Efficient yellow electroluminescence of four iridium(iii) complexes with benzo[: D] thiazole derivatives as main ligands

Article abstract of DOI:10.1039/c8dt01479e

Four bis-cyclometalated iridium(iii) complexes, (bt)₂Ir(tpip), (fbt)₂Ir(tpip), (cf₃bt)₂Ir(tpip) and (dfbt)₂Ir(tpip) (bt = 2-phenylbenzo[d]thiazole, fbt = 6-fluoro-2-phenylbenzo[d]thiazole, cf₃

Full text of DOI:10.1039/c8dt01479e

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Efficient yellow electroluminescence of four iridium(III)
complexes with benzo[d]thiazole derivatives as main ligands
Cite
this:
DOI:
10.1039/x0xx00000x
ZhiꢀGang Niu1,2, Jun Chen1, Peng Tan2, Wei Sun2, YouꢀXuan Zheng*,1, GaoꢀNan Li*,2,
Jingꢀlin Zuo*,1
Received 00th April 2018,
Accepted 00th April 2018
Four bisꢀcyclometalated iridium(III) complexes, (bt)2Ir(tpip), (fbt)2Ir(tpip), (cf3bt)2Ir(tpip) and
(dfbt)2Ir(tpip) (bt = 2ꢀphenylbenzo[d]thiazole, fbt = 6ꢀfluoroꢀ2ꢀphenylbenzo[d]thiazole, cf3bt = 2ꢀ
phenylꢀ6ꢀ(trifluoromethyl)benzo[d]thiazole, dfbt = 5,7ꢀdifluoroꢀ2ꢀphenylbenzo[d]thiazole, tpip =
tetraphenylimidodiphosphinate) have been synthesized and investigated. All complexes emit yellow
DOI: 10.1039/x0xx00000x
www.rsc.org/
lights peak at 564–574 nm with quantum efficiencies (Φem) of 27.1−48.4% and excited state lifetimes
of 2.40−2.81 ꢁs in degassed CH2Cl2 solution at room temperature, respectively. Correspondingly, the
organic lightꢀemitting diodes (OLEDs) using these complexes as emitters achieve yellow
electrophosphorescence with good device characteristics. Due to its highest photoluminescence
quantum yield (48.4%), the device based on (dfbt)2Ir(tpip) displays the best device performances
with a maximum current efficiency (ηc,max) up to 69.8 cd Aꢀ1 and a maximum external quantum
efficiency (EQEmax) up to 24.3%. Furthermore, all devices showed low efficiency rollꢀoff ratios. The
EQE still could be retained at 17.7%, 16.4%, 18.3% and 20.6% for four devices at a luminance of
1000 cd m−2, respectively. These results suggest that these materials have potential application in
efficient OLEDs.
Introduction
Organic lightꢀemitting diodes (OLEDs) have attracted great attempt different types of ancillary ligands. From our previous
interest in the development of modern optoelectronic work, tetraphenylimidodiphosphinate (tpip) derivatives with polar
technologies such as lowꢀcost, moreꢀefficient flatꢀpanel displays P=O bonds and phenyl rings were used as ancillary ligands,16
and the next generation solidꢀstate lighting sources.1ꢀ3 In which can improve the electron mobility of the emitters and
particular, luminescent iridium(III) complexes play an important benefit their device performances.17 Besides, we found that the
role in the fabrication of efficient OLEDs, owing to their high fluorinated main ligands can modify the optical properties of
photoluminescence quantum yields, good stability and flexible iridium complexes. Particularly, the bulky –CF3 group can affect
color tunability.4ꢀ5
the packing of molecular and the according steric protection
In order to achieve highly efficient fullꢀcolor and white OLEDs around metal can restrain the selfꢀquenching of luminescence.
(WOLEDs), the yellowꢀ or orangeꢀemitting Ir(III) phosphors are Again, the C–F bonds with lower vibrational frequency can
also the most important research directions, since they can be reduce the rate of nonradioactive deactivation, which would
used in combination with deep blue emitters to fabricate twoꢀ enhance the photoluminescence quantum yields.18
emittingꢀcomponent WOLEDs.6 In this regard, many different
In consideration of these factors, in this study we synthesized
classes of Ir(III) complexes have been synthesized and four Ir(III) complexes (Scheme 1) with bt derivatives ( fbt = 6ꢀ
investigated.7ꢀ13 Among them, 2ꢀphenylbenzothiazole (bt) is one fluoroꢀ2ꢀphenylbenzo[d]thiazole,
typical ligand framework to construct orangeꢀyellow Ir(III) (trifluoromethyl)benzo[d]thiazole,
cf3bt
dfbt
=
2ꢀphenylꢀ6ꢀ
5,7ꢀdifluoroꢀ2ꢀ
=
complexes. Since the first yellowꢀemitting Ir(III) complex phenylbenzo[d]thiazole) as the cyclometalated ligands and tpip as
(bt)2Ir(acac) (acac = acetylacetonate) was reported,14 a number of the ancillary ligand. All complexes emit yellow lights peak at
bt derivatives with substituents on the ligands have been 564–574 nm in degassed CH2Cl2 solution at room temperature
developed for OLED applications.6, 15 These early examples of and the fluorinated substituents in the main ligands have great
neutral btꢀbased iridium(III) complexes adopt acetylacetonate effects on their photophysical properties. Using these complexes
(acac) as ancillary ligand, and their electroluminescent properties as emitters, all device show good electroluminescence properties
are not very satisfying. To further improve the efficiencies of with maximum current efficiencies (ηc,max) of 57.1–69.8 cd Aꢀ1
yellow and orange phosphorescent materials, it is necessary to
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DOI: 10.1039/C8DT01479E
and maximum external quantum efficiencies (EQEmax) of 18.5– at room temperature and 77 K. The thermoanalytical analysis
24.3%.
(TGA) was performed under a flow of nitrogen using a
simultaneous NETZSCH STA 449C thermal analyzer.
Luminescence lifetime curves were measured on an Edinburgh
Instruments FLS920P fluorescence spectrometer and the data
were treated as oneꢀorder exponential fitting using OriginPro 8
software. Cyclic voltammetry (CV) was performed on a CHI
Experimental section
Materials and measurements
All reagents and chemicals were purchased from commercial
sources and used without further purification. 1H, 19F and 31P
spectra were recorded on a Bruker AM 400 MHz instrument, and
chemical shifts were reported in ppm relative to Me4Si as internal
standard. ESIꢀMS and MALDIꢀTOFꢀMS spectra were recorded
on an Esquire HCT−Agilent 1200 LC/MS spectrometer and a
Bruker AutoflexII TM TOF/TOF instrument, respectively. UV–
vis spectra were recorded on
spectrophotometer. Fluorescence spectra were carried out on a
Hitachi F−7000 spectrophotometer as deaerated CH2Cl2 solutions
1210B electrochemical workstation, with
a glassy carbon
electrode as the working electrode, a platinum wire as the counter
electrode, an Ag/Ag+ electrode as the reference electrode, and 0.1
MnꢀBu4NClO4 as the supporting electrolyte. All solutions were
deaerated by nitrogen bubbling for 30 min before measurements.
The luminescence quantum efficiencies were calculated by
comparison of the emission intensities (integrated areas) of a
standard sample (facꢀIr(ppy)3) and the unknown sample.7
a Hitachi U3900/3900H
Scheme 1. Synthetic routes of four Ir(III) complexes (bt)2Ir(tpip), (fbt)2Ir(tpip), (cf3bt)2Ir(tpip) and (dfbt)2Ir(tpip).
Syntheses
with brine. Then solvent was removed and the resulting solid was
purified by column chromatography (petroleum ether : ethyl
All reactions were performed under nitrogen atmosphere. The
synthetic routes of ligands and complexes are also showed in
Scheme 1. Tetraphenylimidodiphosphinate (tpip) and its
potassium salt (Ktpip) were synthesized according to our
previous work.19 2ꢀAminoꢀ5ꢀfluorobenzenethiol and 2ꢀaminoꢀ5ꢀ
(trifluoromethyl)benzenethiol were prepared according to the
literature methods.20
acetate = 30 : 1) to give the ligands of 3a−3c as white solid.
2ꢀPhenylꢀbenzo[d]thiazole (3a, 0.96 g, yield: 84.2%). 1H NMR
(400 MHz, CDCl3) δ (ppm) 8.07~8.10 (m, 3H), 7.91 (d, J = 8.0
Hz, 1H), 7.46~7.50 (m, 4H), 7.39 (t, J = 7.4 Hz, 1H). MS (ESI):
m/z 212.05 [M+H]+.
6ꢀFluoroꢀ2ꢀphenylbenzo[d]thiazole (3b, 0.73 g, yield: 80.3%).
1H NMR (400 MHz, CDCl3) δ (ppm) 8.04~8.07 (m, 2H), 8.01
(dd, J = 4.8 Hz, 8.8 Hz, 1H), 7.58 (dd, J =2.8 Hz, 8.0 Hz, 1H),
7.50 (t, J = 4.0 Hz, 3H), 7.20~7.25 (m, 1H). MS (ESI): m/z
230.04 [M+H]+.
General syntheses of benzothiazole ligands (3a−3c).
Benzaldehyde (0.60 g, 5.65 mmol) was mixed with a substituted
2ꢀaminothiophenol (0.71 g, 5.65 mmol) in 20 mL of DMSO and
an equimolar amount of Na2S2O3 was then added. The reaction
was heated at 120 °C for 3 hours. After cooling, H2O was added
and the precipitate collected was dissolved in CH2Cl2, washed
2ꢀPhenylꢀ6ꢀ(trifluoromethyl)benzo[d]thiazole (3c, 0.75 g,
1
yield: 82.6%). H NMR (400 MHz, CDCl3) δ (ppm) 8.10~8.20
2 | J. Name., 2012, 00, 1-3
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(m, 4H), 7.73 (d, J = 8.8 Hz, 1H), 7.51~7.53 (m, 3H). MS (ESI): 7.6 Hz, 2H), 7.53 (d, J = 7.6 Hz, 2H), 7.31~7.36 (m, 6H),
m/z 280.03 [M+H]+.
7.21~7.25 (m, 4H), 7.11 (t, J = 7.6 Hz, 2H), 6.81~6.88 (m, 4H),
6.68~6.76 (m, 6H), 6.57 (t, J = 7.6 Hz, 2H), 6.27 (d, J = 7.6 Hz,
2H). 31P NMR (162 MHz, CDCl3): (ppm) 23.06. MALDIꢀTOF
Synthesis of 5,7-difluoro-2-phenylbenzo[d]thiazole (3d)
Nꢀ(3,5ꢀDifluorophenyl)benzamide (1d). Benzoyl chloride (1.14 calcd for C50H36IrN3O2P2S2: 1029.135 ([M+H]+), found:
g, 8.13 mmol) was added dropwise to a solution of 3,5ꢀ 1029.467. Anal. calcd. for C50H36IrN3O2P2S2: C 58.35, H 3.53, N
difluoroaniline (1.0 g, 7.75 mmol) and Et3N (1.18 g, 1.62 mL) in 4.08. Found: C 58.39, H 3.62, N 4.05.
1
dry DCM (30 mL) at 0 oC. After the addition, the reaction
(fbt)2Ir(tpip) (yield: 46.3%) H NMR (400 MHz, CDCl3) δ
mixture was warmed up to room temperature and stirred (ppm) 8.86 (dd, J = 4.8 Hz, 8.8 Hz, 2H), 7.83~7.88 (m, 4H), 7.58
overnight. Then the mixture was diluted with water and extracted (d, J = 7.6 Hz, 2H), 7.35~7.37 (m, 6H), 7.19~7.25 (m, 6H), 6.92
with DCM. The combined organic phases were washed with (t, J = 7.2 Hz, 2H), 6.79~6.86 (m, 6H), 6.59 (t, J = 6.8 Hz, 2H),
brine and evaporated. The crude product was recrystallized from 6.35~6.40 (m, 2H), 6.24 (d, J = 7.6 Hz, 2H). 19F NMR (367 MHz,
the mixture of ethyl acetate and petroleum ether (V : V = 1 : 1) to CDCl3) δ (ppm) ꢀ116.87. 31P NMR (162 MHz, CDCl3): (ppm)
give compound 1d (1.58 g, yield: 87.5%) as white solid, which 23.06. MALDIꢀTOF calcd for C50H34F2IrN3O2P2S2: 1065.116
was used in next step without further purification.
([M+H]+),
found:
1065.324.
Anal.
Calcd.
For
Nꢀ(3,5ꢀDifluorophenyl)benzothioamide (2d). To a solution of
1d (1.50 g, 6.43 mmol) in toluene (50.0 mL) was added
Lawesson’s reagent (2.60 g, 6.43 mmol). The solution was
C50H34F2IrN3O2P2S2: C 56.38, H 3.22, N 3.95. Found: C 56.29, H
3.26, N 3.98.
(cf3bt)2Ir(tpip) (yield: 52.6%) H NMR (400 MHz, CDCl3) δ
1
refluxed for 8 hours, and then concentrated. The residue was
purified by column chromatography (petroleum ether : ethyl
acetate = 10 : 1) to give the product of 2d (1.35 g, yield: 84.2%)
as yellow solid.
(ppm) 8.98 (d, J=8.8Hz, 2H), 7.86~7.91 (m, 4H), 7.78 (s, 2H),
7.66 (d, J = 7.6 Hz, 2H), 7.39~7.40 (m, 6H), 7.20~7.24 (m, 4H),
6.82~6.89 (m, 6H), 6.71~6.73 (m, 4H), 6.62 (t, J = 7.6 Hz, 2H),
6.25 (d, J = 7.6 Hz, 2H). 19F NMR (367 MHz, CDCl3) δ (ppm) ꢀ
61.60. 31P NMR (162 MHz, CDCl3): (ppm) 23.15. MALDIꢀTOF
calcd for C52H34F6IrN3O2P2S2: 1165.110 ([M+H]+), found:
1165.352. Anal. calcd. for C52H34F6IrN3O2P2S2: C 53.60, H 2.94,
N 3.61. Found: C 53.51, H 3.02, N 3.72.
5,7ꢀDifluoroꢀ2ꢀphenylbenzo[d]thiazole (3d). To a solution of
o
K3Fe(CN)6 (5.28 g, 16.05 mmol) in H2O (10 mL) at 90 C was
added dropwise a solution of 2d (1.00 g, 4.0 mmol) and NaOH
(0.96 g, 24.13 mmol) in H2O/EtOH (20 mL / 4 mL). The solution
was heated at this temperature for another 8 h. The mixture was
cooled to room temperature, diluted with water and extracted
with DCM. The combined organic phases were washed with
brine and evaporated. The residue was purified by column
chromatography (petroleum ether : ethyl acetate = 20 : 1) to give
1
(dfbt)2Ir(tpip) (yield: 42.9%) H NMR (400 MHz, CDCl3) δ
(ppm) 8.55 (d, J = 10.0 Hz, 2H), 7.87~7.91 (m, 4H), 7.62 (d, J =
7.2 Hz, 2H), 7.29~7.37 (m, 10H), 6.82~6.94 (m,8H), 6.64 (t, J =
8.0 Hz, 4H), 6.23 (d, J = 8.0 Hz, 1H). 19F NMR (367 MHz,
CDCl3) δ (ppm) ꢀ111.08, ꢀ111.09, ꢀ113.32, ꢀ113.33. 31P NMR
(162 MHz, CDCl3): (ppm) 23.95. MALDIꢀTOF calcd for
C50H32F4IrN3O2P2S2: 1101.098 ([M+H]+), found: 1101.311. Anal.
calcd. for C50H32F4IrN3O2P2S2: C 54.54, H 2.93, N 3.82. Found:
C 54.41, H 2.97, N 3.90.
1
the product 3d (0.75 g, yield: 75.1%) as white solid. H NMR
(400 MHz, CDCl3) δ (ppm) 8.06~8.08 (m, 2H), 7.49~7.60 (m,
4H), 6.93 (dt, J1 = 9.2 Hz, J2 = 2.0 Hz, 1H). MS (ESI): m/z
248.04 [M+H]+.
General syntheses of iridium complexes.
Results and discussion
A mixture of IrCl3 (1.0 mmol) and the ligands 3a−3d (2.2 mmol)
in 9 mL of ethoxyethanol and H2O (V : V = 2 : 1) was heated at
120 °C for 24 h, respectively. Upon cooling to room temperature,
the precipitates were collected by filtration and washed with
water and MeOH. After drying, the cholorobridged dimers were
used directly in next step without further purification. A mixture
of the above dimers (0.2 mmol) and Ktpip (2.5 equiv.) in 10 mL
of ethoxyethanol was heated at 120 °C for 12 h. After the solvent
was removed, water was added and the reaction mixture was
extracted with CH2Cl2 three times, and then evaporated. The
residues were purified by flash column chromatography
(petroleum ether : dichloromethane = 10 : 1) to afford the target
Ir(III) complexes, which were further purified by sublimation in
vacuum.
Preparation and characterization of compounds
The syntheses of the cyclometalating ligands 3a−3d and their
derived complexes (bt)2Ir(tpip), (fbt)2Ir(tpip), (cf3bt)2Ir(tpip)
and (dfbt)2Ir(tpip) are shown in Scheme 1. The ligands 3a−3c
were conveniently synthesized by the improved method, which
used benzaldehyde and the corresponding substituted 2ꢀ
aminothiophenol as raw materials. The difluorinated ligand 3d
was easily synthesized through a traditional threeꢀstep reaction.
First, the amide compounds 1d was prepared between benzoyl
chloride and the corresponding substituted aniline by the
acylation reaction in good yield (87.5%), and the thioamide 2d
was then generated with Lawesson’s reagent in high yield over
80%. Sequential the main ligand 3d was prepared by the
cyclization reaction under K3Fe(CN)6 catalysis. The Ir(III) dimers
[(C^N)2Ir(ꢁꢀCl)]2 were synthesized by IrCl3 with 2.2 equiv of
(bt)2Ir(tpip) (yield: 48.7%) 1H NMR (400 MHz, CDCl3) δ
(ppm) 8.89 (d, J = 8.4 Hz, 2H), 7.81~7.87 (m, 4H), 7.61 (d, J =
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