylacetonate (acac–) ligand. All of the Ir(III) complexes
with boryl-substituted oligothiophene produced are stable in air and can be purified by column
chromatography or preparative TLC plates on silica gel and they are soluble in common
chlorinated hydrocarbons and toluene. The structures were unequivocally characterized by
elemental analyses, mass spectrometry and NMR spectroscopies (see Experimental section).
The facial geometry of homoleptic complexes Ir-1 and Ir-3 is confirmed by the simplicity of 1H
NMR spectral pattern, which indicates that the number of coupled spins is equal to that of the
protons on one ligand because the three ligands are magnetically equivalent due to the 3-fold
symmetry of the molecule [50]. The molecular structure of Ir-2 was further confirmed by X-ray
crystallography (Fig. 1). The molecular structure reveals the central Ir(III) center to be
coordinated by two anionic L2 C^N ligands and one chelating acac anion. The Ir(III) atom
resides in a distorted octahedral environment, with the cis-O,O, cis-C,C, and trans-N,N chelate
disposition. The crystallographic results were tabulated in Table 1. All of the complexes Ir-1 to
Ir-4 exhibit high thermal stability with the decomposition temperatures for a 5% weight loss of
in the range of 301 to 325 ºC.
3.2. Photophysical and electrochemical characterization
The photophysical properties of Ir-1−Ir-4 in CH2Cl2 solution were investigated (Table 2).
The absorption and photoluminescence (PL) spectra of all complexes at 293 K are shown in Fig.
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2. In common with most Ir(III) complexes, the absorption spectra exhibited two major bands.
The intense absorption bands with distinct vibronic features in the UV region are assigned to the
spin-allowed 1π–π* transitions of C^N ligands. The next weaker, low-energy features with peak
wavelengths in the region of 400−600 nm which appear as humps can be ascribed to an
1
3
3
admixture of MLCT, MLCT and π–π* excitations [51]. However, these absorption bands
cannot be distinctly resolved mainly owing to their hidding inside the UV region of the strong
singlet transitions which are of much lower but non-negligible absorptivities. Relatively higher
absorption coefficients for the complexes with L2 than those with L1 in both regions were
observed as the number of thiophene rings increases [52].
Upon photoexcitation, each of Ir-1 and Ir-2 emits an intense red light in CH2Cl2. Both of
them show a strong room-temperature phosphorescence band at 643 and 661 nm, respectively. It
is believed that their emissions arise predominantly from the ligand-centered 3π−π* excited state
since they display a large Stokes shift (> 100 nm) from the lowest energy absorption peak and
vibronic progressions. Also, there is only a small dependence of the PL on solvent polarity and
temperature. The full width at half maximum (FWHM) of the PL is only 52 and 56 nm for Ir-1
and Ir-2, respectively, which precludes the possibility of their emissions to be contributed by the
charge transfer characters since the emissions from CT states are often broad, featureless and
sensitive to solvent polarity and temperature [53]. The vibronic progression of ca. 1267 and
1240 cm−1 (ν0−1) in the emission profile of Ir-1 and Ir-2 corresponds to the aromatic stretching
of the cyclometalated ligands which is diagnostic of the substantial involvement of the
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ligand-centered π−π* transitions in the emission. On the other hand, as compared to the Ir(III)
complexes with B(Mes)2-ppy (Hppy = 2-phenylpyridine) reported previously by us [54], a large
red-shift was observed upon replacement of phenyl ring (λPL = 605 nm) by the electron-donating
thiophene group in Ir-2. This is in agreement with the observable bathochromic shift in the
emission wavelength from [Ir(ppy)3] (λPL = 514 nm) to [Ir(thpy)3] (λPL = 550 nm) [55] due to
the fact that this kind of ligands having intramolecular donor-acceptor (D-A) systems would
exhibit bathochromic shifts in both absorption and emission spectra. Generally,
2-thiophenylpyridine (thpy) itself has a D-A character resulting from the interaction between an
electron-rich group, thiophene, and an electron-deficient group, pyridine. Through the
introduction of B(Mes)2 group, a powerful π-electron acceptor (with vacant pπ orbital on boron)
and an inductive σ-donor (related to the electropositive nature of the boron atom), the D-A
character of the resulting complex is enhanced. Therefore, a further red shift in the emission
wavelength was observed in Ir-1 as compared to [Ir(thpy)3]. Interestingly, the emission profiles
of Ir-3 and Ir-4 are very different from that of Ir-1 and Ir-2, further extending the π-conjugation
to the C^N ligand that results in dual emission. For Ir-3, one emission appears as a band at
around 530 nm and another NIR band at > 750 nm with some evidence of vibrational structure.
The former band corresponds to fluorescence with a small Stokes shift while the latter one to
phosphorescence on the basis of the much larger Stokes shift and long emission lifetime. This
phenomenon was commonly observed in many heavy-metal complexes [56]. Therefore, we
believe that the S1 → T1 intersystem crossing is not fast enough to completely eliminate the
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fluorescence. For Ir-1 and Ir-2, population of the triplet state from the singlet is favored that
fluorescence cannot compete with ISC and no fluorescence can be observed. We anticipate that
there is a weakening of the spin-orbit coupling as the size of the cyclometalating organic group
increases which is consistent with some previously reported data [57,58]. The phosphorescence
of Ir-3 and Ir-4 are red-shifted relative to those of Ir-1 and Ir-2 because of the more extended
π-conjugated structure in the former cases. Their phosphorescence intensities are also weaker
than those of Ir-1 and Ir-2. These results also indicate that the triplet state energy drops more
rapidly than the singlet state upon introduction of a second thiophene ring onto the ligand and
this is well reflected by the larger influence on the phosphorescence energy than on the energy
of the lowest-energy absorption band. On the other hand, the relative ratios of fluorescence to
phosphorescence emission intensities of Ir-3 and Ir-4 are different. Here, the intensity of
fluorescence relative to that of phosphorescence in the homoleptic compound Ir-3 is almost the
same, whereas the emission of the heteroleptic congener Ir-4 is mainly caused by fluorescence
with very weak phosphorescence only. This implies that the nonradiative decay is expected to
increase since the emission energy of the latter is substantially lower than that in the former, in
line with the energy gap law [35]. The lower emission energy also reveals a larger energy gap
between the S1 and T1 states which will also lead to a reduction in the S1 → T1 rate due to the
poorer overlap integral, hence multiplying the above effect. Moreover, the inefficient
intersystem crossing indicates that the rate of the formally forbidden radiative T1 → S0 process
would not be facilitated to the same extent as it was for Ir-1 and Ir-2. The higher ligand-field
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strength of acetylacetonate anion also leads to the bathochromic shifts in λem for Ir-2 and Ir-4
relative to Ir-1 and Ir-3, respectively. The lifetimes of the lower energy peaks of all the Ir(III)
complexes fall in the range of microseconds, suggesting that this band is due to spin-forbidden
triplet excited states. The PL quantum efficiencies of Ir-1−Ir-4 were measured against
Rhodamine 6G standard. Clearly, we can observe that complexes with L1 show much higher
quantum yields than those with L2. This is a manifestation of the fact that decreasing the energy
gap toward NIR may trigger a much faster radiationless decay channel because of the
interaction between ground and emitting states (c.f. energy gap law), giving rise to poorer
quantum efficiencies in Ir-2 and Ir-4.
Cyclic voltammetry was used to investigate the electrochemical behavior of these Ir(III)
complexes. The redox potentials, measured relative to an internal ferrocenium/ferrocene
reference (Fc+/Fc), are listed in Table 3. On the basis of the peak potentials of oxidation and
reduction, the HOMO and LUMO energy levels of these Ir(III) complexes, relative to the
energy levels of ferrocene (4.80 eV under vacuum) were estimated [59]. Upon the anodic sweep
in THF at 298 K, each of these complexes undergoes a reversible one-electron oxidation with
the oxidative potential falling within the range 0.21–0.51 V, mainly involving the Ir-aryl center
according to the previously reports [60]. It is believed that the oxidation potential is strongly
dependent on the electronic environment of the Ir(III) cations, i.e. a better electron donor would
shift the oxidation potential to a less positive value [61]. Herein, with the extension of
π-conjugated aromatic motif by thiophene in the ligand frame, the HOMO levels systematically
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increase from −5.31 to −5.16 V in Ir-1 and Ir-3 and from −5.22 to −5.01 V in Ir-2 and Ir-4 [44].
During the cathodic scan, all complexes undergo reversible reduction processes on the
cyclometalated C^N ligand. This may be attributed to the sequential reduction of thiophene and
boron center, which was commonly observed in other boron-containing compounds [62].
3.3. DFT Calculations
We have examined the electronic structure of Ir-2 by computational methods based on the
experimental geometry obtained from the X-ray crystal data. The time-dependent density
functional theory (TD-DFT) calculations show that the S0 → S1 transitions correspond to the
HOMO → LUMO transitions with non-zero oscillator strengths (Table 4). The results of DFT
calculations show that Ir-2 and the one with B(Mes)2-ppy as previously reported [54] have
similar HOMOs and LUMOs in terms of their orbital character (Fig. 3). The coutour plots show
that the HOMOs mainly consist of the t2g d orbitals from the metal center that mix with the π
orbitals of the thiophene rings that are directly bonded to the metal center. The LUMOs
correspond to the lowest π orbital of the conjugated organic ligands, to which pyridyl ring as
well as electron deficient boron atom contribute more.
3.4. Electroluminescence behavior
To date, there are only limited examples on electrophosphorescent devices based on Ir(III)
complexes functionalized with organoboron-type ligand [54,63,64]. To investigate the
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electroluminescence characteristics of these complexes, multilayer devices with configuration
ITO/PEDOT:PSS (50 nm)/CBP:x% Ir (80 nm)/BCP (15 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100
nm) were fabricated (Fig. 4). The following layers were used:
a
layer of
poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) for hole transport and
smoothing the ITO surface [65], a layer of 4,4’-dicarbazolebiphenyl (CBP) doped with different
concentrations (1 to 20 wt.-%) of Ir(III) complexes as the emitter to optimize the device
efficiency, a thin layer of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) for hole and
exciton blocking to confine the recombination zone inside the doped CBP layer; a layer of
tris(8-hydroxyquinolinato)aluminum (Alq3) for electron transport [66] and LiF as an electron
injection layer. Fig. 5 shows the normalized EL spectra with various concentration of Ir(III)
dopant. The EL color of these complexes varies from single red emission of Ir-1 and Ir-2 to
dual emission bands consisting of both visible and NIR peaks of Ir-3 and Ir-4. The EL emission
peaks are centered at ~650 and 667 nm for Ir-1 and Ir-2-doped devices with low-intensity
shoulders on the red side of the peak near over 715 and 730 nm, respectively. Their EL spectra
are voltage independent and nearly coincident with the PL spectra of the corresponding
complexes in the polymer matrix at room temperature, indicating an effective energy transfer
from the host exciton to the phosphor upon electrical excitation and the effective hole-blocking
effect of BCP. The corresponding Commission Internationale de L’Eclairage (CIE) coordinates
were calculated to be (0.66, 0.31) for Ir-1 and (0.63, 0.32) for Ir-2 at 1 wt.-% doping level.
However, there are some differences between the PL and EL patterns of each of the Ir-3 and
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Ir-4-doped devices, in which two distinct peaks were observed and the relative ratio of the peak
height varied with the doping concentration. The relative intensities for their NIR emissions in
the EL spectra are dramatically higher than those in their PL spectra and increase as the doping
concentration increases. It can be shown that the devices displayed the strong visible and NIR
emissions simultaneously (especially at low dopant concentrations), and the visible green
emission peak can be attributed to either the Alq3 emission or the singlet emission from the
metal complexes. The energy transfer from the host exciton to the phosphor becomes more
significant as the doping concentration increases for Ir-3 and Ir-4, although the degree of such
energy transfer is still much lower than that of Ir-1 and Ir-2.
The performance data for the devices are summarized in Table 5. Fig. 6 shows the current
density and brightness as a function of applied voltage for the devices. The corresponding
current efficiency (ηL), power efficiency (ηP) and external quantum efficiency (ηext) as a
function of current density for the devices are displayed in Fig. 7. As usual, the current density
and brightness increase with the driving potential. The overall efficiencies of the Ir-1-doped
devices (device A1) are the highest among the four metallophosphors. It has a turn-on voltage of
5.9 V, maximum luminance of 451 cd/m2 at 14.6 V, external quantum efficiency (
power efficiency ( P) of 0.82 lm/W and current efficiency ( L) of 1.90 cd/A. On the contrary,
ηext) of 0.58%,
η
η
for complex Ir-2 at the same doping concentration (device B1), it shows a poorer performance
with ηext of 0.23%, ηP of 0.25 lm/W and ηL of 0.67 cd/A. All the devices show a gradual
decrease of the luminance and external quantum efficiency with an increase of doping
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concentration, an effect that is likely due to self molecular quenching at high concentrations.
The devices with Ir-3 and Ir-4 show an overall poorer performance. Their lower PL quantum
efficiencies are probably the reason why the devices have such low efficiencies. We believe
through further material development and device optimization, such as by using different host
materials and assistant dopants, these kind of NIR emissive phosphors would show better
potential in night-vision displays or sensors operating in the NIR spectral regime [67,68].
4.
Conclusions
In conclusion, a new family of borylated Ir(III) cyclometalated complexes with
oligothiophene-based ligands were prepared and characterized. These complexes represent the
first example of this kind of iridium(III)-based NIR-emitting OLEDs using boron directly bound
to the cyclometalating ligand. Ir-1 and Ir-2 display strong red emission at around 650 nm, while
the emission is red-shifted to the NIR region for Ir-3 and Ir-4 after inserting an extra thiophene
unit in the ligand. This approach opens up a new avenue to develop NIR materials by using
simple ligand modification of Ir(III) cyclometalates with electron-withdrawing borylated
substituent.
Acknowledgements
This work was financially supported by Hong Kong Baptist University (FRG2/10-11/101),
Hong Kong Research Grants Council (HKBU202709 and HKUST2/CRF/10) and Areas of
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Excellence Scheme, University Grants Committee of HKSAR, P.R. China (Project No.
[AoE/P-03/08]). We also thank the National Natural Science Foundation of China (NSFC)
(project number: 21029001) for financial support and Prof. Todd B. Marder for helpful
discussion.
Appendix. Supplementary material
CCDC 911166 contains the supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be
found, in the online version, at xxxxx.
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