Synthesis, photophysical and electroluminescent properties of iridium(iii) complexes with 2-aryl-thiazole and oxadiazol-substituted amide derivative ligands

Article abstract of DOI:10.1039/c8nj05370g

By using aryl-thiazoles as cyclometalated ligands and an oxadiazol-substituted amide as the ancillary ligand, three novel iridium(iii) complexes have been synthesized and characterized, and their photophysical and electrochemical properties have been investigated. Complexes 1-3 exhibit efficient phosphorescence emissions at 542, 555 and 585 nm with photoluminescence quantum yields (PLQYs) of 0.04, 0.03 and 0.02 in CH₃CN solutions, respectively. In the 10 wt% doped 2,4-diphenyl-6-bis(12-phenylindolo)[2,3-a]carbazole-11-yl)-1,3,5-triazine (Dictrz) film, the PLQYs (0.88 for complex 1, 0.47 for complex 2 and 0.37 for complex 3) increase significantly owing to the great restriction of rotations and vibrations in the rigid host. The organic light-emitting diodes based on these complexes show good performance with maximum current efficiencies of 20.06, 22.20 and 13.45 cd A^?1 and maximum power efficiencies of 13.54, 17.31 and 9.48 lm W^?1 with CIE coordinates of (0.46, 0.53), (0.49, 0.50) and (0.59, 0.40) for complexes 1-3, respectively.

Full text of DOI:10.1039/c8nj05370g

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X. Dong, B. Yu, M. Zhang, D. Wei, Z. Li, B. Wei, C. Zhang, S. Li, G. Cao and B. Zhai, New J. Chem., 2019,
DOI: 10.1039/C8NJ05370G.
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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
rsc.li/njc

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Synthesis, Photophysical and Electroluminescent Properties of Iridium(III)
Complexes with 2-Aryl-thiazole and Oxadiazol-substituted Amide Derivative
Ligands
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Jiali Yang a, Fuli Zhang b,*, Xiaobin Dong b, Bo Yu b, Mengyang Zhang b, Donghui Wei
c, Zhongyi Li b, Bin Wei a,*, Chi Zhang b, Suzhi Li b, Guangxiu Cao b and Bin Zhai b,*
a Key Laboratory of Advanced Display and System Applications, Ministry of Education,
School of Mechatronic Engineering and Automation, Shanghai University, Shanghai
200072, P. R. China.
b
Engineering Technical Research Centre for Optoelectronic Functional Materials of
Henan Province, Key Laboratory of Biomolecular Recognition and Sensing of Henan
Province, College of Chemistry and Chemical Engineering, Shangqiu Normal
University, Shangqiu 476000, P. R. China.
c
College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou
450001, P. R. China.
*Corresponding author.
E-mail: zzu_zfl@126.com (Fuli Zhang); bwei@shu.edu.cn (Bin Wei);
zhaibin_1978@163.com (Bin Zhai).
Abstract
By using aryl-thiazole as cyclometalated ligand and oxadiazol-substituted amide as
ancillary ligand, three novel iridium(III) complexes have been synthesized and
characterized, and the photophysical and electrochemical properties have been
investigated. Complexes 1–3 exhibit efficient phosphorescence emissions at 542, 555
and 585 nm with photoluminescence quantum yields (PLQYs) of 0.04, 0.03 and 0.02 in
1

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CH3CN solutions, respectively. In the 10 wt% doped 2,4-diphenyl-6-bis(12-
phenylindolo)[2,3-a]carbazole-11-yl)-1,3,5-triazine (Dictrz) film, the PLQYs (0.88 for
complex 1, 0.47 for complex 2 and 0.37 for complex 3) increase significantly owing to
the great restriction of rotations and vibrations in rigid host. The organic light-emitting
diodes based on these complexes show good performance with the maximum current
efficiencies of 20.06, 22.20 and 13.45 cd A-1 and maximum power efficiencies of 13.54,
17.31 and 9.48 lm W−1 with the CIE coordinates of (0.46, 0.53), (0.49, 0.50) and (0.59,
0.40) for complexes 1−3, respectively.
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Keywords: Organic light-emitting diodes; Iridium complex; Aryl-thiazole; Oxadiazol-
substituted amide
1. Introduction
Since the pioneering work done by Forrest and Ma in 1998,1, 2 who used Platinum
and Osmium complexes to fabricate organic light-emitting diodes (OLEDs),
luminescent heavy transition metal complexes have been attracting increasing
attention.3-5 These organo-transition metal compounds exhibit a phosphorescence from
the lowest excited electronic triplet state to the electronic singlet ground state allowed
by spin–orbit coupling (SOC) induced by the central metal. Consequently,
phosphorescent heavy-metal complexes can harvest both singlet and triplet excitons due
to their effective intersystem crossing (ISC) and achieve an maximum internal quantum
efficiency (IQE) of 100% in phosphorescent OLEDs (PHOLEDs), which are four times
than those of the fluorescence-based OLEDs. In comparison with other phosphorescent
emitters, cyclometalated iridium(III) complexes have exhibited enormous potential in
2

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OLEDs owing to their short lifetime of triplet excited states, high photoluminescence
quantum yields (PLQYs), and particularly broad range of emission colors from near
ultraviolet to visible and near infrared regions.6-19
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Generally, the highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) are crucial for the photophysical properties,
some popular strategies are adopted to regulate the light emission color of
cyclometalated iridium(III) complexes. One is to attach electron-donating or electron-
withdrawing moieties of cyclometalated ligands to enlarge or reduce the energy gaps
between HOMOs and LUMOs. For example, the PL spectrum (peak at 475 nm) of
Ir(dfppy)2pic (Firpic, dfppy = 4,6-difluorophenylpyridinato-N,C2ʹ, pic = picolinate) is
blue-shifted relative to that (peak at 506 nm) of the Ir(ppy)2pic (ppy is 2-
phenylpyridine)20 when 4 and 6 positions of cyclometalated ligand ppy are substituted
by two electron-withdrawing fluorine atoms in the phenyl ring.21 Another approach is to
extend or narrow the π-conjunction of cyclometalated ligands by merging or
interrupting aromatic rings. By adopting large conjugated 1-phenylisoquinoline (piq) as
the cyclometalating ligand in place of ppy, the emission peak of Ir(ppy)3 is tuned to 620
nm for Ir(piq)3 in N2-saturated toluene solution.22 As is well known, 2-
phenylbenzothiazole (Hbt) has been widely used as cyclometalated ligand of yellow- or
orange-emitting phosphors due to its convenient modification. Some functional
substituents, such as −OMe, −Me, −CF3 and halogen atoms, have been incorporated to
the frame of (bt)2Ir(acac) (acac is acetylacetone) molecule to develop the new
phosphors.23-25 Interestingly, few reports studied the influence of conjugation degrees of
the 2-aryl-thiazole cyclometalated ligands on the electronic states and optoelectronic
properties of their iridium complexes. In this article, using 2-phenylthiazole (Hpt), Hbt,
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and 2-(2-naphthyl)-1,3-benzothiazole (Hnbt) as cyclometalated ligands and N-(5-
phenyl-1,3,4-oxadiazol-2-yl)-diphenylphosphinic amide (HPOXD) as ancillary ligand,
respectively, three new phosphorescent iridium complexes [Ir(pt)2(POXD)] (1),
[Ir(bt)2(POXD)] (2), and [Ir(nbt)2(POXD)] (3) were designed and synthesized. By
adjusting the conjugation length of the cyclometalated ligands, emissions peaking at
542, 555 and 585 nm, respectively, of complexes 1–3 in CH3CN solution were acquired.
In addition, 1,3,4-oxadiazole derivatives have proved to be good electron injection and
transport materials due to their high electron affinity and good thermal/chemical
stability.26-31 Recently, our group has reported efficient iridium complexes and devices
using HPOXD as ancillary ligands to improve the complexes’ electron transporting
ability and consequently achieve high device performance.17, 32, 33 For example, the
electron mobility of POXD-based complex [Ir(ppy)2(POXD)] is in the range of 7.41–
8.57 × 10–6 cm2 V–1 s–1 under an electric field from 1100 (V cm–1)1/2 to 1300 (V cm–1)1/2,
higher than that (4.74–4.86 × 10–6 cm2 V–1 s–1) of the popular electron transport material
Alq3 under the same electric fields.17 Herein, we present the results of our investigation
on the synthesis, structural characterization, photophysical and electroluminescent
properties of these novel Ir(III) complexes.
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2. Experimental
2.1. General experiments
All solvents and materials were purchased from commercial sources and used as
received unless otherwise stated. The cyclometalated ligands Hpt and Hbt were
purchased from J&K Chemical Ltd. (Shanghai). ESI mass spectrometry was performed
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with a Waters Xevo G2-S QT of mass spectrometer. H NMR spectra were recorded
with a Bruker DPX 400 spectrometer. Elemental analysis was determined with an
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Elementar Vario EL CHN elemental analyzer. The absorption and PL spectra were
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performed with UV–vis spectrophotometer (Agilent Cary 60) and
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fluorospectrophotometer (Hitachi F-7000), respectively. The excited state lifetimes of
the complexes were measured by time-correlated single-photon counting technique at
the peak emitting wavelength upgrade on a transient spectrofluorimeter (Edinburgh
Instruments, FLSP920). PLQYs in CH3CN solutions were measured with quinine
sulfate (φ = 0.545 in 1 M H2SO4)34 as the reference. Cyclic voltammetry was performed
on a CHI 820C electrochemical station (Shanghai Chenhua Co., China) in CH3CN
solutions (1 × 10-3 M) at a scan rate of 100 mV s-1 with a platinum plate as the working
electrode, a Ag wire as the pseudo-reference electrode, and a platinum wire as the
counter electrode. The supporting electrolyte was n-Bu4NPF6 (0.1 M) and ferrocene was
selected as the internal standard. The solutions were bubbled with argon for 10 min
before measurements.
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2.2. Syntheses
Syntheses of ligand Hnbt. A mixture of 2-naphthaldehyde (1.09 g, 7 mmol) and 2-
aminobenzenethiol (0.88 g, 7 mmol) in dimethyl sulfoxide (10 mL) was refluxed at
200°C for 1 h under argon atmosphere. After cooling to room temperature, the mixture
was poured into water, and the formed precipitate was filtered and dried. The crude
compound was purified by column chromatography on silica gel (200–300 mesh) with
petroleum ether/ethyl acetate (15:1) as the eluent, yielding of the product, 2-(2-naphthyl)-
1,3-benzothiazole (Hnbt) as a white solid. Yield: 72%. 1H NMR (400 MHz, CDCl3, δ):
8.57 (s, 1H), 8.22 (d, J = 8.0 Hz, 1H), 8.13 (d, J = 8.0 Hz, 1H), 7.98–7.92 (m, 3H),
7.89–7.86 (m, 1H), 7.56–7.49 (m, 3H), 7.40 (t, J = 8.0 Hz, 1H).
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General syntheses of iridium complexes 1–3. A mixture of IrCl3 (1.00 mmol) and
ligands Hpt, Hbt and Hnbt (2.05 mmol) in 2-ethoxyethanol (20 mL) and water (100 mL)
was refluxed for 24 h. After cooling to room temperature, the mixture was filtered, and
the precipitate was washed with H2O and petroleum ether to give the crude
cyclometalated Ir(III) chloro-bridged dimer. The slurry of crude chloro-bridged dimer
(0.43 mmol) and 2.2 equivalent KPOXD17 (0.95 mmol) were dissolved in 2-
methoxyethanol (20 mL). The mixture was then refluxed at 120 °C for 12 h under an
argon atmosphere. The crude compound was filtered after the solution cooled to room
temperature and purified by column chromatography on silica gel (200–300 mesh) with
petroleum ether/ethyl acetate (4:1) as the eluent, yielding of iridium complex powder.
Ir(pt)2(POXD) (1). Overall yields: 50%. 1H NMR (400 MHz, CDCl3, δ): 8.08 (d, J =
4.0 Hz, 1H), 7.97–7.91 (m, 1H), 7.80 (dd, J = 4.0 and 8.0 Hz, 2H), 7.62 (d, J = 4.0 Hz,
2H), 7.55–7.45 (m, 5H), 7.40–7.29 (m, 7H), 7.16 (d, J = 4.0 Hz, 2H), 6.89 (t, J = 8.0 Hz,
1H), 6.84–6.78 (m, 3H), 6.66 (t, J = 8.0 Hz, 1H), 6.43 (d, J = 8.0 Hz, 1H), 6.19 (t, J =
8.0 Hz, 1H). ESI-MS [m/z]: 872.9 (M + H)+. Anal. found: C 52.31, H 3.10, N 7.99.
Anal. Calcd for C38H27N5O2PS2Ir: C 52.28, H 3.12, N 8.02.
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Ir(bt)2(POXD) (2). Overall yields: 62%. 1H NMR (400 MHz, CDCl3, δ): 9.02 (d, J =
8.0 Hz, 1H), 7.81–7.70 (m, 4H), 7.64 (t, J = 8.0 Hz, 3H), 7.50–7.31 (m, 11H), 7.15 (t, J
= 4.0 Hz, 2H), 7.01–6.92 (m, 6H), 6.75 (t, J = 8.0 Hz, 1H), 6.54 (dd, J = 8.0 and 12.0 Hz,
2H), 6.26 (d, J = 8.0 Hz, 1H). ESI-MS [m/z]: 973.9 (M + H)+. Anal. found: C 56.74, H
3.23, N 7.24. Anal. Calcd for C46H31N5O2PS2Ir: C 56.78, H 3.21, N 7.20.
Ir(nbt)2(POXD) (3). Overall yields: 64%. 1H NMR (400 MHz, CDCl3, δ): 9.12 (d, J =
8.0 Hz, 1H), 8.26 (d, J = 20.0 Hz, 2H), 7.88 (d, J = 8.0 Hz, 2H), 7.79–7.70 (m, 5H),
7.57–7.49 (m, 6H), 7.35–7.28 (m, 6H), 7.20–6.86 (m, 12H), 6.52 (s, 1H). ESI-MS [m/z]:
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1074.1 (M + H)+. Anal. found: C 60.47, H 3.31, N 6.49. Anal. Calcd for
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C54H35N5O2PS2Ir: C 60.43, H 3.29, N 6.53.
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2.3. Quantum Chemical Calculations
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Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations
were conducted to gain insights into the electronic states and the orbital distribution.
The B3LYP35, 36 density functional was used to optimize the geometries of the Ir
complexes in the acetonitrile solvent using the integral equation formalism polarized
continuum model (IEFPCM).37 The 6-31G(d, p) and Lanl2DZ basis sets were applied
for the main group atoms and Ir atom, respectively. All the calculations were performed
using the Gaussian 09 program.38 The visualization of molecular orbitals was
accomplished using the GaussView program.
2.4. Fabrication and characterization of OLEDs
Device was fabricated upon a glass substrate, which was precoated with an about 100
nm thick layer of indium-tin-oxide (ITO) with a sheet resistance of about 10 Ω sq-1, then
degreased with solvents, cleaned by exposure to oxygen plasma and ultraviolet-ozone
ambient. All the organic layers and cathode were fabricated by vacuum evaporation
deposition under a low pressure of 1× 10−4 Pa. The luminance–voltage–current density
characteristics were measured with a PR655 spectrometer and a Keithley 2400
programmable voltage-current source in ambient atmosphere without further
encapsulation.
3. Results and discussion
3.1. Synthesis and Structural Characterization
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Scheme 1 Synthetic routes for complexes 1−3.
Chemical structures and synthetic protocols of iridium compelxes 1−3 are shown in
Scheme 1. The cyclometalated ligand Hnbt and potassium salt KPOXD of ancillary
ligand used in this work were synthesized according to the reported procedures.17, 25
Complexes 1−3 were synthesized by reacting the dimeric iridium(III) intermediates
[(pt)2Ir(μ-Cl)2Ir(pt)2], [(bt)2Ir(μ-Cl)2Ir(bt)2] and [(nbt)2Ir(μ-Cl)2Ir(nbt)2] with KPOXD
according to the literature procedure.17, 39, 40 We purified the crude products by column
chromatography on silica gel (200–300 mesh) with petroleum ether/ethyl acetate as
1
eluent, and fully characterized them by H NMR, ESI (electron spray ionization) mass
spectroscopy, elemental analysis (see the Experimental Section).
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Fig. 1 The molecular structures of complexes 2 (a) and 3 (b). Hydrogen atoms are
omitted for clarity.
The crystal structures (CCDC numbers are 1836142 and 1889387, respectively) of
complexes 2 and 3 were further examined using single-crystal X-ray diffraction
analysis. As shown in Fig. 1, the Ir center exihibits a pseudo-octahedral geometry with
two cyclometalated ligands adopting C,C-cis, N,N-trans configurations, which
resembles those observed in other cyclometalated iridium complexes.40, 41 The Ir–C(C˄N)
,
Ir–N(C˄N), Ir–N(N˄O) and Ir–O(N˄O) bond lengths, as well as the ligand bite angles, are
similar to those of previously reported iridium complexes.32, 33 The downside is that
single crystals of 1 could not be obtained.
3.2. Photophysical Properties
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Fig. 2 Normalized absorption and emission pectra of complexes 1−3 in CH3CN solution
(a) and Dictrz doped film (b).
Table 1. Photophysical characteristics of complexes 1−3.
Electrochemical
Emission at room temperature
λabs [nm]
data[d]
(ε[×104
M-1cm-1]) [a]
λem [nm]/Φem/τ[μs]
Eox [V]
0.58
Ered [V]
CH3CN[b]
Dictrz film[c]
224(5.77)/
292(3.70)/
385(0.44)
217(6.52)/
313(2.83)/
401(0.51)
228(11.66)/
327(6.54)/
418(0.47)
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3
542/4%/0.14
542/88%/1.85
557/47%/1.09
593/37%/1.64
-2.49
-2.21
-2.12
555/3%/0.45
585/2%/0.56
0.71
0.56
[a] In CH3CN solution (1 × 10-5 M). [b] In degassed CH3CN solution. λex: 385 nm. [c] Dictrz film was
[d]
deposited on quartz substrate with thickness of about 100 nm. λex: 385 nm. Electrochemical data
versus Fc+/Fc (Fc is ferrocene) were collected in CH3CN solution (10-3 M).
The absorption and photoluminescent (PL) spectra of complexes 1−3 are shown in
Fig. 2 and detailed photophysical characterizations are depicted in Table 1. The broad
and strong absorption bands in the UV region below about 400 nm assigned to spin-
allowed 1π−π* transitions of the cyclometalated main ligands and ancillary ligands. The
relatively weak absorption bands (see Fig. S1 in Supporting Information) extending to
1
about 525 nm are attributed to excitations to MLCT (metal-to-ligand charge-transfer),
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1LLCT (ligand-to-ligand charge-transfer), MLCT, LLCT, and ligand-centered (LC)
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3π−π* transitions.42-46 It is notable that the absorption peaks shift toward longer
wavelength with the changes of cyclometalating ligand from Hpt to Hbt and Hnbt due
to a gradual enlargement of conjugacy.
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As shown in Fig. 2a, in CH3CN solution, the PL peaks of the complexes have red
shift with the changes of cyclometalating ligand from Hpt to Hbt and Hnbt, which is
consistent with their absorption spectra. Complex 1 shows greenish yellow emission
peaked at 542 nm, with a PLQY of 0.04 and an excited state lifetime of 0.14 μs (see Fig.
S2 in Supporting Information), which resemble to those of previously reported cationic
iridium complex [Ir(ppy)2(bpy)]PF6 (bpy is 2,2'-bipyridine).47 Complex 2 shows yellow
emission peaked at 555 nm, with a PLQY of 0.03 and an excited state lifetime of 0.45
μs. In contrast, complex 3 gives orange emission peaked at 585 nm, with a PLQY of
0.02 and an excited state lifetime of 0.56 μs. Their radiative decay rates (kr) calculated
are 2.86 × 105 s-1, 0.67 × 105 s-1 and 0.36 × 105 s-1, respectively, whereas their
nonradiative decay rates (knr) calculated are 68.57 × 105 s-1, 21.56 × 105 s-1 and 17.50 ×
105 s-1, respectively, from the PLQYs and the lifetime values. The rapid nonradiative
decay is responsible for the low quantum yield of the excited states. The PL spectra of
complexes 1−3 in CH3CN solution are vibronically structured, indicating that at room
3
temperature, the emitting triplet states of these complexes are dominated by LC π−π*
character.
In sharp contrast to the PLQYs in solutions, the doped (10 wt %) films of complexes
1−3 in Dictrz48 exhibit significantly better PLQYs of 0.88, 0.47 and 0.37, respectively,
accompanied by remarkable increase of excited state lifetimes (1.85, 1.09 and 1.64 μs,
respectively, see Fig. S3 in Supporting Information), which plausibly result from the
great restriction of rotations and vibrations in rigid host.42, 49 In addition, as shown in
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Fig. 2b, complexes 1−3 in doped films show more structured PL spectraDwOI:it1h0.10t3h9e/C8NJ05370G
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emission peaks at 584, 505 and 507 nm, respectively, indicating dominant LC π−π*
8
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character in their emitting triplet states.
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3.3. Electrochemical Properties and Theoretical Calculations
The electrochemical behaviors of complexes 1−3 were measured by cyclic
voltammetry (CV) versus ferrocenium/ferrocene (Fc+/Fc) using n-Bu4NPF6 (0.1 M) as
the supporting electrolyte in CH3CN solution. Fig. 3 depicts the CV curves and redox
potentials are summarized in Table 1. All complexes exhibit reversible oxidation
processes with the oxidation potentials at 0.58, 0.71 and 0.56 V, respectively, which are
almost identical to those of other thiazole-based iridium complexes.50 Complexes 1−3
undergo irreversible reduction processes with the reduction potentials at −2.49, −2.21
and −2.12 V, respectively. The HOMO (−5.38 eV for 1, −5.51 eV for 2 and –5.36 eV
for 3, respectively) and LUMO (−2.31 eV for 1, −2.59 eV for 2 and –2.68 eV for 3,
respectively) levels of the complexes were calculated from the oxidation and reduction
potentials.
Fig. 3 Cyclic voltammograms of complexes 1−3. Potentials were recorded versus
Fc+/Fc.
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To provide deeper insight into the photophysical and electrochemical properties of
complexes 1−3, DFT calculations at the B3LYP/6-31G ** + LANL2DZ level were
carried out. The frontier molecular orbital composition, including the HOMOs and
LUMOs of complexes 1−3 are displayed in Fig. 4. HOMOs of complexes 1 and 2 were
composed of phenyl groups of cyclometalated ligands, oxadiazole group of POXD and
the iridium center. For complex 3, HOMO resides on the iridium center and naphthyl
group of Hnbt ligand. LUMOs of complexes 1 and 2 were almost entirely distributed on
one of the cyclometalated ligands. In particular, LUMO of complex 3 was located on
two cyclometalated ligands.
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Fig. 4 Isodensity plots of the frontier orbitals of complexes 1–3. All of the molecular
orbital surfaces correspond to an isocontour value of |C| = 0.025.
The electronic transition dipole of low-lying triplet states of complexes 1−3 were
calculated by TD-DFT approach in order to investigate the nature of the emitting
excited states. The vertical excitation energies and orbitals involved in the excitations
for calculated triplets below 3.0 eV were summarized in Table 2. For T1, T2 and T3
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states of complex 1, the orbitals involved are the HOMO−3, HOMO−2, HOMO−1,
HOMO, LUMO, LUMO+1, LUMO+2 and LUMO+3 orbitals, rendering these states
the nature of mixed 3MLCT (dπ(Ir)→*(pt), dπ(Ir)→*(POXD)), 3LC −* ((pt-based)
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3
and (POXD-based)) and LLCT ((pt) → *(POXD), (POXD) → *(pt)) characters.
3
The T1, T2 and T3 states of complex 2 can be rendered the nature of mixed MLCT
3
(dπ(Ir)→*(bt), dπ(Ir)→*(POXD)), LC −* ((bt-based) and (POXD-based)) and
3LLCT ((bt)→*(POXD), (POXD)→*(bt)) characters. For complex 3, however, the
3
3
T1, T2 and T3 states can be attributed to MLCT (dπ(Ir)→*(nbt)), LC −* ((nbt-
based). Considering that the vibronically structured emission spectra and the red-shift of
PL spectra with the conjugation degrees of cyclometalated ligands increasing, we
tentatively assign the emission of complexes 1−3 to be mainly from cyclometalated
ligand-based 3π−π* states.
Table 2 Selected triplet states for complexes 1−3 calculated from TDDFT approach.
States
T1
E [a] [eV]
2.41
Dominant excitations [b]
H → L (0.46)
Nature
1
dπ(Ir)-π(pt)-π(POXD) → *(pt)
dπ(Ir)-π(pt)-π(POXD) → *(pt)
dπ(Ir)-π(pt) → π*(pt)
H – 1 → L (0.17)
H – 2 → L (0.13)
H → L + 1 (0.59)
H – 2 → L (0.15)
H – 3 → L + 1 (0.14)
H – 1 → L + 2 (0.35)
H – 1 → L + 3 (0.23)
H → L + 2 (0.22)
H → L (0.45)
T2
T3
T1
2.45
2.74
2.40
dπ(Ir)-π(pt)-π(POXD) → *(pt)
dπ(Ir)-π(pt) → π*(pt)
dπ(Ir)-π(pt) → π*(pt)
dπ(Ir)-π(pt)-π(POXD) → π*(POXD)
dπ(Ir)-π(pt)-π(POXD) → π*(POXD)
dπ(Ir)-π(pt)-π(POXD) → π*(POXD)
dπ(Ir)-π(bt)-π(POXD) → π*(bt)
dπ(Ir)-π(bt)-π(POXD) → π*(bt)
dπ(Ir)-π(bt) → π*(bt)
2
3
H – 1 → L (0.31)
H – 2 → L (0.17)
H → L + 1 (0.69)
T2
T3
2.42
2.65
dπ(Ir)-π(bt)-π(POXD) → π*(bt)
H → L (0.52)
H – 2 → L (0.31)
H → L (0.81)
H – 2 → L + 1 (0.14)
H → L + 1 (0.73)
H – 2 → L (0.24)
H → L (1.00)
dπ(Ir)-π(bt)-π(POXD) → π*(bt)
dπ(Ir)-π(bt)-π(POXD) → π*(bt)
dπ(Ir)-π(nbt) → π*(nbt)
dπ(Ir)-π(nbt) → π*(nbt)
dπ(Ir)-π(nbt) → π*(nbt)
dπ(Ir)-π(nbt) → π*(nbt)
dπ(Ir)-π(nbt) → π*(nbt)
T1
T2
T3
2.09
2.14
2.55
[a] Excitation energies calculated for the triplet states. [b] H and L denote HOMO and LUMO,
respectively; data in parentheses are the contributions of corresponding excitations.
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3.4. Electroluminescent Properties of OLED Devices
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In order to evaluate the electroluminescent properties of these complexes, 1−3 were
employed as doped emitters to fabricate the OLEDs with the structure of ITO/HAT-CN
(20 nm)/NPB (10 nm)/TCTA (20 nm)/Dictrz: x% Ir complex (10 nm)/TmPyPB (40
nm)/Liq (1 nm)/Al (100 nm) (HAT-CN is dipyrazino[2,3-f:2',3'-h]quinoxaline
2,3,6,7,10,11-hexacarbonitrile, NPB is N,N'-bis(naphthalene-1-yl)-N,N'-bis(phenyl)-
benzidine, TCTA is 4,4',4''-tris(carbazol-9-yl)triphenylamine, TmPyPB is 1,3,5-tri(m-
pyrid-3-yl-phenyl)benzene). HAT-CN acted as hole-injecting layer; NPB and TCTA
were used as hole-transporting materials; The aromatic thermally activated delayed
fluorescence compound Dictrz have a triplet energy (ET) of 3.0 eV,48, 51 which served as
hosts for complexes 1 (ET = 2.30 eV), 2 (ET = 2.24 eV) and 3 (ET = 2.12 eV). The
higher ET of host can prevent the energy transfer of triplet excitons from the emitters
back to the host material more effectively; TmPyPB was employed as electron-
transporting material; Liq/Al acted as electron-injection layer and cathode. The device
architecture and chemical structures of the materials used are shown in Fig. 5. Doping
concentrations were varied from 1, 5 to 10 wt % in order to optimize the device
performance.
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Fig. 5 Device architecture and chemical structures of the materials used in the OLEDs.
The electroluminescence (EL) spectra and potential (V) –luminance (L) – current
density (J) curves of the OLEDs are depicted in Figs. 6–8. As shown in Figs. 6(a), 7(a)
and 8(a), devices based on 1, 2 and 3 emit greenish yellow, yellow and orange light with
major EL peak at 540, 553 and 594 nm with the Commission International de
I’Eclairage (CIE) coordinates of (0.46, 0.53), (0.49, 0.50) and (0.59, 0.40), respectively,
which closely resemble those of PL spectra in solution and Dictrz doped film, indicating
that EL originated from the triplet excited states of complexes 1–3. However, at low
doping concentrations of the dopant (1% and 5 wt %), a weak emission from host Dictrz
was observed, indicating incomplete energy transfer from Dictrz to complexes 1–3.
With an increase of the doping concentration to 10%, emission from host disappeared.
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As can be seen in Figs. 6(b), 7(b) and 8(b), as increasing the dopant cDoOnIt:e10n.1t03o9/fC8NJ05370G
complexes in OLEDs, evolution trend of luminance with applied bias are similar, but
current density have different evolution trend with applied bias. The diffient evolution
trend of current density may stem from the diffrerent extent of carrier trapping and
energy transfer in the host-guest emitting system of Dictrz-complexes 1–3. From Figs.
6(a), 7(a) and 8(a), we can judge that there existed the incomplete energy transfer from
Dictrz to complexes when the doping concentration is lower than 10%. On the other
hand, the energy barrier (0.6 eV) between electron transport layer and adjacent host is
big. The host doped with a guest molecule of a deep LUMO level can effectively reduce
the barrier and faciliate electron injection. This leads to different evolution trend of
current density with applied bias. The current density within one OLED can be
described as J = JI – Jr, in which JI is the injected current, and Jr is the recombation
current, respectively. As increasing the dopant content of complexes in OLEDs, the
current density for complex 1 slightly decreased from 1% to 5% due to increase in
recombination current (Jr), which can be demonstrated from the efficient energy transfer
as depicted in PL spectra of Fig. 6(a). And then, with the doping concentration
increased from 5% to 10%, the current density rose due to efficient electron injection (JI)
into Dictrz through complex 1. For complex 2, as the ascent concentration from 1% to
5%, the current density increased because the electrons may hop more efficiently
through the channel of complex 2 due to the little gap between the LUMO of dopant
and the adjacent electron transport layer. Despite the elevated recombination current,
the enhanced electrons is bigger than recombined ones in dye sites. While the
concentration of the complex 2 increased to 10%, the accumulated charge may cause the
concentration quenching giving rise to the nonradiative triplet–polaron annihilation
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(TPA), which lead to the severe drop of current density. With regards to compDleOxI: 130.,10t3h9e/C8NJ05370G
current increased dramatically due to dominant trapping effect of electrons in complex 3
for very small barrier. Comparatively, notwithstanding the existence of TPA, the
barrier-free interface facilitate the high efficient injection, leading the more increased
injected charge than the quenching current.
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Fig. 6. (a) normalized EL spectra and (b) L–V–J characteristics of OLEDs with a
structure of ITO/HAT-CN (20 nm)/NPB (10 nm)/TCTA (20 nm)/Dictrz: x% complex 1
(10 nm)/TmPyPB (40 nm)/Liq (1 nm)/Al (100 nm), x = 1, 5 or 10.
Fig. 7. (a) normalized EL spectra and (b) L–V–J characteristics of OLEDs with a
structure of ITO/HAT-CN (20 nm)/NPB (10 nm)/TCTA (20 nm)/Dictrz: x% complex 2
(10 nm)/TmPyPB (40 nm)/Liq (1 nm)/Al (100 nm), x = 1, 5 or 10.
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Fig. 8. (a) normalized EL spectra and (b) L–V–J characteristics of OLEDs with a
structure of ITO/HAT-CN (20 nm)/NPB (10 nm)/TCTA (20 nm)/Dictrz: x% complex 3
(10 nm)/TmPyPB (40 nm)/Liq (1 nm)/Al (100 nm), x = 1, 5 or 10.
Table 3. EL performances of the devices based on complexes 1−3.
Vturn-on [b]
Lmax [c] ηc,max [d]
ηp,max [e]
λEL [f]
[nm]
CIE [g]
(x, y)
x [a]
[V]
[cd m-2]
2400
[cd A-1]
6.25
[lm W-1]
1
2
3
1
5
4.5
4.0
4.0
3.5
3.5
3.4
4.0
4.0
3.5
3.92
9.19
539, 578 (sh)
539, 579 (sh)
540, 580 (sh)
552, 595 (sh)
552, 595 (sh)
553, 596 (sh)
588, 633 (sh)
591, 632 (sh)
594, 633 (sh)
(0.38, 0.48)
(0.43, 0.52)
(0.46, 0.53)
(0.46, 0.49)
(0.48, 0.50)
(0.49, 0.50)
(0.42, 0.45)
(0.56, 0.41)
(0.59, 0.40)
6650
13100
11000
9040
12.85
20.06
13.52
17.01
22.20
4.90
10
1
13.54
9.23
5
10.58
17.31
3.68
10
1
26000
2930
5
4640
7.78
5.43
10
8950
13.45
9.48
[a] x: doping concentration. [b] Vturn-on: Turn-on voltage recorded at a luminance of 1 cd m-2. [c]
Lmax: maximum luminance. [d] ηc,max: maximum current efficiency. [e] ηp,max: maximum power
efficiency. [f] λEL: maximum EL wavelength. [g] CIE: Commission Internationale de l'Éclairage.
As shown in Figs. 6b, 7b, 8b, S4 (in Supporting Information) and Table 3, a
maximum luminance (Lmax) of 13100 cd m−2, a maximum current efficiency (c, max) of
20.06 cd A−1 and a maximum power efficiency (p, max) of 13.54 lm W−1 for complex 1,
a Lmax of 26000 cd m−2, a c, max of 22.20 cd A−1 and a ext, max of 17.31 lm W−1 for
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complex 2 and a Lmax of 8950 cd m−2, a c, max of 13.45 cd A−1 and a ext, max of 9.48 lm
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W−1 for complex 3 are achieved. These devices exhibit inferior EL properties compared
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with those of other POXD-based iridium(III) complexes.17,
The relatively low
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efficiencies of these devices are presumably due to the inefficient energy transfer from
the host to these complexes or TPA in emitting layer with high doping concentration
(vide supra). The devices based on complexes 1 and 2 exhibit better
electroluminescence properties compared with those based on complex 3, which can be
ascribed to the higher PLQYs of Dictrz doped films in complexes 1 and 2.
4. Conclusion
In summary, by adjusting the conjugation degrees of the 2-aryl-thiazole-based
cyclometalated ligands, greenish yellow, yellow to orange phosphorescent iridium(III)
complexes (1–3) bearing N-(5-phenyl-1,3,4-oxadiazol-2-yl)-diphenylphosphinic amide
as ancillary ligand have been synthesized and fully characterized with their
electrochemical, photophysical and device optoelectronic properties. The PhOLEDs
with the structure of ITO/HAT-CN (20 nm)/NPB (10 nm)/TCTA (20 nm)/Dictrz: 10%
complexes (10 nm)/TmPyPB (40 nm)/Liq (1 nm)/Al (100 nm) shows excellent
performance with the maximum current efficiencies of 20.06, 22.20 and 13.45 cd A-1
with the CIE coordinates of (0.46, 0.53), (0.49, 0.50) and (0.59, 0.40) for complexes
1−3, respectively.
Acknowledgements
We would like to thank the National Natural Science Foundation of China (Grant
Nos. 21501117, U1704140, 21571123 and 21601119) and the Program for Science and
Technology Innovation Talents in Universities of Henan Province (19HASTIT034) for
financial support.
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DOI: 10.1039/C8NJ05370G
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By using aryl-thiazoles with different conjugation degree as cyclometalated ligands
and oxadiazol-substituted amide as ancillary ligand, greenish yellow, yellow to orange
phosphorescent iridium(III) complexes have been synthesized and fully characterized
with their electrochemical, photophysical and device optoelectronic properties.